06.03 Homogenisers

8/12/2019 06.03 Homogenisers

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Dairy Processing Handbook/Chapter 6.3 123

Homogenisers

The technology behinddisruption of fat globulesHomogenisation has become a standard industrial process, universally

practised as a means of stabilising the fat emulsion against gravityseparation. Gaulin, who invented the process in 1899, described it inFrench as fixer la composition des liquides.

Homogenisation primarily causes disruption of fat globules into muchsmaller ones (Figure 6.3.1). Consequently, it diminishes creaming and mayalso diminish the tendency of globules to clump or coalesce. Essentially, allhomogenised milk is produced by mechanical means. Milk is forcedthrough a small passage at high velocity.

The disintegration of the original fat globules is achieved by acombination of contributing factors such as turbulence and cavitation. Thehomogenisation reduces fat globule size from an average of 3,5 m indiameter to below 1 m. This is accompanied by a four- to six-fold increasein the fat/plasma interfacial surface area. 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 plasmaphase.

Fox et al.1)studied a fat-protein complex produced by thehomogenisation of milk. They showed that casein was the protein half of thecomplex and that it was probably associated with the fat fraction throughpolar bonding forces. They postulated further that the casein micelle wasactivated at the moment it passed through the valve of the homogeniser,predisposing it to interaction with the lipid phase.

Process requirementsThe physical state and concentration of the fat phase at the time of homo-genisation contribute materially to the size and dispersion of the ensuing fatglobules.

Homogenisation of cold milk, in which the fat is essentially solidified, isvirtually ineffective. Processing at temperatures conducive to the partialsolidification of milk fat (i.e.below 40 C) results in incomplete dispersion ofthe fat phase.

Products of high fat content are more difficult to homogenise and alsomore likely to show evidence of fat clumping, because the concentration ofserum proteins is low in relation to the fat content. Usually, cream withhigher fat content than 20 % cannot be homogenised at high pressure,because clusters are formed as a result of lack of membrane material(casein). Increasing the homogenisation temperature decreases the visocity

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.

Fig. 6.3.1Homogenisation causes

disruption of fat globules into much

smaller ones.

1) Fox, K.K., Holsinger, Virginia, Caha, Jeanne and Pallasch,

M.J., J. Dairy Sci, 43, 1396 (1960 ).


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Dairy Processing Handbook/Chapter 6.3124

Flow characteristicsWhen the liquid passes the narrow gap, the flow velocity increases (Figure6.3.2). The speed will increase until the static pressure is so low that theliquid starts to boil. The maximum speed depends mainly on the inlet(homogenisation) pressure. When the liquid leaves the gap, the speeddecreases and the pressure increases again. The liquid stops boiling andthe steam bubbles implode.

Homogenisation theoriesMany theories of the mechanism of high pressure homogenisation havebeen presented over the years. For an oil-in-water dispersion like milk,where most of the droplets are less than one m (106m) in diameter, two

theories have survived. Together, they give a goodexplanation of the influence of different parameters onthe homogenising effect.

The theory of globule disruption by turbulenteddies(micro whirls) is based on the fact that a lot of

small eddies are created in a liquid travelling at a high velocity.Higher velocity gives smaller eddies. If an eddy hits an oil droplet of its

own size, the droplet will break up. This theory predicts how thehomogenising effect varies with the homogenising pressure. This relationhas been shown in many investigations.

The cavitation theory, on the other hand, claims that the shock wavescreated when the steam bubbles implode disrupt the fat droplets.According to this theory, homogenisation takes place when the liquid isleaving the gap, so the back pressure which is important to control thecavitation is important to homogenisation. This has also been shown inpractice. However, it is possible to homogenise without cavitation, but it isless efficient.

Single-stage and two-stage homogenisationHomogenisers may be equipped with one homogenising device or two

connected in series, hence the names single-stage homogenisation andtwo-stage homogenisation. The two-stage system is illustrated in Figure6.3.5.

In both single-stage homogenisation and two-stage homogenisation, thewhole homogenisation pressure (P

1) is used over the first device. In single-

stage homogenisation, the back pressure (P2) is created by the process. In

two-stage homogenisation the back pressure (P2) is created by the second

stage. In this case the back pressure can be chosen to achieve optimalhomogenisation efficiency. Using modern devices, the best results areobtained when the relation P

2/P

1is about 0,2. The second stage also

reduces noise and vibrations in the outlet pipe.Single-stage homogenisation may be used for homogenisation of

products with high fat content demanding a high viscosity (certain cluster

formation).Two-stage homogenisation is used primarily to reach optimal

homogenisation results and to break up fat clusters in products with a highfat content. The formation and break-up of clusters in the second stage isillustrated in Figure 6.3.3.

