Performance of Commercial Cultures in Fluid Milk Applications

¹ Dairy and Food Culture Technologies, 7119 S. Glencoe Court, Littleton, CO 80122
² North Carolina State University, Southeast Dairy Foods Research Center, Raleigh 27695-7624

Six Lactobacillus acidophilus, 5 Bifidobacterium, and 6 Streptococcus thermophilus strains were studied for characteristics that are important to activity and stability in unfermented fluid milk products. Speciation, strain relatedness, frozen concentrate stability, bile sensitivity, and lactase activity were evaluated. The microbiological stability of a culture-containing fluid milk product was also determined. Two of the bifidobacteria cultures contained >1 strain. Some strains were shown to be closely related or identical by pulsed-field gel electrophoresis of fragmented chromosomal DNA. Selective media that distinguished among all 3 added genera were identified. All lactobacilli and most of the bifidobacteria were resistant to bile concentrations varying from 1 to 3%, and all streptococci were sensitive to bile. Lactase activities were highest for S. thermophilus strains, supporting use of this species in fluid milk and dairy products to aid in the digestion of lactose by consumers. The experimental product evaluated in this study contained 10⁷ cfu/ml of both L. acidophilus and Bifidobacterium spp. and 5 x 10⁷ cfu/ml of S. thermophilus. Lactic, but not psychrotrophic, populations were fairly stable during storage. The results suggest that milk formulated with high concentrations of three different genera of probiotic bacteria can be manufactured with commercial strains.

Key Words: bifidobacteria • lactobacilli • Streptococcus thermophilus • probiotics

Submitted on January 10, 1995
Accepted on January 22, 1996

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