Behaviour of cultures of Bifidobacterium spp. in different conditions of freezing

Seven different strains of the species Bifidobacterium bifidum (ATCC 15696, B1, B3, B4 and B12), B. infantis (ATCC 27920) and B. longum (B6) were subjected, as concentrated cultures in skim-milk, to different freezing treatments (high rate in liquid nitrogen and slow rate in cold air at -30 degrees C), then stored at -80 degrees C and at -30 degrees C for six months. Survival was determined by plate counting in TPY medium with and without bile salts or sodium chloride at the highest tolerated concentration for each strain, to discriminate non-injured population from total population. Fermentative activity was determined as total lactic acid production by thawed cultures under standard conditions. Behaviour of the strains did not resulted homogeneus both in resistance to increasing concentrations of selective agents and in trend of the viability in different conditions of freezing. For most cultures survival and residual activity did not decrease below 50% until the end of the storage time; storage temperature seemed to have a greater effect than cooling rate in viability. Results obtained in media added with bile salts or sodium chloride were unsatisfactory to evaluate the non-injured share of the surviving population.