Effect of homogenisationThe effect of homogenisation on the physical structure of milk has manyadvantages: 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 products

Fig. 6.3.2 At homogenisation, the milk is

forced through a narrow gap where the

fat globu les are split.

Homogenisedproduct

Unhomogenisedproduct

Seat

Forcer

Gap 0,1 mm

Homogenisedproduct

Fig. 6.3.3 Disruption of fat g lobules in

first and second stages of homogeni-

sation.

1 After first stage

2 After second stage

2

1


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Fig. 6.3.4The homogeniser is a large

high-pressure pump with a homogenising

device.

1 Main drive moto r

2 V-belt transmission

3 Gearbox

4 Damper

5 Hydraulic p ressure setting system

6 Homogenising device, second stage

7 Homogenising device, first stage

8 Solid stainless steel pump block9 Pistons

10Crankcase

However, homogenisation also has certain disadvantages: Somewhat increased sensitivity to light sunlight and fluorescent tubes

can result in sunlight flavour (see also Chapter 8, Pasteurised milkproducts).

The milk might be less suitable for production of semi-hard or hardcheeses because the coagulum will be too soft and difficult to dewater.

The homogeniserA high-pressure homogeniser is a pump with a homogenisation device. Ahomogeniser is generally needed when high-efficiency homogenisation isrequired.

The product enters the pump block and is pressurised by the pistonpump. The pressure that is achieved is determined by the back-pressuregiven by the distance between the forcer and seat in the homogenisationdevice. This pressure P

1(Figure 6.3.8) is always designated the

homogenisation pressure. P2is the back-pressure to the first stage.

The high-pressure pumpIn Figure 6.3.4, the piston pump is driven by a powerful electric motor (1),via belts (2) and pulleys through a gearbox (3) to the crankshaft (10) andconnecting-rod transmission, which converts the rotary motion of the motorto the reciprocating motion of the pump pistons (9).

A piston pump is a positive pump and its capacity can only be adjustedby changing the speed of the motor or changing the size of the pulleys. Tohandle higher pressures, pistons with smaller diameter are installed. This willreduce the maximum capacity, as each machine size has a maximumcrankshaft speed. A larger machine has a longer stroke length and/or morepistons. In many cases these pistons also have a larger diameter.

A high-pressure pump has normally three to five pistons (9), running incylinders in a high-pressure block (8). They are made of highly resistantmaterials. The machine is fitted with double piston-seals. Water is supplied

1

10

7

5

3

2

8

9

4

6


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to the space between the seals to lubricate the pistons. A mixture of hotcondensate and steam can also be supplied to prevent reinfection when thehomogeniser is placed downstream in aseptic processes.

A pistonpump will always generate a pulsating flow. The acceleration anddecelerationof the liquid will create a pulsating pressure in the suction pipe.To avoid cavitation in the pump, there is always a damper on the suctionpipe to reduce the pulsation. On the outlet side, the pulsation might createvibrations and noise, why the outlet pipe is also equipped with a damper.

As it is a positive pump, a piston pump should not operate in a series ofother positive pumps, unless there is a bypass otherwise the result can beextreme pressure variations and damaged equipment. If the flow can bestopped downstream of a high-pressure pump, a safety device must beinstalled that opens before the pipe bursts.

The homogenisation deviceFigure 6.3.5 shows the homogenisation and hydraulic system. The pistonpump boosts the pressure of the milk from about 300 kPa (3 bar) at theinlet to a homogenisation pressure of 10 25 MPa (100 250 bar),

depending on the product. The pressure to the first stage before the device(the homogenisation pressure) is automatically kept constant. The oilpressure on the hydraulic piston and the homogenisation pressure on theforcer balance each other. The hydraulic unit can supply both first andsecond stage with an individually set pressure. The homogenisationpressure is set by adjusting the oil pressure. Actual homogenisationpressure can be read on a pressure gauge.

