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

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HEAT TRANSFER METHODS

J Varley, Imperial College London, London, UK

Introduction

0001 Heat transfer is an essential stage of many food-pro-

cessing operations, e.g., cooking, sterilizing, freezing,

baking, and drying. The fundamental principles of

heat transfer and a review of methods for the addition

and removal of heat are considered in this article.

Principles of Heat Transfer

0002 There are three fundamental types of heat transfer:

conduction, convection, and radiation; these can

occur individually or simultaneously.

0003 Heat transfer may be steady-state or unsteady-

state. The equations considered here are for steady-

state heat transfer and can be solved easily. More

complicated solutions arise for unsteady-state heat

transfer (i.e., when temperature varies with time),

but solutions are available for simple geometries,

e.g., slab, cylinder, and sphere.

Conduction

0004 Conduction is the transfer of heat by exchange of

molecular energy from hotter to colder regions,

under conditions in which there is no turbulent

motion of particles. Heat transfer by conduction can

take place in a solid, liquid, or gas. Rates of heat

transfer by conduction are given by Fourier’s law:

q ¼kAðdT=dxÞð1Þ

where q is the heat transfer rate in the positive x

direction, k is thermal conductivity, A is the area

perpendicular to direction of heat flow, T is tempera-

ture and x is distance.

0005For example, for conduction through a slab the rate

of heat transfer is given by the following equation:

q ¼kA½ðT

 T

Þ=xð2Þ

where (T

 T

) is a steady temperature difference

maintained across the slab and x is the thickness of

the slab (Figure 1).

0006Thermal conductivity (k) is influenced by tempera-

ture, pressure, and composition of the food, including

such factors as moisture content and amount of air

trapped between cells. Values of thermal conductivity

are available for some food materials.

0007Fourier’s law indicates that the rate of heat transfer

by conduction is proportional to the driving force for

transfer, and inversely proportional to the resistance

to heat transfer. For conduction in a slab, the driving

force for transfer is the temperature difference, and

the resistance to heat transfer is given by x/k.

Thermal conductivity

of slab

Direction of heat transfer from hotter to colder region

fig0001Figure 1 Heat transfer by conduction through a slab. Repro-

duced from Heat Transfer Methods, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

HEAT TRANSFER METHODS 3029

Convection

0008 Convective heat transfer occurs when molecules or

groups of molecules in a fluid are mixed. In a solid,

molecules are not free to move, and convective heat

transfer will therefore only occur in a liquid or gas.

Natural convection arises when mixing is a result of

density differences caused by temperature gradients.

For forced convection, mixing is brought about

by mechanical means. Forced convection provides

higher rates of heat transfer than natural convection

and is therefore more commonly used in the food

industry, e.g., in air blast freezers, heat exchangers,

agitated retorts, and forced convection ovens.

Examples of natural convection include heat transfer

in domestic freezers or ovens with natural air circula-

tion (i.e., unaided by fans) and heating below boiling

in unstirred, steam-jacketed pans in the food industry.

0009 For heat transfer from a fluid to a surface, rates of

convective heat transfer can be calculated from the

following equation:

q ¼ hAðT

 T

Þð3Þ

where h is the heat transfer coefficient, T

is the

temperature of the fluid and T

is the temperature of

the surface.

0010 For convective heat transfer between a surface and

a fluid, a film (layer of fluid) exists between the bulk

fluid and the surface. At the surface, the velocity of

the fluid will be zero and there will be minimal mixing

of fluid elements. Heat transfer in this film will there-

fore be by conduction only, and the film will provide

the main resistance to heat transfer. The rate of

heat transfer can thus be written in terms of the

conduction equation:

q ¼ðk

ÞAðT

 T

Þð4Þ

where k

and x

are the thermal conductivity and

effective thickness of the film, respectively. The film

heat transfer coefficient is therefore equal to k

for

the film, and the resistance to heat transfer is 1/h.

0011 Typical values of film heat transfer coefficients are

available in the literature.

0012 Determination of heat transfer coefficients The heat

transfer coefficient can be written in terms of the

dimensionless Nusselt number (Nu):

ðNuÞ¼ðhDÞ=k ð5Þ

where D is a characteristic linear dimension of the

heat transfer system, e.g., diameter of a cylinder.

