Homogenization of milk

HOMOGENIZATION OF MILK

• Homogenization is a mechanical process in which milk is forced through a homogenization valve under very high pressure. The milk is thus deflected at right angles through a narrow opening of about 0.1 mm (100(m). As the milk comes out of this valve opening, there is sudden drop in pressure and the milk is subjected to impact against an impact ring. This complete process results in disruption of fat globules leading to decrease in the average diameter (typically from 0.2 to 2 (m) and an increase in the number and surface area of fat globules.
• Homogenization with reference to milk / dairy applications thus refers to a mechanical process that is used to reduce the size of fat globules such that milk fat does not rise to form a cream layer during storage of milk. Although homogenization renders fat globules uniformly distributed in the body of the milk, upon prolonged storage it does not remain completely dispersed.

Definition of Homogenized Milk

• United State Public Health Service has proposed one of the most comprehensive definitions for homogenized milk. It states that Homogenized milk is milk which has been treated in such manner as to ensure break-up of the fat globules to such an extent that after 48 hours of quiescent storage no visible cream separation occurs in the milk and the fat percentage of the milk in the top 100 ml of milk in a quart bottle (946ml), or of the proportionate volumes in containers of other sizes, does not differ by more than 10 per cent of itself from the fat percentage of the remaining milk as determined after thorough mixing.

Principle of Homogenization

• The principle underlying the process of homogenization is to subject the fat globule to enough severe conditions, which disrupts it into smaller globules. The newly formed fat globules are maintained in dispersion for sufficient time to allow milk fat globule membrane (MFGM) to be formed at the fat-serum interface. As the fat globules are subdivided into smaller globules, there is increased surface area of the newly homogenized fat globules. The original milk fat globule membrane (MFGM) material is not sufficient to cover this. Proteins, particularly casein micelles migrate from serum phase to form new membrane material with the existing MFGM. This may result in sharing of the casein micelles and therefore some aggregation of fat globules could take place thus defeating the purpose of homogenization. A second stage homogenization therefore, becomes essential at reduced pressure (almost 20% of the first stage pressure (175 kg/cm2) or upto 35 kg/cm2). This enables aggregated fat globules to be disrupted for formation of stable emulsion of finely dispersed fat globules.

Advantages and Disadvantages of Homogenized Milk

Advantages

1. Prevents removal of fat/cream from milk
2. Homogenized milk results in softer curd and therefore easily digested by infants
3. Churning of fat does not occur during bulk transportation
4. Fat is uniformly distributed and therefore gives uniform consistency
5. Homogenized milk is comparatively resistant to development of oxidized flavour defect

Disadvantages

1. Homogenization offers possibility of incorporation of foreign fat into milk
2. Homogenized milk is prone to development of sunlight or activated flavour defect
3. Homogenized milk if returned unsold from the market is difficult to salvage as centrifugal separation of fat is not possible

Viscolised Milk

• Viscolised milk refers to a product, which has unusually deep cream/fat layer resulting from admixing of homogenized cream, skim milk and/or whole milk. The homogenized fat forms very loose clumps with the unhomogenized fat globules and rise to the surface giving an appearance of deep cream layer. The unfair traders, who first separate the cream, homogenize it and then remix with the skim milk sometimes practice it. This gives the remixed milk a very rich and creamy surface appearance and thus deceives the consumers.

Design and Operation of Homogenizers

• There are several types of homogenizer valves and therefore designs of homogenizers vary depending on the manufacturers. Homogenizers essentially consist of two components  a piston pump to generate high pressure and a Homogenizing valve.The homogenizer pump is generally a positive displacement pump with at least three and sometimes five or seven pistons, which operate consecutively to generate steady pressure. The pump block is generally made of stainless steel but the piston seal rings are of a soft composite material. Homogenizer valves, used for milk may be either a poppet type or ball type. A poppet design has relatively large contact surfaces and provides close fitting seal. Ball valves can exert greater pressure on the much small seal area and are therefore, suitable for high viscosity liquids. Milk from high pressure manifold enters into the centre of the valve seat.  The internal diameter of the valve seat is smaller than the manifold. As it passes into the narrow gap between the fixed and the adjustable faces of this valve, milk velocity gets accelerated. Liquid which passes across the valve at about 200-300 m s-1 suddenly drops in pressure to below saturation vapour pressure. This permits microscopic bubbles to form for a few microseconds before collapsing. The high velocity jet of milk then impinges on a perpendicular impact ring. These effects contribute to the disruption of the fat globules.

