Dairy Ingredients for Food Processing

128 Chapter 5

during heat treatment. More commonly, both

disodium orthophosphate (DSP) and mono-

sodium orthophosphate (MSP) are used. DSP

and MSP have opposite effects on the pH of

the milk. Hence, the appropriate phosphate is

used to give the evaporated milk the pH of

optimum heat stability. The phosphate salts

also may limit binding of ionic calcium to

casein micelles and reduce the possibilities

for subsequent precipitation (Fotheringham

and Choat, 1979 ). Carrageenan is also added

as a thickener.

The heat stability of milk is infl uenced

by several compositional factors including

mineral (ash) content and protein and acid-

ity levels. Natural heat stability also varies

seasonally and is infl uenced by stage of

lactation.

Preheating

Preheating reduces the microbial load, inac-

tivates some indigenous milk enzymes, and

enhances heat stability (i.e., increasing the

resistance of the milk to coagulation during

subsequent sterilization.) In addition to the

primary purpose of increasing heat stability,

preheating also affects viscosity of the fi nal

product. Preheating is done in continuous

heat exchangers (plate or tubular) with long

holding times. The time and temperature

requirements are usually 93 ° C to 100 ° C

(199 ° F to 212 ° F) for 10 to 25 minutes or

115 ° C to 128 ° C (239 ° F to 262 ° F) for 1 to 6

minutes.

Concentration

Next, the heated milk is evaporated under

vacuum (typically using any of the multiple -

effect evaporator types described earlier).

Evaporation under vacuum exposes milk to a

pressure lower than atmospheric pressure and

its boiling point is lowered to approximately

45 ° C (113 ° F). Typical evaporation tempera-

tures used are not less than 45 ° C (seldom

the vapor over one to three effects. In TVR
evaporators, the heating medium in the fi rst
effect is the product vapor from one of the
next calandria, compressed to a higher tem-
perature by steam injection. Vapor generated
from each calandria is the heating medium in
the next, while vapor from the last effect is
condensed and may be used as boiler or
cleaning water or to preheat incoming air for
spray drying. For further reading, see Hess
(1993) , Cari c ́ (1994) , Walstra et al. (1999) ,
and Niro Inc. (2007).

Process for Manufacture of

Evaporated Milk

The manufacture of evaporated milk involves
the following steps:

1. Preparation (i.e., standardization, heat
   treatment, concentration/evaporation,
   homogenization, and cooling) of a con-
   centrated milk


2. Canning and heat sterilization of the
   concentrate


3. Cooling and storage

For good - quality evaporated milk, incom-
ing raw milk must be of good microbial and
organoleptic quality. Although the regulatory
limits for Grade A raw milk in the United
States are microbial counts less than 100,000/
ml, coliform counts not exceeding 10/ml, and
somatic cell counts of less than 750,000/ml,
most Grade A raw milk produced on dairy
farms in many regions of the United States
may be less than 10,000 colony - forming
units (cfu)/ml.
The raw milk is clarifi ed and fi ltered to
remove any debris or foreign matter and stan-
dardized to the desired fat content. For a
stable product and to prevent coagulation
during heat processing, and to minimize age
thickening during storage, the milk is heat
stabilized by adding small amounts of stabi-
lizing agents such as phosphates, citrates, and
bicarbonates to maintain pH 6.6 during 6.7