Afroza
Sultana
a,
Shuji
Adachi
b and
Hidefumi
Yoshii
*c
aDepartment of Food Processing and Engineering, Chattogram Veterinary and Animal Sciences University, Chattogram-4225, Bangladesh
bFaculty of Bioenvironmental Sciences, Kyoto University of Advanced Science, Kameoka, Kyoto 621-8555, Japan
cDepartment of Food Science and Human Nutrition, Setsunan University, 45-1 Nagaotouge-cho, Hirakata 573-0101, Japan. E-mail: hidefumi.yoshii@setsunan.ac.jp
First published on 25th August 2023
Fish oil and essential fatty acids are considered significant in functional foods. Polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (20:
5) and docosahexaenoic acid (22
:
6) in fish oil are prone to oxidative damage. The encapsulation of fish oil by spray drying is a useful method to stabilize fish oil by reducing contact with oxygen. In this review, kinetic analyses of oxidation of fish oil and fatty acids in bulk and emulsion systems and encapsulated fish and krill oils in spray-dried powders were reviewed. Oxidation kinetics of PUFAs could be expressed with the autoxidation equation using the fraction of unoxidized PUFA. The surface-oil ratio could be correlated with the equation 1 − (1 − 2(de/dp))3 using the ratio of oil-droplet diameter, de, to powder diameter, dp. The morphology of the spray-dried powder containing vacuoles affected the surface-oil ratio. The activation energy (Ea) could be correlated with the reconstituted oil droplet diameter of squalene powders. In the estimation of fish oil in spray-dried powder, this review showed that the model of deterioration kinetics of fish oil in the powder is very useful.
Sustainability spotlightThe technological development of “encapsulation” to form functional foods such as fish oil with a high content of polyunsaturated fatty acids (PUFAs) in a stable form is expected to promote the No. 3 purpose, “Ensure healthy lives and promote well-being for the super-aging society,” in SDGs. It is very important to realize “healthy lives for all at all ages” by using stable functional foods. This review paper summarized especially the degradation kinetics of PUFAs in liquids and the emulsified spray-dried powder. We believe that it will be very useful for advancing the basic technology for safe intake of functional food in order to maintain the health of elderly people. |
The main sources of n-3 fatty acids are fish oils, krill oil, and marine oils; they can also be found in plant products, such as flaxseeds and nuts. Fish oil is a good source of long-chain n-3 PUFAs. EPA and DHA have double bond structures that include 5 and 6 double bonds in a carbon chain of 20 and 22, respectively, with n-3 indicating the position of the first double bond in the acyl chain, as shown in Fig. 1.
Fish oils, which are an important source of n-3 fatty acids, are suitable for direct incorporation into foods; however, it is difficult to protect them from oxidation owing to their high degree of unsaturation. Anchovy and sardine oils are major sources of EPA and DHA, and currently, these two species account for 80% of n-3 products in the market.9 These oils were reported to have the highest concentrations of EPA and DHA (over 18% and 12%, respectively).10 Durmus (2018)11 reported that the fatty acid composition of seafood species included 10.69–39.57% PUFAs, 1.72–10.73% EPA, and 4.07–31.44% DHA. All seafood species had high EPA and DHA levels and a significantly higher n-3 PUFA content than the n-6 PUFA content. As EPA and DHA are important functional compounds for human health, the Japanese fisheries company, Maruha Nichiro Corporation, produces high DHA-content oil. This oil is produced by the enzymatic hydrolysis of raw tuna oil using lipase to obtain an undecomposed reaction product containing concentrated free fatty acids and DHA. Fatty acids are removed from this oil mixture to obtain a triacylglycerol fraction. This product is decolourised with active soil and deodorised by steam distillation to produce DHA-45, which is a triacylglycerol comprising 45% DHA and 4.6% EPA.12 Dahiya et al. (2023)13 reviewed the current status and future prospects of bioactive molecules delivered through sustainable encapsulation techniques for food fortification. Eratte et al. (2018)14 reviewed the microencapsulation of n-3 oil through complex coacervation.
