Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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reduces the foaming properties of the product slightly,

giving less volume and/or a longer whipping time

than with the raw egg white.

0028 Protein denaturation may also be used beneficially

to indicate that a satisfactory heat treatment has been

given, using an assay of a suitable enzyme occurring

naturally in the product. The indicator enzyme

should be denatured under conditions slightly more

severe than those needed for microbial stability.

Ideally the activity of the indicator enzyme should

not be subject to wide variation with season or source.

0029 Alkaline phosphatase is denatured under slightly

more severe conditions than are required for destruc-

tion of M. tuberculosis, so the absence of alkaline

phosphatase activity is generally used as an indicator

for satisfactory pasteurization of milk. The presence

of alkaline phosphatase activity may indicate either

an insufficient heat treatment or contamination by

raw milk. The pasteurization of liquid egg in the UK

(minimum 64.4



C for 2.5 min) is not sufficiently

severe to inactivate alkaline phosphatase but will

denature a-amylase, whereas the milder treatment

required in the USA (minimum 60



C for 1.75 min)

aimed at achieving a 9D reduction in Salmonella

enteritidis, will leave residual a-amylase activity.

0030 Bacteria are more resistant to heat treatment when

the a

of the medium is lowered. Thus more severe

heat treatments are normally used for pasteurization

of sweetened products such as icecream and dessert

products. Examples of heat treatments are given in

Figure 4.

0031In fruit juices the pH is usually below 4.5 so that

growth of pathogenic bacteria will not be supported,

though pathogens may survive for some time. Yeasts

and some lactobacilli may grow and cause spoilage of

the juice, plus molds may grow at the surface. Heat

treatments to eliminate yeasts and lactobacilli are

more severe than for elimination of vegetative patho-

gens, e.g., 70



C/60 s or 85



C/30 s for citrus juices.

0032Survival of enzymes can cause problems on storage

of fruit juices. In apple juice extraction, polyphenol

oxidase will cause rapid browning of cold extracted

juices if the juice is not immediately treated with

antioxidants such as ascorbic acid or sulfur dioxide.

HTST treatment at 89



C/90 s will denature polyphe-

nol oxidase as well as potential spoilage organisms.

0033In citrus juices the presence of pectinase will lead to

breakdown of the cloud associated with the fresh

juices. HTST treatment at 90



C/10 s or 85



C/4 min

will denature the pectinase. For many fruit juices,

including apple and orange, the juice is extracted in

the country of origin, heat-treated, and concentrated.

10 20

Time (s)

Temperature (°C)

fig0003 Figure 3 Example of two heat treatment profiles (A and B) with

P* values using the same heating and cooling rates. (P* ¼ 1 for

a 15-s exposure at 72



C with z ¼ 8



C.)

Heating 0.22 2.4

Hold 1 0

Cooling 0.16 1.8

Total P* 1.4 4.2

60 70 80 90

Holding temperature (°C)

1000

100

0.1

Holding time (s)

fig0004Figure 4 Examples of heat treatments used in pasteurization

processes. o Milk (UK); O milk (USA); D liquid egg (UK); ^ cream

(UK); h cream (International Dairy Federation: IDF);

cream

(35%, IDF); X milk products >10% fat or added sugar (USA);

eggnog and frozen mixes (USA); icecream (UK); - - - fruit

juices; P* ¼1 for milk. The dotted boxes indicate the range of

values normally associated with low-temperature long-time

(LTLT) and high-temperature short-time (HTST) pasteurization

processes.

PASTEURIZATION/Principles 4385

The concentrate is then stored and transported in

bulk before reconstitution and final heat treatment.

0034 With any perishable product, the shelf-life is con-

trolled not only by the survivors of the heat treatment

but also by the posttreatment contaminants. With

mild heat treatments such as the pasteurization of

milk the total count may be reduced by two orders so

the shelf-life will be a function of the original count,

the postpasteurization contamination, and the storage

temperature. Fruit juices, though subjected to more

severe treatment, will also have their shelf-lives re-

stricted by posttreatment spoilage so that the same

juice may be packed as a short shelf-life chilled product

or, by aseptic handling and packaging, as a long-life

product stable at ambient temperatures. The advan-

tages resulting from avoiding postprocess contamin-

ation have led to the retention of in-container

pasteurization for many fruit juices and fruit drinks.

