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eventually to the combination of oxygen with the

carbon and hydrogen components of the body’s

fuels to yield the carbon dioxide and metabolically

derived water. The incompletely oxidized nitrogen is

excreted as urea, which is synthesized by the liver and

excreted by the kidneys. The intermediate steps in the

metabolism of the body’s fuels are linked biochemi-

cally to drive the generation of phosphate-containing

organic molecules, such as adenosine triphosphate

(ATP), which in turn serve as the direct energy sources

for all the body’s cell activities, including the synthesis

of complex molecules, the maintenance of tightly

controlled ionic gradients in the cell, and the excre-

tion of ions and molecules outside the cell. Thus, the

oxygen being taken up by the lungs reflects the tissue

metabolism of the fuels needed to regenerate the ATP

used up in either biochemical ‘internal’ work or

mechanical external work undertaken by the body’s

muscles. The rate at which the body burns its own

stored fuels in the fasted, resting and relaxed state,

i.e., in the basal state in a warm room, is called the

basal metabolic rate (BMR). This varies with the age

of the individual, mainly because of the varying sizes

of metabolically very different organs at different

ages. Thus, a child has a relatively large brain, liver,

and intestine with a higher metabolic rate per kilo-

gram of body weight than a more muscular adult.

Body fat cells are metabolically active but contain a

substantial amount of inert fat, so that the larger fat

mass of a woman has a lower BMR per unit weight

than a man, but if the oxygen uptake is calculated in

terms of the metabolically active fat-free mass, then

her metabolic rate is the same. As men and women

age, they tend to lose lean tissue and store extra fat, so

that the BMR on a weight basis falls with age. Again,

the BMR remains the same when expressed in terms

of the fat-free mass of the body.

0004 Figure 1 depicts the changes in BMR of girls and

women as they grow and age. Equations can now be

used to estimate a group’s BMR from their sex, age

and bodyweight (see Table 1), but there is a range of

BMR amounting to + 20% of the mean value at each

weight. Thus, in a young 25-year-old woman of

55 kg, the anticipated mean BMR is 1305 kJ per day

but could vary under normal conditions from 1063 to

1547 kJ per day. The kilojoule is the standard meas-

ure, and 4.184 kJ corresponds to 1 kcal, which was

originally defined in energy terms as that required to

increase the temperature of 1 g of pure water by 1



from 14.5 to 15.5



C. The variation in BMR at a

constant weight in part reflects differences in the fat

content of individuals of the same weight. Thus, the

BMR per unit fat-free mass varies by 12–15% rather

than by + 20% as for weight. About 40% of the

BMR may be explained by differences in the size of

the body’s organs, e.g., liver, intestine and muscle, but

there is a residual difference between individuals,

which seems to be explicable only in terms of differ-

ences in the rate at which every organ of the body

metabolizes its fuel. This in turn is controlled princi-

pally by the circulating concentration of thyroid hor-

mones. Adults with a normal but above-average level

of thyroid hormones in their blood tend to have a

BMR in the upper part of the normal range. Smokers

also have a BMR that is about 5% above the normal,

but whether or not this relates to changes in thyroid

hormones is unknown. Young women who have

normal menstrual cycles show a swing in BMR that

is at its lowest in the late follicular phase, just before

ovulation. On ovulation, the basal body temperature

rises rapidly by about 0.5



C. The BMR is also in-

creased but rises further to a peak in the later luteal

phase, immediately before menstruation. This meta-

bolic cycle with swings of +5% of the mean is inde-

pendent of changes in food intake, but the recognized

tbl0001Table 1 Equations for prediction of basal metabolic rate (used

in Figure 1)

Age range (years) Males Females

0–3 0.255W  0.226 0.225W  0.214

3–10 0.0949W þ 2.07 0.0941W þ 2.09

10–17 0.074W þ 2.754 0.056W þ 2.898

18–29 0.063W þ 2.896 0.062W þ 2.036

30–59 0.048W þ 3.653 0.034W þ 3.538

60–74 0.0499W þ 2.930 0.0386W þ 2.875

75þ 0.0350W þ 3.434 0.0410W þ 2.610

500

1000

1500

2000

BMR (MJ per day)

BMR (kcal per day)

30 40

Bodyweight (kg)

50 60

Females

fig0001Figure 1 Relationship of basal metabolic rate (BMR) with age

and weight in females: —^— 1–3 years; --s-- 3–10 years; —d—

10–18 years; --h-- 18–30 years; --&-- 30–60 years; s--s 60–74

years; s——s over 74 years (data based on equations in

Table 1). Reproduced from Metabolic Rate, Encyclopaedia of

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

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

3864 METABOLIC RATE

5–10% fall in intake during the follicular phase with

a similar rise in the luteal phase may accentuate the

hormonally dependent swing. The effects of contra-

ceptives that inhibit ovulation and the subsequent rise

in basal temperature are unknown. The previous

day’s food intake does not affect the BMR unless

there has been substantial overeating. However, the

mixture of fuels combusted during fasting is influ-

enced by the proportion of the previous 3–4 days’

intakes, which is derived from carbohydrate, much

of the glucose from glycogen being metabolized in the

fasting state if carbohydrate intake was previously

high. When the carbohydrate store of glycogen in

the liver is nearing exhaustion, the body’s output of

carbon dioxide falls as the body switches to using

body stores of fat. The oxygen uptake for combusting

the fatty acids continues since the demand for

regenerating ATP is unaffected by the change in fuel

supply, but a carbohydrate-rich diet tends to induce a

slightly higher fasting metabolic rate than an energy-

equivalent, fat-enriched diet, probably because of a

slight induction of thyroid metabolism by dietary

carbohydrates. (See Hormones: Thyroid Hormones.)

