salad dressings

Salad dressings and mayonnaise are preserved by the addition of about 0.5-1.5% acetic acid or the addition of lemon juice. The microorganisms in these products come from the ingredients, the manufacturing equipment, and from the air. The microflora responsible for the spoilage of salad dressings seems quite restricted and consists of a few species of Lactobacillus, Saccharomyces and Zygosaccharomyces. The source usually can be traced to unsanitary equipment such as the mixing, pumping, and filling machines. Surface spoilage may also result from airborne contaminants such as molds and film yeasts. Ingredients such as the starch paste or the vinegar can also be the source of spoilage organisms. This is the case of spoilage due by Monilliela acetoabutans due to its presence in vinegar stored in wooden tanks.

Microbiological spoilage is frequently manifested by gas formation that forces out the dressing when the container is opened. This is the case of the spoilage by Zygosaccharomyces bailii. Other spoilage organisms are Lactobacillus plantarum and L. fructivorans.

Pathogens such as Salmonella and Staphylococcus die off quickly at pH 3.2. In a recent study, E. coli0157/H7 rapidly died off at normal storage temperatures (22-25°C). However, at lower temperatures it can survive longer. For example, at 5°C it can survive for as long as 35 days.

Three groups of microorganisms are used as an indication of poor sanitation and potential spoilage problems: yeast and molds, lactobacilli, and aerobic bacilli. Any persistence or increase in numbers of yeast and molds to > 10 per g should be a warning of a sanitation problem. The presence of aerobic bacilli in excess of 50 per g should be considered indicative of a sanitation problem.

Recommended tests

Aerobic plate count.
Yeast and molds.

Lactobacilli.

Additional tests

Preservative-resistant yeasts.
Heterofermentative screen.
Total coliform count., E. coli, Salmonella, Staphyloccoccus aureus, Listeria monocytogenes, and Yersinia enterocolitica (if suspected of causing illness).

Adapted from

Smittle, R. B. and Cirigliano, M. C. 2015. Salad Dressings,, chapter 53, p. 541. In F. P. Downes and K. Ito (eds.), Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, Washington, DC.