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

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Flavor Changes

0005 The flavor of food can be retained, modified, or,

occasionally, significantly changed during heat pro-

cessing, with most of the changes seen occurring in

the volatile flavor components.

Lipid Oxidation

0006 Lipids occur in almost all foodstuffs, particularly

meat and fish products, and lipid oxidation can there-

fore occur during canning of most foods. Saturated

fatty acids are relatively stable at the temperatures

used in standard canning operations. However, un-

saturated fats are degraded, under the conditions of

oxygen and heat, to a large number of volatile com-

pounds, which give rise to both desirable and undesir-

able flavors. (See Fatty Acids: Properties; Oxidation

of Food Components.)

0007 The first stage of the oxidation reaction involves

uptake of oxygen in the presence of catalysts, such

as transition metals and hemoproteins, and is initi-

ated by heat or light. Highly reactive hydroperoxides

are formed, which undergo secondary reactions

giving rise to a complex mixture of low molecular

compounds, including aldehydes, ketones, alcohols,

acids, alkanes, alkenes, and alkynes. A certain level

of volatile compounds is generally considered

necessary to give characteristic odor and flavor prop-

erties to many foods, but as many of the volatile

compounds give rise to typical rancid or stale off-

flavors, an ideal balance needs to be achieved in

the food.

Maillard Reaction

0008 In foodstuffs, reactions that occur between reducing

sugars and amino acids or proteins are known as

Maillard reactions. A very important aspect of

the Maillard reaction in food preparation is the

production of flavors and aromas. The rate of Mail-

lard reaction increases with temperature, although

pH and water content also influence the reaction.

The Maillard reaction can be split into three main

stages. The first is a condensation reaction between

the carbonyl group of a reducing carbohydrate and

the free amino group of an amino acid or protein,

followed by rearrangement of the glycosylamines.

These early Maillard reactions can lead to losses in

protein quality but do not give rise to flavors in the

food.

0009 The second stage, the advanced Maillard reactions,

involve many complex reactions and pathways.

The rearranged glycosylamines can degrade via a

multiplicity of routes, including dehydration,

elimination, cyclization, fission, and fragmentation,

which results in a pool of flavor intermediates and

flavor compounds being formed. The flavors

produced by the Maillard reaction can be classified

into four main groups, nitrogen heterocyclics and

cyclic enolenes, which give characteristic flavors

to heated foods, and monocarbonyls and poly-

carbonyls, which include more volatile supple-

mentary flavors not necessarily associated with

product characteristics.

0010The final stage of the Maillard reaction is the poly-

merization of the highly reactive compounds formed

in the second stage, which results in the formation of

brown melanoidin pigments.

0011Most Maillard reaction products contribute highly

desirable flavors to heated foods such as bread, toast,

cereal products, and meat. These flavors are often

described as baked, nutty, roasted, caramel, and

burnt, but even these can be considered to be off-

flavors in certain other foods, e.g., the burnt caramel

taste in heat processed milk.

Taints

0012Other types of off-flavor in canned foods can be caused

by contamination of the product leading to an undesir-

able taint. One particularly unpleasant taint that has

been found in a range of foods is the ‘catty taint.’ This is

caused by a heat-dependent reaction between natural

sulfur-containing compounds of the food and unsatur-

ated ketones, such as mesityl oxide, which are wide-

spread in many solvent-based products.

0013Examples of causes of catty taint in processed meat

products include: meat that has been stored in a cold

store painted with a material containing mesityl oxide

as a solvent contaminant, ox tongues that have been

hung on hooks coated with a protective oil, and pork

packed in cans where the side seam lacquer has been

dissolved in an impure solvent. Catty taint has also

been a problem in canned rice pudding, where the dye

used for printing on the rice sacks contained traces of

mesityl oxide, which was picked up by the rice and

reacted with the trace amounts of hydrogen sulfide in

the milk during processing.

Texture

0014Canning can bring about both desirable and undesir-

able changes in texture of foods, primarily through

starch gelatinization, protein denaturation, and

pectin changes.

Starch Gelatinization

0015Starch gelatinization commences at a range of tem-

peratures dependent on the type of starch, i.e., pro-

portions of amylose and amylopectin present, and the

availability of water. The two components of starch

846 CANNING/Quality Changes During Canning

behave differently on canning, with amylose giving an

opaque solution that sets to a firm gel on cooling and

with amylopectin forming a translucent paste that

remains fluid when cooled. The swelling of the starch

granules during canning or other heat processes

causes cellular disruption, which, together with the

starch gelatinization, gives a softening in texture and

increased palatability to the product. During canning

of vegetables, starch can often be leached into the

brine making it viscous or turbid, e.g., in mature

canned peas. (See Starch: Structure, Properties, and

Determination.)