0005 The BMR falls by between 2 and 5% when indi-

viduals transfer to live in a tropical warm environ-

ment, and in uninsulated houses, seasonal changes in

the BMR were readily seen with a 5–10% rise from

summer to winter being observed in the Japanese

before World War II. The BMR formulae shown in

Table 1 ignore any temperature effects. The BMR of

some people living in the tropics may be as much as

10% below the values shown, but these studies have

been conducted on children and adults who are, or

were, undernourished. Poor nutrition may have both

an immediate and a long-term effect in lowering the

basal metabolic rate. Semistarvation leads to a fall in

BMR from about day 4, and within 2 weeks, the

BMR can fall by 15% as thyroid metabolism changes

and the body’s organs become more efficient. More

prolonged or severe semistarvation leads to a progres-

sive loss of the body lean tissues as well as fat, and the

BMR therefore continues to decline in proportion to

the loss of lean tissues. Body weight can eventually

stabilize at a new low level and, if physical activity is

also reduced, semistarved volunteers can come back

into energy balance on 50% of their initial intake.

However, this requires a 40% loss of weight and

marked lethargy if energy balance is to be preserved

on such a low intake.

The Components of Metabolic Rate

0006 Traditionally, the metabolic rate is divided into three

components: BMR, postprandial thermogenesis,

and physical activity. The BMR usually amounts to

50–60% of an individual’s total energy expenditure,

and postprandial thermogenesis to 10% used for the

metabolic cost of processing, i.e., eating, absorbing,

transporting, and storing food. The remaining energy

is used for physical activity.

Postprandial Thermogenesis

0007The surge in oxygen uptake after a meal, known

as postprandial thermogenesis, has been variously

described as the specific dynamic action of food,

dietary-induced thermogenesis or the thermic effect

of feeding. The last term is particularly favored by

animal nutritionists. It is difficult to measure it accur-

ately because after ingesting the food with minimum

physical effort, an individual has to lie at complete

rest while the oxygen uptake and carbon dioxide

production are monitored for many hours until the

metabolic rate has returned to the basal rate. This

may take more than 10 h, which explains why BMR

is measured after a 14-h fast. Separate feeding of

different fuels shows that the maximum effect on

oxygen uptake occurs after protein. This response is

equivalent to about 30% of the protein’s energy;

glucose induces a 5–10% effect, fat only a 2–5%

effect, consistent with its slow absorption by the

lymphatic tissue, and alcohol a 0–8% effect. Certain

dietary components also increase metabolic rate, e.g.,

a caffeine equivalent to two cups of tea increases

metabolic rate by 1–3%, and spices, such as those

found in an Indian curry, increase it by 25% com-

pared with a nonspiced meal. Moderate exercise

amplifies the metabolic response to a standard meal,

so that the combined effect of exercise and food is

greater than the sum of the response to each stimulus

given separately. The effect, however, is small and

amounts to 2% of the total energy expenditure. (See

Thermogenesis.)

0008Differences in postprandial energy expenditure

have been sought as an explanation for the propensity

of some individuals and animals to obesity. Results

are often conflicting because in any person, the

response tends to vary from day to day and is readily

influenced by changes in gastric emptying. A propor-

tion of obese subjects have a reduced metabolic re-

sponse to a meal; this effect may prove to depend on

the degree of abdominal insulation since the response

is reduced if volunteers are swathed in insulation to

reduce the abdominal heat loss, thereby increasing

the temperature of the blood entering and leaving

the liver. This seems to reduce the stimulus to body

metabolism. Lactating mothers (and pregnant

women) have a lower postprandial thermogenesis

that returns to normal after they have stopped

breast-feeding. Smoking and postprandial thermo-

genesis interact synergistically so the thermic output

METABOLIC RATE 3865

after a meal is enhanced. The small response during

lactation is consistent with that observed in many

species of animal in which brown adipose tissue is

used as the organ for modulating heat production as a

mechanism to maintain body temperature. However,

this organ is not very active in humans.

0009 Prolonged underfeeding and overfeeding lead to

changes not only in postprandial thermogenesis but

also in BMR. The effects of semistarvation (see

above) are modest, but overfeeding can produce a

much greater response, provided that the intensity

of overfeeding (especially with carbohydrate) is

high. Thus, progressive overconsumption of 6.3 MJ

(1500 cal) per day leads to a 33% increase in daily

energy expenditure. This composite response to more

prolonged overfeeding is usually classified as dietary-

induced thermogenesis. Nevertheless, this apparent

mechanism for dissipating excess energy is limited

because, at most, 27% of the excess intake is metab-

olized, the remaining 73% being stored, two-thirds of

it as fat and about a third as lean tissue. The majority

of the excess response in metabolism is accounted for

by the theoretical cost of fat synthesis from carbohy-

drate, although the human capacity to transform

carbohydrate into fat is limited, preference being

given to the selective storage of the fat component

of the ingested energy.

