Filtration and Clarification of Milk

i. Purpose

Raw milk as produced on the farm and transported to the collection centre or a dairy plant generally contains varying amounts of visible, invisible impurities. This foreign matter includes straw and hair pieces, dust particles, leukocytes (somatic cells or white blood cells), insects, etc. If not effectively removed, such extraneous insoluble matter can result in deposits in milk handling equipment such as cooler,etc., and, more importantly, cause unsightly appearance.

Relatively large pieces of such material e.g. straw, hair and insects, are usually removed by ‘straining’ (passing the milk through a fine metal–gauge strainer or metallic sieve on the farm, at the collection centre or at the processing plant.Tubular sieves located in the milk inlet pipe to the processing unit (e.g. pasteurizer) are also used.

However, finer foreign matter to be eliminated requires clarification using a special filter or a centrifuged clarifier. These steps of aesthetic improvement of product are particularly useful for overcoming the problem of sediments in fluid milk and liquid milk products in general, and homogenized milk in particular.

 

ii. Filtration

Filtration (or, clarification using a filter-bag) refers to making the milk pass through a filter-cloth or filter-pad. The filtering medium has a pore size (25-100 mm) that permits most of the foreign matter to be retained on it. The milk filter consists of a nylon filter-bag or a filter-pad supported on a perforated stainless steel (SS) support held in an SS enclosure with a tight-fitting lid, milk distributor, and inlet- and outlet- connections. Milk usually passes from top to bottom. In case of twin filters,three way valves in the inlet and outlet lines enable switching from one filter to the other when the first is to be cleaned. Sometimes, filters may be provided in the form of cylindrical bags or ‘stockings’ fitted over perforated SS tubes as in the modern continuous pasteurizing plants (high-temperature short-time, or HTST pasteurizers

Filtration can be carried out either on cold milk (about 10oC) or warm milk (40-45oC). Since warm milk filtration is more rapid due to lower viscosity of warm milk,it is universally used. For cold filtration, the filter is located in the line connecting the milk receiving tank or holding tank and the pasteurizer. Since warm filtration requires preheating, the filter of this type is placed between the regenerator and the final heating section of the HTST pasteurizer.

The filter-bag must periodically be cleaned. Accordingly, the operation run may vary from 2 to 10 hours depending on the level of foreign matter and the filter pore size. Generally, twin filters located in parallel are employed to permit cleaning of one filter while the other is in use. This enables continuous process run.We should be able to realize that filtration removes only the gross impurities, and does not remove bacteria from milk. Accordingly, it does not improve the keeping quality of the milk. In fact, bacteria may grow in the filters if they are used for unusually long times before cleaning.

 

iii. Clarification

Definition and objective : As an alternative to filtration, clarification can also be employed to remove insoluble impurities especially the finer ones. It involves the use of a centrifugal machine called ‘clarifier’. Thus, clarification is a process of subjecting milk to a centrifugal force in order to eliminate the finer but heavier particles from milk, somatic cells, dust particles, etc. Although part of bacteria are also removed along with the extraneous matter, clarification cannot be considered an effective means of bacteria removal. Hence, one should be aware that it cannot be a substitute for a suitable heat treatment in order to ensure safety against pathogenic (disease-causing) microorganisms.

Principle of clarification : As we have studied, when milk is introduced between two adjacent rotating conical discs (in a stack of several discs) of a centrifuge bowl, it is subjected to a centrifugal force. This force causes the heavier dirt particles to be thrown out into the sludge space surrounding the discs where it is collected during the run, while the comparatively lighter milk continuously flows inward and upward to the outlet. There is no separation of fat globules (cream) and skim milk in a clarifier.

Operation of a clarifier : Raw milk is made to pass usually under a pump pressure, down a central pipe of a rotating bowl and led to the outer edge of the clarifier discs through a distributor in the bottom and then onto the spinning discs,where milk and dirt are separated. The milk is led to the discharge port at the top of the bowl whereas the dirt is accumulated in the sediment space. The accumulated sludge is removed from the bowl by dismantling the clarifier at regular intervals.The interval may range from 1 to 8 hours depending on size of the clarifier and the amount of impurities in the milk. However, most large-size modern clarifiers are self-desludging or ‘partial desludging’ type in which periodical sludge removal takes place during the clarification process, without interruption of the clarifier operation.Such desludging results in about 0.05-0.10% of milk being lost and the sludge being liquid rather than solid as in the non-self-desludging machines.As for the milk filter, clarifier may be located in the raw milk line between the raw milk tank and pasteurizer. Alternatively, milk may be clarified warm/hot by placing the clarifier at a suitable point in the regeneration section of the HTST unit or between the regeneration and heating sections.The clarifier sludge or clarifier ‘slime’ consists primarily of dust and dirt particles,blood cells, microorganisms and milk protein. Its composition will depend on whether it is liquid (82-86% water, 6-8 % protein), or solid (65-69% water, 24-28% protein).