Tannins subclasses

Singleton and Esau (I) reviewed the methods for phenol analysis of wine. They pointed out that study would be greatly advanced if one could determine the total content of phenolic substances and express it in such a way that analysis of subclasses of phenols could be related to the original total and a balance sheet could be obtained. One could then say, for example, this wine has a total phenolic content of 1200 mg/liter calculated as gallic acid, and of that total, cinnamic acid derivatives account for 200 mg/liter, anthocyanins for 300 mg/liter, other small flavonoids for 200 mg/liter, and condensed tannins complete the total with 500 mg/liter of gallic acid equivalent. To accomplish this, the total phenol analysis not only must meet ordinary criteria of reproducibility and precision, but it also must be based on chemical relationships such that fractions determined separately can be converted to units of the total. Of course when clearcut fractionation can be accomplished by... 

Proanthocyanidins are so named because under oxidative and acidic conditions they are converted to anthocyanidins, a subclass of flavonoid (for general structure, see Fig. 11.3.4). Historically, they have also been referred to as leucoanthocyanidins, condensed tannins, or simply tannins because of their ability to fix or tan leather hides. 

Flavan-3-ols represent the most common flavonoid consumed in the American and, most probably, the Western diet and are regarded as functional ingredients in various beverages, whole and processed foods, herbal remedies, and supplements. Their presence in food affects quality parameters such as astringency, bitterness, sourness, sweetness, salivary viscosity, aroma, and color formation [Aron and Kennedy, 2007]. Flavan-3-ols are structurally the most complex subclass of flavonoids ranging from the simple monomers ( + )-catechin and its isomer (—)-epicatechin to the oligomeric and polymeric proanthocyanidins (Fig. 1.10), which are also known as condensed tannins [Crozier et al., 2006b]. 

Depending on their structures, tannins are defined as hydrolyzable (gallotannins and ellagitannins) or condensed (monomers, dimers, oligomers, and polymers of flavan-3-ols). Condensed tannins are also known as proanthocyanidins [3,47]. Proanthocyanidins can be divided into propelargonidins, based on the hydroxylation pattern of the A- and B-rings [3]. Of these, procyanidins constitute the most common subclass of flavonoids in foods, and prodelphinidins and propelargonidins are also present [48,49]. 

The C-glucosidic ellagitannin subclass also includes complex tannins known as flavano-ellagitannins, which are hydrolysable tannin hybrids with a C-glucosidic ellagitannin moiety such as stachyurin (37) or vescalagin (39) and a flavan-3-ol... 

The C-glucosidic ellagitannin subclass also encompasses so-called complex tannins, which are structural hybrids composed, in their simplest variations, of a C-glucosidic ellagitannin moiety derived, for example, from the monomer stachyurin (30) or vescalagin... 

Flavan-3-ols are the most complex subclass of flavonoids ranging from the simple monomers (+)-catechin and its isomer (—)-epicatechin, to the oligomeric and polymeric proanthocyanidins (Figure 1.5), which are also known as condensed tannins. 

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