Stability of Fish Oil Encapsulated in Spray-dried Powders Coated with Starch Particles

Abstract

n-3 Polyunsaturated fatty acids (PUFAs) have important physiological functions. Encapsulation by spray drying is a technique used to protect against oxidation of PUFAs. The present work aimed to investigate the effect of starch coating on the oxidation stability of spray-dried powders containing fish oil droplets.

A mixture (wall material content: 22 wt%) containing maltodextrin (MD, dextrose equivalent (DE) = 25)) or sucrose, hydrolyzed casein (4 wt%), antioxidants (4 wt%) dissolved in water (40 wt%) and fish oil (30 wt%) was homogenized at 100 MPa using a high-pressure homogenizer. The emulsion was spray-dried into powders using a pilot-scale spray dryer with and without starch feeding near the atomizer. The apparent oxidation rate constant depended significantly on the surface-oil content and was larger for the MD-coated fish oil than for the sucrose-coated fish oil.

Introduction

Encapsulation of fish or krill oil by spray drying is an important technology in the formation of functional lipid powders for the development of functional foods. Encina et al. (2016) reviewed conventional spray drying and future trends for microencapsulation of fish oil. They indicated that long-chain polyunsaturated n-3 fatty acids (PUFAs) are essential for human nutrition and summarized the investigations on encapsulation of fish oil by spray drying and commercial microcapsules of fish oil to fortify foods. Recently, encapsulation powders were formed as monodisperse powders (Khalid et al., 2015; Wang et al., 2016) and nano-emulsions (Chen et al., 2017) to encapsulate functional lipids, and the physicochemical properties and gastrointestinal stability of fish oil were studied (Chatterjee and Judeh, 2016). Among the encapsulation techniques, dry powder coating, which facilitates the free-flowing ability of the powder, is a distinctive technique to modify the physical properties of food powders. Poncelet and Bilancetti (2010) reviewed dry powder coatings for food and feed applications, and showed that coatings were suitable for modifying a particle surface. Drusch and Mannino (2009) reported a patent-based review of industrial approaches for microencapsulation of oils rich in PUFAs and indicated that starch coatings on spray-dried powders serve as a double encapsulation system. Shaffner (2004) held the patent for a starch coating method during spray drying from outside of the liquid feeding. Of note was the feeding of the liquid solution in the upper section of the spray dryer through a nozzle, and feeding of the starch powder through a separate inlet. . Bilancetti et al. (2010) reported a statistical approach to optimizing spray drying with a starch particle coating for application as a dry powder coating. They formed starch coatings on spray-dried powders by feeding starch particles during spray drying and investigated the effects of solvent type, feeding rate of coat polymer, and temperature of air drying on the coating efficiency.

The oxidative stability of microencapsulated oil is affected by several factors. In particular, the surface-oil ratio, defined as the fraction of oil exposed on the surface of a microcapsule to the total oil in the microcapsule, is considered the most important factor in the susceptibility of microencapsulated oil to oxidation (Keogh et al., 2001; Ghani et al., 2017a and b). Drusch et al. (2007) reported that the interfacial composition and properties of both the oil-water interface in the parent emulsion and the surface composition of the dried droplets played major roles in the stability of the core material. Kikuchi et al. (2013) examined the effects of the oil fraction and oil droplet size within microcapsules produced by dehydrating O/W emulsions on the surface-oil ratio using a percolation model. They suggested that smaller oil droplets were more favorable for producing microcapsules, wherein the oil was hardly oxidized. Ghani et al. (2017b) reported that the oil droplet diameter (d_e) and particle diameter (d_p) affected the encapsulation efficiency of fish oil and the surface-oil ratio in spray-dried powders. They showed that the surface-oil ratio (s) could be calculated by the equation s=1−(1−2(d_e/d_p))³, and further indicated that the surface-oil ratio was an important factor in forming stable encapsulated fish oil powders.

