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

Подождите немного. Документ загружается.

equivalent sphere is then used to calculate the freezing

time of the food item.

0038 In the equivalent sphere diameter technique, the

volume–surface diameter, D

, is defined as the diam-

eter of a sphere having the same volume-to-surface-

area ratio as the irregular shape:

ð18Þ

where V is the volume of the irregular shape and A

the surface area of the irregular shape. In addition, the

volume diameter, D

, is defined as the diameter of a

sphere having the same volume as the irregular shape:





1=3

ð19Þ

Then, the equivalent sphere diameter, D

eq, s

, is defined

as follows:

eq;s



þ 1



þ 1

ð20Þ

Thus, the prediction of the freezing time of the irregu-

larly shaped food item is reduced to predicting the

freezing time of a spherical food item with diameter

eq, s

See also: Freezing: Blast and Plate Freezing

Further Reading

Becker BR and Fricke BA (1999a) Evaluation of semi-

analytical/empirical freezing time estimation methods,

part I: regularly shaped food items. International Jour-

nal of Heating, Ventilating, Air-Conditioning, and Re-

frigerating Research 5: 151–169.

Becker BR and Fricke BA (1999b) Evaluation of semi-

analytical/empirical freezing time estimation methods,

part II: irregularly shaped food items. International Jour-

nal of Heating, Ventilating, Air-Conditioning, and Re-

frigerating Research 5: 171–187.

Becker BR and Fricke BA (1999c) Freezing times of regu-

larly shaped food items. International Communications

in Heat and Mass Transfer 26: 617–626.

Becker BR and Fricke BA (1999d) Food thermophysical

property models. International Communications in

Heat and Mass Transfer 26: 627–636.

Cleland AC and Earle RL (1977) A comparison of analyt-

ical and numerical methods of predicting the freezing

times of foods. Journal of Food Science 42: 1390–1395.

Cleland AC and Earle RL (1979) A comparison of methods

for predicting the freezing times of cylindrical and spher-

ical foodstuffs. Journal of Food Science 44: 958–963,

970.

Cleland AC and Earle RL (1982) Freezing time prediction

for foods – a simplified procedure. International Journal

of Refrigeration 5: 134–140.

Cleland DJ, Cleland AC and Earle RL (1987a) Prediction of

freezing and thawing times for multi-dimensional shapes

by simple formulae – part 1: regular shapes. Inter-

national Journal of Refrigeration 10: 156–164.

Cleland DJ, Cleland AC and Earle RL (1987b) Prediction of

freezing and thawing times for multi-dimensional shapes

by simple formulae – part 2: irregular shapes. Inter-

national Journal of Refrigeration 10: 234–240.

Fricke BA and Becker BR (2001) Evaluation of thermophy-

sical property models for foods. International Journal of

Heating, Ventilating, Air-Conditioning and Refrigerat-

ing Research 7: 311–330.

Ilicali C and Engez ST (1990) A simplified approach for

predicting the freezing or thawing times of foods having

brick or finite cylinder shape. In: Engineering and Food,

Speiss WEL and Schubert H (eds) vol. 2, pp. 442–451.

London: Elsevier Applied Science.

Ilicali C and Hocalar M (1990) A simplified approach for

predicting the freezing times of foodstuffs of anomalous

shape. In: Speiss WEL and Schubert H (eds) Engineering

and Food, vol. 2, pp. 418–425. London: Elsevier

Applied Science.

Pham QT (1984) An extension to Plank’s equation for

predicting freezing times for foodstuffs of simple shapes.

International Journal of Refrigeration 7: 377–383.

Pham QT (1985) Analytical method for predicting freezing

times of rectangular blocks of foodstuffs. International

Journal of Refrigeration 8: 43–47.

Plank R (1913) Die Gefrierdauer von Eisblocken. Zei-

tschrift fu

r die gesamte Kalte-Industrie 20: 109–114.

Plank R (1941) Beitrage zur Berechnung und Bewertung der

Gefriergesch Windikeit von Lebensmitteln. Beiheft zur

Zeitschrift fu

r die gesamte Kalte-Industrie 10: 1–16.

Blast and Plate Freezing

M F G Boast, MBA Consulting Engineers, Markyate,

Background

0001The provision of low temperature for freezing is

obtained from closed-circuit compression–expansion

refrigeration systems, or by a cryogenic process of

exposing the product to a cold atmosphere of liquid

nitrogen or liquid carbon dioxide.

