THE SURVIVAL OF CULTURE BACTERIA IN FRESH AND FREEZE-DRIED AB YOGURTS

Yoghurt bacteria viability is important in providing a number of thera peutic benefits to consumers. The survival of AB-culture (Lactobacillu s acidophilus and Bifidobacterium spp.) in traditional commercial yogh urts was reported to be unsatisfactory (Rybha, 1994). Two batches of y oghurt were prepared fermented with: (i) L. acidophilus, Bifidobacteri um spp. (mixed B. bifidum and B. longum 10:90) and Streptococcus therm ophilus (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 spps and S. thermop hilus were; after fermentation: 4.0 x 10(7), 9.0 x 10(6) and 2.8 x 10( 9) cfu/mL; after 36 days of refrigerated storage: 10(7), 4.9 x 10(5) a nd 4.5 x 10(8) cfu/mL correspondingly. In traditional yoghurt with AB cultutre, L. acidophilus, Bifidobacterium spp., L. bulgaricus and S. t hermophilus were; after fermentation: 4.0 x 10(6), 8.6 x 106, 1.2 x 10 (8) and 1.6 x 10(9) cfu/mL; after 62 days of refrigerated storage: 1.2 x 10(6), 1.5 x 10(6), < 10(2), 10(8) 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 y oghurt did not decrease during 48 hours of freeze-drying at -40 degree s 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 9 6 hours at -50 degrees C) yoghurt bacteria populations were from 0.25 to 2 log lower than in the fresh liquid product. L. bulgaricus count w as reduced from 12 X 10(8) to 3.0 x 10(5) cfu/mL and these species wer e the most sensitive to freeze-drying. Viable counts of acidophilus an d Bifidobacterium spp. in both commercial and experimental yoghurt pow ders were less than the suggested minimum levels after 27 days of stor age. Reincubation of the powder did not increase viable population of yoghurt bacteria.