So why do winemakers need to remove the stems from the grape bunches before fermentation. Well the simple answer is tannins. Grape stems and seeds contain high concentrations of bitter tannins, and leaving them in the must during fermentation will result in a wine so high in aggressive tannins it will not be pleasant to drink.
Grape skins also contain natural tannins so the winemaker is not eliminating all tannins by removing the stems. The grape skins will provide plenty of tannins without all the aggressive, green, stalky flavours. During the destemming process, this is an ideal time to look over all the grapes you have harvested and are processing as you load them into the auger (the cork-screw hopper), before the fruit goes into the destemming machine.

 

As at this stage you can make sure that nothing unwanted is getting into the tanks and wine such as bugs, green stems, or worse damaged, rotten grapes. The team as the fruit comes into the winery are looking over the grapes for evidence of mould, dehydrated and under-ripe grapes, which do not help in making good wine. The portion, percentage of mould doesn’t need to be all that high for you to taste it in the finished wine. Dehydrated grapes will make for a sweet and raisin wine if in high enough quantities. Which is not a bad thing, if that is the style of wine you are wanting the make.
Destemming consists of separating grapes from the herbaceous parts of the clusters and the stems that are unnecessary. Though it must be stated some winemakers keep small fragments of the stems to increase tannin in some wines. And can vary from parcel to parcel, varietal and vintage. Destemming can be done mechanically or by hand, and like harvesting manually, destemming manually increases quality as only the best grapes (un-ripe, damaged grapes are also discarded) are taken from the clusters.
Typically, the destemming is undertaken before crushing with the purpose of lowering the development of tannins and vegetal flavours in the finished wine. Single berry harvesting, as is done with some wines like the German Trockenbeerenauslese, which avoids this step altogether with the grapes being individually selected.
Grapevine stems are also high in potassium, which can decrease a wines balance of good acidity. Stems can be high in water, which can decrease the colour and alcohol level of the finished wine. Grape stems will also absorb colour and alcohol, so they are usually removed when making red wines. The stems can also take up a great deal of space, so the reduction in volume will also reduce the total tank capacity required for fermentation.
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. For some parcels and wine styles the stems can add additional tannin to the wine, particularly in the case of wet vintages. Plus the stems of some varietals (when ripe) can contribute good subtle tannins and flavours to a finished wine.