is standardized by adding skim milk to whole raw milk to reduce the percent of milkfat,
by adding cream to milk in order to raise the percentage of milkfat, or by passing the
raw milk through a separator to produce standardized milk. Milk procured for the Armed
Forces must contain a minimum of 3.25 percent milkfat.
Separation is the process of removing the cream from whole milk, leaving skim
milk. Separation is more efficient when the milk is warmed, usually between 90 to
100F (32 to 38C), prior to the separation. The separator operates on the principle of
centrifugal force whereby the lighter cream is separated from the heavier milk. The
cream and the milk are removed from the separator by different outlets. Most
separators have an adjustment device by which the percent of milkfat in the cream
removed from whole milk may be regulated.
Homogenized milk is whole milk in which the fat globules are reduced in size and
are evenly dispersed throughout the milk so that after 48 hours of quiescent storage, no
visible cream separation occurs. The milkfat percentage in the top 100 cc will not vary
more than 10 percent from the milkfat percentage in the remaining portion of a 1-quart
bottle of milk. Homogenization is accomplished more efficiently when the milk is warm.
The homogenizer operates by using high pressure to force milk through a minute orifice.
Homogenizers are operated at pressures of 2,000 to 3,000 pounds per square inch.
Milkfat particles normally ranging in size from 1 to 18 microns in diameter (averaging 7
microns) are reduced in size to an average diameter of 1 1/2 microns.
Pasteurization is a process of heating every particle of milk to a definite
temperature for a specific period of time. It is the most effective method of destroying
pathogenic bacteria without materially altering the flavor or consistency of the milk. The
Grade A Pasteurized Milk Ordinance (PMO), U.S. Public Health Service, defines
pasteurization as the process of heating every particle of milk or milk product to at least
145F (63C) and holding it continuously at or above this temperature for at least 30
minutes; or to at least 161F (72C) and holding it continuously at or above this
temperature for at least 15 seconds; or to one of the temperatures given in Table 1-1
and held continuously at or above that temperature for at least the corresponding
specified time. In addition, milk products that have a higher milkfat content than milk or
contain added sweeteners (cream, half-and-half, chocolate flavored milk) must be
heated to at least 150F (66C) and held continuously for at least 30 minutes in the vat
method; or to at least 166F (74.4C) and held continuously for at least 15 seconds in
the high-temperature, short-time (HTST) method. Diagrams of two types of
pasteurizers are shown in figures 1-1 and 1-2. (Figure 1-3 shows the milk flow through
the HTST pasteurizer.) The time and temperature combination of four types of
pasteurizers are shown in table 1-1.