While protein is considered a good thing regarding nutrition, it is not desirable in wine as it can solidify with changes of temperature, particularly in a wine that has been allowed to get too warm (such as when left in a warm car too long), and then chilled down for consumption. This causes either hazy wine or precipitates in the bottle. While not harmful, this is aesthetically unpleasant for the consumer. Protein is a component of the chemical make-up of wine and is present in varying amounts depending upon the variety of grape and where it is grown (terroir). In my experience, some varieties of grapes that are grown in NC show little protein while wine from the same grape grown in PA will need to be treated. So obviously there is a relationship with terroir. Protein and its presence is still a subject of study in the field of wine chemistry and it does not seem to be fully understood yet.
While some people joke about it, the presence of protein does not mean that small critters were not quick enough to avoid the crusher at harvest. So we won’t consider that possibility.
Special Tip: I always test for protein before I start cold stabilizing the wine. That way, should I need to add bentonite (see below), I can add it when I start cold stabilization and both the tartrate crystals and the bentonite precipitate out together, forming more compact sediment. In addition, you eliminate handling the wine twice and risking some damage to it by additional oxygen getting into it.
Protein or heat stability testing is a little trickier than cold stability. First you do an initial test to see if the wine has the protein that will make it unstable. Not all wines do, and in the case of protein stability, we are only talking about white wines. This does not seem to be an issue with red wines. Again, a 100 ml. sample is lab filtered and put into a clear glass bottle. The bottle is then placed in some sort of heating chamber to bring the temperature up to at least 120 degrees, and hold it there for a two to three hours.
Many labs have special (also read expensive) little ovens or incubators for this task. You can make your own by cutting a hole in the top of a small cooler and putting a bare light bulb with a cord that can be plugged into an electrical outlet. The light bulb will create enough heat inside to make a very nice incubator. Since the light bulb itself can get fairly hot, I would not recommend using a Styrofoam cooler.
After heating the wine as stated above, the bottle is then removed and placed in a cool location. I always let it set overnight or even a couple days, to see if protein creates a haze or precipitates out. If there is a haze in the wine or any slight precipitation of something that looks just a bit fluffy, you have protein. Sometimes the precipitate is barely visible. Shine a flashlight into the bottle and give it a gentle shake. If even a little poof of something rises from the bottom, you have protein that must be removed.
If protein is present, the product to use is bentonite, which is a montmorillonite (your new word for the day) clay with great properties for removing protein from aqueous solutions. However, each wine is different, and it is usually a good idea to do bench trials to find out how much bentonite will take out the protein in a particular wine. As an aside, I don’t know the origin of the term “bench trials” as there is no bench involved so that has always made me ponder a bit. Anyway, what I am talking about here is taking several samples of the wine, each the same volume, and putting increasing amounts of bentonite into them, mixing thoroughly, then letting the bentonite settle out. Draw off the clearing wine, lab filter it, and incubate the samples in the same manner stated above. The one that has no protein using the least amount of bentonite is the one you use for your calculations regarding how much bentonite to put into the whole batch of wine. Generally, this is figured in pounds per 1000 gallons of wine. So, if you have say, 750 gallons of wine and the findings show that you need 3 pounds of bentonite per 1000 gallons, the calculation is pretty easy. 750 divided by 1000 equals .75 times 3 pounds equals 2.25 pounds.