Proteins as emulsifiers in retarding oil oxidation in an oil/water emulsion
Abstract
Food proteins form a class of natural emulsifiers. Also, due to their high nutritional value and generally-recognized-as-safe (GRAS) status, they make a popular choice as encapsulating agents for bioactive lipids. However, their efficacy in the ability to confer emulsion oxidative stability is scarcely reported in literature. The main objective of this investigation was to study the influence of processing on the structural characteristics of interfacial food proteins in Menhaden oil-in-water emulsion systems, and relate these characteristics to emulsion oxidative stability. Being the foremost immediate physical barrier around the oil drop, the interfacial protein layer might confer effective protection of the oil from oxidative deterioration if strategically designed. Three different food proteins with distinct characteristics were experimentally studied: (i) sodium caseinate (∼24 kDa)—which has a random-coiled structure; (ii) bovine β-lactoglobulin (∼18.4 kDa)—which is globular and encompasses a β-barrel in its native state; and (iii) soy β-conglycinin (7S) (∼180 kDa in trimeric form), which is globular, much larger in size, and has a predominant β-sheet structure. A number of ways were identified which improved the efficacy of these proteins in protecting the encapsulated Menhaden oil against oxidation, depending on the conformation of the protein. In the case of sodium caseinate, homogenization at higher pressure levels, the use and subsequent vaporization of a food-grade volatile solvent to induce drop shrinkage, and limited transglutaminase treatment of the interfacial layer were all found to be useful in promoting oxidative stability. For the globular β-lactoglobulin, momentary preheating to the molten globule state, before homogenization at ambient temperature, had a noteworthy impact on improving oxidative stability due to increased surface aggregation while retaining the original protein stiffness. As for soy 7S, trypsinization was successful in improving the functionality of the native control at neutral and alkaline pH; it was likely due to the induced formation of β-sheets oriented close to the oil surface which reinforced the protection of oil, in addition to protein unfolding that promoted protein-oil hydrophobic interaction. Based on mathematical modeling, the aforementioned successful methods caused a similar effect, i.e. a decrease in the diffusion coefficient of oxygen in the interfacial protein layer.
Degree
Ph.D.
Advisors
Narsimhan, Purdue University.
Subject Area
Food Science
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