0002Various refrigerants have been used for the compres-

sion–expansion system, which forms the major source

of food freezing, probably 90% of all installations.

0003Ammonia and sulfur dioxide are excellent media

for the refrigeration process, but they carry the

hazard of being inflammable and the risk of explo-

sion. This led to the development during the 1930s of

2718 FREEZING/Blast and Plate Freezing

the first chlorofluorocarbon (CFC) refrigerants, R11

and R12. These refrigerants replaced SO

in domestic

refrigeration and NH

for small freezing systems. The

later development of R22 (a hydrochlorofluoro-

carbon, or HCFC) and R502 (an azeotrope), dis-

placed NH

from all but the largest of industrial

freezing systems. Table 1 shows the characteristics

of common refrigerants.

0004 During the 1980s, the environmental effect of

CFCs and HCFCs, in terms of ozone depletion poten-

tial and contribution to the greenhouse effect, became

apparent. This has led to the restriction in the use of

some refrigerants under the Montreal Protocol, and

the development of new refrigerants called hydro-

fluorocarbons (HFCs).

Refrigerants and the Environment

Ozone Depletion

0005 The ozone-depleting effect of refrigerants is import-

ant, because the ozone layer filters out ultraviolet

(UV) radiation, which might otherwise reach the sur-

face of the earth. This radiation could have seriously

damaging effects on germinating crops and on the

photoplankton of the oceans. In addition, increased

doses of UV light can cause malignant skin cancer in

fair-skinned races.

Greenhouse Effect

0006 Ecological systems of the world are finely dependent

on temperature. Small changes in average temperature

can affect the distribution of crops and animals.

There is evidence that the average global temperature

has been increasing since the mid-nineteenth cen-

tury because of carbon dioxide from the burning

of fossil fuels. If it is not halted, mean sea levels will

rise.

0007The consequences of even a small rise in sea level in

certain areas of the world, where strong seasonal

winds might coincide with high tides, would be disas-

trous. It is impossible to predict the likely rise of mean

sea level, but it could be significant. In the remote

past, the earth had no polar ice caps, presumably

because of high concentrations of carbon dioxide in

the atmosphere, which had not by then been absorbed

by forests and oceans.

0008Chlorofluorocarbons are greenhouse chemicals

more potent than the present greenhouse chem-

icals (carbon dioxide and methane) and should there-

fore not be released into the atmosphere.

Recommendations

0009Refrigerants known as CFCs are typically R11, R12,

R115, R13B1, and R502 and have now been phased

out, although some installations may still exist using

the above refrigerants. New systems are not being

constructed from the refrigerants.

0010The destruction of CFCs present problems in terms

of safety and the need for environmental protection.

Therefore, any system being decommissioned con-

taining CFCs must be addressed in a controlled

manner by personnel with the appropriate experience

and qualifications.

tbl0001 Table 1 Characteristics of common refrigerants

Substance Type Formula MontrealProtocol Ozone depletion potential

(R11 ¼1)

Greenhouse potential

(CO

¼1)

Flammability

R11 CFC CC1

F Yes 1 3 300 No

R12 CFC CC1

Yes 2 10 000 No

R22 HCFC CCHClF

No 0.05 1 100 No

R113 CFC CC1

FCC1F

Yes 0.8 4 500 No

R114 CFC CC1F

CC1F

Yes 1.0 1 300 No

R115 CFC CClF

Yes 0.6 25 000 No

R123 HCFC CHC1

No 0.02 50 No

R124 HCFC CHClFCF

No 0.02 300 No

R125 HFC CHF

No 0 1 900 No

R134a HFC CF

F No 0 900 No

R141b HCFC CH

CCl

F No 0.08 300 Slight

R142b HCFC CH

CClF

No 0.06 1 200 Slight

R152a HFC CH

CHF

No 0 100 Moderate

R500 Azeotrope R12/152a Yes 0.74 7 400 No

R502 Azeotrope R22/115 Yes 0.33 13 300 No

R717 Ammonia NH

No 0 0 Moderate

R407C Azeotrope R32/125/134a No 0 1 610 No

R404A Azeotrope R125/143a/134a No 0 No

R410A Azeotrope R32/125 No 0 1 890 No

FREEZING/Blast and Plate Freezing 2719

0011 The construction of new systems for small to

medium-sized uses refrigerant R404A. Large indus-

trial systems use refrigerant R717 (ammonia).

