Whole Bunch Fermentation has been practiced to produce extraordinarily complex red wine like Pinot Noir in the Burgundy region of France for centuries, as historically they had no de-stemmers. Today, many prominent Domaines are believers in whole bunch (also known as whole cluster) fermentation such as Domaine de la Romanee-Conti.
Whole bunch fermentation refers to the fermentation of intact bunch of grapes as they are picked from the vine with no intervention of machine harvesters - leaving all berries and stems intact. Traditionally, red winemaking begins with crushing of the red grapes to start to release the contents of the berries and de-stemming to remove the grapes from the stem, achieved by using a mechanical crusher/de-stemmer.

 

The stems are often removed in red winemaking before fermentation since the stems have high tannin content and when unripe, stems can give the wine a stalky green aroma. With Pinot Noir, even if the bunches are de-stemmed, the grapes are usually left uncrushed (whole berries) to encourage the development of desirable aromas through partial carbonic maceration.
The benefits of this whole bunch process with Pinot Noir can be seductive aromas of wild strawberries, spice and earthy complexity and a sense of forest floor with wild mushrooms. The weight and body of the finished wine can also be enhanced. However the most beneficial attribute is to increase the silkiness of the palate mouth-feel and provide structure for long ageing. The texture of whole bunch wines can have a soft, fine grainy mouth-feel - like it had been meticulously refined, with good weight, softness and breadth across the whole palate.
Whole bunch fermentation is not adding stems to a fermentation that has been put through a de-stemmer as was carried out in the early 1970′s and 80′s around the world. The stems would break up during pumping over, with the broken and crushed stems imparting unpleasant green herbaceous aromas and flavours into the wine. It is argued to avoid the latter only whole bunch with brown, ripe stems are added to the ferment. Although the berries may be physiological ripe, the stems may not be.
Whole bunch stems in the fermentation slows the ferment, introduces more oxygen and allows some of the whole berries to start fermentation inside the berry - thereby producing fresher more complex fruit aromas and flavours. Wines fermented with stems generally result in a slightly lower alcohol wine. Whole bunch will introduce more complex tannins to the wine filling out the tannin profile, plus there additional stem tannins contain three or more specific molecules which are predominantly astringent and contribute to suppleness.
On the negative side acid in the finished wine can be reduced by the release of potassium from the stems and the colour of the wine can also be lighter. One thing is for certain there is usually a modification of the method from year to year, as in ripe years more whole bunches are added and in less ripe years less if any whole bunches are added to the ferment.
Whole bunch require earlier pressing to make sure aggressive green tannins are not extracted. In a year where the stems are ripe, the wine can gain body and tannin that pair well with secondary fruit flavours with ten years age. The green edge is very hard to predict - one winemakers whole bunch triumph is another winemakers green-stalky atrocity depending on site, bunches and vintage. Many young winemakers largely moved away from whole bunch fermentation or limit it to 10% - 30%. Though there are some believers who use as much as 100% whole bunches in selected wines with favourable results.