Interaction between pH, autolysis promoters and bacterial contamination on the production of yeast extracts

Bakers' yeast (Saccharomyces cerevisiae) cell suspensions were submitted to autolysis at various pH values (4.0, 5.5, 7.0 and 8.5) and with chemical autolysis promoters (ethyl acetate, chitosan). In one series of assays, bacterial contaminants were added at 10(8) CFU/ml. Autolysis yields, total nitrogen (total N) and alpha-amino nitrogen (alpha-amino N) composition as well as turbidity of the yeast extracts (YE) were examined. The addition of bacterial contaminants at 10(8) CFU/ml to the yeast cell suspension did not significantly influence any of the response variables tested. There was a significant effect of pH on autolysis yields, total N and cr-amino N of YE as well as on the turbidity of the YE. There was an interaction between pH and the autolysis promoter. The highest autolysis yields and total N contents of YE were obtained with a combination of pH 5.5 and the addition of ethyl acetate. The alpha-amino N represented between 50 and 60% of total N in the YE produced at pH 5.0 and 7.0, suggesting good peptidase activities at these pH values. The YE produced at pH 7.0 and 8.5 had much more turbidity than those obtained by incubating at pH levels of 4.0 and 5.5. (C) 2000 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.