High-pressure induced gelation of globular proteins

This thesis is focused on the structural and rheological changes in globular proteins when subjected to high pressure processing (HPP), a novel non-thermal food processing technique. Structure-functionality changes associated with thermal processing has been well studied, but little is known for HPP. One of the principal objectives of this thesis was to compare protein structure-functionality relationship between thermal and high pressure processing. Three groups of proteins of varying complexity and source were studied: beta-lactoglobulin, a small well understood protein from cow's milk whey (model system); porcine blood plasma proteins, and soy protein concentrate. Blood is generally considered a waste from the meat industry, plasma is obtained from centrifugation of the blood and is composed mainly of serum albumin and globulins (simplified multi-component system). Soy protein concentrate is a complex system of vegetable proteins composed of a varied mix of large glycoproteins.;Protein gels of comparable strength were produced by thermal and high pressure treatments (concentration ≥ 20%); the heat induced gels were stiffer than their pressure induced counterparts, the latter only modestly generating intermolecular a-sheets albeit a broad destruction of native structures. For heat induced gelation, the change in protein secondary structure resulting in the formation of intermolecular beta-sheets in conjunction with additional intermolecular disulfide bonds, explains the development of the three-dimensional structure responsible for the gel stiffness and viscoelastic parameters increase. On the other hand, for the pressure induced gel formation, changes in the protein secondary structure did not fully explain the creation of gel scaffold; but the augmented hydrophobic interactions due to internal molecular rearrangements creating different electronic densities in the molecule is perhaps important.;Within the secondary structure of plasma proteins, the globulin fraction appears to be more susceptible to changes in pH. At any given pH, the intensity of the native amide I' bands decreased with an increase in temperature. In addition, a decrease in pH resulted in a higher susceptibility to conformational changes during heating. A decrease in pH at room temperature enhanced the denaturation of globulin fraction while an increase in temperature first affected a-helical domains, which are predominantly associated with the serum albumin fraction.;Conclusively, the dynamic rheology indicated a strong influence of protein concentration and pH on both elastic and viscous moduli of soybean protein concentrate (SPC). The structure of the soybean proteins suffered limited changes after HP treatment: hydrophobicity increased, as well as the relative proportion of random coil, while the beta-sheet content decreased. It was envisioned that HP treatment can be used to enhance the viscoelastic behavior of SPC; and the HP treated SPC can then be used to enrich both protein content and improve textural properties of foods. A wide range of gel textures, determining mouth-feel, can be obtained with varying SPC concentrations, pressure levels, holding times and processing temperatures, pH, and additives. Therefore, with a target product on hand, one can aim for specific rheological characteristics.;In general, HP treatment affected the protein structure and functionality, but the specific effects depended on the characteristics of the protein. Pressure-independent parameters, like protein concentration and pH, exerted a major influence on protein denaturation and influenced the HP-induced gel network formation. Rheological characteristics were unaffected by pressures up to 550 MPa. At 600 MPa significant structural changes were observed and these were correlated to changes in the viscoelastic properties. A true gel was only formed after applying 650 MPa to a beta-lg dispersion of 20% concentration, but it did not follow the classic structural changes of thermal induced gel formation, resulting in a significant drop in the intensity of intramolecular beta-sheet and an increase in alpha-helixes and random coil structures. Using mathematical models, the exponential model described reasonably well the higher end HP treatments for prediction of storage modulus; and the cubic model fitted the protein structural data related to beta-structure HP-induced formation.;These results are expected to provide valuable information to the food industry for enhancing the functionality and value of common proteins through the use of HPP. Milder HP treatments promoted creation of soft gels while more severe treatments resulted in a firm gel. In addition, milder HP treatments could be used for modifying the rheological properties of foods at higher the protein content, and with more complex and higher molecular weight proteins.