Building-blocks ofdairy processing

Dr.AyeshaNIFSAT,UAF.

Abbas Ranjha,NIFSAT,UAF.

The technology behinddisruption of fat globules

• Homogenisation has become a standard industrial process, universally practised as a means of stabilising the fat emulsion against gravity separation

• Homogenisation primarily causes disruption of fat globules into much smaller ones

• It diminishes creaming and may also diminish the tendency of globules to clump or coalesce

• Milk is forced through a small passage at high velocity

Homogenisation causesdisruption of fat globules

• The disintegration of the original fat globules is achieved by a combination of contributing factors such as turbulence and cavitation

• The homogenisation reduces fat globule size from an average of 3.5 μm indiameter to below 1 μm

• The newly created fat globules are no longer completely covered with the original membrane material

• Instead, they are surfaced with a mixture of proteins adsorbed from the plasma phase

• Casein was the protein half of the complex and that it was probably associated with the fat fraction through polar bonding forces

• Casein micelle was activated at the moment it passed through the valve of the homogeniser,

• Predisposing it to interaction with the lipid phase.

• The physical state and concentration of the fat phase at the time of homogenisation

contribute materially to the size & dispersion of the subsequent fat globules

• Homogenisation of cold milk, in which the fat is essentially solidified, is virtually ineffective

• Processing at temperatures conducive to the partial solidification of milk fat (i.e. below 40 °C) results in incomplete dispersion of the fat phase

• Products of high fat content are more difficult to homogenise and also more likely to show evidence of fat clumping, because the conc. of serum proteins is low in relation to the fat content

• Usually, cream with higher fat content than 20 % cannot be homogenised at high pressure,

clusters are formed as a result of lack of membrane material (casein)

• Increasing the homogenisation temperature decreases the viscosity of milk and improves the transport of membrane material to the fat globules

• Homogenisation temperatures normally applied are 55 – 80 °C, and homogenisation pressure is between 10 and 25 MPa (100 – 250 bar), depending on the product

Flow characteristics

• When the liquid passes the narrow gap, the flow velocity increases

• The speed will increase until the static pressure is so low that the liquid starts to boil

• The maximum speed depends mainly on the homogenisation) pressure

• When the liquid leaves the gap, the speed decreases and the pressure increases again

• The liquid stops boiling and the steam bubbles collapse

Homogenisers may be equipped with one homogenising device or two connected in series, hence the names • Single-stage

homogenisation and

• Two-stage homogenisation.

Effect of homogenisation

• Smaller fat globules leading to less cream-line formation

• Whiter and more appetizing colour• Reduced sensitivity to fat oxidation• More full-bodied flavour, and better

mouthfeel• Better stability of cultured milk product

The homogeniser in a processing line

• In general, the homogeniser is placed upstream, i.e. before the final heating section in a heat exchanger.

• In most pasteurisation plants for market milk production, the homogeniser is usually placed after the first regenerative section.• In production of UHT milk, the homogeniser is

generally placed upstream in indirect systems but always downstream in direct systems, i.e. on the

aseptic side after UHT treatment

The homogeniser in a processing line

• However, downstream location of the homogeniser is recommended for indirect UHT systems

when milk products with a fat content higher than 6 – 10 % and/or with increased protein content are going to be processed. with increased fat and protein contents, fat clusters

and/ or agglomerates (protein) form at the very high heat treatment temperatures.

These clusters/agglomerates are broken up by the aseptic homogeniser located downstream.

Split homogenisation

An aseptic homogeniser is more expensive to operate. In some cases it is sufficient if just the second stage is placed downstream. This arrangement is called split homogenisation

Partial homogenisation

Partial stream homogenisation means that the main body of skim milk is not homogenised, but only the cream together with a small proportion of skim milk

• This form of homogenisation is mainly applied to pasteurised market milk.

• The basic reason is to reduce operating costs

Determining homogenisation efficiency

• Homogenisation must always be sufficiently efficient to prevent creaming

• A sample of milk is stored in a graduated measuring glass for 48 hours at a temperature of 4 – 6 °C

• The top layer (1/10 of the volume) is siphoned off, the remaining volume (9/10) is thoroughly mixed, and the fat content of each fraction is then determined

• The difference in fat content between thetop and bottom layers, expressed as a of the top layer, is referred to as the homogenisation index• An example: If the fat content is 3,15 % in the

top layer and 2,9 % in the bottom layer, the homogenisation index will be (3,15 – 2,9) x 100: 3,15 =7,9.

• The index for homogenised milk should be in the range of 1 to 10.

Evaporators

• Removal of water• Concentration of a liquid involves evaporation

of a solvent, in most cases water. • Concentration is distinguished from drying in

that the final product – the concentrate – is still liquid

Reasons for concentrating food liquids

• Reduce costs for storage and transportation• Induce crystallisation• Reduce the cost of drying• Reduce water activity to increase

microbiological and chemical stability• Recover valuable substances and by-products

from waste streams

Circulation evaporators

• Circulation evaporators can be used when a low degree of concentration is required or when small quantities of product are processed.

• In yoghurt production, for example, evaporation is utilised to concentrate milk 1,1 – 1,25 times, or from 13 to 14,5 or 16,25 % solids content

respectively• This treatment simultaneously deaerates the

product and rids it from off-flavours

Plate-type evaporator

• Distribution in a plate-type falling-film evaporator can be arranged with two pipes running through the plate pack.

• For each product plate there is a spray nozzle in each product pipe, spraying the product in

a thin, even film over the plate surface.

Deaerators

• More air is introduced into the milk during handling at the farm and transportation to the dairy, and during reception at the dairy.

• It is not unusual for incoming milk to contain 10 % air by volume, or even more

The basic problems caused by dispersed air are• Inaccuracy in volumetric measurement of milk.• Incrustation of heating surfaces in pasteurisers (fouling).• Reduced skimming efficiency in separators.• Loss of precision in automatic in-line standardisation

Deaeration in the milk treatment line

• Vacuum deaeration has been used successfully to expel dissolved air and finely dispersed air bubbles from milk

• Pre-heated milk is fed to an expansion vessel, in which the vacuum is adjusted to a level equivalent to a boiling point about 7 to 8 °C below the pre-heating temperature

• If the milk enters the vessel at 68 °C, the temp will immediately drop to 68 – 8 = 60 °C.

• The drop in pressure expels the dissolved air, which boils off, together with certain amount of the milk.

• The vapour passes a built-in condenser in the vessel, condenses, and runs back into the milk,

• while the boiled-off air, together with non condensable gases (certain off-flavours) is removed from the vessel by the vacuum pump