Table 111

Some Outbreaks of Foodborne Illness Linked to Fresh-Cut Cabbage and Lettuce Products

Etiological agent Commodity Ref.

Listeria monocytogenes Cabbage coleslaw 44

Clostridium botulinum (A) Cabbage salad 45

Shigella sonnei Iceberg lettuce 46

E. coli O157:H7 Mesclun lettuce 47

Hepatitis A Iceberg lettuce 48

E. coli O157:H7 Shredded carrots 49

or as a result of human activity (see Chapter 1). Foodborne pathogens have been isolated from market fresh-cut vegetable products in various parts of the world [18-20]. In addition, processing plants frequently receive raw materials from a variety of sources and the potential exists for cross contamination of product. Despite these apparent vulnerabilities, relatively few documented outbreaks of foodborne illness have been conclusively linked to fresh-cut vegetables. Table 11.1 lists some outbreaks that have implicated fresh-cut carrots, cabbage, and lettuce, and these examples serve to illustrate the potential for transmission of various infectious agents through fresh-cut vegetables. It should be stressed that epidemiological investigations of outbreaks involving such products are fraught with difficulties. Considerable time delays between definitive association and sampling of suspected products may reduce the likelihood of detection. In addition, microbiological analyses often lack the sensitivity required to detect small populations of target microorganisms in environmental samples containing complex background microflora in high populations. This problem is acute for some pathogens, particularly species of shigella, due to a lack of effective selective enrichment protocols. Hence, currently held assumptions about the behavior of human pathogens in fresh-cut vegetable products are largely derived from research.

The behavior of pathogenic microorganisms has been examined in fresh-cut cabbage under laboratory conditions. For example, Kallander et al. [21] were unable to detect Listeria monocytogenes (detection limit 102CFU/g) in shredded cabbage stored under air or under 70% carbon dioxide and 30% nitrogen after 6 days at 25°C. A reduction in pH related to the growth of lactic acid bacteria was evident at this temperature. In contrast, the species was capable of growth in cabbage stored at 5°C, irrespective of atmosphere composition, and little or no growth of lactic acid bacteria was detected. Omary et al. [23] inoculated shredded cabbage with Listeria innocua, a surrogate species for L. monocytogenes. Packaging films with oxygen transmission rates ranging from 5.6 to 6000 cm3 O2/m2/24 hours were employed for storage of samples at 11°C. Listeria innocua populations initially declined but eventually increased in samples subjected to all treatments. Hence, the results of studies carried out in the laboratory may lead to different individual conclusions. Nevertheless, the sum of this research suggests that Listeria spp. can grow in packaged fresh-cut cabbage and that temperature is a critical determinant for the fate of this species.

The behavior of L. monocytogenes in cut lettuce has also been examined and variable results are reported from individual studies. A gradual decline in populations was reported by Francis and O'Beirne [22] at 3°C and by Kakiomenou et al. [8] at 4°C. Beuchat and Brackett [24] found there was no change in cut lettuce stored at 5°C, but Steinbruegge et al. [25] observed exponential growth at higher temperatures. The effect of modified atmospheres on the fate of L. monocytogenes is also unpredictable. Francis and O'Beirne [22] found that growth was enhanced in shredded lettuce stored under 100% N2 instead of an aerobic packaging system. Jacxsens et al. [26] recorded similar observations for shredded iceberg lettuce stored at 7° C under a 2 to 3% O2, 2 to 3% CO2, 94 to 96% N2 atmosphere. However, contradictory conclusions were drawn from the work of Beuchat and Brackett [24] where no difference was found between samples stored at 10°C in air or in a 3% O2/97% N2 gas mixture. Hence, the effects of modified atmospheres on the fate of L. monocytogenes in cut lettuce remain uncertain.

Comparatively little is known about the behavior of human pathogens in fresh-cut carrots, likely due to the limited number of foodborne infections conclusively associated with this commodity. Nevertheless, some interesting observations have been derived from research with inoculated products. Bagamboula et al. [27] found that Shigella spp. populations gradually fell in grated carrots stored at either 7 or 12° C. The spoilage association in the product was dominated by lactic acid bacteria, and it was concluded that a gradual decline in pH was largely responsible for a gradual die-off of the inoculum. Viable cells were recovered from all products after 7 days.

Analysis of published research indicates that temperature has a major influence on the fate of pathogens in fresh-cut vegetable products. The maintenance of low temperatures evidently provides the best means to prevent growth, particularly for members of the family Enterobacteriaceae. Listeria monocytogenes poses a unique challenge given the ability of the species to grow at refrigeration temperatures. However, the plurality of experimental outcomes at higher temperatures or where modified atmospheres are applied to packaging systems suggests that additional factors influence the behavior of this pathogen in fresh-cut vegetable products.

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