This section is from the book "Beverages And Their Adulteration Origin, Composition, Manufacture, Natural, Artificial, Fermented, Distilled, Alkaloidal And Fruit Juices", by Harvey W. Wiley. Also available from Amazon: Beverages And Their Adulteration.
The pomace which is left from the pressing of grapes is called "marc." In many instances the marc is fermented and distilled, forming a pomace brandy, or as it is called in French "eau-de-vie de marc." This brandy has no resemblance to the true Cognac in flavor, taste, or other properties, except the common property of alcohol. It is extremely strong and acrid in taste and can hardly be swallowed by those who are not used to its comsumption. It contains a very high percentage of aldehydes, ethers and acids. Nevertheless, in spite of its somewhat revolting character it is consumed in considerable quantities by many people. In Burgundy the brandy which is made from the marc is very extensively used and is highly thought of by those accustomed to its use.
From the seeds of the grapes an oil is made which is devoted to many industrial uses, but there is no industry of this kind in the Charente.
Marcs are also made use of for the extraction of essences with which to flavor neutral spirit in imitation of brandy. It is needless to say that such an imitation is but a poor substitute for the genuine article.
The residues which are left after the distillation of brandy are called in French "vinasses." The residue of the distillation is still largely liquid, and it consists of all those parts of the wine, including some water, which are not volatile at the temperatures at which distillation takes place. The vinasses contain practically all of the tartaric, malic and succinic acid, and gums which are found in the original wine. Also the whole of the tartrate of potash, the bitartrate of lime, alumina, phosphates of all kinds, and any mineral acids that may have been in the wine or produced by the use of sulphur in its manufacture, notably sulphuric acid. In addition to these, the nitrogenous matters of the wine are also found in the vinasses, as well as those of a pectic character and the tannin derived from the wine, and the glycerin produced during fermentation.