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

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Components; Dextrins; Enzymes: Uses in Food

Processing; Fructose; Honey; Sorghum; Starch:

Structure, Properties, and Determination; Sucrose:

Properties and Determination; Sugar: Sugarcane;

Refining of Sugarbeet and Sugarcane; Sweeteners:

Intensive; Water Activity: Effect on Food Stability

Further Reading

Clarke MA and Godshall MA (eds) (1988) Chemistry and

Processing of Sugarbeet and Sugarcane. Amsterdam:

Elsevier Science.

Marie S and Piggott JR (eds) (1991) Handbook of Sweeten-

ers. London: Blackie.

Pancoast HM and Junk WR (eds) (1980) Handbook of

Sugars, 2nd edn. Westport, Connecticut: AVI.

SYRUPS 5717

TAINTS

Contents

Types and Causes

Analysis and Identification

Types and Causes

B D Baigrie, The Lord Zuckerman Research Centre,

University of Reading, Whiteknights, UK

Background

0001 British Standard 5098, 1974 defines a taint as a taste

or odor foreign to the product, whereas the dictionary

definition of an off-flavor is a flavor that is not nat-

ural or up to standard owing to deterioration or

contamination. Often, the term taint is used specific-

ally to describe a flavor defect that has arisen from

extrinsic sources. Where a flavor defect has developed

internally, e.g., via enzymically induced deterioration

and not by external contamination, the term off-

flavor is used. In practical terms, this differentiation

is not particularly helpful or necessary, and hereafter,

the term ’taint’ will be used to describe any deviation

of flavor from the accepted norm, irrespective of the

cause.

0002 The sources of taints in foodstuffs are manifold.

They can arise through microbial spoilage (bacteria,

yeasts, and molds), and many of the metabolites pro-

duced by these species will often remain in the food,

even after extensive processing. They may also be

formed by internal deterioration of food components

through the intermediacy of enzymes, by other non-

enzymic pathways (Maillard reactions, Strecker deg-

radation, etc.), or by oxidative degradation. In

addition, many food taints or precursors thereof

arise from some source of external contamination.

For example, airborne sources of taint are common.

Water used in food processing, cleansing/sanitizing

agents, and packaging materials are major vectors

also for the transmission of taints to foods. This

article discusses some of the more commonly occur-

ring taints and their origins.

Microbial Metabolites

0003Microorganisms are a common source of taint in

foods. Microbial activity can produce taints in several

ways: by production of primary metabolites; by con-

version of innocuous food components into species

with a low odor threshold, or through residual

enzyme activity after death of the organism. Fish is a

food that is susceptible to tainting by all of these

mechanisms. Fresh fish has little odor, but on storage,

spoilage odors develop, primarily through bacterial

action. Achromobacter, Vibrio,andPseudomonas

bacteria have been shown to be capable of producing

sulfidic taint in fish by the production of volatiles

such as hydrogen sulfide, methyl mercaptan, dimethyl

sulfide, dimethyl disulfide, and dimethyl trisulfide.

A metabolite of Pseudomonas perolens, 2-methoxy-

3-isopropylpyrazine, was found to cause a potato-like

taint in fish, whereas a fruity taint in fish muscle

caused by ethyl esters of acetic, butyric, and hexanoic

acids was produced by Pseudomonas fragi. The clas-

sic ’fishy’ odor of aged fish has been attributed to the

presence of trimethylamine, and it has been estab-

lished that this volatile is produced by the action of

bacterial enzymes on trimethylamine oxide, a natural

constituent of fish muscle.

0004Metabolites from microbial species, including

Pseudomonas, have also been implicated as the

source of specific taints exhibited by various crust-

acea. Dimethyl trisulfide was responsible for an

onion-like flavor, and indole a fecal-like flavor that

had developed in red prawns. The compounds re-

sponsible for a distinctive garlic-like taint also in

red prawns, namely bis-(methylthio)-methane and

trimethylarsine, were believed to arise from microbial

activity. The former chemical was also the principal

cause of an objectionable taint in sand lobster. 2,6-

Dibromophenol was identified as the cause of an

iodoform-like taint in prawns. Marine algae and

bryozoa were found to contain this compound in

concentrations comparable with that found in the

prawns and were implicated as the primary source

of the taint.

0005 Freshwater crustacea (peneid shrimp) and fish

(rainbow trout, catfish, bream) can often acquire

earthy/musty taint via contaminated water. The

compounds held to be primarily responsible for

musty taint from these sources are geosmin ([E]-

1, 10-dimethyl-[E]-9-decalol), 2-methylisoborneol

(1,2,7,7-tetramethyl-exo-bicyclo [2.2.1] heptan-2-ol)

and, to a lesser extent, 2-methoxy-3-isopropylpyra-

zine. In most cases, geosmin and methylisoborneol

are found in eutrophic and hypertrophic waters, and

it has been established unequivocally that the causal

agents of these chemicals are actinomycetes (several

species), cyanobacteria (blue–green algae), or certain

species of fungi. The presence of geosmin or 2-

methylisoborneol in water supplies, particularly that

used for food processing, is the probable cause of

earthy taints in some foods canned in brine (e.g.,

champignons) and syrups.

