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Studies on Starter Metabolism. II. The Influence of Heating Milk on the Subsequent Response of Starter Cultures^{1, 2,}

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

ABSTRACT

1. A study of starter responses in a series of fluid milks which had been subjected to heat treatments ranging from 62° C. for 30 minutes to 120° C. for 60 minutes revealed the following:
   1. Gradual increases in severity of exposure resulted in an initial stimulation, followed by a zone of inhibition, followed by another zone of stimulation, and terminating in a second area of heat-induced inhibition.
      
   2. The primary zone of stimulation occurred at exposures from 62° for 30 minutes to 72° for 40 minutes. The first zone of heat-induced inhibition was initiated at exposures equivalent to 72° for 45 minutes and extended through 82° for 10 minutes to 120 minutes, as well as at 90° from 1 to 45 minutes. This inhibition was arrested and stimulation was reinduced by heating at 90° for 60 to 180 minutes, or autoclaving (120°) for 15 to 30 minutes. The final zone of inhibition occurred after autoclaving for longer than 30 minutes.
      
   
   
2. There was a marked variation in starter responses in raw and pasteurized milk, but the differences between starters were considerably diminished in milks heated to 72 or 95° for 30 minutes.
   
3. The first and second phases of the heat-induced stimulation-inhibition-stimulation-inhibition cycle could be duplicated by the addition of denatured serum protein or cysteine hydrochloride. The transition from stimulation to inhibition occurred at exposures which resulted in concentrations of 0.15 to 0.20 mgm. denatured serum protein nitrogen or from 10 to 20 cysteine per ml.
   
4. Artificially supplemented cysteine augmented the sulfhydryl groups made available by heating. In raw and low-heat milks, cysteine was stimulatory; in highly heated milks, cysteine was inhibitory.
   

¹ Paper No. 3743, 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.