0013 For natural convection, the heat transfer coefficient

is found to depend on the temperature difference,

acceleration due to gravity, and the physical proper-

ties of the food (e.g., density, viscosity, thermal

conductivity, and specific heat). The dependence of

heat transfer coefficient on these factors can be

represented by the following equation:

ðNuÞ¼aðPrÞ

ðGrÞ

ð6Þ

where a, b, and c are constants. Pr and Gr are the

Prandtl and Grashof number, respectively:

Pr ¼ðc

Þ=k ð7Þ

Gr ¼ðD

gbTÞ=

ð8Þ

where c

is specific heat at constant pressure, m is

viscosity, D is the linear dimension of the system,

r is density, g is acceleration due to gravity, and

b is the coefficient of thermal expansion. Values

for the constants, determined experimentally for

some foods at certain conditions, are available in the

literature.

0014For forced convection, gravity and the coefficient

of thermal expansion are much less significant than

for natural convection. Therefore the heat transfer

coefficient does not depend to any great extent on

the Grashof number. Of more importance is the influ-

ence of forced circulation velocities, which can be

represented by the Reynolds number (Re). For forced

convection the heat transfer coefficient can, in gen-

eral, be evaluated from the following dimensionless

equation:

ðNuÞ¼aðReÞ

ðPrÞ

ð9Þ

As for natural convection, values for the constants (a,

b, and c) in the above equation have been determined

for a range of experimental conditions.

Composite Heat Transfer

0015The equations given above for heat transfer by con-

duction and convection show that the rate of heat

transfer is given by the ratio of the driving force for

heat transfer (temperature gradient) and the resist-

ance to heat transfer offered by the material. The

resistance to heat transfer for conduction and

convection is given by x/k and 1/h, respectively.

0016If heat is transferred through a composite material,

the heat transfer resistances behave in a similar

manner to electrical resistances in series. Consider,

for example, heat transfer to a food material from

the surroundings, first through a surface (surface 1),

then through several composite layers (layers 1 and 2)

and, finally, through a further surface film (surface 2).

The overall resistance to heat transfer would be given

by the following equation:

ð1=UÞ¼ð1=h

Þþðx

Þþð1=h

ð10Þ

3030 HEAT TRANSFER METHODS

where 1/U is the overall resistance to heat transfer, U

is the overall heat transfer coefficient, h

and h

are

the surface heat transfer coefficients for surface 1 and

2, x

and x

are the thicknesses of layers 1 and 2, and

and k

are the thermal conductivities of layers

1 and 2.

0017 The overall rate of heat transfer can then be written

as follows:

q ¼ UAT ð11Þ

where DT is the overall temperature difference.

Radiation

0018 Radiation is the transfer of heat by electromagnetic

waves. Radiant heat transfer is often found in the

food industry, especially in baking and drying

operations. Heat transfer by radiation depends on

temperature, surface area, and surface properties of

the body. Rates of heat emission by radiation can

be determined from the Stefan–Boltzmann law:

q ¼ AsT

ð12Þ

where A is the surface area, s is the Stefan–Boltzmann

constant (5.67  10

8

2

4

)andT is the

absolute temperature of the body.

0019 The Stefan–Boltzmann law determines the max-

imum amount of radiation that would be emitted at

a particular temperature by a perfect radiator (a black

body). However, most surfaces do not emit this max-

imum amount of energy, but many do emit an amount

of energy which is directly proportional to the max-

imum value. The proportionality constant is known

as the emissivity of the body (e) and has a value

between 0 and 1. Bodies with emissivities between 0

and 1 are known as gray bodies. For a gray body, the

heat transfer equation for radiation is as follows:

q ¼ eAsT

ð13Þ

Bodies not only emit radiation but also absorb and

reflect radiation. Absorption can be evaluated from

the Stefan–Boltzmann law for black bodies, and from

the modified equation for gray bodies, with the

emissivity replaced by the absorptivity (a). For gray

bodies, it can be shown that e ¼ a. Any radiation

which is received by a body but is not absorbed

must be reflected. For a gray body, the reflectivity is

equal to (1  a) and the amount of heat reflected can

be evaluated by replacing the emissivity in the above

equation with the reflectivity.

0020 Radiative heat transfer can take place between

bodies and also between a body and its surroundings.

As an example, radiative heat exchange between a

body and its black surroundings would be given by

the following equation:

q ¼ A

sðT

 T

Þð14Þ

where A

is the surface area of body 1, e

is the

emissivity of body 1, T

is the temperature of body

1, and T

is the temperature of the surroundings.

0021If the temperature difference (T

 T

) is small, the

above equation can be written as follows:

q ¼ A

T ð15Þ

The above equation shows that the rate of heat

transfer for radiation is directly proportional to the

temperature difference and inversely proportional

to the resistance to heat transfer (as is the case for

convection and conduction). The resistance to heat

transfer is given by 1/(A

). If convection

and radiation are occurring simultaneously, this

radiative heat transfer resistance can be used to

calculate an overall heat transfer resistance (as

described above for simultaneous convection and

conduction).

