DAIRY ENGINEERING

Module 4. Homogenizers

Lesson 14
HOMOGENIZATION: CLASSIFICATION, SINGLE STAGE AND TWO STAGE HMOGENIZER PUMPS, POWER REQUIREMENT FOR HOMOGENIZATION

14.1 Introduction

Homogenizing in the dairy industry is used principally to prevent or delay the formation of a cream layer in full cream milk, by reducing the diameter of the butterfat globules.

The average storage temperature and duration of storage play an important part in determining the requirement of homogenization of milk, whether, as in pasteurized milk, it is stored for 1 to 2 weeks at refrigerator temperatures, or, as in UHT milk, at room temperatures for longer period.

In the past it has been very rare for pasteurized liquid milk to be homogenized, although the flavour of milk becomes fuller by homogeni­zing. A certain amount of cream was permitted to form to show the consumer clearly the full cream character of the milk. However, Homogenization process has become more common for Toned milk also. Sterilized milk, evaporated or condensed milk and sterilized cream are generally homogenized. Ice cream mixes, milk for yoghurt production and milk for milk powder manu­facture are also homogenized.

The purpose for which the following are homogenized is

Milk, cream, condensed milk	Prevention of cream separation
Coffee cream	Improvement in flavour, increased Whitening power, increase in Viscosity
Yoghurt	a more stable gel
Ice cream mix	less fat separation during freezing
Full cream milk powder	less separation of free fat

14.1.1 Mode of operation

In milk greater part of the fat volume consists of globules with a diameter ranging from 2 to 6 mm. A few fat globules may exist which have a diameter of 10 mm or more. Milk fat contains also a large number of small fat globules with diameters down to 0.1 mm, but these do not greatly increase the total volume of the fat. The largest fat globules in liquid milk intended for only a few days' storage need not be smaller than 1 - 2 mm. If sterilized milk is to be made suitable for several weeks' storage, the range of diameter of the fat globules should lie between 0.2 and 0.7mm. Researchers found that the diameter of 0.7 mm is critical for fat clumping. With a diameter less than 0.7 mm the dispersion fat globule-milk serum is stable, since aggregates of fat globules become separated again by Brownian movement. This is not so for formation of clusters at higher fat contents (> 20 %) where the sub-units of the casein micelles keep fat globules together by bridge formation.

Fig.14.1 Principle of fat globule disintegration

Homogenization divides globules into smaller ones with diameters down to <1 µm, depending on the operating pressure. This is done by forcing all of the milk at high pressures through a narrow slit, which is only slightly larger than the diameter of the globules themselves. The velocity in the narrowest slit can be 100 to 200 m/s. This can cause high shearing stresses, cavitation and micro-turbulence. The globules become deformed, then become wavy and then break up (fig 14.1).

Intense research activities in the last few years have been very successful in elucidating the actual mechanism of homogenization. There is a definite relationship between the Laplace number La, a dimensionless homogenizing pressure, and the degree of particle size reduction, irrespective of whether flow is laminar or turbulent.

La= ­­­­­­­­∆p.d_mo / б

where ∆p=pressure difference

d_mo= mean initial particle size.

б =Interfacial tension.

When cavitations were suppressed, the degree of homogenization was the same at the same Laplace number, independent of the type of flow. This shows that turbulence is not a decisive criterion for the results of homogenization. However, the effect of homogenizing can be improved if cavitations take place.

According to the theories of flow mechanics, a particle size reduction should only be possible if the viscosity ratio nOil/n water is less than 4. In the homogenization of milk this ratio is 2 to 4 times larger. Nevertheless, the shearing effect at the homogenizer slit can not be excluded since there are high shear gradients as well as very high velocity gradients which can have an accelerating as well as a retarding action. As soon as a liquid thread is produced by deformation only small forces are needed to break it up. Some research workers established the following proportionality for the mean globule diameter d_m:

