Studies on Preservation of Lactic Acid Bacteria by Freezing

III. Effects of Various Substances on Survival of Streptococci below Freezing Point

Abstract

Experiments were performed to find substances stimulating the growth of bacteria once frozen and thawed in order to modify the preserving method of lactic acid bacteria by freezing.
The test strains used were representative streptococci, including those of Streptococcus lactis, S. cremoris, and S. thermophilus. The freezing-thawing test was carried out by two methods. One was a direct incubating method. In it, each strain was suspended in 10 per cent reconstituted skim milk and four kinds of modified reconstituted skim milk to which had been added glucose, corn steep liquor (C. S. L.), yeast extract, or L-glutamic acid at a rate of 1 per cent, and was kept at -15-20°C for a month. The frozen culture was tested for acid producing ability in direct incubation after it had been thawed.
The other was an indirect incubating method. In it, each strain was suspended in 10 per cent reconstituted skim milk and was kept in a frozen state for a month. The frozen cultures were inoculated into the media mentioned above after they had been thawed and examined for acid-producing ability.
L-glutamic acid had an effect of protecting bacteria from dying due to freezing and promoted acid production during incubation after thawing. S. cremoris was especially susceptible to this compound. Yeast extract was more effective to promote acid production than to prevent death from freezing. C. S. L. was a little and glucose not effective. Both methods gave similar results.