0006 Food production systems that utilize microorgan-

isms to produce flavor, e.g., dairy products, alcoholic

beverages, and cocoa, are particularly susceptible to

the development of taints via excessive microbial ac-

tivity. A selection of some taints developed by these

types of foods is shown in Table 1. Of course, many

other foods can develop fermentation-like taints

through adventitious infection by the appropriate

microorganisms.

Enzymic Deterioration of Food

0007Foods containing lipids are particularly prone to the

development of taints by enzyme-catalyzed degrad-

ation of lipid. Two main types of enzymes are in-

volved in lipid degradation: lipoxygenase and lipase.

The former are particularly common in plant tissues

such as the legumes. For example, the beany flavor of

soya beans and the reversion flavor of soya bean oil

are due to lipoxygenase activity.

0008Lipoxygenase catalyzes fatty acid hydroperoxide

formation in a manner similar to nonenzymic systems.

Polyunsaturated fatty acids such as linoleic and lino-

lenic acid are more susceptible to attack than mono-

unsaturated or unsaturated acids. After initial

formation of the hydroperoxides, decomposition by

pathways similar to those occurring in autoxidation

takes place. This is believed to involve homolytic

cleavage of the oxygen–oxygen bond to yield an

alkoxy radical that decomposes via carbon–carbon

cleavage to give aldehyde or hydrocarbon products.

Other pathways lead to the formation of hydrocar-

bons, alcohols, aldehydes, ketones, esters, and acids.

There is a wide variation between these oxidation

products with respect to flavor threshold values, con-

centration in the food, and flavor character. However,

the D-value parameter, a measure of the relative in-

tensity of flavor compounds extracted from a food,

reveals that unsaturated aldehydes and ketones,

namely (Z)-3-hexenal, 1-octen-3-one, (Z)-1,5-octa-

dien-3-one, (Z)-2-nonenal, (E, Z)-2,6-nonadienal,

tbl0001 Table 1 Microbiologically produced taints in fermented foods

Food Microorganism Tainting species

Butter Bacteria (Streptococcus) Acetaldehyde

Cheddar cheese Bacteria (Nonlactic) Ethyl butanoate and ethyl hexanoate

Cheddar cheese Yeast (Candida) Ethyl acetate and ethyl butanoate

Munster cheese Bacteria (Pseudomonas) 2-Methoxy-3-isopropyl-pyrazine

Yogurt Fungi 1,3-Pentadiene

Wine Yeast (Brettanomyces) 2-Acetyltetrahydro-pyridine

Bacteria (Lactobacillus)

Wine Yeast (Schizosaccharomyces) Organic sulfur compounds, e.g., 2-mercaptoethanol

Wine Bacteria (Pediococcus) Diacetyl

Cider Bacteria (Lactobacillus) Indole

Malt distillate Bacteria (Actinomycetes) Geosmin

Rum Yeast (Saccharomyces) 3-Ethoxypropanal and 1,1,3-triethoxypropane

Beer Yeast (Saccharomyces) Diacetyl and 2,3-pentanedione

Beer Yeast (Saccharomyces) Isoeugenol and other phenols

Beer Yeast (Saccharomyces) Methional

Beer Bacteria (Clostridium) Butanoic acid

2-Methoxy-3-isopropylpyrazine and methylisoborneol

Coffee Molds (Aspergillus) 2,4,6-Trichloroanisole

5720 TAINTS/Types and Causes

and 3-methyl-2,4-nonandione, are the most potent

taints. Flavor descriptors for lipid oxidation volatiles

range from rancid, fishy, painty, cardboard, and

metallic to mushroom, green, and fruity.

0009 Lipase enzymes can produce taint by hydrolysis of

triglycerides and release of fatty acids. The longer-

chain fatty acids have higher flavor thresholds, and

so lipolytic taints are more common in products con-

taining short-chain fatty acids. Dairy products such as

butter, cheese, and milk powder regularly suffer from

taints described variously as rancid, butyric, goaty,

and unclean, which are caused by excessive concen-

trations of free fatty acids of chain lengths C

–C

Products containing coconut or hydrogenated palm

kernel oil (HPKO) are prone to soapy and bitter taints

caused by hydrolytic release of lauric (C

) and myr-

istic (C

) acids from the triglycerides of coconut oil

or HPKO.

