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The survival of culture bacteria in fresh and freeze-dried AB yoghurts

Authors: S. Rybka and K. Kailasapathy


Yoghurt bacteria viability is important in providing a number of therapeutic benefits to consumers. The survival of AB-culture (Lactobacillus acidophilus and Bifidobacterium spp.) in traditional commercial yoghurts was reported to be unsatisfactory (Rybka, 1994). Two batches of yoghurt were prepared fermented with: (i) L. acidophilus, Bifidobacterium spp. (mixed B. bifidum and B. longum 10:90) and Streptococcus thermophilus (yoghurt with ABS-culture); (ii) as in (i) plus L. delbrueckii subspp. Bulgaricus (traditional yoghurt with AB-culture). In yoghurts with ABS culture, L. acidophilus, Bifidobacterium spp. and S. thermophilus were; after fermentation: 4.0x107, 9.0x106 and 2.8x109 cfu/mL; after 36 days of refrigerated storage: 107, 4.9x105 and 4.5x108 cfu/mL correspondingly. In traditional yoghurt with AB culture, L. acidophilus, Bifidobacterium spp., L. bulgaricus and S. thermophilus were; after fermenation: 4.0x106, 8.6x106, 1.2x108 and 1.6x109 cfu/mL; after 62 days of refrigerated storage: 1.2 x 106, 1.5x106, <102, 108 cfu/mL respectively. The strain of L. bulgaricus used was a slow lactic acid producer not sufficiently antagonistic towards AB-culture. ABS-culture counts in a commercial yoghurt did not decrease during 48 hours of freeze drying at -40°C. After 21 days of storage of the freeze dried yoghurt powder only L. acidophilus population met the suggested minimum levels (106 cfu/mL). In freshly prepared experimental yoghurt powders (freeze-dried for 96 hours at -50°C) yoghurt bacteria populations were from 0.25 to 2 log lower than in the fresh liquid product. L. bulgaricus count was reduced from 1.2x108 to 3.0x105 cfu/mL and these species were the most sensitive to freeze-drying. Viable counts of the acidophilus and Bifidobacterium spp. in both commercial and experimental yoghurt powders were less than the suggested minimum levels after27 days of storage. Reincubation of the powder did not increase viable population of yoghurt bacteria.

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