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found in the earth’s crust. The same can be applied to

many metals (such as aluminum, zinc, copper, and so

on) which are unstable.

0003 Thus, corrosion is an unavoidable problem, and

the most important aim is to reduce its occurrence

as much as possible. Annual damage due to corrosion

in the world is very important. Every domain of

industry is affected, including the food industry.

How Corrosion Starts

0004 Prior to corrosion, a disparity or a heterogeneity can

always be found, either physical or chemical. The

most conspicuous physical disparity is obtained by

the contact of two different metals (e.g., tinplate, gal-

vanized steel) present in the same solution (e.g., salt

water). Thereby, an electrochemical cell is formed.

0005 Chemical heterogeneity may be represented by dif-

ferences of ion concentration near identical electrodes

or by the Evans cell, which results from differential

aeration. The case of a salt water drop on a ferrous

material is well known (see Figure 4). The anodic site

(i.e., the area of corrosion) is always at the center of

the drop where there is an oxygen deficit relative to

the peripheral zone where aeration (and thus oxygen)

is more extensive than in the center of the drop.

0006 Microscopic disparities can also be found; there is

no practical industrial process which yields metals

with an absolutely homogenous surface, in the strict

physical sense: a touch of the metal surface with the

hand or simple machining with any tool may be suffi-

cient to damage the surface. A cold-worked zone be-

comes less ‘noble’ that is more easily corroded than the

surrounding zones as the atoms found in stressed crys-

tals tend more easily to leave the metallic crystal lat-

tice. The various chemical treatments sustained by the

metallic material are other sources of heterogeneity.

Electrochemical Basis of Corrosion

0007 A study of corrosion phenomena at a molecular level

shows an electron exchange between donating and

accepting sites. This is a reduction–oxidation or

‘redox’ reaction: the oxidizing part (which acts as a

cathode) is reduced through electron capture, while

electrons are donated by the reducing material (which

acts as an anode), which becomes oxidized (Figure 1).

0008 Some metal atoms lose electrons to form ions much

more easily than others, depending on how strongly

the metal’s positive ion attracts electrons, and this

depends on the nature of the metal. Reactive metals,

such as sodium, are those that ionize very easily,

whereas unreactive metals, such as platinum, are

those that ionize only with difficulty.

0009 sodium atoms ionize very easily;

0010zinc atoms ionize easily;

0011iron atoms ionize fairly easily;

0012copper atoms ionize with some difficulty;

0013gold atoms ionize with great difficulty;

0014platinum atoms ionize hardly at all.

0015Let us take an example: a bar of iron coupled to a

bar of copper. A bar of iron builds up a greater

concentration of electrons than a bar of copper

when dipped in water. In the iron, the electrons are

crowded more closely together and so repel each

other more strongly. That is, the ‘electron pressure’

in the iron is greater than in the copper.

0016The electric potential is more negative (lower) for

iron than for copper. (Figure 2). What can get

the electrons flowing? Something is needed which

consumes excess electrons and prevents them from

accumulating in the copper bar. Hydrogen ions can

do the job. Positive hydrogen ions, short of one elec-

tron, are strongly attracted to electrons and combine

with them to form neutral hydrogen atoms. In acid

water, they are plentiful. At the iron bar, ions continue

to leave the surface and pass into the water, and the

iron bar starts to corrode. The electrons released by

the ionized iron atoms flow through the connecting

wire to the copper bar and combine with the hydro-

gen ions to form gaseous hydrogen. The copper bar

cannot corrode because it is protected by an excess of

electrons coming from the corrosion of iron. The

cathodic protection of iron (in the form of steel) by

sacrificial anode (soluble anode) is usually obtained

through a coating; galvanized steel is an important

industrial example (Figure 3). Galvanized steel (zinc

plating) on steel dustbins, iron rails, etc. is a very

effective form of sacrificial anode protection. It

works even when large areas of zinc have gone. As

long as some zinc is left in contact with iron, the zinc

corodes preferentially. Mildly acidic rain water is

usually the electrolyte. The protection life through a

metallic coating is roughly proportional to its thick-

ness, as it dissolves progressively to protect base

metal. Rust formation, too, is driven by electron

exchange between an electron donor site and an elec-

tron acceptor site (in this case, oxygen; see Figure 4).

0017Moreover, electron exchange may take place in

plain metal when some heterogeneities (physical or

Anode

Excess of electrons

Release of electrons

Oxidation

Destruction of material

Deficit of electrons

Capture of electrons

Reduction

Protection of material

Cathode

(e.g. Sn Sn

+2e

Al Al

+3e

Fe Fe

+ 2e)

(e.g. 2H

+ 2e H

+4e+4H

fig0001Figure 1 Corrosion as an electrochemical process.

1676 CORROSION CHEMISTRY

chemical) are present in the metal structure. Micro-

cells form and may cause in pitting (Figure 5).

Parameters Driving Electron Transfer

0018 The most important factors governing electron

transfer are potential and easiness.

0019 The potential factor is governed by the electron

affinity difference between the two transferring

elements. The greater the difference, the faster the cor-

rosion may develop. The potential factor is often called

the dissolution potential. Consider, for example, a

metal M in a solution containing M

ions. The poten-

tial of the metal is given by the Nernst formula

E ¼ E

log

ða

Þ, ð1Þ

where E

is a constant factor, independent of the

solution concentration, am

is the activity of the M

ions in the solution (activity is proportional to con-

centration), R is the gas constant (8.31 J K

1

mol

1

n is the number of moles of electrons driven by the cell

potential E, F is the Faraday (96 500 coulombs), and

T is the absolute temperature. The Nernst formula is

commonly used as

E ¼ E

0:06

log

ða

Þð2Þ

for convenience. From measurements of differences in

potential between a metal and a reference electrode,

a series of dissolution voltages can be produced

(Table 1). Elements that have a greater tendency than

hydrogen to lose electrons are described as electroposi-

tive, while those that gain electrons are called electro-

negative, e.g., sodium is more electropositive than

aluminum. From this series, it can be seen which of

two metals, if placed in contact with each other, will

become corroded. For example, in this series, zinc has a

much greater dissolution voltage than iron; thus, if a

piece of iron is coated with zinc and placed in water, the

iron will not rust if the zinc coating is scratched, be-

cause zinc loses electrons in preference to iron. This is

the principle behind the galvanizing of steel.

0020The easiness factor reflects any factors affecting the

reaction arising from the conditions of the electrolytic

media where the electron transfer takes place. If the

electron donor (the anodic site) does not easily let

electrons loose, the corrosion is said to be donor

controlled, and there is an anodic overvoltage. In

contrast, the corrosion is controlled by the electron

acceptor if it cannot freely accept the electrons; a

cathodic overpotential then exists.

0021Any corrosion cell can be studied through intensity

voltage curves or polarization plots. In the laboratory,

2OH

−

2OH

−

Rust + H

Iron

fig0004Figure 4 Formation of rust through the formation of a differen-

tial aeration (Evans) cell. Reproduced from Corrosion Chemistry,

Encyclopaedia of Food Science, Food Technology and Nutrition,

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

Press.

Hydrogen

forming

+++++

IronCopper

Acid water

Electron

flow

Iron

corroding

fig0002 Figure 2 Single electrochemical cell. Reproduced from Corro-

sion Chemistry, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

Electron flow

Zn anode

Fe cathode

fig0003 Figure 3 Galvanized steel. Reproduced from Corrosion Chem-

istry, Encyclopaedia of Food Science, Food Technology and Nutrition,

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

Press.

Electron flow

Electrolyte

Micro-anode

Surface

Micro-cathode

Metal

fig0005Figure 5 Microelectrochemical cell. Reproduced from Corro-

sion Chemistry, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

CORROSION CHEMISTRY 1677

special devices quantitatively measure the ease of

electron transfer from an anodic site to a cathodic

state. Evans diagrams are mostly favored as they are

very simple; three examples are shown in Figure 6.

0022 The ideal situation would be a corrosion battery

with the lowest possible activity as a result of strong

anodic and cathodic overvoltages.

0023 The role of corrosion inhibitors is to increase the

overvoltage on the anode, on the cathode, or on both

electrodes simultaneously. They polarize the cell, thus

reducing its electron flow, often through the formation

of an insoluble compound on the surface of one elec-

trode. The corrosion inhibitor behavior (especially the

anodic inhibitors) is sometimes unpredictable as most

of the compounds used, depending on their concen-

tration and other environmental factors, may have a

role as either an accelerator or inhibitor of corrosion.

A corrosion accelerator is the reverse of an inhibitor.

Every electroactive compound in the electrolyte which

is able either to complex the positive ions coming from

the anode or to use the electrons arriving at the

cathode, either directly or through nascent hydrogen

formation, must be considered as a corrosion

accelerator.

Appearance of Corrosion

0024Macroscopically, corrosion can appear in the

following ways (Figure 7):

0025Generalized or uniform corrosion, which appears

at the same speed on the whole surface of the metal.

0026Pitting corrosion, which appears on localized sites,

e.g., at the interface between the metal and an

inclusion, on the breaks of a passivation film or

coatings. Current density is high in the vicinity of

the defect as the anode is small, while the cathode is

very large.

0027Crevice corrosion, which appears in cracks mainly

on stainless steels and under metallic coatings.

0028Intergranular corrosion, which appears only on

intergranular joints.

0029Intragranular corrosion: arborescent corrosion

cracks which appear in metallic crystals. Stress

often initiates this type of corrosion.

Corrosion Reactions in the Food Industry

0030Most foodstuffs are in aqueous solution, they are thus

conductors and, to some extent, active electrolytes.

0031Metals used in the food industry are found chiefly

in two separate areas:

0032metallic cans used for preservation of foodstuffs:

tbl0001 Table 1 The electrochemical series

Metal Electrode

reaction

Standard electrode

potential (V)

(Active end)

Sodium Na !Na

þe 2.712

Magnesium Mg !Mg

2þ

þ2e 2.34

Beryllium Be !Be

2þ

þ2e 1.70

Aluminum Al !Al

3þ

þ3e 1.67

Manganese Mn !Mn

2þ

þ2e 1.05

Zinc Zn !Zn

2þ

þ2e 0.762

Chromium Cr !Cr

3þ

þ3e 0.71

Iron Fe !Fe

3þ

þ3e 0.44

Cadmium Cd !Cd

2þ

þ2e 0.402

Cobalt Co !Co

2þ

þ2e 0.277

Nickel Ni !Ni

2þ

þ2e 0.250

Tin Sn !Sn

2þ

þ2e 0.136

Lead Pb !Pb

2þ

þ2e 0.126

Hydrogen H

!2H

þ2e 0.000 (reference)

Copper Cu !Cu

2þ

þ2e þ0.345

Cu !Cu

þe þ0.522

Silver Ag !Ag

þe þ0.800

Platinum Pt !Pt

2þ

þ2e þ1.2

Gold Au !Au

3þ

þ3e þ1.42

(Noble end)

log

E (mV) E (mV)

E (mV)

Towards positive

potentials

log

i log

(

)(

)

fig0006 Figure 6 (a) Low anodic overvoltage, high cathodic overvoltage; the galvanic couple is under cathodic control. (b) High anodic

overvoltage, low cathodic overvoltage; the galvanic couple is under anodic control. (c) High anodic and cathodic overvoltage; the

galvanic couple is under mixed control. log

i, log

of the corrosion current (or electrons), E

, from the equilibrium potential: anodic

polarization or overvoltage on the electrode supplying the electrons; E

, mixed and common potential of both; E

, from the equilibrium

potential: cathodic polarization or overvoltage on the electrode capturing the electrons. Reproduced from Corrosion Chemistry, En-

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

1678 CORROSION CHEMISTRY

.0033 processing equipment and storage vessels – alumi-

num alloys and stainless steels are most widely used

for this application.

Corrosion of Metal Cans

0034 Worldwide, over 200 billion cans are used every year

to preserve and protect a very wide range of foods.

Preservation of foodstuffs in closed cans by heat was

proposed by the Frenchman Nicolas Appert at the

beginning of the nineteenth century. A few years later,

the first metallic cans were produced in England from

tinplate, a material made from the tin/iron couple used

since the Middle Ages for kitchenware. The preserved

food industry developed most rapidly in the USA.

0035 The shelf-life of cans should be several years, thatis if

therateofcorrosioniskeptaslowaspossible.Themetal

canning of foodstuffs makes use of two base materials:

0036 Steel coated on both sides with a layer of tin of

varying but uniform thickness (0.4 – 1.6 mm). For

some 20 years, tin has been replaced, from time to

time, by a layer of metallic, oxidized chromium not

thicker than 0.015 mm. This composite is usually

called ‘tin-free steel.’

0037 Aluminum-based alloys, which have only been

used extensively since World War II.

0038Tinplate is a very asymmetric material; the behavior

of the tin/iron cell is shown schematically in Figure 8.

Tin in the presence of most foodstuffs behaves as a

sacrificial anode, and steel is thus cathodically

protected. The anodic behavior of tin is due to the

formation of many complexes with organic acids

and phenolic compounds (tannins) found in fruits and

vegetables. The uncomplexed fraction of tin in solution

or in the foodstuff is very low. Following the Nernst

equation, tin is more electronegative than it appears

from the electrochemical series. (See Tin.)

0039Tinplate producers sell about 15 million tonnes per

year, two-thirds of which is used for foodstuff can-

ning. Tinplate is not a single product: many types can

be manufactured, depending on the quality of the

steel, the thickness of the tin coating and sometimes

the quality of the lacquer.

0040Pure tin exhibits good resistance to the acid medium

of foodstuffs without oxidizing agents. This corrosion

is considered as normal. It even has a beneficial effect

on the retention of several organoleptic qualities of

foodstuffs. The most important corrosion accelerators

(that is, electron acceptors) which may be found in

foods are oxygen (air), sulfur dioxide (preservative),

sulfur (pesticide), nitrates (water, fruit, vegetables)

and trimethylamine oxide (fish). Farming techniques

(a) (d)

(e)(b)

(c)

fig0007 Figure 7 (a) Generalized or uniform corrosion of the metal. (b) Pitting corrosion. (c) Crevice corrosion. (d) Intergranular corrosion.

(e) Intragranular corrosion. Reproduced from Macrae R, Robinson RK and Sadler MJ (eds) (1993) Corrosion Chemistry, Encyclopaedia

of Food Science, Food Technology and Nutrition, Academic Press.

CORROSION CHEMISTRY 1679

are always changing, and have an influence on the

chemical composition of foodstuffs. Corrosion prob-

lems have multiplied during the last 20 years as a

result of, for example, fast detinning by nitrates,

which is even faster as the pH is lowered, and the

preferential dissolution of iron due to the presence of

pesticide residues such as dithiocarbamates. Contents

of up to 40 mg per kilogram of foodstuff, which have

no adverse effect on the consumer’s health may, in

some cases, lead to significant dissolution of tin

through corrosion. Similarly, dithiocarbamate resi-

dues (a few milligrams per kilogram) are sufficient to

invert the tin/iron cell and induce corrosion of the

steel. When these additional corrosion risks appear,

organic coatings inside the can are needed. For this

purpose, lacquers (macromolecular compounds or

high polymers) are used. They are insoluble and inert

in aqueous media. On account of the nature of the

foodstuffs and the various mechanical stresses these

lacquers have to withstand, several different types are

used. These are characterized either by their barrier

behavior or by their degree of flexibility: oleoresinous,

organosols, epoxyphenolics, epoxyesters, epoxyurea,

and so on are commonly used. These organic films are

deposited either on the flat sheet or after can manu-

facture. In both cases, they are cured thermally in an

oven, usually at 200



C for 10–15 min. Their thickness

ranges between 5 and 15 mm, depending on the re-

quirements for preservation of the can and/or the

foodstuff. They must also withstand sterilization

temperatures (125–130



C).

0041 Tin-free steels cannot be used alone as they are

much more sensitive to acid corrosion, and an organic

protective coating is thus essential.

0042 Aluminum is widely used in the food industry, either

as a canning material or in various equipment for

preparation, storage, and transport of foodstuffs. For

metallic canning, it is used much less than steel-based

materials: only 2.5 million tonnes per year. In the USA,

about 2 million tonnes per year are used due to the

large market for beverage cans (beer, soft drinks). In

foodstuff cans, aluminum is always used as an alloy

(with magnesium or manganese), the exact compos-

ition depending on the required mechanical properties

needed: for aerosol cans, 99.5% aluminum is used,

and for collapsible tubes 99.7%. Aluminum is also

used in composite packaging materials associated

with polymer films (polyethylene, polypropylene,

polyester, Pet, etc.). Theoretically, aluminum is a

very passive metal as it can be easily covered with an

alumina film (hydrated aluminum oxide). But being

an amphoteric metal, it is still highly sensitive to

corrosion in acid media (aluminum salts) or alkaline

media (aluminates). Pure aluminum is subject to all the

corrosion forms mentioned above (uniform, pitting,

stress, inter- and intracrystal, galvanic). For this

reason, aluminum is often chemically or electrochem-

ically passivated (anodization or chroming) in sulfuric

media. The surface treatments greatly improve the

adhesion of organic coatings (or lacquers) always

used for internal protection of metallic cans to achieve

chemical inertness over several years. Epoxyphenolic,

vinyl organosols or polyesters are usually used, in one

or two layers, as the situation requires. Corrosion of

aluminum cans is very rare with such protection. (See

Aluminum (Aluminium): Properties and Determin-

ation; Toxicology.)

0043Whether the can is in tinplate or aluminum, beer

can only be stored in totally inert cans as it is ex-

tremely sensitive to contamination by iron or alumi-

num, resulting in cloudiness and taints.

0044Low density, high thermal conductivity, and low

sensitivity to atmospheric corrosion are the three main

specific advantages of aluminum over steel for cans.

Equipment Corrosion

0045Many aluminum alloys are still used in tanks and

containers of all kinds (jars, cans, trolleys, tables,

and so on). The meat, fish, milk, cheese, pastry, and

confectionery industries all use such equipment.

However, in the modern food industry, stainless steels

are now more commonly used on account of their

good mechanical properties and robustness to fre-

quent cleaning. Similarly, glass and porcelain are

now more commonly being used.

0046There are two main families of stainless steel which

possess ferritic and austenitic structures. The first is

mostly chromium and iron, while the second contains

additional nickel. In both families, some molybdenum

may be added to improve corrosion resistance. To

prevent corrosion of welded zones, it is necessary

−

Electrolyte

Iron

unattacked

Cathode

−

Anode

Product to be canned

Sn gradually

converted into Sn

Electron capture

by H

, O

oxidizers

Fe Sn

Electron

source

fig0008 Figure 8 Schematic diagram of a tin/iron cell. Reproduced

from Corrosion Chemistry, Encyclopaedia of Food Science, Food

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

(eds), 1993, Academic Press.

1680 CORROSION CHEMISTRY

to reduce the carbon content of austenitic steels or to

stabilize them by adding titanium and/or niobium.

Corrosion resistance and also cost increase with the

amounts of other elements used in alloys. It is thus

essential to evaluate, as far as possible, the risks and

the corrosion resistance properties of the steels used,

to eliminate all technical problems, yet at the same

time avoiding high costs of production and mainten-

ance. Stainless steels can withstand different types of

localized corrosion: intergranular corrosion, pitting,

and cracking. Chloride solutions at high temperatures

can be highly aggressive: any cracks in the passive

layers may lead to rapid corrosion as they become

anodic sites with regard to the remainder of the sur-

face. Depassivation may have a mechanical origin

(abrasion, wear), and the metal is then permanently

depassivated or ‘active.’ It must be kept in mind that

stainless steels have extensive but not universal corro-

sion resistance. Every use of stainless steel in corrosive

media must be treated as a particular problem. The

choice of the material must be made in conjunction

with a metallurgist, the equipment builder, and the

user in order to consider all aspects of the problem

(both economic and technical).

0047 Some food industries experience more corrosion

problems than others, such as pork butchers and

salt meat producers as a result of chlorine- and salt-

containing vapor in plants (from brines).

0048 For a long time, the dairy industry large quantities

of austenitic stainless steel (18% chromium, 10%

nickel) for tubes, heat exchangers, tanks, centrifuga-

tion bowls, etc. New varieties of ferritic steels, con-

taining 17% chromium, a low percentage of carbon

and some titanium, are now being successfully intro-

duced for boilers.

Corrosion by Cleaning and Disinfecting Products

0049 To obtain regular production of good-quality food-

stuffs it is necessary to clean, disinfect, and descale

surfaces in contact with food. The frequency of such

treatments varies from one industry to another: two

to four times a day in the milk industry to once a year

in the sugar industry. These treatments require the use

of chemical products, which may themselves exert a

considerable corrosive action, generating microcav-

ities which will, as the damage increases, be increas-

ingly difficult to clean. (See Cleaning Procedures in

the Factory: Types of Detergent; Sanitization.)

0050 With regard to alkaline cleaning agents, stainless

steels withstand corrosion well, while aluminium

alloys are very sensitive, although corrosion may be

reduced by the addition of silicates.

0051 Corrosion of stainless steels in acidic media varies

with the particular acid, pH, and oxidizing potential

of the solution. In nitric acid solution, austenitic

stainless steels face no generalized corrosion, being

naturally autopassive. In some cases, slight pitting of

the whole surface can be seen. However, aluminum

alloys do not exhibit passivation in nitric acid, but

here too, it is possible to reduce corrosion speed by

adding organic acids such as malic or citric acids.

Cleaning solutions based on sulfuric acid always con-

tain corrosion inhibitors for stainless steels.

0052Chlorination is frequently used by many food in-

dustries, owing to its simultaneous strong and

cheap disinfecting and bactericidal properties. Its

effect is still not fully understood, but it is known

that chlorine is effective between pH 7 and 9. As

sodium hypochlorite, its effect is due to nascent

oxygen, which is very effective against microbial

germs. The problem with the use of chlorine is to

determine the level of addition required. Resistance

to chlorine varies widely between microbial species

which are to be killed. In terms of corrosion, chlorine

is a strong oxidizing agent. For metallic cans, chlorin-

ation of 0.5–2 mg of free chlorine per liter is sufficient

to prevent their recontamination through suction via

seams in the cans due to the increasing vacuum level

developing inside. Higher chlorine levels may induce

corrosion phenomena (detinning and rust on tinplate,

pitting on aluminum cans).

0053With the very high chlorine concentrations (300–

1500 mg l

1

) needed for thorough disinfection (in

dairies, for example), the risk of pitting and crevice

formation on stainless steel is higher as the tempera-

ture and contact durations increase. The lower the

pH value, the higher the risk. Sometimes, corrosion

inhibitors are also needed to improve the chemical

inertness of stainless steels.

0054Iodine containing compounds are considered as

having no action on stainless steels but should not

be used for cleaning and disinfecting aluminum and

aluminum alloys.

0055Solutions of peracetic acid (300 mg l

1

) made from

acetic acid, hydrogen peroxide, and water, which

have very good bactericidal properties, may be used

at room temperature, for short durations (about

20 min) on austenitic steels and aluminum alloys.

0056Bacterial corrosion, although uncommon, may

appear in some food industries, e.g., in buried tubing.

Every material, even metals, may be attacked by

microorganisms adhering to surfaces and, through

their bioactivity, leading to the, accumulation of

acids and dissolved gases. For example, we may

quote ferrobacteria and sulfate-reducing bacteria. Fer-

robacteria, acting on the anodic site, take their energy

from the oxidation of ferrous ions to ferric ions, thus

initiating the rapid formation of rust as they continu-

ously modify the equilibrium by simultaneous anodic

CORROSION CHEMISTRY 1681

and cathodic depolarization. Sulfate-reducing bac-

teria use hydrogen and induce cathodic depolariza-

tion: jelly-like vesicles appear, which are living

bacterial colonies.

Ways to Prevent Corrosion

0057 Some common ways of preventing corrosion of

metals have already been mentioned:

0058 Organic coatings with inert macromolecular poly-

mers used for cans and for steel-based equipment in

food industry plants.

0059 When using unprotected metals (aluminum alloys

and stainless steels), the following must be con-

sidered:

0060 – avoid as far as possible of the joining two metals;

0061 – choose the best suitable material;

0062 – modify aggressive media composition with in-

hibitors;

0063 – use cathodic protection by coupling a sacrificial

anode metal to the material to be protected. The

use of a metal as a sacrificial anode does not sup-

press corrosion and theoretically should not be

used for materials in contact with food (unless the

corroding metal is specifically authorized for con-

tact with food (e.g., tinplate).

