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Pasteurized milk has an initial microflora that consists primarily of thermoduric bacteria and spores such as Bacillus , Micrococcus, Lactobacillus, Microbacterium, Corynebacterium, Streptococcus, Enterococcus, and Arthrobacter spp. Large numbers of these microorganisms in the raw milk supply can contribute significantly to the SPC of pasteurized products. Most thermoduric bacteria grow slowly in refrigerated milk and are outgrow by gram-negative psychrotrophic bacteria (Pseudomonas, Flavobacterium, and Alcaligenes spp. as well as some members of the coliform group) that contaminate pasteurized milk after pasteurization. In the absence of psychrotrophic bacteria or if large number of thermoduric bacteria survive pasteurization, certain thermodurics, particularly psychrotrophic sporeforming Bacillus spp., can grow and cause spoilage (e.g. sweet-curdling). Spoilage by gram-negative bacteria results in fruity, rancid, bitter, and unclean flavors. Generally, populations in excess of 106 per mL are required before flavor defects are detectable. For maximum shelf life, refrigerated storage should be below 4ºC.

Bacteria related to foodborne illness are destroyed by proper pastaurization. Recent outbreaks of salmonellosis and listeriosis in pasteurized milk have been linked to post-pasteurization contamination. Post-pasteurization contamination with L. monocytogenes and Yersinia enterocolitica is of major concern to the dairy industry since these organisms grow at refrigeration temperatures.

Recommended tests:

Aerobic plate count.
Total coliform count.

Additional tests:

Psychrotrophic count
Moseley Keeping Quality test

Salmonella.
Listeria.
Campylobacter.
Yersinia.

Dried products are often used as ingredients of other foods and are subject to further processing. Yet, dried milks are considered sensitive products from a public health aspect because they are often consumed after reconstitution without additional heating. It is well known that dried milk can be a source of foodborne illness because of contamination with Salmonella (environmental contamination) and Staphylococcus intoxication (due to growth of this microorganism and toxin production prior to the heating processing). Because of their low water activity, dried products rarely spoil or deteriorate because of microbial growth. Improper storage of dry milk will promote the growth of molds. Molds can cause discoloration, musty flavors, breakdown of milk components resulting in off-flavors, and possible production of mycotoxins.

Recommended tests:

Aerobic plate count.
Total coliform count.
Yeast and mold count.

Additional tests: