Distillate From Milk Can Affect Growth and Activity of Streptococcus lactis1

Abstract

Raw whole milk was processed under vacuum at 60, 68.3, or 76.6 C and distillate (5 ml/100 ml of milk) was collected in cold traps. Unprocessed and processed samples of milk were autoclaved, steamed, or heated in a waterbath at 60, 68.3, or 76.6 C. Each heated processed milk was subdivided into two portions; distillate (5:95) was added to one portion and distilled (in glass) demineralized water (5:95) was added to the other. All samples were inoculated with Streptococcus lactis 4175 and incubated at 22 C. Growth and activity of S. lactis were monitored by determining the number of organisms, pH, and titratable acidity after 0, 4, 7, 10, and 13 h of incubation. Distillate collected at 60 C slightly retarded growth of S. lactis in steamed milk. Both growth and activity of this organism were reduced when distillate collected at 68.3 C was added to autoclaved and waterbath-treated (68.3 C) milk; this was not true for steamed milk. When milk was processed at 76.6 C, the resultant distillate had only a very slight effect on behavior of S. lactis in autoclaved milk. In steamed milk, although its generation time was unaffected, acid production by S. lactis was enhanced when the distillate was added. When processed milk was heated at 76.6 C, distillate collected at 76.6 C caused a slight increase in the generation time of S. lactis.