Heat Transfer Rates in Heat Exchangers

0022There are a wide variety of heat exchangers in use in

the food industry. These include direct steam injec-

tion, plate heat exchangers, and swept-surface heat

exchangers. The variety in design of heat exchanger

available reflects the wide range of physical proper-

ties of food requiring heat treatment. Most heat

exchangers can be used for both heating and cooling.

Heat transfer rates will, in general, be higher for

heating than for cooling, because the viscosity of a

liquid film will be higher during cooling than for

heating. (See Heat Treatment: Ultra-high Tempera-

ture (UHT) Treatments.)

0023The surface area of a heat exchanger required

to provide a certain heat transfer rate can be deter-

mined from the equation for the overall rate of

heat transfer (eqn 11), if the overall heat transfer

coefficient and the relevant value of the temperature

difference are known. In general, two fluids are

flowing on opposite sides of the walls of a heat ex-

changer, either cocurrently or countercurrently

(Figure 2). The relevant temperature difference for

heat transfer calculations in this case is the log mean

temperature difference (DT

), which is defined as

follows:

T

ðT

 T

ÞðT

 T

lnðT

 T

Þ=ðT

 T

ð16Þ

where T

and T

are the temperatures of fluids 1 and

2 at one end of the heat exchanger and T

and T

are

the temperatures of fluids 1 and 2 at the other end of

the heat exchanger.

HEAT TRANSFER METHODS 3031

Addition and Removal of Heat

0024 For addition or removal of heat to a food material, an

energy source is required. Sources of energy for bulk

generation of heat include gas, oil, solid fuels, electri-

city (microwave or dielectric heating), or infrared

energy. These energy sources may be used directly or

indirectly.

0025 Direct methods of heating involve direct contact

between the food material and the source of heat

energy. The food material therefore comes directly

into contact with the products of combustion of the

fuel. Direct methods may be used for the addition of

heat in food processing, e.g., baking ovens, but they

are rarely used for the removal of heat.

0026 In indirect methods, the food material is separated

by a barrier from the primary source of heat energy

and does not come into contact with the products of

combustion. Heat is generally transferred to the food

via a heat exchanger. The primary energy source may

be used as the heating medium, or it may be used to

heat a secondary medium, which will then be passed

through the heat exchanger. The heat transfer

medium may be a vapor or gas (e.g., steam or air), a

liquid (e.g., water), or electricity (as for resistance

heating systems). Heating of the secondary heat

transfer medium is usually accomplished by means

of a boiler, in which the source of fuel is burnt to

generate heat. Once the heat transfer medium has

been heated, it is pumped to a heat exchanger where

heat transfer takes place between this fluid and the

food material. In some systems, the secondary heat

transfer medium is passed directly over the food, e.g.,

steam blanching.

Removal of Heat

0027Removal of heat is commonly referred to as refriger-

ation or chilling. Removal of heat can be accomplished

by mechanical or nonmechanical refrigeration. Mech-

anical refrigeration is dealt with elsewhere. Typical

methods of nonmechanical refrigeration include heat

removal using ice, chilled brine, or cryogens. (See

Freezing: Principles; Cryogenic Freezing.)

0028Heat removal using ice Ice, at 0



C, is capable of

removing large quantities of heat (the heat capacity of

ice is 68 kJ kg

1

at 0



C) and has therefore been

used as a means of heat removal for many years. In

recent years, the use of ice for heat removal has de-

clined as mechanical refrigeration has grown in popu-

larity. Ice is still used for some chilling applications,

particularly when refrigeration is only required for a

short period of time (e.g., marine fishing operations)

or as a secondary refrigeration system which may be

required to deal with peak loads. There are several

ways in which ice can be employed as a heat transfer

medium. These include the following:

0029Packing crushed ice around the food material.

0030Circulating air over ice blocks before passing it

over the food product.

0031Circulating water over ice blocks prior to flow of

water through a heat exchanger.

The freezing temperature of ice can be reduced (and

hence the potential for heat removal increased) by the

addition of salts. However, corrosion can become a

problem when ice and salt mixtures are used.

