Tannins reactions involving

Other reactions involving this aldehyde include the following (1) combination with sulphite ion, which substantially increases the proportion of bound sulphite in wine (2) formation of addition compounds with some polyphenols such as tannins and procyanidins, where it acts as a bridging molecule (Haslam and Lilley 1998) and (3) chemical oxidation to acetic acid, which only occurs to a small extent and has little influence on wine composition and quality. 

These various reactions involving yeast metabolites and non-pigmented phenolics are also believed to lead to changes that affect the astringency of tannins (Eglinton et al. 2005). Sensory evaluation of wines made with two Saccharomyces yeast which differed in acetaldehyde production led to differences in mouth-feel attributes associated with tannins, namely grainy, silky, velvet, drying and pucker. 

Reactions Involving Tannins with Protein and Polysaccharides... 

The main consequences of these reactions involving phenols in red wines are changes in color intensity, a tendency to develop a yellow-orange hue (generally accompanied by loss of color) and various modifications in the tannins, responsible for their gradual softening. 

The reactions involved in these color changes and the oxidative transformations of phenols in wine mainly involve ethanal. They either result in the formation of an ethyl cross-bond between anthocyanin and tannin molecules (Section 6.3.10), or a cycloaddition to the anthocyanins, producing tannin-pyranoanthocyanins (Atanasova et al.,... 

Tannins may be implicated in other reactions involving plant polymers, proteins (nsed in... 

One of the first reactions described in red wines was the polymerization reaction between anthocyanins and flavanols (catechins and condensed tannins) mediated by acetaldehyde (Timberlake Bridle, 1976). This reaction involves the protonation of the aldehyde, followed by addition of the respective carbocation to the nucleophilic position C-8 and less likely C-6 of the phloroglucinol ring of the flavanol (see Fig. 3.6). Further dehydration and a nucleophilic attack from the anthocyanin give rise to the formation of two new pigments that differ in the stereochemistry of the asymmetric carbon of the interflavonoid linkage [catechin-HC (CH3)-anthocyanin], as demonstrated by several authors (Rivas-Gonzalo etal, 1995 Escribano-Bailonefa/., 1996 Lee etal, 2004). 

Taking advantage of the many aromatic hydroxyl functions in conifer bark tannins, Hartmann (83) used ground bark as a polyol for reaction with isocyanates to prepare urethane foams with particularly good flammability resistance. Most uses for conifer bark tannins that involve reactions with the hydroxyl functions center on their complexation with cations. When sulfonated, condensed tannins can also be used as water-soluble heavy metal complexes. One of the more interesting of these applications is the development of water-soluble heavy-metal micronutrient complexes that have been used to correct iron deficiency in citrus... 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

According to some studies, the compounds form as reaction by-products between anthocyanins and flavan-3-ols, such as catechins and proanthocyanidins (condensed tannins). These reactions may also involve other molecules such as acetaldehyde, pyruvic acid, acetoacetic acid, vinylphenol, vinylguaiacol, vinylcatechol, and dimerization of anthocyanins (Asenstorfer et ah, 2001 Atanasova et al., 2002 Bakker and Timberlake, 1997 Brouillard and Dangles, 1994 Fulcrand et ah, 1996, 1998 He et al., 2006 Liao et ah, 1992 Remy et ah, 2000 Salas et ah, 2004 Schwarz et al., 2003 Timberlake and Bridle, 1976). 

The conversion of animal hides into leather by treatment with water-soluble plant extractives has been practiced since antiquity. This process became known as tanning and obviously involved the reaction of a naturally occurring extractive, tannin, with the protein in the hide. We now know, of course, that tannins comprise a whole spectrum of chemical compounds, but generally they are polyphenolic and polymeric. Tannins have been isolated from a wide variety of raw materials, including insect galls, fruit skins, seed hulls, leaves, bark, and heartwood. Indeed, tannins are of nearly ubiquitous occurrence in higher orders... 

Microoxygenation is a technique that involves the addition of small and controlled quantities of oxygen to the wine. This technique is mainly used to stabilize the colour of red wines, since oxygen in small quantities favours polymerization reactions among anthocyanins and tannins (Atanasova et al. 2002). Globally, total anthocyanins decrease, but what is formed, combined with tannins, leads to a product which is more intensely coloured and more stable over time than the initial compounds. 

---