In this study, the effects of starch coating of spray-dried powders on the surface-oil ratio of the powders and the oxidative stability of fish oil were investigated. During spray drying, corn starch was supplied near the rotary atomizer, and corn starch-coated spray-dried powders could be obtained at two corn starch feeding rates (SFRs) using sucrose or maltodextrin (MD; dextrose equivalent, DE = 25) as the wall material.

Materials and Methods

Materials    Fish oil containing 27 wt% docosahexaenoic acid triglyceride was a product of Maruha Nichiro, Tokyo, Japan. Hydrolyzed whey protein, Emulup^®, was obtained from Morinaga Milk Industry, Tokyo, Japan. Vitamin E (90% γ-tocopherol) was purchased from Mitsubishi-Chemical Foods Co., Tokyo, Japan. MD with DE=25 was gifted by Matsutani Chemical Industry, Itami, Japan. Sucrose was obtained from Dai-Nippon Meiji Sugar Co., Tokyo, Japan. Other reagents were obtained from Wako Pure Chemical Industries, Osaka, Japan.

Preparation of spray-dried powders of emulsified fish oil by spray drying    Fish oil was the core material. Hydrolyzed whey protein (4 wt%) was used as an emulsifier and sucrose or MD (22 wt%) was used as the wall material. Sucrose or MD, hydrolyzed whey protein and antioxidants (4 wt%) were solubilized in distilled water (40 wt%), and fish oil (30 wt%) was added to the carrier solution. The total concentration of the solids in the carrier solution was 60 wt% on a wet basis. The mixture of fish oil and carrier solution was homogenized first using a Polytron homogenizer (PT-6100; Kinematica GA, Littau, Switzerland) at a dial position 8 for 3 min, and then by using a high-pressure homogenizer (Starburst Mini; Sugino Machine, Toyama, Japan) at 100 MPa. The emulsion was spray-dried using an Ohkawara-L8 spray dryer (Ohkawara Kakouki Co., Yokohama, Japan) equipped with a centrifugal atomizer. The operational conditions of the spray dryer were as follows: air inlet temperature, 140 °C; air outlet temperature, 82–96 °C; rotational speed of the atomizer, 10,000 rev/min; feed flow rate, 30 mL/min; air flow rate, 110 kg/h; feed-liquid temperature, 50 °C; and SFR, 20 or 90 g/min. The diameter of the feed pipe used for feeding the corn starch was 20 mm. The horizontal level of this starch feed pipe was the same as that of the rotary atomizer, and the distance between the supply port and rotary atomizer was 7 cm. Corn starch was supplied using a hand-made powder feeder.

Sieving of spray-dried powders    The spray-dried powder with starch coating and starch were separated using a stainless steel sieve. Stainless steel sieves with mesh opening sizes of 212, 106, and 75 µm (70, 140, and 200 mesh of the sieve tray, φ300 × 100 mm; As One Corp., Osaka, Japan) were used to roughly separate the powders into different sizes by hand shaking. After sieving, the spray-dried powders were further sieved using stainless-steel sieves with mesh opening sizes of 212, 150, 106, 75, and 53 µm (70, 100, 140, 200, and 270 mesh of the sieve tray, φ75 × 20 mm by using an electromagnetic vibration sieve unit (M-2; Tsutsui Scientific Instruments, Tokyo, Japan).

Analysis of oil droplet and powder particle diameters    The size distributions of the oil droplets in the feed and reconstituted emulsions were measured using a laser diffraction particle size analyzer (SALD-7100; Shimadzu Corp., Kyoto, Japan). The spray-dried powder with starch coating was dispersed in distilled water in a 50 mL beaker and the beaker was allowed to stand to separate the dispersed solution into three layers: an upper water phase, an oil droplet solution in the middle phase, and a precipitate phase containing starch. The reconstituted emulsions in the middle layer were used to measure the oil droplet distribution. Particle size distribution was measured by dispersing the spray-dried powders in 2-methyl-1-propanol in order to measure particle diameter before and after sieving. The volume-based diameter (d₄₃) was considered the mean diameter in all the measurements.

