Dr. Mohammad Ashraf Paul

Division of Livestock Products TechnologyFaculty of Veterinary Sciences & Animal Husbandry SKUAST-K Alusteng, Ganderbal Kashmir

Homogenization refers to the process of forcing the milk through a homogenizer with the object of sub dividing the fat globules to a size of 2 µ dia. or less.

• Piston driven valve homogenizers are used which subject milk to high velocity turbulent flow fields.

• Homogenizers are sometimes incorporated in the pasteurizer and operated at or near pasteurization Temp.

• This has the advantage of permitting lower pressures to be used and reducing problems due to microbial contamination.

Homogenizer types:1. 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

2. Low pressure- Rotary type: Grinding & shearing action of fat under low pressure (~ 500 psi)

3. The sonic vibrators or oscillator: milk subjected to high frequency vibrations

Mechanism :o As milk first enters the valve, liquid velocity is about 4 to 6 m/s. oIt 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.o The milk then moves across the face of the valve seat (the land) and exits in about 50 microseconds.o The homogenization phenomenon is completed before the fluid leaves the area between the valve and the seat, and therefore emulsification is initiated and completed in less than 50 microsec.

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

Theories, explaining the reduction in size of the fat globules:Turbulence Energy, dissipating in the liquid going through the homogenizer valve, generates intense turbulent eddies of the same size as the average globule diameter. Globules are thus torn apart by these eddie currents reducing their average size.CavitationConsiderable pressure drop with the change of velocity of fluid results in cavitations Cavitations generate further eddies that would produce disruption of the fat globules Increased pressure increases velocity The high velocity gives liquid a high kinetic energy which is disrupted in a very short period of time Dissipation of this energy leads to a high energy density (energy per volume and time). Resulting diameter is a function of energy density.

Factors contributing to stability of homogenized milk :

1. 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”)2. 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

3. Increase in density of globules owing to the adsorption of a protein membrane bringing them closer to the continuous phase

4. Heat pasteurization breaks down the cryo-globulin 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

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 left over portions of two tubes and carry out fat test

Creaming index = [(f1-f2)/f1]X100

Interpretation scale:≤10 = Excellent10-20 = Very good20-30 = Good30 = Fair>30 = Poor e.g.:Fat in upper 5 ml (f1)= 3.5%Fat in lower portion (f2) = 2.9%Creaming index = (f1-f2)/f1x 100 = 3.5 – 2.9 x100 = 17% 3.5 Very good

Alternative Method:Homogenized milk in a measuring

cylinder Keep quiescent for 48 hrs. Two layers are formed Take fat test of both layers

Creaming index = [(f1-f2)/f1]X100

Effects: 6x in Fat Globule surface area Adsorption of major amounts of Casein

& casein micelles on to the newly created milk fat globule surface

Foaming properties Heat stability (of high fat products) Curd tension Viscosity Susceptibility to lipolytic enzyme

action Susceptibility to the formation of

light induced/ oxidized flavor

Fat globule No Homogeniza

tion

15 MPa (2500 psig)

Av. diam. (µ m) 3.3 0.4

Max. diam. (µ m) 10 2

Surf. area (m2/ml of milk)

0.08 0.75

Number of globules (µ m-3)

0.02 12

Homogenization Effects:

Factors affecting homogenization :

Type of valve Pressure Single or two-stage Fat content Surfactant type and content Viscosity Temperature Droplet diameter (the smaller, the more difficult to disrupt), and the log diameter which decreases linearly with log P and levels off at high pressures.