0032Heat transfer using chilled brine Brine, a solution of

salt and water, can be used to remove heat from food

materials, e.g., on fishing vessels. As with ice, mech-

anical refrigeration has largely superseded the use of

brine in refrigeration. Sodium chloride and calcium

chloride are the most widely used brines for heat

removal. The freezing point, and hence capacity to

remove heat, depends on the concentration of the

brine solution. For typical concentrations used for

refrigeration, the minimum freezing point of sodium

chloride and calcium chloride solutions are 15



and 51



C, respectively. Problems with corrosion

will arise when employing brine solutions, if precau-

tions are not taken. Calcium chloride is much more

corrosive than sodium chloride. Appropriate precau-

tions to reduce corrosion include elimination of air,

neutralization to pH 7 or 8 with caustic soda or

hydrochloric acid, and addition of corrosion retard-

ants, e.g., sodium bichromate. A typical system which

Direction of flow for both streams

Direction of flow of heating medium

⬘

(a)

(b)

fig0002 Figure 2 Temperature differences for heat transfer in heat

exchangers: (a) co-current flow; (b) countercurrent flow. Repro-

duced from Heat Transfer Methods, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

3032 HEAT TRANSFER METHODS

uses brine to remove heat would consist of the

following steps:

0033 The brine is cooled to the required temperature in

direct expansion coils.

0034 The brine is then passed through a heat exchange

system which is in contact with the food.

0035 The brine is returned to the expansion system

where heat is removed.

Other media that have been used for chilling include

glycol–water mixtures and methylated spirits.

0036 Heat removal using cryogens In cryogenic systems,

heat is absorbed by the cryogen as it changes phase.

Cryogens commonly used for removing heat include

solid and liquid carbon dioxide, and liquid nitrogen.

Halogenated hydrocarbon refrigerant gases are also

used to a limited extent.

Addition of Heat

0037 Heat can be added by direct or indirect methods (in

direct methods the food material comes into direct

contact with the energy source, whereas for indirect

methods the food material and energy source are

separated by a barrier). (See Drying: Theory of Air-

drying.)

0038 Heat transfer media used in indirect heat addition

to food materials include vapors, gases, and liquids.

Only the most commonly used heat transfer media

will be considered in this article.

0039 Heat transfer using air Air is used to a limited extent

as a heat transfer medium in indirect heating systems,

e.g., in some baking processes, fluidized-bed cooking,

and hot-air driers. Air has the advantage of being

odorless and nontoxic, although it also has low

specific heat and thermal conductivity, which are

disadvantageous for efficient heat transfer.

0040 Heat transfer using steam Saturated steam is a

widely used heat transfer medium in food processing.

Maximum temperatures and pressures of typical

steam supplies generated for heat transfer in food

processing are 200



C and 1.7 MPa respectively.

Steam is used in a wide variety of heating processes

which include steam blanching, pasteurization, heat

processing of food in containers, extrusion cooking,

and evaporation.

0041 Saturated steam has several physical properties

that make it attractive for use as a heat transfer

medium, including the following:

0042 High heat capacity.

0043 Nontoxicity.

0044Stability.

0045No odor.

0046Low cost.

0047Adverse properties of steam for use as a heat transfer

medium include the following:

0048High vapor pressure.

0049Low critical point.

0050Corrosion, formation of scale, or foaming (during

steam generation) may occur as a result of impur-

ities being present in the raw material, i.e., the

water supply.

0051Heat transfer using hot water and other liquid

media Hot water is used as a heat transfer medium

up to temperatures of 200



C in a wide variety of food

processes, including blanching, pasteurization, and

in-container sterilization. Advantages of hot water

as a heat transfer medium include the following:

0052High specific heat and thermal conductivity.

0053Stability.

0054No odor.

0055Simplicity and ease of control of associated equip-

ment.

0056Abundance and low cost.

0057Limited corrosion, scale formation, and foaming,

as compared to steam systems.

Limitations include, for example, the possibility that

if hot water comes into direct contact with the food

material there may be substantial loss of water-

soluble nutrients (vitamins, salts, and minerals).

0058Other liquids that are used as heat transfer media

include mineral oil, chlorinated diphenyls, organo-

silicate, o-dichlorobenzene, and diphenyl or diphe-

nyloxide. All these liquids have the disadvantage of

giving off odors, but with the exception of diphenyl

and diphenyloxide they have the advantage of pos-

sessing low vapor pressures, which allows design of

high-temperature, low-pressure systems.

See also: Drying: Theory of Air-drying; Freezing:

Principles; Cryogenic Freezing; Heat Treatment: Ultra-

high Temperature (UHT) Treatments

Further Reading

Cengel YA (1998) Heat Transfer: A Practical Approach.

New York: McGraw-Hill.

Cleland AC (1989) Food Refrigeration Processes – Analy-

sis, Design and Simulation. Essex: Elsevier Applied

Science.

