Freshcut Lettuce

Iceberg, leaf, and mixed lettuces remain the most common fresh-cut products in the marketplace. Unlike carrot or cabbage products, raw materials for their manufacture are not stored for extensive periods of time prior to processing.

Field lettuce carries populations of microorganisms derived from soil [12]. Gram-negative bacteria, particularly fluorescent pseudomonads, and smaller numbers of yeast make up the bulk of the initial microbial flora, which ranges in density between 104 and 106 CFU/g [2,13-15]. Densities of microorganisms also vary with location in the plant, and outer leaves tend to carry higher microbial populations.

The need to control browning reactions catalyzed by oxygen in packaged cut lettuce has led to extensive use of technologies designed to maintain anoxic atmospheres during storage. Control over browning can be achieved by vacuum packaging, flushing with nitrogen, or the incorporation of atmospheres containing various levels of oxygen, nitrogen, and carbon dioxide. Selective pressures exerted by gas composition of the atmosphere are expected to influence the spoilage association. However, observations drawn from numerous studies on the microbiology of lettuce packed under oxygen-reduced atmospheres suggest that storage temperature exerts more influence on the development of the spoilage association than gaseous composition. Mesophilic and psychrotrophic populations tend to be similar in stored shredded lettuce, while yeast and mold and lactic acid bacteria populations generally remain low [2,13,14]. Spoilage microflora are always dominated by species of psychro-trophic Pseudomonas spp. [14-16]. Evidence of microbial spoilage appears earlier in products with high initial microbial loads and at higher storage temperatures.

The effects of unit operations on the microbiology of fresh-cut lettuce are not well characterized. Bolin et al. [17] examined the influence of cutting method, washing, centrifugation, initial microbial load, packaging, and temperature on the storage stability of iceberg lettuce. Unfortunately, microbiological assessments were not carried out for all experimental treatments. Deleterious quality effects were associated with tissue damage induced by cutting with dull blades, inadequate water removal, and storage at high temperatures. There is little doubt that some of these factors could influence the development of microbial populations in the package. High initial microbial loads were also correlated with reduced keeping quality.

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