Moisture content of spray-dried powders    The moisture content of the spray-dried powders was measured using a moisture analyzer (HB43; Mettler-Toledo, Tokyo, Japan) at 160 °C. When the value of the weight change was below 1 mg per 50 s, the moisture analyzer indicated the moisture content.

Viscosity measurement    The viscosity of the feed emulsion was measured at 50 °C using an R/S Plus Rheometer model DV-II (Brookfield Engineering Laboratories, Inc., MA, USA) with a R/S-CPS cone plate.

Measurement of surface-oil ratio and total oil content of spray-dried powders    The spray-dried fish oil powders (0.3 g) were dispersed in 5 mL of hexane and vortexed for 15 min (Vortex Gene2; Scientific Ind., Inc., NY, USA). The dispersion was centrifuged using a Kubota 2010 centrifuge (Kubota Laboratory Centrifuges, Tokyo, Japan) at 3,000 rpm. A 1-µL aliquot of the supernatant hexane was used to measure the surface-oil content. Total oil was analyzed by dissolving 20 mg of powder in 1 mL of dimethylformamide in a glass bottle, as described by Shiga et al. (2014). Then, hexane (1 mL) was added and the glass bottle was vortexed using the vortex mixer. The post-wash hexane (1 µL) or the extracted oil in hexane was spotted onto a rod (S-III chromarod, Iatroscan; LSI Medience Corp., Tokyo, Japan), and the oil content was quantified for the rods using an Iatroscan MK-5 TLC-FID (LSI Medience Corp.). Surface oil and total oil were quantified using calibration curves.

Morphology of starch-coated spray-dried powders    The morphology of the starch-coated spray-dried powders was evaluated using a scanning electron microscope (SEM) (JSM 6060; JEOL, Tokyo, Japan). The microcapsules were set onto the SEM sample holder using double-sided tape and coated with Pt-Pd using an MSP-1S magnetron sputter coater (Vacuum Device Inc., Tokyo, Japan). The coated samples were analyzed at an operational voltage of 8 kV.

Measurement of fish oil stability in spray-dried powders by Rancimat test    A hand-made Rancimat apparatus was used to measure the oxidative stability of the fish oil in the spray-dried powders. The spray-dried powders (2 g) were placed in test tubes (φ24 × 90 mm) held at 105 °C using a heating block (DTU-1B; Taitec Corp., Saitama, Japan). The air flowed through the test tube to the conductometric beaker containing up to 80 mL of deionized water. The air was flushed through the powder in the tube at a rate of 50 mL/min. The conductivity of the distilled water was measured using a conductivity meter (SevenCompact S230; Mettler-Toledo).

Measurement of starch content in spray-dried powders    About 2.5 g of spray-dried powders was weighed in a 50 mL conical beaker and 25 mL of distilled water was added to the beaker. After stirring for 1 min, suction filtration was performed using a 500-mL suction filtration bottle with No. 2 filter paper (φ70 mm; Toyo Roshi Kaisha, Ltd., Tokyo, Japan), and the filtrate was washed twice with approx. 50 mL of distilled water. The filter cake was dried at 105 °C for 24 h using a MOV202 dryer (Sanyo, Osaka, Japan). The weight fraction of starch was defined as the ratio of the weight of dried starch to that of the spray-dried powders. Powder recovery was defined as the ratio of the total sample weight of the spray-dried powder in the pot and the bottom of the spray dryer (on a dry basis) to the feeding weight of the solid content in the emulsion solution.

Results and Discussion

Formation of spray-dried powders coated with starch    Table 1 shows SFRs, the average oil droplet sizes of the feed emulsion and the reconstituted emulsion of the powders, and the viscosities of the feed solutions. The viscosities of the sucrose feed solutions were about 90 mPa.s, which was about half of those of the MD solutions. Following high-pressure homogenization, average oil droplet sizes of about 170–180 nm were obtained. Meanwhile, reconstituted oil droplet sizes were in the range of 200–240 nm.

Table 1. Physical properties of feed emulsion and reconstituted oil-droplet diameter.

