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

Подождите немного. Документ загружается.

Nelson PE, Chambers JV and Rodriquez JH (eds) (1987)

Principles of Aseptic Processing and Packaging. Wash-

ington, DC: Food Processors Institute.

Reuter H (1989) Aseptic Packaging of Food. Lancaster:

Techomic.

Chemical and Microbiological

Changes

C James and S James, University of Bristol,

Langford, North Somerset, UK

Introduction

0001 Temperature controls the rate of all physicochemical

reactions and consequently has a profound effect on

all biological systems. It influences such fundamental

properties as hydrogen bonding, solubility of solute

molecules, viscosity, density, and osmotic properties

of cell membranes.

0002 The prime purpose of most heat treatments is to

make the food safe to eat by reducing the numbers of

pathogenic organisms present in the food, if not com-

pletely eliminating them. Many food sterilization and

pasteurization processes have been developed to

achieve this aim without unacceptable changes to

the structure or flavor of the food. However, in

many cooking processes the aim is to cause structural

and chemical changes that will make the food more

palatable.

0003 Every operation has, however, side-effects which

adversely influence product properties. This is

because during every heat treatment many types of

process take place simultaneously, i.e., mechanical,

physical, chemical, biochemical, and microbial.

Chemical

0004 The adverse chemical effects of heat treatment may

be classified as:

0005 Enzymic and nonenzymic browning reactions

0006 Vitamin destruction

0007 Destruction of amino acids

0008 Oxidation of lipids, flavor components, etc.

0009 Hydrolysis of carbohydrates, etc.

0010 Destruction of pigments, such as chlorophyl, caro-

tenoids, and flavonoids

0011 Development of undesirable flavors by reaction

products

0012 Destruction of carbohydrates and proteins

0013 Retrogradation of starch

0014Cross-linking reactions of polymers, including pro-

teins and carbohydrates, which adversely affect the

water uptake of the product.

Changes in Proteins

0015Heating causes dramatic changes in most foods. The

heating of meat is accompanied by changes in appear-

ance, flavor, texture, and nutritive value. The most

drastic changes in meat during heating, such as

shrinkage and hardening of the tissue, release of

juice, and color changes, are caused by changes in

the muscle proteins. It is difficult to investigate heat-

induced changes in the complicated and highly organ-

ized protein structure found in meat. Therefore, many

of the experiments have been carried out by heating

isolated proteins. These studies show that the most

drastic changes of the myofibrillar proteins occur

between 30 and 50



C, reaching almost completion

at 60



C. These changes are characterized by an

unfolding of the protein molecules, accompanied by

an association of molecules and resulting in coagula-

tion and loss of enzyme activity. At temperatures

above 70



C more severe changes such as oxidation

of sulfhydryl groups occur.

0016Home cooking and commercial canning proced-

ures have little effect on the nutritive value of food

proteins. However, other industrial processes that

involve the use of excessive heat at low moisture

levels, such as drying, and subsequent storage of

these products may result in severe nutritional

damage. Milk is the only important naturally occur-

ring protein food that has a high content of reducing

sugar. Liquid milk appears virtually unaffected by

industrial pasteurization and only slightly affected

by autoclaving and canning. However, the protein in

dried milk can deteriorate during manufacture and

storage.

0017Maillard reactions occur during the baking of

bread and biscuits and during the production of

breakfast cereals. A 10–15% loss of lysine can occur

during the baking of bread with the loss occurring

mainly in the crust. Biscuits are an attractive product

to enrich the protein content of diets, but are sub-

jected to high temperatures and baked to low mois-

ture contents. Losses of up to 50% reactive lysine can

occur during the production of biscuits enriched with

15% skimmed milk powder.

0018Lysine is the most sensitive amino acid to Maillard

damage but all amino acids can be reduced in avail-

ability when the heat process is increased.

Changes in Carbohydrates

0019Heat affects the properties of different carbohydrates

in foods to different extents, and not always in the

HEAT TREATMENT/Chemical and Microbiological Changes 3039

same direction. The effect can best be considered

under four areas – taste, caramelization, pyrolysis,

and interaction with other food components. These

naturally are not independent but are related to

a greater or less extent, depending on chemical

structure.

0020 It is important when considering the effect of tem-

perature on taste to distinguish between relative and

absolute sweetness. The former is usually determined

with reference to sucrose, and the relative sweetness

of most sugars decreases with increasing temperature.

However, the absolute sweetness of sugars usually

increases with increasing temperature.

0021 Caramelization or scorching of carbohydrates is a

result of the partial thermal breakdown of carbohy-

drates. The brown product produced, caramel, con-

sists of dehydration products and colored substances,

the proportion of which depend on the extent of

heating.