Cavitation occurs when the kinetic energy (m.v²/2) increases during flow through the slit and when, according to Bernoulli's equation, disregarding friction and deformation losses, the potential energy (P.v) de­creases to such an extent that the static pressure becomes as low as the vapour pressure of the liquid. The pressure distribution during flow through a homogenizer slit is qualitatively shown in Fig.14.2 Shortly after the liquid enters the homogenizer slit, the initial homogenizing pressure P1 decreases sharply due to the sudden increase in velocity. Depending on the value of the back pressure P2 which exists outside the slit, the pressure can drop to as low as the saturated vapour pressure. P_2k shows that a critical back-pressure must be present for cavitation to occur. For the formation of vapour bubbles due to cavitation it is necessary to have a local pressure of less than Pv and gas nuclei to trigger it off. De-gassing of milk can influ­ence cavitation negatively. Cavitation does not take place if the back pres­sure P₂ is higher than P_2K .

Fig. 14.2 Homogenization

Implosion, of cavitation bubbles is one of the main indications of good homogenization. This is shown by a steep rise in pressure. Similar sudden pressure changes are also known to happen in the Laval valve which is of the continuously widening type. There, compression occurs when a uni-phase flow accelerated up to supersonic velocity abruptly changes into sonic velocity. Figure 14.3 shows further that the cross-sectional area of a homogenizer valve behaves like a Laval valve. Using Bernoulli's equation one finds, from the relationship.

√v≈ 2 p

ρ

that the homogenizing velocity can be as high as about 250 mls. The requirements for a com­pression shock can be fulfilled also in homogenizing, depending on the shape of the slit and the pressure conditions. The intensity of the compression shock depends on the geometry of the slit and it can be influenced as by the value of the applied back pressure .As in the valve with varying gap, the compression shock can also reach a maximum within the homogenizer slit, as shown by the dotted line. These are conditions which give optimum homogenizing results.

It is not known how implosions of bubbles disintegrate droplets. The following hypotheses have been proposed: Extremely high local pressure gradients form during spreading of the shock waves. An overlapping of many shock waves pro­duces cavitation noises. The implosion of a single bubble cannot contribute to the disintegration of the fat globule. According to one of the hypothesis of researchers only the overlapping of shock waves, which cause cavitation noises, stimulates the fat globules to resonate near their own frequencies and disintegration occurs when the critical amplitude is exceeded.

With regard to shear stress, fat globule disintegration occurs mainly in flows which have high shear gradients. Flows which cause cavitation are responsible for pressure fluctuations. Water is required in medium sized homogenizer for cooling of piston rods.

14.2 Types of Homogenizer Valves and their Profiles

Figure 14.3 shows types of homogenizer valves. Valve plugs are pressed with an adjustable force F onto the corresponding valve seat. In this way the homo­genizing slit is formed when the incoming liquid has adjusted to the pressure P₁,

Fig. 14.3 Homogenizer valves and their profiles

A conical shape, (b) causes changes in direction due to its profile, (c) shows a simple plate valve and (d) a conical shape but with a grooved valve face which forces alternating stresses onto the liquid to be homogenized, e shows a valve with a breaker ring which has a flat valve face and a conical seat. Changes in the radial cross section can be brought about by means of the cone. By altering the internal diameter of the breaker ring, the back pressure P₂ and therefore the position of the zone of cavitation can be adjusted.

Fig. 14.4 shows the mechanism of a 3 stage piston pump with two homogenizing steps joined in series. The 120 phase shift of each working piston respective­ly, sucks in the liquid while the valves open on their suction side and are closed on their delivery side. The pistons finally force the liquid through the homogenizer value when the value positions are reversed. The required pressure can be regulated from outside by pressure springs while the machine is in the operation. Beside triple stage piston pumps, five- stage ones are often used for homogenizing because of their even feed characteristics.

Two-stage homogenizing is not always necessary. It is used. however, if the broken up fat globules have a tendency to agglomerate after the first homogen­izing stage (150 - 200 bar) in order to re-disperse them at 20 - 40 bar in the second stage. By a suitable construction of the valve, both processing stages can be combined, A homogenization procedure of two or several stages has only a slight effect on the mean particle size. In modern homogenizer, the adjustment of pressure is done by pneumatically operated valve. (Fig. 14.5)

The throughput through a homogenizer can be regulated by either adjusting the piston stroke of a piston pump or by an infinitely variable speed regulating device.