Pectin Changes

0016 Canning of plant materials can lead to loss of semi-

permeability of cell membranes, and solubilization

and breakdown of pectic substances in the cell walls

and middle lamellae. The resultant cell separation

leads to a loss of crispness and a softening of the

product. This is generally a desirable effect, improv-

ing the palatability of foods, but overprocessing can

lead to excessive softening of fruits and vegetables.

Low-temperature blanching (* 70



C) of fruits and

vegetables can cause firming, due to enzymic

demethylation of pectins followed by calcium bridges

forming across the liberated carboxylate groups. (See

Pectin: Properties and Determination.)

Protein Denaturation

0017 Application of heat in meat canning causes protein

denaturation, which leads to changes in texture. The

hydrogen bonds maintaining the secondary and

higher structure of the protein rupture and form into

a predominantly random coil configuration, which

affects the solubility, elasticity, and flexibility of the

proteins. The sarcoplasmic and myofibrillar proteins

in meat coagulate during heat processing, resulting in

a firming of the texture, while the collagen proteins

become more soluble, softening as they take up water.

The resultant texture of canned meat is similar to that

of conventionally cooked products.

Struvite

0018 Canned seafood products such as tuna, salmon, and

shrimps can occasionally contain small crystalline

fragments of magnesium ammonium phosphate,

commonly known as struvite. Struvite is formed

during the canning process by the union of mineral

elements that occur naturally in the flesh of seafood.

0019 The formation of struvite is a rare event that

appears to occur sporadically. Although struvite has

no odor or taste and is harmless, it does resemble

broken glass and, therefore, is undesirable from the

consumer’s point of view.

Color

0020Color changes during canning can be brought about

by the breakdown of natural pigments, by the pro-

duction of colors due to oxidation reactions, by the

Maillard reaction, and by interactions between

product constituents, e.g., metals and polyphenolic

compounds.

Chlorophyl

0021Chlorophyl pigments are present in all green vege-

tables, but canning leads to breakdown with the as-

sociated color change from bright green to olive green

or brown. The chlorophyl is converted to pheophytin

by the loss of magnesium ions (Mg

2þ

), with heat

and low pH during processing greatly accelerating

this change. The addition of alkaline salts in the can-

ning liquor to maintain a pH of 6.2–7.0 and high-

temperature, short-time (HTST) processing have both

been used in canning to reduce chlorophyl degrad-

ation. (See Chlorophyl.)

Heme Pigments

0022The red coloration in meat is due to two heme pig-

ments, hemoglobin in the blood and myoglobin in the

muscle tissue; as most of the blood is removed after

slaughter, the main pigment is myoglobin. Canning

causes oxidation of myoglobin to metmyoglobin,

which gives the typical brown color of cooked

meats. It is this reaction that is also the main cause

of color change on canning of dark fleshed fish, e.g.,

tuna and mackerel. Green discolorations can also

occur as a result of a reaction of myoglobin with

sulfur compounds.

Carotenoids

0023Carotenoids are fat-soluble, highly unsaturated red,

orange, or yellow pigments that are susceptible to

oxidation and isomerization under the conditions of

heat and low pH, such as those used during the

canning process.

0024Carotenoids are generally found complexed with

either proteins or fatty acids, which protect them

from oxidation. It is the breakdown of these com-

plexes during processing that leads to degradation of

the carotenoids resulting in bleaching or discolor-

ation of the product.

0025In crustaceans, the denaturation of carotenopro-

tein on heating releases the carotenoid astaxanthin,

which causes a change in color from natural blue–

grey to pinky red. Two types of isomerization, cis–

trans and epoxide, can also occur, giving rise to a

slight lightening in color in canned fruits, e.g., pine-

apple. (See Carotenoids: Occurrence, Properties, and

Determination.)

CANNING/Quality Changes During Canning 847

Anthocyanins

0026 Anthocyanins are water-soluble, red–violet pigments

that can take part in a wide range of reactions during

canning. A combination of heat and oxygen can lead

to the hydrolysis of the glycosidic bonds, resulting

in loss of color and formation of yellow or brown

precipitates, but a low pH gives rise to greater color

stability. Aldehydes, produced by the breakdown of

sugars during canning, and ascorbic acid both accel-

erate the breakdown of the anthocyanins. These color

losses are a particular problem in canned red fruits,

e.g., strawberries.

0027 Anthocyanins can also be produced, as a result of

excessive thermal processing, from leucoanthocyani-

dins, giving rise to defects such as red gooseberries

and pink pears. (See Phenolic Compounds.)

Tin and Iron

0028 Both desirable and undesirable color effects can occur

in canned goods due to reactions involving metal ions.

