Proteins are complex in composition and in their behavior in food products. Careful attention needs to be paid to the preparation of protein- rich food because the quality of the final product is influenced to a very large degree by the treatment of the protein. The proteins in food are unforgiving of abuse during preparation; heating for too long a total time or heating to high temperature can cause some highly detrimental changes in the proteins of a food. The significance of the heat treatment will become apparent as the structure and behavior of proteins are presented. The importance of pH in dealing with protein rich foods also will become evident.
When heat or other energy such as beating is applied to a food containing protein, the protein begins to denature or gradually relax from tertiary form to secondary, low energy structure. When continued energy input, molecules may crosslink with each other and precipitate to provide a notable thickening and loss of solubility. Coagulation also occurs very readily when proteins are at their isoelectric point, because the reduction in electrical charges on the molecular permits them to clump together rather than to repel from each other.
Proteins function in important way in food preparation. A particularly important function is the formation of foams. Egg whites and gelatin are especially useful in foaming foams. Egg white foams can be denatured and coagulated to give a very stable product. Gelatin foams give stability when cooled. Egg proteins often used as thickening agents and milk proteins can be precipitated to form the curd used in making cheese.