EFFECT OF FREEZE-DRYING AND STORAGE ON THE MICROBIOLOGICAL AND PHYSICAL-PROPERTIES OF AB-YOGURT

Experimental yoghurts fermented with L. acidophilus, Bifidobacterium s pp.(mixed B. bifidium and B. longum) and S. salivarius spp. thermophil us (yoghurt with ABT culture); and ''traditional'' yoghurt fermented w ith L acidophilus, Bifidobacterium spp.(mixed with B. bifidum and B. l ongum), S. salivarius spp. thermophilus and L. delbrueckii spp, bulgar icus (''traditional'' yoghurt with AB culture) were prepared and freez e dried for 96 h at 50 degrees C, in freeze dried yoghurt powders, bac terial population was 0.25 to 2 log lower than in the fresh product. L . bulgaricus was the most sensitive bacteria to freeze drying. Viable counts of L. acidophilus and Bifidobacterium spp. in the yoghurt powde rs were less than the recommended minimum (10(6)cfu/ml) after 27 days of storage. Reincubation of the yoghurt powder did not increase viable population of yoghurt bacteria, The texture of reconstituted freeze d ried yoghurt was poorer than that of the fresh yoghurt. Reincubation o f freeze dried yoghurt powders did nor result in the improvement of te xtural characteristics of reconstituted yoghurt. Population of tactic acid bacteria in a commercial ABT yoghurt did not decrease significant ly during freeze drying (48 h at -40 degrees C). However, after 21 day s of storage, only L, acidophilus was at >10(6) cfu/ml, and after 31 d ays of storage no lactic acid bacteria were detected. L. acidophilus d id not survive at sufficient populations in yoghurt prepared from comm ercial yoghurt powders after 1 week of refrigerated storage.