2009_Book_FoodChemistry

10.2 Dairy Products 527
Fig. 10.25. Production of condensed milk
45–50% of the weight of the end-product. Homogenization
and sterilization steps are omitted.
To avoid graininess caused by lactose crystallization
– the solubility limit of lactose is exceeded
after sucrose addition – the condensed milk is
cooled rapidly, then seeded with finely pulverized
α-lactose hydrate. Seeding ensures that the lactose
crystal size is 10 µm or less.
The critical quality characteristics of condensed
milk are the degree of heat damage (lysine degradation),
prevention of separation during the storage
life, absence of coarse crystallized lactose, as
well as color and taste. These criteria are influenced
not only by the process management (heat
treatment during evaporation and sterilization and
suitable selection of the homogenization temperature
and pressure), but also by the source of the
milk (feed) and the producer’s ability to maintain
hygienic conditions.
orated in a continuously operated vacuum evaporator
at 40 to 60 ◦ C. In comparison with previously
used circulation, riser, and flat-plate evaporators,
film evaporators are mainly employed today.
Several units (up to seven stages) are usually
connected in series, each unit being heated by the
vapor from the previous stage. The temperature
and pressure decrease from stage to stage. Optimal
energy utilization is achieved by mechanical
or thermal vapor compression. Fat separation
is prevented by homogenization at 40–60 ◦ C
(12.5–25 MPa). The resulting evaporated milk,
with a solid content of 24–31% or more, is homogenized,
poured into lacquer (enamel)-coated
cans made of white metal sheets, and is sterilized
in an autoclave at 115–120 ◦ C for 20 min.
Continuous flow sterilization followed by aseptic
packaging is also used. To prevent coagulation
during processing and storage, Na-hydrogen
carbonate, disodium phosphate and trisodium citrate
are incorporated into the condensed milk.
These additives have a dual effect: pH correction
and adjustment of free Ca 2+ ion concentration,
both aimed at preventing casein aggregation
(cf. Fig. 10.3). The additives are in the range of
0.2–0.8g/1.Theyarecontrolledbylaw.
In the production of sweetened condensed milk,
after a preheating step (short-time heating at 110–
130 ◦ C), sucrose is added to a concentration of
10.2.5 Dehydrated Milk Products
Skim milk powder and whole milk powder are
used either for the reconstitution of milk in countries
that for climatic reasons have no dairy farming
or as intermediate products for further processing
into infant milk products, milk chocolate
etc. The quality of these instant products depends
on the durability, redissolution capacity (cold and
warm), taste, microbiological characteristics, and
preservation of essential constituents (proteins,
vitamins) during production.
The main drying process used is spray drying.
However, drum drying (with and without vacuum)
and fluid-bed drying (foaming with inert
gas N 2 or CO 2 ) are used for special purposes.
Freeze drying offers no particular advantages
over the less expensive spray drying process and
is only of interest for special products.
Using film evaporating systems, the milk is first
preconcentrated to 30–55% solids.
In drum drying, the liquid (30–40% solids) is applied
in a thin layer to a heated drying cylinder
(100–130 ◦ C) and, after a defined residence time
(rotation, 2–3 s), removed with a scraping knife.
The liquid film can be applied in various ways.
In drum drying, relatively large particles are obtained.
The thermal exposure (temperature, time)
is considerably higher than in spray drying, which