As bacteria grows on the media, a mass forms that is visible to the naked
eye. This mass, known as a colony, corresponds to one group of bacteria originally
present in the milk added to the media. With the aid of a magnifying lens, the number of
colonies are counted and multiplied by a dilution factor. This gives the estimated
number of bacteria per milliliter of milk. This test is not performed on cultured products
where bacteria are intentionally added (such as buttermilk, cottage cheese, and sour
cream).
(2) Coliform test. The coliform test is a bacteriological examination of dairy
products to detect bacteria of the coliform group. Bacteria of both fecal and nonfecal
origin are members of this group. This test provides information on contamination of
milk after pasteurization. The coliform test is not specifically intended to detect coliform
organisms but to measure the sanitary practices used to control bacterial contamination
of dairy products. Since those organisms are easily destroyed by pasteurization, their
presence in pasteurized products usually indicates contamination following
pasteurization. The test procedures are similar to those for the standard plate count
(SPC) except that the media is prepared by using violet red bile agar or desoxycholate
lactase agar. The plates are incubated at 90F (32C) for 24 hours (2 hours). Coliform
organisms appear as dark red colonies on the red agar media.
(3) Yeast and mold counts. An estimate of the yeast and mold count of dairy
products is sometimes desirable if the product is contaminated. Such counts are made
on manufactured dairy products, such as cheese, butter, condensed and dry dairy
products, and cultured dairy products such as cottage cheese. A positive test indicates
contamination after pasteurization. Yeast and mold are easily destroyed by
pasteurization; therefore, their presence in a finished product is an indication of
improper processing and/or plant sanitation. An acidified potato glucose agar is used to
prepare the media for yeast and mold growth. The plates are incubated at 75F (3),
which is 24C (2), for five days, and the colonies are reported as combined yeast and
mold count.
(4) Phosphatase test. The phosphatase test is a chemical test to determine
the efficiency of pasteurization. Phosphatase is an enzyme found in raw milk that is
inactivated when the proper time and temperature combinations are reached for
adequate pasteurization. Therefore, the presence of the phosphatase enzyme in a
pasteurized product is an indication of improper pasteurization or contamination of a
properly pasteurized product with raw product.
(5) Keeping Quality Testing (KQ). The Mosely Keeping Quality Testing, a
special program, will provide valuable objective indications of how well a plant's
sanitation program and pasteurization equipment function to preclude
post-pasteurization contamination. While no regulatory or contractual action can be
taken based strictly on this information, it is the best objective measure of plant
performance that we currently have available. Three samples of regular milk, lowfat, or
skim milk, are selected for submission to the laboratory. The samples must be
maintained at 40F or less until received by the laboratory. One sample of each line
MD0715
1-22