Liqian Wangab,
Yuxiu Wenab,
Caihong Suab,
Yuan Gaoab,
Qi Liab,
Shuangkui Duab and
Xiuzhu Yu*ab
aCollege of Food Science and Engineering, Northwest A&F University, 22 Xinong Road, Yangling, Shaanxi 712100, P. R. China. E-mail: xiuzhuyu@nwafu.edu.cn; Fax: +86-29-87092486; Tel: +86-29-87092308
bEngineering Research Center of Grain and Oil Functionalized Processing, Universities of Shaanxi Province, 22 Xinong Road, Yangling 712100, Shaanxi, P. R. China
First published on 23rd March 2022
This study aimed to investigate the effect of water content on the properties and structure of oleogels by developing walnut oleogel based on potato starch and candelilla wax (CW). Physical, thermal, rheological and microstructure characteristics of the walnut oleogel were determined by texture analyzer, differential scanning calorimeter, rotary rheometer, X-ray diffractometer and optical microscope. Results showed that with increased water content, the hardness of the oleogel increased from 123.35 g to 158 g, whereas the oil loss rate decreased from 24.64% to 10.91%. However, these two values decreased slightly when the ratio of oil to water was 1:
1. The prepared oleogels have a high elastic modulus, and the flow behavior of all walnut oleogels conformed to that of a non-flowing fluid. Microstructure observation indicated that the crystal size and quantity increased with an increase in water content, and the liquid oil was wrapped in the crystal network by CW and potato starch, forming solidified droplets to further promote gelation. In conclusion, when the ratio of oil to water is 39%, the oleogel has good physical properties and stable crystal structure. These findings can provide an indication of water content in the composition of oleogels.
The preparation process of oleogels requires gelling agents.8 Marangoni and Garti (2011) found that the most promising food-grade gels are monoglycerides, waxes, lecithin and sorbitol tristearates, long-chain fatty acids, fatty alcohols,9 protein, polysaccharides or their mixtures, phytosterols, and glutamate, because they are edible and can form stable oleogels. However, wax has many promising properties. For example, it can form a high oil-binding capacity and thermo-reversible oleogel at very low concentrations.10 Meanwhile, a high concentration of wax may result in rougher waxy texture.
Recently, hydrocolloids have become very popular in the development of novel gel systems due to their excellent thickening, gelling and texture properties.11 Silva et al. (2021) studied emulsion-filled hydrogels composed of potato starch (PS) and alginate, in which starch partially replaced alginate, thus changing the hydrogel network. It was reported that natural (sodium alginate) and synthetic (hydroxypropyl methylcellulose) polymer had been applied to develop a polymer-fish oil bigel (hydrogel/oleogel colloidal mixture). The results indicated that hydrogels are essential for the gel structure because of hydrogen bonding.12 In addition, much research proved that methylcellulose and starch are promising hydrocolloid-based oleogelators in the food industry.13–15 Potato starch is a main component of Chinese traditional staple food. Compared to other low-molecular-weight organogelators, PS is cheap, safe and healthy.16–18 Besides, Zhai19 et al. (2016) found that the addition of starch significantly affects the storage modulus of the emulsion during heating and cooling and increases the hardness, viscosity, and chewiness of the emulsion gel.
As for the rheological, thermal stability, and textural properties of oleogel, aside from gelators, oil is another key factor.20 The oleogel can be made from nutrient-rich vegetable oils, such as those rich in mono and polyunsaturated fatty acids.21 Walnut oil (WO) contains mainly unsaturated fatty acids, such as oleic acid (14.9–28.7%), linoleic acid (50.6–66.6%), and linolenic acid (9.2–16.4%).22 These unsaturated fats play an vital role in regulating blood lipids, cleaning blood clots, and improving body immunity.23–25 In addition, WO is rich in micronutrients, such as phytosterols, squalene, polyphenols, and tocopherols, and its flavonoid content is significantly higher than that of peanut oil, corn oil, and other common vegetable oils. It is currently mainly consumed as a health oil and low-temperature cooking oil, but deep processing remains lacking.26 WO possesses high biological activities, including good nutritional function and antioxidant property. Therefore, it is a new kind of oleogel material with development potential.
