Dairy Business - Introduction

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1.Milk Procurement

2.Dairy Products (Production Technique)

3.Milk Testing Procedure

Homogenization of Milk and Milk Products

Homogenization of Milk and Milk Products


The following topics will be covered in this section:
Introduction
Homogenization Mechanism
turbulence
cavitation
Effect Of Homogenization
fat globule properties
surface layers

Introduction
Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skimmilk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) oweing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise.


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 is 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.
Homogenizer and Valve 17 KB


It is most likely that a combination of two theories, turbulence and cavitation, explains the reduction in size of the fat globules during the homogenization process.
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.
Cavitation
Considerable 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 a high energy density (energy per volume and time). Resulting diameter is a function of energy density.
In summary, the homogenization variables are:
type of valve
pressure
single or two-stage
fat content
surfactant type and content
viscosity
temperature
Also to be considered are the 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.

Effect of Homogenization:
Fat globule No Homogenization 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
Surface layer
The milk fat globule has a native membrane, picked up at the time of secretion, made of amphiphilic molecules with both hydrophilic and hydrophobic sections. This membrane lowers the interfacial tension resulting in a more stable emulsion. During homogenization, there is a tremendous increase in surface area and the native milk fat globule membrane (MFGM) is lost. However, there are many amphiphilic molecules present from the milk plasma that readily adsorb: casein micelles (partly spread) and whey proteins. The interfacial tension of raw milk is 1-2 mN/m, immediately after homogenization it is unstable at 15 mN/m, and shortly becomes stable (3-4 mN/m) as a result of the adsorption of protein. The transport of proteins is not by diffusion but mainly by convection. Rapid coverage is achieved in less than 10 sec but is subject to some rearrangement.
Surface excess is a measure of how much protein is adsorbed; for example 10 mg/m2 translates to a thickness of adsorbed layer of approximately 15 nm.

Milk Pastuarization Technique in India

Plate Heat Exchangers

Plate heat exchangers offer better economy, maximum efficiency and easy maintenance.

The plate heat exchanger is widely recognized today as the most economical and efficient type of heat exchanger on the market. With its low cost, flexibility, easy maintenance, and high thermal efficiency it is unmatched by any type of heat exchanger. The key to plate heat exchanger's efficiency lies in its plates. With corrugation patterns that induce turbulent flow, it not only achieves unmatched efficiency it also creates a self-cleaning effect thereby reducing fouling.

WCR plate corrugations are available in different pressing depths and patterns depending on the application. All are designed to achieve turbulence across the entire heat transfer area in order to get the highest possible heat transfer coefficients with the lowest possible pressure drop and allow for close temperature approaches. Consequently this means smaller heat transfer area, smaller heat exchangers and sometimes even less heat exchangers.

Plate heat exchanger consists of a number of gasketed plates which are fixed between a top carrying bar and a lower guide bar. The plates are compressed by means of tie bolts between a stationary frame part (called the head) and a movable frame part (called the follower). Fluids enter the plate heat exchanger through frame connections and are distributed to plates. The flow to alternate passages between the plates is controlled by alternating the placement of gaskets.

The plate heat exchanger is designed with either single-pass or multi-pass flow, depending on the duty. For most duties single-pass is suitable and often the preferred solution as it keeps all connections on the stationary frame part and consequently makes disassembly easier. Multi-Pass however, is required when flow rates are low or when approach temperatures are close. Other factors such as building ceiling height or space limitations for handling of large plates often leads to the decision to use multi-pass and thereby more and smaller plates.

On all WCR plate designs, gaskets provide a peripheral seal while separating product flows. The space between gaskets is vented to the atmosphere, eliminating cross contamination of fluids. This is an ideal feature where product contamination cannot be tolerated.

Easy to Remove and Clean

You simply remove the tie bolts and slide back the movable frame part. Now the plate pack can be inspected, pressure cleaned, or removed for refurbishment if required.

Expandable

A very significant feature of the plate heat exchanger is that it is expandable. Increasing your heat transfer requirements means simply adding plates instead of buying a new heat exchanger, saving time and money.

High Efficiency

Because of the pressed patterns in the plates and the relative narrow gaps, very high turbulence is achieved at relative low fluid velocity. This combined with counter directional flow results in very high heat transfer coefficients.

Compact Size

As a result of the high efficiency, less heat transfer area is required, resulting in a much smaller heat exchanger than would be needed for the same duty using other types of heat exchangers. Typically a plate heat exchanger requires between 20-40% of the space required by a tube & shell heat exchanger.

Close Approach Temperature

The same features that give the plate heat exchanger its high efficiency also makes it possible to reach close approach temperatures which is particularly important in heat recovery and regeneration applications. Approach temperatures of 0.5ºC is possible.

Multiple Duties in a Single Unit

The plate heat exchanger can be built in sections, separated with simple divider plates or more complicated divider frames with additional connections. This makes it possible to heat, regenerate, and cool a fluid in one heat exchanger or heat or cool multiple fluids with the same cooling or heating source.

Avoid cross contamination

Each medium is individually gasketed and as the space between the gaskets is vented to the atmosphere, cross contamination of fluids is eliminated.

Less Fouling

Very high turbulence is achieved as a result of the pattern of the plates, the many contact points, and the narrow gap between the plates. This combined with the smooth plate surface reduces fouling considerably compared to other types of heat exchangers.

Lower Costs

High heat transfer coefficients mean less heat transfer area and smaller heat exchangers, and sometimes even less heat exchanger.