0035 The principle of pasteurization has also been

applied to the surface treatment of beef and other

carcasses following slaughter. Both steam and hot

water at 80



C have been used for reduction of the

surface microflora, including Escherichia coli. This

treatment reduces both the spoilage rate and the po-

tential public health risks associated with raw meat.

See also: Enzymes: Functions and Characteristics; Uses

in Food Processing; Fermented Foods: Origins and

Applications; Heat Transfer Methods; Heat Treatment:

Ultra-high Temperature (UHT) Treatments; Chemical and

Microbiological Changes; Electrical Process Heating;

Lactic Acid Bacteria; Milk: Processing of Liquid Milk;

Mycobacteria; Pasteurization: Pasteurization of Liquid

Products; Pasteurization of Viscous and Particulate

Products; Other Pasteurization Processes; Spoilage:

Chemical and Enzymatic Spoilage; Bacterial Spoilage

Further Reading

Cunningham FE (1986) Egg-product pasteurisation. In:

Stadelman WJ and Cotterill OJ (eds) Egg Science and

Technology, 3rd edn. Westport: Avi Publishing.

Dahlberg AC (1932) New York Agr. Expt. Sta. Tech. Bull.

203.

Dubos RJ (1960) Louis Pasteur, Freelance of Science. New

York: Da Capo Press.

Hammer (1928) Dairy Microbiology. New York: John

Wiley.

IDF (1986) Bulletin 200: Monograph on Pasteurized Milk.

Brussels: International Dairy Federation.

Kessler HG (1981) Food Engineering and Dairy Technol-

ogy. Freising: Verlag A. Kessler.

Michalski CB, Brackett RE, Hung YC and Ezeike GOI

(1999) Use of capillary tubes to validate US Department

of Agriculture pasteurization protocols for elimination

of Salmonella enteriditis from liquid egg products. Jour-

nal of Food Protection 62: 112–117.

Rees JAG and Bettison J (1991) Processing and Packaging

of Heat Preserved Foods. Glasgow: Blackie.

Robinson RK (ed.) (2002) Diary Microbiology Handbook,

3rd edn, pp. 765 New York: John Wiley.

Wilbey RA (1996) Estimating the degree of heat treatment

given to milk. Journal of the Society of Dairy Technol-

ogy. 49(4): 109–112.

Pasteurization of Liquid

Products

R A Wilbey, The University of Reading, Reading, UK

Background

0001Liquid products are relatively easy to pasteurize, since

their flow properties permit fast heat transfer by a

mixture of convection and conduction. Three basic

types of process have evolved to meet the require-

ments of the food industry: these are batch, in-con-

tainer, and continuous processes.

Batch Pasteurization

0002Batch processing represents the simplest approach,

where the bulk of the liquid is heated, and possibly

cooled, within a vessel with either a jacket or a

heating coil. This method was originally used for

pasteurization of milk and is still used for small-

scale production of icecream and yogurt mixes.

0003Batch processing suffers from a number of disad-

vantages. The product flow rate within the tank is

relatively slow, and so the heat transfer rates are low,

whereas increasing the severity of mixing will in-

crease the power consumption of the plant and may

damage the product. The overall heat transfer is also

slow because of the relatively low surface:volume

ratio, a problem that worsens exponentially with

increasing vessel size. Slow heating and cooling rates

contribute to a relatively high level of chemical

changes in relation to the biocidal effects of the heat

treatment, which can limit both the temperature and

applicability of the process. Thermal efficiency of

batch processing is usually low, as heat is not easily

or economically recovered during the cooling process.

0004Against these disadvantages, batch pasteurization

has the advantage of relative simplicity and lower

capital cost. Thus, it has been used widely for small-

scale processing, particularly where daily outputs are

likely to be less than 1000 l per day.

0005The basic batch pasteurizing setup will usually

consist of a jacketed tank with a mixer, temperature

sensor and recorder, plus a source of heat and control

4386 PASTEURIZATION/Pasteurization of Liquid Products

system. The mixer should be mounted off-center to

avoid excessive cavitation and swirling. A control

system is essential to maintain the product at the

desired holding temperature for what is usually an

extended period, e.g., 30 min at 63



C for milk or



C for icecream mix. With many small batch pas-

teurizers, the cooling is carried out within the vessel,

and the control system will actuate the changeover

from heating to cooling medium.