Physical Activity

0010 The energy cost of physical activity is predictable, but

in order to obtain a reasonable estimate of energy

expenditure on a daily basis, an analysis of activity

patterns on a minute-by-minute basis is required.

Children can be very active, making a detailed analy-

sis difficult because the type of activity needs to be

specified if an energy cost is to be assigned to each

type of activity. Weight-bearing movement and anti-

gravitational moves, e.g., walking up a hill with a

load, are particularly expensive. The simplest way of

estimating individual costs is to use the extensive

tables now collected on the energy cost of different

movements in children and adults. For simplicity,

these can be expressed as a ratio of the BMR, since,

in this way, differences between the sexes and indi-

viduals of varying size are removed. Table 2 illus-

trates how this is achieved for a female domestic

worker. The physical activity ratios (PARs), i.e.,

energy costs in relation to BMR, can be assigned on

the basis of extensive measures of similar activities. If

the average energy requirement of the individual is to

be estimated, account must be taken of the different

types of work involved throughout the year. Thus, the

individual shown, working in a Mediterranean envir-

onment, worked both as a domestic help and as an

agricultural seasonal laborer. Each type of day is

compartmentalized on a minute-by-minute basis, a

division being made for convenience between occupa-

tional and other work. Maintenance of the individ-

ual’s household varies depending on the day of the

week and the season. Socially desirable activities en-

compass a variety of activities, including, for example,

visiting friends, walking, churchgoing or dancing.

The assumption is also made that some intensive

exercise, amounting, on average, to 15 min daily, is

undertaken; this is desirable if muscle tone and phys-

ical fitness are to be preserved. After integrating all

tbl0002 Table 2 Energy requirements of a female domestic helper (age, 25 years; weight, 60 kg; BMR, 5.8 MJ day

1

)

Integrated

EnergyIndex

Daytype1

Housewife

Day type 2

Domesticlabor

Day type 3

Agriculturallabor

hMJh MJ h MJ

In bed 1.0 8 1.93 8 1.93 8 1.93

Occupational activities

Household work 2.7 2 1.31 1 0.65 1 0.65

Domestic labor 2.8 8 5.41

Tomato harvesting 3.0 8 5.80

Discretionary activities

Household maintenance 2.0 2 1.00

Socially desirable 1.7 4 1.64 2 0.82 2 0.82

Cardiovascular and muscular maintenance 6.0 0.25 0.36

Rest of day 1.4 7.45 2.62 5 1.69 5 1.69

Daily total PAL

(MJ)

Integrated PAL (MJ) is 1.69 (9.78) 1.53 (8.86) 1.81 (10.50) 1.88 (10.89)

Day type 1, 4 out of 7 days for 10 months (48% of the year); day type 2, 3 out of 7 days for 10 months plus 1 out of 7 days for 2 months (38% of the year); day

type 3, 6 out of 7 days for 2 months (14% of the year).

Integrated Energy Index (IEI) is the energy cost of the activity including pauses for rest, expressed on a minute or hourly basis and calculated as a ratio

of BMR.

PAL, physical activity level.

3866 METABOLIC RATE

these activities, the ratio of the total energy expend-

iture to the BMR is found to average 1.69; the ratio is

designated the physical activity level (PAL). Table 3

provides a listing of PAL values for adults of all ages

according to their activity patterns. Thus, by knowing

the sex, age, and bodyweight of individuals, it is

possible to estimate their BMR (see Table 1). Given

this BMR figure in MJ per day, multiply by the PAL

value shown in Table 3, and the energy needs can be

estimated. Note that individuals vary in their energy

needs by +15%, so that an individual’s needs cannot

be predicted very accurately unless his or her BMR is

measured. Nevertheless, the total energy expenditure

of an individual child or adult is remarkably consist-

ent from day to day, varying by only 1–2% provided

that food intake and physical activity are meticu-

lously standardized, and account is taken in women

of the stage of the menstrual cycle. Thus, the energy

needs of individuals are surprisingly stable, and once

this BMR is measured, the total energy needs of the

child or adult can be readily estimated from a knowl-

edge of their activity patterns. The factors modulating

BMR or the metabolic response to food are many, but

their effect is small, so that it is not surprising that

energy expenditure is very predictable; the human

body is therefore a finely tuned and well-regulated

machine. Abnormalities of regulation, e.g., in obesity,

only arise because of a consistent discrepancy be-

tween the physiological controlled intake and ex-

penditure, which, if discrepant by 1–2%, produces a

2–5 kg weight change in a year. (See Exercise: Muscle;

Metabolic Requirements.)

0011New measures of total energy expenditure esti-

mated over 2–3 weeks can be obtained by the use of

the double-labelled water technique, which relies

on the difference in labelling of urine or saliva with

the two heavy isotopes of water, deuterium and

The differential dilution of deuterium and

Oin

urinary water is monitored over a 2–3-week period

following a single oral dose of D

O. The

O con-

tent is diluted more rapidly than the deuterium

because the oxygen in water exchanges rapidly with

the body’s bicarbonate pool, which is turning over

rapidly as carbon dioxide is produced by tissue me-

tabolism. Thus, the difference in dilution rates of

and deuterium provides a measure of the rate

of carbon dioxide production. The technique is

expensive and difficult to perform analytically but

very convenient for the subject being studied, since

only single daily or occasional urine or saliva speci-

mens are needed over the period of observation.