0029 Tin coating of the internal can surface produces a

chemically reducing environment, which minimizes

oxidation and helps prevent color and flavor degrad-

ation in certain products, e.g., tomato-based products

and asparagus. Asparagus canned in a lacquered can

may develop a dark discoloration due to the forma-

tion of a complex between rutin (a flavanol glycoside)

and iron, whereas even partial exposure to tin, such

as one plain can end, will retain product color.

0030 Certain anthocyanin pigments can form metal com-

plexes with tin and iron produced through internal

can corrosion, causing pink discoloration, especially

in pears and peaches, and bluing of red fruits.

0031 Current UK legislation limits the level of tin in

canned foods to 200 mg kg

1

. Tin can cause short-

term gastric irritation and nausea, but it is not

absorbed by the gut in significant quantities, and

there is no evidence of any long-term or cumulative

toxic effects.

Maillard Reaction

0032 The Maillard reaction, the principles of which are

discussed earlier in this article, can cause off-colors,

particularly browning on canning of a wide variety of

products.

0033 Color compounds can be grouped into two general

classes, low-molecular-weight compounds and the mel-

anoidins, which have much higher molecular masses.

0034 The browning of navy beans in tomato sauce is

primarily due to the formation of melanoidins

through the Maillard reaction. Melanoidins are also

partly responsible for the browning effect seen in

canned apricots. Browning during canning of dark

flesh fish such as mackerel and tuna is of little

consequence. However, in white fish, such discolor-

ation is a major problem, and for this reason, white

fish is not routinely canned. Canning of milk can also

lead to a brown tint, although cream is less affected.

Betalains

0035Betalains are water-soluble and split into two main

groups, red betacyanins and yellow betaxanthins.

The most important pigment in this group is betanin,

the red pigment in beetroot which is often used as a

natural colorant. Betanin is susceptible to oxidation

during canning, which leads to a loss of color, al-

though this is often not noticeable due to the large

amount present.

0036The occurrence of a black ring discoloration in

canned beetroot is due to enzymatic oxidation of

polyphenols that occur during steam peeling, and

not to betalain reactions.

Changes in Nutritional Properties of

Foods

0037Both physical and chemical changes can occur during

the canning of food, which may affect its nutritional

status (Table 2). As well as damage to heat-labile

tbl0002Table 2 The effect of heat processing on the major nutritional

components

Nutrient Effect

Water Loss of total solids into canning liquor

Dilution

Dehydration

Proteins Enzymic inactivation

Loss of certain essential amino acids

Loss of digestibility

Improved digestibility

Carbohydrates Starch gelatinization and

increased digestibility

No apparent change in

content of carbohydrate

Dietary fiber Generally no loss of

physiological value

Lipids Conversion of cis fatty acids

to trans fatty acids

Loss of essential fatty acid activity

Water-soluble vitamins Large losses of vitamins C and

due to leaching and

heat degradation

Increased bioavailability of

biotin and nicotinic acid due to

enzyme inactivation

Fat-soluble vitamins Mainly heat-stable

Losses due to oxidation of lipids

Minerals Losses due to leaching

Possible increase in sodium and

calcium levels by uptake

from canning liquor

848 CANNING/Quality Changes During Canning

nutrients and physical loss of nutrients due to leach-

ing, there are many reactions that occur on canning

that affect the availability of nutrients within the

foodstuff and therefore their usefulness to the body.

0038 If comparing the nutritive value of canned foods

with that of fresh foods, it is also important to con-

sider any changes that occur during conventional

preparation and cooking techniques.

Moisture

0039 The movement of water and solids during canning

can cause major changes in the nutritional status of

the foodstuff. If the complete can contents are con-

sumed, these changes can be largely disregarded, but

in products where the canning liquor is discarded, the

effects of dilution, dehydration, and loss of total

solids must be taken into consideration. Dilution or

dehydration will affect the relative proportions of

other constituents in the food, while soluble nutrients

can be leached into the liquor.

Proteins

0040 Heating of proteins, as in canning, causes denatur-

ation, i.e., rupturing of the hydrogen bonds and other

noncovalent bonds, leading to changes in the

conformation of the protein. The degree of protein

denaturation depends on the level of heat treatment

applied, but it can also be caused by oxidation and

reaction with other food constituents, e.g., reducing

sugars and lipid oxidation products. The total level of

crude protein is generally unaffected by canning, but

both desirable and undesirable changes can occur in

its nutritive quality and availability. Mild heating of

proteins leads only to changes in tertiary structure,

which have little nutritional effect, although there is

usually a loss in solubility. More severe heating, as in

canning of vegetables, results in the Maillard reaction

and the consequent loss in protein quality. These

reactions occur mainly between lysine and sugars

and cause a loss in availability of lysine, through

cross-linking, with a loss of up to 40% being seen

on canning of potatoes. Canning of meat also leads to

the reduction in availability of lysine and other essen-

tial sulfur containing amino acids and can lead to a

reduction in the digestibility of the meat. (See Protein:

Chemistry; Functional Properties.)