Experimental condition		Oil-droplet diameter (µm)			Viscosity of feed solution (mPa.s)
Wall material	starch feed rate (g/min)	Mechanical homogenization	High-pressure homogenization	Reconstituted emulsion	Mechanical homogenization	High-pressure homogenization
Sucrose	90	1.10 ± 0.05	0.168 ± 0.008	0.206 ± 0.008	73.1 ± 3.2	90 ± 3.2
Sucrose	20	1.14 ± 0.06	0.169 ± 0.007	0.235 ± 0.009	66.7 ± 3.5	91 ± 3.8
MD (DE=25)	90	0.78 ± 0.04	0.174 ± 0.008	0.23 ± 0.009	152.3 ± 4.2	187 ± 4.2
MD (DE=25)	20	0.73 ± 0.03	0.179 ± 0.009	0.239 ± 0.009	158.3 ± 4.5	223 ± 5.0

Figure 1 (a) shows the distribution of the reconstituted oil-droplet and particle diameters of the sieved powders with a diameter of 93 µm and 90 g/min SFR. In the figure, the solid circles and squares represent the MD solutions, whereas the open circles and squares represent the sucrose solutions. Figure 1(b) shows the distribution of the reconstituted oil-droplet and particle diameters of the sieved powders with a diameter of 110 µm and 20 g/min SFR. The ratios of the average oil-droplet diameter to average particle diameter were about 0.001, which is highly similar to those of commercial fish-oil powders (Encina et al., 2016) By using a starch coating, sucrose (as the wall material) could form spray-dried powders with a small ratio of oil-droplet diameter to particle diameter.

Fig. 1.

Distribution of oil-droplet and particle diameters of the sieved powders of (a) for a 90 g/min SFR, and (b) for a 20 g/min SFR. ○, ●: reconstituted oil-droplet diameter distribution in the spray-dried powder using sucrose and MD, respectively, as the wall material; □, ■: particle-diameter distribution of the spray-dried powder using sucrose and MD, respectively. ɛ, ▴: oil-droplet diameter of feed emulsions.

The characteristics of the spray-dried powders are shown in Table 2. The recovery rate of the powder without a starch coating was either extremely low or no powder was recovered. When sucrose was used as the wall material, spray-dried powders could not be obtained without a starch supply. Because the outlet air temperature ranged from 84 to 96 °C and the glass transition temperature (T_g) of sucrose is about 60 °C (Simperler et al., 2006), almost all the powders would adhere to the inner wall of the spray dryer. Bhandari and Howes (2005) showed the relationship between the stickiness of foods undergoing drying and dried products, and their viscous and glass transition properties. Sugar-rich foods cannot form spray-dried powders because of their high sugar content, and organic acids cannot form powders due to a high moisture sorption (Truong et al., 2005, Lay Ma et al., 2008).

Table 2. Physical properties of non-sieved spray-dried powder and sieved spray-dried powder.

		Non-separated spray-dried powder			Sieved spray-dried powder
Wall material	starch feed rate (g/min)	Water content (wt%)	Recovery percentage (wt%)	Starch content (wt%)	Mesh opening sizes (µm)	Averaged powder diameter d43 (µm)	Total oil content (g-oil/g-powder)	Surface oil content (g-oil/g-powder)	Surface oil ratio (%)
Sucrose					75–106	79 ± 3.9	0.226	0.00445	1.97
	90	4.66 ± 0.24	78.2 ± 4.3	45.5 ± 3.5	106–150	93 ± 4.1	0.197	0.00163	0.83
					150–212	145 ± 5.2	0.247	0.0014	0.56
					75-106	79 ± 3.8	0.388	-	0
	20	3.29 ± 0.20	47.7 ± 3.1	26.4 ± 2.4	106-150	85 ± 4.0	0.4	-	0
					150-212	104 ± 4.5	0.396	-	0
MD (DE=25)					75-106	81 ± 3.8	0.359	0.00177	0.49
	90	4.36 ± 0.25	57.1 ± 2.7	31.3 ± 2.2	106-150	110 ± 5.3	0.271	0.0024	0.88
					150-212	153 ± 5.1	0.379	0.00144	0.38
					75-106	85 ± 4.2	0.437	0.00728	1.67
	20	3.82 ± 0.19	47.7 ± 3.4	14.5 ± 1.3	106-150	110 ± 5.1	0.497	0.0042	0.84
					150-212	117 ± 6.2	0.404	0.00413	1.02