0006 Apart from the risk of contamination of the prod-

uct towards the end of the holding period, there are

two potential design/operational faults that can occur

with batch pasteurizers. Inadequate heating of the

head space within the pasteurizer may lead to conden-

sation dripping back into the product with a conse-

quent risk of contamination. The potentially more

serious risk is that of cross-contamination from raw

materials. The raw material feed must be kept separ-

ate from the pasteurized product exit pipe, normally

by using a top feed, bottom drain setup, as illustrated

in Figure 1. Furthermore, the drain valve must be

located close to, or preferably integral with, the

vessel, so that all liquid in the drain pipe between

the vessel and valve will also receive an adequate

heat treatment. The control system and/or the plant

operating instructions must insure that the drain

valve is closed before any raw materials are filled

into the batch pasteurizing vessel.

0007 The different mixing requirements and shear sensi-

tivities have led to a wide range of types of mixers

being used in batch pasteurizers. For icecream mix

manufacture, an emulsification stage is required,

which, on a small scale, can be carried out by using

a high-shear mixer. (See Emulsifiers: Organic Emulsi-

fiers; Uses in Processed Foods), and thus high- and

low-shear mixers may be placed into the same vessel.

Pasteurizer vessels for shear-sensitive products such

as cream have used large, slow-moving paddles and

rocking coils, though there have been contamination

problems resulting from leaks when passing coolant

through the paddles.

0008Where outputs of about 1000 l h

1

are required,

some processes use two batch pasteurizers in a semi-

continuous or flip-flop mode. The vessels are equipped

for mixing and heating, together with a temperature

sensor, recorder, and control system. In the case of

icecream, the emulsification is left to the end of the

heat treatment (66



C for 30 min or 72



C for 10 min),

the heat-treated mix being pumped to a high-pressure

homogenizer then cooled by passage through a plate

heat exchanger. This plant configuration is outlined in

Figure 2. A flip-flop mode of operation is sometimes

used for heat treatment of yogurt mix, where holds of



C for 30 min may be applied to the mix (though it

is advisable to homogenize the mix before the hold).

0009Although the semicontinuous mode of operation

overcomes some of the objections to batch heat treat-

ment, there remain two serious disadvantages in

terms of economics and product quality. It is difficult

to recover heat during the cooling phase, thus raising

the cost of processing. Product quality can be com-

promised by the need to empty the tank, after hold-

ing, at a rate equivalent to the time taken to prepare,

heat, and hold the succeeding batch. This means that

the actual hold used will vary from the nominal hold

to greater than twice that time when the last part of

the batch is removed. Though this excess holding time

can be reduced by using three tanks rather than two,

there remains a significant average over-processing

that can lead to cooked flavors and textural defects

in the product. (See Browning: Nonenzymatic; En-

zymatic – Biochemical Aspects; Protein: Interactions

and Reactions Involved in Food Processing; Vitamins:

Overview.)

In-container Pasteurization

0010Batch pasteurization of bulk liquids has a further

disadvantage: the pasteurized product must then be

dispensed into containers for storage, distribution,

and sale. This introduces the likelihood of recontami-

nation of the product and, without the use of aseptic

handling systems, limits the shelf-life and requires

refrigerated storage to minimize the growth of con-

taminants. Such postpasteurization contamination

(PPC) may be avoided by moving the heat treatment

downstream so that pasteurization is carried out in

the final container.

0011The principle of in-container pasteurization is iden-

tical to that of batch pasteurization, but in this case,

the heat-exchange surface is the container wall,

Mixer

Raw materials

Control panel

with recorder

Refrigeration

Heating

fig0001 Figure 1 Outline of a simple batch pasteurizer.

PASTEURIZATION/Pasteurization of Liquid Products 4387

whether plastic, glass, aluminum, or coated steel. The

simplest operations may use a tank of hot water or a

steam chamber for heating, followed by immersion in

cold water for cooling. Adequate disinfection of the

cooling water is essential to avoid contamination,

either directly via pinhole leaks or indirectly via con-

taminated films on the surface of the pack. Unlike

bulk batch pasteurization, the product temperature

cannot be readily measured during the heat treat-

ment, so test containers with the product must first

be set up with temperature sensors and run through

the process to establish safe processing conditions.

