Evaluation of the impact of short-term temperature abuse on the microbiology and shelf life of a model ready-to-use vegetable combination product

The growth dynamics of indigenous aerobic mesophilic populations (AMP), lactic acid bacteria (LAB) and inoculated (Listeria spp.) microbial populations on cooked and fresh vegetable products, packaged as separate entities and in combination, subjected to temperature fluctuation, were assessed. Microbial proliferation was temperature and product dependent, being most pronounced at 12°C in all products with maximum growth rates of 0.140, 0.175 and 0.126 log10 CFU/g per h being identified for Listeria, aerobic mesophilic and LAB populations, respectively. Listeria spp. and AMP generally demonstrated higher rates of growth within products containing cooked vegetables. Prolonged storage at 3°C resulted in a reduced ability by AMP and Listeria spp. to proliferate upon exposure to growth temperatures; this was not the case with LAB populations. Comparison of Listeria population estimates made using selective (Oxford) and non-selective (nutrient agar) identified reduced recovery on the former. The magnitude of the deviation increased with the duration of exposure of Listeria populations to 3°C with recoveries on selective systems being reduced by 6.3% immediately after inoculation and 82.3% after 168 h at 3°C, respectively. Growth of populations associated with exposure to abuse temperatures was not accompanied by significant changes in product colour (P<0.05).