i. Definition of Homogenized Milk
United State Public Health Service has proposed one of the most comprehensive definitions for homogenized milk. This has been the most widely accepted and referred definition. 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.
ii. Theories 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. The following theories have been proposed to be responsible for the entire phenomenon.
Shearing or Grinding: As milk is passed at high pressure (velocity ~ 200-300 m s-1) through the homogenizer valve (~ 100 mm gap), fat globules undergo shearing action. The shear between fat globules and the surface of the homogenizer wall coupled with wire drawing effect results in elongation of the fat globules which progressively becomes unstable. These phenomenon result in subdivision of the fat globules. Furthermore, the difference in velocity of the faster moving serum phase at the centre of the liquid stream as compared to the liquid near the edge of the stream causes the fat globules to grind against each other. The turbulence created by the difference in velocity and eddy currents of the liquid add to the shear effects and thus enhance the process of disruption of the fat globules.
Exploding: This theory suggests that during homogenization, there is build up of tremendous pressure. When this pressure is suddenly released, the internal pressures within the fat globules pull the globule apart with exploding effect. This results in disintegration or subdivision of fat globules into smaller globules.
Splashing/Shattering: As the high homogenizing pressure is attained in the homogenizer, the homogenizing valve releases the highly compressed milk at very high velocity. The liquid suddenly strikes a retaining wall/ perpendicular surface.This causes splashing or shattering effect on the fat globules resulting in break down of globules into smaller sizes.
Acceleration and Deceleration: This theory relates sudden change in velocity of milk as it passes through homogenizer to the homogenization effect. When milk enters the homogenizer valve, velocity of milk changes from almost static to very high velocity. As it comes out of the valve, there is sudden deceleration at a rate at which it was accelerated. This sudden change in velocity results in shattering effect leading to division of fat globules.
Cavitation: It is postulated that as the milk passes through the homogenization valve, the initial homogenization pressure decreases sharply due to sudden increase in the velocity of milk. Depending on the back pressure that exists outside the homogenizer valve, the pressure can drop to as low as the saturated vapour pressure of liquid. This leads to formation of vapour bubbles due to cavitation. Cavitation generates shock waves, which could be in excess of 1600 kg/cm2 in intensity. Due to overlapping of these shock waves, disintegration of the fat globules may occur.
iii. Advantages and Disadvantages of Homogenized Milk
Advantages
- Prevents removal of fat/cream from milk
- Homogenized milk results in softer curd and therefore easily digested by infants
- Churning of fat does not occur during bulk transportation
- Fat is uniformly distributed and therefore gives uniform consistency
- Homogenized milk is comparatively resistant to development of oxidized flavour defect
Disadvantages
- Homogenization offers possibility of incorporation of foreign fat into milk
- Homogenized milk is prone to development of ‘sunlight’ or ‘activated’ flavour defect
- Homogenized milk if returned unsold from the market is difficult to salvage as centrifugal separation of fat is not possible
iv. 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.
v. Design and Operation of Homogenizers
There are several types of homogenizer valves and therefore designs of homogenizers vary depending on the manufacturers. However, many homogenizers used in the dairy industry have been developed based on the principles introduced by Gaulin.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. Single piston pumps generate pulsating output with fluctuating pressure thereby resulting in poor homogenization. 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.If maintained properly, ‘poppet’ valves give better performance with low viscosity liquids like milk. ‘Ball’ valves can exert greater pressure on the much small seal area and are therefore, suitable for high viscosity liquids or suspensions with smaller particles.
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. The gap is maintained against the feed pressure by a counter force exerted by an adjustable heavy duty spring. Shear effects result from the high velocity gradients between the liquid and the surface of the homogenizing valve. Turbulence also results from the high velocity of the liquid in the valve,causing eddy currents within the flow. 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. Homogenizer valves are made of very tough corrosion resisting alloys such as stellite. Better resistance to corrosion can be achieved by using tungsten carbide and ceramic valves, which are used by many manufacturers in modern homogenizers.
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| Sectional view of single-stage Gaulin type homogenizing valve |
vi. High Pressure Homogenization Technology
With significant improvement in understanding of machine design, material strength and fluid mechanical knowledge, homogenizers with higher pressure capabilities have been developed. Such high pressure homogenizers could be operating based on two principles: (1) Conventional valve type homogenizer operating at a far higher pressure; (ii) Micro-fluidization based on the principle of collisions between high speed liquid jets.
Conventional Valve Type High Pressure Homogenizers (HPH): Such high pressure homogenizers work on the principle of conventional ball-and-seat type homogenizer valve. Highly abrasive resistant and durable components of high pressure homogenizers are made from best quality stainless steel, high alloy compositions and new ceramic materials. This allows these systems to operate at pressures upto 2550 kg/cm2 or more. Besides the regular use in emulsion formation, these high pressure homogenizers find applications in inactivation of enzymes and bacteriophages and also in destruction of micro-organisms. Destruction of bacterial cells by HPH is due to several physical phenomenon viz., pressure drop, cavitation, shearing,turbulance and collision. These systems can be therefore, used as a combined process for pasteurization and homogenization.
Microfluidization Technology: The microfluidizer operates under a different principle as in this case the liquid being processed is divided into micro streams that are so projected that these collide with each other. The essential design features of micro-fluidizers include a double acting intensifier pump and an interaction chamber. The intensifier pump, which is either air-driven or electric-hydraulic driven,forces milk/product at high pressure through the interaction chamber. The interaction chamber has fixed-geometry micro-channels, which divides the product into streams.These streams, which accelerate to a very high velocity, are made to collide against each other. Shear and impact that occur lead to homogenization effect. These micro-fluidizers are capable of generating pressures upto 2800kg/cm2. As in case of conventional valve homogenizers, microfluidizers too bring about changes in the fat and protein fractions of milk thereby altering some physico-chemical properties of milk.
vii. Vacuum Homogenization
This innovation in the homogenization technology is based on the discrete pulse energy input theory. In vacuum homogenization, energy is introduced discretely into the liquid (milk) through powerful short-time impulses. The homogenizer unit, placed in the HTST pasteurization line has two condensing chambers. The chilled milk is first pumped to condenser-I where it is heated to 20oC and subsequently to 30oC in condenser-II. Milk then enters regeneration section of the pasteurizer where it is heated to 65oC. It is then delivered to 1st stage vacuum (homogenization) chamber through a special nozzle. As a result of flashing effect, bubbles are formed in the milk as it falls in the vacuum chamber maintained at 0.15 to 0.2 kg/cm2. Due to the pressure changes taking place, the bubbles either show high frequency pulsation and release energy or collapse producing shockwave effect in the product. The bubbles therefore burst into smaller units and the fat globules are divided into smaller globules. As the milk enters 2nd stage homogenization chamber, further breakdown of fat globules takes place. The outgoing milk passes through regeneration section followed by chilling section to finally attain 5oC temperature. Besides the generally accepted homogenization effect, the other major advantages offered by this vacuum homogenizer are deodourization, reduced acidity and partial suppression of microbial activity. These systems also claim to be economical as it consumes almost 2.5 times less power than the valve type high pressure homogenizer.
viii. Checking the Efficiency of Homogenization
The method recommended by the United States Public Health Service has been the most widely used for checking the homogenization efficiency. 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.
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