Nutritional quality of diacetyl/urea treated caseinates

Diacetyl markedly improves the heat stability of milk, especially in the presence of urea. The present study was undertaken to investigate the effects of severe heat treatments in the presence of diacetyl and urea on the nutritional value of casein. Acid casein solutions (7% w/v protein) at pH 6·6 were heated at 120°C for 20 min in the presence of the following: (1) no additive; (2) 10 mm urea; (3) 1·5 mm diacetyl; (4) 5 mm diacetyl; (5) 1·5 mm diacetyl + 10 mm urea; (6) 5 mm diacetyl + 10 mm urea. All caseinates treated with diacetyl or diacetyl plus urea were visibly browned. When compared with caseinate heated alone, available lysine was reduced by 8%, 9·5%, 8% and 7% in the caseinates treated with 10 mm urea, 1·5 mm diacetyl + 10 mm urea, 5 mm diacetyl and 5 mm diacetyl + 10 mm urea, respectively, while treatment with 1·5 mm diacetyl, 5 mm diacetyl and 5 mm diacetyl + 10 mm urea reduced arginine content by 10%, 15% and 11%, respectively, and tryptophan content by 5%, 8% and 8%, respectively. PER, NPR and available methionine were unaffected by diacetyl and/or urea except for a 9% reduction in PER in caseinate treated with 5 mm diacetyl + 10 mm urea.