0041 The losses in protein availability that occur under

normal canning conditions are, however, quite small

and not nutritionally significant for most people in

developed countries, as lysine is rarely the limiting

amino acid in the diet. Canning can, in fact, lead to

improved protein availability and digestibility by

denaturing antidigestive factors and by denaturing

proteins. Some examples of advantageous effects of

heat processing are mentioned below.

0042Heating of milk results in the proteins being

precipitated by stomach acids as finely dispersed

particles, making attack by digestive enzymes more

effective than in raw milk. This can also enhance

the formation of disulfide bonds, e.g., between b-

lactoglobulin and K-casein, which leads to greater

stability of the normally unstable b-lactoglobulin.

The canning of legumes improves their digestibility

by unfolding the major seed globulins, as well as

increasing nutritional availability by inactivation of

trypsin inhibitors.

Lipids

0043The nutritive value of the fat content of foods is not

generally significantly altered during normal heat

processing. Hydrolysis reactions can occur, resulting

in separation of the fatty acids from the glycerol unit,

but this does not adversely affect the nutritional value

of the fat as the resulting free fatty acids are available

for digestion.

0044Saturated lipids are relatively stable, but unsatur-

ated lipids are prone to oxidation when heated in the

presence of oxygen or air. The exclusion of oxygen or

use of antioxidants prevents the oxidation of lipids

during canning, so that losses in the nutritional value

of fats are unlikely to be significant. It is worth con-

sidering the effects of lipid oxidation, however, as any

contact with oxygen during the history of the food

can be sufficient for oxidation reactions to occur.

0045The major effect of lipid oxidation is related to the

flavor of food, but it can also result in the conversion

of cis fatty acids to trans fatty acids. The nutritional

value in terms of energy is similar for the two fatty

acid types, but the trans fatty acids do not generally

possess essential fatty acid activity. The availability

of the fat-soluble vitamins A, D, and E, as well as

vitamin C and folate, can also be reduced during

lipid oxidation.

Carbohydrates

0046Carbohydrates have numerous characteristic proper-

ties, and the effects of canning are therefore varied.

The levels of total and available carbohydrates have

been found to be largely unaffected during canning of

fruit and vegetables. In general, the effects of canning

on carbohydrates are related not directly to their

nutritional value but to their interaction with other

food constituents and to the overall eating quality of

the foodstuff.

0047Losses on canning can be caused by reducing

sugars reacting with protein through the Maillard

reaction, which also causes a loss in availability of

certain amino acids.

0048Gelatinization of the starch granules improves the

texture and, thereby, palatability of the food; it also

CANNING/Quality Changes During Canning 849

aids the digestibility of some foods (e.g., potatoes,

rice) that are largely indigestible in the raw state.

0049 The other major constituent of carbohydrate in

food is the indigestible dietary fiber, which consists

mainly of cellulose. Cellulose and other polysac-

charides, i.e., hemicelluloses and pectins, are largely

responsible for the texture and structure of plant

foods.

0050 Canning appears to have little effect on total diet-

ary fiber levels, but the exact effects of canning on the

various constituents of dietary fiber and the effect of

heat-induced fiber breakdown are not fully known.

(See Carbohydrates: Classification and Properties;

Dietary Fiber: Properties and Sources.)

Minerals

0051 Total mineral levels are not generally adversely

affected by the canning process as they are relatively

stable under conditions of heat, acid, or alkali

(Table 3). However, minerals are susceptible to

changes in bioavailability due to interactions with

other food components. The bioavailability of iron

can be enhanced during canning in the presence of

vitamin C, or reducing sugars with which it forms

available complexes. Both oxalates (which occur nat-

urally in many acidic plants) and phytates (from

cereals) can inhibit calcium bioavailability.

0052 The major changes that can occur in mineral levels

on canning are caused by movement between the

foodstuff and the canning liquor. Certain minerals,

especially sodium and calcium, can be taken up by the

food from the canning liquor; this can be seen espe-

cially during the canning of vegetables in brine. Min-

erals can also be leached out from the foodstuff into

the canning liquor. Potassium is particularly prone to

leaching with losses of between 15 and 50% seen on

canning of vegetables. Zinc, manganese, and cobalt

are also susceptible to leaching.

0053 No further substantial changes in sodium or cal-

cium levels are seen on storage of canned vegetables,

but slight further leaching of potassium and zinc does

occur.