Homogenisation always takes place in the first stage. The second stagebasically serves two purposes: Supplying a constant and controlled back-pressure to the first stage,

giving best possible conditions for homogenisation Breaking up clusters formed directly after homogenisation as shown in

Figure 6.3.3.The parts in the homogenisation device are precision-ground. Its seat is atan angle that makes the product accelerate in a controlled way, therebyreducing the rapid wear and tear that would otherwise occur.

Fig.6.3.5 The components of a two-

stage homogenisation device.

1 First stage forcer

2 Second stage forcer

3 Seat

4 Gap

5 Hydraulic actuato r

Note that the homogenisationpressure is the pressure beforethe first stage, not the pressuredrop.

5

1

2

4

3

3

5

4


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Milk is supplied at high pressure to the space between the seat andforcer. The distance between the seat and the forcer is approximately 0,1mm or 100 times the size of the fat globules in homogenised milk. Thevelocity of the liquid is normally 100 400 m/s in the narrow annular gap.The higher the homogenisation pressure, the higher the speed.Homogenisation takes 10 15 microseconds. During this time, all thepressure energy delivered by the piston pump is converted into kineticenergy. Part of this energy is converted back to pressure again after thedevice. The other part is released as heat; every 40 bar in pressure dropover the device gives a temperature rise of 1 C. Less than 1 % of theenergy is utilised for homogenisation, but nevertheless, high-pressurehomogenisation is the most efficient method available.

Homogenisation effic iencyThe purpose of homogenisation varies with the application. Consequentlythe methods of measuring efficiency also vary.

According to Stokes Law, the rising velocity of a particle is given by:v

g= velocity

g = force of gravityp = particle size

hp= density of the liquid

lp

= density of the particlet = viscosityin the formula:

It can be seen that reducing the particle size is an efficient way of reducingthe rising velocity. Therefore, reducing the size of fat globules in milkreduces the creaming rate.

Analytical m ethodsAnalytical methods for determining homogenisation efficiency can bedivided into two groups:

Studies of creaming rateThe straight forward way of determining the creaming rate is to take apackage, store it at the recommended storage temperature until the lastday of consumption, open it and check if the cream layer is acceptable or

not.The USPH method is based on this. A sample of, say, 1 000 ml is stored

for 48 hours, after which the fat content of the top 100 ml is determined, aswell as the fat content of the rest. Homogenisation is reckoned to besufficient if 0,9 times the top fat content is less than the bottom fat content.

The NIZO method is based on the same principle, but with this method,a sample of 25 ml is centrifuged for 30 minutes at 1 000 rpm, 40 C and aradius of 250 mm. The fat content of the 20 ml at the bottom is divided bythe fat content of the whole sample, and the ratio is multiplied by 100. Theresulting index is called the NIZO value. The NIZO value of pasteurised milkis normally 50 80 %.

Size distrib ution analysisThe size distribution of the particles or droplets in a sample can bedetermined in a well defined way by using a laser diffraction unit (Figure6.3.6), which sends a laser beam through a sample in a cuvette. The light

Fig. 6.3.6Particles analysis by laser

diffraction.

Sensors

Laser light

Sample

Scattered light

vg

= x gp2x (

hp

lp)

18 x t

or vg = constant x p2


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will be scattered, depending on the size and numbers of particles in thesample.

The result is presented as size distribution curves. The percentage of thevolume (fat) is given as a function of the particle size (fat globule size). Three

typical size distribution curves for milk are shown in Figure 6.3.7. It can beseen that the curve shifts to the left as a higher homogenisation pressure isused.

Note that fat globules can aggregate during storage and that this canincrease the creaming rate.

Energy consumpt ion and influenceon temperature

Flowl/h

Qin

10 000

TempC

Tin

65

Pressurebar

Pin

2

Pressurebar

P1

200

Pressurebar

P2

40

Pressurebar

Pout

4

Temp

C

Tout

70

Electric effectkW

E68

Pistonpump

1sthomogenisation

stage

2ndhomogenisation

stage

Fig. 6.3.8Energy, temperature and pressure in a homogenisation example.

The electrical power input needed for homogenisation is expressed by theformula:

Example:

E = Electrical effect, kWQ

in= Feed capacity, l/h 10 000

P1

= Homogenisation pressure, bar 200 (20 MPa)P

in= Pressure to the pump, bar 2 (200 kPa)

pump

= Efficiency coefficient of the pump 0,85

el. motor= Efficiency coefficient of the

electrical motor 0,95

The efficiency coefficients are typical values. From the figures for feedcapacity and pressures given on the right above, the electric powerdemand will be 68 kW. Of this, 55 kW is used for pumping and converted to

Fig. 6.3.7Size distribution curves.

