Growth characteristics of bifidobacteria in ultrafiltered milk.

Five replicates of raw skim milk were ultrafiltered at 54 degrees C to total protein concentration ratios of 2:1, 3:1, 4:1, and 5:1. Bifidobacterium bifidum and Bifidobacterium longum were inoculated at 5% into skim milk that was not ultrafiltered (1:1) and into ultrafiltered skim milks followed by incubation at 37 degrees C. The mean maximum bacterial count (colony-forming units per milliliter) range and protein concentration ratio were from 6.25 x 10(8) to 1.69 x 10(9), skim milk; 4.42 x 10(8) to 3.56 x 10(9), 2:1; 2.62 x 10(8) to 3.94 x 10(9), 3:1; 6.67 x 10(8) to 1.88 x 10(9), 4:1; and 2.90 x 10(9) to 3.59 x 10(9), 5:1. The mean developed acidity at maximum B. bifidum population in skim milk was .16%, and pH was 5.55. The 5:1 concentrate had a higher mean developed acidity of .57% at pH 5.35, which was similar to that of the skim milk. Trends were similar for B. longum. Because of the increased buffering capacity of highly concentrated ultrafiltered milks, pH 5.5 or higher was maintained longer, along with high developed acidity. Scanning electron micrographs showed distinct morphological variations between bifidobacteria grown in broth versus those grown in the milks.