The development of radio telemetry and solid-state

logging systems now facilitates continuing checks on

product temperatures, but the main control is via the

temperature of the heating medium.

0012 The majority of in-container pasteurization oper-

ations have moved to continuous processes, based

on the conveyance of the containers through a tunnel

where the heating and cooling operations may be

carried out by spraying water over the containers.

The total process time is approximately 1 h, requiring

a long tunnel to give the desired holding time and

capacity. The width of such tunnels increases with

the capacity, as a number of containers will be treated

in parallel, so that machines can be up to 25 m long

and 7 m wide. Modular construction is essential.

0013 Typical products handled in tunnel pasteurizers are

soft and carbonated drinks, fruit juices, beers, sauces,

and, occasionally, semisolid products such as jams

and puddings. Pasteurization of acid products in this

manner can yield products of a very high microbio-

logical quality, where refrigeration is not essential for

a long shelf-life.

Tunnel Pasteurizer

0014 The product must first be filled into clean containers

and sealed. A scanner or check-weigher is used to

guard against under-filling. The stream of containers

must then be split into groups corresponding to the

number of lanes through the pasteurizer. The con-

veyer system must be open to permit the heat ex-

change liquid to drain through and must be made of

corrosion resistant material, e.g., stainless steel. A

combination of fixed and moving grid may be used

to convey the containers on an intermittent rather

than continuous basis. The conveyer speed is variable

to allow for changes in throughput and holding time.

0015Heating is carried out by a series of zones in which

increasingly hotter water, e.g., 25, 36, and 47



C, is

sprayed over the containers, as illustrated in Figure 3.

This incremental heating has two advantages: glass

bottles are unlikely to suffer thermal shock, and the

incremental system can allow surplus hot water (usu-

ally generated by steam injection) to cascade over into

the next hottest reservoir, thus reducing the energy

loss. The relatively poor heat-transfer characteristics

of the containers require an extended heating time,

since product flow within the container will be

relatively poor, being primarily due to convection

currents.

0016Some energy may be saved by using warm water

from the cooling section to preheat the incoming con-

tainers, the cooled water then being returned to the

cooling part of the tunnel. Process integrity demands

that not only water temperature at the various stages

be monitored but the flow checked, for instance by

monitoring the feed pressure to the spray bars.

Control of In-container Pasteurization

0017Many tunnel pasteurizers are built for processing a

specified product and packaging system and are

optimized for that combination. Nevertheless, the per-

formance of the machine must be checked in order for

the safety of the process to be proved. This is done by

monitoring the center temperature of test containers

passing through the pasteurizer under operational

conditions. An example of a temperature–time plot

Control panel

with recorder

Control panel

with recorder

Hot water

supply

Hot water

supply

Hot water

return

Hot water

return

Transfer pump Homogenizer Cooler

Product exit

Chilled water

fig0002 Figure 2 Outline of a semicontinuous pasteurizer.

4388 PASTEURIZATION/Pasteurization of Liquid Products

for a soft drink is given in Figure 4. The temperature–

time data must then be converted into a relevant

measure of the lethality of the process, such as the

use of pasteurization units (PU). For beers, the PU

used is a 1-min exposure at 60



C, assuming a z value

of 7



C, 20 PU being accepted as a safe process with

little effect on flavor, providing the dissolved oxygen

has been kept low, preferably below 0.1 p.p.m. (See

Pasteurization: Principles.)

0018 Major problems can occur when the tunnel pas-

teurizer stops, whether through a fault in the pasteur-

izer itself or in the production system downstream of

it. The risk then is that the containers within the

system will be over-processed if no remedial action

is taken. Most systems are arranged to shut off the hot

water sprays in the event of a stoppage, thus avoiding

further heating. Some systems may have extra cooling

sprays, the best systems using a computer model of

the process to model the actual process during the

stoppage, predict when the held product has had

sufficient heat treatment, and modify the heat treat-

ment given to the material on start-up, including

variation of the conveyer speed.

