A single method to remove efficiently microbial cells or spores from all types of fruits and vegetables for the purpose of detection or enumeration would be ideal, but this may not be an achievable goal. Differences in size, shape, and surface morphology of fruits and vegetables complicate the protocol. The ratio of surface area to weight of individual produce items varies substantially, raising the need to establish a basis (CFU/g or CFU/cm2) to be used to record and report data.
The procedure for preparing the sample for analysis may affect the efficiency of retrieval of microbial cells from produce, as well as dispersal before preenrichment, enrichment, or direct plating. Homogenization of a standard weight of a fruit or vegetable using a standard volume of diluent would be a simple procedure for selecting sample size and method of preparation of samples. Problems, however, may be associated with homogenized, blended, or macerated plant tissues. These include the potential lethal effect of naturally occurring antimicrobial compounds against pathogens or other microflora targeted for detection or enumeration. When microbial cells on the surface of produce tissues come in contact with organic acids or other antimicrobials naturally present in tissue fluid, or produced in the form of phytoalexins as a result of rupture of cells or invasion with insects or molds, death may occur .
Acids and phenolic compounds are naturally present in plant stems, leaves, flowers, and fruits. These compounds may interfere with detection and enumeration of pathogenic and spoilage microorganisms. The low pH of produce tissues, particularly those in many fruits, is attributable to a wide range of organic acids they may contain. Garlic, onion, and leek are probably the most widely consumed vegetables that have antimicrobial activity. Allicin, a diallyl thiosulfate, is not present in intact tissues but is produced when the tissues are disrupted. Plant tissues used largely as seasoning agents may also be inhibitory to pathogenic and spoilage microorganisms (Table 24.3). Spices
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