Gross interference by sulphur dioxide in standard determinations of wine phenolics

Abstract

There is a very strong interaction between sulphur dioxide (SOa) and o-dihydroxy phenols in response to the Folin-Ciocalteu colorimetric assay, currently the established procedure for analysis of total phenols in wines and plant extracts. Because of this previously unsuspected interference, the method is shown to be generally quite invalid in application to white wines; measures of total phenols may be magnified several-fold by the SO2 artefact. By use of model solutions, it has been shown that the extent of interference depends primarily on the molar ratio of SO2:o-dihydroxy phenols. The mechanism of the artefact can be interpreted with reference to the chemistry of the photographic process, in which the oxidation-reduction reactions involved in image development are similarly mediated by the presence of bisulphite in alkaline solution.

Phenolic Composition of Natural Wine Types

“…S02 is invariably used in commercial vinification, and this molar ratio may be as high as 10: 1 in white wines. Thus the method was shown to be generally invalid in application to white wines; measures of "total phenolics" are frequently magnified more than twofold by this previously unsuspected S02 artefact (Somers and Ziemelis 1980). As red wines have much higher phenolic extract than whites, the molar ratio S02: o-dihydroxy phenols is very low, and error from the S02 artefact is then consequently small (Fig.…”

Effect of Sulfites on Antioxidant Activity, Total Polyphenols, and Flavonoid Measurements in White Wine

“…Interference of sulfites and other non-phenolic reducing compounds in the FC assay, used for polyphenols determination in foods and beverages, particularly white wine, has occasionally been reported [ 15 , 22 , 23 , 24 ]. Recently, an analysis of white wine with the DPPH and FC assays has been found to overestimate the contribution of phenolics to the antioxidant activity [ 25 ].…”

Wine Phenols

“…The interference can be so large as to invalidate results if the 55°e 29°e 55°e 25°e 55°e 2h 5 min 2h 5 min 2h 5 min 100 5 20 10 30 20 60 200 10 40 15 50 40 75 500 20 85 30 120 50 190 1000 30 105 60 150 100 250 2000 60 200 120 250 200 340 phenol content is low and the free S02 content high (Somers and Ziemelis 1980). This is not the typical situation, however.…”

“…This synergistic effect is related presumably to effective reduction of quinoid oxidation products enabling reoxidation by the reagent. When 100 mg 1-1 of free S02 was added to different phenols so that there was 3.1 mol of S02 per mol of phenol, the color increase was 91 to 101 mg gallic acid equivalent (GAE) 1-1 for gallic acid, caffeic acid or catechin, but only 40 mg GAE 1-1 for p-coumaric and 16 mg GAE 1-1 for resorcinol (Somers and Ziemelis 1980). S02 was only 1/3 as effective in generating color on a molar basis as was gallic acid.…”