Checking the Efficiency of Homogenization

• It is performed by subjecting a specified volume (one quart) of milk to quiescent storage for 48 hours and then testing the fat in upper 100 ml and the remainder of milk. For properly homogenized milk, the percent difference in both the top 100 ml and the remainder milk should not be more than 10 per cent.

Factors Affecting Homogenization Efficiency

• Design of homogenizer valve affects homogenization efficiency. Grooved valves require less homogenization pressure to attain same degree of homogenization pressures as compared to either simple valve with flat seat or needle valve.
• The recommended pressure ranges for homogenization of milk is 140-175 kg/cm2. If the homogenizer is in perfect working condition i.e. the homogenizer valves are not worn out and are well seated, a homogenizer pressure of 175 kg/cm2 should give good homogenization efficiency.
• Two stage homogenization is often recommended because broken fat globules after first stage homogenization (175 kg/cm2) may have a tendency to agglomerate. In order to re-disperse them, homogenization at reduced pressure (35 kg/cm2) may be thus necessary in the second stage.
• Homogenization becomes less effective with increasing fat content. When high fat milk is homogenized, the newly created total fat globule surface becomes so large that materials required to form new membranes for all the fat globules is not sufficiently availably in the serum phase.
• The homogenization temperature in excess of 50oC is often recommended which is necessary to inactive nature lipases. The recommended temperatures for attaining high degree of homogenization (80-90%) are therefore between 60 and 70oC.

Effect of Homogenization on Physico-Chemical Properties of Milk

• Homogenized milk with normal fat content does not have marked clustering of fat globules. Proper homogenization however, does not cause any change in important fat constants or physico-chemical properties.
• Only 2% casein in milk is adsorbed on the fat globules in un-homogenized milk, in homogenized milk almost 25% of casein is adsorbed as part of fat globule membrane. Homogenization is often associated with destabilization of proteins.
• Homogenization results in more uniform, opaque and whiter milk which make the product more acceptable to the consumers. The increased whitening is due to the increase in number and total surface area of fat globules, which reflect and scatter more light.
• It is practically not possible to churn homogenized milk. However, with increasing fat content, the emulsion stability decreases.
• Homogenized milk has greater tendency to form coagulum and requires less coagulating agent. The resultant coagulum has lower curd tension and a soft, spongy body.
• Single stage homogenization causes increase in viscosity because of formation of fat clusters. When the milk is subjected to second stage homogenization, the fat globule clusters are disintegrated/ broken down resulting in decrease in viscosity.

Problems/ Defects Associated with Homogenized Milk

• Homogenized milk is some times more susceptible to curdling when it is used in certain food preparations requiring cooking.
• Milk fat is difficult to separate from the homogenized milk. The recovery of fat from homogenized milk is a serious problem for commercial dairies which receive significant quantities of processed milk as returns from the market.
• Sometimes, fat rising in homogenized milk is to such an extent that a compact ring of creamy material is visible under the container closure often referred as cream plug.
• Appearance of sediments is often ascribed to settling of the extraneous matters. However, clarification of milk before homogenization reduces the amount of deposits significantly whereas clarification after homogenization prevents this defect entirely.
• Excessive foaming in homogenized milk poses handling difficulties. This is due to inclusion of air or homogenizing the air into the milk during processing. However, improving the handling procedure during homogenization can largely eliminate this problem.
• The most important flavour defect associated with homogenized milk is sunlight flavour, sometimes referred as tallowiness, burnt like or activated flavour. This develops due to oxidation of free methionine and formation of free SH compounds from sulfur containing amino acids.

Saiyad Azharuddin
B.tech (Dairy Technologist)