Grapes arrive at a winery on stems, and removing them is a choice rather than a necessity. Leaving some or all of them in the vessel changes the wine in several distinct ways at once.
The stem is a different material entirely
Stems contain their own tannin, coarser and more herbaceous than skin tannin, and they carry it in significant quantity. A whole-bunch batch is a substantially more tannic starting point.
Ripeness of the stem itself matters as much as ripeness of the fruit. Green, sappy stems contribute a bitterness that woody, lignified brown ones do not, and the two ripen on different schedules.
This is why the decision is made in the vineyard as much as the cellar. A producer working with whole clusters is watching stem colour through the final weeks of ripening.
Intact berries ferment from the inside
Where whole bunches go into a sealed vessel undamaged, the berries at the bottom are crushed by weight while those above stay intact.
Inside an unbroken berry, enzymes convert sugar to alcohol without yeast in a process that operates only in the absence of oxygen.
That internal pathway produces a distinctive set of aromatic compounds, often described as floral or spicy, which cannot arise from a crushed and destemmed batch.
Stems alter the chemistry of the must
Stems absorb some alcohol and dilute the concentration slightly, which lowers the finished strength of the wine.
They also absorb potassium and colour, and they raise pH, which softens the perception of acidity and affects long-term stability.
The lower final alcohol and altered acid balance are frequently as noticeable as the aromatic change, though they attract less attention.
Practical effects in the tank
Whole bunches create channels through the fermenting mass, which improves drainage and makes pressing easier at the end.
The stems also moderate temperature by physically separating the fruit, slowing the rate at which heat concentrates in the middle of a tank.
Against this, whole clusters occupy far more volume than destemmed fruit. A producer using them fills fewer litres of wine into the same tank.
Why proportion is the real decision
Very few producers work entirely with whole clusters or entirely without them across every batch and every vintage.
The common approach is a percentage, adjusted year by year according to how ripe the stems became and how much structure the fruit already has.
Reading that percentage on a technical sheet tells a drinker more about the vintage conditions than about the house style. It moves with the growing season.