Mayonnaise and salad dressing commercially produced in the United States are defined in accordance with the Food and Drug Administration Standard of Identity. The microbiological content of these products is dictated primarily by the high acetic acid concentration found in their aqueous phase. The overall microbiological content of mayonnaise and salad dressing is low with a very low incidence of spoilage. Lactobacilli, yeasts, and bacilli are the organisms commonly found. The organisms most frequently isolated from spoiled products are yeast and, to a lesser extent, lactobacilli. The major preservative effect is from the acetic acid content with a minor influence from salt or sugar concentration. Mayonnaise and salad dressing produced in the United States are inimical to bacteria, especially food pathogens. The acetic acid levels used by the major producers, 0.31–0.32% for mayonnaise and 0.90–0.928% for salad dressing, are effective in destroying salmonellae and staphylococci. Salad dressing and mayonnaise used to prepare salads and sandwiches have an inhibitory effect on pathogenic bacterial growth in these products, which is attributed to the acetic acid from the mayonnaise and salad dressing. Contrary to popular opinion, mayonnaise and salad dressing when added to salads or sandwiches will not increase spoilage or public health hazards, but actually retard spoilage and growth of pathogenic microorganisms.

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