Green DW (ed.) (1984) Perry’s Chemical Engineering

Handbook, 6th edn. New York: McGraw-Hill.

HEAT TRANSFER METHODS 3033

Hayes GD (1987) Food Engineering Data Handbook. New

York: Longman Scientific and Technical.

Incropera FP and De Witt DP (1981) Fundamentals of Heat

Transfer. New York: John Wiley.

Loncin M and Merson RL (1979) Food Engineering –

Principles and Selected Applications. London: Academic

Press.

Mohsenin NN (1980) Thermal Properties of Foods and

Agricultural Materials. New York: Gordon and Breach.

Munton R and Stott JR (1978) Refrigeration at Sea, 2nd

edn. Essex: Elsevier Applied Science.

Winterton RHS (1997) Heat Transfer. New York: Oxford

Science Publications.

HEAT TREATMENT

Contents

Ultra-high Temperature (UHT) Treatments

Chemical and Microbiological Changes

Electrical Process Heating

Ultra-high Temperature (UHT)

Treatments

K R Swartzel, North Carolina State University,

Raleigh, USA

Introduction

0001 Commercially sterile foods are processed and pack-

aged in a manner that leaves the food free of micro-

organisms of public health significance, and inhibits

the growth of any microorganism under normal non-

refrigerated storage and distribution. Process methods

include both thermal and nonthermal. These are ac-

complished by either: (1) heating the product in a

hermetically sealed container so that the package

and contents are sterilized at the same time; or (2)

separately sterilizing the food and the container and

then aseptically filling and sealing the sterile product

in the sterile container.

0002 The former sterilization process relates to conven-

tional in-can or in-bottle processes, while the latter

refers to ultrahigh-temperature (UHT) processing. By

heating product during continuous flow, product

temperatures of 130–150



C are achieved. Due to

the relatively high sensitivity of vegetative bacterial

cells and spores to these high temperatures, less chem-

ical change occurs than with conventional canning

to achieve the same sterilizing effect. This results in

increased retention of nutrients, color, flavor, and

texture. Time needed for UHT processes is short, in

the order of seconds. By understanding product

constituent temperature-dependent changes, equip-

ment performance and process design, the desired

balance between desirable effects (inactivation of bio-

logical materials, i.e., enzymes, microorganisms, and

their spores), and undesirable effects (loss of product

quality, i.e., flavor, color, and flow properties) can be

achieved.

0003This section discusses thermal aseptic process

equipment, processing with and without particulates,

temperature measurement, and control consider-

ations. Product quality, microbiological consider-

ations, and aseptic packaging are discussed in

separate sections.

Equipment

0004Equipment required for continuous thermal steriliza-

tion of foods includes the following: product supply

pump, heat exchanger for preheating product,

metering pump, product sterilizer, holding section,

heat exchangers for cooling product, and aseptic op-

tional surge tanks (Figure 1). Components which may

be incorporated into a continuous sterilization system

include orifice plate or backpressure valve for main-

taining the pressure necessary to prevent the product

from boiling; homogenizer; and steam seals on

pumps, valves, connection or access ports; and a

monitoring and control system to insure the proper

thermal treatment.

0005The product supply pump and preheat exchanger

can be conventional food-handling equipment since

product has not been sterilized when it moves

through these components.

3034 HEAT TREATMENT/Ultra-high Temperature (UHT) Treatments

Metering Pump

0006 The metering pump determines the flow rate of prod-

uct through the holding tube. A positive displacement

pump is employed to maintain a constant flow rate

that is relatively independent of the load on the pump.

Rotary pumps are normally specified for pressures up

to 68.94 kPa. For higher pressures, plunger or piston

pumps are used. The metering pump is located up-

stream from the holding section. (The holding section

will be described in detail later.) If the pump has a

speed control device for changing the flow rate, some

mechanism is employed to insure that the pump does

not operate at a speed higher than that needed to

give product the required thermal treatment time in

the holding section. A lock on the speed control is the

most satisfactory method of preventing unauthorized

changes which would reduce residence time.

Direct and Indirect Heaters

0007 The sterilizer is a heat exchanger used to heat product

to the sterilization temperature. There are two basic

types of commercial sterilizers available: direct and

indirect units. Sterilizers may be categorized as direct

contact heaters (steam injectors and steam infusers)

and indirect heaters (tubular heaters, plate heaters,

and scraped surface heaters).

0008Steam or hot water is normally the source of heat

for continuous thermal sterilization. Direct contact

heating involves mixing steam and product together.

