Ingredients, structure and reconstitution properties of instant powder foods and the potential for healthy product development: a comprehensive review

Abstract

Instant foods are widely presented in powder forms across different food segments, which potentially can be formulated with functional or beneficial compounds to provide health benefits. Many reconstituted instant powder foods form colloidal suspensions with complex structures. However, designing instant powder food could be challenging due to the structural complexity and high flexibility in formulation. This review proposed a new classification method for instant powder foods according to the solubility of ingredients and the structure of the reconstituted products. Instant powder foods containing insoluble ingredients are discussed. It summarised challenges and current advances in powder treatments, reconstitution improvement, and influences on food texture and structure to facilitate product design in related industries. The characteristics and incorporation of the main ingredients and ingredients with health benefits in product development were reviewed. Different products vary significantly in the ratios of macronutrients. The macronutrients have limited solubility in water. After being reconstituted by water, the insoluble components are dispersed and swell to form colloidal dispersions with complex structures and textures. Soluble components, which dissolve in the continuous phase, may facilitate the dispersing process or influence the solution environment. The structure of reconstituted products and destabilising factors are discussed. Both particle and molecular structuring strategies have been developed to improve wettability and prevent the formation of lumps and, therefore, to improve reconstitution properties. Various types of instant food have been developed based on healthy or functional ingredients and exhibit positive effects on the prevention of non-communicable diseases and overall health. Less processed materials and by-products are often chosen to enhance the contents of dietary fibre and phenolic compounds. The enrichment of phenolic compounds, dietary fibres and/or probiotics tend to be simultaneous in plant-based products. The process of the ingredients and the formulation of products must be tailored to design the desired structure and to improve the reconstitution property.

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1. Introduction

Instant foods are pre-processed or pre-cooked products that require minimal preparation before consumption. Many instant foods can be processed into powders, such as beverages, milk, soups and gravies. These powders can be described by different terms such as powder or granules depending on their particle sizes and sources of the materials.¹ The formulation and source of ingredients of instant powder foods are complex. Various types of instant food have been developed across different regions and countries with different ingredients tuning the taste, appearance, package sizes and health benefits according to the local dietary culture and the various needs of the local market.

The increasing awareness of the link between diet and health has led to a growing demand for healthy food options. The value of the global market of health and wellness food was 841 billion US $ in 2022 and is expected to rise to one trillion US $ by 2026.² Many instant powder foods are produced as dry mixes with high flexibility in formulation. They can be produced either by directly mixing different ingredients in powder forms or by mixing in liquid forms and then drying. These products are light in weight, small in volume, have long shelf lives, are easy to transport and carry, and are ready to be reconstituted by consumers. Additionally, instant foods are highly diverse in formulation, flavour and texture, and food instantisation can be applied in a wide range of product categories. Therefore, they have the potential to be formulated into excellent carriers of different functional ingredients to deliver health benefits to different consumers. One of the strategies towards foods with health benefits is the inclusion of beneficial or functional ingredients in the formulation. The nutrients and functional substances added to instant foods include dietary fibres, probiotics, phenolic compounds and unsaturated lipids. Saikia, et al. (2020)³ have developed fibre-enriched fruit beverage powder, which exerts positive influences on lipid profile, liver function and colon health. Wolever, et al. (2018)⁴ have developed bran-enriched instant oatmeal with a glycaemic response-lowering effect. Materials with high contents of phenolic compounds, such as berries, cherries, buckwheat and oat brans are widely investigated to produce functional instant foods, which exert health benefits on blood pressure, inflammation levels, and hypercholesterolemia.^5–7 Health benefits and functions, such as reducing body weight, suppressing chronic inflammation and reducing cardiovascular risks, of fermented or probiotic-enriched products have also been proved.^8–10 Milk powder is one of the oldest dairy powders developed for storage purposes. Milk powder and other dairy-based ingredients, which are mainly casein and whey proteins, have been widely used in various types of instant powder food due to their high nutritional quality, various functionalities and pleasant flavour.¹¹ Plant-based diets have been gaining attention due to their potential health benefits and low environmental impact. Plant-based materials or plant proteins can be applied either as alternatives to dairies to avoid the risks of lactose intolerance and milk allergies or as independent products. Techniques, such as glycation, fine milling, direct steam injection and particle surface modification, have been investigated to improve the solubility, dispersibility or encapsulating properties of plant proteins.^12–16 Additionally, plant-based fermented products and probiotic-enriched products are also widely studied.^10,17,18

This review, for the first time, proposes a novel classification of instant powder foods based on the solubility of the ingredients and the structure of the reconstituted products. With an emphasis on the type that contains insoluble ingredients, the objective of the review is to summarise current advances and challenges in the designing of instant powder foods in the related industry with a focus on reconstitution properties, structure and the potential for the development of healthy products.

2. Instant powder foods on the market

Products in different food segments can be processed into instant powder foods because of their high flexibility in formulation, flavours and texture and their instant and convenient properties. Therefore, the classification of instant powder foods can be complicated and varies significantly across different categorisation systems. The three macronutrients, which are carbohydrate, protein and fat, are of the highest content in most instant powder food products. However, different products vary significantly in their macronutrient composition, which then influences the structure of the reconstituted products and contributes to the characteristic texture. According to public awareness and categorisation by food retailers, nine types of mostly seen instant powder food, which are instant porridges, instant coffee mixes, instant hot chocolates, milkshakes, instant desserts, instant soup, milk powder, meal replacements and protein shakes, are selected and listed in Fig. 1. Fig. 1a illustrates the ratios of protein, carbohydrate and fat in these nine types of instant powder food. The ratios are calculated from the nutrition labels of commercially available products at the time of writing this review. The carbohydrate ratio is calculated from the total carbohydrate contents including dietary fibres. The carbohydrate ratio tends to be the highest in most products, regardless of the product types, which can be sourced from starch, sugar, thickeners and dietary fibres. Within the group of instant porridges, the ratio of carbohydrate, fat and protein varies significantly across different formulations. Some products exhibit a high ratio of carbohydrates, while some products have higher ratios of fat and protein accompanied by a lower ratio of carbohydrates. Instant oats, a type of instant porridge, are one of the most common instant foods on the global market. They are usually a combination of oat flakes, sugar and milk powder. Instant porridges of other cereals, legumes and seeds, such as wheat, corn, rice and sesame, are cereal-based drink powders with plain or sweet flavours available on the East Asian market. These types of product contain high amounts of starchy ingredients which are adequately gelatinised during processing and contribute to the viscosity, stability, texture, appearance and eating quality after being reconstituted. Products in the groups of instant coffee mixes, instant hot chocolates, instant desserts, milkshakes and instant soups shown in Fig. 1a exhibit very high ratios of carbohydrates (higher than 60%). As for protein and fat in these five types of product, instant coffee mixes and soups incline toward either higher ratios of protein or fat. Instant soup powder is widely accepted in Western countries and usually has a creamy texture. It usually has long ingredient lists, containing vegetables, cereals, legumes, dairy, meat, fat, herbs and spices. Instant desserts are more likely to be high in fat. They generally have a higher fat content than milkshakes. However, some milkshake products tend to have higher protein ratios. Milk powder is a traditional type of instant powder food. Most milk powders have a fat ratio of approximately 30% in the three macronutrients, which is higher than most of the other products in the nine types of instant powder food analysed, with varying ratios of protein and carbohydrate. Meal replacement products are another type of instant food, commonly, in powder forms that are formulated into low-calorie and very-low-calorie diets for body weight control.¹⁹ The protein and carbohydrate ratios vary significantly in different products and most products contain a certain amount of fat. Protein shakes, as a protein supplement, are mostly high in protein (approaching 100%), while some products are formulated with high amounts of carbohydrates. It is noticed in Fig. 1b that only some products of instant porridges, meal replacements and protein shakes have high proportions of dietary fibres in their total carbohydrates, while dietary fibres in other products constitute less than 20% of the total carbohydrates. Although differences exist across food categories, regions and countries, the mechanism for food structuring, desired qualities and trends for healthy products are similar.

Fig. 1 (a) Ratios of protein, carbohydrate and fat in nine types of instant powder food. (b) Ratios of dietary fibre and other carbohydrates in nine types of instant powder food.

3. Classification and overview of the structure

For a better understanding of the structure of the reconstituted products and to facilitate the strategical quality improvement and the strategical loading of health benefits, we propose a novel classification method according to the solubility of ingredients and the structure of the reconstituted products. The first type includes, for instance, instant tea, instant juice and electrolyte powder, of which all the ingredients are soluble. The products can be fully dissolved in water and form clear solutions with low viscosity after being reconstituted. The second type contains insoluble ingredients that can be dispersed after being reconstituted and the products form colloidal suspensions with complex structures. Instant soup, instant porridge, milk powder, milkshakes, and meal replacement powder, for example, belong to this type. Several instant powder foods in the literature that can be classified into the second type are listed in Table 1. Their formulation contains various types of ingredient that have limited solubility in water. Some studies listed in Table 1 evaluated the influences of different processes or ingredients on the physical and structural properties of the products. The measurements of the contents of nutrients and functional compounds were sometimes included. Other studies focused on the health benefits evaluated by clinical trials or animal experiments. A few recent studies included both evaluations of physical and structural properties and the health benefits.^4,20–22 The second type of instant powder food is extensively discussed in the present review.