0 1 2 3 4 5 6 7 8

Volumedistribution of fat, %

Globule size, microns

Homogenised at 250 bar

Homogenised at 100 bar

Unhomogenised milk

E = kW36 000 x

pumpx

el. motor

Qinx (P

1 P

in)


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heat in the homogenisation device, and 13 kW is released as heat to thecooling water and to the air.

As was mentioned above, part of the pressure energy supplied isreleased as heat. Given the temperature of the feed, T

in, the

homogenisation pressure, P1, the pressure after homogenisation, P

out, and

that every 4 MPa (40 bar) in pressure drop raises the temperature by 1 C,the following formula is applicable:

The energy consumption, temperature increase and pressure decrease areillustrated in Figure 6.3.8.T

in= 65 C

P1

= 200 bar (20 MPa)P

out= 4 bar (400 kPa)

resulting inT

out= 70 C

The homogeniser in a processing lineIn general, the homogeniser is placed upstream, i.e.before the final heatingsection in a heat exchanger. In most pasteurisation plants for market milkproduction, the homogeniser is usually placed after the first regenerativesection.

In production of UHT milk, the homogeniser is generally placed upstreamin indirect systems but always downstream in direct systems, i.e.on theaseptic side after UHT treatment. In the latter case, the homogeniser is ofaseptic design with special piston seals, sterile steam condenser andspecial aseptic dampers.

However, downstream location of the homogeniser is recommended forindirect UHT systems when milk products with a fat content higher than6 10 % and/or with increased protein content are going to be processed.The reason is that 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 homogeniserlocated downstream.

Split homogenisationAn aseptic homogeniser is more expensive to operate. In some cases it issufficient if just the second stage is placed downstream. This arrangementis called split homogenisation.

Note that the whole section, including the heat exchanger, between thefirst and the second stage in the homogeniser, has to withstand a fairly highpressure.

Full stream homogenisationFull stream or total homogenisation is the most commonly used form ofhomogenisation of UHT milk and milk intended for cultured milk products.

The fat content of the milk is standardised prior to homogenisation, as isthe solids-non-fat content in certain circumstances, e.g.in yoghurtproduction.

Partial homogenisationPartial stream homogenisation means that the main body of skim milk is not

homogenised, but only the cream together with a small proportion of skimmilk. This form of homogenisation is mainly applied to pasteurised marketmilk. The basic reason is to reduce operating costs. Total power

Tout

= + Tin

P1 P

out

40


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consumption is cut by some 80 % because of the smaller volume passingthrough the homogeniser.

As sufficiently good homogenisation can be reached when the productcontains at least 0,2 g casein per g fat, a maximum cream fat content of18 % is recommended. The hourly capacity of a homogeniser used forpartial homogenisation can be dimensioned according to the followingexample.

Example:Q

p= Plant input capacity, l/h 10 000

Qsm

= Output of standardised milk, l/hQ

h= Homogeniser capacity, l/h

frm = Fat content of raw milk, % 4,0fsm

= Fat content of standardised milk, % 3,5fcs

= Fat content of cream from separator, % 35fch

= Fat content of cream to be homogenised, % 18

The hourly output of pasteurised standardised milk, Qsm

, will be approx.9 840 l. Inserted into Formula 2, this gives an hourly homogeniser capacityof approx. 1 915 l, i.e.about one-fifth of the output capacity.

The flow pattern in a plant for partially homogenised milk is illustrated inFigure 6.3.9.

The formulae for the calculations are:

1. Qsm

=Q

px (f

cs f

rm)

fcs

fsm

Qh=

Qsm

xfsm

fch

2.

Fig. 6.3.9Produc t flow at partial stream

homogenisation.

1 Heat exchanger

2 Centrifugal separator

3 Automatic fat standardisation device

4 Homogeniser

1 2

3

4

Raw milk, 4 % fat

Cream, 35 % fat

Skim milk, 0,05 % fat

Cream, 18 % fat

Standard ised milk, 3,5 % fat

Cooling media

Heating media

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06.03 Homogenisers