1. A satisfactory method was developed for the isolation of acid-proteolytic streptococci from dairy products. The organisms were found in milk and various derivatives of it, but always comprised only a relatively small percentage of the total flora; they were most regularly isolated from ripened cheddar cheese.
2. Litmus milk was reduced by some cultures before coagulation and by others after coagulation. The latter, from the standpoint of reduction, are comparable to Streptococcus lactis var. anoxyphilus. The acid-proteolytic streptococci coagulated milk by enzyme action rather than by the formation of acid. The titratable acidities of 15 cultures at the time of coagulation averaged 0.27 percent, while the pH values averaged 5.90.
3. The general characters of the 101 cultures studied were found to be identical, except for the rapidity of reduction of litmus milk and the fermentation of sucrose. The organisms were identified as Streptococcus liquefaciens Orla-Jensen.
4. When incubated 7 days at 21° C., the 101 cultures varied widely in the production of volatile acid, carbon dioxide and acetylmethylcarbinol in skimmilk. Diacetyl was not produced by any of a number of cultures investigated. Volatile acid production was not correlated with the fermentation of sucrose or the rapidity of reduction of litmus milk. The production of carbon dioxide and acetylmethylcarbinol did not vary directly with the volatile acid produced; although high carbon dioxide and acetylmethylcarbinol production t ended to accompany high volatile acid production, considerable intergradation existed. The production of volatile acid, carbon dioxide and acetylmethylcarbinol was generally lower at 37° C. than at 21° C.
5. With an incubation of 7 days at 21° C., the addition of 0.2 (or in a few cases 0.4) percent citric acid to skimmilk at the time of inoculation generally resulted in a decreased volatile acid production with the 39 cultures studied, although an occasional culture gave a definite increase. The production of acetylmethylcarbinol was increased by the addition of 0.2 percent citric acid with 9 of the 33 cultures employed, 4 of the increases being significant. The four cultures giving the significant increases were the ones that gave an increased production of volatile acid with added citric acid.
6. With an incubation of 7 days at 21° C., the addition of 0.15 ml. of acetaldehyde to 300 ml. of skimmilk at the time of inoculation generally resulted in a decreased volatile acid production with the 33 cultures studied, while the addition of either 0.15 or 0.18 ml. of acetaldehyde increased the yield of acetylmethylcarbinol with 6 of the 24 cultures employed, 3 of the increases being striking. In a study of two cultures that were known to give an increased production of acetylmethylcarbinol on the addition of acetaldehyde to skimmilk cultures, the time of adding the aldehyde was found to have a pronounced effect, the highest production being obtained when the aldehyde was added 14 or 16 hours after inoculation, rather than either earlier or later.
7. The neutralization of two well ripened cultures to about the original acidity of the milk and subsequent incubation at 21 ° C. resulted in a slightly increased acetylmethylcarbinol production rather than in a destruction of the product. None of the five. cultures studied produced 2,3-butylene glycol.
8. Either d lactic acid or d and i acid was produced by the 15 cultures examined.
9. Butterfat and cottonseed oil were not hydrolyzed by the 101 cultures while tripropionin and tributyrin were hydrolyzed by some cultures and not by others.
10. The four cultures studied were found to greatly increase the soluble nitrogen in milk. Amino nitrogen was significantly increased as well as the fractions soluble in trichloracetic acid and the fractions soluble and insoluble in ethyl alcohol or phosphotungstic acid. Proteolysis was largely complete after a comparatively short incubation period. The distribution of the soluble nitrogen into various fractions was practically the same with the different cultures and was essentially the same at 37° and 21° C.
11. S. liquefaciens lived in cultures much longer than S. lactis. It remained viable at about 5° C. over a long period of time.
12. Resistance to heat was found to vary considerably with the culture used and with its age. All six of the cultures studied were killed in 40 minutes at 65.6° C.
Long, H. F. and Hammer, B. W.
"Classification of the organisms important in dairy products I. Streptococcus liquefaciens,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 18
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol18/iss206/1