Table 1 Several instant powder foods developed in the literature

Product	Main process	Main ingredients	Physical and structural properties	Nutrients, functional compounds and health benefits	Ref.
Mix fruit beverage powder	Spray drying	Carambola, watermelon pineapple, carambola pomace and maltodextrin	N/A	Decreasing levels of cholesterol, high-density lipoprotein, low-density lipoprotein and triglycerides. Improving liver functions and colon health	3
Probiotic fermented juçara pulp	Juçara pulp was extracted, fermented and spray-dried with gum Arabic, maltodextrin or gelatin	Lactobacillus reuteri strain LR92, juçara fruits, gum Arabic, maltodextrin and gelatin	The powder produced with gelatin is less soluble	The powder produced with gelatin exhibits higher probiotic survival during drying	128
Black tea powder	Black tea was powdered using a shear fibre machine	Black tea	Powder with smaller particle sizes has higher swelling ability and wettability	Powder with higher particle sizes has higher contents of total polyphenols, fibre, catechins and thearubigins and is more effective in reducing body weight. Powder with lower particle sizes regulates blood lipids in hyperlipidaemic rats	20
Functional synbiotic drink powder	Gum was extracted from quince seeds. Lactobacillus casei was encapsulated in quince seed gum–alginate beads. The powder was prepared by freeze-drying	Lactobacillus casei (free or beads), extract powders of beetroot peel, pomegranate peel and grape pomace, quince gum, stevia and mint	Moisture content, solubility and wettability of drink powders were evaluated	Total phenolic content, oxygen radical absorbance capacity and DPPH radical scavenging activity were evaluated. Encapsulation increases the probiotic survival rate	129
Instant-mix probiotic beverage	Fruit powder was freeze-dried. Quinoa was roasted and fermented using Lactobacillus plantarum and freeze-dried	Freeze-dried fruit powder and freeze-dried fermented quinoa flour	N/A	The fermentation and roasting processes significantly reduce phytate contents, which increases the estimated mineral absorption. High probiotic viability was obtained	130
Goat milk powder with branched-chain fatty acids	Fresh goat milk was mixed with branched-chain fatty acids and spray-dried	Goat milk, branched-chain fatty acids and soy lecithin	The milk powder has smaller particle sizes, lower solubility and higher viscosity. It is stable during 2-year storage	N/A	131
Synbiotic milk powder	N/A	Milk powder, Lactobacillus plantarum JJBYG12 and isomaltooligosaccharide	N/A	Increasing calcium absorption, serum calcium and phosphorus levels and bone mineral density in calcium-deficient mice	93
Probiotic almond milk powder	Almond milk powder was prepared by spray-drying	HydroSOStainable almond and Lactobacillus plantarum ATCC 8014	N/A	HydroSOStainable almond milk powder has higher contents of total phenolic and polyunsaturated fatty acids, higher polyunsaturated fatty acids/saturated fatty acids ratio and higher polyunsaturated fatty acids/monounsaturated fatty acids ratio. It is also a good source of Ca, K and Zn	132
Bambara groundnut yogurt powder	Bambara groundnut milk was extracted and spray-dried, which was reconstituted, fermented and foam-mat dried to produce yogurt powder	Bambara groundnut	Water absorption, water solubility, glass transition temperature and particle size were evaluated	Nutrient composition was evaluated	133
Bambara groundnut powdered drink mix	Bambara groundnut milk was extracted and dried. Bambara groundnut was mixed with soy powder	Bambara groundnuts and soybean	Products with higher Bambara groundnut ratios have higher water absorption capacity and gelation capacity but lower oil absorption capacity	Reducing total cholesterols in rats	21
Avocado powder drink	Avocado was mixed with milk, sugar and maltodextrin, homogenised and spray-dried	Avocados, maltodextrin, sugar and cow milk	Higher drying temperatures and smaller droplets lead to higher yields, lower moisture content and lower particle density	Contents of protein, ascorbic acid and phenolic compounds were evaluated	134
Functional drink powders	Oat bran and highland barley bran were processed by vertical stone mills. Products were prepared by co-rotating twin-screw extrusion	Oat bran powder and oat flour; highland barley bran powder and highland barley flour	N/A	High in protein, fibre, resistant starch and β-glucan. Low in starch contents. Low starch digestibility and postprandial glycaemic response	59
Powdered drink	Wheat bran powder was obtained using a vertical stone mill and extruded in a co-rotating twin-screw extruder	Wheat bran powder	Extruded wheat bran powdered drink has a lower viscosity	Lower in vitro digestibility, lower human glycaemic response and higher gut microbiota fermentation compared with wheat flour noodles	22
Oat β-glucan beverage	N/A	N/A	N/A	Reducing low-density lipoprotein cholesterol by 6% and cardiovascular disease risk by 8% in healthy adults	135
Instant asparagus soups	Extraction, drying and high-speed milling of asparagus fibre	Asparagus fibre, asparagus powder, potato starch, salt, maltodextrin and flavour mix	The addition of asparagus fibres leads to a thicker perception. Insignificant influence on measured viscosity and mean particle diameter	N/A	51
Instant egusi soup	Egusi flour was defatted in a supercritical fluid, cooked using different processes and mixed with other ingredients	Egusi grit, egusi flour, egusi oil, hydrocolloid and spices	N/A	High in minerals. Containing all essential amino acids	136
Instant ogbono soup	Ingredients are powdered and mixed	Ogbono, stock fish, cray fish, Cameroon pepper, ugwu leaves, salt, seasoning and locust bean	High water absorption, swelling capacity and bulk density are associated with the formula with high crude fibre	High in minerals and vitamins	137
Functional instant soup	N/A	Chickpea, vegetables (mushroom, parsley, dill and celery), olive oil and by-products (outer leaves of lettuce, onion peels, banana peels, whey protein and brewery yeast)	N/A	Showing preventive effects on hyperglycemia, hyperlipidemia and constipation	88
Fermented instant soup powders	Fermentation at 35 °C for 48 h and drum drying	Mung bean, pregelatinised rice, strained yoghurt, salt, powders (onion, tomato, paprika, mint), instant bread yeast and guar gum	N/A	Increased contents of total phenolics, protein and dietary fibre	112
Instant soup	Chickpea flour was prepared by cooking and freeze-drying	Potato starch, chickpea flour, maltodextrin, sunflower oil creamer and roux	Powder flowability, viscosity and particle size were evaluated	N/A	138
Instant soup	The instant soups were prepared using foam-mat drying	Cowpea and glycerol monostearate	N/A	N/A	139
Pearl oyster mushroom soup powder	The mushroom was freeze-dried. Vegetables were air-dried and ground. Legumes were cooked, dried and milled	Pearl oyster mushroom, Mung bean, Dutch pea, red kidney bean, black turtle bean, vegetables and full cream milk powder	N/A	Total energy, protein, carbohydrate and lipid contents were evaluated	140
Instant soup for the elderly	All ingredients were mixed, homogenised and drum-dried	Brown rice, pumpkin, sweet corn, red tilapia, rice bran oil and inulin	N/A	A rich source of antioxidants and β-carotene	141
Instant kheer (dessert) mix	Extraction and drying of bottle gourd pomace	Bottle gourd pomace powder, milk powder, sugar and cardamom	The incorporation of the bottle gourd pomace improves the consistency, texture and overall acceptability	Increasing the content of total phenols and antioxidant activity	52
Cassava–chia seed instant flour	Single screw extrusion	Cassava roots and chia seeds	The increase in chia seed incorporation increases the bulk density and water absorption index, while the influences on solubility and swelling powder are insignificant	N/A	62
Buckwheat enriched instant porridge	Buckwheat is extruded	Buckwheat, dried apples, oat flakes, oat bran, soy protein isolate, casein, xylitol and cinnamon	N/A	Increasing contents of phenolic compounds, dietary fibres and buckwheat protein. Improve lipid profile and fat-free mass in participants with mild to moderate hypercholesterolemia	7
Instant cassava–soy porridge	Co-rotating twin screw extrusion	Cassava flour, defatted toasted soy flour and wheat bran	Extrusion increases the content of soluble dietary fibres and the viscosity of products	High viscosity increases the oral exposure time and bites during consumption, enhances satiety and suppresses appetite	58
Instant oatmeal	N/A	Commercial instant oatmeal and oat bran	The incorporation of oat bran increases viscosity	Reducing glycaemic response	4
Instant caffè latte beverage	Pomegranate (Punica granatum) seed oil microparticles were prepared by spray-drying or by complex coacervation followed by spray-drying	Pomegranate seed oil microparticles, soluble coffee and powdered milk	Microparticles have higher solubility and thermal stability	Caffè latte is fortified with conjugated linolenic acid	127
Meal replacement powder	N/A	Almased Vitalkost® meal replacement powder	N/A	Meal replacement and low-intensity lifestyle reduce body weights and cardiovascular risk factors in overweight and obese patients	142
Meal replacement powder	N/A	SlimStyles® meal replacement powder and PGX® fibre	N/A	Reducing body weights, waist and hip circumference and body mass index in overweight and obese participants	143

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The reconstitution of instant foods requires the addition of water at varying temperatures depending on the product types and the preferences of the consumers. Different ingredients have different behaviours upon the addition of water. The second type of instant food can contain a diverse range and varying amounts of soluble compounds, including salt, sweeteners, flavouring agents, thickeners and dispersing aids. These polymers and compounds hydrate and dissolve quickly into the continuous phase. They also contain insoluble substances, including insoluble polysaccharides, plant cell wall materials, insoluble protein aggregates and oil droplets. These insoluble substances are wetted, dispersed, and form the dispersed phase (Fig. 2). Some of them that have high water absorbability and water holding capacity will absorb water and swell with an increase in their phase volumes. The presence of the dispersed phase and the increase in phase volume leads to an increase in bulk viscosity (Fig. 3), which is a critical part of structure and texture development. Fig. 3 illustrates the relationship between the viscosity and volume fraction of disperse systems. At low volume fractions, the slow increase in viscosity is attributed to the increased flow resistance of individual particles. With the increase in volume fraction, the dispersed particles start to collide and the particle–particle interaction contributes to the faster increase in viscosity. It becomes dominant approaching the maximum pack fraction (ϕ_m), which leads to a dramatic increase in viscosity, and the system starts to show solid-like behaviour. The relationship of monodisperse hard spheres can be described by the Einstein–Batchelor equation for low volume fractions and the Krieger–Dougherty equation for high volume fractions.²³ Various models have been developed for more complex disperse systems, such as emulsions,²⁴ and dispersions of starch (swelling granules),²⁵ plant cell wall materials (soft deformable particles)²⁶ and cellulose nanocrystals (rod-like particles),²³ which are all potential ingredients for instant powder food formulation.

