SURVIVAL OF PROBIOTIC MICROBIAL STRAINS IN A CHEESE MATRIX DURING RIPENING - SIMULATION OF RATES OF SALT DIFFUSION AND MICROORGANISM SURVIVAL

The growth and survival behaviours of Bifidobacterium lactis and Lacto bacillus acidophilus in a semi-hard Gouda cheese at various axial loca tions during 9 wk of ripening at 13 degrees C were assessed using non- linear regression analysis. The final average salt levels ranged in 2- 4%(w/w). Viable numbers of both probiotic strains underwent a slow dec line during the first 3 wk followed by a sharper decrease towards the end of ripening; such decrease was more substantial for the outer than for the inner cheese portions. Salt transport was successfully descri bed by Fick's second law of diffusion, and the cheese was considered a s a finite slab for modelling purposes. Salt diffusivity remained cons tant with time and was estimated to be 0.2 cm(2)/day. Theoretical salt concentration profiles were in good agreement with experimental data. The mathematical models postulated and fitted to the microbial viabil ity data encompassed both a linear relationship between specific death rate and salt concentration and a constant death rate, following a me thodology of increasing model complexity. Decision on the better model was taken based on a F-test of the ratio of incremental sum of square s of residuals to sum of squares of residuals of the more complex mode l, and it was concluded that viability of the probiotic strains was be tter described by a first order process independent of local salt conc entration. Prediction of profiles of viable numbers of B. lactis and L . acidophilus in cheese with respect to both ripening time and axial l ocation for several overall salt concentrations is useful in attempts to predict potential viability by the time of consumption. (C) 1998 El sevier Science Limited. All rights reserved.