This method is likely to be increasingly used, but

classical methods have allowed current estimates of

energy needs to be calculated. These are accurate for

groups of people but not for individuals who vary

not only in their physical activity patterns but also

in their metabolic rate at rest. Figure 2 gives an

indication of the decline in energy needs during

adult life. This results from the atrophy of the lean

tissues, which may be related to the fall in physical

activity. Lack of exercise is therefore a handicap

because it directly reduces energy expenditure, and

it may also lead to a slow shrinkage of tissues, such

as muscle, thereby producing a long-term fall in

tbl0003 Table 3 Development of the International Labour Organization’s classification to derive simple estimates of PAL values

for each

occupational group separated into averages for developed countries (DC) and less developed countries (LDC)

Occupational group

Occupational PALvalues

Males Females

DC LDC DC LDC

Professional, technical, and related workers 1.55 1.61 1.56 1.58

Administrative and managerial 1.55 1.61 1.56 1.58

Clerical and related workers 1.55 1.61 1.56 1.58

Sales workers 1.67 1.78 1.60 1.64

Service workers 1.67 1.78 1.60 1.64

Agricultural, animal husbandry, forestry, fishing, and hunting 1.78 1.86 1.69

Production and related transport equipment operators and laborers 1.78 1.86 1.64

1.69

Housewives 1.56 1.64

Students 1.55 1.61 1.56 1.58

Unemployed 1.55 1.61 1.56 1.58

Subsistence farmers 1.78 1.86 1.64 1.69

Domestic helpers 1.78 1.60 1.64

Elderly (>65 years) 1.51 1.51 1.56 1.56

PAL, physical activity level. These values are based on different proportions of the population being assumed to be engaged in light, medium, and heavy

activities. The assumption is made that in less developed countries, there will be a greater demand for physical activity because the mechanization of

transport, work, home, and leisure time activities will be appreciably less than in developed countries.

Excludes female laborers.

METABOLIC RATE 3867

metabolism at rest. Unless people adapt their intake

extraordinarily well to this progressive decline in

energy output, energy storage, weight gain, and

obesity are inevitable.

Extra Energy Costs

0012 The cost of growth amounts to 20 kJ per gram of new

tissue deposited; the value can be higher if fat with

little lean tissue is laid down. A newborn has a high

energy requirement of about 480 kJ kg

1

, but by

1 year of age, this has fallen to about 400 kJ day

1

as growth slows. Without sufficient energy, a child

will fail to grow, but the causes of growth failure

usually relate to a deficiency of other nutrients or to

infection, rather than to a lack of dietary energy.

Adolescents, particularly boys, who are physically

very active, may have a high demand for energy.

However, the actual cost of even rapid growth rates

at this age is modest. (See Adolescents; Children:

Nutritional Requirements.)

0013Traditionally, pregnancy is considered a time of

great demand for food. Good nutrition is extremely

important, but the need for additional energy as such

is small. The total cost of the 9 months of pregnancy

has been estimated as 293 MJ (70 000 kcal) once the

cost of tissue deposition (167 MJ) and increase in

BMR (126 MJ) have been allowed for. In practice,

however, physical activity declines, particularly in

late pregnancy, and some enhanced metabolic effi-

ciency, with an early fall in BMR in early pregnancy,

seems to occur. The overall extra demand is very

small, amounting to 420 kJ day

1

in the last third of

pregnancy only. (See Pregnancy: Safe Diet.)

0014Lactation imposes a greater demand on mothers,

since their milk contains 1.9 MJ day

1

after birth,

rising to about 2.3 MJ day

1

on exclusive breast-feed-

ing at 3 months. Extra energy is involved in making

this milk, and the total extra energy demand is 2.4–

2.9 MJ day

1

. Part of the energy for this comes from

the extra fat stored by the mother during pregnancy, so

that the effective extra need amounts to 1.9 MJ at birth

and 2.4 MJ in the third to sixth month of exclusive

breast-feeding. This explains why mothers are more

hungry when nursing their child than when pregnant.

(See Lactation: Physiology.)

0015Convalescent patients who need to put on weight

need extra food, but the cost of this weight gain

amounts to between 20 and 40 MJ per kilogram

weight gain. If 1 kg is gained per month, the extra

food needed therefore amounts to about 1 MJ day

1

See also: Adolescents; Children: Nutritional

Requirements; Energy: Measurement of Food Energy;

Exercise: Muscle; Metabolic Requirements; Hormones:

Thyroid Hormones; Lactation: Physiology; Obesity:

Etiology and Diagnosis; Pregnancy: Safe Diet;

Thermogenesis

Further Reading

Erlichman J, Kerbey AL and James WPT (2002) Physical

activity and its impact on health outcomes. Paper 2:

prevention of unhealthy weight gain and obesity by

physical activity: an analysis of the evidence. Obesity

Reviews 3: (in press).

FAO/WHO/UNU (1985) Energy and protein requirements.

Report of a Joint Expert Consultation. Technical Report

Series, No. 724. Geneva: WHO.

James WPT and Schofield EC (1990) Human Energy

Requirements. Oxford: Oxford Medical Publications.