Vitamins

0054Most vitamins are unstable under conditions of heat

and are susceptible to loss during the canning process

(Table 4). The fat-soluble vitamins are generally more

stable than the water soluble vitamins, but losses can

occur during canning due to oxidation. Carotenoids

are particularly prone to oxidation during heat pro-

cessing, but this can be greatly reduced by the add-

ition of antioxidants, e.g., vitamin C.

0055Losses of water soluble vitamins during canning

can be quite considerable, with vitamin C being the

most labile. Vitamin C can be lost through the

following processes: (1) oxidation, which can occur

in the early stages of heat treatment before the ascor-

bic oxidase is inactivated; (2) chemical degradation,

e.g., losses due to nonenzymatic browning reactions

in fruit products with high vitamin C levels; or (3)

through leaching into the canning liquor, which is

generally the major cause of vitamin C loss in canned

fruits and vegetables. The levels of vitamin C

remaining in canned vegetables have been shown to

be as low as 20% of the vitamin C found in the fresh

raw produce; however, vitamin C is lost during all

stages of fresh storage, preparation, cooking, or pro-

cessing, and some canned vegetables have levels of

vitamin C similar to the fresh product, which has

been stored prior to cooking (Table 5). Thiamin (vita-

min B

) is the most heat-sensitive of the B vitamins,

especially under alkaline conditions and in the pres-

ence of bisulfite ions. Thermal degradation of thiamin

involves the cleavage of its methylene bridge, which

gives rise to many volatile products, producing a

meat-like aroma in cooked foods. In the presence of

reducing sugars, thiamine may take part in nonenzy-

matic browning reactions, and it also reacts with

aldehydes in the presence of ascorbic acid. Thiamin

can also be lost through leaching, but it is less soluble

than vitamin C, and retention levels of between 60

and 90% are usual on canning.

0056Folic acid is lost on canning through heat degrad-

ation and oxidation, although it is stabilized in

the presence of ascorbic acid, whereas pyridoxine

tbl0003 Table 3 Mineral content in freshly cooked and canned cooked

peas on a wet-weight basis

Mineral (mg per100 g)

Sample Ca Na K Zn Fe

Fresh 48 65 179 0.82 1.4

Zero time canned 47 320 152 1.0 1.4

Canned stored

3 months 40 315 79 0.72 1.3

6 months 31 82 0.44 0.9

9 months 28 295 84 0.53 1.5

12 months 280 108 0.55 1.2

tbl0004Table 4 Vitamin losses on canning of vegetables

Vitamin Typicallosses (%)

Vitamin C 30–90

Thiamin 16–83

Riboflavin 20–70

Nicotinic acid 20–70

Folic acid 30–80

Panthothenic acid 30–85

Vitamin B

30–90

Biotin 0–80

Vitamin A 0–80

850 CANNING/Quality Changes During Canning

(vitamin B

) can be lost through heat degradation and

leaching. Losses of these two vitamins during canning

of fruits and vegetables range between 30 and 80%.

Losses on canning of meat can be very considerable,

with up to 90% losses.

0057 Riboflavin (vitamin B

) and nicotinic acid are both

relatively heat-stable, with minimal losses on canning

of meat products; however, losses of between 20 and

50% have been found on storage of canned meats.

Losses seen during the canning of fruits and vege-

tables, ranging from 25 to 70%, are mainly attributed

to leaching. Nicotinic acid and riboflavin both show

very high retention levels during milk processing,

but riboflavin is lost from bottled milk as it is very

sensitive to sunlight.

0058 Heat processing generally has a detrimental effect

on most vitamin levels, but mild heating conditions

can have a beneficial effect due to enzyme inactiva-

tion and the breakdown of binding agents, which

increases the bioavailability of certain vitamins, e.g.,

biotin and nicotinic acid.

Conclusions

0059 The canning of foods has resulted in a wider choice of

nutritious, good-quality foods being available all year

round in a convenient form for the consumer. Al-

though, with better global links and improved agron-

omy, many foods are now available fresh all year

round, canned foods still form an important part of

the food marketplace.

0060 When considering the quality of canned foods, it is

important to compare them with fresh or frozen

foods at the point of consumption. Many of the

changes that occur in both sensory and nutritional

aspects do so during any thermal process, whether

it is conventional cooking, blanching, or canning.

For most foods, the canning process replaces a

conventional cooking process, and any mild reheating

stage has no further significant effect on quality.

0061Losses in heat-labile nutrients such as vitamins can

be significant. However, as canned products are usu-

ally produced from materials at optimum maturity

and immediately after harvest, levels can often be as

high as the ‘fresh’ material purchased from the green-

grocers and prepared in the home.