Continuous Pasteurization Processes

0019Continuous pasteurization processes take advantage

of the shorter exposure times needed at higher

1st 2nd 3rd 4th

1st Super-

heating

2nd Super-

heating

Pasteuri-

sation

1st 2nd 3rd

CoolingHeating

Water

supply

Steam supply

Condensate

4th

fig0003 Figure 3 Schematic layout of the water circulation system in the Etna P85 tunnel pasteurizer. (Courtesy of Simonazzi srl.)

4.5

Times and temperatures in balanced condition.

Total time 53.5 min

4.5

Spray temperature (C)

Inner temperature (C)

4.5 4.5 12.5 5 4.5 4.5 4.5 4.5

26.5

33.5

39.5

48.5

12.5

32.5

fig0004 Figure 4 Time–temperature plot for an eight-stage tunnel pasteurizer. Dotted lines indicate water temperatures. (Courtesy of

Simonazzi srl.)

PASTEURIZATION/Pasteurization of Liquid Products 4389

temperatures, and these are often referred to as high-

temperature, short-time (HTST) processes. The de-

velopment of HTST processes has been dependent

on the development of hygienic heat exchangers,

most notably the plate heat exchanger.

0020 The concept of the plate heat exchanger is brilliantly

simple. A pair of plates are separated by a thin elasto-

mer seal, so that the liquid flows as a thin layer, 1.5–

5 mm thick, depending upon the seal design,

minimizing the distance through which heat must

travel. This gives an exceptionally high surface:vo-

lume ratio, typically in excess of 500:1 (compared

with 5:1 for a 1000-l batch pasteurizer). The plates

are rippled, e.g., as shown in Figure 5, so that the flow

is broken up, and turbulent flow can be maintained at

relatively low velocities over a larger surface area than

suggested by the overall dimensions of the plate.

0021All the plates in the heat exchanger can be pressed

with one tool, different heat exchanger configur-

ations being achieved by the presence or absence of

the ports cut at the corners of the plate and by the

design of the elastomer seal. Plates are now designed

so that the seals are held in place without the need for

adhesive, speeding up maintenance. The range of

flow rates over a plate is limited by the efficiency of

heat transfer at lower flow rates, while the pressure

drop and possible product damage provides the upper

limit. This capacity limitation may be overcome by

arranging plates in parallel so that there is an optimal

flow across each plate. Similarly, the time permitted

fig0005 Figure 5 (see color plate 121) Heat exchanger plates. (Photo by courtesy of Tetra Pak Processing UK.)

4390 PASTEURIZATION/Pasteurization of Liquid Products

for heat exchange can be lengthened incrementally by

arranging plates in series, as illustrated in Figure 6.

0022 The heat-exchange medium is normally arranged

to flow countercurrent to the product. This reduces

the temperature differential between the two media,

increasing the overall efficiency of heat transfer, as

illustrated in Figure 7. In most cases, the heat-transfer

medium will be flowing at a higher rate than the

product, but with extremely heat-sensitive products

such as fermented milks, similar flow rates must be

used to minimize the temperature differential.

0023 With heat-sensitive foods the build-up of a fouling

layer will limit the running time of the HTST pasteur-

izer, owing to both pressure build-up and loss of heat

exchange. The nature of the fouling layer will vary

with the medium but is most commonly protein, with

or without mineral deposition. Fat and carbohydrate

components may become associated with fouling

layers as they build up. Mineral deposits are common

where hard water is used for cleaning and disinfec-

tion; additional sequestrant in the cleaning solution

or periodic acid cleaning may be required. The

running time may be extended by using a wider gap

between the plates, though this would require a cor-

respondingly larger heat exchanger to compensate for

lower heat-transfer rates. Larger gaps have also been

used when liquids have small quantities of suspended

solids, e.g., orange juice containing cells that can

become lodged within the plate pack and even with

back flushing are very difficult to remove. Tubular

heat exchangers may be used as an alternative to

plates where fouling is a serious problem but thermal

efficiency would suffer.

0024The use of a plate heat exchanger has made it much

easier to recover heat by using the hot heat-treated

product to heat the incoming untreated liquid. Heat

recoveries of up to 95% may be achieved by this 1:1

flow, the recovery being limited by the increasing

capital cost of providing further heat exchange cap-

acity as temperature differentials decrease and the

hygiene issue of the relatively low flow rates permit-

ting the build-up of a biofilm at temperatures that will

permit microbial growth and may eventually cause

significant spoilage of the product.

