Milk is a highly perishable food item. If left alone, milk is fermented or
putrefied rapidly by micro-organisms especially bacteria. Various methods like
boiling, pasteurisation, sterilisation etc are used to preserve milk for a
longer period.
Boiling
Milk contains 87 per cent water, 4 per cent proteins and 5 per cent lactose
(milk sugar). When we boil milk, the fat, sugar, proteins and minerals get
separated. Since they are lighter than milk they collect on the surface in the
form of cream.
During heating some amount of water gets converted into vapour and the bubbles
of water vapour rise to the top but the heat is conducted away by the layer of
water and by the fat droplets that have a higher boiling point than water.
The vapour gets trapped in the creamy upper layer. As the milk is heated further
the water vapour expands and thick foam is produced on the top.
As the milk is boiled continuously the water, which boils at 100 degrees
Centigrade, produces more water vapour and pressure builds up in the boiling
milk so that the vapour pressure raises the creamy layer. So the milk pushes the
creamy layer out and milk spills out.
Condensed milk and Fermentation
fermented milk whole or skim milk curdled to beverage or custardlike consistency
by lactic-acid-producing microorganisms. Many forms of fermented milk were used
by early nomadic herders, especially in Asia and S and E Europe, Scandinavia,
Africa, and South America. Such milks are believed to have medicinal value in
the control of intestinal fermentation by contributing bacteria that aid in
digestion. Fermented milks include acidophilus milk; cultured buttermilk; kumiss
(koumiss), probably originated from mare's milk by western Mongols, effervescent
and of acrid flavor and containing alcohol produced by yeasts; the similar kefir
of Central Asia; yogurt, similar to the Armenian matzoon; and the Scandinavian
beverages, kaeldermaelk and filbunke.
Drying of milk
This is Drying milk and reducing it to a powdered concentrate. It can be found
in two forms, regular and instant. They are both made from milk in a
spray-drying process, but the instant variety has been given further processing
to make it more easily soluble in water than regular dry milk. Both types have
the same nutrient composition. The regular variety is more compact and requires
less storage space than the instantized variety, but it is more difficult to
reconstitute. The most easily found variety is the instant, available in nearly
any grocery store. The regular variety has to be sought out from baking and
restaurant suppliers and storage food dealers. There is a retail brand by the
name of "Milkman" that has a bit of fat content that makes it similar to 1%
milk. The fat content means it should be stored like whole milk,
Pasteurisation
Along with correct cooling, pasteurisation is one of the most important safety
measures in milk processing. Pasteurisation is a heat treatment that destroys
unwanted or disease-causing bacteria without reducing the nutritional quality of
milk. If carried out correctly, pasteurisation will ensure milk quality and a
longer shelf life. All Pura milk is pasteurised.
Pasteurisation temperature and time are the two most important factors which
must be specified precisely in relation to the quality of the milk and its shelf
life requirements. For example, High Temperature Short Time (HTST) pasteurised
milk is usually heated at 72°C - 75°C for 27 seconds.
While pasteurisation kills harmful bacteria in milk, as soon as you open a milk
container at home, new bacteria are introduced to the milk. Eventually, these
will grow large enough in number to cause the milk to go 'off'.
Sterilisation
A process for high-temperature sterilisation of products such as milk and the
like comprises the steps of directly injecting steam into the product for
heating it to a lower temperature than the sterilisation temperature, making the
product stand for a definite period of time, making the product pass into an
infuser to further heat it to the sterilisation temperature, keeping it to this
temperature for a predetermined period of time, admitting the product to a
vacuum chamber for sudden cooling of same. It kills all micro-organisms and
spores. The main disadvantages are the vitamin content is reduced as well as the
biological value of protein becomes low.