Fig. 2 Schematic representation of reconstituted instant food. Each symbol represents a group of materials. The symbol sizes do not represent the actual ratios of the particle sizes between the components.

Fig. 3 The relationship between the viscosity and volume fraction of disperse systems. ϕ_m is the maximum pack fraction.

Different phase volume fractions are required in different types of food to achieve the desired texture and function. Fig. 3 lists several types of instant food within the characteristic volume fraction of their dispersed phase. Instant coffee mixes, reconstituted milk and similar products usually contain lower volume fractions of the dispersed phase, which are mostly oil droplets and colloidal protein aggregates. The volume fractions of fat and casein micelles in milk are 0.043 and 0.106, respectively.²⁷ The volume fraction of particles can be up to 0.3 in soymilk.²⁸ These products are commonly described by smoothness and creaminess. In meal replacement powder, instant soup and porridges, the dispersed particles mostly have high volume fractions or are closely packed. These products usually have high viscosity and consistency and are required to provide certain satiety as they are mainly consumed as a part of meals. According to Calton, et al. (2019)²⁹ “liquid”, “soft” and “semi-solid” food products typically have apparent viscosity lower than 0.1 Pa s, between 0.1 to 1 Pa s, and between 1 to 4 Pa s, respectively. Additionally, the shape, size and hardness of these dispersed particles further influence rheological properties and the texture of the systems.

However, the presence of insoluble particles may lead to challenges in achieving a stability and homogeneity of the reconstituted products, and results in an undesired appearance and mouthfeel.^3,30,31 As a colloid system, the destabilising factors in reconstituted products may include sedimentation, creaming, flocculation, phase separation and coalescence (Fig. 4). Unexpected gelation and over-thickening may also occur and lead to inferior quality (Fig. 4).³² The most prevalent destabilising factors are creaming and sedimentation, which are caused by differences in density between the dispersed particles/droplets and the continuous phase (a water solution), especially when the particle/droplet sizes are large. Oil droplets and particles associated with gas bubbles have a lower density than the continuous phase and they may rise to the top of the reconstituted products. Particles such as protein isolates and plant particles have a higher density and they tend to sediment to the bottom. Flocculation occurs when the attractive forces overcome the repulsive forces between particles and droplets. The flocs formed may then sediment to the bottom or rise to the surface when associated with bubbles incorporated by stirring during reconstitution. Phase separation is caused by thermodynamic incompatibility between different components in a system. The homogeneous mixture is converted into multiple phases that are rich in different components with different rheological properties and appearances. Coalescence is the process by which two or more approaching oil droplets merge into a big droplet. It is more likely to occur when the attraction forces between droplets overcome the repulsive forces and when the interface around the droplets is weak.

Fig. 4 Schematic representation of the destabilising factors of reconstituted instant foods.

Stokes’ law calculates the terminal velocity of the sedimentation or creaming of droplets in diluted dispersions. According to Stokes’ law, strategies to prevent creaming or sedimentation are: reducing the density difference between the dispersed phase and continuous phase (less practical); reducing the sizes of the dispersed droplets and particles; and increasing the viscosity of the continuous phase by adding thickeners.³³

With the increase in the volume fraction of the dispersed phase, the Stokes’ velocity is reduced due to the hydrodynamic interaction between the particles/droplets.³³ The interparticulate interaction is also promoted with the reduction of the distance between the particles, which leads to possible instability. Flocculation is a common instability issue found in disperse systems, which is caused by attraction and interaction between particles or droplets. The possible attraction factors include van der Waals bonds, hydrogen bonds, electrostatic attraction, hydrophobic attraction and depletion forces, while the possible repulsive factors are electrostatic repulsion and steric repulsion. Multiple forces and interactions usually exist in a food system and cause different structural behaviours. These factors are considered and assessed when the stability of a disperse system is discussed. However, the destabilising rate must be taken into consideration. The reconstitution is usually completed in a few seconds (such as milkshakes and instant coffee) to three minutes (such as instant oats and soups). Nevertheless, they are consumed for a longer time and the stability is usually monitored within 1 to 2 hours, during which the structure and texture, such as viscosity, keep developing. For an electrostatically stabilised disperse system with a medium phase volume (0.5) and small particles (0.1 μm), according to the equation proposed by Smoluchowki (1927), the reduction in particle numbers by half due to flocculation only takes 1.5 ms in the absence of the potential barrier described in the DLVO theory.³³ The inappropriate formulation may lead to instant flocculation during reconstitution. However, the destabilising factors listed in Fig. 4 are usually studied in ready-to-drink beverages to evaluate long-term stability and shelf life but are less evaluated in reconstituted products.

The bulk viscosity increases with the increase in the volume fraction of the insoluble particles/droplets and dramatic viscosity rise occurs when the volume fraction is reaching the maximum pack fraction (ϕ_m) (Fig. 3).^24,33 The maximum pack fractions of randomly and hexagonally packed hard spherical particles (monodisperse) are 0.64 and 0.74, respectively.³³ Polydisperse particles or deformable particles have higher ϕ_m, while non-spherical particles or particle aggregates have lower ϕ_m. It is noted that many viscosity–volume fraction graphs display relative viscosity instead of bulk viscosity, which is the ratio of the bulk viscosity to the viscosity of the continuous phase. In most reconstituted food systems, the dispersed particles absorb and compete for water with the continuous phase, which further increases the concentration and, hence, the viscosity of the continuous phase and leads to a more pronounced increase in the bulk viscosity. When the volume fraction of the dispersed phase is close to or above the maximum pack fraction, the system is prone to show solid-like properties, which are commonly characterised using oscillatory shear rheometry. The bulk rheological behaviour of a system can be also influenced by particle size, particle hardness and slipping between particles. One of the most controllable parameters is particle size. It has a more pronounced influence on bulk viscosity at high volume fractions.³³ It is reported that small particle sizes are associated with high viscosity due to the high specific surface area.³⁴ However, the strong interaction between particles or a highly packed structure due to a high degree of particle swelling leads to undesired gelation or over-thickening.

A concentrated dispersion may exhibit yield stress when the dispersed particles and droplets are closely packed or when interparticulate interaction and a weak network exists. Meena, et al. (2019)³⁴ observed a higher yield stress of reconstituted dispersion of diafiltrated milk protein concentrates compared with untreated protein concentrates. Shevade, et al. (2018)³⁵ reported yield stresses of reconstituted composites of fermented milk and cereals, which is attributed to the close-packing of swollen starch granules. Additionally, a concentrated dispersion may show thixotropic behaviour as the interfaces of droplets and particles or the dangling polymers on the outer layers may interact and form weak structures that can be destroyed during shearing but restored during resting.

4. Reconstitution properties

The instant powder food is intended to be rehydrated and reconstituted by consumers via simple procedures, which are adding water at a certain temperature range and stirring. Therefore, instant reconstitution is a critical factor for high consumption quality. The powder reconstitution or rehydration generally undergoes overlapped phases of wetting (wettability), sinking (sinkability), dispersing (dispersibility) and dissolving (solubility) and the time scale can be influenced by multiple factors as listed in Fig. 5.³⁶ Swelling may occur instead of dissolving when the ingredients are insoluble.

Fig. 5 Reconstitution process and factors influencing the process, which are discussed by Fang, et al. (2008)³⁶ and strategies to improve the reconstitution property.

