Folin-Denis procedure

The Folin-Denis assay is used as a procedure for the quantification of total phenolics in plant materials, food, and beverages. Reduction of phosphomolybdic-phosphotungstic acid (Folin-Denis reagent) to a blue-colored complex in an alkaline solution occurs in the presence of phenolic compounds (Folin and Denis 1912). 

The Folin-Denis reagent was used by Rosenblatt and Peluso (1941) for the colorimetric determination of tannin in wines. Careful attention to details of temperature, time, and concentration is necessary, but the procedure is reported accurate to within... 

Fig. 4. Procedure for the measurement of soluble tannins (Folin-Denis method)...

Despite the overall limitations of these procedures in producing quantitative analyses, certain amino acids that contain appropriate functional groups were readily determined at considerably more sensitive levels by the use of colorimetric methods. Folin and Denis (1912) introduced such an approach to the measurement of tyrosine and tryptophan, and procedures for the determination of arginine (Sakaguchi, 1925), histidine (Koessler and Hanke, 1919 Kapeller-Adler, 1933), and phenylalanine (Kapeller-Adler, 1932) were subsequently reported. The highly reactive thiol group of cysteine has also... 

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