Module 6. Common dairy operations

Lesson 18

18.1 Introduction

During handling of milk on farm and its transportation, certain visible particles and dirt may gain access into the milk which may be removed by either filtration or centrifugal clarification. Filtration/clarification equipment has been designed for both cold and warm milk. Since fluidity of warm milk is more, its separation process is more efficient. However, warming of milk for this purpose requires additional equipment. It also poses the risk of bacterial growth unless handled properly. Handling at higher temperatures may also affect creaming property of the milk besides dissolving some of the extraneous matter.

18.2 Pre-heating

This term refers to heating of milk before the operation which follows immediately. The milk is pre-heated to about 35-40°C using plate or tubular heater for efficient filtration/clarification. Pre-heating becomes essential, if the incoming milk is cold. As the temperature of the milk increases, the viscosity of milk decreases resulting in more efficient filtration/clarification.

18.3 Straining

The practice of straining milk was introduced to remove some of the large particles of foreign material such as straw, hair, insects, grass, dirt, flies, etc., so that the visible sediment in milk might be reduced. The straining in the ordinary sense is accomplished on the dairy farm by means of pieces of cloth, cotton, wire gauge or specially prepared strainers/strainer pads.

18.4 Filtration

Filtration of milk is carried out to remove visible sediment (foreign matter) from the milk to improve the aesthetic quality of milk. This may be removed either by filtration or centrifugal clarification. While filtration removes suspended foreign particles by straining process, clarification removes the same by centrifugal force. There are two types of filters or clarifiers viz., those that operate with cold milk and those that operate with warm milk. The advantages of filtration are that preheating is not essential and there is less likelihood of soluble dirt going into the solution. However, the major disadvantage is the flow of milk is slow. Broadly there are two types of filters.

18.4.1 Tubular sieve

It removes dirt by sieving and is placed in the inlet pipe in the processing section. This permits the removal of coarser particles only.

18.4.2 Important features of a filter
  • A filter cloth or pad of the desired pore size is used that can retain the smallest particle.
  • A frame or support to compress and hold the margins of cloth or pad, so that milk can pass through pores
  • A perforated metal or other support for the cloth or pad which will not tear or break under the pressure of milk
  • An enclosure to confine both the filtered and unfiltered milk in closed system fitted suitably with inlet and outlet connections for sanitary piping
  • A continuous operation is essential to handle large volumes of milk; 2 or more filters may be used without interruption.
For cold filtration, an in-line filter may be installed in the milk receiving line between the raw milk dump tank, unloading pump and chiller or raw milk storage tank. Warm milk filters may be installed in the pasteurization circuit.

In order to achieve the desired filtration effect, the filter material must have pores of 25 - 100 µ. The smaller the pores, the greater are the separation effect and filtration time. For a filtration installation with a flow of 10,000 L/h, we can assume 1 - 2 h time with pores size of 45 µ and pores size of 100 µ up to 10 h. Changing of filter after every 6 h of operation is recommended.

The filter consists of stainless steel body wherein a filter with a small pore nylon cloth is placed and closed with a tight fitting lid (Fig18.1). Milk passes from the top to bottom. After 3 - 4 h of operation, the filter bag must be cleaned. For a continuous process, a double filter must be installed. This would enable cleaning of one filter while the other is being used. The flow of milk when using such type of filter can be up to 15000 L/h.

Fig. 18.1

Fig. 18.1 Milk filter

18.5 Clarification

Clarification is more efficient than filtration for the removal of dirt and foreign matter from milk. Clarification removes leucocytes, udder tissues, other large cells and fine dirt. The objective of clarification is to improve the appearance and marketability of milk.