0012 Refrigerant R717 should only be used by com-

panies and specialists with knowledge of this refriger-

ant owing to its pungent smell and hazard potential.

Maintenance

0013 Plants should be constructed to the appropriate codes

of practice and standards. Routine leak testing must

be conscientiously carried out. In addition to being

environmentally correct, this will produce cost

savings and improve plant reliability.

0014 Leak detectors should be fitted to all systems with a

large refrigerant charge and should report to a central

monitoring station.

Substitutes for CFCs

0015 The substitute for refrigerant 12 is refrigerant

R134A. The substitute for refrigerant R502 is

refrigerant R404A. These refrigerants are not suitable

for use with mineral oils, and polyester lubricants

have been developed. Changing the system from re-

frigerant R12 or refrigerant R502 requires specialist

knowledge and should only be carried out by suitably

experienced companies and personnel. Changing

the system refrigerant from R12 or R502 requires

system flushing and thoroughly cleaning to insure

that all traces of the previous refrigerant have been

removed.

0016 At the present time, systems using refrigerant R22

can continue to operate and can be serviced with new

virgin refrigerant until the year 2005.

Freezing Considerations

Freezing Times

0017 Freezing times vary, and each product has an associ-

ated freezing time that will depend on its compos-

ition, dimensions, and packaging. Dimensions and

packaging have a direct effect on the freezing time,

as heat can only leave the product through its surface.

Products that are unpacked and have a large surface

area-to-weight ratio, such as peas, will freeze in a

minimum time, whereas a dense block of meat prod-

ucts, with heavy packaging or boxing, may take days

to be fully frozen to the core.

0018 The influence of freezing time is more apparent on

some products than others. For strawberries, the drip

loss is reduced from 20% to nearly 0% with shorter

freezing times. A difference in drip loss from 20% for

strawberries frozen in 12 h, to 8% for strawberries

frozen in 15 min, is significant. Cryogenic systems

perform the same freezing function in 8 min or less,

reducing the drip loss to less than 5%.

0019A modern mechanical freezer or a cryogenic freezer

can crust products very rapidly, minimizing the loss of

natural juices and locking in the qualities that make

the product more marketable.

Packaging

0020Packaging must be of good quality, odor- and taint-

free, and it must totally enclose the product. The

container should not be capable of being opened

without recognizable damage. It should also protect

the product against normal transit and storage

hazards and inhibit dehydration by incorporating a

moisture vapor barrier. All packaging should carry

clear identification and should be coded so that

stock rotation can be carried out. The inner carton

should be marked with an appropriate identification

code that would enable the producer concerned to

establish the date of production and the location of

the producing factory, and gain easy access to daily

production records.

0021As the product is wrapped, the operator must

insure that the inner layer is in good contact with

the actual food and that all air has been excluded.

Unless this is established, reasonable cooling rates

will not be obtained. The packages should then be

placed and arranged to insure good contact with the

freezing surface in the case of a plate freezer, or be in

the air flow of an air blast freezer.

0022Cooked food must be chilled to around 10



Cor

lower before it enters the freezer. The placing of a hot

product in the freezer will considerably slow the

freezing process as the outer surface will quickly

freeze, leaving the hot interior to give up initially

larger amounts of sensible heat than would have

been necessary had the product been allowed to

stabilize and cool prior to freezing.

0023Food cooked in a domestic kitchen should be first

allowed to cool close to ambient temperature and

then transferred to the refrigerator to chill down

before being placed in the freezer. The use of an

immersion thermometer to check core temperatures

prior to freezing is recommended. (See Packaging:

Packaging of Liquids; Packaging of Solids.)

Weight Loss and Economics

0024Freezing equipment will represent the single largest

investment in a food production line, yet its operating

costs will normally only be 3–5% of the total product

cost. Packaging costs vary widely but are normally

several times greater than the total cost of freezing.

0025An important factor to consider when selecting

freezing equipment is the weight loss of the product

2720 FREEZING/Blast and Plate Freezing

that occurs during freezing. This loss may be about

the same as the operating costs of the freezing facility.

This applies to inexpensive products like peas and is

even more significant for expensive products, such as

seafoods and soft fruits.

