Polyphenols in whites
For whites an acid is necessary, perchloric acid (HClO4) which, in addition to acidifying, has the property of precipitating all proteins and thus inhibiting all PPO (polyphenoloxidase) and lacassa activity. 37% concentrated HCl can also be used.
The procedure is simple:
Prepare an S solution of diluted acid at 1% w/v (10 g of acid per liter of water).
- At the time of extracting the must, dilute it with S solution (dilution d between 10 and 50 times*): the must is stabilized and should not evolve further (it can be kept for short periods in the refrigerator but without freezing, since phenolic acids would precipitate with tartaric acid).
- Before measuring, clarify the diluted must by filtration or centrifugation (be careful as with measurements in UV the clarification must be impeccable).
- With the dilution at room temperature, measure absorbances at 280 and 320 nm (E280 and E320) in a quartz container applying the dilution correction: A280 = d x E280 and A320 = d x E320 and possibly that of the optical path if it is different from 10 mm (for example, for 1 mm containers, the result must be multiplied by 10).
- Use data with the attached table: the values of phenolic acids and tannins are expressed in mg/L.
* The dilution should be such that E280 and E320 are between 0.1 and 2 in absorbance. We think that a dilution of 20 is suitable for the majority of musts but preliminary tests are recommended.