Nonenzymic Deterioration of Food

Autoxidation

0010 Autoxidation of lipid is initiated by the formation of

lipid radicals and, subsequently, lipid peroxy radicals

and lipid hydroperoxides. Volatile taints are gener-

ated from the lipid hydroperoxides via the secondary

reactions described in the previous section. High-

energy (short-wavelength) light is an effective initi-

ator of lipid autoxidation. However, many other

factors also influence the rate of oxidation: the

number and type of double bonds; the presence of

trace metals such as copper, cobalt, and iron; water

activity (low-moisture foods oxidize at higher rates),

temperature, and the presence of antioxidants. While

dry foods such as milk powder, crisps, and breakfast

cereals are particularly susceptible, fish and meat

products are also prone to lipid autoxidation. (See

Fatty Acids: Properties.)

Nonenzymic Browning

0011 The importance of nonenzymic browning, also

known as the Maillard reaction, in the development

of desirable food flavors is well established. However,

it is also recognized that the Maillard reaction is a

major route for the development of taints in food.

0012 The formation of both desirable flavors and taints

via the Maillard reaction starts with a condensation

reaction between a carbonyl group, usually from

reducing sugars, and an amino group, most com-

monly from peptides or proteins. A complex sequence

of chemical rearrangement, fragmentation, cycliza-

tion, and polymerization reactions occurs, which gen-

erates the flavor volatiles and the characteristic

brown coloration. The generation of specific flavor

chemicals depends on factors such as time, tempera-

ture, water activity, pH, and sources of carbonyl and

amino groups.

0013Any food that undergoes substantial heat treatment

during processing can develop a Maillard reaction-

type taint. Maillard chemistry can also proceed sig-

nificantly at room temperature and often causes taint

in low-moisture foods that have been stored for long

periods. For example, staleness in stored dry milk

powder was thought to be caused by 2-aminoaceto-

phenone, a Maillard degradation product of trypto-

phan. The same chemical, together with some

furanoid species, was thought to contribute to the

gluey flavor of aged casein. The concentration of

furfural, a classic Maillard reaction product, in

canned citrus juices was found to increase during

storage and could be correlated with the development

of taint in these products. Similarly, heat-treated

orange juice and blackcurrants were found to contain

flavor compounds thought to be derived by Maillard

reactions. (See Browning: Nonenzymatic.)

Direct External Contamination

Airborne

0014Airborne contamination is one of the most frequent

causes of direct tainting of food, and many such cases

have been documented. Growing crops have been

contaminated by airborne industrial discharges. For

example, grapes inadvertently contaminated in this

way produced wine exhibiting a taint. Exposure of

food during processing and packaging operations to

contaminated air either from sources external to the

factory or from localized sources within a factory has

resulted in many incidents of taint. Baked goods and

chocolate confectionery, products that are often

cooled or set using chilled air, are particularly suscep-

tible to tainting by this route.

0015In one example, chocolate confectionery was found

to be contaminated with a wide range of alkyl

phenols. The source of the phenols was traced to

an area of flooring some distance from the cooling

confectionery. The flooring was found to contain a

phenolic-based bonding resin that was slowly releas-

ing alkyl phenols to the atmosphere. These chemicals

were subsequently adsorbed by the confectionery. Air

diffusion within warehouses or transit containers

contaminated by other commodities stored either

contemporaneously or at some stage in the past

often results in tainting. A good illustration of this

source of taint is afforded by the case of white

beans that acquired a disinfectant-like taint after

being transported in containers. The primary taint

was identified as 6-chloro-2-methylphenol. This

TAINTS/Types and Causes 5721

compound was an impurity in a herbicide, which,

during a previous voyage, had contaminated the

wooden floor of the container and subsequently

transferred to the beans. Examples of taints arising

by these routes are outlined in Table 2.

Water

0016 Water is used extensively in the production or pro-

cessing of many foods and can be a major source of

taint. Waterborne, earthy/musty taints have been dis-

cussed in the section on microbial taints. However,

another common type of waterborne taint is the

disinfectant-like taint caused by phenolic compounds,

particularly chlorophenols. Mono-, di-, and trichlor-

ophenols can be formed by spontaneous reaction of

phenol with free chlorine present in chlorinated

mains water. Plastic fittings and hoses or phenol-

based resins and paints used as protective coatings

on processing plant often function as ready sources

of phenol.

0017 Boiler treatment chemicals have been documented

as the source of free phenol that caused chlorophenol-

like taints when the boiler water was used for direct

steam injection of brines and syrups. Another docu-

mented example of chlorophenol contamination of

food by water involved the accidental introduction

of algaecide-treated water into the processing water

supply. The algaecide was found to contain tri-,

tetra-, and pentachlorophenol. Disinfectant solutions

used as plant-sanitizing agents have been implicated

in many cases as sources of chlorophenolic taints.

Several documented cases of tainting by chlorophe-

nols are listed in Table 2.

