HOMOGENISATION:

Homogenization is the process of breaking up the fat globules in cream to such a small size that they remain suspended evenly in the milk rather than separating out and floating to the surface. Homogenization (chemistry), or homogenisation, is any of several processes used to make a chemical mixture the same throughout.

Homogenization is intensive mixing of mutually insoluble phases (sometimes with addition of surfactants) to obtain a suspension or emulsion.

THEORIES OF HOMOGENISATION

The reduction in size of the fat globules during the homogenization process is most likely explained by the combination of two theories i.e. turbulence and cavitation.

TurbulenceEnergy, 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 charge of velocity of fluid. Liquid cavitates because its vapor pressure is attained. Cavitation generates further eddies that would produce disruption of the fat globules.

The high velocity gives liquid a high kinetic energy which is disrupted in a very short period of time. Increased pressure increases velocity. Dissipation of this energy leads to high energy density (energy per volume and time). Resulting diameter is a function of energy density.

HOMOGENIZERThe machine which carries out the operation of homogenisation is known as Homogeniser3.It consists of a high pressure piston pump which forces the milk at high pressures (and velocity) through a narrow opening between the homogenizing valve and its seat; the fat globules in the milk are thereby sub-divided into smaller particles of more uniform size. The homogenizing valve is held down by a heavy pressure spring against the seat of the valve. The valve and its seat are made of extremely hard material (e.g. steliye) and the contact faces are carefully ground so that the valve site accurately on its seat.

SINGLE-STAGE AND TWO-STAGE HOMOGENISATIONHomogenisation device can be equipped with one homogenising device or two connected in series, hence the name single stage homogenisation and double stage homogenisation is given. In single-stage and two-stage homogenisation the total homogenisation pressure is measured before the first stage, P1, and the homogenisation pressure in the second stage is measured before the second stage, P2. The two-stage method is usually chosen to achieve optimal homogenisation efficiency. Best results are obtained when the relation P1 / P2 is about 0.2. In two stage homogenizer, a pressure of 2000 - 2500 psi is on the first stage and 500 - 1000 psi on the second stage. Temperature is around 1200F/480C.In first stage fat globules join together to form clumps and this process is known as post clumping. To overcome the situation of clumping homogenisation is done.Single-stage homogenisation may be used for homogenisation of:– Products demanding a high viscosity (certain cluster formation).

Two-stage homogenisation is used for:– Products with a high fat content– Products where high homogenisation efficiency is desired.

HOMOGENIZATION MECHANISM

Auguste Gaulin's patent in 1899 consisted of a 3 piston pump in which product was forced through one or more hair like tubes under pressure. It was discovered that the size of fat globules produced were 500 to 600 times smaller than tubes. There have been over 100 patents since, all designed to produce smaller average particle size with expenditure of as little energy as possible. The homogenizer consists of a 3 cylinder positive piston pump (operates similar to car engine) and homogenizing valve. The pump is turned by electric motor through connecting rods and crankshaft.

To understand the mechanism, consider a conventional homogenizing valve processing an emulsion such as milk at a flow rate of 20,000 l/hr. at 14 MPa (2100 psig). As it 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 are increased to 120 meter/sec in about 0.2 millisec. The liquid then moves across the face of the valve seat (the land) and exits in about 50 microsec. The homogenization phenomena 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. The whole process occurs between 2 pieces of steel in a steel valve assembly. The product may 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 clumping or clustering of the reduced fat globules. The second stage valve permits the separation of those clusters into individual fat globules.

EFFECT OF HOMOGENISATIONHomogenisation effects on the physical structure of milk have many advantages:

Smaller fat globules leading to no cream –line formation, Whiter the more appetizing color, Reduced sensitivity to fat oxidation, More full-bodied flavor, better mouth feels, Better stability of cultured milk products.

However, homogenisation also has certain disadvantages:

Homogenized milk cannot be efficiently separated. Somewhat increased sensitivity to light –sunlight and fluorescent tubes- can result in

“Sunlight Flavor”. Reduced heat stability, especially in case of single- stage homogenisation, high fat

content and other factors contributing to fat clumping. The milk will not be suitable for production of semi-hard or hard cheese because the

coagulum will be too soft and difficult to dewater.

FACTORS INFLUENCING HOMOGENIZATION

(a) Temperature of homogenization:

At the time of homogenization, milk should be at a temperature above the melting point of fat, viz., above 330C (910F). This is because fat should be in the liquid state for proper sub-division. The enzyme lipase should be inactivated, preferably prior to homogenization or immediately afterwards. This can be achieved by heating the milk to a temperature of 550C (1310F) or above. In routine practice, the milk is heated to 65-700C (149-158*F) for homogenization. (The danger zone for lipase activity, viz., temperatures 38-490C (100-1200F), should be avoided during or after homogenization.)

(b) Pressure of homogenization:

In a single stage, upto 6 percent fat milk, usually 2000-2500 psi pressure is sufficient. Higher pressures may increase the tendency for the milk to curdle when cooked, due to the increased destabilizing effect on milk-proteins. For liquid products with more than 6 per cent fat, two-stage homogenization is needed to prevent fat clumping: 2000 psi at the first stage and 500psi at the second.

Note:

The man considerations are: (i) All homogenized milk must be pasteurized either prior to homogenization or immediately after; (ii) the homogenized milk should preferably be clarified after homogenization; (iii) under Indian conditions, the sequence underlined above is desirable, so as to avoid any post-pasteurization contamination.

DISADVANTAGES OF HOMOGENIZATION:

Simple proteins rarely survive digestion in a balanced world. When milk is homogenized, it passes through a fine filter at pressures equal to 4,000 pounds per square inch, and in so doing, the fat globules (liposomes) are made smaller (micronized) by a factor of ten times or more. These fat molecules become evenly dispersed within the liquid milk. Milk is a hormonal delivery system. With homogenization, milk becomes a very powerful and efficient way of bypassing normal digestive processes and delivering steroid and protein hormones to the human body (both your hormones and the cow's natural hormones and the ones they were injected with to produce more milk). Through homogenization, fat molecules in milk become smaller and become 'capsules' for substances that bypass digestion. Proteins that would normally be digested in the stomach or gut are not broken down, and are absorbed into the bloodstream.

USES OF HOMOGENIZATION :

Homogenization is used for making milk more stable against creaming and for giving the product a richer mouthfeel due to a slight increase in viscosity. Homogenization is also used in ice cream making. The homogenized mix gives a creamier product, when the mix is frozen. However, experienced ice cream makers know, that it is beneficial to store homogenized ice cream mix a few hours at low temperature (4°C) before freezing. This aging process permits any added emulsifying agent (glycerol mono- and di-stearates) to partially displace some of the casein submicelles from the fat surfaces. The reason, this is beneficial, is not well understood, and is still a subject of research.

ABHISHEK GOYAL

B.TECH FOOD TECHNOLOGY

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