Studying the effect of temperature on microbial growth using multiplicative model.

The specific growth rates of Brochothrix thermosphacta, Listeria monocytogenes, Yersinia enterocolitica, Bacillus cereus, Escherichia coli O157:H7, Salmonella spp., Staphylococcus aureus and Clostridium perfringens at various temperatures were taken from the Food MicroModel database, and the data sets of specific growth rate versus temperature were fitted using the multiplicative model (r = a Td, r = specific growth rate; T = temperature; a, d = regression parameters). The exponential d-value derived from microbial growth at suboptimum temperatures reflected the effectiveness of temperature in enhancing growth. A microorganism with a large d-value exhibited a large increment of growth rate as temperature increased. The d-value of a microorganism was related to the temperature range for growth. The temperature range for the growth of psychrotrophs was usually narrow for B. thermosphacta and Y. enterocolitica; hence the d-values of these two psychrotrophs were close to 1 whereas d-values of mesophiles, such as B. cereus, E. coli O157:H7, Salmonella spp., and S. aureus, were 2.31-2.90, and the d-value of C. perfringens, a thermophile, was 3.29. The values of parameter a of the model were affected by extra salt added into cultures. For all the strains mentioned above, the a-values decreased when the cultures contained higher levels of salt. The lowering of the a-value implied that the influence of temperature on the growth rate in the model was reduced. The change of the d-value was dependent on the capability of the microorganism to overcome the obstacle to growth and was affected by the composition of the nutrients and by inhibitory factors in the culture. The influence of environmental factors on the d-value was also found in Chinese sausages. The d-value of a dominant spoilage strain of Enterococcus sp. derived from sausages was 0.833 whereas the d-value derived from MRS cultures was 2.36. In refrigerated foods which usually contained some preservatives and were stored at low temperature, the d-value of psychrotrophic spoilage bacteria was around 1. In this case, the linear model could be a reasonable choice for predicting the proliferation of spoilage bacteria.