At 20 g/min SFR for sucrose or MD as the wall material, the recovery percentage of the powders was approximately the same at 49%. When the spray-dried powders were coated with starch, powders with good flowability were obtained. Moreover, the starch contents coated on the spray-dried powders were about 45.5 and 26.4 wt% for sucrose powder and 31.3 and 14.5 wt% for MD powder at SFRs of 90 and 20 g/min, respectively. The amount of starch adhered to the spray-dried powders was higher with sucrose than with MD. The stickiness of the spray-dried powders depends on the T_g of the wall material. The T_gs of sucrose and MD under dry conditions are about 60 and 121 °C (Bhandari and Howes, 2005), respectively. The T_g of the wall material might affect the amount of coated starch. The moisture content in the spray-dried powders was about 3–5 wt%, which indicated that the starch coating did not affect the moisture content.

The spray-dried powders were sieved using stainless-steel sieves with mesh opening sizes of 75, 106, and 150 µm. Table 2 shows the average diameters (d₄₃) of the sucrose powders were 79, 93, and 145 µm for 90 g/min SFR and 79, 85, and 104 for 20 g/min SFR, respectively. The MD powders had d₄₃ = 81, 110, and 153 µm for 90 g/min SFR and 85, 110, and 117 µm for 20 g/min SFR. The average powder diameter obtained using the Ohkawara-L8 spray dryer was typically approximately 20–80 µm. Larger powder sizes could be obtained by starch-coating spray drying than by conventional spray drying.

The surface-oil content in spray-dried powders is the most important factor in evaluating the stability of functional oils, such as fish oil, in spray-dried powders. In Table 2, the total oil content, surface-oil content, and surface-oil ratio, which is the ratio of surface oil content to total oil content, are shown for the sieved powders. The retention of fish oil during spray drying was assumed to approach 100%, since volatile compounds were negligible in fish oil. The surface-oil ratios of the (d₄₃ = 79 µm) sucrose powder at 90 g/min SFR and the (d₄₃ = 85 µm) MD powder at 20 g/min SFR were 1.97 and 1.67%, respectively. The surface-oil ratios of the other powders were below 1–2%. Surface oil could not be detected for sucrose powders at 20 g/min SFR.

Morphology of starch-coated spray-dried powders    Figure 2(a) shows the powder morphologies of the surface (a–f) and cross sections (a'–f') for sucrose with SFRs of 90 g/min (a–c, a'–c') and 20 g/min (d–f, d'–f'). Figure 2(b) shows the surface (g–l) and cross section (g'–l') images for MD with SFRs of 90 g/min (g–i, g'–i') and 20 g/min (j–l, j'–l'). For sucrose at 90 g/min SFR, the surfaces of the spray-dried powders were well coated with starch particles; however, at 20 g/min SFR, few starch particles were coated, as shown in images d–f of Figure 2(a).

Fig. 2 (a).

Powder morphologies of the surface (a–f) and cross sections (a'–f') for sucrose with SFRs of 90 g/min (a–c, a'–c') and 20 g/min (d–f, d'–f').

Fig. 2 (b).

Powder morphologies of the surface (g–l) and cross section (g'–l') images for MD with SFRs of 90 g/min (g–i, g'–i') and 20 g/min (j–l, j'–l').

Starch adhered on the powder surfaces when supplied at a flow rate of 90 g/min for sucrose and MD as an excipient. However, at 20 g/min, the amount of adhered starch was extremely low. As can be seen in images of particle cross sections, smooth, solid spray-dried powders are obtained.

