(Wine-Searcher) - Why would anyone want to put grape stems in their wine? Richard Hemming investigates.

Stems? As in the bits of a bunch of grapes you throw away?

Yes! There is a growing trend for chucking the stems in the fermentation vat along with the grapes, rather than chucking them away. When making red wine, most grapes go through a machine called a crusher-destemmer, when they reach the winery. This process separates the stems from the skin and pulp of the fruit, which are normally discarded, and often become used for fertilizer or mulch back in the vineyard but some winemakers think they're worth keeping.

So go on – how do stems influence the way a wine tastes?

The most obvious stylistic effect is a herbal character in the wine. Jeremy Seysses of Burgundy's stem-loving Domaine Dujac describes it as "sappy and spicy" bringing aromas of menthol and jasmine. If it becomes excessive, this green element can give the impression of underripeness and become a negative attribute: "If it goes wrong, it's vegetal. But when it works the highs are higher," says Seysses.

But it's not just the flavor. Stems can influence the structure of a wine too, especially its tannins. Matt Stafford, chief winemaker at New Zealand's Craggy Range, says that "stems lengthen the structure and give a savory, rather than sweet, finish to the tannins".

Meanwhile Gavin Monery of urban winery London Cru suggests that adding more stems doesn't automatically equate to a harder texture. In fact, he feels that greater stem content can create smoother texture. That sounds counter-intuitive, but it may be explained by the fact that stems contain potassium. This has the effect of lowering the acidity of the wine, which may explain how the perception of tannic feel is altered. However, Monery adds that post-fermentation maceration is best avoided where stems have been used, as ethanol can extract harsher tannins.

How should I use the stems?

Good question. There's an important distinction between whole-bunch fermentation and adding stems back. In the latter case, grapes are destemmed as normal, but the stems are then returned to the fermentation vat. This is claimed to be a poor substitute for the whole-bunch method. Stafford describes it as "no different to a powdered tannin addition". For improving the quality of a wine, whole-bunch (sometimes called whole-cluster) fermentation is the best way to potentially benefit from the influence of stems.

All right then, what happens with whole-bunch fermentation?

Here comes the science: firstly, using whole bunches of grapes means that a certain degree of carbonic maceration takes place at the beginning of the process. This is the practice made famous by Beaujolais. It involves intracellular fermentation within unbroken grapes, which is supposed to extract lots of color but minimal tannin from the skins. Stafford also believe it "lifts the floral notes" of the fruit.

At this stage, the stem is incidental to the action; necessary to facilitate whole bunches of grapes, but without having a direct influence on the fermentation. After this intracellular stage, however, stems take on a more active role. Spread throughout a fermentation, they break up the mass of skins and pulp, thereby affecting temperature and movement of two key gases – oxygen and carbon dioxide.

Seysses observes that the temperature of whole-bunch fermentations is more regular throughout the tank, because the liquid circulates more easily. This avoids the hotspots that can accumulate in destemmed fermentations. Furthermore, the overall temperature tends to be lower so the process of extracting tannin and color is more gentle.

Stems also allow gases to move more freely. That means the escaping carbon dioxide doesn’t produce such a thick cap of skins, and it also allows oxygen to permeate the fermentation to keep yeast active. However, Monery warns that if there are too many stems in a tank, pumping over becomes impossible, which could actually reduce the oxygen in the ferment.

That’s why 100 percent whole-bunch fermentations are quite rare, and it is more common to have a proportion of whole bunches, and therefore stems, among otherwise destemmed fruit.