Proteins

Among the various degradative reactions that take place in foods, the most difficult to arrest are those between proteins and reducing carbohydrates. This situation is even more aggravated when foods, such as combat rations are stored for extended periods without refrigeration. Even when reducing carbohydrates are not part of product formulations, hydrolysis of the polysaccharides and oligosaccharides may occur during processing or during prolonged storage, and then protein quality loss will follow.

Maillard Reaction. Although it is generally recognized that protein quality losses occur during processing and storage through the Maillard reaction, the quantitative aspects of the protein-reducing sugar reaction, which leads to a decline in lysine availability, are subject to interpretation depending on the methodology employed (15). In the initial phase of the reaction, the free epsilon amino groups of lysine and N-terminal amino groups in the protein react with the reducing sugar to form a Schiff base, which cy-clizes then rearranges to form a ketose sugar derivative, commonly referred to as the Amadori compound. This compound is not digested by mammalian proteolytic enzymes and, therefore, becomes biologically unavailable. Furthermore, the amino acids adjacent to the blocked lysine in food proteins may also be unavailable due to steric hindrance to enzymatic action. Acid hydrolysis of foods that have suffered quality loss due to Maillard reaction leads to the formation of a cyclic compound, furosine, in 20% yield, which may serve as an indicator of unavailable lysine (16). It has been used in Natick as a marker to determine protein quality losses in stored dairy bars.

Aminocarbonyl Interactions in Model Systems. Investigations focused on interactions in model systems between lysine, acetyllysine or albumin, and glucose at 40, 50, and 60°C and a low water activity, aw = 0.20. The reaction was monitored by measurement of color, fluorescence, reducing capacity, furosine, glucose, and lysine. There was poor correlation of fluorescence and color increase with lysine loss. Both furosine and reducing capacity correlated highly with lysine loss (17).

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