Vanillin assay

The use of HPLC for quantification of phenols is often limited to a single class of phenolics and then often only to low-molecular weight compounds that are available as standards. It is, therefore, often necessary to use colorimetric assays such as the Folin-Ciocalteau assay which rely on the reducing ability of phenols to quantify the amount of total phenolics in a sample (Waterman and Mole, 1994 Singleton et al, 1999 Schofield et al, 2001). The degree of condensation of polyphenols can be quantified by colorimetric assays such as the acid-butanol assay and the vanillin assay (Waterman and Mole, 1994 Schofield et al, 2001). 

Sun BS, Ricardo-da-Silva JM and Spranger MI. 1998. Critical factors of vanillin assay for catechins and proanthocyanidins. J Agric Food Chem 46 A261-A21A. 

The following functional group assays are used proanthocyanidin assay, vanillin assay and dimethylaminocinnamaldehyde (DMACA) assay. Details to the experimental designs are listed in Tab. (3.1) to (3.3). 

Vanillin assay in samples in glacial acetic conducted in glacial acetic acid, 0.5 % 133, see text... 

The reaction kinetic of the vanillin assay depends on the chemical structure. For (+)-catechin the reaction was terminated after 7 minutes,... 

Conventional assays in hydrochloric acid work best, if moisture is rigorously excluded. Since procyanidins are most effectively extracted with aqueous organic solvents, this is a major drawback. For this reason Makkar and Becker [143] developed a procedure which also allows the performance of the vanillin assay in aqueous methanol or acetone. The use of aqueous acetone is delicate, since it is known that acetone reacts with acidified vanillin to produce a chromogen exhibiting a A,max at 548 nm which may interfere determinations substanially [143]. 

Although, during the last twenty years numerous attempts have been made to improve vanillin assays with respect to reproducibility and accuracy, resulting in a series of different protocols, adaptions are still necessary for certain matrices [144]. 

Several methods have been described in the literature to quantify proanthocyanidins (condensed tannins) however, the most common are the acid-butanol assay and vanillin assay. 

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... 

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