The effect of heat stress on freeze-drying and conservation of Lactococcus

The production of lactic acid bacteria isolated from Tunisian fermented milk (Lben): Lactococcus lactis subsp. lactis var. diacetylactis (SLT6) was investigated in fed-batch process. The final biomass production was 2.2710(11) cfu/ml. The effect of heat stress, after fermentation, on the survival rate after freeze-drying and during storage was studied. The survival rate after freeze-drying of SLT6 subject of a thermal treatment was 38.8% whereas it was only 21.9% for unprocessed SLT6. The accelerated shelf life testing (ASLT) method was used to study the effect of heat stress on conservation of Lactococcus. The bacterium was stored at four different temperatures of 4, 15, 25 and 37 degrees C. Lactococcus cell kinetic behaviour suggested a first order reaction, and the effect of storage temperature was quantified by determination of the half-life period and the activation energy. The beneficial effect of heat stress is clearly associated with a half-life period of 86 days for SLT6T and only 48 days four SLT6NT stored at 4 degrees C. (c) 2005 Elsevier B.V. All rights reserved.