Viability of Lactobacillus acidophilus and Lactobacillus casei in fermented milk products during refrigerated storage.

The viability was investigated of five strains of Lactobacillus acidophilus and one strain of Lactobacillus casei that were added as adjuncts to yogurt and cultured buttermilk during 28 d of refrigerated storage at 5 to 7 degrees C. A modification of LBS (Lactobacillus selection) agar was used for the enumeration of L. acidophilus and L. casei. The medium allowed the colony formation of the adjunct bacteria while preventing colony formation of the traditional yogurt or buttermilk starter cultures. At each sampling period, colonies from the selective agar medium were isolated for confirmation of identity to confirm that only L. acidophilus and L. casei were enumerated, that their characteristics did not change during storage, or both. The strains of L. acidophilus varied in both cultured products. In buttermilk, L. acidophilus MUH-41, O-16, and L-1 exhibited no significant loss in viability, but strains 43121 and La-5 did. No significant loss in viability of L. acidophilus MUH-41 and L-1 occurred in yogurt prepared using culture CM2; however, strains 43121, O-16, and La-5 lost viability. In the yogurt prepared using culture YC-4, L. acidophilus 43121 exhibited no significant loss in viability, but MUH-41, O-16, L-1, and La-5 did. There was no loss in viability of L. casei GG during storage of any of the cultured products.