By Sara Rennekamp, Inside Science - 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 wine makers to remove unwanted compounds from wine either before or after the juice is fermented. There are several reasons that a wine maker might elect to fine his wine, according to Jim Law, the owner and wine maker 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.

To fine a wine, wine makers add a fining agent to either the freshly pressed juice – also called the "must" – or the fermented wine. These fining agents are usually made of one of four kinds of animal protein: egg whites, gelatin, casein – which is derived from milk, and isinglass – obtained from the swim bladder of fish.

"Don't ask me how they figured that one out," said Law, on when swim bladders were first used in the wine-making process.

Once upon a time, blood was also used as a fining agent, according to Law, but that is now illegal.

There are a few fining agents available that are not made of animal proteins. One type often used by home and professional wine makers is bentonite, a clay that forms from weathered volcanic ash. There are also synthetic fining agents available. But, when wine makers decide to fine their wine they will often use an animal protein.

When added to the must or the fermented wine, the agents will bond with the offending compounds and sink to the bottom of the liquid.  

Each of the different fining agents have a different effect on the wine and target different substances, so it is crucial that a winemaker know what issue or problem he or she wants to correct and use the fining agent that addresses that specific problem. In some cases, winemakers will run fining trials in which they use several fining agents on the same batch of wine to see which one produces the desired results. 

Fining practices vary from winery to winery and are used (or omitted) entirely at the discretion of the winemaker.

Law explained that the necessity of fining has diminished in the last 50 years as refrigeration and pressing technology improved.

"The pressing process used to be really hard on the grapes," Law explained, resulting in more leftovers from seeds, stems and skins, which usually meant more sediment and a higher possibility of funky flavors or hazy wine. Just imagine all the stuff left over in the must Lucy stomped out in "I Love Lucy." But, today's modern pressing technology is much gentler on the grapes, meaning fewer of the unwanted materials get through into the must and final product.

The grapes also benefit from modern refrigeration, explained Law. Must pressed from warm grapes under warm conditions tends to have more sediment.

Both of these advances mean that some winemakers forgo the fining process all together. Law said that he used to fine some of his wines, but hasn't done so in more than 10 years simply because he didn't think it was necessary and was happy to omit any steps he could in the wines' processing.

"My goal is to make the best wine I can that expresses the vineyard site." Some wineries still fine their wines and if you adhere to a vegan diet, cater to a vegan population or are simply curious, it can be difficult to find out which ones do and which don't. But, it's even rarer for vegan customers to request unfined wines for dietary reasons.

"They don't even think along those lines, they think that wine is vegan," said Phillip J. Heyser, sommelier at Elizabeth's Gone Raw, a raw food restaurant in Washington D.C. Heyser said that he gets customer requests or questions about vegan wine about once a month on average.

When he does get a request or question about vegan wine, Heyser does have a go-to. One of Heyser's favorite unfined wines is a pinot noir called Swine Wine from the Oregon vineyard EIEIO.

"I know, the name is kind of ironic," said Heyser.

However, EIEIO does not market their wine as vegan, and neither does Law. Since fining practices vary from vineyard to vineyard (and sometimes from vintage to vintage) the best way to find out if a wine is vegan or not is to ask the person who made it.