Tannin aldehyde reactions

The reaction of the aldehyde with tannin and with low-moleeular-weight tannin aldehyde condensates, whieh are responsible for the aldehyde eonsumption. 

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. 

Aldehyde-tannin and aldehyde-anthocyanin condensation reactions result in polymer formation (Figure 1). These polymers may be responsible for haze formation in wine and the polymers may eventually precipitate out of solution (26). The polymerized tannins have different flavor properties than the monomeric starting units (21-29) and formation of anthocyanin polymers affects wine color. In addition, these reactions may result in a reduction of aldehyde flavors in the wine. These condensation reactions are discussed more fully in other chapters of this volume. The formation of strong covalent bonds between the aldehyde and the tannin or anthocyanin makes recovery of the bound aldehydes difficult. 

We have examined these lethal reactions biochemically (93, unpublished). In all cases spontaneous defense reactions (phytoalexin synthesis, tannin synthesis and tannin oxidation) occur concurrent with the onset of symptoms. The quantities of terpenoid aldehydes and tannins found in tissues of severely affected plants are usually very similar to those found in severely diseased plants dying from microbial infections. Thus, it is possible that many of the disease symptoms seen in cotton are due to the toxic terpenoids and tannins formed in response to infections. 

Within the realm of polymer science and technology, the major application of tannins is in adhesives, mostly for wood, because of their aptitude to react with formaldehyde, although considerable research has been devoted to reduce the use of the latter by optimizing the conditions favoring tannin self-condensation and/or reactions with other aldehydes, as reviewed recently by Pizzi [50], the most authoritative expert in the field. 

Formaldehyde is generally the aldehyde used in the preparation, setting, and curing of tannin adhesives. It is normally added to the tannin extract solution at the required pH, preferably in its polymeric form of paraformaldehyde, which is capable of fairly rapid depolymerization under alkaline conditions, and as urea-formalin concentrates. Hexamethylenetetramine (hexamine) may also be added to resins due to its potential formaldehyde releasing action under heat. Hexamine is, however, unstable in acid media [24] but becomes more stable with increased pH values. Hence under alkaline conditions the liberation of formaldehyde might not be as rapid and as efficient as wanted. Also, it has been fairly widely reported, with a few notable exceptions [25], that bonds formed with hexamine as hardener are not as boil resistant [26] as those formed by paraformaldehyde. The reaction of formaldehyde with tannins may be controlled by the... 

Possible crosslinkers are formaldehyde as aqueous solution [268], paraformaldehyde [263,265,267,269], UFC [270,271], UF resins [272], aqueous formaldehyde solution emulsified in an oil [273], dimethylolurea [274] or urea and phenol methylols with longer chains to overcome steric hindrance. Tannins can also be hardened by addition of hexamethylenetetramine (hexamine) [275], whereby these boards show a very low formaldehyde emission [269,275 281]. The autocatalytic hardening of tannins without any addition of formaldehyde or other aldehyde as crosslinker is possible, if alkaline Si02 is present as a catalyst at high pH or just as a consequence of the catalysis of the reaction induced by a lignocellulosic surface [282]. 

Compared to previous gluten based adhesives the present resins are applicable in liquid form, thus without any need for modifications of the application systems in particleboard factories. Relative to other protein adhesives such as soy-based adhesives based on reaction with formaldehyde some of the resins presented here have several advantages (i) they cannot and do not produce any aldehyde emission as neither formaldehyde nor any other volatile aldehyde was used in some of the formulations (ii) the percentage of natural materials was increased up to 70% for one type of formulation and up to 95% for others. Furthermore, in relation to resin formulations based on different cross-linking reactions other than those with formaldehyde the resins presented here have other advantages they are competitive with alternate natural resin systems such as those based exclusively on tannins and/or lignins. 

The vanillin procedure involves reaction of an aromatic aldehyde, vanillin, with the meta-substituted ring of flavonols to yield a red adduct. Although the vanillin assay is used for the estimation of condensed tannins (proanthocyanidins), the... 

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). 

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