Application of ultra high-pressure homogenization (UHPH) in the production of submicron/nano-oil-in-water emulsions using vegetable oils and milk proteins as emulsifiers

Abstract

The overall goal of the present PhD thesis was to study some factors related to the choice of emulsifier (whey protein isolate or sodium caseinate) concentration, oil-phase volume fractions (10-50%) and homogenization conditions (100-300 MPa) that could influence physical stability and lipid oxidation in nano/submicron oil-in-water emulsions by using a rotor-startor system (colloidal mill, CM, at 5000 rpm for 5 min) for obtaining the coarse emulsions and stabilized by ultra high-pressure homogenization (UHPH), in comparison to conventional homogenization (CH, 15 MPa). Emulsions were characterized for their physical properties (droplet size distribution, microstructure, surface protein concentration, emulsifying stability against creaming and coalescence, and viscosity) and oxidative stability (hydroperoxide content and TBARs) under light (2000 lux/m2 for 10 days). The first study focused on using whey protein isolate (WPI) as emulsifier in different concentrations (1, 2 and 4%) with a fixed oil concentration (20%) of sunflower and olive oils (3:1). UHPH produced emulsions with lipid particles of small size in the sub-micron range (100-200 nm) and low surface protein with unimodal distribution in emulsions treated at 200 MPa using whey proteins at 4%. Long term physical stability against creaming and coalescence was observed in UHPH emulsions, compared to those obtained by CM and CH. Oxidative stability of emulsions was also improved by UHPH compared to CM and CH, especially when 100 MPa was applied. All emulsions exhibited Newtonian behavior (n ≈ 1). These results led us to use the best conditions obtained in the previous work (4% of protein concentration and pressure treatments of 100 and 200 MPa) to study the physical and oxidative stability of emulsions containing different oil-phase volume fractions (10, 30 and 50%). Increasing the oil concentration from 10 to 50%, in general, increased the particle size, decreased the surface protein concentration and resulted in a high degree of flocculation and coalescence, especially in emulsions treated at 200 MPa. All UHPH emulsions, except those treated at 100 MPa containing 10% oil, and CH emulsions with 50% oil displayed an excellent stability vs. creaming during storage at ambient temperature. The lowest oxidation rate was observed in UHPH emulsions, especially those containing 30% oil. The third study was conducted on using sodium caseinate (SC) as emulsifier in different concentrations (1, 3 and 5%) with a fixed oil concentration (20%) of sunflower and olive oils (3:1). UHPH emulsions containing 1% protein presented a high particle size (especially in emulsions treated at 100 MPa) but increasing the protein content to 3 and 5% in UHPH emulsions reduced the particle size, and tended to change the rheological behaviour from Newtonian to shear thinning, improving the creaming and oxidative stabilities of emulsions. From the previous study, the best droplet breakdown, physical and oxidative stability were obtained with pressures in the range of 200 and 300 MPa and sodium caseinate (5%). Therefore, the objective of the last study was to evaluating the emulsions containing different oil-phase volume fractions (10, 20, 30 and 50%) treated by UHPH in the conditions above mentioned, in comparison to CH emulsions containing 1 and 5% SC. Increasing the oil content to 50% tended to produce emulsions with a gel structure such as a mayonnaise type product so, the results of this study focused only on emulsions containing 10, 20 and 30% oil. CM and CH emulsions containing 1% SC and different oil contents (10, 20 and 30%), exhibited a Newtonian flow behavior with a slow creaming rate, whereas the oxidation rate was faster in these emulsions. On the other hand, high degree of flocculation with a shear thinning behavior, higher creaming rates, but low oxidation rates were observed in CH emulsions containing 5%. UHPH-treated emulsions containing high oil contents (20 and 30%) exhibited excellent creaming stability, and with a shear thinning rheological behavior only in emulsions containing 30% oil. UHPH produced stable emulsions against oxidation, especially when high oil contents (20-30%) were used. Increasing the oil concentration from 10 to 30%, in general, resulted in an increase in the oxidative stability in all emulsions, except in CH emulsions containing 1% of SC. Emulsions produced by both whey protein (4%) and caseinate (5%), and treated by UHPH have a good physical stability to flocculation, coalescence and creaming and also high stability to lipid oxidation, opening a wide range of opportunities in the formulation of emulsions containing bioactive components with lipid nature.