| China is the world's largest meat producer; muscle foods contribute significantly to a person's everyday nutrition. Lipid and starch are often added to meat products, providing good flavor, juiciness, and tenderness. However, the mechanisms underlying the improvements of textural properties of processed muscle foods, particularly comminuted products (e.g., frankfurters), by lipid or starch have not been clearly elucidated. Of particular importance is the gel-forming ability of muscle proteins. Myofibrillar protein, the most abundant protein group in meat, can form a viscoelastic gel capable of binding meat pieces, stabilizing fat particles, and immobilizing water. The objective of the dissertation study was to investigate the physicochemical roles of pre-emulsified lipids and native or modified starches, imbedded in the pork myofibrillar protein (MP) gel matrix, in the development of well-textured muscle foods. The ultimate goal was to establish a theoretical basis on which to produce novel and nutritious meat products.In Experiment 1, emulsified fat (lard) and oil (peanut oil) were investigated for their roles in the rheology and microstructure of MP gels. Dynamic rheological testing upon temperature sweeping (20 to 80℃at 2℃/min) showed substantial increases in G'(an elastic modulus) of MP sols/gels with the addition of emulsions. Gel hardness was markedly enhanced (P< 0.05) by incorporating≥10% emulsions, and the composite gel with 15% lard was 33% more rigid (P< 0.05) than that with 15% peanut oil. Incorporation of both emulsions at 10% or higher levels improved the water-holding capacity of the gels by 28-44%(P< 0.05). Light microscopy revealed a compact gel structure filled with protein-coated fat/oil globules. The results indicated that physical packing effect contributed to the enhancements in the rheology, moisture-retention, and gel properties in the MP-emulsion composite gels.In order to understand the molecular mechanism of lipid droplet stabilization in MP gels and the gel texture enhancement, the sulfhydryl/disulfide bond chemistry involved in the interaction between peanut oil emulsion membrane and the MP gel matrix was studied (Experiment 2). MP-coated peanut oil emulsion was treated with 0,1,3,5 and 10 mM N-ethylmaleimide (NEM, a sulfhydryls-blocking agent) and subsequently incorporated into an MP sol to produce 5%-lipid,2%-protein composites at pH 6.2. About 69% of sulfhydryls in the emulsion (1% protein) were blocked by 1 mM NEM, and almost all were bound by≥3 mM NEM. The loss of free sulfhydryls resulted in a significant drop in G'and harness of the composite gels. Microstructural examination revealed pores and oil leakage from emulsion droplets by NEM treatments, and the phenomenon was aggravated with increasing amounts of NEM, corresponding to the decline in rheological properties of the MP-emulsion composites. The results supported the hypothesis that cross-linking of MP-coated oil droplets with the surrounding protein matrix via disulfide bonds was important for the stabilization of oil and reinforcement of MP-based composite gels formed in comminuted muscle foods.In Experiment 3, the effect of emulsion droplet size on the rheological and microstructural properties of MP composite gels was investigated. Composite gels were prepared from MP imbedded with pre-emulsified olive oil and peanut oil stabilized by 1% MP or 1% Tween 80. Emulsions with various droplet sizes were prepared by different homogenizations. Dynamic rheological testing upon temperature sweeping showed substantial increases in G' of MP sols/gels with the addition of emulsions, and the G' increases were inversely related to the emulsion droplet size. Furthermore, gels containing emulsified olive oil had a greater (P< 0.05) hardness than those containing emulsified peanut oil. Regardless of oil types, MP-coated oil droplets exhibited stronger reinforcement of MP gels than Tween 80-stablized oil droplets; the latter composite gels also had considerable syneresis. Light microscopy with paraffin sectioning revealed a stable gel structure when filled with protein-coated oil droplets, compared to gels with Tween 80-treated emulsions that showed coalesced oil droplets. These results suggested that rheological characteristics, hardness, texture, and water-holding capacity of MP gels were influenced by type of oils, the nature of interfacial membrane, and the size of emulsion droplets.In Experiment 4, rheological properties of MP gels as affected by the heat-induced swelling of imbedded native and modified starch granules were explored to illustrate the importance of starch particle size and gelatinization temperature in MP gelation, as was similarly observed in previous experiments with lipid droplets. Native and modified potato, tapioca, corn and rice starches were studied. The results showed that gel strength of all MP-starch composites was directly related to the pasting temperatures of respective starches and their viscosity and increased with the size of swollen starch granules until the point of gelatinization. Starches also improved water-holding of the MP gels at temperatures near or higher than their respective gelatinization temperatures. Of all starches, esterified potato starch improved the rheological properties of MP gel the most, while native and esterified tapioca starches gave rise to the highest water-holding in MP gels formed by heating at 70-80℃. The results offered an insight into the mechanism of gelation in starch-added Western-style comminuted meat products.The overall findings from this dissertation research substantiate the importance of lipid emulsion droplets and starch granules, in both particle size and concentration, for the reinforcement of mechanical strength and viscoelasticity of MP gels. The elucidation of the critical role of disulfide linkages existing between oil/fat globule membrane and the MP matrix in the stabilization of MP-lipid or MP-starch composite gels contributes to the fundamental understanding of the mechanism on lipid- and starch-mediated improvements of textural quality of processed meat products.
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