AbstractsBiology & Animal Science

The stability of diacetyl in fermented milk and the removal of diacetyl from beer were studied. A convenient method for the determination of diacetyl in beer, established by Owades and Jakovac, was modified and applied for flavor analyses of dairy products. Through this method, diacetyl in 12 samples could be determined simultaneously, facilitating the comparative study of diacetyl production and stability in milk during fermentation. A general parabolic curve for the synthesis and destruction of this compound was observed during the fermentation of milk by single or mixed-strain lactic streptococcus starter organisms at 21°C. The streptococci destroyed diacetyl by means of the enzyme diacetyl reductase. Lyophilized crude enzyme extracts of Aerobacter aerogenes was used as a source of diacetyl reductase and some characteristics of this enzyme were studied. The diacetyl reductase had an optimum activity at a pH between 6.0 and 7.0, while its activity was remarkably inhibited at pH values below 5.5. The crude enzyme preparation was quite stable during storage at -20°C. The rapid destruction of diacetyl in milk at 21°C could be prevented by cooling the culture promptly (2°C) after the maximum production of diacetyl. Apparent chemical conversion of precursor to diacetyl was also observed at this low temperature during storage. About 7.5 ppm of diacetyl was found in cottage cheese dressed with cultured cream prepared in this manner using Streptococcus diacetilactis and held at 5°C for 20 days; only a trace amount (0.2 ppm) of diacetyl was found when the cheese was dressed with non-cultured cream. Another method for the enhancement of diacetyl in fermented milk was developed; heated (121°C for 13 min) nonfat milk (100 ml) cooled to 25°C was treated for 20 minutes with 0.03 percent hydrogen peroxide, and was then exposed to sufficient concentration of catalase to destroy the oxidant. The milk, in tightly capped containers, was inoculated with one percent of a mixed-strain starter culture containing S. diacetilactis and held at 21°C. Diacetyl level rose rapidly to at least 14 ppm within 15 hours and decreased slowly to 9 ppm upon holding for eight days at 21°C. Nonperoxide-treated controls produced less total diacetyl (5 to 8 ppm), which was rapidly reduced within 24 hours to less than 2 ppm. Also, the amounts of diacetyl desired in the finished product could be controlled by adjusting the concentration of hydrogen peroxide. Reduction of the level of hydrogen peroxide from 0.03 percent to 0.015 percent, lowered diacetyl synthesis, and the stability of diacetyl in culture was also reduced to about one-half. The reduced effect on the stability of diacetyl at 21°C when milk was treated with lesser amounts of hydrogen peroxide was remedied by combining the cooling process with this treatment. Diacetyl level in a mixed-strain culture held for five days at 21°C was about 3 ppm when the milk was treated with 0.015 percent of hydrogen peroxide. However, more than 5 ppm of diacetyl was…