As steam condenses, its latent heat is transferred to

the product. When no wall separates the steam and

product, the heat transfer rate is high and product is

heated rapidly. Product is diluted by the steam con-

densate and vacuum equipment is generally used for

partial cooling and removal of excess moisture. Steam

is injected into a continuous stream of product in the

steam injector. The steam infuser introduces product

with a large surface area into a steam atmosphere

under sufficient pressure to achieve the desired steril-

ization temperature. Steam for direct contact heating

must be produced from potable water in a culinary

steam generator or in a boiler using accepted boiler

treatment compounds.

Raw product supply

Preheating

Metering pump

(constant product flow)

Sterilizing

Microwave or

electrical resistance heating

Holding

Cooling

Aseptic storage

Aseptic filling

Multiple product streams

sterilized separately

Direct heating Indirect heating

fig0001 Figure 1 Flow chart of continuous-flow aseptic processing.

HEAT TREATMENT/Ultra-high Temperature (UHT) Treatments 3035

0009 In the injector steam enters the product in pulses,

and excessive vibration, pressure fluctuations, and

temperature variation may result. To stabilize oper-

ation, orifices may be installed in the product en-

trance and discharge ports of the injector to isolate

the injection chamber. The orifices are approximately

equal in size and designed to produce at least a

68.94 kPa pressure drop across the injector during

normal operation.

0010 In indirect heating, product is heated by trans-

ferring heat from water or steam through a heat

exchange wall. The wall may be a tube, a plate, or a

scraped surface with each having its own

time–temperature profile. Tubular and plate heaters

used as sterilizers have relatively small flow channels

for product. This gives a large surface area to product

volume ratio for rapid heating. The high tempera-

tures used for sterilization require heat exchangers

to be built to handle the associated high pressures

required. Plate and tube designs incorporate varying

machining (plate patterns, tube dembling, tube hel-

ical patterns) in an effort to promote mixing, resulting

in more uniform product thermal treatment.

0011 Tubular and plate sterilizers lend themselves to

regenerative heating and cooling as an energy-saving

feature. Regenerative heating uses the heat of the hot

product to heat cold incoming product. Longer times

are required to heat product to the sterilization tem-

perature than in direct contact heaters, and deposit of

product on hot heat exchange surfaces sometimes

severely limits the operating time and makes cleaning

more difficult. (See Cleaning Procedures in the

Factory: Overall Approach.)

0012 Scrape surface heat exchangers (SSHEs) provide a

moving blade inside an elongated circular chamber

which scrapes the outer heated surface. SSHEs are

used with highly viscous products or products with

particulates.

0013 In the direct heating systems (steam injection or

infusion), the heating and cooling at higher tempera-

tures take place very rapidly compared with indirect

systems (tubes, plates, SSHE). In the direct systems,

virtually all of the thermal effects on product occur in

the holding section after the product has reached the

scheduled process temperature. In the indirect system,

substantial thermal effects can occur during heating

and some additional effects may occur during

cooling. For public safety reasons, however, usually

only the heating effects in the holding tube are

considered.

Holding Section

0014 The holding section is a pipeline section located

immediately downstream from the sterilizer. It is

designed to insure that all product is held at, or

above, the sterilization temperature for the time

equal to, or greater than the scheduled process time.

The tube is sloped upward toward the discharge end.

The upward slope insures that vapor cannot be

trapped. A vapor pocket in the holding tube would

permit product to move through the tube faster than

if the holding tube contained product only.

0015For public safety (destruction of pathogenic organ-

isms, mainly Clostridium botulinum), regulatory spe-

cification calls for a holding tube to be sized to hold

the fastest-moving particle in laminar flow for the

minimum specified time. (See Clostridium: Botulism.)

Heat Exchangers for Cooling Product

0016Product is cooled either by evaporative cooling in a

vacuum chamber or in indirect heat exchangers simi-

lar to those used for product heating and sterilization.

A vacuum chamber is often used when the sterilizer is

a steam injector or infuser. In addition to cooling

product, the vacuum chamber removes moisture.

When vacuum chamber pressure (or temperature) is

controlled properly, the same amount of moisture

added during heating by the condensing steam can

be removed. The operating pressure or temperature is

a function of product flow rate and properties, steam

properties and heat losses from the sterilizer, holding

tube, and vacuum chamber. The correct pressure

(or temperature) must be established for each system.

0017Since the vacuum chamber is normally controlled

at pressures below atmospheric pressure, aseptic

seals are used on all ports for pipeline connections,

sensors, sight glasses, and manholes. Steam seals are

commonly employed.

