VALIDATION OF A MODEL FOR PREDICTING THE COMBINED EFFECT OF 3 ENVIRONMENTAL-FACTORS ON BOTH EXPONENTIAL AND LAG PHASES OF BACTERIAL-GROWTH - TEMPERATURE, SALT CONCENTRATION AND PH

A linear-arrhenius predictive model, hypothesised previously, for the combined effect of three or more environmental factors on bacterial growth was assessed against 66 growth curves of Salmonella spp. as effected by temperature (283-303 K), sodium chloride concentration (0.7-4.56% wt/v) and pH (5.63-6.77) for both the exponential (growth) phase and lag phase of growth. Lag times were determined by both traditional (subjective) microbiological methods and by the use of the Modified Gompertz equation. No significant difference (P less than or equal to 0.05) was found between values of the lag time determined by either of these two methods. Model predictions gave a very high degree of fit to these data explaining 97.6% of the per cent variance accounted for (%V) between observed and predicted values for the growth phase, and 94.6%V for the lag phase. The quadratic form of the model involves six terms to account for the three environmental factors: 1/T, 1/T-2, salt, salt(2), pH, pH(2). These findings appear to confirm the generalised additive model proposed earlier (Davey, 1994). Advantages of the model include its accurate prediction of rate coefficient for growth and the duration of the lag time, parsimony (small number of coefficients) and ease of formulation and of use.