Evaluating the effect of inoculation method and validating existing combase models for listeria monocytogenes on ten whole intact raw fruits and vegetables

US regulatory agencies have established a zero-tolerance policy for Listeria monocytogenes in ready-to-eat foods, including fresh fruits and vegetables. Dry inoculation methods can influence microbial behavior in low moisture foods, but few studies have investigated dry inoculation methods for fresh produce. This study elucidates how growth observed on the surface of ten different fresh produce items is influenced by inoculation method, temperature, and food matrix. Whole, intact blueberry, broccoli, carrot, cauliflower, cherry, mandarin orange, lemon, raspberry, and tomato were investigated. A cocktail of 5 rifampicin resistant L. monocytogenes outbreak strains suspended in 0.1% peptone water was used as the wet inoculum. Dry inoculum was prepared by mixing the wet inoculum with sterile sand and drying at 40°C for 24 h before use. Six replicates of each produce type were inoculated (~3.5 log CFU/sample), incubated at 2, 12, 22, 30 and 35°C and enumerated over time. Growth rates were estimated with DMFit and compared with ComBase modeling predictions for Listeria. Carrots did not support L. monocytogenes growth regardless of temperature or inoculation method, possibly due to the presence of phytoalexins. Apparent growth was generally observed for all temperatures except at 2°C and on other produce types. Inoculation method had a significant effect on the average maximum increase of L. monocytogenes on all commodities and at all temperatures (P < 0.05), and wet inoculation often led to greater increases in L. monocytogenes concentration. ComBase gave fail-safe predictions under all conditions except for tomatoes at 30 and 35°C. Inoculation method is a key variable and its effect should be considered in studies on fresh produce. Use of wet inoculation methods may significantly overestimate L. monocytogenes growth potential in fresh produce.M.S.Includes bibliographical reference