Schofield WN, Schofield C and James WPT (1985) Basal

metabolic rate: Review and prediction. Human Nutri-

tion: Clinical Nutrition 39C: 1–96.

MJ per day

kcal per daykcal per day per kg fat-free mass

kJ per day

100

250

200

150

Intense exercise

Energy expenditure

Discretionary

Occupational

Sitting, coffee,

smoking

Dietary-induced

thermogenesis

Basal metabolic

rate

Intense exercise

In terms of fat-free mass

Discretionary

Occupational

Sitting, coffee,

smoking

Dietary-induced

thermogenesis

Basal metabolic

rate

70 kg

Aged 25 years

Aged 70 years

Aged 25 years

70 kg

Aged 70 years

2000

fig0002 Figure 2 Different components of energy expenditure in a

young man and the impact of aging. The energy expenditure

falls mainly because of a decline in physical activity. There is

also a fall in the basal metabolic rate, but this is in line with the

reduction in the lean tissues, i.e., fat-free mass, in the elderly.

Reproduced from Metabolic Rate, Encyclopaedia of Food Science,

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

MJ (eds), 1993, Academic Press.

3868 METABOLIC RATE

METALS USED IN THE FOOD INDUSTRY

Ph Decle

ty, Chemin de la Treille, Vernouillet, France

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 When talking about materials in the food industry, it

is practically impossible to avoid use of the term

‘stainless steel,’ as so much of this material is used in

the fabrication of storage, processing, handling, and

other plant and equipment essential to the industry.

After a brief recall of the definition and characteris-

tics of the stainless steels, reference will be made to

their properties relevant to food industry engineering,

and the main applications where these properties are

utilized will then be surveyed.

Definition and Classification of Stainless

Steel

0002 Stainless steels make up a family of corrosion- and

heat-resistant iron-based alloys containing a min-

imum of about 12% chromium. The corrosion resist-

ance is improved by increasing the chromium

content. Both corrosion resistance and fabricating

characteristics are further improved by addition of

nickel. Other important alloy additions include

molybdenum, titanium, and nitrogen.

0003 Stainless steels may be divided into three groups

according to composition and metallurgical charac-

teristics:

0004 Austenitic stainless steels contain chromium and

nickel, frequently with additions of other elem-

ents. These steels are normally nonmagnetic and

can be hardened only by cold working. The com-

monly used grades containing 18% chromium and

8–10% nickel account for about two-thirds of

stainless steel produced. They are in general use

in food industry engineering. Steels containing up

to about 2.5% molybdenum are used when the

corrosive environment is particularly severe. For

specialized applications involving the most corro-

sive environments, higher-alloy grades containing

up to 20% chromium, 25% nickel, and 6%

molybdenum are available.

0005 Ferritic stainless steels contain chromium as the

principal alloying element. They are magnetic and

can be hardened only by cold working. The most

commonly used ferritic grade contains 17% chro-

mium with a carbon content of about 0.05%. The

ferritic steels are for general use where the corro-

sive environment is not too severe.

0006Martensitic stainless steels are iron–chromium

alloys containing a significant level of carbon,

which allows the steel to be hardened by heat

treatment. Other alloying elements are sometimes

added. The most commonly used steels of this

family contain 12–13% chromium. Mechanical

parts subject to heavy wear and knife blades are

their principal applications. The martensitic steels

are magnetic. They are not so resistant to corro-

sion as the ferritic and austenitic grades.

0007To these three main groups must be added a fourth

class, the duplex stainless steels, which have a mixed

austenitic/ferritic structure. These steels have a higher

chromium content and a lower nickel content than

the austenitic stainless steels and may have additions

of molybdenum, nitrogen, and copper. They are char-

acterized by good corrosion resistance and high

strength. Use of these relatively new materials may

be expected to increase.

0008Table 1 gives the compositions of the main stainless

steels available for food industry applications.

Stainless Steel Properties

Physical and Mechanical Properties

0009Tables 2 and 3 list the physical and mechanical prop-

erties of the stainless steels most commonly used in

the food industry. The following points should be

noted:

0010the relatively low thermal conductivity and high

coefficient of thermal expansion of austenitic

steels;

0011the high strength of the stainless steels relative to

carbon steel;

0012the high tensile ductility of the austenitic steels,

which, coupled with good weldability, contributes

to the excellent fabricability of these steels.

0013It should be noted that the austenitic stainless steels

offer mechanical strength and ductility also at high

temperatures.

Corrosion Resistance

0014The corrosion resistance of the stainless steels results

from the formation of an invisible protective surface

oxide film, which is able to repair itself by contact

with an oxygen-containing environment if acciden-

tally destroyed. A minimum of 12% chromium is

METALS USED IN THE FOOD INDUSTRY 3869

necessary to form this ‘passive film,’ and, as indicated

earlier, its protective properties are improved by an

increase in the chromium content and by addition of

other alloying elements such as nickel and molyb-

denum. (See Corrosion Chemistry.)

0015 Ferritic stainless steels are used for food industry

equipment where service is not particularly onerous,

but, in general, nickel-containing austenitic stainless

steels must be selected. In the presence of most food-

stuffs, AISI type 304 grade offers a good resistance to

general corrosion and pitting corrosion, and it is only

for some particularly aggressive products or processes

that recourse to more resistant materials containing

molybdenum, notably AISI type 316, is necessary.

0016In welded constructions, a particular type of corro-

sion can occur in the zone affected by heat during

welding. This type of corrosion, termed ‘intergranu-

lar corrosion,’ is due to precipitation of chromium

carbide at the grain boundaries, which results in chro-

mium depletion of the adjacent material. This chro-

mium-depleted zone is subject to corrosive attack if

exposed to some aggresive media. To avoid the risk of

this form of corrosion, it is necessary to prevent the

carbide precipitation, and this is achieved by lowering

the carbon content or by adding a stabilizing element

(titanium or niobium). In practice, extra low-carbon

grades (types 304L and 316L) or stabilized grades

(types 321 and 316Ti) should be used for welded

structures that are to be exposed to aggressive media.

0017Stress corrosion cracking can occur if a structure

has residual surface tensile stresses (resulting from

welding or a deep-drawing operation or, in service,

from differential expansion effects or vibration)

and is exposed to particular chloride corrosive

environments at temperatures above about 60



C. In

particular, care is needed to remove any traces of

chlorine-containing cleaning agents (which can result

readily in chloride contamination) after a cleaning

operation. Enhanced resistance to chloride stress

tbl0001 Table 1 Main stainless steel grades used in food industry engineering

Grade designation Composition (%)

AISI BS Euronorm C (Max.) Cr Ni Mo Other Typicaluses

Austenitic steels

304 304S16 X6CrNi8–10 0.06 17–19 8–11 General use

304L 304S11 X3CrNi18–10 0.03 17–19 9–12 General use

Extra low-carbon

grade

321 321S31 X6CrNiTi18–10 0.08 17–19 9–12 Titanium content is

over five times the

carbon content

General use

Ti-stabilized grade

316 316S31 X6CrNiMo17–12–2 0.06 16–18.5 10.5–13.5 2–2.5 Severe

environments

316L 316S11 X3CrNiMo18–12–2 0.03 16–18.5 11–14 2–2.5 Severe

environments

Extra low-carbon

grade

316Ti 320S31 X6CrNiMoTi17–12–2 0.08 16–18.5 10.5–13.5 2–2.5 Titanium content is

over five times the

carbon content

Severe

environments

Ti-stabilized grade

Ferritic steels

409 409S19 X8CrTi12 0.08 10.5–12.5 Titanium content is

over six times the

carbon content

For painted welded

structures

430 430S17 X8Cr17 0.08 16–18 Claddings, not very

severe

environments

Martensitic steels

420 420S37 X20Cr13 0.20 12–14 Mechanical parts

Other stainless steels are available for specialized applications and enhanced resistance to the most aggressive environments.

AISI, American Iron and Steel Institute.

tbl0002 Table 2 Physical properties of main stainless steel grades

used in the food industry

Stainless steel

types

Density

(kg dm

3

)

Coefficients of

thermalexpansion

(20^100



(10

6



1

)

Thermal

conductivity

at 20



(Wm

1



1

)

Austenitic steels 7.9 16–16.5 15

Ferritic steels 7.7 10–12 26

Martensitic steels 7.7 10 26

3870 METALS USED IN THE FOOD INDUSTRY

corrosion cracking is shown by the relatively new

high-strength duplex stainless steels (e.g., 22% chro-

mium, 5.5% nickel, 3% molybdenum, nitrogen), and

this grade has been used for, inter alia, sugar industry

applications in which stress cracking had been a prob-

lem. The higher alloy austenitic grades (e.g., 20%

chromium, 25% nickel, 4% molybdenum, copper,

and 20% chromium, 18% nickel, 6% molybdenum,

copper, nitrogen) also offer enhanced resistance to

chloride cracking, and their use for critical applica-

tions is expected to develop.

0018 Crevice corrosion can occur in stainless steels when

a chlorine-containing solution is trapped under stag-

nant conditions within a confined crevice, as may be

formed under a bolt head or under deposits on a

stainless steel surface. Laps and cold shuts in poor-

quality welds, inadequately dressed, can form typical

sites for crevice corrosion. Design and manufacture of

plant and equipment intended to work with chlorine-

containing solutions must avoid such crevices, and if

deposits are likely to occur during operation of the

plant, arrangements should be made to remove these

on a regular basis. To avoid risk of crevice corrosion,

particular care should be taken to eliminate chlorine-

containing solutions after a cleaning operation. In

general, it is good practice to avoid prolonged contact

with stagnant chlorine-containing solutions.

Stainless Steel Products

0019Stainless steel is available in all forms known for

carbon steel: hot-rolled and cold-rolled flat products;

seamless and welded round tubes; square and rect-

angular welded tubes; flat, round, square, and hex-

agonal bars; wires; forgings; castings; hot-rolled or

extruded sections.

0020As far as flat products are concerned, cold-rolled

sheets and strip thicknesses range usually from 0.25

to 6 mm. For thicknesses over 6 mm, sheets are de-

livered in the hot-rolled condition.

0021Thin wall sections can be obtained from sheets and

strips by brake or roll forming.

Plates, Sheets, and Strip Surface Finish

0022The standard mill finishes available are described in

Table 4.

0023Special finishes are available from mills, steel ser-

vice centers, or independent finishing specialists. Such

finishes range from highly reflective electropolished

and mirror-polished surfaces to textured and colored

surfaces, and epoxy and tern-coated surfaces.

