Partial or complete removal of grape stalks prior to fermentation, usually by machine. Results in reduced tannins, making for softer, earlier maturing, fruitier red wines. Always done for white wines. Some stems may be left in to keep the ‘marc’ loose during pressing.
The long-held belief that all wine should be aged for long periods hearkens back to when winegrapes were pressed in whole bunches -- stems, seeds and all. The classic image of Oporto’s “black feet” stomping grapes doesn’t show that the stems are being stomped just as aggressively as the grapes. And the same with primitive pressing technology: the stems were included with the grape mash. Often the result was wines that had very high tannin content. In fact, many old style wines were so tannic that they were hard to tolerate until some of the tannins had softened from cellar ageing -- possibly for many years.
A fairly modern improvement in grape processing is the crusher/destemmer. This machine crushes the grape bunches to release the juice, and then strips away the stems, leaving fermentable juice that is relatively stem free. As a red wine macerates and ferments, it extracts tannin from the stems and skins, and from the pits to some degree (although these too are often removed). When the stems are removed before fermentation, the wines emerge less tannic, softer, and earlier maturing. A classic example is Brunello. Traditionally these wines took 10 to 20 years to soften to drinkability, but with more modern techniques -- including égrappage -- the wines can be ready to open well before their tenth birthday. (I have had Brunello as young as 5 years that was perfectly balanced and drinkable.)
Égrappage gives the winemaker another area of control over the wine: Remove all the stems for an early-drinking, fruit forward wine; leave in a small percentage of stems for added structure and longevity; or maximize stem content to create a traditional cellar monster.