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

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, Bifido-bacterium spps and S. thermophilus were ; after fermentation : 4.0x10 7 , 9.0x10 6 and 2.8x10 9 cfu/mL ; after 36 days of refrigerated storage : 10 7 , 4.9x10 5 and 4.5x10 8 cfu/mL correspondingly. In traditional yoghurt with AB cultutre, L. acidophilus, Bifidobacteriumspp., L. bulgaricus and S. thermophilus were ; after fermentation : 4.0x10 6 , 8.6x10 6 , 1.2x10 8 and 1.6x10 9 cfu/mL ; after 62 days of refrigerated storage : 1.2x10 6 , 1.5x10 6 , < 10 2 , 10 6 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 (10 6 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 1.2x10 8 to 3.0x10 5 cfu/mL and these species were the most sensitive to freeze-drying. Viable counts of acidophilus and Bifidobacterium spp. in both commercial and experimental yoghurt powders were less than the suggested minimum levels after 27 days of storage. Reincubation of the powder did not increase viable population of yoghurt bacteria.