Fresh raw milk is free from contamination till it is inside the animal’s udder. However, as soon as the milking takes place, the milk comes in contact with the environment and gets contaminated with microorganisms from air, falling cow hair, or straw and insects from the surroundings. This needs to be clarified before processing in order to remove the visible and invisible extraneous matter. There are two processes employed for the purpose. For the removal of visible extraneous matter like hair, insect, and straw, the technique employed is FILTRATION, whereas, for the removal of non-visible foreign matter like sediments and blood cells, the CLARIFICATION technique is used. It overcomes the problem of sediments in fluid milk products in general and homogenized milk in particular.
In filtration, there is a filter bag/ filter pad, of pore size 25-100μm, mounted on stainless steel support, held in a stainless steel enclosure. The sieve retains foreign matter in itself, thus making the milk appear clean.
However, the milk still has bacteria, and white blood cells, along with fine dirt particles that need to be removed. For this purpose, clarification is done.
Clarification is the process of subjecting milk through a centrifugal force in a machine called a clarifier in order to separate the finer but heavier dirt and foreign matter from milk. Though some bacteria also get separated away during clarification, it is still not an effective method for the removal of microorganisms. For that, methods like bactofugation have to be applied. The objective of clarification is also to improve the aesthetic appearance and marketability of milk.
Principle of Clarification- When the milk is introduced between two adjacent rotating conical discs of a centrifuge bowl (in a stack of several discs), it is subjected to centrifugal force. Due to this force, the heavy dirt particles are thrown into sludge space, surrounding the disc. The comparatively lighter milk flows inward and upward to the outlet.
How does a clarifier operate?
Raw milk falls under pump pressure, down a central pipe of a rotating bowl, and is then led to the outer edge of the clarifier disc through a distributor in the bottom.
Then they enter the spinning discs, enclosed by baffles between them. One baffle leads to rotation, whereas the other baffle leads to the flow to the center.
The dirt particles having specific gravity above that of milk, get separated from the milk and fall off into the sludge space.
The clarified milk rises to the outer surface of the distributor, reaches the baffle, and ejects out. The pressure is about 5.4 bars.
Cleaning- The accumulated sludge needs to be removed from time to time from the bowl by dismantling the clarifier. These days, however, large-sized self-desludging clarifiers are available in which periodical sludge removal takes place during clarifier operation. The clarifier sludge basically consists of dust and dirt particles, blood cells, microorganisms, and milk protein.
Location of Clarifier- The position where clarifier will be placed in the milk processing depends upon the type of clarification employed, i.e., Cold clarification, or Warm clarification.
In cold clarification- A clarifier is placed between the storage tank and the pasteurizer. Or the clarifier is between the receiving room and storage tank in plants where steady receiving operation is maintained.
The efficiency of clarification is low, as the viscosity of milk is low (Due to the milk being cold).
Operation time is also more as the casein particles are thrown out of the milk.
In Warm clarification- A clarifier is placed between the preheater and the pasteurizer. It may also be placed between the regeneration section of the HTST pasteurizer and the heating section.
It can also be placed between the final heater and the holding tube of the HTST pasteurizer.
The efficiency is high due to low milk viscosity.
Running time is also reduced as the slime build-up is rapid.
Factors affecting clarification-
1. Viscosity- The setting of particles by centrifuging depends upon the size, density of particles, and viscosity of the fluid in which they are suspended.
2. Temperature- The viscosity decreases as the temperature increases, which leads to better, and faster clarification.
3. Bowl Speed- The higher the speed, the better is the efficiency because centrifugal force is directly influenced by the bowl speed.
4. Microbial Load- The type of microbes and the state of microbes decide the efficiency of clarification. For example- The bacterial spores being denser are thrown into the slime more easily.
Now, after reading the information, you must be aware of the action of clarifiers in the milk processing line. It, however, must be kept in mind that neither filtration nor clarification is used to improve the keeping quality of the milk. They are used to improve aesthetic appeal and marketability.
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