See also: Dietary Fiber: Properties and Sources; Fatty

Acids: Properties; Oxidation of Food Components;

Pectin: Properties and Determination; Phenolic

Compounds; Starch: Structure, Properties, and

Determination

Further Reading

Bender AE (1978) Food Processing and Nutrition. London:

Academic Press.

Hall MN, Edwards MC, Murphy MC and Pither RJ (1989)

Technical Memorandum No. 553. CFDRA. A Compari-

son of the Composition of Canned, Frozen and Fresh

Garden Peas as Consumed. Chipping Campden:

CCFRA.

Harris RS and Kamas E (eds) (1988) Nutritional Evaluation

of Food Processing, 3rd edn. West port, CT: AVI.

Lund D (1982) Effect of processing on nutrient content and

nutritional value of food. In: Heat Processing.

Moyem T and Kvale O (eds) (1977) Physical, Chemical and

Biological Changes in Food Caused by Thermal Process-

ing. Essex: Applied Science Publishers Ltd.

Priestley RJ (ed.) (1979) Effects of Heating on Foodstuffs.

Essex: Applied Science Publishers Ltd.

Rees JAG and Bettison J (eds) (1991) Processing and Pack-

aging of Heat Preserved Foods. Glasgow, UK: Blackie.

Richardson T and Finley JW (eds) (1985) Chemical

Changes in Food During Processing. AVI: Westpart, CT.

Canola See Rape Seed Oil/Canola

tbl0005 Table 5 Vitamin C content of fresh and canned peas and carrots (mg per 100 g)

Carrots Garden peas

Pea canning liquorUncooked Cooked Uncooked Cooked

Fresh 9 8 30 16

Stored for 7 days at ambient 3 4 17 16

Canned 2 3 20 12 11

Canned, stored for 6 months 2 2 15 2 10

Canned, stored for 12 months 2 3 8 5 5

CANNING/Quality Changes During Canning 851

CARAMEL

Contents

Properties and Analysis

Methods of Manufacture

Properties and Analysis

P Tomasik, University of Zimbabwe, Harare,

Zimbabwe

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 Home cooks used to burn sugar to obtain so-called

caramel for flavoring food. Commercial caramel

differs from the domestic product by its method of

preparation and consequently in many of its proper-

ties. Several products called caramel are available

commercially. Tables presented in this article sum-

marize their physical and chemical properties and

analytical problems are also discussed.

Definition of the Product

0002 Caramel is a brown product originating from various

sugars when they are heated either dry or in concen-

trated solutions and alone or with certain additives. It

is designed as a food additive or ingredient for

coloring and/or flavoring food.

0003 In some countries the name ‘caramel’ is reserved

for products manufactured from saccharides in the

absence of nitrogen-containing compounds. Such

products are used as flavoring ingredients. The prod-

ucts resulting from heating sugars with nitrogen-

containing additives are called ‘sugar colors’ and

they serve as coloring additives. (See Colorants (Col-

ourants): Properties and Determination of Natural

Pigments; Flavor (Flavour) Compounds: Structures

and Characteristics.)

Types and Standardization

0004 The origin and properties of caramel are laid down in

many producing countries. Also, the use of only a few,

among many reported additives, is permitted in the

manufacture of caramels. The World Health Organ-

ization and the Food and Agriculture Organization

Joint Expert Committee on Food Additives distin-

guishes three general kinds of caramel: (1) caramel

color, plain (CP), (2) caramel color, ammonia process

or caustic (AC), and (3) caramel color, ammonium

sulfite process (SAC), depending on the additive used

in their manufacture. Both the European Technical

Caramel Association (EUTECA) and the Inter-

national Technical Caramel Association (ITCA)

have standardized the properties of four classes and

10 types of caramel (Table 1).

00054(5)-Methylimidazole in ammonia and ammonium

sulfite caramel colors cause a special problem. This

compound, which is formed from sugars and ammo-

nia during manufacture of the caramel, is strongly

neurotoxic. For this reason the daily intake of food

containing ammonia caramel is controlled in some

countries and in others ammonia caramel is banned.

Physical Properties

0006Caramel is polymeric in its character. Three main

components of the plain type are called caramelan,

caramelen, and caramelin. Ammonia caramels add-

itionally contain melanoidin, which is much darker in

color than the three other components mentioned

above. Hence, ammonia caramels are the most inten-

sively colored.

0007Depending on their isoelectric points (pIs) caramels

may be roughly divided into positive (pI 5.0–7.0),

negative (pI 4.0–6.0) and spirit (pI < 3.0) types. The

pI determines the possibility of application of cara-

mels. The average molecular weight of compounds in

caramels is between 5 kDa (electropositive caramels)

and 10 kDa (electronegative caramels). Other import-

ant properties are pH (Table 1), aqueous solubility

(should be completely soluble), specific gravity

(usually 1.315–1.345), color intensity (Table 1)or

tinctorial strength, hue index (called ‘redness’)as

well as flavor and aroma. These last two organoleptic

properties consist of two components: taste arising

from the acidity (modifiable) and a taste contribution

attributed to the nature of caramel (nonmodifiable).

