Food Biochemistry and Food Processing (2 edition)

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37 Natural Food Pigments 717

TANNINS

Properties and Functions

Tannins are a heterogeneous group of water-soluble polyphe-

nolic compounds present in many plants. The term “tannin”

originated from the ancient practice of using plant extracts to

“tan” leather, thus the name “tanning” is used to describe the

process. This process makes use of the capacity of tannins to

form cross-links with other biomolecules such as proteins or

polysaccharides to “toughen” the textures of otherwise soft ma-

terials. Tannins occur mainly in tea, coffee, immature fruits,

leaves, and the bark of trees where their function is believed

to be one of defense. They have an astringent and bitter taste

that is unpleasant to prospective consumers. The best known

members of this family of compounds include tannic acid and

its breakdown products (ellagic acid, gallic acid, and propyl gal-

late; Fig. 37.9). Tannic acid is used as a food additive. Foods

known to be high in tannins include pomegranates, persimmons,

cranberries, strawberries and blueberries, hazelnuts, walnuts,

smoked meats and smoked fish, spices (e.g., cloves, tarragon,

thyme, and cinnamon), most legumes, and beverages such as

wine, cocoa, chocolate, and tea.

Tannins are classified into two main classes, (1) the hydrolyz-

able and (2) the condensed (nonhydrolyzable) tannins. The hy-

drolyzable tannins are those that on heating with dilute mineral

acids or bases form carbohydrates and phenolic acids (i.e., gal-

lic or egallic acid). The condensed tannins form phlobaphenes

(e.g., phloroglucinol) when heated with dilute acids or bases. It

Gallic acid

OH

OH

OH

O

O

O

HO

HO

OH

OH

HO OH

O

O

O

O

HO

HO

Ellagic acid

Propyl gallate

Figure 37.9.Structures of some common tannins.

is the condensed tannins that are more important in alcoholic

beverages such as wines.

Some tannins such as tannic acid and propyl gallate display

antimicrobial activity toward foodborne bacteria, aquatic bac-

teria and off-flavor-producing microorganisms, e.g., tannic acid

reduces staphylococcal activity (Akiyama et al. 2001). Thus,

they can also be used in food processing to extend the shelf-life

of some food products.

Tannins and Health

Some of the health benefits of tanninsinclude their anti-

inflammatory, antiseptic, and astringent properties. They also

are effective against diarrhea, irritable bowel syndrome, and

skin disorders. However, tannins are metal ion chelators and

their binding to metal irons make these mineral elements not

readily bioavailable. High intake of tannins could hamper the

absorption of minerals such as Fe to result in anemia. They can

also cause protein precipitation to interfere with the uptake of nu-

trients in some ruminants. Foods with high levels of tannins are

judged to be of poor nutritional value, because they reduce feed

intake, protein digestibility, feed efficiency and growth rates,

among other things (Chung et al. 1998). These observations

have been linked to tannins decreasing the efficiency in con-

verting absorbed nutrients into new body components (Chung

et al. 1998). The intake of foods high in tannins such as herbal

teas is also thought to increase the incidence of cancers (e.g.,

esophageal cancer) and this has led to the suggestion that these

compounds may be carcinogenic, but other reports attribute the

apparent carcinogenic activity observed to constituents associ-

ated with tannins and not the tannins per se (Chung et al. 1998).

Some of these observations contradict findings alluding to the

anticarcinogenic behavior of tannins (Nepka 2009), or their ca-

pacity to reduce the mutagenicity of certain compounds (Chen

and Chung 2000). The anticarcinogenic and antimutagenic ac-

tivities of tannins and related compounds have been attributed

to their polyphenolic nature that enables them to behave as an-

tioxidants to check lipid peroxidation (Hong et al. 1995) and

the damaging effects of reactive O 2 species (Nakagawa and

Yokozawa 2002) in biological tissues. Tannins are also reported

to reduce blood clotting, blood pressure, and serum lipid levels

(Akiyama et al. 2001). They also induce liver necrosis (Goel and

Agrawal 1981), and modulate immunoresponses (Marzo et al.

1990).

QUINONES

Properties and Functions

Quinones are a class of heterocyclic organic compounds with

structures like cyclic conjugated diones. They have been de-

scribed as secondary metabolites and are found in several or-

ganisms including flowering plants, bacteria, fungi, and to a

small extent in animals like insects, sea urchins, corals, and

other marine animals (Thompson 1991). The compounds dis-

play a plethora of colors from yellow to orange to intense

purple to black. Examples of quinones include anthraquinone,