0018The heat exchangers used for cooling, whether

plate, tubular, SSHE, or vacuum chamber, are

designed to withstand temperatures and pressures

required for equipment sterilization before product

flow is initiated. (See Sterilization of Foods.)

Aseptic Surge Tank

0019The function of the aseptic surge tank is to act as a

reservoir for product between processing and pack-

aging. This may or may not be a part of all systems.

Operation of the tank is as critical as any other part of

the aseptic system.

0020Sterilization of the surge tank is independent of the

rest of the system. Four modes are utilized: steam and

vent, sterilizing, cooling, and tank sterile. The steam

and vent cycle allows the previously cleaned and sani-

tized tank to be vented of any gases and drained of

any liquids or condensates during heat-up. This cycle

continues until predetermined sterilizing conditions

(temperature and pressure) are met. The sterilization

cycle maintains this temperature and pressure for the

3036 HEAT TREATMENT/Ultra-high Temperature (UHT) Treatments

sterilization time. The tank is then cooled, but kept

pressurized with filtered sterile air or inert gas (i.e.,

nitrogen). The air pressure also provides a means to

move the sterile product to the filler without the use

of pumps.

Processing Considerations

Thermal Evaluation

0021 Techniques for evaluating thermal systems have been

the subject of elaborate developments in process

calculation procedures. Continuous-flow thermal

processing offers new challenges to the traditional

batch (retorting) process.

Backpressure Requirements

0022 For process temperatures above the boiling point of

the liquid, backpressure devices must be used. The

backpressure system assures proper pressure to

achieve and maintain the desired temperature. For

direct steam heating, condensation of the steam may

not be complete. Lack of complete condensation

inside the injector would cause temperature vari-

ations in the holding tube that could lead to some

fluid particles being processed below the desired

process thermal treatment.

0023 For steam injection units, the product and the

steam flows must be isolated from pressure fluctu-

ations inside the injection chamber. Supplementary

orifices on the product inlet and the heated product

outlet is one method of isolation. The orifices are

approximately equal in size and yield a minimum

product pressure drop of 68.94 kPa across the

injector during normal operations.

0024 For holding tubes in all systems, the product pres-

sure must be of sufficient magnitude to keep heated

product in the liquid phase. If this pressure is too low,

the resultant vaporization in the holding tube will

substantially reduce residence times. A minimum

product pressure in the holding tube of 68.94 kPa

for operating temperatures from 88



C through

100



C is satisfactory. For temperatures above

100



C, a minimum pressure of 68.94 kPa above the

product saturation value pressure is needed to keep

the heated product in the liquid phase.

0025 No noncondensable gases can be tolerated in the

holding section. Any two-phase flow caused by the

noncondensable gases would displace the product in

the holding tube, resulting in reduced residence times.

Holding Tube Calculations for Direct Systems

0026 As with the indirect systems, holding tube lengths

for direct systems are calculated as twice the length

required to hold the average measured flow. How-

ever, with the steam injection process, the holding

time is adjusted because the product volume increases

as the steam condenses to water during heating in the

injector. This surplus water is evaporated as the prod-

uct is cooled in the vacuum chamber. With as much as

60–70



C increase by steam injection, which is prob-

ably the maximum temperature rise that will be used,

a volume increase of 12% would occur in the holding

section. For holding time calculations, the volumes

per unit time are adjusted accordingly.

Consideration for Processing Fluids with Particles

0027Special equipment is usually incorporated for the

processing of fluids containing particles. As with all

UHT systems, the required minimum heat treatment

for the system must be achieved for all parts of each

particle. This means that for systems with particles,

the spot receiving the least thermal treatment (the

‘cold spot’) is generally in the center of the largest

particles with the lowest heat of conduction traveling

with a minimum of residence time (fastest particle).

To achieve this, the carrier fluid is often overpro-

cessed for the desired end result. For many products,

this is not a problem and SSHEs are the equipment

choice. For heat-sensitive carrier fluids, systems have

been developed that keep the particles and final car-

rier fluid separate during heating. In these systems the

solids are processed in a processing carrier fluid. The

end carrier fluid is heat-treated in a conventional

manner. After the particles have been properly pro-

cessed the processing fluid is drained off and the end

carrier fluid is then added. This also provides for

minimum mechanical damage. In other systems,

other means of heating are incorporated for rapid,

uniform heating, such as electrical resistance heating,

microwave heating, and/or radiofrequency heating.

With these systems, due to the unique form of heating

with conventional cooling, the surface of the particles

may define the cold spot.

0028Numerous special pieces of equipment have been

developed for moving the two-phase material and

controlling the flow. One such item relates to recipro-

cating piston pumps capable of pumping 2–3-cm-

diameter particulates. Moving chamber devices have

been developed to allow for different holding times

for different-size particles.

