microbial curve, log N versus time, obtained for eachcombination of environmental factor levels, is fittedby a primary model. Recently, Baranyi et al. (1993)proposed a sigmoidal curve based on the logisticcurve (Tomassone et al., 1993) with an additiveterm, a(t), which allowed lag phase period to be introduced as an adaptive period for micro-organismsbefore growing