Media for Routine Microbiological Analyses

Methods have been developed and validated to analyze raw and processed foods of animal origin and processed foods of plant origin for the presence and populations of pathogenic and spoilage microorganisms. Methods for analysis of raw fruits and vegetables, in contrast, are not well defined. Procedures for enrichment and direct plating of raw produce samples in the U.S. are generally modifications of those outlined for processed fruits and vegetables in the Bacteriological Analytical Manual (BAM) of the U.S. Food and Drug Administration [5] or the Compendium of Methods for the Microbiological Examination of Foods published by the American Public Health Association (APHA) [6]. A 25 g analytical unit diluted at a 1:9 ratio (weight:volume) of sample:diluent or sample:broth is prescribed for most foods. The food and diluent or broth are then mixed, swirled, soaked, or blended before withdrawing samples for direct plating or incubated for a specified time at a given temperature for preenrichment or enrichment. In part because of the lack of a standard method(s) to analyze raw fruits and vegetables for pathogens and spoilage microorganisms, researchers have modified BAM and APHA methods to suit their needs to analyze specific types of produce, often without validation of the efficiency of detection or recovery of the test microorganism or group of microorganisms under investigation. Substantial variations in methods to select and process samples for preenrichment, enrichment, and direct plating have been used to analyze raw produce for the presence and populations of pathogens. To illustrate these variations, Table 24.2 gives some examples of procedures used by researchers to analyze produce for salmonella.

The inability of injured or stressed cells of pathogenic bacteria to resuscitate and grow on selective media is too often not recognized. Selective media recommended for foods other than raw fruits and vegetables have been formulated and evaluated for that purpose. The same media may not perform well for detection and enumeration of pathogens or other microorganisms on produce. The number of stressed salmonella recovered from tomatoes [24,31,32], lettuce [33], alfalfa sprouts and seeds [32,35], and parsley [33], for example, is significantly less on selective versus nonselective media. Recovery of Escherichia coli O157:H7 and Listeria monocytogenes from tomatoes [31], lettuce, and parsley [33] treated with sanitizers has also been shown to be less on selective media.

While progress has been made in developing media for enumerating yeasts and molds in a wide range of foods and beverages [36], less attention has been given to evaluating the performance of diluents in removing and dispersing fungal propagules on fruits and vegetables. Standard methods for determining yeast and mold counts in foods recommend 0.1% peptone as a diluent [5,37,38]. A study done by Beuchat et al. [39] was aimed at determining the retention of viability of mycoflora recovered in seven diluents used to wash seven types of raw fruits as affected by composition of diluents.

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