Influence of Processing on Protein Quality

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Abstract
In the first part the reactions and interactions of protein with macroconstituents of our food during processing are exposed from the chemical point of view. The reactions involving only protein (formation of isopeptides, of lysinoalanine, racemization) and the interactions with carbohydrates (Maillard reaction), oxidized lipids and polyphenols are briefly presented. Emphasis is put on the Maillard reaction since it is the most frequent reaction occurring during food processing and storage. The key compound rendering lysine unavailable in processed and stored foodstuffs is N.epsilon.-fructoselysine (FL). Its oxidative degradation product, N.epsilon.-carboxymethyllysine (CML) is found in variable but significant amounts in heat processed proteins. An interesting newer finding is that tryptophan can participate in a Maillard reaction with its indole-NH-group. In the second part an overview is given on the impact these reactions have on the two components of protein nutritive value, namely digestibility and biological value. Again, most examples with be related to the Maillard reaction. Protein digestibility may be reduced by the modification of the protein molecule (blocking of active amino acid side-chains, establishment of crosslinks) or by the formation of compounds that inhibit digestive enzymes. (Inhibition of aminopeptidase by an advanced Maillard derivative of lysine). Biological value may be diminished by the loss of essential amino acids and/or their reduced specific availability. Ion-exchange chromatography of the protein hydrolyzate is the method of choice to determine amino acid losses. It also provides some clues for the type of processing damage by the presence of unusual amino acids in the chromatogramme (e.g., furosine, lysinoalanine). Global amino acid bioavailability is defined. It is of a complex nature and can only be truely determined in a bioassay in the animal. Specific availability of an amino acid is linked to particular structural features. Thus, specific lysine availability is determined by the presence of a free or "reactive" .epsilon.-amino group. This is the basis for the analytical methods for available lysine. In the third part, the practical application of this knowledge to processed foods is shown using milk and vegetable protein as examples. Figures for the reduction in available lysine (blocked lysine) in different milk products processed according to conventional procedures are given and discussed. More subtile effects of milk processing on milk digestibility and stomach emptying are mentioned. The effects on protein nutritional value of extrusion-cooking of legume seeds and cereal flours are, then, presented. Whereas sulphur amino acid bioavailability and protein digestibility in legumes are improved by appropriate extrusion-cooking, extensive lysine loss and nutritional damage can take place when cereal flours are extruded under severe conditions. It is concluded that, nowadays, extensive knowledge of the reaction mechanisms, appropriate analytical methods and flexible processing operations are available to prevent to a large extent protein nutritional losses.

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