0025This heat recovery, often referred to as ‘regener-

ation,’ may also pose a hygiene risk. This risk arises

because, in a simple flow-through system, the pres-

sure of the raw liquid will be greater than that of the

heat-treated liquid downstream of it. Thus, any leak-

age could be from the raw into the pasteurized prod-

uct and could have serious public health implications.

The design of heat exchanger plates seeks to avoid

this problem by insuring that the two liquids are

separated by either the heat exchanger plate or by

two seals, one for each product with a vent to the

atmosphere between. Partial failure of one seal is not

a major problem as the product would leak on to the

floor, but the development of a crack or pinhole crev-

ice could lead to cross-contamination. Most heat ex-

changer plates are made from a corrosion and stress

crack-resistant stainless alloy (e.g., AISI 316), but

failure can eventually occur, and checks are needed.

The risk can be avoided by using a second pump to

drive the downstream part of the process at a higher

pressure, raising the pressure in the downstream

fig0006 Figure 6 Arrangement of heat exchanger plates in parallel and series. 1, Cold product in; 2, Cool water out; 3, Hot water in; 4, Hot

water out; x, blanked-off ports.

Cooling medium

Distance through heat exchanger

Hot product in

Temperature

fig0007 Figure 7 Illustration of co-current (——) and counter-current

(——)flowthroughaheatexchangerusingequalflowrates.

PASTEURIZATION/Pasteurization of Liquid Products 4391

regeneration section either through the design of the

final cooling section or by incorporating a back pres-

sure valve into the heat exchanger discharge, when

this will not damage the product. An alternative

system now available is to use double plates; these

plate pairs have no internal gaskets other than around

the ports, so that any leakage, whether from a port or

through a crack in the plate, would leak out of the

heat exchanger stack and become obvious. The intro-

duction of the air gap between the pairs reduces the

heat transfer rate and would need more plates to

achieve the same regeneration efficiency. It must be

remembered that the savings from the use of regener-

ation are twofold; the first saving is from a reduction

of heat input, and the second, often greater, saving

comes from reduction in the refrigeration require-

ment.

0026 Figure 8 illustrates a typical arrangement for the

pasteurization of liquid milk. Raw milk should

be taken from well-mixed bulk storage, e.g., a

100 000–200 000 l silo tank, and fed to a float balance

tank, which provides a constant head to the pasteur-

izer feed pump. Centrifugal pumps are frequently used

to feed pasteurizers because of their relatively low

cost, but their output is dependent on the pressure

that they operate against. In the case of beer, the

pump arrangement must allow an over-pressure to

maintain the carbon dioxide in solution throughout

the process, giving a total working pressure in excess

of 8 bar. Shear-sensitive products such as cream re-

quire the use of a gentler pump such as a lobe pump,

which also has the advantage of a constant output

against moderate variations in pressure. Where a cen-

trifugal pump is used, it must be accompanied either

by a flow controller or by a second constant-output

pump to guarantee a fixed flow rate. This requirement

for a constant flow rate is set by the need to guarantee a

constant residence time in the holding tube at the

completion of heating. The heating is normally

divided into two sections, the first preheating section

using regeneration and the second heated by hot water

or steam.

0027Sometimes, the preheating section is partially by-

passed by a length of pipe with a fine control valve.

This may be used to reduce the thermal efficiency

during cleaning, but the main use is to control the

final temperature of products such as milk for cheese-

making, where the milk will be cooled to incubation

temperature (e.g., 30



C) without the need for final

cooling and without upsetting the pasteurization tem-

perature (typically 72–73



C). A small flow of cold

raw milk past the preheater can lower the regener-

ation efficiency and hence raise the final milk

temperature. This implies that the limit for thermal

Raw milk pump

Chilled water

Flow

controller

Regeneration

Control panel with recorder

Hot water pump

Holding Tube

Steam

Flow diversion

valve

Filter/clarifier/separator,

standardization &

homogenization options

Raw milk

Float balance tank

Pasteurized

milk

fig0008 Figure 8 Schematic diagram of a milk pasteurizer.

