Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 1996
Publication Date: N/A
Interpretive Summary: Fresh-cut spinach can be contaminated with food borne pathogens, such as Listeria monocytogenes. The growth rate or pattern of L. monocytogenes on spinach leaves is unknown. Thus, a study was undertaken to determine if something in the spinach or naturally occurring microorganisms on spinach would have an influence on the growth of L. monocytogenes. Results indicated that the spinach leaves either developed or contained a product which had an inhibitory effect on the increasing population of L. monocytogenes. The naturally occurring microorganisms also had an inhibitory effect on the increasing population and the effect was greater when the population of the naturally occurring microorganisms was increased. These results indicate that it would be desirable to maintain the growth of naturally occurring microorganisms to control the growth of pathogens that occurs from contamination. This information will be useful to the industry in understanding the importance of natural microorganisms in controlling the growth of food borne pathogens.
A study was undertaken to investigate the cause of the bacteriostatic activity of fresh-cut spinach leaves against Listeria monocytogenes. L. monocytogenes was cultivated in 1) pure tryptic soy broth used as a monoculture, 2) in tryptic soy broth containing 10 mg.ml-1 of autoclaved or non autoclaved freeze-dried spinach powder, and 3) in tryptic soy broth in mixed cultures with different microorganisms isolated from fresh-cut spinach including Pseudomonas fluorescens biovar I (Ps.I), P. fluorescens biovar III (Ps. III), Staphylococcus xylosus (St.), and an undefined culture of mesophilic aerobic microorganisms (MAM) isolated from freeze-dried spinach powder. These microorganisms were inoculated at the population of 4.4 Log10 CFU.ml-1 and L. monocytogenes was inoculated at two populations, 2.4 or 4.4 Log10 CFU.ml-1. After 24h of incubation at 30C, the Log10 CFU.ml-1 of L. monocytogenes increased to 9.0 and 9.6 in the control, 5.4 and 7.5 in non autoclaved spinach powder cultures, 8.8 and 9. in autoclaved spinach powder cultures, and in mixed cultures, 7.4 and 8.6 (Ps. I), 7.7 and 9.1 (Ps. III), 7.8 and 9.2 (St.), and, 7.1 and 8.0 (MAM), in the low and high listerial inoculum cultures respectively. The LSD (.05) of the means were 0.5 and 0.6, respectively. The freeze-dried spinach powder had an inhibitory effect on the growth of L. monocytogenes. The inhibitory effect was greatly decreased when the native microorganisms were almost eliminated by heating or irradiation. These results indicate that if L. monocytogenes is present as a contaminant on fresh-cut spinach, its growth probably will be restricted by native microorganisms.