Wine Sugar analysis: It's Just a Matter of… Fehling

The approach of the conclusion of the alcoholic fermentation is a turning point in the wine-making process, and it is at that point that the sugar content is of capital importance for its outcome, both with red and with white wines.

Must sugar assaying through both enzymatic and chromatographic (HPLC) methods allows for an accurate estimation of fermentable sugars, that is glucose and fructose, ignoring pentose sugars, sugars with 5 carbon atoms..

On the other hand, an analysis of the total reducing components OIV-MA-AS311-01ª (Fehling method), carried out with a titration, to assay the sugar content at the end of alcoholic fermentation will yield a value inclusive of the pentose sugars, since these are reducing sugars, too, even though they are not fermentable since they cannot be assimilated by the yeasts.

Pentose sugars are reducing compounds that are present in the grapes at constant concentrations varying between 1 and 3 g/l. This value is affected by several factors such as the vine variety, the soil, geographical latitude, and so on.

Since pentose sugars are not involved in alcoholic fermentation, their concentration remains constant throughout the process, and comparing the results of the three different sugar residue assay methods at the end of the process, it can be observed that the results given by the Fehling method are consistently above the other two by anywhere between 1 and 3 g/l.

This explains why it is of the utmost importance to know what method was used to assay the residual sugars at the end of fermentation, so that the value may be correctly interpreted.

This is a classic example comparing the test results with the three different sugar assaying methods:

As shown in the above table, the HPLC method and the enzymatic method only measure the fermentable sugars, while the Fehling test yields a higher value comprehensive of pentose sugar content.

For a more precise determination of the conclusion of the alcoholic fermentation process, HPLC and enzymatic methods can be seen to be superior, since they only assay the concentration of sugars that are actually assimilated by yeasts, and are therefore involved in the fermentation process.

CDR WineLab® Wine Testing System allows a precise assaying of the actual fermentable sugar content during alcoholic fermentation via an easy, fast (6-11 minutes) and reliable method based on enzymatic analysis (correlated with the OIV-MA-AS311-03 method), to be performed directly in the winery. 

CDR WineLab® is a a photometric technology analyzer that uses optimized methods. It can be used by anyone without specific experience of chemical analysis, and no specific dedicated laboratory is required.