0026 Weight loss during freezing may be caused by

mechanical losses, downgrading, and dehydration.

Mechanical loss refers to products dropping to the

floor, sticking to conveyor belts or dripping juice. A

modern freezer should have almost no loss in these

categories. Downgrading losses refer to damaged

products, breakages, and similar occurrences that

render the product unsaleable at the top quality

price. Dehydration losses will always be present in

any freezing system. The evaporation of water vapor

from unpacked products during freezing can be seen

as frost on the refrigeration plant freezing surfaces.

This frost is also caused by excessive infiltration of

warm, moist air into the freezing chamber. Still air

inside the vapor-proof product carton can create

larger dehydration losses than an unpacked product

frozen in a cryogenic quick freezer. The heat transfer

is poor because no circulation of air occurs within the

package. The result is evaporation of moisture, which

can be significant, with the frost staying inside the

carton.

0027 A poorly designed freezer or freezing tunnel can

have dehydration losses as high as 5%, as against a

well-designed freezing system in which losses are

maintained below 1.5%. Cryogenic liquid nitrogen

tunnels normally operate with a dehydration loss of

between 0.25 and 1.25%. These losses usually occur

when the nitrogen gas circulates over the product at

the in-feed end of the freezer. It may be necessary

at the in-feed end to temper the product so as to use

the heat capacity of the nitrogen more efficiently.

Nitrogen immersion freezing tends to have even

lower dehydration losses but uses more liquid nitro-

gen. Carbon dioxide freezing operations use jet im-

pingement, and dehydration losses are similar to that

experienced with liquid nitrogen.

Freezing Methods

0028 Freezing equipment can be divided into groups with

regard to the basic method of extracting heat from the

product to be frozen:

0029 Air blast freezing: Air at high velocity (between 4

and 7 m s

1

) is circulated over the product. The air

picks up heat, which is then recooled by means of

an air-to-refrigerant heat exchanger before the air

is recirculated.

0030 Contact freezing: Food (packed or unpacked) is

placed between metal plates or surfaces, and the

heat is extracted by direct conduction through the

metal surface, which is refrigerated by the circula-

tion of a cooling medium.

0031Immersion freezing: Food is immersed in a low-

temperature, nonfreezing solution that is cooled

by evaporators in a conventional refrigeration

system.

0032Cryogenic freezing: Food is exposed to an atmos-

phere below 60



C, which is achieved by

spraying liquid nitrogen (LN) or liquid carbon

dioxide into the freezing chamber.

All of the above methods are used in food freezing

processes. However, the more favored systems are

those that can be operated in line with the proceeding

processing and preparation operations and the subse-

quent packing functions.

Types of Freezing Equipment

Storage Rooms

0033The storage room must not be considered as freezing

equipment, although it may sometimes be used for

this purpose. Freezing in a storage room has many

disadvantages and should only be used in exceptional

cases. The freezing process is slow, and the quality of

almost all products suffers. If products are already

being stored in the room, their quality is jeopardized,

because flavors may be transferred from the warm

products yet to be frozen.

0034Because the storage room is not designed for freez-

ing, the cooling coils may frost up quickly, and total

refrigeration capacity may be reduced. The tempera-

ture of the products already frozen may rise consider-

ably, affecting their quality. Without doubt, freezing

in a storage room leads to a lower product quality.

Blast Freezer

0035A blast freezing room is usually equipped with more

forced-air cooling units or larger coolers than would

normally be found. These coolers are often of suffi-

cient size to warrant floor mounting. The air coolers

are equipped with fans that create considerable air

turbulence. Products may be laid in trays that are then

loaded into a freezing trolley or rack; this is moved in

and out of the blast freezer by hand or with a forklift

truck.

0036The racks must be designed to provide air space

between the trays, which is equal to approximately

50% of the product thickness. Because there is no

control over air circulation, the resulting heat transfer

of the product to air may be less effective. Blast

freezer rooms offer acceptable conditions for a

limited range of products with the freezing cycle and

FREEZING/Blast and Plate Freezing 2721

the quality of freezing dependent on the experience of

the operator.

Stationary Freezing Tunnels

0037 A stationary freezing tunnel is the simplest type of

freezer (Figure 1), designed to produce satisfactory

results for the majority of products. It consists of an

insulated enclosure, equipped with refrigeration coils

and fans that circulate air in a controlled pattern

over the product. The air circulation system design

influences the freezing rate and the product weight

loss.

