By way of Real Clear Science, Sara Rennekamp explains how a product made of (mostly) grapes might still not be acceptable for dedicated vegans:
Is your wine vegan? It seems like an odd question: wine is made of grapes, grapes fall solidly under the “not an animal product” label, therefore it would seem that wine is a vegan-friendly beverage.
However, many people who adhere to a vegan diet refrain from consuming any food or drink that is processed using animal products as well as the animal products themselves. Unfortunately for vegans, some wines are processed using animal products.
The culprit: a process called fining.
The fining process, when it is used, allows winemakers to remove unwanted compounds from wine either before or after the juice is fermented. There are several reasons that a winemaker might elect to fine his wine, according to Jim Law, the owner and winemaker of Linden Vineyard in Virginia. One reason is to clarify the wine. Another reason might be to fine-tune the taste or correct an imperfection. There are other reasons, but most fining is done to address these two issues.
But before getting into the whys of fining, it’s helpful to know what the fining process does.
Freshly pressed grape juice contains a lot more than just juice. As the grapes are pressed bits of seeds, stems and skin can all make their way through the press and into the liquid wine-to-be. These add-ons can form unsightly sediment in the wine and produce an unwanted flavor to the wine, oftentimes bitterness or astringency.
Some wine makers fine their wine to get the interloping roughage out of the wine before it ends up in your glass. Wines can also be filtered to remove particles and some unwanted sediment, but only the fining process can remove imperfections like unwanted compounds that are too small to be caught by a filter, such as tannins and other phenols, and proteins.