18.5.1 Milk clarifier

In general, the clarifiers are quite similar to cream separator. The difference between clarifier and cream separator are as follows:

  • In clarifiers there is only one inlet and one outlet. In cream separator there is one inlet and two outlets (cream and skim milk).
  • The discs in the clarifier are smaller in diameter to provide a large space for the accumulation of sludge than separators.
  • The milk distribution holes are at the outer edge of the disk in the clarifier but near the axis of rotation in the separators.
18.5.2 Clarification process

In a clarifier, the ratio of bowl diameter to disc diameter is greater than in cream separator, resulting in a larger sludge space. In most clarifiers, 10 - 20 discs have no holes and on their surface are 6 - 12 baffles with a thickness of 1-3 mm which are evenly spaced. These baffles determine the disc distance and influence the flow pattern. Raw milk is pumped through central pipe through the rotating bowl via a distributor and passed through the small opening into the sludge area. The dirt separation takes place in the disc assembly where milk is enclosed by two adjacent baffles between discs. One baffle leads to rotation whereas the other baffle leads to the flow to the center. The dirt particles which have a higher specific gravity are separated by the centrifugal force. The clarified milk rises to the outer surface of the distributor and reaches the baffle and ejects out. The pressure is about 5.4 bars. The amount of sludge is influenced by:

  • The amount of foreign matter
  • Condition of the udder
  • Stage of lactation
  • The bacterial count and acidity of the milk
  • The clarifying temperature
  • The speed of the bowl
  • Amount of milk run through the bowl
A clarifier may be operated depending on the size of the machine, for a period ranging from 2 - 8 h for cold milk (5 - 10°C) and 1-4 h for warm milk (57°C), without cleaning.

18.5.3 Location of clarifier

Clarifiers can be located in any one of the following ways: Cold clarification
  • Between the storage tank and the pasteurizer
  • Between the receiving room and the storage tank. This arrangement is applicable only in those plants where a steady receiving operation is maintained. Warm clarification
  • Between the pre-heater and the pasteurizer.
  • Between the regeneration section of high temperature short time (HTST) pasteurizer and heating section.
  • Between the final heater and the holding tube of the HTST pasteurizer. Difference between cold and warm milk clarification

a) Cold milk clarification
  • Efficiency is lower since viscosity of milk is high.
  • Operation time is more because less free casein particles are thrown out as slime.
  • Quality of the processed milk is better as dirt is removed from the milk.
b) Warm milk clarification
  • Efficiency is higher due to lower viscosity of milk.
  • Running time is reduced because of rapid free slime build-up
  • Operation of the entire system is more critical with a high pressure drop through the clarifier on one side of the pump and suction on the other side.
18.5.4 Factors affecting clarification Viscosity

The viscosity of milk is an important factor to be considered in clarification, since the suspended foreign particles are removed by centrifugal sedimentation. Settling of particles by centrifuging depends upon their size, density and viscosity of the fluid in which they are suspended. Temperature

In general, the viscosity of liquid decreases as the temperature increases. Hence, milk is usually heated to a temperature of 32-35°C before it is subjected to clarification. A high temperature must be avoided as it adversely affects the creaming property of milk. Bowl speed

The higher the speed, the better is the efficiency as the centrifugal force is directly influenced by the speed of the bowl. Microbial load

The type and state of microbe influences the efficiency of clarification. The bacterial spores being denser are thrown into the slime more easily.

18.5.5 Effect of clarification on the bacterial quality

Since a large number of microbes are thrown into the clarifier ‘slime’, there will be fewer microbes in clarified milk. Due to the breaking up of the clumps of bacteria during the process, there may be an apparent increase in the plate count of clarified milk. Hence, it apparently seems that the plate count of milk is higher after clarification, while the actual number of organisms in the clarified milk is considerably lower.

Table 18.1 Composition of clarifier slime

Table 18.1

Separator slime is usually considered to be identical with clarifier slime. The removal of slime does not affect the composition of milk, since the loss of solids resulting from clarification is usually < = 0.01%. Neither filtration nor clarification improves the keeping quality of milk.

Last modified: Tuesday, 9 October 2012, 10:19 AM