0038 Products are placed on trays that are then placed in

a rack. The racks are arranged to provide an air space

between each level of trays and are moved in and out

of the tunnel manually. The human element becomes

important when positioning the racks inside the

tunnel to prevent air bypass; this is essential for effi-

cient freezing processes.

0039 Most products can be frozen in a stationary freez-

ing tunnel. Whole, sliced, or diced vegetables may be

frozen either in cartons or unpacked, provided that

the layers are limited to 30–40 mm in depth on the

trays. Spinach, broccoli, meat pate

s, fish fillets, and

prepared foods are often frozen in packages, using

this type of equipment. By using different rack

designs, thick packages and whole meat carcasses

can also be frozen. The system capacity depends on

the product thickness and composition, as well as on

the existence of packaging.

0040This type of freezer is reasonably flexible in use,

making it suitable for the initial development stage of

the new frozen food product, but does require heavy

outlay for manpower and may result in considerable

weight loss, if not properly used.

Push-through Tunnels

0041Some simple mechanization is achieved in a push-

through tunnel (Figure 2). Tracks are provided in

the floor, and the racks are fitted with casters or

wheels. The mechanization system is often hydraulic-

ally powered with a special stroking facility that

feeds the racks through the freezer. The time of the

hydraulic stroking operation can be adjusted to vary

the time the product is in the freezer.

0042This type of equipment may require open ends to

allow product entry and exit. If doors are provided,

they have to be interlocked with the hydraulic track

system, so that they open and close, permitting a rack

to be drawn into the tunnel and also ejected at the

other end. This type of freezer has a number of ad-

vantages over a stationary tunnel: there is improved

Air deflectors

Rubber flaps

Air flow

Cooler

Fan

fig0001 Figure 1 Stationary freezing tunnel (Michael Boast Associates). Reproduced from Blast and Plate Freezing, Encyclopaedia of Food

Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

2722 FREEZING/Blast and Plate Freezing

air circulation over the product as it moves at a steady

rate through the tunnel; labor costs are considerably

decreased; and there is added flexibility with the

facility of varying the freezing time.

0043 Some tunnels may have three or four lines, each

with independent hydraulic stroking facilities, and

this makes different product freezing times available

in a single installation.

Belt Freezers

0044 Modern belt freezers use vertical air-flow patterns

and require uniform distribution of product over the

entire belt surface for effective freezing.

0045 The single-belt freezer, multiple-belt freezer, and

spiral-belt freezer are the main types available. The

simplest is the single-belt freezer, consisting of the

single belt exposed to an updraught of air. This is

suitable for deep-fried or relatively dry products that

do not tend to freeze to each other or form clumps,

i.e., fish sticks, French fried potatoes, and bakery

products.

0046 Multiple-belt freezers are suitable for individual

freezing of fried fish sticks, fish portions, bakery

items and other products.

004 7 The spiral-belt freezer (Figures 3 and 4) offers a

design that maximizes the belt surface area in a given

floor space. It is achieved by using a product belt

that can bend around a rotating drum; by stacking

up to 40 tiers of belt on a drum, floor space occupied

is kept to a minimum. The continuous belt eliminates

product transfer points internal to the system. The

product is transferred only at the in-feed and out-feed

ends of the freezer. Spiral-belt freezers are suitable for

unpacked meat, fish, and poultry products, including

meat pate

s, meat balls, fish fillets and chicken por-

tions.

Fluidized Bed Freezers

0048Fluidization is defined as a method to keep solid

particles floating in an upward direction (Figure 5)

fig0002 Figure 2 Push-through tunnel (Michael Boast Associates). Reproduced from Blast and Plate Freezing, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

Air

Product

Air flow

Product

Belt

fig0003Figure 3 Spiral-belt freezer (Michael Boast Associates). Re-

produced from Blast and Plate Freezing, Encyclopaedia of Food

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

and Sadler MJ (eds), 1993, Academic Press.

FREEZING/Blast and Plate Freezing 2723

in a flow of gas or liquid. In freezing, fluidization

occurs when particles of a similar shape and size are

subjected to an upward stream of low-temperature

air. At certain air velocities, particles float in the

airstream, with each particle be separated from the

other but surrounded by air and free to move. In this

state, the particle mass assumes the properties of a

liquid or a fluid. By utilizing low-temperature air to

achieve fluidization, the product is simultaneously

frozen and conveyed by the same air without the aid

of a mechanical belt.