Measurement of fish oil stability in spray-dried powders by Rancimat test    Oxidation stabilities of the powders were measured using spray-dried powders at 90 g/min SFR according to the Rancimat method. During spray drying, the fish oil was exposed to a high temperature for about 10–30 s. However, the oxidation of fish oil during spray drying was assumed to be negligible due to the low moisture content in the spray-dried powder. The spray-dried powders were separated by sieving of each powder, containing sucrose or MD, and the results of the Rancimat data of d₄₃ = 79, 93, and 145 µm and d₄₃ = of 81, 110, and 153 µm, respectively, were plotted as conductivity vs. reaction time (Fig. 3). The electrical conductivities of sucrose were smaller than those of MD. The Rancimat method is based on the conductometric determination of volatile degradation products, wherein the conductivity is automatically plotted against time. Conductivity was automatically recorded. The inflection points of the plotted curves (the intersection point of the two extrapolated sections of the curve) were taken as the induction times of fish oil. The apparent oxidation rates of the spray-dried powders were obtained from the reciprocal of the induction time of the electric conductivity change obtained by the Rancimat method. Figure 4 (a) shows a plot of the apparent oxidation rates against the powder diameters. Figure 4 (b) shows the values plotted against the surface-oil content of the powders. In order to confirm the influence of particle diameter and surface oil on oxidation rate, the Rancimat method was carried out using particles prepared with a commercial spray dryer and the apparent oxidation rate was measured. The squares in Fig. 4 (a) show the powders formed by the spray dryer with 20 kg/h moisture evaporation. However, the amount of surface oil could not be measured because of the low amount of powders. The apparent oxidation rate of sucrose was lower than that of MD. The apparent oxidation rate constant was dependent on the powder diameter for sucrose but not MD. The apparent oxidation rate for sucrose appears to be diffusion-limited, possibly due to the difference in surface-oil content in the powders. The rate-limiting step of oxidation might depend on the initiation, propagation, and termination of oxidation as well as the diffusion of oxygen in the powder. The mechanism of fish oil oxidation is very complex in spray-dried powders. However, the surface oil, as a non-encapsulated oil, on the spray-dried powder might be oxidized with the initiation step of the reaction as the rate-limiting step. In this experimental system, when plotting the relationship between the apparent oxidation rate constant, k, and the surface-oil content, q, a saturation-type correlation k = 0.012q^0.378 was obtained with respect to the amount of surface oil. The surface-oil content was assumed to be Freundlich-type (Ahmad et al., 2005) because there might be an adsorption energy distribution due to the various shapes of the powder. This result suggests that the stability of fish oil depends mainly on the surface-oil content in the spray-dried powders.

Fig. 3.

Measurement induction periods in the incubation of the spray-dried powder with a 90 g/min SFR at 105 °C using Rancimat test.

Dashed lines are for MD (average powder diameter = 81 µm) and sucrose (79 µm). Solid lines are for MD (average powder diameter = 110 µm) and sucrose (= 93 µm). Dotted line is for MD (153 µm) and sucrose (145 µm).

Fig. 4.

Relationship between apparent oxidation rate and (a) powder diameter or (b) surface-oil content.

○: Sucrose, SFR = 90 g/min; △: Sucrose, SFR =20 g/min; □: Sucrose, spray-dried powder supplied by the company; ●: MD, SFR = 90 g/min; ▴: MD, SFR = 20 g/min; ■: MD, spray-dried powder supplied by the company.

Conclusion

Employing starch feeding, starch-coated spray-dried powders were generated during spray drying. Sucrose could be used as a wall material to produce spray-dried powders with a starch coating, with a moderate powder recovery.

A SFR of 90 g/min resulted in the starch being closely attached to the powder surface, with sucrose or MD as the wall material. The apparent oxidation rate constant of fish oil in the spray-dried powders with MD was higher than that with sucrose. The apparent oxidation rate constant depended significantly on the surface-oil content.

Acknowledgements    This research was partly supported by JSPS KAKENHI (Grant Number JP15K07455) and by grants from the Project of the NARO Bio-oriented Technology Research Advancement Institution (the special scheme project on vitalizing management entities of agriculture, forestry and fisheries).

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