Starters and Bacteriophages in Lactic Acid Casein Manufacture

Abstract

The bacteriological composition and acid producing activity of mixed strain starters used for commercial lactic acid casein manufacture were investigated. Rates of acid production by mixed starters from eight New Zealand casein factories were similar under standard laboratory conditions. Streptococcus cremoris was the predominant species comprising some 70–90% of total bacteria in all but one mixed casein starter. Most isolates in a random selection of 160 individual S. cremoris colonies took longer than the parent mixed starter to coagulate autoclaved skimmilk at 22 C. More than half required 2–3 days and some up to 7 days of incubation, in contrast to the 17–20 h required by the mixed starters. Only 20% of the S. cremoris isolates had the coagulation times and reached maximum cell densities characteristic of the parent mixed cultures. The slow-coagulating isolates grew to only low population densities in milk although acid production continued in stationary-phase cultures. The maximum cell density and rate of acid production could be increased by culturing together with a fast-coagulating strain, by addition of hydrolysed milk proteins or amino acids. Whey samples from the casein precipitation silos of eight commercial factories were examined for bacteriophage. Virulent phages were found in all samples.