0049Using the fluidization principle for freezing pro-

vides several advantages over the conventional belt.

Most products that tend to stick together, i.e., sliced

green beans, sliced carrots, and sliced cucumbers, are

individually quick-frozen. The fluidized bed freezer

(Figure 6) is effective and reliable in freezing wet

products, as it can accept products with a high surface

water content. Another advantage of the fluidized

bed freezer is its complete independence from product

variations and flow. Even when running at reduced

capacity, the same evenly distributed air pattern is

maintained.

Plate Freezers

0050In a plate freezer (Figure 7), the product is pressed

firmly between top and bottom metal plates. The

refrigerant is circulated in channels, housed inside

the plates. This insures good heat transfer and

reasonable freezing times.

0051Because heat transfer at the surface is gradually

reduced with increasing product thickness, package

Air flow

fig0005 Figure 5 Fluidization principle (Michael Boast Associates).

Reproduced from Blast and Plate Freezing, Encyclopaedia of

Food Science, Food Technology and Nutrition, Macrae R, Robinson

RK and Sadler MJ (eds), 1993, Academic Press.

fig0004 Figure 4 Diagrammatic arrangement of a spiral-belt freezer.

Reproduced from Blast and Plate Freezing, Encyclopaedia of Food

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

and Sadler MJ (eds), 1993, Academic Press.

Product inflow

Product outflow

Air flow

fig0006Figure 6 Fluidized bed freezer (Michael Boast Associates).

Reproduced from Blast and Plate Freezing, Encyclopaedia of

Food Science, Food Technology and Nutrition, Macrae R, Robinson

RK and Sadler MJ (eds), 1993, Academic Press.

2724 FREEZING/Blast and Plate Freezing

thickness has to be limited to a maximum of 50 mm.

Pressure from the plates has the additional advantage

that it eliminates the bulging of the packages during

the freezing cycle, so that the packages are discharged

with straight sides and within close tolerances.

0052 The two main types of plate freezers are horizontal

or vertical in design; either type can be manual or

automatic. In general, manual horizontal plate

freezers have 15–20 plates. The product is placed on

metal trays at the end of a packaging line, loaded in a

rack or trolley, and transported to the freezer. The

trays are loaded manually between the plates.

0053 The vertical plate freezer was developed specific-

ally for the freezing of fish at sea; it comprises a series

of vertical freezing plates that form partitions in the

container. Products are fed from the top, and the

finished block of frozen products are discharged to

the side, top, or bottom. Usually, this operation is

mechanized. Whole unpacked fish are frozen, but

fillets may also be handled in a vertical plate freezer.

The block thickness varies from 25 to 100 mm.

Drum Freezer

0054 An alternative to the flat plate freezer is the drum

freezer, consisting of a rotating drum containing a

refrigerant at low temperature, resulting in a cold

surface for freezing. The product to be frozen is

placed on the stainless steel refrigerated drum,

which rotates at a preset speed depending on freezing

time required. The product must be frozen, within

one rotation. This rotation will last for less than

1 min if the product is only 3 mm thick, such as

chopped spinach paste. A 6-min rotation would be

required for a 10-mm-thick plaice fish fillet and

16-min rotation for a 20-mm-thick cod fillet.

Immersion Freezing

0055For irregularly shaped products such as chickens and

turkeys, high heat transfer can be achieved in an

immersion freezer, which normally consists of a tank

that houses a refrigerated brine. The brine is most

often of glycol or sodium chloride solution. The prod-

uct is immersed in the brine or sprayed while it is

conveyed through the tank.

0056Immersion freezers are most commonly used for

plastic-bagged chickens or turkeys. The product must

be protected from contact with the brine by using a

high-quality packaging that gives an absolute tight

seal. The brine residues on the packages are normally

washed off with water at the freezer exit.

See also: Freezing: Principles; Operations; Cryogenic

Freezing; Storage of Frozen Foods

Further Reading

ASHRAE (1998) Refrigeration Systems and Applications.

Atlanta, GA: American Society of Heating Refrigeration

and Air Conditioning Engineers.

