EXTENT OF DAMAGE TO AMINO-ACID AVAILABILITY OF WHEY-PROTEIN HEATED WITH SUGAR

The effect of heat treatments, at various water activities (a(w)), on digestibility and on the availabilities of amino acids of whey protein samples in the presence of lactose was estimated by an in vitro digestion method with continuous dialysis. Four a(w) (0.3, 0.5, 0.7 and 0.97), three temperatures (75, 100 and 121-degrees-C) and three heating periods (50, 500 and 5000 s) were selected. The initial lysine:lactose molar ratio was 1:1. Amono acid profiles showed that excessive heating of whey (121-degrees-C, 5000 s) destroyed a significant proportion of cystine at all a(w), lysine at a(w) 0.3, 0.5 and 0.7, and arginine at a(w) 0.5 and 0.7. At a(w) 0.3, 0.5 and 0.7, protein digestibility decreased (P < 0.05) as the temperature increased from 75 to 121-degrees-C for a heating period of 5000 s, and as the heating time was prolonged from 500 to 5000 s at 121-degrees-C. Excessive heating also decreased (P < 0.05) the availabilities of all amino acids at a(w) 0.3, 0.5 and 0.7. The availabilities of lysine, proline, aspartic acid, glutamic acid, threonine, alanine, glycine and serine were particularly affected. Severe heating at a(w) 0.97 did not seem to favour the Maillard reaction, but the availabilities of cystine, tyrosine and arginine were decreased, probably as a result of structural modifications of the protein upon heating. Heating whey protein concentrates in the presence of lactose not only affected lysine, but also impaired enzymic liberation of other amino acids, according to the severity of heat treatments and a(w).