Toxic phenolic substances

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More than 800 phenolic substances have been detected in plants. Many of them contribute to the (bitter) taste and flavor of foods, and some also contribute to color.

These substances can be divided into two major groups on the basis of frequency of occurrence, structural relationship, and relative toxicity:

1. widespread phenolic plant substances, often used in the production of foods and beverages. About 25 have been identified and only a few are present in relatively high concentrations in certain plant foodstuffs; the majority are only present in trace amounts. This group includes phenolic acids such as caffeic acid, ferulic acid, gallic acid, flavonoids, lignin, hydrolyzable and condensed tannins, and derivatives. At the levels present in food, these substances are devoid of acute toxicity. Presumably, evolutionary adaptation gave animals and man the ability to detoxicate them;

2. a more heterogeneous group of highly toxic phenolic substances. Examples are coumarin, safrole, myristicin, and phenolic amines also known as catecholamines, and gossypol (see Chapter 3).

A number of phenolic substances will be discussed in more detail in the following subsections.

2.2.1.1 Flavonoids

A class of plant pigments that are widely present in human food, are the flavonoids. These pigments are polyhydroxy-2-phenylbenzo-y-pyrone derivatives, occurring as aglycones, glycosides and methyl ethers. They are divided into six main subgroups (see Figure 2.1). Most flavonoids are present as P-glucosides. The enzymes catalyzing the formation of the active agents have not yet been found.

Flavanone 3-OH: Flavanol

Flavone 3-OH: Flavonol

Anthocyanidin 3-OH: catechin 3-OH: condensed tannins

Anthocyanidin 3-OH: catechin 3-OH: condensed tannins

Isoflavanone

Chalcone

Aurone

Chalcone

Aurone

Figure 2.1 Flavonoid classification.

Because of their wide distribution, the effects of the flavonoids on human well-being are of considerable interest. More than 1 g of various flavonoids are ingested daily in the diet of the Western world.

A group of yellow pigments that occurs abundantly is the flavones. Examples are nobiletin, tangeretin and 3, 3', 4', 5 ,6 ,7, 8-heptamethoxyflavone. The former two are found in citrus fruits such as tangerines, mandarines and oranges, the latter in grapefruit.

OCH3

h3co

OCH3

H3CO

OCH3

och3 o

Tangeretin

OCH3

OCH3

OCH3

OCH3

h3co h3co

OCH3

OCH3 O 3,3',4',5,6,7,8 - Heptamethoxyflavone

OCH3

The flavones are generally located in the oil vesicles of the fruit peel. Flavones are apolar, and therefore readily soluble in the oil. They can be found in the juice after pressing the peel. The oily material from orange peel can contain about 2 mg nobiletin per 100 ml oil and 0.3 mg tangeretin per 100 ml oil.

The flavones group has been extensively investigated for mutagenicity. A well-known mutagenic representative is quercetin, occurring, for example, in cereal crops. Quercetin is the only flavonoid shown to be carcinogenic in mammals after oral administration. Its structure can be found in Chapter 3.

2.2.1.2 Tannins

Tannins are a heterogeneous group of broadly distributed substances of plant origin. They are considered to include all polyhydric phenols of plant origin with a molecular mass higher than 500. Two types of tannins can be distinguished on the basis of degradation behavior and botanical distribution, namely hydrolyzable tannins and condensed tannins.

The hydrolyzable tannins are gallic, digallic, and ellagic acid esters of glucose or quinic acid. An example of this group is tannic acid, also known as gallotannic acid, gallotannin,

H3CO

or simply tannin. Tannic acid has been necrosis and fatty liver.

reported to cause acute liver injury, i.e., liver

ch2o-dg

ch2o-dg

O-DG

H O-DG Tannic Acid

H O-DG Tannic Acid

O-DG

digallic

The condensed tannins are flavonoids. They are polymers of leukoanthocyanidins (Figure 2.1). The monomers are linked by C-C bonds between positions 4 and 6, or positions 4 and 8.

OH

Proposed structure of a condensed tannin

Proposed structure of a condensed tannin

Tannins occur in many tropical fruits, including mango, dates, and persimmons. The contribution of the tannins in tea, coffee, and cocoa to the total tannin intake by humans is of particular importance. A cup of regular ground coffee was found to contain 72 to 104 mg of tannins, a cup of instant coffee 111 to 128 mg, and a cup of decaffeinated instant coffee 134 to 187 mg. One particular brand of cocoa was found to contain 215 mg tannins per cup. Tea has the highest tannin content. It has been shown that on maximum extrac

tion, black tea leads to 431 to 450 mg tannin per cup. Green tea may yield more soluble tannins, while black tea contains tannins with a higher molecular mass, as a result of oxidation of phenolic precursors during fermentation. From these data it can be estimated that a person may easily ingest 1 g or more tannins per day. Other important sources of tannins are grapes, grape juice, and wines. The tannins in grapes are mostly of the condensed type. The highest levels are found in the skin of the fruit. On average, grapes contain 500 mg per kg, and red wines 1 to 4 g per l of wine. Tannins are also found in large amounts in ferns.

2.2.1.3 Coumarin, safrole, and myristicin Natural toxins can also be found among the flavorings. Three examples will be discussed in this section: coumarin, a chroman derivative, and safrole and myristicin, both methylenedioxyphenyl substances.

Only a few flavor components of natural origin have been toxicologically evaluated. One reason for this may be that isolation of sufficient quantities for testing is often difficult. More is known about those flavor components that are also synthetically made. This concerns coumarin and safrole.

Coumarin

Coumarin

Safrole

OCH3

Myristicin

OCH3

H2C-CHCH2

Coumarin widely occurs in a number of natural flavorings, including cassis, lavender, and lovage. These flavorings are extensively used in sweets and liquors. Traces of coumarin occur in citrus oils and some edible fruits.

Safrole has been shown to cause liver tumors in rats. It is found in the oil of sassafras and in black peppers. Both coumarin and safrole are still allowed for food use in the European Community. They are prohibited in the US though, as they have been found to cause liver damage in rats.

One of the most common other methylenedioxyphenyl substances is myristicin. It is found in spices and herbs such as nutmeg, mace, black pepper, carrot, parsley, celery, and dill. It has been suggested that myristicin contributes to the toxicity of nutmeg. After nutmeg abuse, tachycardia, failing salivation, and excitation of the central nervous system have been reported. Nutmeg has been abused as a narcotic.

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