Pomace (or in French marc) is the remaining solid material left after pressing grapes for juice. It is made up of the skins, pulp, seeds and stems of the fruit. The word Pomace stems from the Latin word ‘pomum’ (meaning fruit or fruit tree). Grape pomace has traditionally been used to produce pomace brandy (such as orujo, grappa, zivania or törkölypálinka) and also grape-seed oil. Today, it is mostly used as castle feed or natural fertilizer.
The Ancient Greeks and Romans used pomace to create a wine that was known as Piquette, it was a very simple wine served to slaves. After wine grapes had been pressed twice, the pomace was soaked in water, then allowed to fermented and pressed for a third time. The resulting liquid was mixed with more water to produce a rough, weak wine.

 

In the ‘Middle Ages’ pomace wine with a low alcohol content of around 3 - 4% was very common. Typically wines of the time were only partially fermented - resulting in a pomace that would have some unfermented residual sugars.
Pomace in winemaking differs, depending upon whether white wine or red wine is being produced. In red wine production, pomace is produced after a period of time the juice is poured off, leaving behind dark-red debris consisting of grape skins and stems. The resulting pomace is more alcoholic and tannic than pomace produced from white wine production.
In white wine production, grapes are quickly pressed after crushing to avoid skin contact with pomace as a by product of the pressing. The resulting debris is a pale, green-brown in colour and contains more residual sugars than it contains tannins and alcohol. It also has nitrogen, amino acids and other goodies that did not make it into the fermentation tank. This is the pomace normally used in brandy production.
During wine production pomace is produced in large quantities, with its disposal an important environmental decision. Some wineries use the material as fertilizer, while others sell it to Bio-Gas companies for renewable energy to produce methane gas that could be used to generate power. Oenocyanin a natural red dye and food-colouring agent, is produced from grape pomace. Tartrates (potassium bitartrate, 'cream of tartar') and grape polyphenols can also be manufactured from grape pomace.
Pomace, even when fermented, contains a staggering variety of chemical components. The skins mainly consist of cellulose, tartaric acid; trace amounts of other organic acids, unfermentable sugars, tannins, anthocyanins and some aromatic phenolic substances. The seeds, which remain largely intact during pressing and fermentation, are gold mines of bitter and astringent tannins. The seeds also contain large amounts of nutritious oil, which is popular with chefs due to its high cooking-smoke point. Stems contain tannins and if the pomace is pressed from a fermented wine, it will contain water and alcohol.
White or red, pomace also varies in moisture content depending on how hard it has been pressed. White pomace, often pressed gently to avoid extracting bitter compounds into a delicate white wine, can be juicy and sticky with sugar. On the other hand, red pomace often forms large clumps and can be quite dry. Anything added during the winemaking process - like enzymes, bentonite or yeast - will be present in the pressed pomace.
In Verona, Italy winemakers re-use skins by placing the pressings from red wine, into another fermentation tank to get more mileage from the grapes. By crushing fresh red must over already-pressed and fermented red pomace, it is possible to extract even more of the colours, tannins and - if the grapes were at all raisined and not completely fermented sugars. The most famous and delicious examples of this technique is the Italian ‘Ripasso’ wines where the sweet, raisined pomace from Amarone della Valpolicella is added to other red ferments of lighter style red wine. These wines, having been ‘re-passed’ over the sweet, flavourful Amarone skins, are richer and have more depth than they would have had on their own.