Factors affecting the growth of Listeria monocytogenes on minimally processed fresh endive.

The influence of various factors on the fate of Listeria monocytogenes on cut leaves of broad-leaved endive has been studied. Factors considered were temperature, characteristics of the leaves (age, quantity and quality of the epiphytic microflora) and characteristics of the L. monocytogenes inoculum (concentration, strain). The increases in numbers of L. monocytogenes were lower than those of the aerobic mesophilic microflora at 3 degrees, 6 degrees, 10 degrees and 20 degrees C. Doubling times of the populations of L. monocytogenes were in the same order of magnitude as those of aerobic bacteria at 10 degrees and 20 degrees C, but longer at 3 degrees and 6 degrees C. There were positive significant correlations between growth of L. monocytogenes and populations of aerobic bacteria, and between growth of L. monocytogenes and extent of spoilage on the leaves. Of 225 bacteria isolated from the leaves, 84% were identified as fluorescent pseudomonads; there was no difference in the species isolated from leaves that showed a low growth of L. monocytogenes and leaves that showed a high growth of L. monocytogenes. Populations of L. monocytogenes increased faster during the first 2 and 4 d of storage at 10 degrees C on leaves inoculated with 10-10(3) cfu g-1 than on leaves inoculated with about 10(5) cfu g-1, but the population reached after 7 d was lower. The behaviour of L. monocytogenes was similar among the three strains tested.