Changes In Protein Structure During Food Processing

Physical treatments that modify protein structure include the use of thermal energy, mechanical energy, or pressure. Common examples of physical processes that alter food proteins are heating, freezing, radiating, extrusion, salting, fiber spinning, sonication, whipping, emulsification, and storage. These processes result in denaturation of the proteins. Denaturation is usually accompanied by unfolding of the protein molecules, without any apparent loss in solubility as long as they are monomeric. The unfolding step is frequently followed by aggregation, which may lead to loss of solubility. Thus, changes in solubility are frequently used as an indicator of protein denaturation. Determination of molecular weights by gel filtration chromatography and gel electrophoresis has been commonly used also for investigating protein denaturation (48).

The presence of a partially folded conformation called a molten globule state, that is, a kinetic intermediate of reversible denaturation equilibrium, was reported in many globular proteins (49). The molten globule has a nativelike backbone secondary structure, and the side chain's environment undergoes a denaturation-like alteration. Hydrophobic clusters are exposed, as reflected in increasing binding of a hydrophobic fluorescent probe. It is postulated that molten globules are involved in the functional properties (eg, emulsification, foaming, and gelation) of food proteins (49).

In emulsions of oils in protein solutions, the proteins change from their native conformations to forms that depend largely on the hydrophobic interactions between the surface and the amino acid side chains in the protein. Ideally, hydrophobic side chains will be close to the oil surface and hydrophilic residues will favor the aqueous phase, but this will be constrained by the distribution of the amino acid residues in the protein. The conformation that is adopted also depends on the surface area that the protein is required to cover (47,50). Dickinson (51) classified milk proteins into two distinct classes: the disordered casein and the globular whey proteins. Substantial differences exist between these two classes in terms of adsorbed layer structure and surface rheological properties at the oil-water interface. Computer simulation showed promise for modeling the behavior of hypothetical proteins and peptides (51).

In the case of foam formation, the adsorption of proteins at the air-water interfaces has often been described in terms of three processes. They are transportation from bulk solution to the interface, penetration into the surface layer and reorganization of structure (surface denaturation) of the protein in the adsorbed layer (47,52). This process is similar to the case of emulsion formation, but the detail in structure changes at the air-water interface is unclear, mainly because of difficulty in analysis.

Conformational changes of proteins when the protein solutions are gelled upon heating are reviewed by Matsu-mura and Mori (53). It is interesting to note that /i-sheet structure, which is mostly association units for gel formation, seems to be due to the formation of an intermolecular yS-sheet structure induced by association between denatured molecules. That is, /i-sheet formation itself probably has no direct relation to the denaturation step of globular proteins.

A similar phenomenon of aggregation of ^-structure-forming (ie, cross /i-fold) is observed in amyloid fibrinoge-nesis. Tissue deposition of soluble autologous proteins as insoluble amyloid fibrils is associated with serious diseases, including systemic amyloidosis, Alzheimer's diseases, and transmissible spongiform encephalopathy. Two naturally occurring human lysozyme variants were both amyloidogenic (54).

Modeling has been attempted to study the relationships between processing conditions and the resultant structural changes, using a variety of multivariate analysis techniques (55). The most advanced methods may be partial least squares regression (PLS), artificial neural networks, and genetic algorithm. Many commercial computer programs for these modeling methods are available.

For assessing the folding stability of protein molecules, in addition to the traditional spectroscopies (UV, fluorescence, and CD) calorimetry, solvent stabilization (56), a novel method of hydrogen exchange using NMR (57), and electrospray mass spectrometry (58) are being used. The exchange of the protons of buried backbone NH groups with those of the solvent is measured. Peptide NH groups that are exposed to solvent exchange rapidly, but those that form stable hydrogen bonds within the protein, or on occasion are simply buried, exchange slowly. The rate of H ^ D exchange of individual groups can be measured conveniently using either normal protein dissolved in D20 or D-labeled protein dissolved in H20.

600 Chocolate Recipes

600 Chocolate Recipes

Within this in cookbook full of chocolate recipes you will find over 600 Chocolate Recipes For Chocolate Lovers.

Get My Free Ebook

Post a comment