Why do winemakers destem their grapes?
Tannins! Not that tannins are inherently bad. They’re contained also in grape skins and seeds and help give a wine firm, textural backbone for structure. They can also help with endurance in the cellar thanks to their antioxidant qualities. But too much is too much, and often stem tannins will taste green and harsh, depending on the physiological ripeness of the cluster. In certain regions, depending on weather, grapes can develop high sugar ripeness before the stems fully “ripen” or turn from green to brown.
There’s another advantage to using a destemming machine. Cellar hands can better inspect the individual berries on a sorting table before they’re crushed and fermented. Mouldy or overripe or damaged grapes can thus be removed. When you’re dealing with the whole cluster, it can be difficult to see the mould or rot.
Destemming really only started to become a big thing since about the middle of the past century with the introduction of automated machines and also mechanical harvesting tractors, which shake and strip berries right off their stems.
But as with many aspects of winemaking, there’s a contrarian philosophy, too. Some producers believe that destemming is not always the best route. There’s been something of a return to what’s called whole-bunch, or whole-cluster, fermentation, particularly with such grapes as pinot noir and syrah. As long as the clusters are picked when the stems themselves are fully ripe and brown, many vintners believe they can add softness to the texture as well as more complex aromatics. I’m told by winemakers that stems can help moderate temperature swings and spikes within the tank, slow down fermentation and even help introduce oxygen in a beneficial way. A lot of pinot producers will hedge their bets and use a combination of destemmed berries and whole bunches in the same fermentation.