The most used
construction of roller drier in the dairy industry is an atmospheric double
drum drier made of cast iron. The product at 74-85oC
is pumped into the reservoir between the two drums. Milk is continuously
brought to the heated surface of the rollers where it is dried. Since the drum
speed, on an average at 14-19 rpm,is normally constant, the principal
adjustment is the steam pressure during operation.
This should be
changed with care to maintain desired 3.0-4.0 % moisture in the powder. The
temperature of the milk reaches almost the temperature of steam during drying.
After being dried, milk is continuously scraped off the roller by sharp knives in
the form of a thin film. Dry film, scraped off by the knives, falls on a spiral
conveyer that is along the side of each roller. In this procedure, the film is
broken into smaller particles and transported to the hammer milk for
pulverizing. The powder is usually sifted through a system of sieves to be
classified according to particle size. The product is usually packed in 25 kg
kraft bags with a plastic liner reasonable impervious to moisture vapours.
The quality of
roller dried milk powder depends on many factors. Higher drum temperature or
slower speed will cause scorching of the product despite insufficient drying.
In addition to the temperature and duration of drying, the temperature and concentration
of milk feed, the thickness of the milk layer over the rollers, and the uniformity
of the milk feed play a major role.
Direct contact of the pre-concentrated milk
with the hot drum surface results in some irreversible changes in the milk.
Caramelization of lactose, Maillard’s browning reactions, and denaturation of
proteins are the most important changes affecting the quality of roller dried
milk powder. Products of the Maillard type reactions(interactions of lactose
with the amino groups of lysine) may cause a scorched flavor of the powder.
Denaturation of proteins decreases the solubility of the powder. In spite of
these negative effects, roller drying process is used for the following advantages of this
process:
- Relatively low capital and operating costs
- Relatively smaller floor space requirement
- Economical processing, easy operation and maintenance
- Suitable for processing smaller quantities of milk
- Production of dried milk with longer storage life
- Advantageous in some applications: such as in the production of certain confectioneries or bakery products.
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