0022 After polymerization, depolymerization, and

dehydration reactions, secondary thermal reactions

cause carbon–carbon bond cleavage and many

pyrolysis products of low molecular weight are

produced in a similar way from both mono- and

polysaccharides.

0023 The most important reaction of carbohydrates

with other food components is the so-called non-

enzymic browning or Maillard reaction. This occurs

between reducing sugars and nitrogenous com-

pounds, in particular amino acids and proteins. The

reaction is highly temperature-dependent and may

increase two to three times for each 10



C rise in

temperature in model systems and even more in real

foods.

Changes in Vitamins

0024 Three vitamins, vitamin E in its ester form, riboflavin,

and niacinamide, are extremely heat-resistant, where-

as thiamin is the most labile of the vitamins. Some

vitamins show a gradual degradation as the tempera-

ture is raised from 100 to 130



C (vitamins A, D, B

and C). Other vitamins show a sudden drop above a

critical temperature – folic acid at 110



C, and pyrid-

oxine and pantothenic acid at 120



0025 Vitamin A and carotene lose activity when heated

in the presence of oxygen, but are fairly stable to

ordinary cooking. Insignificant losses are found in

milk during pasteurization, sterilization, and spray-

drying.

0026 Carotene is well retained during the dehydration

of vegetables if enzymes have been inactivated by

blanching. Synthetic b-carotene has good stability

when used as a coloring agent in canned soups. Re-

tention in cakes baked at 175–200



C was 90–95%,

whereas in white bread it was only 50–80%.

0027Frying of oils and fats may lead to a considerable

loss of carotene and vitamin A. Vitamin A appears

to be retained as long as water is present but after

40 min at 130–160



C or 20 min 175–200



C about

90% had been destroyed. It has been reported that a

40% loss of carotene occurs during the frying of

margarine for 5 min with the loss increasing to 60%

after 10 min.

0028Less information is available on the thermal stabil-

ity of vitamin D in foods but it is though to be similar

to vitamin A. Its retention during smoking of fish,

spray-drying of eggs, and pasteurization and steriliza-

tion of milk is satisfactory.

0029Vitamin E in food products occurs in the form of

free tocopherols. These survive domestic cooking

processes far more than commercial processing.

Losses of up to 95% have been measured in canned

vegetables (Figure 1). However, oils and cereals are a

more important source of vitamin E than vegetables.

Almost 100% of the free tocopherols were destroyed

when vegetable oil was heated for 3 h at 200



whilst 47% of tocopherols in unbleached flour were

destroyed during the baking of bread.

0030High losses occur in water-soluble vitamins during

slow heating, long cooking, and slow cooling pro-

cesses. Much of the loss is due to leaching of soluble

matter into the cooking water. When processing

of vegetables prior to freezing, steam blanching

followed by air-cooling is recommended.

0031Blanching of fruits and vegetables inactivates the

ascorbic acid oxidase and stabilizes vitamin C. Under

the most favorable conditions, blanching losses can

be less than 10%; however, under severe conditions

they can exceed 50%. Similar differences in vitamin C

content can result in domestic cooking (Table 1).

0032The thermal degradation of thiamin vitamin (B

)

results in numerous reaction products from the

cleavage of the vitamin molecule to its pyrimidine

and thiazole moieties. Vitamin B

losses in high-

temperature, short-time (HTST) canning processes

are far less than in conventional canning. Losses in

vegetables are in the 13–16% range compared with

Canned corn

Frozen corn

Canned green beans

Frozen green beans

Canned green peas

Fresh green peas

Total tocopherol (mg%)

1.5 20.5

fig0001Figure 1 Total tocopherol content after processing of vege-

tables.

3040 HEAT TREATMENT/Chemical and Microbiological Changes

over 40% in conventional systems. With meat, losses

are under 10%, compared with approximately 20%

conventionally.

0033 Less severe heat treatments such as sous-vide

processing have been shown to increase retention

of vitamin B

and folate acid. After 5 min of boiling,

vitamin B

and folate losses were 55 and 39%

respectively. Steaming for the same time reduced the

losses to 17 and 12% and sous vide to 3 and 3%

respectively.

0034 Overall, vitamins C, B

, and folic acid are the

most vulnerable vitamins in thermal processing.

However, major losses of vitamins during thermal

processing can be taken as a sign of poor treatment

methods.

Changes in Enzymes

0035 The purpose of most pasteurization processes is

to kil microorganisms and inactivate deteriorative

enzymes. Often the inactivation of certain enzymes

is used as a processing index to verify that the food

has been subjected to the desired heat treatment.

0036 Different heat treatments have different effects on

enzymes and within processes, process temperature is

critical (Table 2).