Wetting is considered to be the rate-controlling phase of reconstitution.³⁶ It is the process during which the particle surface is covered by a liquid phase and it is dependent on the interfacial tension between liquid, solid and air. The substance on the surface of the particles has a significant influence on the interfacial tension and wettability of the powder. The presence of hydrophilic substances such as sugar (lactose in the case of milk powder), salt and whey protein on the particle surface increases the wettability of the powder, while hydrophobic substances decrease it. The migration and accumulation of lipids on the particle surface, which occur during the drying and storage of milk powder and encapsulated lipid powder, lead to low lipid encapsulation and reduced wettability. The free fat also leads to the association of particles, which results in low dispersibility and lump formation.³⁷

High wettability is favourable as it indicates quick hydration of the powder. However, particles with high wettability tend to form lumps, which lead to low dispersibility and undesired consumption experiences. The formation of lumps is usually observed during the reconstitution of the powder of hydrophilic polymers because part of the polymers quickly absorbs water, swells and forms barriers hindering the further hydration of the polymers inside the lumps. An increase in the percentage of small particles (with a particle size lower than 90 μm) could also lead to low dispersibility.³⁶ Additionally, the association of fine particles with larger particles or other fine particles also leads to poor dispersibility or the formation of lumps.³⁷

To overcome these issues, particle structuring strategies and molecular structuring strategies are developed. Particle structuring strategies includes coating the particles, controlling the particle density and particle agglomeration. Powder wettability, a crucial parameter while designing instant powder products, is significantly influenced by capillary flow and models have been developed to predict the wetting rate, which indicates that the powder wettability is increased by smaller contact angles, larger particle or agglomerate sizes and higher porosities.³⁶ The coating of milk powder using micronised lactose decreases the contact angle and increases the reconstitution rate.³⁸ The wettability of milk protein isolate was proved to be increased by coating using Tween 80 and lecithin.³⁹ Cold plasma treatment has been recently investigated by Bormashenko, et al. (2021)¹⁶ for the surface hydrophilisation of soy protein isolate and milk protein concentrate powders. The generated reactive oxygen and nitrogen species during the treatment interaction with the particle surface, which potentially etch the surface, introduce polar groups and unfolding proteins.⁴⁰ Therefore, the treatment significantly increased the wettability of the powders and the effect remained up to one month without hydrophobic recovery.¹⁶ Particle coating using a hydrophilic or amphiphilic substance and surface treatments such as cold plasma is expected to increase the hydrophilicity of the particle surfaces, decrease the contact angles and, therefore, improve the powder wettability. Particle agglomeration, as a particle-structuring process, has been widely utilised to achieve an appropriate porosity and pore structure for better reconstitution properties.⁴¹ These conditions that increase wettability also accelerate the sinking of the powder. Additionally, air occlusion decreases the powder density and, therefore, reduces the powder sinking rate. A hollow structure is frequently observed in spray-dried powders, which is caused by the early formation of an elastic skin and quick expansion during the final drying stage of the particles.⁴¹ It can be utilised to adjust the powder density and solubility.

The molecular structuring strategy is based on modification at the molecular level. The drying process is necessary for producing food powder. However, the high temperature during most drying processes may induce adverse effects, especially the denaturation of proteins, which leads to poor solubility, dispersibility and reconstitution properties. The glycation of proteins by the Maillard reaction for the enhancement of their favourable properties has been widely investigated. The glycation of proteins increases the denaturation temperature threshold, which prevents the protein from denaturing at the drying temperature.¹² By conjugating with proteins, saccharides prevent the interaction between the protein molecules and their hydrophilic nature improves the solubility of the conjugates.^12,42 Spray-dried soy beverages made from dextran–soy protein isolate conjugates were proved to show dramatically higher values of solubility index, wettability and dispersibility.¹² Rice protein is another widely used high-quality plant protein. Ultrasonic-assisted glycation of rice protein with dextran was investigated and optimised by Chen, et al. (2022)⁴² and the obtained protein has improved solubility. The Maillard reaction has also been proved to be enhanced via cold plasma treatment by increased exposure of the internal amino acid.⁴³ Cold plasma-assisted glycation has a more pronounced influence on rheological behaviour compared with untreated samples and glycated samples.⁴³ The solubility improvement of protein by glycation has also been recorded between zein and chitosan (by transglutaminase),⁴⁴ and between oat protein isolate and β-glucan.⁴⁵ Zhang, et al. (2022)⁴⁶ complexed starchy kudzu powder with fatty acids and the obtained powder has improved reconstitution properties because of the formation of the starch–fatty acid complexes on the surface of the powder particles. The complexation with starch also increases the thermal stability of the fatty acids. It prolongs the rehydration of individual particles even in hot water, which improves powder dispersibility.⁴⁶ In conclusion, the complexation of polysaccharides and proteins or between polysaccharides and fatty acids via covalent and non-covalent interactions can improve solubility and dispersibility at a molecular level, which can be adopted in product designing for instant powder foods.

5. Insoluble components

5.1. Insoluble polysaccharides

Polysaccharides are the main components contributing to the thickness or viscosity of reconstituted instant food products. Starch is the energy storage polysaccharide produced by green plants via photosynthesis. It is one of the main energy and nutrient sources for humans, livestock and many microorganisms. Starch is added in instant foods, such as instant porridges and soups, as thickeners to achieve the desired texture and stability. These products usually need to be reconstituted in hot water to cook the starch and the starchy components. When heated in hot water, starch granules absorb water and swell followed by the release of amylose, which results in significant increases in viscosity. Native starch granules are not readily dissolved or dispersed in cold water. Therefore, starch is usually chemically or physically modified to achieve instant properties. Pregelatinisation is a physical modification of starch to achieve solubility at a temperature below the gelatinisation temperature. Pregelatinised starch is commonly produced by heating a starch slurry followed by drying in a drum dryer.⁴⁷ Alternatively, it can also be produced by extruding a starch–water mixture or by heating with superheated steam in a spray-dryer.⁴⁷ Due to the high hydration rate of the pregelatinised starch (instant starch), it may form lumps, which hinder further hydration and dispersion in water especially when the powder has smaller particle sizes.⁴⁷

Granular cold-water-swelling starch is another type of modified starch, which maintains an intact granular structure. It can be easily dispersed in cold water without the formation of lumps, swell, and cause an increase in viscosity. The methods to prepare cold-water-swelling starch include heating in aqueous ethanol, alcoholic-alkaline treatment, high temperature and pressure in aqueous ethanol, spray drying, and the instantaneous controlled pressure drop method. As cold-water-swelling starch forms a dispersion in water and increases the viscosity via granule swelling instead of the dissolution of polymers, it is more efficient in stabilising dispersion systems, such as cake and muffin batters, and preventing flour particles, chocolate chips, fruit pieces and other particles from sinking to the bottom. It is also added to instant foods such as instant soup and baby foods. Calton, et al. (2019)²⁹ have developed semi-solid model products of instant food by thickening with 7% cold water swelling starch, which swells and provides a viscous and short texture.

The solubility of polysaccharides depends on the interaction with the solvent and the intermolecular association. One of the typical insoluble polysaccharides is cellulose. Cellulose powder, which is a purified cellulose product, is commercially available. Cellulose is rich in plant cell wall materials or plant fibres, which are usually by-products of juice, soup or cereal industries. Citrus fibre, apple fibre, oat bran and potato fibre are some of the commercially available plant fibres. Citrus fibre contains various amounts of cellulose, hemicellulose, pectin and lignin, of which the contents are dependent on the extraction and purification processes.⁴⁸ Plant fibres usually have a certain water-holding capacity, swelling capacity and fat adsorption capacity.⁴⁹ They not only are a natural source of dietary fibres but also affect the structure development within the food systems. Figuerola, et al. (2005)⁴⁹ found that apple fibres and citrus fibres have a water retention capacity between 1.62 and 2.26 g g⁻¹, a swelling capacity between 6.11 ml g⁻¹ and 9.19 ml g⁻¹, and a fat adsorption capacity between 0.6 g g⁻¹ and 1.81 g g⁻¹. Higher water holding capacity and water swelling capacity were reported by Zhang, et al. (2020)⁴⁸ and the value can be significantly increased by alkaline hydrogen peroxide treatment and homogenization, which exposes more hydrophilic hydroxyl groups of polysaccharides. The fruit and vegetable fibre concentrates show a certain resistance to compression forces and the ability to increase viscosity.⁴⁹ Dispersions of plant cell wall particles at the concentration of 0.5% to 8% as studied show structured fluid properties with the elastic moduli higher than loss moduli in small deformation rheological measurements, while the viscosity exhibits a sharp decrease when the yield stress is reached.⁵⁰ Considering the high water holding capacity and swelling capacity, the fruit and vegetable fibres can be added to instant soups, instant porridge and other instant foods to increase the viscosity, stabilise the structure, improve mouthfeel and provide a source of dietary fibre. Carambola pomace fibre, which was extracted, purified and dried by Saikia, et al. (2020),³ was homogenised and spray-dried with fruit juices to produce a fibre-fortified beverage powder. Pegiou, et al. (2023)⁵¹ prepared asparagus fibres by hot-air drying and high-speed milling and the obtained fibres were used to prepare instant asparagus soups. The addition of asparagus fibres leads to a thicker perception although the influence on measured viscosity and mean particle diameter were insignificant.⁵¹ Pomace powder has also been prepared by air-drying the waste of pressed bottle gourds and used to prepare instant kheer. The incorporation of the bottle gourd fibres improved the consistency, texture and overall acceptability of the product.⁵²