ASHRAE (2001) Refrigeration Fundamentals. Atlanta, GA:

American Society of Heating Refrigeration and Air Con-

ditioning Engineers.

Boast MFG (1986) Refrigeration and Air Conditioning.

Oxford: Butterworth Heinemann.

Boast MFG (1991) Refrigeration Note Book. Oxford:

Butterworth Heinemann.

Robinson RK (ed.) (1985) Microbiology of Frozen Foods.

Barking, UK: Elsevier Science.

Cryogenic Freezing

C James and S James, University of Bristol,

Langford, UK

Introduction

0001Freezing methods can be divided into two main

classes: (1) those involving direct contact between

Refrigerant

supply

Refrigerant

return

Hydraulic ram

Flexible refrigerant lines

Refrigerated platesInsulated cabinet

Plate guides

Hydraulic power

and control

fig0007 Figure 7 Horizontal plate freezer (Michael Boast Associates).

Reproduced from Blast and Plate Freezing, Encyclopaedia of Food

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

and Sadler MJ (eds), 1993, Academic Press.

FREEZING/Cryogenic Freezing 2725

the refrigerant and the food; and (2) those involving

the use of a secondary medium, e.g., air, brine, or

metal plate, which is cooled by the refrigerant.

0002 Cryogenic freezing uses refrigerants, such as liquid

nitrogen or solid carbon dioxide, directly. The boiling-

off of the refrigerant when it comes in contact with

the product brings about cooling. As well as using the

latent heat absorbed by the boiling liquid, sensible

heat is absorbed by the resulting cold gas.

0003 Most cryogenic systems use total loss refrigerants,

i.e., the refrigerant is released to the atmosphere and

not recovered. Due to environmental and economic

factors, total loss refrigerants must be both readily

available and harmless, which limits the choice to

atmospheric air and its components.

History

0004 Cryogenic freezing was first carried out commercially

using liquid air, in the 1930s. However, liquid air

contains a high proportion of liquid oxygen, which

is a powerful oxidizing agent. Theoretically it can be

produced on site, eliminating the need to purchase

and store ‘gas.’ Although companies have promoted

the use of liquid air, in practice it has been superseded

by less harmful liquid nitrogen and liquid, or solid,

carbon dioxide.

0005 Food freezing using liquid nitrogen or carbon

dioxide became established in Europe from the early

1960s onwards as manufacturers looked for uses of

these byproducts. Liquid nitrogen is the byproduct

produced when liquid oxygen is made by air dis-

tillation, while carbon dioxide is a byproduct of

fermentation processes and a constituent of nat-

ural gas. By the early 1990s an estimated 10% of

all frozen food in the UK was produced using

cryogens.

0006 An alternative to the air-based total loss cryogens,

liquid dichlorodifluoromethane (food grade R 12,

freon, CCl

), was introduced commercially in

1968 by Du Pont. R 12 had the advantage over

other cryogens of being easily recoverable and having

a higher operating temperature (29.8



C), reducing

the problems of thermal shock. It was claimed that

such gentle handling allows ‘true’ individual quick

freezing (IQF) of products such as soft berries, meat

patties, raw shrimps, breaded scampi, fried onion

rings, etc. in less than 6 min. However, its use was

not permitted in all countries and it was not widely

used.

Physical Properties

0007 The major problem with most cryogenic materials

is that they are extremely cold (Table 1). Direct

immersion can be inadvisable since an extremely

high temperature gradient can be imposed on the

food, which is often sufficient to cause it to disinte-

grate. Research studies have concluded that the ideal

cryogen would have a boiling point of 50



Canda

latent heat of evaporation as high as possible.

0008Liquid nitrogen is produced by liquefaction of air,

either as a principal product or as a byproduct of liquid

oxygen. Nitrogen is the main constituent of atmos-

pheric air and at atmospheric pressure liquefies at a

temperature of 196



C. It is usually supplied and

stored at a pressure of 3–6 bar, with corresponding

boiling points of 185



Cto177



C. A useful rule of

thumbisthat1 t h

1

ofliquidnitrogenisapproximately

equivalent to 100 kWof mechanical refrigeration.

0009Since liquid nitrogen is made from air, its costs are

entirely those of manufacture, i.e., the capital cost of

the plant, electricity, and transportation. The energy

required to produce 1 kg of liquid nitrogen is around

3000 kJ, or eight times the consequent stored refriger-

ating effect.

