Doctoral Dissertations

Date of Award

8-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Food Science

Major Professor

Qixin Zhong

Committee Members

Tao Wu, Vermont P. Dia, Douglas G. Hayes, Bin Zhao

Abstract

Caseins aggregate and precipitate at acidity near the isoelectric point of pH 4.6. As emulsifiers, the emulsifying and stabilizing properties of caseins are to be improved. Polysaccharides are commonly studied to improve the acid-stability and functional properties of caseins based on non-covalent and covalent interactions. The overall goal of this dissertation was to fabricate casein-polysaccharide nanocomplexes to improve the acid-stability of caseins, and conjugates to improve emulsifying properties of caseins. To prepare stable and translucent casein dispersions at pH 4.5, carboxylmethylcellulose (CMC), dextran sulfate (DS), and propylene glycol alginate (PGA) were studied individually to form casein-polysaccharide nanocomplexes following the pH-cycle method, by increasing dispersions pH to 11.0 and subsequently decreasing to 4.5. For each casein-polysaccharide system, macroscopic appearance and turbidity of dispersions as affected by casein:polysaccharide mass ratio were correlated to molecular and mesoscopic structures characterized for molecular weight, dimension, morphology, and zeta-potential, and the significance of different physical forces was examined. CMC is a known anionic polysaccharide capable of stabilizing caseins at pH 4.5 and was studied for the charge density effect. A higher charge density of CMC resulted in smaller nanocomplexes more stable at increased ionic strengths. Both CMC and DS, with strongly ionized sulfate groups, had the properties to chelate calcium in casein micelles to assist dissociation and formed the shell of nanocomplexes through electrostatic interactions to stabilize caseins at pH 4.5. Stable casein-PGA nanocomplexes were contributed by both covalent and non-covalent (electrostatic and hydrophobic) interactions. Formation of covalent bonding between caseins and PGA via the transacylation reaction at pH 11.0 inspired the synthesis of caseinate-alginate conjugates containing 52.8-76.5% proteins. The conjugates had exceptional emulsifying properties that varied with the protein content of conjugates. Emulsions prepared with conjugate-to-oil ratio of 0.75:100 (w:v) were stable at wide ranges of pH and ionic strength, and after thermal pasteurization. To further study the molecular basis, conjugation of individual α-, β-, and κ-caseins with PGA was studied, and the interfacial and emulsifying properties of the conjugates were mainly decided by casein content, not the casein type. The present dissertation may be significant for novel applications of casein-based ingredients.

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