• It is a physical process that breaks down fat globules into tiny, uniform droplets, preventing cream from separating and rising to the top, resulting in a consistently smooth texture, richer color, and stable emulsion
• For this, milk is forced through a small passage at high speed to ensure the breakup of fat globules such that no visible cream line occurs after 48 h at 40ºF (4.4ºC)

Effects of homogenization:

– Whiter color and no cream line formation due to smaller fat globules

– More full-bodied flavor, better mouthfeel

Process requirements:

– Homogenization is most efficient when fat phase is in a liquid state

– Cream >12% fat cannot be homogenized at normal pressure, high pressure homogenization process is necessary

Homogenization

Homogenizer types:

High Pressure:

• Single acting triplex positive piston pump (Similar to a car engine) with each cylinder having a suction and discharge valve leading to a common discharge pipe containing 1or 2 homogenizing valves. Pump is turned by motor through connecting rods and crankshaft
• Milk forced under pressure(~500-5000psi) through an orifice
• Fat globules are broken apart and remain dispersed in a stable emulsion in the milk serum

Low pressure (Rotary type): Grinding and shearing action of fat under low pressure (~500 psi)

Sonic vibrators or oscillator: milk subjected to high frequency vibrations

Source: Dairy Processing Handbook, 1995

Mechanism :

• When milk first enters the valve, liquid velocity is about 4 to 6 m/s
• It then moves into the gap between the valve and the valve seat and its velocity is increased to 120 meter/sec in about 0.2 milliseconds
• The milk then moves across the face of the valve seat (the land) and exits in about 50 microseconds
• The homogenization is completed before the fluid leaves the area between the valve and the seat, and therefore homogenization is initiated and completed in less than 50 microsec

• The whole process occurs between 2 pieces of steel in a steel valve assembly. The product then pass through a second stage valve similar to the first stage
• While most of the fat globule reduction takes place in the first stage, there is a tendency for clustering of the reduced fat globules
• The second stage valve permits the separation of those clusters into individual fat globules

Factors contributing to stability of homogenized milk:

• Decrease in the mean dia. of FG (a factor in stokes law “velocity at which a sphere will rise or fall in a liquid varies as the square of its diameter”)
• Decrease in the size distribution of FG- causing the speed of rise to be similar for majority of the globules so that they don’t tend to cluster during creaming
• Increase in density of globules owing to the adsorption of a protein membrane bringing them closer to the continuous phase
• Pasteurization breaks down the cryoglobulin complex ( IgM + lipoprotein complex) which tends to cluster fat globules causing them to rise

Efficiency of homogenization:

Farrall index: Efficiency is excellent when upon microscopic examination average diameter of 90% fat globules ≤ 2 µ

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Creaming index:

• 50 ml homogenized milk in 2 centrifuge tubes
• Centrifuge (1200 – 1500 rpm/ 15min.)
• Take upper 5 ml from each tube and test for fat
• Mix the rest of the two leftover portions of two tubes and carry out fat test

Creaming index = [(f ₁ -f ₂ )/f ₁ ]×100

E.g. Fat in upper 5 ml (f ₁ )= 3.5%

Fat in lower portion (f ₂ ) = 2.8%

Creaming index = (f₁ -f₂ )/f₁ × 100

= (3.5 – 2.8)/ 3.5 x100 = 20%

Very good

Alternative Method:

• Homogenized milk in a measuring cylinder
• Keep quiescent for 48 hrs.
• Two layers are formed, perform fat test of both layers

Creaming index = [(f ₁ -f ₂ )/f ₁ ] ×100

Effects of Homogenization:

• 6x↑ in Fat globule surface area
• Adsorption of major amounts of casein and casein micelles on the newly created milk fat globule surface
• ↑ Foaming properties
• ↓Heat stability (of high fat products)
• ↓ Curd tension
• ↑ Viscosity
• ↑ Susceptibility to lipolytic enzymatic action
• ↑ Susceptibility to the formation of light induced/ oxidized flavor

Homogenization Effects:

Factors affecting homogenization:

• Type of valve
• Pressure
• Single or two-stage
• Fat content
• Viscosity
• Temperature (50- 70°C)
• Droplet diameter (the smaller, the more difficult to disrupt)