A rather simple colorimetric method according to N. Nelson (J. Biol. Chem. 153 (1944) 375-380) relies on the reduction by the sugars of Cu2+ to Cu+, which is then oxidised by arsenate in the presence of molybdate, which in turn is reduced to molybden blue. This can be measured spectrophotometrically at 500-520 nm with glucose as a standard. This method is a simplification of the Nelson-Somogyi method. See the attached file. Several complications may be encountered:
- The own colour of the wine may interfere with colour development, or interfering components may be present. This can be investigated by doing a standard addition series with added glucose and extrapolating to zero added glucose. The slope of the calibration curve must be equal to that of a set of pure glucose solutions.
- It has to be borne in mind that reducing sugar content is expressed as glucose. Fructose and pentose sugars may give a different response to the Nelson reagent.
- Turbid samples may have to be clarified by high speed centrifugation.
- Full characterisation of the reducing sugar profile may require advanced methods like HPLC.
In EU, the official methods for measuring reducing sugars in wine are described in Commission (EEC) Regulation 2676/90 . Make note that only the English text is the official but in most cases you can use whatever language you choose. Furthermore the method you chose may differ according actual sugar content of wine.
http://eur-lex.europa.eu/legal-content/en/ALL/?uri=CELEX%3A31990R2676
official methods of analysis are described in OIV compendium; you should refer to HPLC assessing of sugars or even to Reducing substances (which are actually not just sugars).
note that all methods report values in g/L: that means you should divide the result by 10 to convert them in % W/V
http://www.oiv.int/en/technical-standards-and-documents/methods-analysis/compendium-international-methods-analysis-wines-and-musts-2-vol
There are enzimatic kits that work great in wine. You also can use NIR
Not only HPLC-RI but also HPLC-CAD-based methods are also useful, please take a look at
https://www.researchgate.net/publication/282153636_Analysis_of_selected_chemical_parameters_in_Piemontese_wines
and
https://www.researchgate.net/publication/316280268_Analysis_of_selected_qualitative_and_sanitary_parameters_of_dry_wines
Article Analysis of selected chemical parameters in Piemontese wines...
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