Regulations upon Materials in Contact

with Foodstuffs

0064 Corrosion may cause damage during food processing

through steam or cooling fluid conveying tubes (per-

forations or breaks). Equipment failure as a result of

corrosion may disrupt the processing of sensitive

foodstuffs. If these are to be processed as soon as

possible, significant losses may result.

0065 In addition, certain foodstuffs may pick up certain

metals. Although some metals may have no toxic

effects, they may alter the organoleptic characteristics

of the food. For example, beer can turn cloudy in the

presence of even small amounts of iron. Also, iron has

a deleterious influence on the color of some fruits and

vegetables that are rich in phenolic compounds (tannic

compounds). Gray or black complexes may form and

change the foodstuff hue. For example, white coffee

prepared with milk containing very small amounts of

iron can take on a grayish appearance.

0066 Sometimes, the presence of metals may have bene-

ficial effects. For example, tin picked up by a food-

stuff from the can may preserve the color of clear

fruits and vegetables (such as mushrooms, pears,

asparagus, and pineapple), owing to the reducing

effect of stannous ions (Sn

2þ

0067Firm regulations are always based on a definite

listing: any material or object that is not precisely

authorized is forbidden. Some local regulations stipu-

late limits for global or specific migration levels.

These vary from one country to another. On a world-

wide basis, the Codex Alimentarius is the reference.

(See Legislation: Codex.)

0068Traces of incompletely rinsed cleaning agents may

pollute the foodstuff, although this operation is man-

datory. The choice of corrosion inhibitors to be added

to the cleaning products must always be done with

reference to the list of compounds authorized for

cleaning.

See also: Aluminum (Aluminium): Properties and

Determination; Toxicology; Cleaning Procedures in the

Factory: Types of Detergent; Legislation: Codex;

Sanitization; Tin

Further Reading

Bosich JF (1970) Corrosion Prevention for Practising

Engineers. New York: Barnes & Noble.

Brun S (1983) Les mate

riaux en contact avec les aliments.

Colloque CNERNA, Paris les 9, 10, 11 fe

vrier 1983,

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Crab See Shellfish: Characteristics of Crustacea; Commercially Important Crustacea; Characteristics of

Molluscs; Commercially Important Molluscs; Contamination and Spoilage of Molluscs and Crustaceans;

Aquaculture of Commercially Important Molluscs and Crustaceans

Crackers See Biscuits, Cookies, and Crackers: Nature of the Products; Methods of Manufacture; Chemistry

of Biscuit Making; Wafers

Cranberries See Fruits of Temperate Climates: Commercial and Dietary Importance; Fruits of the

Ericacae; Factors Affecting Quality; Improvement and Maintenance of Fruit Germplasm

Crayfish See Shellfish: Characteristics of Crustacea; Commercially Important Crustacea; Characteristics of

Molluscs; Commercially Important Molluscs; Contamination and Spoilage of Molluscs and Crustaceans;

Aquaculture of Commercially Important Molluscs and Crustaceans

CREAM

Contents

Types of Cream

Clotted Cream

Types of Cream

C Towler, P A E Cant and K R Palfreyman, Fonterra

Research Centre, Palmerston North, New Zealand

Range of Products and Compositional

Data

0001 A variety of creams, with different compositions,

functions, and types of packaging, can be obtained

for consumption (Figure 1). Cream consists of

emulsified globules of fat in a skim milk serum. A

typical particle size distribution in cream is shown in

Figure 2. Particles less than 1 mm in diameter represent

mainly casein micelles; the fat globules are mostly in

the 1–10 mm range. The fat provides flavor, and the

emulsion form gives characteristic textural attributes

and functional properties. The legal classification of

products is generally based on fat content. As each

country has different laws regarding the composition

of different creams, this is not covered here, but

Table 1 gives typical values for the fat contents of

different cream types. Other regulations may cover

the heat treatment that might be applied to the cream

– untreated, pasteurized, sterilized or ultrahigh tem-

perature (UHT)-treated – as well as the limits on

CREAM/Types of Cream 1683

the permitted additives and the normal regulations

relating to labelling. Several countries do not allow

the sale of unpasteurized cream. Low-fat creams are

used as pouring creams for desserts or for addition to

coffee or tea. Creams with a higher fat content may be

used as whipping creams. Permitted additives vary

from country to country. Typical additives are sugar,

stabilizers, emulsifiers, and stabilizing salts.