Being different from fruit and vegetable fibres, cereal brans are separated from grains as a by-product of the grain industry. Arabinoxylan and β-glucan are two of the hemicelluloses in cereal brans that are of interest to researchers for their nutritional benefits⁵³ and their influences on food structure and texture. Cereal brans also contain protein, fat, wax, vitamins, saponins, minerals and phenolic compounds. The instant properties and structuring properties, including the solubility, water holding capacity, and swelling capacity, of cereal bran and whole grain powders can be improved by fine milling, which decreases particle sizes, and by high temperature, high pressure, high shearing and cavitation treatments, which affect the molecular structure and interaction, and assisted by enzymatic treatment or germination.⁵⁴ Extrusion is one of the most investigated high temperature and pressure treatments. It was proved to increase the soluble dietary fibre of rice bran by over 30%.⁵⁵ Microfluidisation has been proved to increase the water holding capacity and swelling capacity of insoluble dietary fibres from oats and peaches.⁵⁶ Dynamic high-pressure microfluidisation has been proved to increase the water holding capacity, cation exchange capacity, and cholesterol absorbing ability of rice bran.⁵⁷ Physical and chemical treatments on dietary fibres have been thoroughly reviewed by Gan, et al. (2021).⁵⁴ In terms of product development for instant powder foods, brans are often extruded with other ingredients. Oladiran, et al. (2018)⁵⁸ developed instant cassava–soy porridge by incorporating wheat bran up to 200 g kg⁻¹ during extrusion. Extrusion increases the content of soluble dietary fibres by depolymerisation of the insoluble dietary fibres and, hence, the incorporation of wheat bran during extrusion significantly increased the viscosity of the reconstituted products.⁵⁸ Oat bran and highland barley bran processed by vertical stone mills were proved to have higher thermal stability, higher protein contents, higher dietary fibre contents, higher β-glucan contents and lower starch contents compared with corresponding commercial flours.⁵⁹ Drink powders were then prepared from the brans and flours by co-rotating twin-screw extrusion, which increased the soluble β-glucan levels.⁵⁹

Soluble polysaccharides interact with cellulose and are trapped in the cellulosic matrix in plant cell wall materials. Therefore, they may not readily dissolve in water without extraction. Psyllium husk is a natural source of dietary fibre, which is commercially available in a non-extracted and non-purified form, and it is mostly consumed or applied in food manufacturing in powder forms. The powder quickly absorbs water and swells in cold water forming a dispersion of soft gel particles with insoluble cores.⁶⁰ The thickening effects of psyllium husk are partially attributed to the swelling of the soft gel particles and the increased volume fraction. Interparticulate interaction was also observed,⁶⁰ which further increases viscosity and alters the rheological responses and the texture of the products. Psyllium husk powder has been used to thicken and stabilise paste-like food materials, such as cake batters, gluten-free bread doughs and ice cream. Other mucilage-producing seeds and seed husks include chia seeds and flaxseed.⁶¹ The instant flour of cassava–chia seeds prepared by extrusion exhibits a significant increase in the water absorption index as the proportion of chia seeds increases while the influences on solubility and swelling powder are insignificant.⁶²

The drying process has a significant impact on the microstructure of the powder. A compact structure with less porosity caused by drying processes such as air drying leads to slow rehydration. Reconstitution of instant foods is usually a short-term process, without sufficient shearing, and, sometimes, at a low temperature. The powder of a polysaccharide with a compact structure may not fully dissolve in these conditions especially when the molecular weight is high and the hydration of the outer layer molecules may prevent the further hydration of the inner molecules. For each hydrated particle, it forms a concentration gradient from the outer layers to the particle core. In this case, the thickening effect of a soluble polysaccharide is also partially attributed to the swelling of particles.

5.2. Lipid droplets

Another group of insoluble substances are droplets of lipids that naturally occur in ingredients, are required in the formulation, or are used as carriers for lipophilic functional ingredients. The naturally occurring fat comprises 26.8% of whole milk powder¹¹ and significantly influences the taste, reconstitution properties and nutritional quality. The fat is dispersed and forms droplets in the system during the reconstitution of milk powder.⁶³ Soy is another ingredient widely used to produce instant powder food. The lipid content of soy milk ranges from 1.34 to 3.17 g per 100 mL.⁶⁴ It exists as oil bodies in soy drinks with diameters of 200 nm to 500 nm and is stabilised by naturally occurring surface proteins.⁶⁵

Encapsulation of oil ingredients, such as fish oil, is proved to facilitate their dispersibility in water, mask unfavourable odours, and prevent oil oxidation during storage, which allows their incorporation in instant food products. Some nutrients and functional compounds such as vitamin A, vitamin E, vitamin D, curcumin and resveratrol are oil-soluble and their solubility and dispersibility in water need to be improved to be added in instant foods that require water to reconstitute. The improvement of the encapsulation process usually targets a higher efficiency and dispersibility. The typical process for lipid encapsulation is emulsification followed by spray drying and the choice of emulsifiers and wall materials is critical for efficient encapsulation, shelf life and dispersibility.⁶⁶ The wall materials are usually polymers such as polysaccharides and proteins, while small molecule surfactants are usually added to facilitate the initial emulsification process and improve the encapsulation behaviour. After being spray-dried, the lipid is embedded in a solid matrix formed by wall materials. Oil leakage may occur due to interface damage and droplet collapse during drying, which leads to low encapsulation efficiency and oil oxidation. Additionally, spray-dried complex materials usually have fat and protein coverages on the particle surface, which lead to fat oxidation, powder stickiness, deposition on the container walls, low flowability, powder caking and slow dissolution. Two or more wall materials showing interaction, such as whey protein isolates/hydrolysates and lactoferrin,⁶⁷ are usually chosen to improve the encapsulation and rehydration properties. After being reconstituted, the oil droplets are dispersed in water with emulsifiers and wall materials absorbed on the droplet surface.⁶⁶ The wall materials may not be fully dissolved but form sphere particles and are dispersed in water. The reconstituted emulsion has lower viscosities and viscoelastic responses than the freshly prepared ones due to the poor dissolution of the wall materials, which are usually polymers, in the continuous phase and the disruption of the structure or network formed in the freshly prepared samples.⁶⁸ Baraki, et al. (2021)⁶⁸ prepared dried oil powder from a Pickering emulsion stabilised by the complex of regenerated chitin and hydroxypropyl methylcellulose, which form a strong network on the droplet surface and stabilise the system during shearing, drying and redispersing. The obtained oil powder is easily dispersible and droplets in the reconstituted emulsion are similar to those in fresh emulsion with a spherical shape and a slight increase in size. It suggests that the Pickering emulsion provides a superior surfactant-free system, which may reduce oil leakage, improve the reconstitution property and stability, and retain the size and shape of droplets in the reconstituted emulsion. There is a trend to use plant proteins for oil encapsulation. Soy protein isolate aggregates have also been used as Pickering stabilisers and hence they have the potential to be used to prepare oil powders. However, the emulsifying properties and solubilities of many commercial soy protein isolate powders are impaired during processes. Glycation with carbohydrates has been proved to improve emulsifying properties, encapsulation properties, redispersion properties and dissolution properties, although the storage stability is impaired at high humidity, causing oil leakage and loss of the structure.¹³

5.3. Protein aggregates

The proteins in instant foods are commonly sourced from dairy protein, legume protein and cereal protein. Dairy products, depending on the ingredients and processes, are categorised as whole milk powder, skimmed milk powder, milk protein isolate, milk protein concentrates, whey protein isolates, whey protein concentrates, casein and caseinates.¹¹ Milk powder, originally developed for storage purposes, contains 24 to 27% protein, which is composed of 20% of soluble whey proteins and 80% of insoluble caseins. The insoluble caseins form colloidal aggregates in reconstituted milk, which are larger than those in fresh milk due to the coating of aggregated whey proteins on the surface and the aggregation of a large proportion of casein micelles.⁶⁹ The solubility of milk protein concentrate powders can be increased by increasing the pH to the natural pH of milk or removing calcium ions before spray drying, which reduces protein denaturation and aggregation.³⁴ Additionally, an improvement in the instant properties has also been reported by physical and mechanical treatments. In addition to cold plasma treatment on particle surfaces as discussed in section 4, positive effects have also been reported using microfluidisation,⁷⁰ ultrasound⁷¹ and hydrodynamic cavitation⁷² treatments prior to drying. Microfluidisation disaggregates casein micelles and increases the solubility of milk protein concentrate powders.⁷⁰ Ultrasound treatment decreases particle sizes, increases surface hydrophobicity, and, hence, increases the solubility of milk protein concentrate powders. It also increases spray-drying efficiency by decreasing the feed viscosity.⁷¹ Hydrodynamic cavitation by nitrogen before spray drying was found to significantly alter the surface composition and structure of the powder particles along with increased specific surface area and porosity.⁷²

Plant proteins are widely used in instant powder food and, with the current plant-based trend, are used as alternatives to dairy proteins. Proteins from legumes are predominantly albumins (water-soluble) and globulins (salt-soluble), which can be easily extracted in water or salt solution.⁷³ These proteins are commercially available as concentrates or isolates. Soy is the primary source of plant proteins. Soy protein concentrates (about 70% protein) and isolates (more than 90% protein) are widely used to produce instant foods for which an appropriate protein content is required, such as sports nutrition drinks. Pea proteins are also widely added in formulating drink powder or meal replacement powders for sports and exercises due to their high levels of leucine, isoleucine and valine (known as three essential branched-chain amino acids), which can promote muscle growth.⁷⁴ However, protein products are mostly produced in dry form and the drying process may expose the hydrophobic groups/residues, lead to hydrophobic interaction, and reduce the solubility of the products for reconstitution. The high temperature during spray drying causes denaturation and aggregation of the proteins (pea protein for example) and leads to low solubility compared with laboratory freeze-dried pea proteins.⁷⁵

