Browsing by Author "Dr. Brian E. Farkas, Committee Member"

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    Aggregation Mechanisms of a Whey Protein Model System at Low PH
    (2009-12-14) Mudgal, Prashant; Dr. Orlin D. Velev, Committee Member; Dr. Brian E. Farkas, Committee Member; Dr. Allen E. Foegeding, Committee Co-Chair; Dr. Christopher R. Daubert, Committee Chair
    MUDGAL, PRASHANT. Aggregation Mechanisms of a Whey Protein Model System at Low pH. (Under the direction of Dr. Christopher R. Daubert and Dr. E. Allen Foegeding) Recently, there has developed increasing interest in cold-thickening whey protein ingredients for food applications where heat may not be desirable and because of their nutritional benefits over conventionally-used starches and hydrocolloids. In previous studies, a procedure allowing for the production of a cold-thickening whey protein ingredient, without any addition of salt or heat, was developed. This procedure involved pH adjustment to 3.35, thermal gelation, followed by drying and milling to a powder. Originally, this procedure was applied to whey protein isolates, but also worked with concentrates. Although, these modified powders provide certain benefits, such as instant thickening without addition of heat or salts, these ingredients were prepared at low pH and yield astringent flavors that limit use in practical applications. To effectively tailor the original modification process to expand the functionality and utility of cold-thickening modified whey protein ingredients, the basic mechanism behind the cold-thickening must be explained. β-Lactoglobulin (β-lg) is the major component of whey protein products and was selected as a model system to investigate the cold-thickening mechanisms. Concentration effects on β-lg modification were studied at low pH using capillary and rotational viscometry, transmission electron microscopy (TEM), and high performance liquid chromatography coupled with multi-angle laser light scattering (HPLC-MALS). From the results of the capillary viscometry, a critical concentration (Cc ~ 6.4 % w/w protein) was identified below which no significant thickening functionality could be achieved. Microscopy revealed formation of flexible fibrillar network at pH 3.35 during heating at all concentrations. These flexible fibrils had a diameter of approximately 5 nm and persistence length of about 35 nm as compared to more linear and stiff fibrils formed at pH 2 and low ionic strength (I) conditions. Under similar heating conditions at concentrations above Cc, larger aggregates similar to microgels were observed. However, at concentrations below Cc, isolated fibrils were observed with an average contour length of about 130 nm. In further investigations, concentration effects were studied together with ionic strength (CaCl2) effects on the β-lg cold-thickening mechanism using light scattering, microscopy, and rotational viscometry. A slight increase in I (up to 20 mM) with β-lg concentrations above the critical concentration resulted in an increased thickening function from modified powders. The network characteristics remained fine stranded with small addition of CaCl2, and increased aggregation was observed at all concentrations. The aggregates formed during heating persisted in dried powders; however, freeze concentration effects during freezing further enhanced thickening function as obtained from the reconstituted modified powders. The size of the aggregates formed during the modification process was found to be dependent on the initial protein concentration from kinetic studies using light scattering and rheology. A nucleation and growth mechanism best fit the aggregation at pH 3.35, and nucleation was found to be a rate limiting step below the critical concentration. Above the critical concentration, nucleation occurred rapidly leading to a higher degree of aggregation and formation of large aggregates. By manipulating concentration and heating times, it seemed feasible to control the degree of aggregation and the size of aggregates between 106 to 108 Da. According to electrophoretic studies, disulfide linked aggregates were formed upon heating during the modification procedure. However, disulfide interactions alone could not explain the observed concentration dependent thickening differences. Some acid hydrolysis occurred at pH 3.35 and with additional hydrolysis of β-lg with the enzyme pepsin, solution viscosities increased by about two logs. Likely, formation of pre-aggregates during hydrolysis may explain increased aggregation and higher viscosities.
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    Factors Affecting Water Holding Capacity and Texture in Cooked Albacore Tuna (Thunnus alalunga)
    (2009-09-22) Ruilova-Duval, Maria Esther; Dr. Andy Hale, Committee Member; Dr. Tyre C. Lanier, Committee Chair; Dr. Brian E. Farkas, Committee Member; Dr. Josip Simunovic, Committee Member
    The goal of this research project was to determine if temperature during precooking and cooling process, initial quality of the fish, muscle location and initial meat pH promoted the activity of heat-stable endogenous proteases in albacore tuna. Proteolytic activity was measured as the rate of MHC degradation. Albacore tuna was exposed at cooking temperature range of 50-60ºC reported in prior studies as optimum for proteolytic enzyme activity and responsible for textural degradation of fish meat. Also, tuna samples were exposed to high temperature (70ºC) to minimize meat texture degradation by stopping or slowing down their activity. Meat texture degradation during the cooling process was also measured by placing precooked meat into isothermal cooling temperatures ranges that tuna would experience during this process. Meat textural degradation was tracked by measuring MHC by SDS-PAGE, texture quality by using the Kramer shear press and sensory analysis of texture. Initial meat quality and postmortem pH of fresh muscle albacore did not affect the rate of proteolytic degradation. Rate of degradation of belly meat was higher than rate of degradation of tail and dorsal meat. Meat from all three body positions degrades the most when precooked at temperatures lower than 70ºC. However, precooking albacore tuna at 70ºC did not inactivate proteolytic enzymes, since MHC degraded even when the cooked meat was cooled at lower temperatures. Degradation of MHC observed in precooked albacore tuna was highly related to muscle texture properties. Belly and dorsal albacore tuna muscle precooked at 50C resulted the less strong in texture, as measured by Kramer press and sensory analysis, and with less moisture content than meat precooked at 70C, and also show evidence of larger grittiness and a more grainy mouthfeel. The softening of albacore tuna muscle may be explained by heat-stable protease activity, while the relative toughening of tuna muscle is probably due to decrease of proteases activity and only protein denaturation had significant effect on texture quality. In the second study, meat textural degradation as consequence of autolysis during precooking was traced by measuring piece integrity by applying a methodology prior developed in our lab for skipjack tuna. Also can yield was measured as an additional a way to follow textural degradation, which could vary if the texture of meat causes it to decrease its ability to hold water. Drained and press weight data of canned albacore tuna allowed to determine albacore tuna can yield. Dorsal and belly meat of albacore tuna, when precooked at 50ºC evidenced the highest rate of autolysis, giving as result a greater percentage of small flakes, plus lower yield after both precooking and canning.