0002As the fat content increases, the viscosity of the

cream increases, and creams with a fat content greater

than 60% can be used as spreads. However, the fat

content is not the only determining factor in consist-

ency, and spreadable creams can be made with a

fig0001 Figure 1 Consumer creams.

0.1

1.0

10.0 100.0

Particle diameter (µm)

Volume (%)

fig0002 Figure 2 Particle size distribution in cream.

tbl0001 Table 1 Typical fat content of different types of cream

Cream type Fat content (%, w/w)

Clotted 55

Double 45

Whipping 35

Aerosol whipping 32

In-can-sterilized 23

Single 18

Half 12–18

1684 CREAM/Types of Cream

lower fat content by reducing the fat globule

diameters (homogenization) and adding thickeners.

Although cream is generally defined by its fat content,

the suspending serum is also important. This serum

consists largely of water (approximately 91%)

containing lactose (approximately 5%), protein (ap-

proximately 2.8% casein and 0.8% whey protein)

and other minor constituents such as minerals

(0.7%) and vitamins. The levels of fat and milk

solids-not-fat (MSNF) in milk are influenced by

breed, nutrition of the cow, and lactational or sea-

sonal factors. Another very important component of

cream is the membrane that surrounds the fat glob-

ules, and this has been the subject of many scientific

studies. The major components of this membrane are

proteins (41%), phospholipids (27%), neutral glycer-

ides (14%), water (13%), cerebrosides (3%), and

cholesterol (2%). Many of the properties of cream

are influenced by the membrane and its surface-active

components as they affect the stability of the globules

and their tendency to agglomerate. Vitamins, min-

erals, and enzymes are important minor components

of the fat and the membrane.

Production and Packaging of Cream

Separation and Standardization

0003 Cream is produced from whole milk by ‘separation,’

which relies on the density difference between the fat

and the aqueous serum. The fat globules will rise in

milk according to Stokes’ law:

¼ðd

ðr

 r

ÞgÞ=18,

where u

¼ velocity of globule (m s

1

); d ¼ diameter

of globule (m); r

¼ density of globule (kg m

3

); r

density of serum (kg m

3

); g ¼ acceleration due to

gravity (m s

2

); Z ¼ viscosity of serum (kg m

1

Note that u

is negative as the equation represents a

velocity of settling: r

< r

0004The rate of separation can be increased by applying

a centrifugal force field, and this provides the basis of

the milk separator:

¼ðd

ðr

 r

Þro

Þ=18 ,

where r ¼ radial distance of the globule from the axis

of rotation (m); o ¼ angular velocity (rad s

1

0005The continuous separation of the fat-rich fraction

(cream) and the serum (skim milk) is achieved

through a stack of rotating disks into which the milk

is distributed (Figure 3). Each gap between the disks

acts as a zone of separation. Separation takes place in

the gaps between the disks, where the denser aqueous

phase moves outwards at a greater velocity than the

fat globules and is channeled via the underside of the

disks to the outside and the skim milk outlet. The fat

globules concentrate toward the axis of the spinning

disks and are channeled out via the upper surface of

the disks to the cream outlet. The position of the

rising channels is important in maximizing the separ-

ation efficiency, and their position on the disks should

be in relation to the flows of the two products.

0006The rotational energy of the streams can be con-

verted to hydrostatic pressure by paring disks (centri-

petal pumps) and used to pump the products away.

Efficiency of separation is measured by the fat content

in the skim milk, usually in the form of very small fat

Rising channels

Spacers (caulks)

Feed

Skim milk discharge

Cream discharge

Centripetal pump

(paring disk)

Skim milk pump

Cream pump

Disks

Soft-stream inlet

Sediment holding

space

Sediment

ejection

ports

Liquid

Skin milk

Cream

fig0003 Figure 3 Elements of a milk separator. From Lehmann HR and Zettier K-H (1994) Separators for the Dairy Industry,Technical Scientific

Documentation No. 7, 4th revised edn. Oelde, Germany: Westfalia Separator AG, with permission.

CREAM/Types of Cream 1685