Nuts and cereals have high contents of prolamins and glutelins in their protein fraction, which have low solubility.⁷³ Hence, the protein isolation of these materials could be challenging. Additionally, nuts are mostly of high value and well accepted in less processed forms. Hence, they are mostly available as the flour or powder of whole seeds. With the development of the food industry, the further fractionation of food resources is currently required and viable at commercial scales. Many protein products from nuts and cereals are available, such as rice protein, oat protein and peanut protein. Rice proteins are extracted from the by-products of glucose and starch manufacturing and are widely used in food production. Rice proteins have multiple benefits such as a high production of rice crops, low cost, hypoallergenicity and easy digestibility. They contain relatively high amounts of the limiting amino acids (such as lysine and threonine) in other cereal proteins. Therefore, rice protein concentrates or isolates can be blended with other proteins (such as pea protein⁷⁴) to fulfil the nutritional requirements of certain populations. They are widely used in infant formulas, protein shakes and sports drinks. Peanut protein isolates are another plant-based protein, which are the by-product of peanut oil manufacturing. They can be produced by milling the cold-pressed and defatted peanut protein or by protein extraction followed by drying. Fine milling can increase the solubility of the peanut protein powder.¹⁴

The reconstitution properties of proteins can be improved by glycation as discussed in section 4. Additionally, the parameters can also be improved by other methods such as controlling the denaturation, optimising the pH, altering the salt type and concentration, and mixing with polysaccharides. Protein and polysaccharide form complexes at acidic conditions. Negatively charged polysaccharides interact electrostatically with the positively charged patches on protein molecules and form soluble complexes at a certain pH range, which significantly increases the solubility, dispersibility and stability.⁷⁴ Additionally, exposing blends of pea protein and rice protein to high temperatures for a short time is suggested to improve the solubility.⁷⁴ Pietrysiak, et al. (2018)¹⁵ heated the protein blend at high pH (107 °C and pH 11 as optimised) by direct steam injection followed by freeze drying and the obtained powder exhibited improved solubility and other functional properties. Direct steam injection processes the protein blends at a high temperature for a short period, avoiding extensive protein thermal denaturation which decreases solubility. The high pH, high temperature, and high shearing conditions disrupt high molecular weight aggregates, allow the rearrangement of the protein molecules, reduce the surface hydrophobicity, allow the formation of disulphide bonds including the bonds between pea legumin and rice glutelin and, therefore, improve the solubility of the blends.¹⁵

6. Soluble components

Many soluble polymers, mostly polysaccharides, are incorporated as stabilisers and thickeners. Instant foods also contain small soluble molecules of functional compounds, flavouring compounds and colouring compounds.

The small molecules may be added as individual ingredients or added to the complex with other insoluble ingredients. For the individual ingredients, it is critical to achieve fast dissolution during reconstitution. Many small molecules, such as sugars, salts and acidity regulators, are widely available in crystalline forms. Compared with amorphous forms, the powders in crystalline forms have low hygroscopicity and high flowability, which are favourable to powder food production. Nevertheless, they tend to have a slower dissolution rate than amorphous states due to limited interface, a tight molecular structure and the low energy level of the crystalline state.¹ Amorphous powders or amorphous regions can be generated by fast supercooling, fast drying, adding impurities and grinding.¹ Grinding also significantly reduces the particle sizes and increases the surface area for faster dissolution. In the complex with insoluble polymers, these small molecules, especially the ones on the particle surfaces, modify the hydration behaviour and dispersibility of the insoluble particles (section 4). The coating of whole milk powder with micronised lactose, especially amorphous lactose, significantly reduces the reconstitution time and affects the melting of the surface fat.⁷⁶ The soluble small molecules may also play the role of dispersing aids. Microparticulated and formulated fine green tea powder using glucose as the core can be instantly dispersed in cold water without forming lumps.³⁰ In terms of the influence of the overall structure of reconstituted foods, most small molecules do not directly influence the structure and texture of the reconstituted products. However, they may influence the pH, the ionic environment or the hydration environment, which affects the inter-or intra-molecular interaction of polymers or the molecular conformation and spatial arrangement.^77,78

The soluble oligomers and polymers are incorporated to provide nutrients, assist in production processes or improve texture. They might be found within the complexes with insoluble components, such as amylose in starch, and whey protein in milk powder. Oligomer dietary fibres are of interest in many instant foods because they provide nutritional benefits and contribute less to viscosity and the overall structure. The polymers, mostly polysaccharides, are used as thickeners to increase the viscosity of the continuous phase, prevent sedimentation and creaming, and stabilise the structure. They are also incorporated for texture improvement, or satiety enhancement, or for modification of swallowing properties for dysphagic patients.^79,80 Additionally, polymers with lower molecular weights and higher solubilities are preferable to be used as carriers in spray-drying due to their low viscosities at high concentrations.

Both soluble polymers and swollen particles of less soluble polymers are utilised as thickening agents. However, the solution thickened by soluble polymers and the dispersion thickened by swollen particles exhibit different rheological and textural characteristics. As discussed in section 3, swollen particles significantly thicken the system at a relatively high volume fraction where extensive interparticulate interactions occur. The particles form a three-dimensional physical network, which is able to store part of the deformation energy and show gel-like/solid-like rheological properties at small deformation. They may not exhibit a zero-shear viscosity (the viscosity of the system at rest when the shear rate is zero). Other possible rheological characteristics of disperse systems include yield stress and thixotropy. As for the soluble polymers, the intermolecular entanglement contributes to the viscosity increase at a higher concentration (higher than the critical concentration). The deformation energy is mostly released and the thickened system is a viscous fluid. Hence, it requires further consideration in choosing thickeners to achieve the desired structure, texture and mouthfeel.

7. Potential for health benefits

Noncommunicable diseases lead to 74% of all deaths globally.⁸¹ A large part of the ‘healthy diet’ concept targets the prevention of non-communicable diseases and the improvement of overall health. Functional compounds and beneficial materials contribute to the prevention of diseases by modulating certain physiological responses or pathways,⁸² balancing microbiota and adjusting digestion and absorption. Instant powder foods are well accepted in different food segments and are superior in storage stability compared with other food products. Therefore, they are excellent carriers for functional compounds and beneficial materials to deliver health benefits. Dietary fibres, probiotics, phenolic compounds and polyunsaturated lipids are four types of the most known material that exert positive effects on health, which are covered below with an emphasis on the potential of instant powder foods developed based on them.

7.1. Dietary fibres

A high intake of dietary fibre and whole grains was found to be associated with a 15–30% reduction in the incidence of and mortality caused by several non-communicable diseases including cardiovascular diseases, stroke, type 2 diabetes and colorectal cancer.⁸³ It is suggested that the daily intake of total dietary fibre should be 25 to 29 g for adults, which can be further increased for additional benefits.⁸³ The mechanism of the benefits of the enrichment of dietary fibre is complicated and may include modifying the chewing and ingestion of foods, enhancing satiety, influencing the absorption of nutrients and affecting gut microbiota and their metabolites. Fermentable dietary fibres (prebiotics), which are commonly soluble, can be utilised by the beneficial bacteria in the colon and increase the microbial mass. The fermentation process in the colon leads to higher levels of short-chain fatty acids produced by the microbes. The short-chain fatty acids are absorbed in the intestine and provide energy. They are also proved to have positive effects on several diseases. The high water-holding capacities of the fibres and the bacterial mass cause increases in stool volume and stool frequency. Non-fermentable dietary fibres, typically cellulose, also contribute to stool bulking. The dietary fibres with high degrees of polymerisation are usually added in instant foods as thickeners to adjust mouthfeel and increase stability. The viscosity of dietary fibres affects their functions related to digestion, laxation and enzymatic activity.

However, the legislation in many countries requires that the addition level of fibre in food needs to be high enough (3 g of fibre per 100 g of product in Europe⁸⁴ and China,⁸⁵ 2.5 g per serving in the U.S.,⁸⁶ 2 g per serving in Canada⁸⁷) to be claimed as a ‘source of fibre’ and the requirement for ‘high fibre’ is even higher. Only low addition levels of gums and thickeners are required to achieve the desired thickness or viscosity and the products might be over-thickened if the addition level reaches the ‘source of fibre’ level. Therefore, saccharides, including fructooligosaccharides, resistant dextrin and polydextrose, which have high solubility, low molecular weights and a limited contribution to viscosity, are commonly used for the high-fibre purpose.