Chemical Nature

0008The products of caramelization are distributed

among volatile and nonvolatile fractions. The volatile

fraction consists mainly of water, carbon mono-

xide, carbon dioxide, formaldehyde, acetaldehyde,

852 CARAMEL/Properties and Analysis

tbl0001 Table 1 Classes of caramel color according to International Technical Caramel Association/European Technical Caramel Association

Class Sort Type Color intensity

at 610 nm)

EBC units (10

) Dryresidue (%) 4 -Methylimidazole

(mgkg

1

)

Total nitrogen (%) Total sulfur (%) Ammonia

nitrogen (%)

Sulfur dioxide (%)

I Caramel color, plain CP-1 5–35 2–12 55–75 <25 <0.1 <0.1 <0.01 <0.005 (<0.1)

CP-2 40–80 15–25

II Caramel color, caustic

sulfite

CCS-1 40–80 15–25 62–82 <25 <0.1 0.15–2.5 <0.01 <0.15

III Caramel color, ammonia AC-1 60–90 16–24

;

55–75

<200 0.5–3.0

;

(<0.7) <0.5 <0.015 (<0.02)

AC-2 100–140 27–37.5 0.5–5.0 <0.3

AC-3 150–200 40–54 1.5–6.5

IV Caramel color,

ammonium sulfite

SAC-1 35–70 8–16

;

55–75

0.1–1.3 0.3–2.0

;

0.3

;

0.08

;

SAC-2 75–100 17.5–23 0.5–2.8 0.8–3.2 (<0.7) 0.5 (<0.5) 0.12 (<0.1)

SAC-3 105–150 22.5–37 0.8–2.8 1.0–4.0 0.8 0.15

SAC-4 210–270 40–52 47–57 2.0–4.0 1.0–5.0 1.5 0.28

The values in parentheses are Food and Agriculture Organization/World Health Organization (FAO/WHO) proposals dating from 1980. Permissible content (in mg kg

1

) of the heavy metals in caramel according to

FAO/WHO standards are: total, 25; copper, 20; lead, 5; astatine, 3; mercury, 0.1. e

, Molar extinction coefficient; EBC, European Brewery Convention.

methanol, and ethanol. The composition of the low-

molecular-weight portion of the nonvolatile fraction

depends on the type of caramel. The plain caramel

nonvolatile fraction contains the following sacch-

arides: d-fructose, d-glucose, kojibiose, isomaltose,

nigerose, sophorose, laminarabiose, maltose, gento-

biose, cellobiose, isomaltotriose, panose, and other

oligosaccharides. There are also several carboxylic

acids: formic acid and higher fatty acids, succinic,

fumaric, pyruvic, levulinic, and furancarboxylic

acids. Oxaheterocycles are represented by 2-furalde-

hyde, 5-(hydroxymethyl)-2-furaldehyde, 2,3-dihy-

dro-4-hydroxy-5-methylfuran-3-one, 2,3-dihydro-

4-hydroxy-2-(hydroxymethyl)-5-methylfuran-3-one,

bis(5,5-formylfurfuryl) ether, and 2,3-dihydro-3,5-

dihydroxy-6-methyl-4H-pyran-4-one. There are also

various other aliphatic carbonyl compounds like glu-

coreductone (2-hydroxypropandial), pyruvaldehyde,

and a,b-unsaturated carbonyl compounds. A number

of other components which are products of reversion

and polymerization remain uncharacterized. The

low-molecular-weight fraction of caustic caramel

Sugar + amine

1-amino-1-deoxy-2-hexulose

(1,2-enol form)

5-(hydroxymethyl)-

2-furaldehyde or

2-furaldehyde

schiff base

5-(hydroxymethyl)

2-furaldehyde or

2-furaldehyde

Aldimine

Aldose

Glycosylamine

Rearrangement

Melanoidin

Aldimine

(ketimine)

Sugar

condensation

products

Strecker

degradation

Dehydroreductone

Nitrogenless

aldol or

a polymer

+ Aldehyde

Nitrogen-containing polymers and copopolymers

Reductone

Amadori

rearrangement

−3H

O −2H

α-amino acid

−H

−2H

−Amine

−nH

+Amine

Ketos-1-ylamine

(a)

(b)