However, there are very few papers on the oxidation kinetics of PUFAs in spray-dried powders.
These functional food materials such as fish oil, krill oil or PUFAs are very prone to oxidation. In this paper, kinetic analyses of oxidation of fish oil and fatty acids in bulk and encapsulated fish and krill oils in spray-dried powders are reviewed.
Initiation:
![]() | (1) |
Propagation:
![]() | (2) |
![]() | (3) |
Termination:
![]() | (4) |
![]() | (5) |
![]() | (6) |
Lipid oxidation comprises complex reactions in which the aforementioned three processes are intricately intertwined; hence, it is difficult to analyse the rate of lipid oxidation.
The autoxidation of PUFAs or their esters is a complicated process, and the aforementioned kinetics are typically considered for describing the individual steps. Özilgen and Özilgen (1990)18 proposed a kinetic model to describe the entire oxidation process of lipids. Adachi, Ishiguro, & Matsuno (1995)19 proposed an autocatalytic-type equation for describing the entire oxidation process of n-6 PUFAs and the first half of the oxidation process of n-3 PUFAs based on the kinetic equation proposed by Bolland and Gee (1946)17 for the propagation step of lipid oxidation. The oxidation rate of PUFA was assumed to be proportional to the product of the concentrations of unoxidised and oxidised PUFAs.
![]() | (7) |
dY/dt = − kY(1 − Y) | (8) |
![]() | (9) |
![]() | (10) |
The parameter Y0, which reflects the initial state of the PUFA to be oxidised, is close, but not equal to 1. Eqn (8) describes the entire oxidation process of PUFAs and their esters. Fig. 2 shows the autoxidation processes of ethyl arachidonate (EtARA) and ethyl docosahexaenoate (EtDHA) at 50 °C and 75% relative humidity. The inset of Fig. 2 shows the applicability of eqn (9) to the oxidation process. The equation was applicable to the entire oxidation process of EtARA and the first half of the oxidation process of EtDHA. The latter half of the oxidation process of EtDHA cannot be described by eqn (8), but it can be simply expressed by first-order kinetics. The kinetic equations were also applicable to the autoxidation processes of other PUFAs and their acylglycerols.20
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Fig. 2 Changes in the fraction of unoxidized (△) ethyl arachidonate (EtARA) and (○) ethyl docosahexaenoate (EtDHA) during autoxidation at 50 °C and 75% relative humidity. Inset: applicability of eqn (9) to the autoxidation processes of EtARA and EtDHA. The closed symbol (●) indicates that the latter half of the oxidation of EtDHA obeyed the first-order kinetics (Adachi et al., 1995).19 “Reproduced from ref. 19 with permission from JAOCS., copyright 1995”. |
PUFAs increase in weight as they are oxidised, and the autoxidation of PUFAs is exothermic. The autoxidation processes of the ethyl esters of PUFAs were monitored using thermogravimetry, calorimetry, and gas chromatography to investigate the stoichiometric coefficient of the PUFA and oxygen.21 The change in weight (Δw) during the oxidation is related to the fraction of the unoxidised PUFA (Y), as shown in eqn (11).
![]() | (11) |
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Fig. 3 Relationships between the relative weight gain Δw/w0 and fraction of the unoxidised substrate (Y) for (△) ethyl EtARA and (○) EtDHA at 50 °C (Adachi et al., 1995).21 “Reproduced from ref. 21 with permission from Food Sci Technol Int, copyright 1995”. |
The oxidation behaviours of unsaturated fatty acids were represented by the autocatalytic reaction using Y, except when Y of n-3 unsaturated fatty acids is above 0.5. This formula could also be applied to powder using the model in which the free energy of activation for the rate constant is assumed to obey a Gaussian distribution (Ishido et al., 2002).22
Low-molecular-weight sugar and maltodextrin do not have antioxidant ability. Proteins as wall materials could not form dense films, but whey protein or caseinates have antioxidant and also emulsifying abilities. The selection of wall materials should be determined by whether or not a stable fish oil powder can be obtained.