Microbiological

0037 The use of heat to inactivate vegetative cells in pas-

teurization processes and to inactivate both vegeta-

tive cells and endospores in sterilization processes is

the basis of many food production systems. The aim

of thermal processing is to deliver sufficient heat to a

food to reduce the chance of survival of an organism

that will grow in the food to an acceptably low level.

The rational for deciding what is an acceptably low

level has developed over many years since initial stud-

ies on the thermal inaction of Clostridium botulinum

in the 1920s.

Heat Activation Kinetics

0038It is generally observed that the number of viable

organisms in a uniformly heated population of a

single strain of microorganisms decreases exponen-

tially with time, and over many orders of magnitude,

with a slope usually designated as D in minutes. It is

also generally observed that the rate of inactivation

increases exponentially with rise in temperature, with

a slope usually designated z in degrees.

0039Much careful experimentation has shown that there

are clear departures from these simple log: linear

relationships in practice. The two most common

types of deviation are shown in Figure 2. When large

inactivations have been achieved and survivor curves

followed down to very small surviving fractions, ‘tails’

are commonly reported. Rising counts or ‘shoulders’

during the initial heating periods are also common.

A number of reasons for these deviations have been

put forward. They include assumptions that they are

artifacts caused by counting colony-forming units

rather than actual survivors. Alternatively, heterogen-

eity of resistance within the population or changes in

resistance during heating have been suggested to cause

deviations from the log:linear relationship.

tbl0001 Table 1 Typical vitamin C losses during cooking

Method Vitamin C (%)

Destroyed Extracted Retained

Green vegetables –––

Boiling (long time,

much water)

10–15 45–60 25–45

Boiling (short time,

little water)

10–15 15–30 55–75

Steaming 30–40 <10 60–70

Pressure cooking 20–40 <10 60–80

Root vegetables

Boiling 10–20 15–25 55–75

Steaming 30–50 <10 50–70

Pressure cooking 45–55 <10 45–55

tbl0002Table 2 Effect of different heat treatments on enzymes

Process Influence on enzymes

Sterilization Extremely thermostable enzymes

survive

Pasteurization Thermostable enzymes survive

Blanching Quality-changing enzymes

inactivated

Cooking/frying/baking Most enzymes inactivated

Drying Dependent on process temperature

Extraction/concentration/

distillation

Dependent on process temperature

Log numbers of

colony-forming units

Time of heating

fig0002Figure 2 Commonly observed deviations from classical heat-

inactivation kinetics. Continuous line, log:linear; dashed line, tail;

dotted line, shoulder.

HEAT TREATMENT/Chemical and Microbiological Changes 3041

0040 Deviations from the log:linear relationship are

more often reported and occur to a greater degree in

vegetative cells than in endospores. This is particu-

larly prevalent during lower-temperature, longer-time

treatments during which vegetative cells continue to

metabolize. This may result in changes in compos-

ition and structure and hence thermal tolerance

during the heating process.

0041 In vegetative cells the shoulders and tails can be

greatly influenced by the heating menstruum and the

conditions of growth prior to heating.

Mechanism of Heat Inactivation and Injury

0042 DNA, RNA and ribosomes, cytoplasmic membranes,

and specific enzymes are the primary sites for lethal

and nonlethal injury of microorganisms.

0043 Studies on spores as well as vegetative cells have

been inactivated by heating and have shown single

strand breaks in DNA. There is a substantial differ-

ence between dry and moist heating. Higher tempera-

tures are needed in dry heating but very high levels of

mutants in spore populations are generated. Because

of this, heat-injured spores may require additional

nutrients for recovery and the production of colony-

forming units. It is thought that recovery, in particu-

lar with respect to DNA repair, plays an important

role in determining the overall resistance of bacterial

spores to thermal stress.

0044 Mild heating accelerates the degradation of riboso-

mal RNA, but this often precedes loss of viability and

is not therefore thought to be a prime cause of cell

death.

0045 Much early work showed that after heating vege-

tative cells generally became leaky, losing ions, amino

acids, and low-molecular-weight nucleic compounds.

The rapid loss of such materials from heated cells

has long suggested that heating interferes with the

permeability of cytoplasmic membranes. Procedures

that minimize leakage have been shown to reduce the

level of injury. However, correlations between cell

death and leakage are poor. Therefore, membrane

damage as a major site of the lethal effects of heat

is unlikely.

0046 After some heat treatments, it has been observed

that a fall in the rate of germination is preceded by a

reduction in colony count. Since DNA is not involved

in the initiation of germination this would support the

view that damage to DNA is a prime cause of heat

inaction. Conflictingly, there are also data that show

that in some cases some part of the germination

system itself may be especially heat-sensitive and the

key site of heat inactivation. The strong inference

from such data is that heat inactivates the spores by

destroying the activity of an enzyme(s) on the germin-

ation pathway. Unfortunately, since the mechanism

of germination is not completely understood, the

identity of the heat-sensitive step(s) is not known.