Product Deposition

0029During the heating of foods, product may deposit on

the heat exchange surface. The deposit fouls the heat

exchange surface and is undesirable for several

reasons. First, the deposit acts as an extra layer of

resistance to heat transfer. Product temperature is

HEAT TREATMENT/Ultra-high Temperature (UHT) Treatments 3037

raised enough to compensate for the increased resist-

ance. In several cases, the deposit can effectively

reduce the cross-sectional area available to flow.

Thus, not only is product temperature affected, but

so is residence time. Second, flakes of fouled material

can break off from the wall of the heat exchanger and

be carried and deposited elsewhere by the product

flow (i.e., homogenizing values, product packages,

etc.). Finally, the burnt material can give off undesir-

able flavors to the product.

0030 Direct systems are not as susceptible to fouling,

though fouled material may be deposited in the hold-

ing tube, and sometimes in the vacuum chamber.

Temperature Sensing and Utilities

0031 Temperature sensors are installed at appropriate

locations in the product flow stream for controlling

and/or recording temperatures to insure sterilization

of all contact surfaces. During equipment steriliza-

tion, hot water under pressure is circulated through

the processing system and interconnecting piping

with the aseptic filler.

0032 The sterilization temperature is monitored at the

coldest point in the system that must be sterilized.

This point is usually in the return line from the filler.

0033 The proper temperature in the holding section is

very important in achieving the commercial sterility

of a product. It is important that this temperature be

closely controlled, monitored, and recorded.

0034 Steam requirements for aseptic systems vary with

the type of system being used. Steam supply must be

of the proper pressure and flow rate to satisfy process

needs. Product heating and steam seals make up the

two major uses of steam in UHT processing. Steam

that comes in contact with product must be culinary

(food-grade).

0035 Steam seals provide a safety barrier at fittings,

valves, and other locations on the sterile side of

the system. They protect against microbiological

contamination from the environment. The seal as-

sembly provides a steam channel between two

gaskets, allowing live steam to enter and fill the

groove, thus forming a microbial barrier. These seals

are located on the potential problem areas of UHT

systems. Usually they are found in low-pressure areas

(i.e., vacuum chambers) and around moving system

components.

0036 Compressed air is used in pneumatic systems such

as controllers, recorders, valves, and sensors. Manu-

facturers of processing and pneumatic systems specify

compressed-air requirements for their equipment.

Because processing plants have existing air supply

systems, it must be determined whether sufficient air

volume and pressure are available. If not, corrective

procedures should be implemented. Pneumatic

systems are very susceptible to entrained particles of

moisture, oil, or dirt. Proper filtering devices should,

therefore, be installed so specifications for particu-

lates can be adhered to. A pneumatic system jammed

by dirt or oil can cause expensive and frustrating

shutdown of equipment.

0037Compressed sterile air or inert gas is also used by

manufacturers to maintain positive pressure in

aseptic tanks. This positive pressure serves two pur-

poses. First, it acts as a barrier to contamination.

Should there be a crack or failure in the tank or

valves, the positive pressure would blow sterile air

out of the tank, tending to prevent entrance of micro-

organisms. Second, the pressure is used to push the

product from the aseptic tank to the filler.

0038Since the air is in contact with sterile product, it

must also be sterile. Air is sterilized by using filters

and/or heaters. Air, to be sterilized, must itself be of

good quality so removal of moisture, oil, and particu-

lates is essential. The filters used have filter pores of a

size smaller than microorganisms, thus preventing

their passage through the filter. In addition, an air-

sterilizing assembly consisting of a heater, holding

tube, and cooler can be used to sterilize the air.

0039Safeguards must be developed to insure that prod-

uct is rejected if all conditions for sterile-aseptic pro-

cessing are not achieved. These conditions include

maintenance of scheduled process time and tempera-

ture, correct functioning of steam seal and/or sterile

air pressurization systems, or sufficient product pres-

sure downstream from the sterilizer to eliminate any

possibility of sterile product contamination. All im-

portant and necessary aspects of UHT processes must

be documented, with proper records maintained to

demonstrate safe operation.

0040The following sections will examine kinetic aspects

of product processing, including quality retention,

microbial destruction, and aseptic packaging.

See also: Canning: Principles; Cleaning Procedures in

the Factory: Overall Approach; Clostridium: Botulism;

Packaging: Packaging of Solids; Aseptic Filling;

Pasteurization: Principles; Quality Assurance and

Quality Control; Sterilization of Foods