The Distribution of Certain Amino Acids in Soluble Fractions of Milk Cultures of Streptococcus Lactis

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Abstract
The literature contains numerous reports ascribing to Streptococcus lactis slight proteolytic action as determined by increases in soluble nitrogen in milk cultures. The results of these studies have been reviewed by Hammer and Babel (4). The proteolysis is carried to the amino acid stage, but no information has been available regarding the extent to which the various amino acids are liberated from milk proteins. Harper and Swanson (6) and Reihard and Garey (12) have recently demonstrated the presence of various free amino acids in cheddar cheese at different stages of ripening. The extent to which the proteolytic enzymes of S. lactis are involved in the liberation of free amino acids in cheese is not definitely known. In the present investigation determination of the distribution of amino acids in some of the soluble fractions of S. lactis milk cultures was undertaken in hopes of obtaining further information concerning the proteolytic properties, of this organism. EXPERIMENTAL PROCEDURE Preparation of S. lactis cultures. Colonies typical for S. lactis were picked from tomato juice-peptonized milk agar platings of different sour cream samples and transferred to litmus milk. After several transfers, cultures typical for S. lactis by their action in litmus milk, morphology, ability to grow at 10 ° C. and failure to grow at 45 ° C. were transferred in duplicate. These cultures were incubated at 30 ° C. for 24 hr., frozen and held for future use at-10 ° C. Two hundred-ml, portions of fresh skimmilk were dispensed into pint screwcapped bottles; these were weighed to the nearest 0.1 g. Sterilization was accomplished by heating at 100 ° C. in flowing steam for 40 minutes on 3 successive days. After the last heat treatment the bottles were allowed to cool, then placed in a 21 ° C. incubator over night. Frozen cultures of the S. lactis strains to be used as inoculum were allowed to thaw at room temperature and carried through four transfers at 21 ° C. before being used in the experiment. Duplicate 200-ml. quantities of sterile skimmilk were inoculated with four drops of a 24-hr. litmus milk culture of each of the strains of S. lactis to be studied. These large cultures and duplicate sterile controls were incubated at 21 ° C. for the desired length of time. Preparation of culture filtrates. After incubation all cultures and sterile controls were made up to their original weight by addition of distilled water.

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