De-stemming

Grapevine stems are:

• High in tannins, but these are usually bitter, low molecular weight tannins.
• High in potassium, which decreases wine acidity
• High in water, which decreases the colour and alcohol level of the wine.

Stems will also absorb colour and alcohol, so they are generally removed in red winemaking. The reduction in volume of the vintage will also reduce the total tank capacity required.

However, stems may be left on, or added back after de-stemming in order to:

• Reduce cap compaction, thus making phenolic extraction easier and enabling the heat generated by the alcoholic fermentation to dissipate more easily through the cap.
• Add extra tannin to the wine, particularly in the case of rotted vintages.

The stems of some varieties (when ripe) can contribute good tannins and flavours.

Crushing

The aims of crushing in red winemaking are to:

• Increase the extraction of phenolics during the alcoholic fermentation
• Partially homogenise and aerate the vintage in order to ensure a prompt and effective start to the alcoholic fermentation.

Crushing can be carried out by:

• Crusher rollers, often incorporated into a de-stemmer
• A must pump transferring vintage to the fermentation tank.
• Human feet.

Crushing should always be gentle so that any solid parts of the vintage (stems, pips, MOG) are not damaged, and the intensity of crushing has a significant effect on wine flavour: Too little crushing will lead to low phenolic extraction, too much could lead to harsh over-tannic wines. A proportion of the vintage can be fermented without crushing in order to give carbonic maceration flavours.