0047Overall, heat is assumed to cause inactivation by

destroying a key lethal target in the cell. This is most

often directly or indirectly the functionality of the

DNA. At the same time, the heat is also destroying

many other targets that may be present in larger

numbers in the cell. Destroying these targets is there-

fore less critical unless their numbers are reduced to

very low levels or unless the cell becomes further

stressed. A heat-treated cell may therefore have a

reduced probability of growth after treatment.

Practical Processing

0048The prime purpose of cooking or other heat treatment

is to reduce, if not completely eliminate, any patho-

genic bacteria present in the food. To quote from the

Chilled Food Association guidelines: ‘The heat pro-

cess must be validated and critical parameters must be

controlled and recorded. These must take account of

the worse conditions likely to occur with respect to

heat transfer (for instance, the use of frozen raw

materials or the use of large pieces of meat). All

parts of the product must receive a minimum heat

process appropriate to the target organism to be con-

trolled and for the targeted shelf life.’

0049From a pathogenic survival view, heat treatments

are divided into three types:

tbl0003Table 3 Time required to produce a 10

reduction in Listeria

monocytogenes and Clostridium botulinum type B at different

temperatures

Temperature Time (min) to produce 10

reduction

Listeria monocytogenes Clostridium botulinum

type B

60 43.5

62 23.3

64 12.7

66 6.8

68 3.7

70 2.0

72 1.0

74 0.6

76 0.3

78 0.2

80 0.09 270.3

82 0.05 138.9

84 0.03 71.9

86 37.0

88 19.2

90 10.0

92 6.3

94 4.0

96 2.5

98 1.6

100 1.0

3042 HEAT TREATMENT/Chemical and Microbiological Changes

1.0050 If the food temperature is less than 70



C for 2 min

then there is a possibility that all pathogens

present will survive.

0051 In foods heated to 70



C for 2 min (or equivalent)

then all pathogens present will be reduced to an

acceptable level (6 log reduction), e.g., Listeria mo-

nocytogenes, Staphylococcus aureus, salmonella,

verocytotoxigenic Escherichia coli. However,

spores and preformed toxins may persist.

0052 In foods heated to 90



C for 10 min (or equivalent)

in addition to vegetative pathogens, spores of psy-

chotrophic Clostridium botulinum will be reduced

to an acceptable level (6 log reduction). However,

more heat-resistant spores, e.g., strains of Bacillus

cereus and some preformed toxins, may persist.

0053 The equivalent times at different temperatures are

shown in Table 3. However, care must be taken when

using these equivalent values. Values for temperatures

above 70



C for Listeria have been extrapolated from

the measured data. The Clostridium values have been

calculated using a z-value of 7



C below 90



C and



C above 90



0054 Rates of thermal inactivation of a five-strain mix-

ture of L. monocytogenes were determined in

ground-beef roast (Table 4). D-values for L. mono-

cytogenes Scott A in naturally ground beef were also

determined. Large differences in D-values were found

when using different media for enumeration of the

surviving bacteria.

0055 Comparing the results of this study with those

obtained for salmonella at similar temperatures in

ground-beef roast it was noted that L. monocyto-

genes is about four times more heat-tolerant than

salmonella (Table 5).

0056 Although Salmonella spp. in general are not as

heat-resistant as Listeria monocytogenes, one species,

S. senftenberg 775W (Table 6), has been found to be

more heat-resistant than L. monocytogenes.

0057 Concern about the manufacture of ‘rare’ roast

beef in the USA (a process requiring relatively low

temperatures, usually below 60



C) initiated studies

into the effect of low temperatures on the survival of

salmonella in meat systems. Results (Figure 3) showed

that adequate processing of meats at temperatures

below 60



C required substantially longer processing

times than at temperatures greater than 60



0058Campylobacter is the most significant source of

food poisoning in the UK, with the number of cases

now exceeding those caused by salmonella. In the

USA it was found that Campylobacter jejuni was

present in 38% of pigs, 24% of sheep, and 2% of

the beef carcasses examined. Studies carried out in

New Zealand found that C. jejuni was commonly

present in the feces of unweaned calves and from

two of four groups of sheep. Poultry is also a major

reservoir of Campylobacter species. Campylobacter

species have a low heat resistance, with a D-value of

2.12 min at 55



C and 0.79 min at 57



C, and should

be destroyed by adequate cooking.

tbl0004 Table 4 Thermal inactivation rates (D-value) of Listeriamonocytogenes in meats determined using different media for enumeration of

surviving L. monocytogenes

Type of meat Temperature (



c) D-value (min)

