Interactions Between Microorganisms And Plant Tissues

The shelf life of fresh-cut vegetables including lettuce, cabbage, and carrots is limited mainly by the appearance of enzymatically induced discolorations. Control over these quality defects currently relies on the maintenance of low-oxygen atmospheres or the application of dips containing enzyme inhibitors, but additional means to alleviate the problem are under investigation.

The application of mild heat treatments or heat shocks before packaging shows promise for this purpose [28-31]. Heat shocks applied between 47 and 50°C for 90 to 180 seconds delay browning in iceberg lettuce by several days in refrigerated product. In addition, these treatments improve disinfection of the product, and reductions of up to 3 log CFU/g are readily achieved. Unfortunately, the advantages of heat treatments are negated by accelerated microbial growth during subsequent refrigerated storage. Faster growth of spoilage bacteria has been observed, and inoculation with L. monocytogenes and E. coli O157:H7 confirmed that growth is enhanced by prior heat treatment of the lettuce [28,32]. A similar effect occurs in shredded cabbage subjected to heat treatments. The example provided in Figure 11.1 shows total aerobic microbial populations in shredded cabbage stored under refrigeration in low oxygen permeability films after treatment in 100 mg/ml NaOCl solutions heated to various temperatures (Delaquis, unpublished data). Evidently, washing in a cold chlorinated water solution had little effect on the development of the spoilage association. Application of heated chlorinated water reduced the initial population by 2 log CFU/g or more. However, the rate of population growth and the ultimate size of the spoilage population were greater in cabbage subjected to the heat treatments.

Removal of a substantial part of the native microflora could provide a competitive advantage to survivors of heat treatments or to contaminants acquired postprocess. There is evidence that additional factors are responsible for these effects, however. The classic plant pathology literature describes numerous constitutive and induced antimicrobial systems in plants [33]. The economic impact of species that are pathogenic to crops in the field or those responsible for losses during prolonged storage has stimulated considerable

| Unwashed Cold wash (5° C) 49° C, 90 seconds 51° C, 90 seconds

| Unwashed Cold wash (5° C) 49° C, 90 seconds 51° C, 90 seconds

FIGURE 11.1 Total aerobic populations in shredded cabbage stored at 1°C in oxygen-permeable film after treatment in a 100 mg/m; NaOCl solution heated to various temperatures. The cabbage was dried in a centrifuge prior to packaging. (P.J. Delaquis, 2004. Unpublished data.)

FIGURE 11.1 Total aerobic populations in shredded cabbage stored at 1°C in oxygen-permeable film after treatment in a 100 mg/m; NaOCl solution heated to various temperatures. The cabbage was dried in a centrifuge prior to packaging. (P.J. Delaquis, 2004. Unpublished data.)

FIGURE 11.2 Fate of Listeria monocytogenes in aqueous extracts prepared from packaged, shredded cut iceberg lettuce stored aerobically for 0, 1,2, and 3 days at 15° C. (P.J. Delaquis, 2004. Unpublished data.)

FIGURE 11.2 Fate of Listeria monocytogenes in aqueous extracts prepared from packaged, shredded cut iceberg lettuce stored aerobically for 0, 1,2, and 3 days at 15° C. (P.J. Delaquis, 2004. Unpublished data.)

research in this area. In contrast, comparatively little is known about the influence of intrinsic plant defense mechanisms on the fate of significant spoilage microorganisms or foodborne pathogens in fresh-cut vegetable or fruit products. Reports of antimicrobial activity in vegetable extracts provide evidence that some plant constituents may have a role in the microbial ecology of these products. Conner et al. [34] found that an unidentified cabbage juice extract inhibits the growth of Listeria monocytogenes. Carrot extracts have also been widely reported to inhibit fungi [35], foodborne bacteria and yeast [36], and Listeria monocytogenes [36-40].

Accelerated development of microbial populations in products subjected to heat shocks hints that intrinsic barriers to growth normally present in physiologically intact plant tissues may be disrupted by processing. This hypothesis was tested in our laboratory by inoculation of Listeria monocytogenes in iceberg lettuce tissue extracts prepared from tissues stored aerobically for up to three days, as shown in Figure 11.2. The results of these experiments provided evidence that an antilisterial factor or factors is elaborated by cut lettuce tissues stored under aerobic conditions. Application of heat treatments before storage and preparation of the extracts reduces this effect, however, as shown in Figure 11.3. The chemical nature of the inhibitor(s) responsible for this effect in iceberg lettuce is not yet known. There is little doubt that heat shocks and unit operations applied in processing and preservation have a major impact on the physiology of plant tissues. For example, heat shocks [41] and modified atmospheres [42] inhibit the activity of phenylalanine ammonia lyase (PAL), a key enzyme in the development of discolorations in cut lettuce tissues. The enzyme catalyzes the first step in a series of complex reactions that leads to the accumulation of phenylpropanoid intermediates, including phenolic compounds such as caffeic, ferulic, and chlorogenic acids. Several of these

FIGURE 11.3 Fate of Listeria monocytogenes in aqueous extracts prepared from packaged, shredded iceberg lettuce stored aerobically for 0 and 3 days at 15° C following a three-minute wash in cold (4°C) or warm (47°C) water. (P.J. Delaquis, 2004. Unpublished data.)

♦ Untreated, day 0 A Hot wash, day 0 ■ Cold wash, day 0

❖ Untreated, day 3 A Hot wash, day 3 □ Cold wash, day 3

FIGURE 11.3 Fate of Listeria monocytogenes in aqueous extracts prepared from packaged, shredded iceberg lettuce stored aerobically for 0 and 3 days at 15° C following a three-minute wash in cold (4°C) or warm (47°C) water. (P.J. Delaquis, 2004. Unpublished data.)

compounds exhibit antimicrobial activity in vitro [43], but it remains unknown whether they influence the fate of microorganisms in packaged cut lettuce.

Clearly, much remains to be learned about the interaction between microorganisms and plant tissues. Improved characterization of intrinsic factors, which affect the fate of microorganisms in fresh-cut vegetables, will undoubtedly enhance our understanding of fundamental interactions in these complex microbial ecosystems. Furthermore, a more complete understanding of these interactions could lead to significant practical outcomes. In the future, it may be possible to exploit intrinsic barriers to restrict microbial growth for the development of novel preservation processes for fresh-cut vegetables.

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