VIABILITY OF BIFIDOBACTERIUM-LACTIS AND LACTOBACILLUS-ACIDOPHILUS IN MILK - SODIUM-CHLORIDE CONCENTRATION AND STORAGE-TEMPERATURE

The growth and viability of Bifidobacterium lactis and Lactobacillus a cidophilus per se and for both strains as a coculture were studied in reconstituted skim milk so as to assess their dependence upon temperat ure (5, 10 and 15C), NaCl concentration (0, 0.51 and 1.03 mol L-1), an d storage time (0-8 weeks). Cultures of B. lactis exhibited no signifi cant loss of viability either with increasing NaCl concentration or in creasing storage temperature within the ranges studied. On the other h and, viability of L. acidophilus decreased with increasing temperature and increasing NaCl concentration under similar conditions. When cocu ltured with L. acidophilus, B. lactis was significantly less tolerant to higher NaCl levels and higher temperatures than when in pure cultur e, although final numbers were still above the threshold required for commercial application. Coculturing with B. lactis had no detrimental effect on viability oft. acidophilus. A mechanistic model, which consi ders the behavior of the pure and mired microbial populations is descr ibed by specific death rates which vary with temperature (following Ar rhenius relationships) and NaCl levels (following simple inhibition ki netics) in the milk medium. Activation energies for the death rates of B. lactis and L, acidophilus were 14 kcal mol(-1) and 9-15 kcal mol(- 1), respectively.