Optimization and pH- and temperature-stability of the complex coacervation between soy protein isolate and inulin containing fish oil

Fish oil is an abundant source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). However, these bioactive compounds are quickly oxidized, causing loss of their functional value. To protect them, microencapsulation by complex coacervation can be applied. On the other hand, the application of microparticles in food matrices is a critical process, factors such as pH or heat treatment can be detrimental in terms of controlled release of the active encapsulated. The objectives of this study were to optimize the fish oil microencapsulation by complex coacervation, using soy protein isolate (SPI) and inulin (Inu); and to determine the effect of pH and temperature on the oil retention (%OR) in the microparticles. The coacervation between the biopolymer solutions was carried out at pH 4.0, then, the process yield (%PY) and the encapsulation efficiency (%EE) were determined. The optimized microparticles were visualized by optical microscopy. A central composite rotatable design was applied, having as variables the Inu:SPI ratio (0.5–1.5) and the amount of oil added (20%–80%). The results were analyzed through ANOVA, and the optimal conditions were estimated by regression analysis and response surface methodology. For the stability of the microparticles, three pH (5.5, 6.0, 6.5) and four temperature (Control, 50°C, 75°C, 100°C) levels were studied. Subsequently, the determination of %OR was performed. The results of %PY show that the maximum (55.60%) and the minimum (36.19%) %PY were obtained at Inu:SPI=0.5 and 80% oil, respectively. For %EE, the maximum (69.32%) and the minimum (36.13%) %EE were obtained at Inu:SPI=0.6 and 80% oil, respectively. In ANOVA, it was observed that the data of %EE presented significant lack of fit, discarding the optimization of %EE. The R2 for the %YP was 0.9234, and the response surfaces indicate that, the lower the amount of Inu used, the higher the %PY. In this regard, the optimized microparticles were produced at Inu:SPI=0.4 and 20% oil; then, %YP was 60.74±0.41% and %EE was 93.57±1.48%, and their morphology show oval and spherical shapes of various sizes. For %OR, there were differences between the pH values, ranging from 51.34% to 80.54% for pHs 6.0 and 5.5, respectively. The temperature significantly decreased the %OR at pH 5.5 (~34.33%–36.62%), while at pH 6.0 the %OR was significantly increased at 100°C (72.24±4.45%). The results of %YP and %EE could be dependent on some properties of Inu, such as the degree of polymerization (DP); that is, the greater the DP, the greater the interaction sites between Inu and SPI. For %OR, pH values above the pH of the coacervation (pH 4.0) and the temperature could decrease the electrostatic interaction between the biopolymers. However, other types of interactions may govern, such as hydrogen bonds at pH 5.5, and hydrophobic interactions induced by temperature at pH 6.0. Furthermore, the optimization of the complex coacervation was mainly dependent on the amount of Inu. On the other hand, the pH and temperature affected the %OR, which is an indicator to give better resistance to microparticles with additional treatments, such as the cross-linking.