0024In selecting a surface finish, it is important to rec-

ognize that a bright finish offers, generally, superior

corrosion resistance and superior cleanability. It is

also important when using oriented polished finishes

tbl0003 Table 3 Mechanical properties of main stainless steel grades used in the food industry (cold-rolled sheets and strip)

Grade designation Condition Tensile

strength

(Nmm

)

Proof stress

¼0. 2 (min)

(Nmm

2

)

Elongation

A (min) (%)

AISI BS Euronorm

Austenitic steels

304 304S16 X6CrNil8–10 Softened 500–680 230 48

304L 304S11 X3CrNil8–10 Softened 480–630 230 44

321 321S31 X6CrNiTi18–10 Softened 540–690 245 35

316 316S31 X6CrNiMo17–12–2 Softened 510–700 240 43

316L 316S11 X3CrNiMo18–12–2 Softened 490–670 240 43

316Ti 320S31 X6CrNiMoTi17–12–2 Softened 510–765 270 37

Ferritic steels

409 409S19 X8CrTi12 Softened 350–520 200 20

430 430S17 X8Cr17 Softened 430–590 240 20

tbl0004 Table 4 Standard mill finishes of stainless steel flat products

Finish designation Description

AISI BS

1 1 A rough dull surface produced by hot rolling, followed by annealing and descaling

2D 2D A dull finish produced by cold rolling, followed by annealing and descaling

2B 2B A bright finish produced by cold rolling, followed by annealing, descaling and final light cold

rolling on polished rolls

BA (bright

annealed)

2A A bright, cold-rolled, highly reflective finish obtained by annealing under a controlled

atmosphere

3 3A A cold-rolled polished surface obtained by finishing with an approximately 100 grit abrasive

4 4A A cold-rolled polished surface obtained by finishing with an approximately 150 grit abrasive or

finer

METALS USED IN THE FOOD INDUSTRY 3871

to arrange for the direction of polishing to be vertical

if this is possible. It is also important to be aware that

polishing is a costly operation and that, from an

economic point of view, it is preferable to select a

bright, cold-rolled mill finish whenever possible.

0025 Finish 1 (Table 4) is a rough finish, specific to hot-

rolled plates of thicknesses over 6 mm. It is not the

best finish from the cleanability point of view and

would require extensive grinding and polishing to

achieve a satisfactory finish. Where a heavy plate

(over 6 mm thick) is necessary, the most economic

solution is often to use carbon steel plate clad with

thin-gauge stainless steel rather than a solid stainless

steel plate.

Properties Particularly Relevant to the

Food Industry

Chemical Neutrality Towards Foodstuffs

0026 Stainless steels of the appropriate grade are resistant

to attack by a very wide range of foodstuffs and can

be regarded as chemically neutral. Specifically, pro-

vided the correct grade is selected, there is no signifi-

cant transfer of metallic elements, and the taste and

color of the product are not affected.

Cleanability and Retention of Bacteria

0027 Soil contamination on a surface can harbor bacteria

and also hinder removal of bacteria that may be

attached directly to the surface. A surface with desir-

able hygiene characteristics must, therefore, readily

release soil contamination on cleaning and also

readily release bacteria attached to the surface. Fur-

thermore, the surface must be resistant to any disin-

fectants used to kill residual bacteria.

0028 The stainless steels meet these requirements to a

high degree. Specifically, the smooth surface of the

steels facilitates removal of contamination, and, with

the hardness and impact resistance of stainless steels,

this surface characteristic is little affected during ser-

vice. The high resistance to corrosion of the stainless

steels allows more aggressive cleaning and disin-

fectant solutions to be used to ensure effective

disinfection.

0029 The cleanability of stainless steel as a food contact

surface has been studied by several authors. One

study concluded that stainless steel was not as clean-

able as glass or china, but was more cleanable than

aluminum and four other plastics tested. More re-

cently, it has been shown that stainless steel domestic

sinks retain 10 times fewer bacteria than other sinks

(in enameled steel, polycarbonate plastic and min-

eral–resin composite) after standardized simulated

wear, contamination, and spray wash treatments,

and that stainless steel with its high resistance to

abrasion and impact damage is more likely to retain

its hygienic characteristics throughout a working life

(see Figure 1).

0030The choice of cleaning and disinfectant products

for use with stainless steels depends on the nature of

the contamination. Some products containing chlor-

ide, iodine, or peracetic acid require care in their use.

They may cause crevice corrosion if allowed to

remain in contact with surfaces under stagnant con-

ditions, for example at seals. Chlorine-containing

solutions can lead to stress corrosion cracking if

allowed to remain in contact with stressed surfaces

at elevated temperatures. There are three precautions

to be observed when using such products: the tem-

perature should be kept low and not higher than



C, exposure time should not exceed 30 min, and

the plant or equipment should be thoroughly rinsed

with clean potable water after the cleaning operation.

(See Cleaning Procedures in the Factory: Types of

Detergent; Types of Disinfectant.)

Mechanical Properties

0031The high strength, toughness, and wear resistance,

particularly of the austenitic stainless steels, offer

important advantages for applications in the food

industry.