Packaging Materials

0018One of the primary functions of food packaging is to

prevent the ingress of contaminants and taints into

the product. Unfortunately, the reverse often occurs

with packaging materials frequently functioning as

abundant sources of tainting chemicals. This is due

in part to the ever-expanding range of materials used

to package food and the increasing sophistication of

packaging construction, e.g., laminates, metallized

films, etc. The most common examples of direct con-

tamination from packaging involve solvent residues

from printing inks, lacquers, resins, and adhesives.

0019Synthetic polymers frequently used in packaging

such as polythene, polystyrene, polypropylene, poly-

vinyl chloride, and poly(ethyleneterephthalate) often

impart chemical or plastic-like odors to foods. Re-

sidual levels of monomer are often the cause of the

taint. Styrene-based polymers, for example, are

known to transfer styrene monomer, which has a

relatively low odor threshold and imparts an objec-

tionable taste to foods. Other polymeric packaging

materials have been found to contain a wide range of

volatile organic compounds, many of which could

cause taint.

tbl0002 Table 2 Taints from external sources

Food contaminated Compoundsinvolved Origin

Biscuits Chlorophenols Airborne from nearby herbicide factory

Chocolate confectionery Xylenols Airborne from adjacent area of composite flooring

Chocolate crumb 2,4- and 2,6-dichlorophenol Airborne from walls of metal container

White beans 6-Chloro-2-methyl-phenol Airborne from wooden floor of metal container

Bakery and confectionery products Petroleum hydrocarbons Airborne from petrol spillages in warehouse

Canned vegetables Tri-, tetra-, and pentachlorophenol Algaecide in process water

Canned fruit Di- and trichlorophenols In-plant reaction of steam contaminated with phenol and

chlorinated water

Fruit juice 2,4- and 2,6-dichlorophenol Phenol from in-plant fitments reacting with chlorinated

water

Distilled spirit Di- and trichlorophenol Phenol in spirit chlorinated by sanitizing agent

Jam filling 6-Chloro-2-methylphenol Tanker with residual traces of disinfectant

Channel catfish 2-Methylisoborneol Pondwater contaminated with phytoplankton

Water clams Geosmin Marshwater

Cocoa powder Chlorophenols and chloroanisoles Multiwalled paper sacks with glued seams

Milled flour Various Jute sacks

Chocolate assortments Aliphatic hydrocarbons Lithographic printed cartonboard

Icecream 2-Butoxyethanol UV-cured printed cartonboard

Cereal Terpenes Resin in laminated liner

Sugar confectionery Aliphatic aldehydes Printed cellophane bag

Soft drink 2-Methylphenol Can lacquer

Milk beverage Isophorone Can lacquer

Biscuits Styrene Polystyrene trays

Spring water 2-Ethylhexanol and hexanal Liner of bottle closure

5722 TAINTS/Types and Causes

0020 Another major route for contamination of food

products with chlorophenols, apart from water as

discussed previously, is via packaging materials and

materials encountered during transport of foods.

Cork closures, Kraft bleached paper sacks, cardboard

boxes, jute sacks, fiberboard containers, and wooden

pallets have all been shown to contain chlorophenols,

which, in many cases, were the primary tainting

species and, in others, precursors of the eventual

tainting species.

0021 Paper-based packaging materials, particularly

those made from recycled paper and pulp, have re-

cently been found to contain 2,4,6-tribromophenol in

high concentrations. Given that it is a powerful taint

with an odor threshold in water of 0.6 mgl

1

, this

compound will also impart a medicinal taint if trans-

ferred to foods.

002 2 Tables 2 and 3 show that nearly all food packaging

materials, except perhaps glass, can cause tainting of

food products. Invariably, the tainting chemicals will

be present in packaging materials at substantially

higher levels than are found in the food. Therefore,

monitoring of packaging for tainting chemicals by

sensory evaluation or instrumental analytical methods

may prevent any potential problems.

Indirect External Contamination

Polyhaloanisoles

0023 The polyhaloanisole family contains some of the most

potent odor compounds known and are notorious as

the source of musty or moldy taints in foods and

beverages. Unlike many of the other tainting chem-

icals discussed previously, the presence of polyhalo-

anisoles in food or food-associated materials is

completely adventitious. As chemicals, they perform

no known useful function, and their presence in food

systems invariably occurs indirectly, since their gen-

esis requires both a microbiological vector (to effect

the methylation stage) and a source of halophenols.

Timber-based packaging materials have been impli-

cated in many of the documented cases of haloanisole

taint. The first examples of a chloroanisole taint in

food occurred in eggs and broiler chickens. It was

shown that both tetra- and pentachloroanisole had

been formed by microbial methylation of the corres-

ponding chlorophenols present in wood shavings

used as poultry litter.

0024A more recent occurrence has been the emergence

of polybromoanisoles, and particularly 2,4,6-tribro-

moanisole, as powerful taints rivalling the polychlor-

oanisoles in terms of potency. As with the

chloroanisoles, timber-based packaging has been

shown to be the primary source of the bromoanisole

precursor, i.e., the corresponding bromophenol.

