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Studies on Starter Metabolism. III. Studies on Cysteine-Induced Stimulation and Inhibition of Starter Cultures in Milk^{1, 2,}

Department of Dairy Husbandry, University of Minnesota, St. Paul

ABSTRACT

During a study of the response of lactic starters to heat-induced changes in milk, and the ability of added cysteine to duplicate some of these heat-induced changes, it was found that: (1) The stimulatory effect of either heat or cysteine could be duplicated by, nutritional supplementation with milk protein hydrolysate; expelling oxygen by nitrogen or hydrogen ebullition, and the addition of cystine, glutathione, or thioglycolate. (2) The inhibitory effect of excess heat (below 100° C.) or excess cysteine could be duplicated by ebullition with hydrogen sulfide, and was closely related to the concentration in milk of heat-evoked volatile sulfides.

The following hypothesis was advanced to explain the response of starters in heated milk:

1. The initial stimulation is due to a variety of factors acting independently and in conjunction with each other. Among these factors are the lowering of Eh due to oxygen expulsion; the destruction of heat-labile inhibitors native to raw milk; the partial hydrolysis of protein, and the denaturing of serum protein.
   
2. The first zone of inhibition is due to excess concentration of cysteine with the concomitant increase in toxic volatile sulfides.
   
3. The second zone of stimulation is due to a heat-induced diminution in toxic sulfides.
   

¹ Paper No. 3744, Scientific Journal Series, Minnesota Agricultural Experiment Station.

² Taken from data submitted to the graduate faculty of the University of Minnesota by the senior author, in partial fulfillment of the requirements for the Ph.D. degree.

³ Present address: Department of Bacteriology, Southwestern Louisiana Institute, Lafayette, Louisiana.