Optimized Formation of Electrostatic Complexes Using Sodium Caseinate and Polysaccharides and Physicochemical Properties of Curcumin or Ellagic Acid-Incorporated Complex

Abstract

Bioactives such as polyphenols are known to have many health benefits including antioxidant and antiinflammatory activities. However, low solubility and stability of the polyphenols hinder their use in food matrixes. One of the ways to overcome these drawbacks is to bind these polyphenols to protein-polysaccharide electrostatic complexes. The ultimate objective of this study was to find optimum condition to form electrostatic complexes using sodium caseinate (NaCas) and polysaccharides, and to increase stability, solubility and utilization of curcumin and ellagic acid (EA) using the complexes. The objective of study I was to find optimum condition to form electrostatic complexes between NaCas and polysaccharides. Also, NaCas and NaCas-polysaccharide electrostatic complexes were compared for their ability to bind and stabilize curcumin. Despite many reported bioactivities of curcumin, its application is limited due to its low bioavailability, solubility and stability. Proteins have been reported to stabilize curcumin in aqueous media. Stabilization of curcumin could be enhanced when proteins form an electrostatic complex with polysaccharides. In this study, electrostatic complexes of NaCas were prepared using high-methoxyl pectin (HMP and NaCas-HMP) and carboxymethyl cellulose (CMC and NaCas-CMC). NaCas and polysaccharide ratio of 1:2 resulted in the lowest turbidity and sedimentation. The electrostatic complexes were more stable than native NaCas against changes in pH and ionic strength. Binding of curcumin to NaCas and the electrostatic complexes were confirmed by UV-vis and fluorescence spectra and Fourier transform infrared spectroscopy (FT-IR). The electrostatic complexes showed a higher binding constant and protected curcumin better than the native NaCas. This study suggests that the electrostatic complexes may be a superior carrier to NaCas in an acidic environment. The objective of study II was to find optimum pH for the electrostatic complexes by evaluating effect of pH on encapsulation efficiency, particle size, zeta potential and heat stability and apply curcumin bound to the complexes as a food colourant to a model beverage. Effect of pH on the characteristics of the complex was evaluated, finding pH 4 was optimum. Zeta potential of NaCas-CMC (-33.59) was larger than that of NaCas-HMP (-22.19) at pH 4, implying higher colloidal stability. The complexes protected curcumin from heat treatment. Antioxidant activity of curcumin bound to the complexes was similar to that of native curcumin. Incorporation of sucrose partially prevented freeze-drying-induced aggregation of the complex, especially for NaCas-HMP. In a model beverage, curcumin bound to the complexes showed higher colour stability. In vitro bioaccessibility of curcumin bound to NaCas-HMP (53.0%) and NaCas-CMC (51.6%) was higher than the native curcumin (21.4%). This study suggests that curcumin bound to the complexes, especially NaCas-HMP-bound curcumin may be used as a potential food colourant, where transparency is needed. The objective of study III was to incorporate EA into NaCas and use EA-incorporated NaCas and polysaccharides to increase oxidative stability of emulsions. EA was incorporated into NaCas using a pH cycle method, a method involving higher solubility of EA and dissociation of NaCas in alkaline media. Fluorescence spectra showed interaction between NaCas and EA. FT-IR showed incorporation of EA into NaCas. EA-incorporated NaCas was used as an emulsifier to evaluate effect of EA on the oxidative stability of an emulsion. HMP or CMC was added to increase stability of the emulsion at acidic pH. A stable emulsion was formed when the ratio of NaCas to the polysaccharide was 1:1 at pH 4. EA did not affect creaming index of the emulsion. However, formation of lipid hydroperoxides in the NaCas-HMP and NaCas-CMC-stabilized emulsions with EA was reduced by 22.5% and 24.0%, respectively. Volatile lipid oxidation products were also produced less in the emulsions with EA than without it. These results suggest that the pH cycle method may be used to incorporate EA into NaCas, which could be used as an emulsifier to increase oxidative stability of an emulsion. Results of this study show that electrostatic complexes could be formed using NaCas and HMP or CMC, and curcumin and EA could be incorporated into the complexes. The complexes were able to bind and stabilize curcumin, and curcumin bound to the complexes could be used as a food colourant in food systems including beverages where transparency is preferred. Also, emulsions stabilized by EA-incorporated NaCas-polysaccharides showed higher oxidative stability than the emulsions without EA. These results suggest that the complex between NaCas and polysaccharides could be used to deliver water-insoluble polyphenols into food systems. Also, incorporated polyphenols could act as a food colourant or antioxidant in food matrixes, providing additional benefits.