Thus, the biocidal agent 2,4,6-tribromophenol can

be readily converted by microbial methylation to

2,4,6-tribromoanisole.

0025As the sensitivity and selectivity of analytical tech-

niques for the detection of polyhaloanisoles in food

have improved, so the number of documented cases

of taint by these chemicals has increased. Some of the

more recent examples of foods affected by haloani-

sole taint are listed in Table 3.

tbl0003 Table 3 Foods tainted by haloanisoles

Food contaminated Haloanisolesdetected Origin

Wine 2,4,6-TCA

Cork closures containing the chlorophenols

2,3,4,6-TeCA

Cognac 2,4,6-TCA Cork closures

Dried fruit 2,4,6-TCA Packaging adhesive containing chlorophenols

2,3,4,6-TeCA

PCA

Raisins 2,3,4,6-TeCA Paper/timer packaging materials

Cocoa powder 2,4- and 2,6 DCA

Multiwalled paper sacks containing chlorophenols

2,4,6-TCA

2,3,4,6-TeCA

Desiccated coconut 2,4,6-TCA Multiwalled paper sacks containing chloroanisoles

2,3,4,6-TeCA

Cheese 2,4,6-TCA Wooden shelves containing chlorophenols and chloroanisoles

2,3,4,6-TeCA

Spring water 2,3,4,6-TeCA Cardboard boxes used to store empty bottles

Canned beer 2,4,6-TBA; 2,3,6-TBA Can lids

Skimmed milk 2,4,6-TBA Unknown

Flour 2,4-DCA; 2,4,6-TCA Jute sacks containing chloroanisoles and chlorophenols

2,3,4,6-TeCA; PCA

Wheat/wheat flour 2,4,6-TCA Unknown – but not packaging materials

DCA, dichloroanisole; TCA, trichloroanisole; TeCA, tetrachloroanisole; PCA, pentachloroanisole; TBA, tribromoanisole.

TAINTS/Types and Causes 5723

Catty Taint

0026 The development of offensive catty odors in foods

such as meat, canned vegetables, and cheeses is an-

other example of indirect tainting. In these cases, a

relatively innocuous substance was transferred to the

food, where it was transformed into a powerful taint

by reaction with a minor component of the food. The

compound responsible for the catty taint was 4-

mercapto-4-methylpentan-2-one, formed by the add-

ition of hydrogen sulfide across the double bond of

mesityl oxide. The latter was present in polyurethane

paint, solvents used to dilute can lacquers and plastic

packaging, and was presumed to have been trans-

ferred to the foodstuff from these materials.

Geranium Taint

0027 The development of geranium-like taint in German

wines occurred, as above, by reaction of two innocu-

ous components to form the tainting substance. In

this case, one of the reactants had been formed by

acid-catalyzed rearrangement of a specific microbial

degradation product of the preservative used in

the fermentation. Sorbic acid (the preservative) was

enzymically reduced to 2,4-hexadien-1-ol, which

underwent rearrangement to 3,5-hexadien-2-ol. This

product reacted with ethanol to form 2-ethoxyhexa-

3,5-diene, the compound held to be responsible for

the taint.

Medicinal Taint

0028 Another good example of taint arising indirectly

through a cascade of chemical reactions between os-

tensibly innocuous food components is afforded by

the example of marinated herring that developed

an offensive, medicinal taint. The taint was caused

by 2-bromophenol. This chemical was formed by the

reaction of bromine with phenol, the latter being

present as a minor impurity in the acetic acid used for

the marinade. The bromine was thought to be formed

via the reaction of the hydrogen peroxide used to

bleach the herring with bromide ion present as an

impurity in the brine component of the marinade.

See also: Browning: Nonenzymatic; Toxicology of

Nonenzymatic Browning; Contamination of Food;

Spoilage: Chemical and Enzymatic Spoilage; Bacterial

Spoilage; Fungi in Food – An Overview; Molds in

Spoilage; Yeasts in Spoilage; Taints: Analysis and

Identification

Further Reading

Heath H and Reineccius GA (1986) Flavour Chemistry and

Technology. New York: Van Nostrand Reinhold.

Nijssen B (1991) Off-flavours. In: Maarse H (ed.) Volatile

Compounds in Foods and Beverages, pp. 689–735. New

York: Marcel Dekker.

Reineccius G (1991) Off-flavours in foods. Critical Reviews

in Food Science and Nutrition 29: 381–402.

Saxby MJ (1996) Food Taints and Off-flavours, 2nd edn.

Glasgow: Blackie Academic & Professional.

Whitfield FB and Last JH (1986) Off-flavours encountered

in packaged foods. In: Charalambous G (ed.) The Shelf-

life of Foods and Beverages, pp. 483–524. Amsterdam:

Elsevier.

