Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


1. Certain cultures of S. lactis definitely proteolyzed milk while others did not.

2. Proteolysis by S. lactis did not require extended incubation periods but was evident even in as short a time as one and one-half days without the addition of CaCO3 to the milk.

3. Proteolysis was evident in freshly coagulated butter cultures grown in pasteurized or sterilized milk.

4. With both S. lac tis and butter cultures, proteolysis was more pronounced on extended holding when CaC03 was added to the milk than when it was not.

5. The organisms associated with S. lactis in butter cultures, e. g. S. citrovorous and S. paracitrovorous, did not definitely cause proteolysis when grown in milk.

6. Sterile lactic acid added to milk in quantities sufficient to bring the final acidities to 1 or 2 percent did not increase the amount of soluble or amino nitrogen after a holding period at room temperature.

7. The air supply did not influence the proteolysis brought about by S. lactis or butter cultures.

8. The addition of 0.1 percent peptone or alanine to the milk slightly retarded the protein decomposition by S. lactis.

9. The S. lactis cultures studied were of two types, proteolytic and non-proteolytic; the first rapidly coagulated milk at room temperature while the second showed considerable variation in the rate of coagulation but was never as rapid at this temperature as the proteolytic type.

10. The general correlation between the proteolytic activity of S. lactis and the rate of coagulation did not hold at 30° or 37° C.

11. Incubating at 30° or 37° C., repeated transferring at room temperature or at 37° C., or holding in soil or CaC03 milk for four months without transferring did not influence the inherent proteolytic properties of the S. lactis strains used.

12. There were no consistent differences in the soluble and amino nitrogen values between the butter made with a proteolytic strain of S. lactis and that made with a non-proteolytic strain.

13. Proteolysis by S. lactis was not evident in butter stored at different temperatures for varying periods, whether it was made in small lots or under creamery conditions.

14. There were no differences in the flavor and aroma between the butter made with a proteolytic strain of S . lactis and that made with a non-proteolytic strain.

15. In general, the quality of the butter decreased during storage but the deterioration was not due to proteolysis by S. lactis.

16. Apparently S. lactis strains causing proteolysis in milk are of no significance from the standpoint of the keeping quality of butter.



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