Tryptose agar LPMagar

Ground-beef roast 54.4 22.4 + 1.1 19.3 + 4.3

57.2 15.7 + 4.1 17.6 + 9.8

60.0 4.47 + 1.60 3.5 + 1.13

62.8 2.56 + 1.04 2.82 + 1.29

Naturally 54.4 12.5 + 6.5 6.84 + 1.79

contaminated 57.2 3.41 + 1.42 1.97 + 0.80

ground beef 60.0 1.62 + 1.10 1.11 + 0.44

62.8 0.73 + 0.32 0.47 + 0.35

tbl0005Table 5 Comparison of heat tolerance of salmonella with that

of Listeria monocytogenes in ground-beef roast

Te m p e ra t ur e (



c) D-value (min)

Salmonella L. monocytogenes

57.2 3.8–4.2 17.6

62.8 0.6–0.7 2.6–2.8

tbl0006Table 6 Comparison of heat tolerances (D-values) of

Salmonella typhimurium and S. senftenberg 775 W

Te m p e ra t ur e (



C) D-values (min)

S. typhimurium S. senftenberg 775W

60 6.3

62.8 0.11

71.7 0.003 0.09

Reproduced from Shapton DA and Shapton NF (eds) (1991) Principles and

Practices for the Safe Processing of Foods. Oxford: Butterworth–

Heinemann, with permission.

HEAT TREATMENT/Chemical and Microbiological Changes 3043

0059 E. coli is a common organism found in the gut of

animals and humans. E. coli is mostly harmless in

humans, but enterohemorrhagic E. coli and, in par-

ticular, E. coli serotype O157:H7, is recognized as

being of significance to human health. Many food-

poisoning outbreaks and a number of deaths have

been associated with its survival after poor cooking.

The US Food and Drug Administration temperature

and time recommendations for cooking ground-beef

patties to control E. coli O157:H7 are shown in

Table 7.

Overall

0060 Most heat treatments for food have been established

to increase the safe high-quality storage life of the

product. The severity of the treatment should be suf-

ficient to reduce, if not completely, eliminate patho-

genic organisms likely to be present in the food of

interest.

0061 Cooking is a heat treatment with a dual role.

Cooking should render the food safe by reducing

pathogen levels. At the same time, it facilitates chem-

ical changes that improve the flavor and texture of the

food.

0062Chemical changes in proteins, carbohydrates, vita-

mins, and enzymes occur in all heat processes. In

many cases these changes are undesirable and careful

control of the heating regime is required to minimize

thermal changes.

See also: Canning: Principles; Cans and their

Manufacture; Quality Changes During Canning;

Carotenoids: Occurrence, Properties, and

Determination; Cooking: Domestic Techniques;

Enzymes: Functions and Characteristics; Heat

Treatment: Ultra-high Temperature (UHT) Treatments;

Electrical Process Heating; Listeria: Properties and

Occurrence; Pasteurization: Principles; Pasteurization of

Liquid Products; Pasteurization of Viscous and Particulate

Products; Other Pasteurization Processes; Protein: Heat

Treatment for Food Proteins; Salmonella: Properties and

Occurrence

Further Reading

Anonymous (1997) Guidelines for Good Hygienic Practice

in the Manufacture of Chilled Foods, 3rd edn. London:

Chilled Food Association.

Gaze JE, Brown GD, Gaskell DE and Banks JG (1989)

Heat resistance of Listeria monocytogenes in homogen-

ates of chicken, beef steak and carrot. Food Microbiol-

ogy 6: 251–259.

Gould GW (ed.) (1989) Mechanism of Action of Food

Preservation Procedures. Barking: Elsevier Science.

Høyem T and Kva

le O (eds) (1977) Physical, Chemical and

Biological Changes in Food Caused by Thermal Process-

ing. London: Applied Science.

James C and James SJ (1997) Meat Decontamination – The

State of the Art. University of Bristol: EU concerted

action programme.

McMeekin TA, Olley JN, Ross R and Ratkowsky DA (eds)

(1993) Predictive Microbiology – Theory and Applica-

tion. Taunton: Research Studies Press.

Shapton DA and Shapton NF (eds) (1991) Principles and

Practices for the Safe Processing of Foods. Oxford:

Butterworth Heinemann.

