Summary
Recent outbreaks and recalls of produce due to Listeria monocytogenes contamination have increased the need to understand its potential for growth on fresh-cut produce at both ideal and abusive temperatures. Fresh-cut produce has provided an easy and convenient way for consumers to increase consumption of healthy foods. To meet consumer demands for “superfoods” and to create sustainable products by utilizing more of the whole plant new salad blends have been developed from non-traditional fresh-cut salad ingredients. Many of these ingredients are not normally consumed raw or may not have even been widely consumed. New salad ingredients include beet greens, kale, Brussel sprouts, and shredded broccoli stalk. While researchers have investigated the growth of L. monocytogenes on common fresh-cut salad ingredients like spinach and iceberg lettuce research on these new salad ingredients is limited. This project will determine if and under what conditions L. monocytogenes will grow on non-traditional salad ingredients. This will be accomplished by placing a known number of L. monocytogenes cells on the selected produce and monitoring the population under ideal, abusive and “real-world” storage conditions. Bagged salad producers will be able to use this data to develop management strategies to minimize food safety risk.
Technical Abstract
Consumer demand for bagged salad has moved beyond shredded iceberg and chopped romaine to more nutritionally dense greens with bold flavors. New salad ingredients include beet greens, kale, Brussel sprouts, and broccoli stalk. Many of these ingredients have not normally been consumed raw or may not have even been widely consumed. The purpose of this project is to determine the fate of L. monocytogenes on non-traditional salad ingredients under ideal, abusive and “real-world” storage conditions and to understand the influence of the produce microbiome on L. monocytogenes behavior. Beet greens, kale, Brussel sprouts, and broccoli stalk will be inoculated with L. monocytogenes and incubated at 4, 12, 22 and 35°C. L. monocytogenes populations and the microbiome will be monitored over the incubation period. To further assess L. monocytogenes growth risk product will be tested under simulated storage and distribution conditions. This part of the study will be done with L. innocua after conducting a challenge study to confirm its acceptability as a surrogate. Then to assess the potential growth of L. innocua under simulated conditions, product inoculated with the surrogate will be subjected to physical and temperature abuse typically experienced during shipment from the distribution center to the retail outlet. Results from this project will provide the fresh-cut produce industry with knowledge on the growth or no growth of L. monocytogenes on non-traditional salad ingredients that can then be used to develop data driven risk management practices.
Research Objectives
1. Determine the ability of Listeria monocytogenes to grow, survive or die-off in fresh-cut broccoli stalk, Brussels sprouts, kale, and beet greens at recommended and abusive storage temperatures
2. Determine if the growth characteristics of L. innocua on the above products are equivalent to L. monocytogenes.
3. Determine the diversity and dynamics of microbial communities (microbiome) present on the selected produce during storage conditions, and determine correlates of community structure with changes in L. monocytogenes populations.
4. Determine the growth potential of L. innocua on the selected products under simulated storage and distribution conditions.
Findings & Recommendations
These findings support the need for close temperature control of RTE salads that may contain kale, Brussels sprouts, broccoli stalk, and/or beet greens. These data can be used to help assess the potential for increased risk of L. monocytogenes growth when loss of temperature control occurs. In addition, the data showed that even in the presence of ideal storage temperature (4°C), L. monocytogenes can still grow in these products. Depending on the vegetable type, growth occurred before (broccoli stalk, 10 d), around (kale and beet greens, 17 d) or after (Brussels sprouts, 25 d) the typical shelf-life. Storage and distribution results indicated that for the tested products, exposure to physical abuse during storage and distribution did not significantly increase the risk beyond what was observed as a result of chopping, slicing, or shredding.