Identification of the major drivers of 'phenolic' taste in ... - GWRDC

AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines
A Preparative and analytical laboratory
Methodologies
A.1 Measurements of Total Phenolics by Spectrometry:
the Rationale Behind Somers Measures
Somers and Ziemelis (1985) proposed the simple measurements of total phenolics, total
hydroxycinnamates and the calculation of the flavonoid extract to assess musts and pressings. The
clarified juice or wine is measured directly in a one or 2mm quartz cuvette (with the result multiplied by
10 or five to give the 10mm equivalent reading of absorbance at 280nm (A280)) and 320nm (A320) or
0.5mL diluted into 10mL 3% aqueous acetic acid, and read in a 10mm quartz cuvette depending on the
sample.
Total phenolics = A280-4 absorbance units (au)
Total hydroxycinnamates = A320-1.4 au
Flavonoid extract = (A280-4) – 2/3(A320-1.4) au (A=10 mm equivalent)
The correction factors of 4 and 1.4 are estimates to allow for the small contribution to absorbance from
non-phenolics (Somers and Pocock 1991). Besides phenolic compounds, some other types of compounds
absorb in the UV-Vis region (200-600 nm), and contribute in a small way to the absorbance. For example,
tannin-protein complexes, nucleic acids, and other compounds such as SO2 contribute to A280 (Myers and
Singleton 1979). Sorbic acid absorbs at 280 nm and can influence the result significantly, so wines with
added sorbic acid are not suitable for these UV-Vis measurements.
A280 is a measure of total phenolics and includes all the different classes of phenolics including flavonoids
and non-flavonoids. A320 is a reasonably specific measure of hydroxycinnamates. The flavonoid extract
includes flavanols, condensed tannins, and flavonols (aglycones and glycones), that are located in the
solid parts of the grape and, therefore, are extracted into the wine or juice with skin contact and pressing.
The flavonoid extract can account for 0-80% of A280, related to the degree of extraction from the grape
solids (Somers and Pocock 1991). Measuring the hydroxycinnamates by A320, total phenolics by A280 and
calculating the flavonoid extract is, therefore, a simple method to characterise the broad phenolic profile
of a sample.
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