Cereal Chem 54:238 - 245. | VIEW
Fortification of Soy Protein with Cheese Whey Protein and the Effect of Alkaline pH.
O. De Rham, P. Van De Rovaart, E. Bujard, F. Mottu, and J. Hidalgo. Copyright 1977 by the American Association of Cereal Chemists, Inc.
Replacement of soy protein with whey protein in the range of 0 to 40% increased the protein efficiency ratio (PER) from approximately 1 to up to 3. PER improvement was similar for both "heat-denatured" and "nonheated" (recovered by ultrafiltration) whey protein, and was due to a better balance of the essential amino acids in the protein mixture, as compared to that of the soy protein alone. Of special importance was the contribution of the whey protein in methionine (1.8 g/100 g protein) and in cysteine/cystine (2.8 g/100g protein) which are the limiting amino acids of the soy protein (1.3 and 0.9 g/100 g progein, respectively). Exposure of the individual soy and whey proteins, as well as their mixture to alkaline pH, caused a loss in PER which was originated mainly by the destruction of cysteine/cystine. The influence of pH, temperature, and the time-length of the exposure to alkali on this destruction was measured at a constant concentration of 14% protein. At pH 12.5 and room temperature, the damage was small even after 15 min; damage increased with temperature, and at 65 C, 0.4-0.6 g/100 g protein of these amino acids were destroyed in the first 90 sec, causing a 10% drop in the PER. The critical pH for both proteins was higher than 11, since at pH 11 only a very slight damage was observed under similar conditions. Destruction of cysteine/cystine yields mainly dehydro-alanine, and this compound reacts readily with several amino acids. For example, we have found that after 2 hr at pH 12.5, 65 C, approximately half of the decomposed cysteine/cystine had reacted with the epsilon-amino group of lysine to form lysinoalanine (LAL). After 90 sec at pH 12.5 and 65 C, the amounts of LAL found in the samples were approximately 0.2 g/100g of soy protein, and 0.5 g/100g of whey protein. These findings should provide a valuable guideline for further development and improvement of the spinning technology.