(1) Curds and Whey. The liquid portion of milk left after curd formation is
termed whey. Curds and whey are often used to make other dairy products under
controlled processes. After pasteurization, the lactic acid bacteria are used to ferment
the milk. These fermented dairy products--that is, buttermilk, cottage cheese, sour
cream, and yogurt--are called cultured dairy products.
(2) Buttermilk and Sour Cream. Cultured buttermilk is made by fermenting
skim milk; sour cream, by fermenting milk, followed by separation, consolidation, and
conditioning the curd.
PERIODIC TESTS OF MILK
There are various laboratory tests performed on milk to indicate its quality and
a. The Phosphatase Test. A very sensitive, practical method used to
determine whether milk has been properly pasteurized is known as the "phosphatase
test," which consists of quantitatively determining the activity of the enzyme
phosphatase in a sample of milk. Phosphatase itself is an enzyme that is always
present in raw milk but is destroyed by the heat if pasteurization is effective. This
enzyme is more resistant than any pathogenic bacterium to pasteurization. The test
itself, then merely decides if the milk has been heated adequately during the
pasteurization process by determining if the enzyme phosphatase has been inactivated.
b. The Standard Plate Count Test. The standard-plate count test is given to
indicate whether the milk is considered Grade A or not is the standard plate count test.
This gives an indication of the approximate number of bacteria in the milk. Obviously,
very clean milk will have lower bacterial counts than milk that has been collected or
handled under unsanitary conditions. The count is carried out by diluting samples of
milk to be tested and mixing the measured quantity with a melted nutrient medium.
After incubation, the original counts per milliliter (ml) of milk are determined by
multiplying the colony count by the dilution factor. For example, 1 ml of a 1:100 dilution
of milk is mixed with a nutrient medium and incubated. If 30 colonies appear on the
plate, the standard plate count for the sample of milk is 3,000. In order for milk to be
classified as Grade A, the raw milk must not exceed a bacterial growth count of more
than 100,000 per milliliter. The same milk after pasteurization cannot exceed a count of
over 20,000 per milliliter.
c. The Coliform Test. Another important test run of milk is the coliform test. By
coliform, we mean the organisms Escherichia coli and Enterobacter aerogenes, both
being normal inhabitants of the large intestine. Their presence indicates fecal
spread of enteric diseases. These organisms are common sources of acute diarrhea.
The laboratory method is quite complex but can be summarized by the following
description. Milk is measured into small quantities and inoculated onto media that have
indicators for coliform bacteria. Certain substances in the media inhibit the growth of