Dairy Ingredients for Food Processing

114 Chapter 4

Figure 4.12. The complete process for in - line standardization of milk and cream. 1, density transmitter; 2, fl ow
transmitter; 3, control valve; 4, control panel; 5, constant pressure valve; 6, shut - off valve; 7, check valve.
Reproduced with permission from Tetra Pak.

1 2

3

3

5

4

6

2

7

2 Standardized

milk

Whole milk

Standardized

surplus cream

The continuous pasteurization process
is known as high - temperature - short - time
(HTST) pasteurization and entails heating
milk to 71.5 ° C (161 ° F) and holding the milk
for a minimum of 15 seconds prior to cooling
and storage. Yogurt manufacture necessitates
the holding of milk for longer periods of time
to denature the whey proteins and thus
improve the gel strength of yogurt. Therefore,
in yogurt manufacture milk may be held at
71 ° C for 30 minutes or it may be heated to
90 ° C (194 ° F) and held for 10 minutes. The
HTST process involves plate heat exchang-
ers; the PMO has prescribed various controls
and requirements for the equipment.
The effect of heat treatment on milk is to
reduce the rate of deterioration due to micro-
bial and enzymatic action. In addition, the
milk may look whiter, appear more viscous,
and have appreciable fl avor changes and a
decrease in nutritive value. The effectiveness
of pasteurization is estimated by assaying for
the enzyme phosphatase. No phosphatase

activity is detected in fresh properly pasteur-

ized milk. Sometimes microbial phospha-

tases or the milk phosphatase itself can regain

some of its activity during storage. If the

presence of phosphatase is detected in stored

pasteurized milk, further tests are often con-

ducted to determine the cause of this positive

test.

In the HTST pasteurization process

(Figure 4.13 ), cold milk enters a balance tank

with a fl oat valve. The balance tank (also

known as a constant level tank) maintains a

constant level of milk in the plate heat

exchanger, because the pasteurizer should be

fi lled at all times during operation to prevent

the product from burning onto the plates. The

balance tank may be fi tted with an electronic

sensor that transmits a signal to the fl ow

diversion valve. If the level in the balance

tank drops below a certain level and fresh

milk is does not come in to make the level

up, this electrode transmits a signal for the

fl ow diversion valve to open and to return the