Electrical Process Heating

P J Fryer, University of Birmingham, Birmingham,

Edgbaston, UK

Introduction

0001In electrical process (‘ohmic’) heating of foods, heat is

generated by the passage of electrical current through

a food. Heat is generated within the food; this over-

comes the limitations of slow thermal conduction

52.0

200100

Time (min)

54.0

56.0

58.0

60.0

62.0

64.0

Temperature (⬚C)

fig0003 Figure 3 Thermal death curve for a 7-D kill of salmonella in

cooked beef.

tbl0007 Table 7 US Food and Drug Administration temperature–time

recommendations for cooking ground-beef patties to control

Escherichia coli O157:H7

Te m p e ra t ur e (



C) Cooking time

60.0 8 min 20 s

62.2 2 min 7 s

62.6 32 s

68.3 8 s

3044 HEAT TREATMENT/Electrical Process Heating

within solids, and makes it possible to process solids

and liquids at similar rates. This article reviews the

principles that govern electrical heating, the factors

which control the heating rate, and the design of

processes.

Electrical Heating

0002 Thermal processing aims to reduce contamination so

that the food will not cause a health hazard during its

shelf-life. Each part of the material must be processed

to a level prescribed to insure product safety. Heat

transfer to solids is slow, however; by the time enough

heat is conducted to the centre of a solid to insure

sterility, much of the rest is overcooked.

0003 Several sets of reactions occur when a food is

heated: those which reduce the level of bacterial con-

tamination, those which result in losses in product

quality, in terms of nutrition, taste, and texture, and

some that improve taste and texture. The reactions

which reduce the number of bacterial spores have a

higher activation energy than those which lower

product quality: to maximize quality for a given level

of sterility it is best to process at as high a temperature

as possible. Continuous ultrahigh-temperature (UHT)

or high-temperature, short-time (HTST) processes

exploit this to produce food of a higher quality than

canning. In these processes, food is rapidly heated to

c. 140



C, held there for a short period and then

rapidly cooled. At 140



C, food can be sterilized in a

few seconds, rather than the several minutes needed

at canning temperatures. This requires rapid heating

rates to minimize time spent at high temperatures,

and thus quality loss, and is best done in a continuous

process.

0004 Processing single-phase liquid foods this way is

straightforward, because they can be heated and

cooled rapidly. High heating and cooling rates

(> 1



1

) are possible. This type of process can

efficiently cook liquids such as milks, fruit juices,

soups, and sauces, but cannot readily be applied to

foods which contain particles, because of the slow-

ness of thermal conduction into the solid. Conduction

heating requires a temperature driving force between

particles and liquid, and for particles larger than

about 2 mm, rapid heating is not feasible. It is pos-

sible to use conduction/convection methods to pro-

cess foods with a high solids fraction (for example, in

scraped-surface exchangers), but low heating rates

and long hold times are needed, giving poor product

quality.

0005 The need to conduct heat is the limiting factor in

the thermal processing of particles, and thus for

solid–liquid mixtures. Volumetric heat generation

techniques can solve this problem. Various techniques

are available which use electric fields. In microwave

or RF (radio frequency) heating, a high-frequency

electric field excites the water molecules within

the material, whilst in electrical resistance heating

(ohmic heating), however, the passage of electrical

current results in heating throughout the material.

The process is more energy-efficient than microwave

heating, because nearly all of the electrical energy

goes to heat the food. It requires the passage of

electric current through the material; electrodes

that make good contact with the food are required,

unlike microwave heating which needs no physical

contact. When an electric current flows through a

material, heat is generated according to the familiar

Ohm’s law:

W ¼ I

R ð1Þ

Here W is power, I is current, and R is the resistance.

Heat is generated throughout the material as a result

of its inherent electrical resistance.

0006If the electrical conductivities and thermal capaci-

ties of different components of a food are similar, they

can heat at similar rates. In practice, as a result of

their thermal capacities and electrical conductivities,

solids can heat faster than liquids, a result which is

impossible by conventional means. Figure 1 shows

that rapid heating rates are possible for real foods.

Short process times thus make it possible to apply

UHT techniques to particulate foods.

100

0 50 100 150 200 250 300

Time (s)

Temperature (⬚C)

fig0001Figure 1 Electrical heating of a piece of lamb meat 20  20 

15 mm, electrical conductivity 0.45 S m

1

at 15



C, in a saline

solution, conductivity 0.454 S m

1

at 15



C. Applied voltage

gradient 11 V cm

1

. Dashed line, solid; continuous line, liquid.

Reproduced from Heat Treatment, Electrical Process Heating,

Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

HEAT TREATMENT/Electrical Process Heating 3045

Electrical Heating Processes

0007 The advantages of electrical heating are such that

many attempts have been made to commercialize it,

in areas such as baking, thawing, and as alternatives

to conventional heating processes. An electrical pas-

teurization process was successful in the USA in the

1930s, and applications have been found in areas

such as fish processing. For a process to be commer-

cially acceptable, a number of process and safety

criteria must be satisfied:

0008 electrical design to avoid electrolysis and product

contamination;

0009 effective control of food heating and flow rates;

0010 efficient aseptic packaging techniques for a two-

phase mixture;

0011 overall cost-effectiveness.

