Dairy Ingredients for Food Processing

Concentrated Fluid Milk Ingredients 129

used to standardize of the fat:SNF ratio. The

salt balance in the milk, hence pH, also is

adjusted with stabilizing salts to ensure that

the product withstands further intense heat

treatment. Because the salts are added in

aqueous form, the milk is concentrated to

higher TS content so that re - standardization

and stabilization is used to bring the solids

content to desired levels.

Packaging

The most common method for packaging

evaporated milk is canning with a suitable

fi lling machine with lid seaming. Typically

the can is manufactured with a lid that has a

center hole (diameter 2 to 3 mm), through

which the can is fi lled. Necessary precautions

are taken during fi lling to prevent foaming.

After fi lling, the can is immediately sealed

hermetically (i.e., avoiding the passage of air

or fl uid in either direction) by soldering.

Because the milk is cold before it is fi lled in

the can, suffi cient head space equal to

approximately 10% of the can volume must

be allowed for product expansion during ster-

ilization. A large head space may result in

excessive foaming, clotting, and brown

deposits forming around the corners of the

can. Hence, the head space can be partially

evacuated by the injection of steam into the

head space of the can at the time of closing.

A suitable vacuum level is 2 to 4 inches of

Hg (6.773 × 1 0^3 to 1.355 × 1 0^4 Pa) in the can

after cooling.

Sterilization

The canned product is sterilized in situ at

115 ° C to 121 ° C (239 ° F to 250 ° F) for 15 to

20 minutes followed by cooling for about 15

minutes to 25 ° C to 30 ° C (77 ° F to 86 ° F). This

is called in - container sterilization. It can be

done in batches or continuously. Sterilization

kills bacterial spores and inactivates heat -

stable indigenous milk enzymes such as

exceeding 54.5 ° C; 130 ° F) to prevent growth
of microorganisms. Evaporation under partial
vacuum also helps prevent undesirable
changes to milk components (e.g., heat
damage to milk proteins). The milk is con-
centrated to 30% to 40% TS. Although con-
centration by evaporation is the most common
method, the milk also may be concentrated
by reverse osmosis, a membrane fi ltration
process used in the dairy industry to remove
water from milk and whey at lower energy
costs compared to evaporation.

Homogenization

Next, the milk is homogenized at high pres-
sure, i.e., 15 to 25 MPa (200 to 250 bar) fi rst
stage and 5 to 10 MPa second stage. The pre-
ferred homogenization temperature matches
the evaporation temperature of more than
45 ° C (113 ° F). Homogenization prevents
creaming and coalescence. It breaks down
the fat globules from an average size of 3 to
5 μ m or larger into smaller sizes of less than
1 μ m, resulting in improved color (natural
white to light cream color) and stability of
the milk. However, excessive homogeniza-
tion pressure may result in an irreversible
destabilization effect and reduction in heat
stability of the product. Following homoge-
nization the product is cooled and placed into
storage, where the fi nal standardization of
composition takes place.

Second Standardization

and Stabilization

During the second standardization and stabi-
lization, the total solids content in the product
is readjusted to meet required standards of
the fi nished product. The process of standard-
ization can take several hours because the
evaporated milk in storage tanks must be
analyzed prior to standardization. Water,
skim milk, evaporated milk, retentate, per-
meate, and homogenized cream are often