Whitfield FB and Tindale CR (1984) The role of microbial

metabolites in food off-flavours. Food Technology in

Australia 36: 204–232.

Whitfield FB, Hill JL and Shaw KJ (1997) 2,4,6-Tribromoa-

nisole: a potential cause of mustiness in packaged food.

Journal of Agricultural and Food Chemistry 45:

889–893.

Analysis and Identification

B D Baigrie, The Lord Zuckerman Research Centre,

University of Reading, Whiteknights, UK

Background

0001The wide diversity of chemical compounds that can

cause taints and off-flavors in fresh and processed

food, the various mechanisms and routes by which

they may be imparted to foods, and the inherent

complexity of food flavors all conspire to make the

analysis and identification of taints and off-flavors a

challenging and often time-consuming exercise re-

quiring the application of a unique combination of

scientific skills and analytical expertise.

Sensory Evaluation

0002Food manufacturers who operate to high standards of

good manufacturing practice with a well-established

quality assurance program and a proactive quality

control scheme will normally detect grossly tainted

product, usually by sensory evaluation, before it

leaves the factory. However, some taints may occur

intermittently within a production batch or only arise

during the shelf-life of the product as a consequence

of storage conditions or some other parameter. The

manufacturer may then only be alerted to a taint

problem indirectly through merchandise returns

from retailers or directly via consumer complaints.

0003The first clues to the identity of the taint may come

from the complainant’s comments. Nevertheless, it

is unwise to rely solely on untrained consumers for

5724 TAINTS/Analysis and Identification

an accurate description of a taint, even if large

numbers of complaints about taint in a product have

been received. Invariably, untrained consumers will

generate a lengthy list of taint descriptors, which are

often confusing, sometimes contradictory, and there-

fore of limited value to the analyst. The first stage in

the analysis of a taint, therefore, is sensory evaluation

of the foodstuff using a panel of trained experts.

0004 It is important to use a panel of experts specifically

trained to detect and describe taints, because, to any

particular chemical, there will exist within a popula-

tion of consumers a wide range of sensitivities. Taints

are more likely to be detected by those in the most

sensitive region of the distribution, and hence panels

should comprise individuals with sensitivities consid-

erably greater than that of the population mean.

0005 For well-characterized types of taint, e.g., TCP,

medicinal, antiseptic, panelists can be selected for

their sensitivity to the taint species in question

(usually halogenated phenols or halogenated alkyl-

phenols). However, it should be recognized that sen-

sitivity to a specific taint does not necessarily imply

high sensitivity to taints in general.

0006 Selection of a panel normally involves a measure-

ment of their threshold to that taint. This requires

the panelists to assess, in a specified medium, a

range of stimulus concentrations ranging from zero

to concentrations that can be clearly detected by all

the panel. A threshold is usually defined as the con-

centration of taint that 50% of the panelists can

perceive. For taints, two types of threshold are

typically used, namely detection threshold and recog-

nition threshold.

0007 The ability of panelists to describe accurately

the sensory properties of a taint and, if possible, to

relate it to known standards is particularly useful,

since it can provide vital information to the analyst

or flavor chemist about the chemical structure of the

taint. In practice, therefore, greater credence is placed

on the comments of individual panelists who are

known to be highly sensitive to certain taints, even

if they constitute a minority opinion among the

panelists.

0008 Since most compounds responsible for taint are

volatile, aroma assessment of the suspect food is

mandatory. However, in some cases, for example

taint caused by polychloroanisoles, the taint may

only manifest itself when the food is held in the

mouth or as a characteristic aftertaste after swal-

lowing.

0009 Once sensory evaluation has confirmed that a

product is tainted and perhaps provided some clues

to its nature, the next requirement is to establish

its chemical structure and then to identify the

source of the taint. This involves isolation of a

concentrated flavor extract and separation of the

individual flavor components. If the extract is par-

ticularly complex, fractionation into functional

group classes followed by separation of individual

components in each class may be necessary. Finally,

identification of the taint is usually achieved using an

appropriate spectroscopic technique. (See Sensory

Evaluation: Aroma.)

Extraction of Taint

0010The first major challenge for the flavor analyst is to

obtain a volatile flavor extract that represents the

characteristic flavor of a food. There can be no abso-

lute certainty that a valid extract (i.e., one character-

istic of the food flavor) will be obtained. Nor is there

any assurance that subsequently identified com-

pounds were actually present in the food since arte-

facts can be introduced by many routes. However,

with tainted foods, it is much easier to ascertain if

an extract is valid, simply by assessing the odor or

taste of the extract with a panel of assessors sensitive

to the taint.

0011A wide variety of techniques exists for ex-

tracting taints from foods. Some of the isolation

methods most frequently used are summarized in

Table 1.

