10.2 Dairy Products 527Fig. 10.25.Production of condensed milk
orated in a continuously operated vacuum evap-
orator at 40 to 60◦C. In comparison with previ-
ously used circulation, riser, and flat-plate evapo-
rators, film evaporators are mainly employed to-
day. 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. Op-
timal energy utilization is achieved by mechan-
ical or thermal vapor compression. Fat separa-
tion 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 ho-
mogenized, poured into lacquer (enamel)-coated
cans made of white metal sheets, and is steril-
ized in an autoclave at 115–120◦C for 20 min.
Continuous flow sterilization followed by asep-
tic packaging is also used. To prevent coagula-
tion during processing and storage, Na-hydrogen
carbonate, disodium phosphate and trisodium cit-
rate are incorporated into the condensed milk.
These additives have a dual effect: pH correc-
tion and adjustment of free Ca^2 +ion concentra-
tion, 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 ofsweetened condensed milk,
after a preheating step (short-time heating at 110–
130 ◦C), sucrose is added to a concentration of
45–50% of the weight of the end-product. Ho-
mogenization and sterilization steps are omitted.
To avoid graininess caused by lactose crystalliza-
tion – 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 lac-
tose crystal size is 10 μm or less.
The critical quality characteristics of condensed
milk are the degree of heat damage (lysine degra-
dation), prevention of separation during the stor-
age life, absence of coarse crystallized lactose, as
well as color and taste. These criteria are influ-
enced not only by the process management (heat
treatment during evaporation and sterilization and
suitable selection of the homogenization tempera-
ture and pressure), but also by the source of the
milk (feed) and the producer’s ability to maintain
hygienic conditions.10.2.5 Dehydrated Milk ProductsSkim milk powder and whole milk powder are
used either for the reconstitution of milk in coun-
tries that for climatic reasons have no dairy farm-
ing or as intermediate products for further pro-
cessing 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 vac-
uum) 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 ap-
plied 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 ob-
tained. The thermal exposure (temperature, time)
is considerably higher than in spray drying, which