The emulsification process is aimed at reducing the oil droplet size and producing a stable emulsion solution. The emulsification process is crucial because it affects the powder properties significantly. In particular, the oil droplet size in an emulsion is an important factor that determines the proportion of fish oil in the spray-dried powder and the proportion of unencapsulated fish oil represented by the surface-oil ratio. Therefore, the selection of an emulsifier is important for the formation of fish oil powder. In the food industry, emulsified functional oil is prepared using a high-pressure homogenizer using about 20 MPa. In the formation of emulsified fish oil, whey protein or caseinate protein at about 3–6 wt% is used as an emulsifier. But octenyl succinic acid anhydride modified starch (OSA-starch) is often used instead of protein emulsifiers due to the increasing number of allergic patients recently.
![]() | (12) |
EE is affected by several parameters, such as the oil droplet diameter, solid and oil contents, and the processing conditions of spray drying. The surface-oil ratio is the most important parameter for estimating the shelf life of fish oil in spray-dried powders. The surface-oil ratio, s, is defined as the ratio of the amount of oil exposed on the surface of a microcapsule to the entire amount of oil within the microcapsule.
s = (surface oil powder)/(total oil in powder) | (13) |
The surface oil is related to the susceptibility of the microencapsulated oil to oxidation.
Jafari et al. (2008)26 showed that nanooil droplets, with a diameter in the range of 210–280 nm, contain the lowest surface-oil ratio in encapsulating fish oil by spray drying. Ahn et al. (2008)27 indicated that low microencapsulation efficiency of powder may lead to a higher surface-oil ratio and indicated higher lipid oxidation in the encapsulation of sunflower oil and its powder storage at 60 °C for 30 days. Carneiro et al. (2013)28 indicated that a high encapsulation efficiency and a minimal surface-oil ratio in the spray-dried powder are essential to produce a stable powder. These studies suggest that the surface-oil ratio is crucial for evaluating the stability of encapsulated oil in spray-dried powder. Shiga et al. (2014)29 proposed a simple method for determining the flaxseed or fish oil content in microcapsules prepared by spray drying using N,N-dimethylformamide (DMF). The DMF method (because DMF is used to dissolve microcapsules) reduces solvent consumption, increases the extraction yield, and enhances the extract. The extraction of surface oil from spray-dried powders by dispersion with hexane under various conditions has been reported by our group.29–32
Furthermore, hexane is known to be more effective than petroleum ether to measure the surface-oil ratio, if the samples are stored for any time duration.33 Abd Ghani et al. (2017)34 indicated that it was difficult to remove the surface oil of nano-sized oil droplets by hexane washing of spray-dried powders because of the higher stability of smaller oil droplets. Several researchers have measured the surface-oil ratio (extractable oil) of fish oil encapsulated with wall materials such as modified starch,35,36 modified starch or glucose,37 modified starch and whey protein,26 and whey protein and sodium caseinate (NC).38 Their data indicated that a higher oil droplet diameter in the spray-dried powder corresponded to a higher surface-oil ratio. Abd Ghani et al. (2017)32 correlated the average reconstituted oil droplet diameter to the surface-oil ratio using the data reported above. No systematic investigations have been conducted on the effects of oil droplet and powder diameters on the surface-oil ratio and the encapsulated efficiency in spray-dried powders.
Fig. 5 shows an electron micrograph of the cut surface of an emulsified spray-dried fish oil powder. Many oil droplets were observed in the shell of the spray-dried powder and one large vacuole was found in the centre of the powder. Oil droplets on the surface of the vacuole can be seen in this figure.