0012Where taints are volatile and readily detected by

odor in a food or beverage, headspace sampling is

often the technique of choice. Static headspace sam-

pling is simple to perform and allows a truly valid

flavor extract to be obtained. However, the quantity

of the tainting species delivered to the analytical in-

strument is often too low to enable detection and

identification to be achieved.

tbl0001Table 1 Methods for isolation and/or concentration of taints

1. Headspace sampling

. Static headspace

. Dynamic headspace

. Purge and trap

. Closed-loop stripping

. Solid-phase microextraction

2. Distillation

. Steam distillation

. Simultaneous steam distillation–extraction with Danish–

Kuderna apparatus

. Vacuum distillation

3. Liquid–liquid extraction

. Discontinuous extraction with immiscible solvent

. Continuous extraction with immiscible solvent and

Kutscher–Steudel apparatus

4. Liquid–solid extraction

. Solvent extract of solid in Soxhlet apparatus

. Solid-phase microextraction

. Stir bar sorptive extraction

TAINTS/Analysis and Identification 5725

0013 Dynamic headspace methods such as purge and

trap, closed loop stripping, and solid-phase micro-

extraction (SPME) are used instead to overcome the

problem of adequate detection limits inherent to

static headspace sampling. In dynamic sampling,

headspace volatiles are removed from the gas phase

above the food and concentrated on a suitable

adsorbent (activated charcoal, porous polymers,

macroreticular resins, polymeric gas chromatography

phases) and then desorbed either thermally or with

a solvent.

0014 As with static headspace, these dynamic collec-

tion techniques are simple and cause minimal

artefact formation. SPME, which is a recent techno-

logical innovation, was introduced by Janusz

Pawliszyn, University of Waterloo, Ontario as a

solvent-free sample preparation technique. The

method utilizes a fused silica fiber coated with an

adsorbent polymeric phase. The fiber is mounted

for protection in a syringe-like device, and by de-

pressing the plunger of the syringe the fiber can be

exposed to the headspace above the sample matrix.

After partition of the headspace volatiles into the

coating, the fiber is retracted into the needle and the

device interfaced with the appropriate analyt-

ical instrument. SPME has been utilized extensively

in the analysis of taints and other contaminants

in foods. For example, it is used to monitor the

presence of 2,4,6-trichloroanisole, the species

responsible for cork taint in wines and distilled

beverages.

0015 Where the tainting species is only moderately or

poorly volatile, headspace methods usually provide

insufficient quantities for detection or identification.

In these circumstances, quantitative removal of the

taint from a larger sample size is a prerequisite,

and the method most frequently chosen, particularly

if the food matrix is solid, is simultaneous steam

distillation-extraction (SSDE). This technique has

the added advantage of being able to be operated

under reduced pressure, thus minimizing possible

thermal decomposition of the tainting species. SSDE

has been used extensively to extract chlorophenol,

chloroalkylphenol, and chloroanisole-type taints

from various food matrices.

0016 If the taint is not steam volatile, techniques based

on direct extraction with solvents such as liquid–

liquid, Soxhlet extraction, or extraction using the

sorptive properties of gas chromatography (GC)

polymeric phases, e.g., SPME in matrix immersion

mode or stir bar sorptive extraction (SBSE), can

be used.

0017 Liquid–liquid and Soxhlet extraction are trad-

itional methods with a long history of usage, but

extracts obtained by these techniques may contain

variable amounts of nonvolatile material, and add-

itional separation techniques may need to be applied.

SBSE is a recently developed extraction technique,

similar in principle to SPME, but offering potentially

much lower detection limits than SPME. In this

technique, a glass magnetic stir bar coated with a

polydimethylsiloxane (PDMS) stationary phase is

stirred in the sample for several minutes. The analytes

of interest come into contact with the PDMS

phase and are extracted. Thermal desorption and

online gas chromatography-linked mass spectro-

metry (GC–MS) analysis complete the analytical

methodology.

Fractionation of Flavor Extracts

0018Fractionation of a flavor extract is best achieved

using high-resolution GC on fused silica columns.

However, despite recent advances in column tech-

nology in terms of stability, inertness, resolution,

and sample capacity, major difficulties can still be

encountered due to the inherent complexity of

flavor extracts from many foods and the presence

within the extract of only minute quantities of

the tainting chemical relative to the other flavor

components.

0019To minimize these difficulties, flavor extracts can

be fractionated prior to analysis by GC. For

example, flavor extracts, particularly those from

SSDE, can be readily fractionated on the basis of

chemical functionality into acids, phenols, bases,

and neutrals. Fractionation of flavor extracts by

conventional open column chromatography on

silica gel is also well established. More recently,

fractionation of food extracts by high-performance

liquid chromatography (HPLC) has been utilized,

but to date, this approach has found limited

applicability.