4392 PASTEURIZATION/Pasteurization of Liquid Products

efficiency is 63% when pasteurizing milk for

cheesemaking.

0028 In the simpler plants, preheated milk may be

filtered before being subjected to final heating, etc.

Many modern plants, however, have replaced filtra-

tion by centrifugation, sometimes simply for clarifi-

cation but more often to separate the cream. The

cream may be fed back into the skimmed milk to

produce standardized milks for subsequent process-

ing. Standardization may be carried out as a continu-

ous process, the milk then passing to a high-pressure

homogenizer that will provide a constant output irre-

spective of back-pressure (thus requiring a pressure-

relief valve). The temperature provided by preheating

is usually sufficient for homogenization of milk or

cream, and incorporation of homogenization at this

point avoids contamination from the homogenizer.

Higher temperatures (65–80



C) are desirable for

icecream emulsification, so the homogenizer may

then be placed at the beginning of the holding tube.

0029 Hot water is the most common heating medium,

energy being provided by steam injection and

the condensate being returned to the boilers. Water

temperature is controlled by sensing the water tem-

perature, the controller generating a signal to open

the pneumatically operated steam control valve when

the temperature drops below the preset level. Some

small pasteurizers, e.g., 200 l h

1

, may use elec-

trical heating only. A few larger pasteurizers may

use low-pressure steam as the heating medium,

though this is less easy to control and is best avoided

for temperature-sensitive products.

0030 Most pasteurizer designs require the product to be

held for a fixed period of time in order to achieve the

desired lethality. The most reliable method of achiev-

ing this is to use a holding tube, the design of which is

critical. Any bacteria present in the liquid to be pas-

teurized can be treated as solid particles in suspen-

sion, and the rate at which they progress along the

holding tube will depend upon the flow pattern that is

induced within it. The flow pattern may be deduced

by estimating the Reynolds Number (Re) for that

liquid under those process conditions, i.e.,

Re ¼ v r d=m,

where v is the velocity, r is the specific gravity at the

holding temperature, d is the pipe diameter, and m is

the dynamic viscosity at the holding temperature.

When Re 2000 for flow in tubes, the flow is said

to exhibit streamline or laminar flow, where there is a

large variation in velocity across the tube, with the

velocity in the center being twice the average velocity,

i.e., the minimum time taken for a particle to pass

along the tube would be half that of the average, as

illustrated in Figure 9. Values of 2000–4000 are said

to characterize critical or intermediate flow patterns,

where the geometry of the tube, e.g., the incidence of

bends, will affect the flow pattern. At Re 4000, the

flow in a tube will exhibit turbulent flow, where

the distribution of velocities is much narrower, and

the minimum residence time may be as high as 0.83

that of the mean. Turbulent flow is designed into most

holding tubes, but the holding efficiency is assumed to

be 0.75. The advantages of turbulent flow in holding

tubes are tighter control of the lethality of the process

and less chemical damage to the product for that

lethality.

0031The second issue for holding tubes is heat loss.

Heat is readily lost even from polished stainless sur-

faces, particularly with smaller plants where the sur-

face:volume ratio is higher. Holding tubes must never

be situated in draughty locations such as close to

doors, since sudden draughts can lead to a drop in

holding temperature that will cause the process to

fail. Where cold and variable ambient conditions are

expected, the holding tubes must be insulated, and if a

long hold is being used, e.g., holding whole liquid egg

at 65



C for 150 s in the UK or 60



C for 210 s in the

USA, auxiliary heating of a box around the holding

tube should be considered.

0032The temperature of the hot product is normally

sensed by a fast responding probe close to the end of

the holding tube. The signal generated is used to both

provide a permanent record of the heat treatment

and to generate a signal to the flow diversion valve.

Should the temperature in the holding tube fall out-

side a preset range (e.g., 72–78



C for milk) or a

failure in any of the services (steam, air, power), the

Average velocity

Maximum under turbulent flow

Maximum under streamline flow

Streamline

Turbulent

fig0009Figure 9 Illustration of the relationships between flow veloci-

ties in a tube under turbulent and laminar flow regimes.

PASTEURIZATION/Pasteurization of Liquid Products 4393

valve must move into the divert position and the

heated product be sent back to the balance tank.