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Fig. 5 Scanning electron micrograph of the cut surface of spray-dried powder containing emulsified fish oil. |
Fig. 6 shows the locations of oil presence in spray-dried powder containing emulsified oil. While encapsulating functional oil by spray drying, the surface-oil ratio is important for evaluating the stability of functional oil in spray-dried powders. The encapsulated oil is the inside oil in closure within the wall material of the spray-dried powder. The surface oil present as adsorbed oil and contact oil exists mainly on the surface of the spray-dried powder and inside the vacuole.
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Fig. 7 Oxidation processes at 50 °C of MeLA encapsulated with MD at a weight ratio of 0.35. Microcapsules were prepared from the O/W emulsions with median oil droplet diameters of (○) 0.02, (△) 0.6, and (□) 1 μm (Nakazawa et al., 2008).39 “Reproduced from ref. 39 with permission from J Oreo Sci., copyright 2008”. |
The oxidation was more suppressed for the microcapsules with a lower weight ratio of oil to the wall material. The fraction of unoxidised MeLA levelled off at Y∞ upon prolonged storage. The dependence of Y∞ on the weight ratio was analysed using two- and three-dimensional percolation models. These models were also applied to the oxidation processes of linoleic acid (LA) encapsulated with polysaccharides of different weight ratios by a single droplet drying method.40 The effects of oil fraction and oil droplet size in microcapsules on the surface-oil ratio were examined by simulating two- and three-dimensional percolation models.41 The surface-oil ratio was lower when the oil content was lower in the microcapsules, particularly with smaller oil droplets. The simulation results indicated that smaller oil droplets were more favourable for suppressing the oxidation of microencapsulated PUFAs.
The surface-oil ratios of microcapsules with different oil droplet-to-microcapsule size ratios were estimated based on the two-dimensional percolation model, assuming that the frequency function of the oil droplet-to-microcapsule size ratio can be expressed by a log-normal distribution.42 The variance in the distribution of the oil droplet-to-microcapsule size ratio had no significant effect on the surface-oil ratio in the microcapsules at any oil fraction.
Oil isolated from the microcapsule surface was hard to oxidise. The two-dimensional percolation model was also applied to statistically calculate the interior oil fraction in a microcapsule, which is defined as the ratio of the volume of oil isolated from the microcapsule surface to the whole volume of the microcapsule.43 As the entire oil fraction increased, the interior oil fraction first increased and then sharply decreased after reaching a maximum value. The reduction of the oil droplet size was effective in suppressing the oxidation owing to the higher interior oil fraction.
Fig. 8 shows the volume-based distribution of the reconstituted oil droplet diameter and particle diameter in the spray-dried powder. This figure shows the particle diameter and reconstituted oil droplet diameter for each DE of MD were almost the same (approximately 1 μm).
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Fig. 8 Volume-based distribution of the reconstituted oil droplet diameter and particle diameter in spray-dried powder. Closed keys (solid lines) show the reconstituted oil droplet diameter and thin-colour keys (dotted lines) show the particle diameter distribution (Abd Ghani et al., 2017).32 “Reproduced from ref. 32 with permission from J Chem Eng Soc. Jpn., copyright 2017”. |
However with an increase in the DE of MD, these phenomena might occur because the water diffusion coefficient increases with an increase in the DE of MD. The vacuole diameter significantly depended on the DE of the MD. This vacuole affects the surface-oil ratio because the oil droplets exist at the shell of the spray- dried powder. Abd Ghani et al. (2017)32 correlated the surface-oil ratio (s) and the ratio of the vacuole diameter (dv) to the particle diameter (dp) and obtained a linear correlation equation, s = 0.42(dv/dp).
The s can also be correlated with the ratio (E) of the oil droplet diameter (de) to the particle diameter (dp) as follows:
s = 1 − (1 − 2E)3 | (14) |
These results indicate that a higher encapsulation efficiency may be obtained with a larger particle diameter, smaller oil droplet diameter, and smaller vacuole diameter in spray-dried powder.
