Author ORCID Identifier

https://orcid.org/0000-0001-7571-7207

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department/School/Program

Animal and Food Sciences

First Advisor

Dr. Youling L. Xiong

Abstract

Polyphenol-rich spices and extracts of phenolic compounds are widely utilized in meat processing to modify product flavors. Chemically, polyphenols are reactive with myofibrillar protein (MP), the most functional fraction of all muscle proteins responsible for texture development in comminuted meat products. Such protein–polyphenol interaction is prevalent under oxidative conditions that are common in meat processing. As a large group of phytochemicals with diverse structures, phenolic compounds are known to interact with MP with varying efficacies. Yet, the structure-function relationship of polyphenols in eliciting modification of MP is poorly understood. The overall objective of this dissertation research was to elucidate the effect of structurally related phytophenols on the physicochemical properties of MP and resultant changes in protein functionalities, i.e., gelation and emulsification.

To establish appropriate testing conditions, a mild oxidative environment was introduced using glucose oxidase (GOx), and the simplest phenolic compound, gallic acid (GA), was used to investigate the effect on the physicochemical and gelling behavior of MP. Compared with non-oxidized (control) MP, GOx-mediated oxidation facilitated both covalent and noncovalent interactions between GA (6, 30, and 60 μmol/g protein) and protein through promoting protein structural unfolding. Such modifications significantly enhanced the gelling capacity of MP, which was evidenced by up to 86% and 53% increases (P < 0.05) in gel elasticity (G′) and breaking strength, respectively.

Based on the above observations, six structurally related monophenolic acids varying in hydroxyl substitution and sidechain groups, i.e., GA, syringic acid (SA), coumaric acid (CMA), caffeic acid (CFA), ferulic acid (FA), and chlorogenic acid (CA), were examined for their effects on MP conformation and gelation under GOx oxidative stress. The elasticity and breaking strength of MP gels were markedly enhanced by all phenolic acids, of which GA and CA induced the highest final G′ values of 291 and 281 Pa (P < 0.05), respectively, as compared with 214 Pa of the control MP sample without phenolic addition. Different reaction modes were evident for these two most effective phenolic acids in improving protein gelation. With the least structural hinderance, the smallest GA facilitated protein cross-linking through covalent adduction to amino acid sidechains. On the other hand, having a bulky sidechain group, CA was the most effective in promoting protein unfolding due to the multiple functional groups, including 5 hydroxyl groups and 1 extra hydrocarbon ring (quinic acid). The findings of structure-dependency of phenolic activity prompted the following experiment where phenolic compounds with more than one phenol structures were included to investigate their influence on MP functionalities.

Here, in addition to three monophenols, i.e., GA, CA, and propyl gallate (PG), two diphenols, i.e., quercetin (QT) and catechin (CC), and one triphenol, i.e., (–)-epigallocatechin-3-gallate (EGCG), were selected to further explore the structure-activity relationship of phenolic compounds on MP functionalities under GOx oxidation. MP-stabilized oil-in-water emulsions were prepared to assess protein emulsifying properties, and an emulsion-filled composite gel system was adopted as a model to mimic comminuted meat products in which MP acted as both an emulsifier and a building block for the protein matrix within the gel. In the emulsion system, phenolic compounds with less polarity, i.e., PG, QT, and CC, significantly improved the emulsifying capacity of MP by increasing protein partition at the oil-water interface by 15, 17, and 23%, respectively (P < 0.05). In the MP–emulsion composite gel system, all three monophenols (GA, CA, and PG) and the diphenol QT increased the MP gel strength to a greater extent than CC (diphenol) and EGCG (triphenol). The flavanol structure in CC appeared to interfere with gel structure development. The multiple phenol structures in EGCG caused protein aggregation so severe that both emulsifying and gelling properties of MP were weakened. Lipid oxidation was retarded by all phenols in MP–emulsion composite gels during storage at 4 °C for 7 days with PG and QT being the most effective.

The above findings established that the type and size of the sidechain groups, the number of hydroxyl attached to the benzene ring, as well as the number of the phenol moiety have an important role in affecting phytophenol–MP interaction and the protein functionality under oxidative condition. Small-sized phenolic compounds tend to promote MP gelation and emulsification, and larger sized (such as EGCG) exhibited negative effects due to the propensity to facilitate extensive protein aggregation.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.228

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