A number of early processes did not meet these

criteria, for example, by using direct current supply

(which leads to electrolysis), or by using packages

containing expensive electrode material. The absence

of efficient packing plant also limited success.

0012 The resurgence of interest in the process in the last

15 years is due to the development of a commercial

system by APV Baker, which incorporates solutions to

the problems described above. The APV Baker ohmic

heater was originally developed by EA Technology

at Capenhurst, UK. In 1984, APV Baker secured a

license for the heating system and then developed a

commercial process. Commercial-scale systems are

available with power outputs of 75 and 300 kW,

corresponding to product capacities of c. 750 and

3000 kg h

1

. A schematic process flowsheet is given

in Figure 2. The system incorporates heat, hold, and

cool sections. Food passes from a product pump into

a vertical or near-vertical pipe containing a series of

electrodes between which current flows. Sufficient

pressure is maintained to insure that the material

does not boil; this can be up to 4 bar for sterilization

at 140



0013The process was originally designed to sterilize

foods of high solids fraction, up to about 60%, at

heating rates in the region of 1



1

. Large particles,

up to 25 mm in diameter, can be processed, and the

technique has found commercial application in

the UK, the USA, Europe, and Japan. It has been

developed for a number of different applications:

0014aseptic processing of high-added-value ready-

prepared meals for storage and distribution at

ambient temperatures;

Holding tubes

Product outlet

Heating tube

Electrode

Power

supply

Product pump

Product inlet

Catch tank

Recycle for

CIP and/or

sterilization

To filler

Aseptic surge

vessel

Coolers

fig0002 Figure 2 Schematic process flowsheet for the APV Baker ohmic heater. CIP, cleaning in place. Adapted from: Parrott DL (1992) Use

of ohmic heating for aseptic processing of particles. Food Technology 46: 68–72 with permission.

3046 HEAT TREATMENT/Electrical Process Heating

.0015 pasteurization of particulate food products, for

example fruit pieces;

0016 preheating of food products prior to in-can steril-

ization;

0017 the hygienic production of high-added-value ready-

prepared meals for storage and distribution at

chilled temperatures.

Rapid pasteurization of particles gives very high

product quality and has proved very successful. Elec-

trical heating involves short process times: different

parts of a meal may require different pretreatments.

For example, to produce tender meat and crisp vege-

tables in the same formulation it will probably be

necessary to precook the meat. It is important to

prove that an ohmic heater carries out the required

treatment. Understanding the principles of electrical

heating is necessary both for process design and for

proving that it is acceptable to regulatory bodies.

Factors that Control Electrical Heating

0018 Ohmic and conventional processes differ significantly.

The process is also significantly different from micro-

wave heating, which has a finite penetration depth of

energy into any food; electrical heating has no such

limitation. Ohmic heating offers the possibility of

rapidly heating solid–liquid mixtures on a commer-

cial scale. The process does however lead to problems

for formulation designers and food engineers. Some

of these are conceptual; many of the situations pos-

sible in ohmic heating are not generally encountered

in food processing.

0019 To design a food formulation to exploit the advan-

tages of electrical heating requires understanding of

the process and the factors which affect product ster-

ility and quality. The process is controlled by the rate

of heat generation, which is governed by a number of

factors, the most important of which is the electrical

conductivity of the food material. However, tempera-

ture is also affected by the way food flows through the

heater, and thus the residence time within the plant.

To design a food and process requires the factors

which affects the heating rate of the material, and

the time that it spends in the heater, to be understood.

0020 The heating rate can be found from eqn 1; a more

useful form of Ohm’s law considers voltage gradients,

where the heating rate is given as:

Q ¼ kE

¼ kðgrad V  grad VÞð2Þ

where E is the voltage gradient (V m

1

) and k the

electrical conductivity (S m

1

). The voltage field

within a system in which the electrical conductivity

varies with position is found by solving Laplace’s

equation:

rðkrVÞ¼0 ð3Þ

throughout the material with appropriate boundary

conditions, such as constant voltage on the elec-

trodes. The distribution of electrical conductivities

thus controls the voltage distribution, and critically

affects the heating rates of the different phases. Local

voltage gradients may well be different to global ones

because of local conductivity changes.

0021The temperature field that results from heating can

be found by solving the equation for thermal trans-

port; for example, in a solid:

ð4Þ

where r is the density, c

the specific heat, and l

the thermal conductivity of the material. Eqns 2–4

are coupled through the temperature dependence of

the physical properties, particularly electrical con-

ductivity.