0010Carbon dioxide’s physical properties are unusual,

in that it does not exist in liquid form at atmospheric

pressure. If stored as a pressurized liquid and released

into the atmosphere, the liquid changes partly to gas

and partly to a frozen solid at 78.5



C, which

sublimes directly into gas without going through a

liquid phase.

0011Liquid carbon dioxide is generally supplied either

at ambient temperature (e.g., 25



C and 65 bar),

giving a refrigerating capacity of 199 kJ kg

1

,orat

16



C and 22 bar, giving a refrigerating capacity of

311 kJ kg

1

. At the point of use, spray nozzles reduce

the pressure of the liquid, generating a mixture of

cold vapor and solid carbon dioxide ‘snow.’

0012The majority of the ‘refrigeration effect’ stored in

solid CO

is latent, whereas in liquid nitrogen almost

half the effect is due to sensible heat transfer to the

cold gas. The available refrigerating effect from 1 kg

tbl0001Table 1 Properties of cryogenic materials

Material Boiling point



Cat1atm Latentheatof

vaporization

(kcal kg

1

)

Helium 268.8 108.8

Nitrogen 195.8 47.3

Carbon monoxide 190.6 53.5

Argon 184.4 37.0

Methane 161.1 136.0

Ethane 88.9 109.0

Nitrous oxide 88.9 90.0

Propane 42.2 100.6

Carbon dioxide 57.6 (5 atm) 75 (approx.)

Carbon dioxide (solid) 79 (sublimation point) 135.4

2726 FREEZING/Cryogenic Freezing

of both liquid and solid carbon dioxide and liquid

nitrogen is shown in Figure 1.

Comparison of Cryogenic Freezing

Systems with Mechanical Freezing

Systems

0013 The generally perceived advantages of cryogenic com-

pared with conventional refrigeration are given in

Table 2.

Capital Cost

0014 The low capital cost and simplicity of design permit

cryogenic systems to be used in a standby or back-up

role in emergencies or for ‘peak lopping’ during

periods of exceptional freezing demand.

0015 The overall economics can be favorable for short

road deliveries. For static standby installations, or

those with very low loading factors, low capital cost

can be a major attraction.

Running Costs

0016 In overall energy or fuel terms, there is no way in

which cryogenic refrigerants can compete with the

best mechanical refrigeration systems. Generally

speaking, cryogenic refrigeration offers lower capital

and maintenance costs but incurs higher running

costs. Many cryogenic freezers are leased from the

gas suppliers, reducing capital cost further.

0017The consumption of the cryogen (Table 3) con-

tributes the major proportion of the freezing costs

(about 75%). Refrigerant consumption is measured

as a ‘consumption ratio’ equal to weight of refrigerant

used divided by the weight of product frozen. Table 4

shows some approximate quantities of liquid nitro-

gen and carbon dioxide required to freeze a range of

products. High refrigerant consumption can be

Liquid CO

1 kg

−18 ⬚C, 20.7 bar

Soild CO

1 kg

−78.5 ⬚C

Liquid N

1 kg

−196 ⬚C, 1.01 bar

575 kJ

24 kJ 45 kJ 180 kJ

268 kJ

313 kJ

313 kJ 620 kJ 378 kJ

198 kJ

Gas

−78.5 ⬚C

Gas

−78.5 ⬚C

−18 ⬚C

−18 ⬚C −18 ⬚C

−18 ⬚C

1 kg

Gas

−78.5 ⬚C

0.534 kg

Gas

−195 ⬚C

1 kg

Solid

0.466 kg

−78.5 ⬚C

fig0001 Figure 1 Available refrigeration effect from 1 kg of total loss refrigerant.

tbl0002Table 2 Advantages and disadvantages of total loss

refrigerants in comparison with mechanical refrigeration

Advantages Disadvantages

Low capital investment High operating cost

High refrigerating capacity High refrigerating capacity

Compact (small ‘footprint’) Poor temperature control

Flexible Limited duration without filling

Low weight when out of use High weight at start of use

No residual weight (dry ice) Reduced humidity

Silent operation Suffocation hazard

Advantageous storage Limited availability

atmosphere (N

)

Bacteriostatic affect (CO

)

Low maintenance requirements

Long life

Foolproof once installed (dry ice)

FREEZING/Cryogenic Freezing 2727