Jm. Farber et al., CHANGES IN POPULATIONS OF LISTERIA-MONOCYTOGENES INOCULATED ON PACKAGED FRESH-CUT VEGETABLES, Journal of food protection, 61(2), 1998, pp. 192-195
A variety of wholesale and retail packaged vegetables and salads were
inoculated with a mixture of strains of Listeria monocytogenes and inc
ubated at 4 and 10 degrees C. Whole rutabagas, butternut squash, and o
nions, as well as packaged Caesar salad, carrots, coleslaw mix, and st
ir-fry vegetables were purchased from local supermarkets in the Ottawa
area. L. monocytogenes population levels remained constant on all fre
sh-cut vegetables stored at 4 degrees C for 9 days, except for carrots
and butternut squash: counts of cell numbers declined on carrots and
increased on the butternut squash. Fresh-cut vegetables stored at 10 d
egrees C, however, supported good growth oft. monocytogenes on all veg
etables tested, except for chopped carrots, where the population decre
ased approximately 2 log units over a 9-day storage period. As in the
situation with the produce stored at 4 degrees C, butternut squash sup
ported the highest rate of cell growth. In addition, Caesar salad and
coleslaw mix were kept at 25 degrees C for 1 or 2 days before subseque
nt storage at 4 or 10 degrees C to simulate extreme temperature-abuse
conditions. In Caesar salad stored at 4 degrees C, by day 6 an initial
24- and 48-h temperature abuse at 25 degrees C led to a 1.21- and 2.5
5-log-unit population increase, respectively, over the control. Simila
r increases were observed on Caesar salads stored at 10 degrees C. Com
pared to Caesar salad, coleslaw mix temperature-abused at 25 degrees C
and then stored at 40C supported slightly greater increases in the po
pulation of L. monocytogenes, i.e., a 3.22- and 3.83-log-unit increase
over the control for the 1- and 2-day abused samples, respectively. C
oleslaw mix samples temperature-abused and then stored at 10 degrees C
, however, only showed log unit increases of 1.75 and 1.94, respective
ly compared to the controls. These results point to the importance of
strict temperature control to prevent or reduce the growth of L. monoc
ytogenes cells on fresh-cut vegetables.