Destemming is the technical term for the process in which the grapes are separated from their stalk structure, the so-called rape.
This can be done by hand either in the vineyard or in the wine cellar, or mechanically directly during the grape harvest in the vineyard. Thanks to destemming or destemming machines, which were invented in the 1960s, the latter is now the case in around 70% of vineyards worldwide. In Valais, however, the grapes are still mainly harvested by hand on the slopes due to their inaccessibility. While white wine grapes are always destemmed, this is only the case for the majority of red wine grapes.
The difference between de-stemming and not de-stemming is very noticeable in the later wine due to a hard or fine tannin structure, which in the worst case could even lead to excessive bitterness in the glass. Some white wine grapes do not necessarily need to be destemmed, as their stems contain very little tannin, as is the case with Riesling, for example. However, a rule of thumb states that the tannin content can be reduced by up to 20 percent during destemming, which underlines the importance of the destemming process.
The reason for this is that the tannins (phenols) and tannins contained in the stems are extremely intense, which could result in an overly dominant influence of undesirable tannins during fermentation. Destemmed grapes result in wines with a purer tone.
In addition, unnecessary mechanical stress on the grapes should be avoided wherever possible, as this leads to increased lees and an increased risk of oxidation. In this respect, every winemaker must consider carefully whether mechanical destemming is really necessary.