Products of condensation

of amines with

products of degradation

Polymerization

Melanoidins

Products of degradation

Diketos-1-ylamine

diamino sugars

+Amine +Amine

+2H

Glycosylamine

Amino acid

fig0001 Figure 1 Theoretical schemes for the formation of melanoidins according to (a) Hodge and (b) Reynolds. Reproduced from Caramel/

Properties and Analysis, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

854 CARAMEL/Properties and Analysis

contains about 50 components. Ammonia caramel

contains in this fraction a certain amount of 4(5)-

methylimidazole, 2-acetyl-4(5)-(1,2,3,4-tetrahydrox-

ybutyl)imidazole, and 3-hydroxy-6-methylpyridine,

as well as at least 143 other components, including

those mentioned above. (See Carbohydrates: Classifi-

cation and Properties; Fatty Acids: Properties; Fruc-

tose; Sucrose: Properties and Determination.)

0009 The high-molecular-weight, nonvolatile fraction of

plain caramels is composed of 1,6-anhydro-a-d-

glucose, caramelan (formed in the reaction shown in

eqn (1)), caramelen (resulting from the reaction

shown in eqn (2)), caramelin (being the product of

reaction shown in eqn (3)), structures of which

remain unknown, and polymers composed of furan

ring units.

!

12H

ð1Þ

!

18H

ð2Þ

!

27H

ð3Þ

0010 The interpretation of carbon-13 cross-polarized

magic-angle sample spinning nuclear magnetic reson-

ance (

C CP/MAS NMR) spectra of plain caramels

leads to the conclusion that there are 8–9% carbonyl

and aldehyde carbon atoms, 7–7.7% carbon atoms in

ester groups, 33–31% heterocyclic carbon atoms, and

52.4–51.2% alkyl groups bonded to carbon atom

and to oxygen. It may also be deduced that the

furan system prevails among other heterocyclic

systems present in caramel. (See Spectroscopy: Nu-

clear Magnetic Resonance.)

0011 Two consecutive steps may be distinguished in

the formation of caramels without participation of

ammonia, ammonium sulfite, amines, or amino

acids and their salts, which may also be used as

the catalysts. They are degradation reactions which

lead to colorless or yellow compounds. First, water

and carbon dioxide are evolved and furan deriva-

tives are formed (2-furaldehyde from furanoses and

5-(hydroxymethyl)-2-furaldehyde from pyranoses).

This step is a function of both temperature and

time; atmospheric oxygen plays some role in the

final stage. This is followed by the second step, poly-

merization and condensation reactions forming

highly colored compounds (caramelan, caramelen,

and caramelin as well as furan polymers).

0012 In caramels manufactured with basic nitrogen-

containing additives, melanoidins are the most im-

portant components. Their structure is unknown,

but there is evidence that they have, at least in part,

an amide structure. Moreover, they contain unpaired

electrons, i.e., they have a free radical character.

There is a great similarity between this product and

humic acids resulting from the Maillard reaction.

Two consistent schemes for the formation of mela-

noidins are proposed by Hodge (Figure 1(a) and Rey-

nolds (Figure 1(b)). Both theories differ in the number

of steps involved in the route from the preliminary

reaction between sugar and amine to give glycosyla-

mine and the final product, melanoidin. Recent stud-

ies have revealed that the possibility of the Strecker

degradation as one of the processes involved should

be rejected. (See Browning: Nonenzymatic.)

Analysis of Caramels

0013The use of caramel is first of all limited by its type,

which affects its use for a given food. For instance, a

caramel may precipitate on contact with a product

subjected to coloration, as a result of discharge of

caramel micelles. The type and quality of caramel

may easily be recognized by means of the citric acid,

alcohol, and Lassaigne tests (Table 2). However, the

complexity of caramel composition makes distin-

guishing between the different types difficult unless

combined analytical methods are applied. These

include chromatography of dilute solutions of cara-

mel followed by thin-layer electrophoresis and size

exclusion chromatography. Thus, full standardiza-

tion of caramel may require many other determin-

ations of chemical components as well as fractions,

as shown in Table 3. This table presents several prop-

erties of certain types of caramel manufactured from

sucrose. It is apparent that, for instance, two I CP-2

tbl0002Table 2 Tests for particular types of caramel

Typeof caramel

Test

Citric acid

b,c

Alcohol

b,c

Lassaigne

Spirit caramel

IA þ

IB 

IC þ

Nonammonia caramel

II þ

Beer caramel

III A  + þ

III B þ

III C þþ

Soft-drink caramel

IV A þþ

IV B þþ

Consult also Tables 1 and 3 for classification of types.

Formation of a precipitate.

Solubility in citric acid–monosodium phosphate at pH 2.5.

Solubility in 13:7 (v/v) ethanol–water.

CARAMEL/Properties and Analysis 855