0022Figure 3 demonstrates the interaction between par-

ticle shape and alignment of the electric field using

experimental results; the same particle will underheat

the fluid if placed parallel to the electric field or

overheat if placed at right angles to the field. This

arises because the solid is less conductive than the

liquid: current is diverted past the particle when it is

parallel to the field, but has to pass through the solid

when it is at right angles to the field. The reverse is

seen when the particle is more conductive than the

surrounding liquid. Particles with a smaller aspect

ratio, such as cubes, will have less heating rate vari-

ation with orientation, but some effect will always be

seen around sharp edges.

Electrical Conductivity of Foods

0023In electrical heating it is obviously necessary to know

the overall electrical conductivity of the mixture. This

is key in determining the power consumption and

mean heating rate of the process. It is also necessary

to know the local conductivity of the different com-

ponents of the food. Foods generally have electrical

conductivities in the region of 0.01–10 S m

1

; these

often increase linearly with temperature, with about

2% increase per degree increase. However, where

there is a physical or chemical change in the food,

complex changes in conductivity can occur, such as

when there is a phase change (such as melting of fats)

or a change in the structure of the food (such as the

breakdown of cell walls).

0024For a commercial situation, electrical conductivity

needs to be measured experimentally for all the

components of a mixture. Care should be taken

HEAT TREATMENT/Electrical Process Heating 3047

here; electrical conductivity can be a strong function

of the frequency of the supply, and can be strongly

anisotropic, because of the structure of food. For

example, the electrical conductivity of carrot at



C was found to be 0.025 S m

1

along the axis of

the carrot, and 0.02 S m

1

perpendicular to the axis.

Table 1 shows typical electrical conductivities for

foods; these are guides only. It is possible to change

the conductivity to some extent, for example by salt

infusion, but the result may not be acceptable to

consumers. Insulators (such as glass or plastic) and

conductors (such as metals) have electrical conduct-

ivities several orders of magnitude different to those

of foods. In each case, these inclusions will not heat at

a significant rate in an electric field; metal has too low

a resistance to heat, whilst no current flows through

insulators to give rise to heating. Care must be taken

to avoid the presence of these inclusions to eliminate

cold spots in the system.

Heating of Solid–Liquid Mixtures

0025The equations show that the distribution of tempera-

ture in a food mixture depends on the distribution

of electrical conductivity. This depends on the

electrical conductivities of the individual phases,

the shape of the particles, and their orientation to

the field. Figure 3 has shown this effect for materials

where the electrical conductivity of the two phases

is significantly different. In practice, if the elec-

trical conductivities of the two phases are within

about 10%, then the temperatures will be reasonably

uniform.

0026Eqn 2 can only be solved analytically in very sim-

plistic situations, such as when the physical properties

are constant. In practice the commercial ohmic heater

involves a flowing mixture where the electrical con-

ductivities vary with temperature: flow through the

heater must also be considered in detail. Full solution

of the electric, temperature, and flow fields is not

feasible given the complexity of the problem and the

coupling of the equations via the variation of viscosity

and electrical conductivity with temperature.

0027Various approximations have been tested to predict

how flowing particle–liquid mixtures behave. Finite

element models have been used to develop solutions

for ohmic heating of solid–liquid mixtures: this type

of model can simulate both complex shapes and tem-

perature-dependent physical properties. Such models

solve the Navier–Stokes equations for flow, but can

also solve energy and potential field equations to

predict temperatures. In each simulation, the vessel

geometry can be discretized by a mesh, and the

material properties and boundary conditions given

as input data.

0028In flow situations, the full Navier–Stokes equation

for fluid flow must be solved together with the

temperature and voltage field. The assumption of no

fluid motion can simplify modeling considerably,

100

0 50 100 150 200

Temperature (⬚C)

Time (s)

(a)

100

0 50 100 150 200

Temperature (⬚C)

Time (s)

(b)

Field direction

Electrode

Particle

fig0003 Figure 3 The electrical heating of potato particles, 3  4 

0.75 cm, as a function of orientation to the applied field. Overheat-

ing and underheating of a particle (circles), in liquid (continuous

line), in different orientations to the electric field (shown in

insets). Reproduced from Heat Treatment, Electrical Process

Heating, Encyclopaedia of Food Science, Food Technology and Nutri-

tion, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

tbl0001Table 1 Electrical conductivity data for typical foods

Product type Conductivity at 25



C(Sm

1

)

Pickles and chutnies 2.0–3.0

Savory sauces 1.6–1.8

Soups 1.4–1.8

Meats 0.8–1.2

Full-cream milk 0.52

Desserts; vanilla, rice, custard 0.38–0.5

Beaten egg 0.4

Vegetable pieces 0.06–0.1

Fruit pieces 0.05–0.15

Margarine 0.027 (too low for successful

heating)

3048 HEAT TREATMENT/Electrical Process Heating