0020An elegant way to optimize the separating power

of modern GC capillary columns is fractionation of

flavor extracts on two different columns – so-called

multidimensional gas chromatography (MDGC). In

this technique, initial separation is effected on a

precolumn, and then selected fractions are introduced

on to a second analytical column, where they are

further separated into individual components. The

columns can be located in the same oven or in two

different ovens: the precolumn can be a packed

column, a wide-bore capillary column, or even a

high-resolution analytical capillary column coated

with a different stationary phase from the second

column. The selection of fractions of interest can be

optimized by the introduction of a splitting device

between the precolumn and detector so that GC ef-

fluent from the precolumn can be assessed at a sniff

5726 TAINTS/Analysis and Identification

port. (See Flavor (Flavour) Compounds: Structures

and Characteristics.)

Detection and Identification of Taints

0021 In addition to the superiority enjoyed by GC over

other analytical techniques as a means of fractionat-

ing complex flavor extracts, it also possesses major

advantages in terms of detection and identification

of taints.

GC Detectors

0022 The ubiquitous flame ionization detector (FID) is

sensitive and is virtually a universal detector for vola-

tile taints. It responds to most organic compounds but

not to air, water, or light fixed gases. Complementing

this detector are a variety of GC detectors, which

are more specific and, in many cases, more sensitive

as well.

0023 The thermionic ionization detector (NPD) is spe-

cific for compounds containing nitrogen and/or phos-

phorus. Known fecal-like tainting compounds such as

indole or skatole can be readily detected with the

NPD, as can 2-acetyltetrahydropyridine, the cause

of mousy taint in wines and cider. More recently, a

nitrogen-specific detector based on chemilumines-

cence, which has a linear, equimolar response and is

not quenched by hydrocarbons, has been developed.

It has been used to monitor nitrogen-containing

flavor volatiles in wine extracts.

0024 The flame photometric detector (FPD) is a very

selective and sensitive detector that measures chemi-

luminescence above a hydrogen-rich flame. By the

use of filters and a photomultiplier, it can readily

measure nanogram quantities of phosphorus or

sulfur-containing compounds. It has been used suc-

cessfully in many taint investigations and notably in

the identification of taints in beer caused by sulfur-

containing species.

0025 Recent advances in the development of sulfur-

selective detectors have culminated in a new sulfur

chemiluminescence detector, which provides a linear

and equimolar response to sulfur-containing com-

pounds without interference from most sample

matrices.

0026 The photoionization detector (PID) can be used

to analyze a wide variety of compounds, including

aliphatic and aromatic hydrocarbons, amines, and

organic sulfur compounds. Key features of the PID

that make it suitable for detecting taints are its

sensitivity, generally lower than an FID or FPD,

and its wide linear dynamic range. It is also non-

destructive and can be coupled in series with other

detectors.

0027Several common tainting species containhal-

ogen atoms, e.g., bromo/chlorophenols and bromo/

chloroanisoles, and are therefore readily detected

using an electron capture detector. This detector has

probably the lowest detection limits of all the

common GC detectors and is highly sensitive and

selective for molecules containing electronegative

atoms; it is, however, very limited in a linear range

and requires extensive calibration for quantitative

results.

0028The atomic emission detector is perhaps the

most versatile of the selective detectors, since it

can be tuned to detect virtually all of the elements

in compounds eluting from a gas chromatograph

(except helium). With an excellent sensitivity and

selectivity, it can be used as a screen for elements to

locate target or unknown peaks of interest in complex

food matrices.

0029Despite the many advantages offered by this variety

of electronic detectors, biological detectors such as

the nose and tongue are probably best suited for

detecting the presence of taints in food/flavor ex-

tracts. The technique of sniffing the effluent gas as it

emerges from the GC – so called odor port gas chro-

matography (GC-O) – is commonly used in aroma

analysis and is particularly useful for locating the

position of tainting odorants in a gas chromatogram.

The area(s) of the chromatogram that correspond to

the taint can be identified, and the number and nature

of the tainting species can often be ascertained. This

greatly simplifies the primary task of the spectrosco-

pist, which is to establish a chemical structure for

each taint.

0030The usefulness of GC-O was readily illustrated in a

recent study of a distilled beverage with an earthy,

musty taint. Preliminary analysis of the flavor extract

from the tainted beverage identified three members of

the polychloroanisole family of taints, but all at sub-

threshold levels. Analysis by GC-O identified a region

in the chromatogram with a strong musty aroma

similar to that possessed by the beverage. The GC

retention time was different from that of any of the

three characteristically musty polychloroanisoles but

identical to that of 2-methylisoborneol, a known

powerful taint with a musty/camphoraceous odor

and taste. Mass spectroscopic data were consistent

with the presence of MIB in the tainted beverage.

When the untainted beverage was spiked with MIB

at 5 mgkg

1

, the taint was reproduced. (See Chroma-

tography: Gas Chromatography.)

Mass Spectrometry

0031In view of the extremely low levels at which taint in

food can be detected by the human sensory organs

TAINTS/Analysis and Identification 5727