Ask Wine Wizard

Is it possible for a plastic fermenter to impact the effectiveness of isinglass as a fining agent?



Dear Wine Wizard,

I used isinglass as a clarifier in my Viognier. I have it in a 55-gallon plastic food-grade barrel. It started to settle out some of the particles but never brought the wine to clear. I then transferred five gallons of it to a glass carboy and within one week it was as clear as any commercial wine I’ve seen. Could it be possible that there is some kind of electric charge in the plastic barrel disrupting the reaction that the isinglass should have with the proteins in the wine? The wine has a wonderful nose and tastes great. No spoilage of any type was ever present.

Jerry Dolley
Auburn, California

Wine Wizard replies: Interesting question you have. We’ve all heard the old warning about never filling up a spare gas can (even if it is plastic and not metal) on the bed liner of a truck as static electricity could possibly spark a fire. If any readers out there work in clean rooms (common in computer or electronics manufacturing plants and engineering labs) you know what a problem static electricity can be around plastics. Charges can build up and have no where to go because plastic is not conductive. This can lead to a spark the moment it comes into contact with anything remotely conductive. Want a more simple example? Shuffle your sock-clad feet across the carpet in a dry room, then touch a light socket plate and zowie! You’re probably in for a shock. Because of this strong connection between plastics and electricity you certainly do have good reason to wonder if your plastic drum might be causing a charge differentiation.

Isinglass (a very pure protein isolated from the swim bladder of fish) is one of the most effective fining agents available to home and commercial winemakers. All protein fining agents have selective affinity for wine polyphenols and, when added to wine, will bond with these high-molecular-weight phenolic compounds and precipitate to the bottom of the container. Protein molecules use the strength of a hydrogen bond (here’s the electric charge part) to latch on to the hydroxyl groups of the phenolic compounds. Related is the concept of the isoelectric point of proteins. The isoelectric point is an inherent characteristic of all proteins and is the point at which the distribution of positive and negative charges are equal. Proteins dissolved in wine are least soluble at their isoelectric points and the pH of many proteins is close to wine pH (3.0–3.7) – i.e. most proteins are not that soluble in wine and will not dissolve easily in it. Temperature and alcohol concentration are other main factors, besides pH, that will determine protein solubility.

The primary sensory effects of protein fining are a decrease of perceived astringency due to the fact that polymers (molecules of condensed, phenolic compounds that have a higher molecular weight and are perceived as more astringent) are usually pulled out preferentially by the protein. This leaves many phenolic monomers (single-phenolic molecules with small molecular weight) behind. Secondary effects of protein fining can be enhanced clarification and settling of wines, especially with white wines. Viognier is often a very tannic grape variety (as white grapes go, reds are naturally higher, of course) and isinglass fining is relatively common practice in its production.

Back to the situation and your question: However attractive (pun definitely intended) the electrical charge theory might be, I’m not quite sure it explains the difference in the clarification of your racked and non-racked Viogniers. Water (and hence, wine) is ultra-conductive and any static charge that might have built up in the plastic container would have been discharged and negated when you filled it with wine.

My guess is that it is the difference between the two lots (one being racked, the other not) that is the cause of the settling seen in the racked wine. You probably added a little oxygen into the wine (which is known as a catalyst for a multitude of chemical reactions) and a condensation followed by subsequent settling of proteins in the wine may have resulted.

Another question to ask is: Did you move the racked carboy to another part of the winery, house or garage? If so, you may have changed the temperature of the wine. Even slight changes in temperature can force proteins out of solution causing them to condense into large molecules and settle out as visible precipitate. Proteins are less soluble at higher temperatures and even a few degrees here or there can make a difference.

It’s common for any wine to throw precipitates with temperature changes or changes in levels of oxygen. Wine is a weird, wonderful and often maddening creation.

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