The effect of fish oil content added to spray-dried powder on the surface-oil ratio was investigated using DE = 19 of MD.
Fig. 9 shows the relationship of the ratio (E) for various fish oil contents in spray-dried powder. In the region where the value of E was 10−2 or less, the surface-oil ratio was as low as 0.05, whereas in the region where E was greater than or equal to 10−2, the surface-oil ratio increased sharply.
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Fig. 9 Relationship of the ratio (E) of oil droplet diameter (de) to particle diameter (dp) and surface-oil ratios for varying fish oil content in spray-dried powder. (Abd Ghani et al., 2017)32 “Reproduced from ref. 32 with permission from J Chem Eng Soc. Jpn., copyright 2017”. |
The changes in the peroxidation value (PV) were measured during the storage of spray-dried powder of emulsified fish oil at 105 °C using MD (DE19). The PV of the surface-oil was measured using fish oil in the hexane-washing solvent. The PV of the encapsulated oil was measured using a solubilised solvent of hexane-washed powder with DMF. The PV of the total oil was measured using a solubilised solvent of spray-dried powder with DMF.
Fig. 10 shows PV changes during the storage separately as the surface oil, the encapsulated oil, and total oil in spray-dried powder.45 The X-axis was plotted with the number scale of sampling time. Surface oils were observed to be significantly oxidised. The PVs of surface oils increased after storage at 105 °C. The PVs of the encapsulated oils were one-order of magnitude lower than the value of surface oil. As shown in Fig. 10, the surface oil oxidised initially. Small oil droplets in spray-dried powder decrease the surface-oil ratio as shown in Fig. 9. However, smaller oil droplets had larger specific surface area, which promotes oxidation. To investigate the effect of oil droplet size on the stability of squalene (SQ) in spray-dried powder of emulsified SQ, three different types of reconstituted SQ oil droplet diameters were formed using the pressure change in a high-pressure homogeniser (20, 50 and 100 MPa). Spray-drying emulsions were prepared using 40 wt% solid content, with 40 wt% SQ in the solid, 3–8% NC, 0.4 wt% lecithin, and 56.6–51.6 wt% MD (DE = 19). SQ can be analysed with gas chromatography, and the retention of squalene in the powder can be easily measured.
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Fig. 10 Peroxidation value changes in spray-dried powder of emulsified fish oil powder during storage at 105 °C. The blue line denotes surface oil, red line denotes total oil, and purple line denotes encapsulated oil (DE of MD = 19) (Abd Ghani et al., 2017)45 “Reproduced from ref. 45 with permission from Oxford Univ. Press., copyright 2017”. |
Fig. 11 shows the stability of SQ in spray-dried powder. By assuming a first-order kinetics for oxidation of SQ in the powder, the first-order kinetic constants were obtained.
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Fig. 11 Stability of squalene in spray-dried powder at three homogenisation pressures, with high-pressure homogenisation of the emulsion at 50, 70 and 105 °C. These powders contained 5 wt% NC in the solid powder. (Yoshii et al., 2023)46 “reproduced from ref. 46 with permission from Taylor and Francis, copyright 2023”. |
At 105 °C, the SQ sample with smaller oil droplets in the powder was more stable than SQ with larger oil droplets. This result suggests that the radical change in the degradation rate of SQ in the powder might be a rate-limiting process in the oil droplet in the emulsified SQ powder. At 50 °C, the powder with larger oil droplets had a lower oxidation rate because of the smaller specific surface area of the oil droplets. These results revealed that the oil droplet diameter plays a major role in the stability of the encapsulated SQ.
Fig. 12 shows the Arrhenius plot of the oxidation rate constants for 3, 5, and 8 wt% of the emulsifier NC. The regression straight lines for 100 MPa and 20 MPa homogenisations intersect with each other. The dependence of oil droplet size on SQ stability differs between high and low temperatures because of the difference in the oxidation reaction mechanism at different temperatures, as described above.