This study aimed to prepare complex oleogels and explore the influence of different water contents on the properties and structure of oleogels in an oil–wax–starch mixture. The oleogels were formed by mixing the oil and water phases. To evaluate the potential applicability of oleogels in food, we evaluated the binding ability of oleogels. In addition, the rheology, thermodynamics, and microstructure of the oleogels were characterized. This study aimed to provide a reference for expanding the application range of WO and transforming the inherent morphology of WO. The effects of mixed components on the three-dimensional network structure and gel mechanism of oleogels are discussed to provide technical reference and theoretical basis for the development of the low-oil oleogel industry.
An AH-2010 high-pressure homogenizer (Antrox nanotechnology (Suzhou) Co., Ltd, China) was used to process the pre-emulsion by pressurizing it twice (100/500 bar). After homogenization, the resulting emulsion was cooled to room temperature (20–25 °C). The cured emulsion was then stored at 4 °C before characterization. These conditions were determined after preliminary trials. The schematic diagram for oleogel preparation is shown in Fig. S1.†
DSC STARe software was used to calculate the initial temperature, maximum value, enthalpy of melting and enthalpy of crystallization of each peak. Each experiment was repeated three times.
The hardness of oleogels increased first and then decreased with an increase in water content, whereas the OLR rate of oleogels showed an opposite trend. When the ratio of oil-to-water was 6:
4 (39%), the maximum hardness of oleogel was 158 g, and the lowest OLR rate was 10.91%, forming a relatively stable gel structure. With the increase of water content, the oleogel system became unstable, and oil–water stratification appeared (Fig. 1a). This finding contradicted previous results27 possibly because the addition of water affects the gel system, and a proper moisture content could produce an optimal oleogel system. The specific interactions among starch, wax, and oil need further exploration.
Fig. 2a shows the amplitude scanning information of the oleogel samples with different oil-to-water ratios. G′′ refers to the solid/elastic character of oleogels: the more resistant the oleogel is to deformation, the higher the G′′ value.29 As can be seen from Fig. 2a, in the linear viscoelastic region, all oleogels showed a dominant ‘solid’ behavior (G′ > G′′); especially, when the ratio of oil to water was 6:
4, G′ and G′′ reached the maximum. This finding was obtained possibly because the addition of starch and water enhanced the intermolecular region in the noncovalent bond.30 Under this condition, it formed more self-assembled structures of the oleogel. This made the oleogels become more stable and harder to deform.29
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Fig. 2 Rheological behavior of oleogels with different oil-to-water ratios. (a) Amplitude sweep, (b) frequency sweep, and (c) flow measurement curves. |
To evaluate additional information on oleogel samples, frequency scanning was performed. Results showed that the values of G′ and G′′ had a low frequency dependence, and the G′ values were always greater than the corresponding G′′ and independent of frequency (Fig. 2b), which is characteristic of the gel and consistent with their strain scan results (Fig. 2a). Therefore, water can induce the flow of droplets to form solidified droplets and promote interactions between droplets to form a network. Interestingly, none of the curves intersected (G′ = G′′), which indicated that the oleogels were not converted from gel to higher-frequency sol.5
The apparent viscosity of the oleogels increased with the increase of water content, and all samples exhibited significant shear thinning behavior (Fig. 2c), indicating that the shear process would lead to disintegration of the network in the oleogels.31 At the initial stage of shear, large shear stress was needed to destroy the gel network structure, and then increasing starch molecules changed from the initial network structure to a directional flow, showing a “liquid” behavior, resulting in shear stress tending to smooth.
The curve of shear stress τ and shear rate was fitted using power law equation via regression. The Herschel–Bulkley model can be expressed as τ = τ0 + Kγn, where τ is the shear stress/Pa, τ0 stands for the yield stress/Pa, K is the consistency coefficient/(Pa s), γ is the shear velocity/s−1, and n represents the flow index. The Herschel–Bulkley model parameters of oleogels with different oil-to-water ratios are shown in Table S1.†
For all oleogel samples, the correlation coefficient R2 was high (0.965–0.997), indicating that the flow behavior of oleogels obeyed the power law equation. The value of K is related to the thickening ability. When K increases, the thickening ability of the emulsion also increases. When the oil-to-water ratio was 5:
5, the fitting degree (∼0.975) was not very high, and the previous rheological behavior was consistent with this finding. This finding was obtained possibly because of the formation of large and irregular droplets in the oleogel (Fig S3e†) and the existence of oil–water stratification. N is the fluidity characteristic index, which indicates the degree of difference between non-Newtonian and Newtonian fluids. Based on Table S1,† the non-Newtonian exponent n of the oleogel was less than 1 and in the range of 0.14–0.39, which indicated that it is a pseudoplastic fluid, and this result is consistent with the shear thinning result from the flow curve (Fig. 2c). The smaller the n was (the farther away from the Newton type it was), the easier the liquid undergoes shear thinning, and the greater the pseudoplasticity. When the oil-to-water was 6
:
4 (39%), the value of K was maximum (21.90), and the value of n was minimum (0.12). According to the results of rheology analysis, PS-CW-oil formed a type of composite oleogel, which is further proved in the following results.
