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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. 2001. Salad Dressings, 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.
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