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Fig. 12 Arrhenius plots of the degradation rate constant at 50, 70, and 105 °C. The powders were prepared at □: 20 MPa, △: 50 MPa, and ○: 100 MPa. (Yoshii et al., 2023)46 “Reproduced from ref. 46 with permission from Taylor and Francis, copyright 2023”. |
The activation energy values were estimated using these regression lines.
Fig. 13 shows the relationship between the activation energy (Ea) [kJ mol−1] and the reconstituted oil droplet diameter (dr) [μm] of the SQ powders. Ea can be correlated with the following equation:
Ea = 43.1dr + 22.2 | (15) |
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Fig. 13 Relationship between activation energy and reconstituted oil droplet diameters of the SQ powders. Powder prepared by homogenisation at □, ●, ●: 100 MPa, △, ▲, ▲: 50 MPa and □,■,■: 20 MPa with 3 wt% NC is denoted by clear colour symbols, 5 wt% NC by light colour symbols, and 8 wt% NC by dark colour symbols. The rhombus key indicates the activation energy for the oxidation of the bulk oil of SQ. (Yoshii et al., 2023)46 “Reproduced from ref. 46 with permission from Taylor and Francis, copyright 2023”. |
This equation indicates that the SQ oxidation kinetics can be estimated using the oil droplet diameter in the spray-dried powder. The oil droplet diameter may affect the radical transfer rate, specific surface area of the oil droplet, and/or kinetic constant of the oil volume. The frequency factors, k0 [1/day], of SQ in these SQ systems of spray-dried powder were also correlated with the activation energies, Ea as follows:
ln![]() | (16) |
The encapsulation process affected the stability retention of SQ in the spray-dried powder. The stability of SQ retention in the spray-dried powders, which had reconstituted oil droplet diameters of 0.69–0.77, 0.47–0.61, and 0.19–0.27 μm, was investigated at 50, 70, and 105 °C for 28 days. Larger oil droplet diameter SQ powders had a lower degradation rate at 50 and 70 °C, and smaller oil droplet diameter powders were more stable at 105 °C.
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Fig. 14 EPA retention behaviour in spray-dried powder of emulsified krill oil at storage temperatures of 25, 50, and 70 °C. Grey lines represent the samples with a solid content of 50% that were subjected to mechanical homogenisation. Dot patterns represent the samples with a solid content of 50% that were subjected to high-pressure homogenisation. Symbols filled in black represent the samples with a solid content of 60% that were subjected to mechanical homogenisation. Symbols filled in white represent the samples with a solid content of 60% that were subjected to high-pressure homogenisation. Symbols filled in grey represent krill oil (Sultana et al., 2021).47 “Reproduced from ref. 47 with permission from Elsevier, copyright 2021”. |
The solid lines in Fig. 14 are correlation lines calculated using the Avrami equation as follows:
R = exp(−(kt)n) | (17) |
As mentioned above, the oxidation rate of a PUFA in a binary system was expressed using a kinetic model in which the rate was proportional to the product of the concentration of the unoxidised PUFA and the sum of the concentrations of the two oxidised PUFAs. The model was adopted to analyse the oxidation rates of EPA and DHA in microencapsulated Isada krill oil. Based on the model, the relationship between the fractions of unoxidised EPA (YE) and unoxidised DHA (YD) is expressed by the following equation (Jimenez et al., 2021)52
YE = (YE0/YD0κ)YDκ | (18) |
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Fig. 15 Relationship between the fraction of unoxidised DHA (YD) and unoxidised EPA (YE) in microencapsulated Isada krill oil stored at (□) 25 °C, (△) 50 °C and (○) 70 °C. The open and closed symbols indicate microcapsules with 50% and 60% solid content, respectively (Jimenez, Miyagawa, Yoshii, & Adachi, 2021).52 “Reproduced from ref. 52 with permission from J. Oreo Sci., copyright 2021”. |
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