Sample | Ton/°C | Tp/°C | Tend/°C | Tg/°C | ΔH/w/g |
---|---|---|---|---|---|
a Different letters indicate significant differences between oleogel samples at p < 0.05. Crystallization and melting onset temperature (Ton), peak temperature (Tp), terminal temperature (Tend), and enthalpy change (ΔH) of different oleogel samples. | |||||
Crystallization | |||||
9![]() ![]() |
−13.9 ± 0.33e | −14.86 ± 0.35c | −22.76 ± 0.44e | −22.04 ± 0.23a | 19.11 ± 1.12e |
8![]() ![]() |
−15.98 ± 0.29d | −19.28 ± 0.45b | −23.53 ± 0.37d | −16.58 ± 0.12d | 30.31 ± 1.98c |
7![]() ![]() |
−18.93 ± 0.24b | −21.27 ± 0.56ab | −26.24 ± 0.41c | −22.00 ± 0.55a | 47.16 ± 2.65b |
6![]() ![]() |
−19.04 ± 0.37a | −22.71 ± 0.20a | −27.58 ± 0.39b | −19.31 ± 0.46b | 59.10 ± 3.10a |
5![]() ![]() |
−18.17 ± 0.43c | −18.995 ± 0.32b | −28.67 ± 0.36a | −17.29 ± 0.32c | 20.0 ± 1.47d |
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|||||
Melting | |||||
9![]() ![]() |
−6.57 ± 0.11b | −1.54 ± 0.02a | 3.61 ± 0.21c | −1.49 ± 0.11a | 47.30 ± 2.79e |
8![]() ![]() |
−5.47 ± 0.17d | −1.41 ± 0.10a | 3.94 ± 0.22b | −1.39 ± 0.45c | 64.05 ± 2.55d |
7![]() ![]() |
−5.47 ± 0.14d | −1.51 ± 0.04a | 4.38 ± 0.27a | −1.48 ± 0.26ab | 71.97 ± 2.94c |
6![]() ![]() |
−6.78 ± 0.20a | −1.49 ± 0.09a | 3.18 ± 0.30d | −1.46 ± 0.37b | 83.99 ± 3.13b |
5![]() ![]() |
−6.02 ± 0.25c | −1.46 ± 0.07a | 3.18 ± 0.41d | −1.45 ± 0.04bc | 91.68 ± 3.45a |
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Fig. 3 FTIR spectra of walnut oleogels prepared at different oil-to-water ratios ((a): PS, CW, WO; (b): oleogels). |
From Fig. 3(a), the broad band at approximately 3304 cm−1 corresponds to the stretching vibration of O–H in starch, and the absorption peak at 2935 cm−1 may be attributed to CH– stretching vibration. The peak at 1641 cm−1 indicates the aldehyde group vibration peak of starch. The absorption peak at 1000–1300 cm−1 was in touch with the stretching vibration peak of the C–O–C bond.33 Based on the spectra of WO and CW, C–H stretching vibration peaks of unsaturated carbon chains were observed at 3008 cm−1, while anti-symmetric and symmetric stretching vibration peaks of saturated carbon chains were observed at 2924 and 2854 cm−1. At 1745 cm−1, a large spike associated with CO stretching was found, and these peaks are obtained from the WO. The peaks at 1452 and 1381 cm−1 confirmed the presence of C–H bending of methyl and methylene groups. The band at 1125–1095 cm−1 is associated with the C–O stretching vibration peak and the methylene C–H bending vibration peak in triglycerides.34
The FTIR spectra of walnut oleogels are shown in Fig. 3(b). The oleogels exhibit weak and wide O–H absorption from 3650–3200 cm−1, and a strong absorption peak at an oil-to-water ratio of 6:
4. A wide spectrum peak appeared in PS, indicating that intramolecular or intermolecular hydrogen bonds exist in the oleogels originated from starch.34 Hydrogen bonding is very important for forming the oleogel network.35 The semicrystalline structure of PS mainly comes from the hydrogen-bonded polymer entanglement. Oil droplets are trapped in the oleogel because of the hydrogen bonding between starch and water. In the study of ethyl cellulose oleogels, hydrogen bonds between polymer chains were found to be responsible for forming a network connecting liquid oils.36 The significant absorption peaks at 3,008, 2,924, 2,854, and 1745 cm−1 were associated with WO and CW. These peaks were significantly lower in strength than those under other conditions, indicating that the oil droplets were firmly trapped and tightly bound. In the sample with 6