An alternative way is to use less processed materials and by-products of agricultural and food industries, such as citrus fibre, apple fibre, fruit and vegetable powders, brans and whole grains. Mixed fruit beverage powder enriched with carambola pomace fibre was proved by animal tests to decrease the levels of cholesterol, high-density lipoprotein, low-density lipoprotein and triglycerides.³ The decreases in serum glutamic pyruvic transaminase and glutamic oxaloacetic transaminase suggest positive effects on liver functions by reducing serum glucose and cholesterol levels.³ Short chain fatty acids were detected in rat caecal matter, which indicates favourable effects on colon health.³ Functional instant soup mixtures for the elderly formulated from nutritious high-fibre ingredients and by-products show the ability to prevent hyperglycemia, hyperlipidemia and constipation in geriatric rats.⁸⁸ In another study, a reduction of glycaemic response by 20% can be achieved by adding 5.72 g oat bran (containing 1.6 g β-glucan) in instant oatmeal compared with β-glucan-free cereals.⁴ They also suggested a relationship between the thickening effect of brans and glucose absorption kinetics. A powder wheat drink was enriched with wheat bran by extrusion and the products had lower digestibility, a lower human glycaemic response and higher fermentability by gut microbiota compared with wheat noodles.²² Liu, et al. (2021)⁵⁹ developed cereal-based high-fibre drink powders from oat or highland barley enriched with their brans, respectively. The products were proved to have a low starch digestibility and postprandial glycaemic response.⁵⁹ In addition to fibres from brans, the extrusion process significantly increases amylose and resistant starch contents and leads to the formation of starch–protein complexes (starch embedded by protein), which lower the digestibility of starch and provide more carbohydrates to support increased colon fermentation.^22,59 Additionally, the extruded instant cassava–soy porridge with added wheat bran exhibits higher viscosity, which leads to increases in oral exposure time and bites during consumption, and, therefore, enhances satiety and suppresses appetite.⁵⁸

7.2. Probiotics

Ingestion of probiotics influences the microbiota in the gastrointestinal tract, improves the gastrointestinal environment, and then causes subsequent beneficial impacts on health, which include immunomodulatory activities, defending against antigens and pernicious bacteria, reducing obesity, lowering cholesterol levels, improving liver health, and improving neural and brain health and functions via the ‘microbiota–gut–brain-axis’, which has been reviewed by Das, et al. (2022)⁸⁹ and Snigdha, et al. (2022)⁹⁰ Probiotic intervention has also be found to be effective for the prevention and treatment of urogenital infections in women.⁹¹ The microorganism genera used as probiotics are Bifidobacterium, Lactobacillus, Saccharomyces and Bacillus. The health benefits and functions could be linked to specific probiotic strains, which need to be taken into consideration during the formulation.

Fermented dairy products have been one of the most preferred systems for probiotic loading as a result of the involvement of the probiotic microorganisms in the fermentation process and the protective effects of the dairy matrices on the probiotics in the gastrointestinal tract. Many dairy-based probiotic drinks are in the ready-to-drink form. A meta-analysis by Companys, et al. (2020)⁹² exhibited an association between fermented milk consumption with reduced cardiovascular risk and an association between probiotic-enriched dairy matrices with improved lipid profile in hypercholesterolemic subjects. In a clinical trial, inhibitory effects of fermented milk containing Lactobacillus paracasei and Glycyrrhiza glabra on Helicobacter pylori infection have been reported with a significant improvement in chronic inflammation, gastrointestinal symptoms and life quality.⁹ As for products in instant powder forms, the health benefits of skimmed milk powder containing Lactobacillus plantarum on overweight adults have been evaluated by Rahayu, et al. (2021)⁸ and a significant reduction in body weight and body mass index were observed. Jia, et al. (2023)⁹³ has developed synbiotic milk powder fortified with protein, minerals, vitamins, Lactobacillus plantarum and isomaltooligosaccharide. The number of viable probiotic bacteria was significantly increased by adding prebiotics and the synbiotic product improved calcium absorption in calcium-deficient mice followed by increased serum calcium and phosphorus levels and bone mineral density.⁹³Lactobacillus paracasei addition in milk powder exhibits a preventive effect on dental caries by inhibiting the growth of mutans streptococci and increasing salivary IgA.⁹⁴

Many plant-based products are high in fibres that naturally serve as prebiotics and may be rich in other beneficial compounds such as phenolic compounds. Supplementation of pineapple waste powder to yoghurt fermentation increased the probiotic population by 0.3–1.4 log cycles with an improvement in antioxidant and antimutagenic activities.⁹⁵ Plant-based probiotic products have also been widely investigated and may serve as an alternative to dairy-based products. An eight-week intervention with apple puree fermented by Lactobacillus rhamnosus in subjects with cardiovascular disease risks led to a more significant impact on the increase in high-density lipoprotein cholesterol and decrease in trimethylamine-N-oxide levels compared with the unfermented apple puree and probiotic alone groups.¹⁰ Long-term supplementation with a fermented product containing soya flour, alfalfa meal and barley sprouts in diabetic and fatty rats was found to reduce glucose absorption and improve glucose homeostasis.¹⁷ Multiple health benefits, including adipocyte size reduction, body weight reduction, faecal microbiota modulation and immune profile modulation, of a soy-based product fermented with Enterococcus faecium and Lactobacillus helveticus with added Bifidobacterium longum have been illustrated.¹⁸

Despite the health benefits of probiotics and probiotic foods, it is also critical to preserve the viability during drying and storage and in the gastrointestinal tract. The drying process is necessary to add probiotic ingredients to instant food products. Although the dehydrated condition increases the stability of the microorganisms during storage, the high temperature, dehydration, osmotic pressure, and oxidation during drying could lead to a significant reduction in viability. The most common drying methods applied are freeze drying and spray drying. Except for the high cost, freeze-drying is adopted to avoid thermal damage and oxidation at high temperatures. Freezing and dehydration lead to damage to cell structure and functions. Protective agents such as sugar, maltodextrin, fructooligosaccharides and sodium caseinate are used to prevent the fusion of cells during dehydration and preserve their integrity to increase their viability.⁹⁶ Oluwatosin, et al. (2022)⁹⁷ compared sucrose, maltodextrin and inulin to skim milk in preparing freeze-dried Lactobacillus plantarum powder and found that skim milk is superior in preserving cell viability during freeze-drying and storage. Inulin is comparable to skimmed milk as a cryoprotectant, while sucrose preserves the cells during storage. β-Glucans from Brewer yeast spent are proved to protect probiotic cells during freeze drying by adherence of the probiotic cells to the β-glucan molecules.⁹⁸ Spray drying is also widely applied to encapsulate and dry probiotics due to its low cost and high efficiency, and the fast drying rate prevents the loss of viable cells at high temperatures. Huang, et al. (2017)⁹⁹ reviewed the choice of strains, culturing conditions, choice of devices, utilisation of protective agents, drying parameters before and during spray drying and storage conditions for the spray-dried probiotics.

It is also important to deliver the probiotics to the targeted position in the gastrointestinal tract by protecting them from the acidic environment, digestive enzymes, bile acids, lysozymes and other components in the oral, gastric and intestinal environment. Encapsulation has been proved to be an effective method to preserve probiotic viability. The encapsulating materials are usually proteins and/or polysaccharides. They form a dispersion with the target probiotics followed by drying to achieve a structure where probiotics are imbedded in a matrix of protein and polysaccharides.¹⁰⁰ Some polysaccharides used for encapsulation are prebiotics, which enhance the survival of the cells.¹⁰⁰ Additionally, the microbeads formed by silica and alginate are proved to protect microbes (L. rhamnosus GG) from the intestinal environment and boost the microbe metabolism in the colon.¹⁰¹ Screening more resistant probiotics or acid treatment may be other solutions. Zhang, et al. (2020)¹⁰² developed fermented noni juice powder with added L. plantarum and L. rhamnosus via freeze drying and found that L. plantarum is more resistant to the low pH, the digestive conditions and the antibacterial effect of noni juice. Srisukchayakul, et al. (2018)¹⁰³ found that acid pretreatment on Lactobacillus plantarum at the stationary phase significantly improves cell survival in highly acidic environments.

7.3. Phenolic compounds

Phenolic compounds are widely occurring in plants as a group of secondary metabolites, which can be classified into simple phenolics, polyphenols, phenolic acids, coumarins, xanthones, stilbenes, flavonoids and isoflavonoids, lignans, and phenolic polymers.^104,105 Phenolic compounds are strong hydrogen- or electron-donating agents and, therefore, are widely incorporated in foods as antioxidants. They also chelate metal ions and hence reduce metal-catalysed oxidative reactions. The imbalance between the free radical production and antioxidant defence systems of living organisms results in oxidative stress and nitrosative stress, which may cause many health issues and diseases.¹⁰⁶ Therefore, the antioxidant capacity of phenolic compounds has been stressed by researchers and relevant industries for their health benefits. Recent studies suggest that the functions of phenolic compounds are not limited to antioxidation. Phenolic compounds, such as flavonoids, are reported to regulate certain enzymes, bind to membrane transport proteins and influence membrane permeability, affect immune and inflammatory cell functions, maintain vascular homeostasis, improve vascular function, lower CHD risk, lower type 2 diabetes risk, increase apoptosis of cancer cells and reduce cell proliferation, protect neuronal function and retard cognitive decline.^107–109 Chen, et al. (2021)¹¹⁰ illustrated that procyanidins protect neural systems by promoting the levels of multiple antioxidant enzymes and by participating in and enhancing the Nrf2/ARE antioxidant pathway. Phenolic compounds are also proved to influence gut microbiota, which then participates in microbial catabolism and metabolism, and the produced metabolites also exhibit health benefits.¹⁰⁷

