| Oxidation of lipids and proteins occurring during meat processing and storage has a major impact on the functional, nutritional, and sensorial properties of meat products. Synthetic antioxidants have traditionally been used to prevent oxidation; however, their potential role in carcinogenicity has raised increasing concerns for consumer safety, leading to the current trend of adopting natural antioxidants as an alternative meat quality control mechanism. Of various natural antioxidants, plant-derived phenolic compounds are the most widely used for their broad physiological activities identified, for example, anti-oxidation, antimicrobial, anti-inflammatory, anti-allergenic, anti-artherogenic, anti-thrombotic, and anti-cancer properties.Plant extracts abundant in polyphenols have been widely incorporated into meat product formulations to inhibit oxidative processes and extend products’ shelf-life. This antioxidant strategy has been shown to be effective for retarding lipid oxidation but not always for protein oxidation. In fact, accelerated protein carbonylation and sulfhydryl loss caused by phenolic acids have been observed in some cases. In proteinaceous foods, plant phenolics can interact with proteins through both noncovalent and covalent bonds to modify protein functional groups, structure stability, aggregation, and solubility, functionality and susceptibility to the digestive enzymes. However, existing reports are inconsistent and the underlying mechanisms have not been fully elucidated. Also, there is no literature report on in vitro digestion behavior of muscle protein gels formed in the presence of phenolic compounds.The objective of this study was to investigate dose-dependent effects of phenolic compounds on the chemical and structural stability of myofibrillar protein(MP), gelation and emulsification of the MP sol, in vitro digestibility of the heat-induced gels. The effect of plant extracts abundant in polyphenols(green tea extracts and licorice root extracts) incorporation at different concentrations on meat products(pork sausages and turkey patties) texture and oxidation stability was subsequently investigated.In experiment 1, the effect of chlorogenic acid(CA, C6-C3) at different concentration levels(0, 6, 30, and 150 μmol/g protein) on porcine MP gelling potential in relation to chemical and structural changes was investigated. The results showed that CA generally inhibited protein carbonyl formation but did not prevent sulfydryl and amine losses caused by oxidation. The presence of CA intensified oxidation-initiated loss of ?-helix conformation as well as tertiary structure of MP. CA at 150 μmol/g produced the greatest increase in MP surface hydrophobicity and insolubility. The physicochemical changes with 6 and 30 μmol/g CA led to a remarkably enhanced gelling capacity of MP and augmented the positive effect of oxidation in building an elastic gel network. However, CA at 150 μmol/g was detrimental to the MP gelation.In experiment 2, the effect of gallic acid(GA, C6-C1) at different concentratons(0, 6, 30 and 150 μmol/g protein) on chemical changes and gelling properties of oxidatively-stressed MP and in vitro digestibility of the gels were investigated. The addition of GA inhibited lipid oxidation and protein carbonyl formation but promoted the loss of sulfydryl and amine groups, destabilization of the tertiary structure, aggregation, and crosslinking. The gelling potential(storage modulus) of MP was increased by nearly 50% with 6 and 30 μmol/g of GA, corresponding to enhanced protein unfolding and aggregation and formation of disulfide-dominant covalent bonds. However, GA at 150 μmol/g induced macroscopic aggregations and insolubility of MP, resulting in poorly structured gels. Despite the oxidative changes, MP gels did not show reduced susceptibility to digestive enzymes in vitro.In experiment 3, the does-dependent effects of(-)-epigallocatechin-3-gallate(EGCG, C6-C3-C6, 50, 100, 200, 500, and 1000 ppm) on the physical and oxidative stability of porcine MP-soybean oil emulsions were investigated, and compared to a control without EGCG. The binding of EGCG to MP leads to fluorescence quenching of MP which is a static type and reduced surface hydrophobicity of MP. Overall, the presence of EGCG at 50 ppm had no significant effect on sulphydryl loss and polymers formation when compared to control, while at higher concentration(> 100 ppm) of EGCG resulted in further sulphydryl loss and cross-linking including myosin heavy chain(MHC) and actin. As a result, addition of EGCG at high levels(500 and 1000 ppm) distinctly suppressed the emulsion activity and stability, as the emulsions showed lower EAI and ESI, and the gels resulted in higher cooking losses but lower storage modulus and puncture forces, due to the excessive masking of hydrophobic region and polymerization. The incorporation of EGCG at all levels significantly improved the oxidative stability of meat emulsion, as evidenced by the depressed formation of TBSAR as a measure for lipid oxidation. These findings showed that EGCG at appropriate contents can be used to improve oxidative stability of meat emulsion without jeopardizing the textural stability.In experiment 4, the effects of replacing pork backfat with canola oil or pre-emulsified canola oil and green tea extracts on the quality and lipid oxidation stability of reduced-fat frankfurters were investigated. The canola oil replacement led to a reduced hardness, cohesiveness and breaking strength, and an increased cookingloss. The pre-emulsification of canola oil with SPI or WPI partially alleviated these negative effects. The incorporation of green tea extracts has a tendency to damage the sausage texture and increase the cooking loss, especially at high concentrations; but there is no significant difference among these treatments. Canola oil substitution improved the lipid oxidation stability of frankfurters, the pre-emulsification of canola oil with SPI further improved lipid oxidation stability throughout the whole storage period, however, pre-emulsification with WPI only showed positive effect in the early stage. The addition of green tea extracts at all concentrations almost completely inhibited the lipid oxidation during the whole storage period.In experiment 5, the efficacy of phosphates and licorice extract(LE) individually or in combinations to curtail lipid and protein oxidation of uncooked mechanically separated turkey meat(MSTM) patties during refrigerated storage was investigated. MSTM patties were formulated with 0.35% phosphates or 0.05% and 0.10%(meat basis) LE alone or in combinations. Raw patties were packaged in polyvinylchloride overwrapped trays and stored at 2 ?C up to 7 d. Cooking loss was similar among the treatments without phosphates, while phosphate groups had significantly lower cooking loss(P < 0.05). LE has a superior radical scavenging activity, and phosphates have a strong metal ion chelating ability. During storage, lipid and protein oxidation of control patties was significantly higher than antioxidant groups(P < 0.05). Treatments of phosphates and LE in combinations had better oxidative stability than that of phosphates alone. The results indicate that phosphates and LE have synergism between the antioxidants in inhibiting oxidation.In summary, phenolic compounds can effectively inhibit lipid oxidation and protein carbonyl formation, but the effect of polyphenols incorporation on other oxidative stability and functional properties of myofibrillar protein was dose-dependent. This study elucidated the reactions between phenolic compounds and MP, clarified the dose-dependent mechanism, and studied the in vitro digestibility of heat-induced MP gels for the first time. The incorporation of green tea extracts or licorice extract can significantly improve the oxidative stability of meat products; licorice extract when used in combination with compound phosphates, can produce a synergistic inhibition on lipid and protein oxidation. Hence, this study provides some theoretical and application basis for the application of phenolic compounds as a natural antioxidant in meat products. |
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