:
4 ratio, a peak associated with (CH2)n bending was observed at 879 cm−1, and this peak may be related to the presence of starch. By comparing the FTIR spectra of the components, we found that the absorption of the oleogels was mainly due to the functional groups of starch and WO; similar results were found in gels made from beeswax.37
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Fig. 4 XRD images of oleogels with different oil/water ratios. ((a): CW; (b): PS; (c): different oil/water ratios). |
For a comprehensive comparison of the changes in XRD characteristics, we set the XRD patterns of CW and PS as controls. In Fig. 4(a), two peaks were discovered at 3.72 and 4.0 Å in CW. For PS, a wide peak was observed at 2θ = 20.0 Å (Fig. 4b). These results supported the finding that most of the carbohydrates were amorphous.38 As shown in Fig. 4(c), a broad peak at 4.41 Å can be discovered in the oleogel sample, which is related to the main amorphous structure. The finding indicated the presence of a liquid triglyceride in the gel. A sharp peak at 4.12 Å and a weak peak at 3.72 Å were observed, indicating the presence of β-crystals with an orthorhombic perpendicular (O⊥) subcellar structure.39,40 The β-polycrystals have an orthogonal-to-vertical subcellular structure and a uniform and creamy texture, which is most suitable for commercial margarines and daubs. The oleogels had a broad peak at around 20 Å, because of the starch molecular chain in the oleogel structure of the extension and winding caused by the conformational change.41 Starch molecules formed a protective layer around the oil droplets and interact to form an ordered structure. There were no obvious differences among XRD patterns of the oleogels with different water contents. When the oil-to-water ratio was 6:
4, peak strength was largest, accounting for the maximum hardness and lowest OLR rate. Overall, the results of XRD patterns indicated that all the oleogels were regularly structured and retained liquid oils effectively.
As shown in Fig. S3(a)–(e),† with the increase of water content, the droplets in the oleogel gradually grew larger, and the droplet morphology gradually changed from spherical to non-spherical.42 When the oil-to-water ratio was 9:
1, a slight reflective phenomenon was observed on the surface, which is consistent with the observed phenomenon in Fig. 1(a). Moreover, the droplets became more compact, especially when the oil-to-water ratio was 6
:
4 (Fig. S3d†); all the droplets seemed to interact and connect, in accordance with the results of FTIR. Similar gelation behavior of starch nanocrystals stabilized as high-in-phase emulsions was reported in micrographs.41 When the oil–water ratio was 5
:
5, the particle distribution was larger, indicating that the droplets may form large clusters and show obvious droplet flocculation and coalescence. Based on Fig. S3,† the oleogel droplet size was slightly more dispersed in an oil-to-water ratio of 9
:
1. When the oil-to-water ratio was 6
:
4, the droplet distribution was more uniform, forming a more stable gel structure.
CW | Candelilla wax |
WO | Walnut oil |
PS | Potato starch |
OLR | Oil loss rate |
DSC | Differential scanning calorimetry |
XRD | X-ray powder diffraction |
Ton | Onset temperature |
Tp | Peak temperature |
Tend | Terminal temperature |
ΔH | Enthalpy change; |
G′ | Storage modulus |
G′′ | Loss modulus |
η | Complex viscosity |
ω | Angular frequency |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d2ra00920j |
This journal is © The Royal Society of Chemistry 2022 |