Only when the temperature is in the desired range

should the valve permit product to flow forward into

the downstream, cooling sections of the plant. This

action protects the downstream part plant from con-

tamination by under-processed material and hence

avoids risk to the consumer. The recorder fitted to

the pasteurizer must record the pasteurization tem-

perature, final product temperature, and status of the

flow diversion valve throughout the operation.

0033 In most large-throughput plate heat exchangers,

the bulk of the cooling will be achieved by regener-

ation, with final cooling by chilled water. Chilled

water may be produced directly by refrigeration or

indirectly via an ice bank. Where cooling by glycol or

brine solutions is employed, more stringent controls

are needed to avoid freezing. In all cases, the coolant

must be of good microbiological quality and product

contamination avoided. With rheologically simple

liquids such as beer and milk, the cooling would use

countercurrent flow to achieve the optimal exit tem-

perature. However, with more complex liquids such

as cream, where the viscosity is dependent on many

factors, including the time–temperature–shear profile

during cooling, a mixture of co- and counter-current

cooling may be employed. For the thickest creams,

there is only partial cooling in the heat exchanger,

with the remaining cooling being achieved after

filling, in the retail container. This approach demands

the highest hygiene standards during filling and

effective cooling of the containers.

Tubular Heat Exchangers

0034 Although plate heat exchangers are excellent ma-

chines for pasteurization of low-viscosity liquids,

there can be problems with those liquids either dis-

playing higher viscosities, a tendency to foul at modest

temperatures or containing suspended particles. The

introduction of tubular heat exchangers enables a

greater path width, e.g., 5–10 mm, so that fouling

becomes less critical and particles less likely to

become trapped.

0035 In the case of fruit juices containing pulp, simple

tubular heat exchangers provide an effective means of

pasteurization though with higher capital and run-

ning costs. This may be attributed to lower surface:-

volume ratios than are possible with plate heat

exchangers and to problems in heat recovery. The

surface:volume ratio can be increased by adopting

an annular design, and heat exchange may be im-

proved by adopting a rippled wall profile to promote

turbulence. Detail improvements in the design of tu-

bular heat exchangers have reduced the cleaning

problems, particularly in permitting product–product

heat exchange, and so direct heat regeneration has

become possible. Control systems are similar to those

for plate heat exchangers.

0036Although most fruit juices have pH values below

4.5 and are not considered high risk, pear juice,

banana pure

e, and tomato juice may be low acid.

Regardless of the public health risk, heat treatment

is needed if rapid spoilage is to be avoided. Pas-

teurization at over 70



C for 15 s should inactivate

vegetative spoilage organisms such as yeasts, mold

mycelia, and Lactobacillus fermentum in a high

acid juice. More severe conditions such as 87



C for

15 s would be needed for inactivation of spoilage

enzymes, and for inactivation of Byssochlamis spp.

ascospores, temperatures in excess of 100



C would

be needed. Tomato juice has been processed at 115



for 15 s.

Concluding Comments

0037Low-acid products must be kept refrigerated after

pasteurization and have a relatively short shelf-life.

Postpasteurization contamination poses a major

threat to product quality and can only be avoided

by using either in-container treatments or aseptic

filling techniques. The advantages and disadvantages

of the pasteurization processes are summarized in

Table 1.

tbl0001Table 1 Summary of the main advantages and disadvantages

of pasteurization systems

Advantages Disadvantages

Batch tank Batch tank

Low capital cost High energy cost

Low maintenance cost Poor cooling characteristics

Very flexible

Suitable for small quantities

PPC risk depends on

downstream hygiene

In-container tunnel In-container tunnel

Avoids PPC High capital cost

High capacity possible High space requirement

Consistent lethality

More efficient than batch

Product containers must be

heat-resistant

Low cleaning requirement Less efficient than HTST

Higher maintenance than

others

Plate heat exchanger Plate heat exchanger

Moderate capital cost Viscosity limit

Low maintenance cost Particulates limit

Low space requirement Shear can damage product

High energy efficiency

Consistent lethality

PPC risk depends on

downstream hygiene

Fouling problems with

temperature-sensitive

products

4394 PASTEURIZATION/Pasteurization of Liquid Products