The enrichments of phenolic compounds are often simultaneous with dietary fibres as many plant-based materials are rich in both compounds. Plant-based probiotic products tend to have high contents of the three of them. Phenolic compounds can be delivered as extracts or in the food matrix of raw materials that are rich in phenolic compounds. Most plant powders are rich in phenolic compounds with the total content ranging from approximately 130 mg kg⁻¹ (green pea) to around 930 mg kg⁻¹ (spinach).¹¹¹ Dark sweet cherries, for example, are rich in dietary fibre and polyphenols and supplementation of the powder with 3 g day⁻¹ decreases blood pressure and inflammation levels in obese adults.⁵ Blueberry and blackcurrant powders were mixed with oat bran to enhance the nutritional benefits of the products, which were proved to have higher phenolic contents and dietary fibre contents, a lower extent of starch degradation and a lower release of reducing sugar during in vitro digestion, as a result of the interaction between the phenolic compounds and amylase or glucosidase.⁶ Buckwheat, oat flakes and oat brans are high in phenolics and fibres and the instant porridge developed from them with other ingredients was proved to improve lipid profile and increase fat-free mass in participants with mild to moderate hypercholesterolemia after a 5-week consumption, which is ascribable to the high content of phenolic compounds and dietary fibres and low digestibility of buckwheat proteins.⁷ Instant soup mixtures for elderly individuals formulated with chickpeas, vegetables, olive oil and by-products are high in both phenolic compounds and dietary fibres. The product exhibits high antioxidant activity, causes low postprandial glycaemic responses and reduces cholesterol levels.⁸⁸ Fermented plant-based products tend to be rich in phenolics, fibres and probiotics. Mung bean was mixed with pregelatinised rice, yoghurt, vegetables, yeast and guar gum to produce gluten-free fermented instant soup powder and the obtained products had high contents of total phenolics, protein and dietary fibre.¹¹² Phenolic compounds that are absorbed by the small intestine and enter the systemic circulation can exert health benefits such as reducing cardiovascular risks.¹⁰ However, they interact with soluble and insoluble fibres in plant-based products and form complexes, which reduces the bioaccessibility of phenolic compounds in the small intestine but increases that reaching the large intestine and exerts relevant health benefits such as reducing colorectal cancer rates.¹⁰ The fermentation process by microbes releases the phenolic compounds from the complexes and increases the levels of free phenolics available for absorption.¹⁰

One of the challenges of enriching phenolic compounds in foods is the bitterness and astringency of many phenolic compounds, typically proanthocyanidins and epigallocatechin gallate. Zhuang, et al. (2020)¹¹³ identified that stronger astringency of different teas is related to the condensation of flavan-3-ols, hydroxylation in the B-ring of flavonoids, acylation of polyphenols and glycosylation. The perception of astringency is caused by the interaction between the salivary components (mainly mucin) and the phenolic compounds, which bind to the oral surfaces and result in the loss of lubricity.¹¹⁴ Mišan, et al. (2017)⁷ added dried apples, sweetener and cinnamon to mask the bitterness of buckwheat. Whey proteins and their hydrolysates form complexes with polyphenols and reduce astringency.¹¹⁵ Other techniques used to mask astringency include encapsulation, adjusting the pH¹¹⁶ and using polysaccharides and glycoproteins as astringency modulators.¹¹⁷

7.4. Polyunsaturated lipids

Lipids, including acylglycerols, cholesterols, phytosterols and phospholipids, are an essential part of diets. They also contribute to the creamy texture and flavour of instant foods. Triacylglycerols are the most common lipids in diets, which are composed of fatty acids and glycerol. Polyunsaturated fatty acids are mainly found in plants, animals, fungi and algae. The linoleic acid level is high in a diet that contains various seed oils and fish oils, while α-linolenic acid is rich in certain seeds such as flaxseed, perilla, chia seed and linseed.^118,119 Recently, many studies have focused on microalgae, oleaginous bacteria, yeasts and fungi as sources of essential fatty acids and investigations have been done to increase the yield by optimising the composition of nutrients and growing conditions.¹¹⁹ Δ12 fatty acid desaturase (FADS12) and Δ15 fatty acid desaturase (FADS15) are the main enzymes to synthesise linoleic acid and α-linolenic acid in the aerobic pathway in many species. Therefore, genetic engineering has been applied to enhance the production by overexpressing FADS12 and FADS15 in certain strains of yeasts, fungi or microalgae or by the transgenesis of plants.¹¹⁹

It is generally recommended that the intake of lipids should comprise less than 30% of the total energy (FAO/WHO) and should contain a high portion of unsaturated fat.¹¹⁸ The increased intake of n-3 polyunsaturated fatty acids is found to have beneficial effects on cardiovascular diseases, diabetes mellitus, breast cancers, respiratory diseases, cognitive function attention deficit, hyperactivity disorder, the development of the neural system, depression and anxiety, nonalcoholic fatty liver disease, chronic kidney disease, and inflammation.¹¹⁸

The positive effects of n-3 polyunsaturated fatty acids on cardiovascular diseases have been proved in numerous epidemiological studies and meta-analyses. Different n-3 polyunsaturated fatty acids potentially show complementary benefits.¹²⁰ Docosahexaenoic acid reduces triglyceride concentration, promotes vasodilation by enhancing the production of nitric oxide, and regulates the expression of several related genes and hence inhibits TNF-α-induced endothelial dysfunction and senescence.¹²⁰ Eicosapentaenoic acid restores vascular endothelium-dependent vasodilation, with the strongest inhibitory effect on insulin-mediated endothelin-1 expression, and suppresses palmitic acid-induced endothelial dysfunction.¹²¹ Additionally, it is also proposed that n-3 polyunsaturated fatty acids have stimulative and inhibitive effects on the synthesis of prostacyclin (a vasodilator) and thromboxane (a vasoconstrictor and potent hypertensive agent), respectively, and show effects on β-adrenergic signalling transduction in cardiomyocyte membranes, which lowers the blood pressure.¹²² The meta-analysis conducted by Guo, et al. (2019)¹²² shows that supplemental eicosapentaenoic acid and docosahexaenoic acid dramatically reduce systolic blood pressure and diastolic blood pressure, respectively. Hence, both eicosapentaenoic acid and docosahexaenoic acid reduce blood pressure, especially in dyslipidaemia patients.¹²² As for α-linolenic acid, it reduces serum total cholesterol, low-density lipoprotein cholesterol and triglycerides and inhibits inflammation and ameliorates vascular dysfunction in healthy populations, the population during weight loss and patients with diabetes.¹²⁰

The importance of n-3 and n-6 polyunsaturated fatty acids on brain development and function has also been investigated. Docosahexaenoic acid is the main fatty acid in the brain and neural cells. It plays significant roles in brain development, neurotransmission and multiple brain and neural functions, including neurogenesis, neuronal migration and outgrowth of the hippocampus and cortex, and deficiency may cause various diseases and disorders.¹²³ Arachidonic acid is the second most abundant polyunsaturated fatty acid in the brain and a balanced level of arachidonic acid and docosahexaenoic acid is important for brain health and function.¹²⁴ According to meta-analyses summarised in Li (2022),¹¹⁸ increasing the intake of long-chain n-3 polyunsaturated fatty acids is beneficial to the cognitive function of children in certain developmental stages and mitigates attention deficit hyperactivity disorder-related symptoms in children and adolescents but may have no or limited impact on cognitive impairment of older adults.

Supplementation of fish oil, which is rich in eicosapentaenoic acid and docosahexaenoic acid, was proved to improve glycaemic control and decrease blood lipid levels in type 2 diabetes patients at 2 g day⁻¹ consumption level.¹²⁵ A three-month study reveals that the supplementation of fish oil reduced hepatocellular damage and plasma triacylglycerol levels in patients with non-alcoholic fatty liver disease at a 3 g day⁻¹ consumption level.¹²⁶ The supplementation of fish oil with vitamin D₃ (1680 IU) was found to have positive effects on insulin resistance and inflammatory factors.¹²⁶ The polyunsaturated lipids can be encapsulated and converted into powder, which can be formulated into instant powder foods.¹²⁷

8. Conclusion, trends and opportunities

Instant powder foods can be excellent carriers for the delivery of functional compounds to achieve certain health benefits. They can be categorised based on the solubility of ingredients and the structure of the reconstituted products. Instant powder foods containing insoluble substances form complex colloidal structures after being reconstituted. The viscosity, texture and stability-influencing factors are closely related to the volume fraction of the dispersed phase, which needs to be adjusted to match the desired quality of different products. The reconstitution is a multistep process and is critical for product quality, which can be influenced by many factors. Both particle structuring strategies and molecular structuring strategies are investigated and applied to improve the reconstitution properties. Insoluble polysaccharides, lipid droplets and protein aggregates are the three main insoluble substances, which disperse, swell and contribute to the dispersed phase of reconstituted instant foods. The soluble components can be dispersing aids or influence the solution environment. Soluble and insoluble polymers are both used as thickening agents but they exhibit different rheological behaviours, which need to be considered to achieve the desired texture.

Dietary fibres, probiotics, phenolic compounds and unsaturated lipids are proved to have positive effects on the prevention of non-communicable diseases and overall health, based on which various functional or healthy instant powder foods have been developed. Plant-based products are often rich in both phenolic compounds and dietary fibres. Less processed materials and by-products are often chosen for their full nutritional profile, low cost, clean label and valorisation. The processes of different ingredients need to be tailored according to the properties of the raw materials and the targeted substances.

Further research may focus on systematic evaluations of the treatments and properties of beneficial ingredients, reconstitution properties of the products and structural development of the reconstituted products for the optimisation of the consumption properties. It may also focus on the combination of multiple nutrients to improve the overall function and enhance the health benefits of the end products. Furthermore, most instant foods are high in carbohydrates, which is unfavourable for weight control, glycaemic control and the prevention of most non-communicable diseases. Sugar and starch reduction in foods have been one of the main interests of researchers. However, future challenges may still lie in balancing low sugar/starch and desirable tastes.

Conflicts of interest

There are no conflicts of interest to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3fo04216b

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