0032The high strength coupled with the absence of need

for corrosion allowances permits the use of thinner

gauges and sections when compared with carbon

steel, for example. This brings about savings in weight

and costs. The good impact and wear resistance

3.5

4.5

5.5

6.5

10 15 20 25

Cleaning time(s)

Mean log

bacteria cm

−2

30 35 40 45

MR 5

MR 6

SS 1

SS 2

fig0001Figure 1 Comparative cleanability of abraded sink material

after spray treatment. Mean logarithmic bacteria retained per

square centimeter (15 trials) versus cleaning time for a range of

abraded sink materials. SS, stainless steel; E, enamelled steel; P,

polycarbonate; MR, mineral resin. The numbers refer to the

sink number. Holah JT and Thorpe RH (1990) Cleanability in

relation to bacterial retention on unused and abroaded domestic

sink materials. Journal of Applied Bacteriology 69: 599–608 with

permission.

3872 METALS USED IN THE FOOD INDUSTRY

coupled with the high resistance to corrosion and ease

of cleaning results in low maintenance costs and long

life. Overall, stainless steel will often prove to be the

best material choice on a lifecycle cost basis.

Main Stainless Steel Applications

Dairy Industry

0033 Stainless steel is used for almost all equipment along

the chain of milk production, milk processing and

milk distribution. (See Milk: Processing of Liquid

Milk.)

0034 At the farm, milking machines as well as piping for

transporting the milk up to the refrigerated storage

tank are in type 304 stainless steel. The inside of the

storage tank is always made from type 304 stainless

steel, but type 430 ferritic grades can be used for the

outside cladding.

0035 For collecting milk at the farm, stainless steel

tankers of suitable size are used. They are fitted with

stainless steel accessories: piping and refrigerating

circuits, pumps, automatic cleaning equipment, etc.

0036 At the dairy processing plant, all equipment is in

stainless steel: storage tanks, pasteurizing plate heat

exchangers, piping, pumps, cleaning circuits,

etc. Type 304 stainless steel is of general use, but

type 316 molybdenum-containing stainless steel is

sometimes used for heat exchanger plates to prevent

any risk of stress corrosion cracking when aggressive

solutions are employed for disinfection.

0037 Equipment for butter processing is also made from

type 304 stainless steel, but type 316 molybdenum-

containing grades must be selected for equipment

used for processing salted cheese because of the better

resistance to chloride corrosion. It is also worth

noting the use of stainless steel wire for cheese trays.

0038 Generally speaking, surface finish in the dairy in-

dustry must be of a high quality in order to facilitate

the rigorous cleaning and disinfection required.

Wine Industry

0039 In the wine industry, it is of prime importance to

prevent the deterioration of taste and color of prod-

ucts, which can occur through loss of flavoring esters,

and through contamination, notably by iron and

copper. (See Contamination of Food.)

0040 Stainless steels meet these requirements with their

chemical neutrality when in contact with a wine: they

do not modify the pH of the wine, they do not trans-

fer any metallic taste or induce ‘ferric breaking’ to

white wines, and they do not interfere with the

biochemical fermentation process.

0041 Many tests have been conducted on stainless steel

behavior in wines of various qualities. Their results,

confirmed by a long experience, can be summarized

as follows:

0042For the storage of red wines in full tanks, for the

wine-processing tanks and for wine tankers, aus-

tenitic chromium nickel type 304 stainless steel

without molybdenum can be used.

0043For the storage of sulfited white wines, when the

tanks are not completely full, molybdenum-

containing stainless steel type 316 must be used

because of the risk of corrosion by sulfur dioxide

condensates above the wine surface. In order to

reduce cost, it is current practice to limit the use of

type 316 to the upper parts which may be exposed

above the wine surface; the lower submerged parts

are made with type 304 stainless steel. (See Wines:

Production of Table Wines; Production of Spark-

ling Wines.)

0044The surface finishes currently used on cold-rolled

sheets in the wine industry are 2B finish and bright

annealed finish. Weld beads must be ground and pol-

ished in order to eliminate oxides, laps, and similar

defects.

0045It should be noted that stainless steel offers the

possibility of setting up storage tanks outside protect-

ive buildings. The atmospheric corrosion resistance of

type 304 stainless steel is generally quite satisfactory,

but the molybdenum-containing type 316 steel would

be necessary if the tanks are to be exposed to marine

environments.

0046An important advantage of stainless steel over non-

metallic wine tanks is the ease of fitting an external

double wall where a refrigerating fluid is circulated

for the control of the fermentation temperature. Such

control is increasingly used for the production of

quality wines.

0047Vinegar obtained from wine is more aggressive

than vinegar obtained from alcohol. It contains elem-

ents originating from the wine, such as iron, copper

and manganese salts, and, sometimes, chlorides and

fluorides, which activate the corrosive action of acetic

acid. Type 316 molybdenum-containing stainless

steel must be used for the processing and storage

equipments of wine vinegar. (See Vinegar.)

0048Brandy and cognac are particularly sensitive to

contamination from the organoleptic point of view

and may be spoiled by a very small quantity of iron.

For this reason, the use of type 316 stainless steel is

advisable for production and storage equipment

(Figure 2). (See Brandy and Cognac: Armagnac,

Brandy, and Cognac and their Manufacture.)

Beer-making Industry

0049Today, stainless steel has replaced copper, which was

the traditional material used for beer-making plant

METALS USED IN THE FOOD INDUSTRY 3873