You’ve got a case of the common “tartrate fall-outs.” The “fine sand” sediment you’re seeing in your bottled Concord wine is probably small tartrate crystals — or solidified tartaric acid. I’m sure you’re wondering how, when you’ve so assiduously clarified your wine, you could wind up with sand in the bottle later. I want to say two things to start out with: 1) it happens to the best winemakers in the world (you know, the folks who get $150 for a bottle of their wine) and 2) it’s entirely natural and to be expected, so you don’t have to worry about trying to correct it if it doesn’t bother you too much.
Concord grapes — along with all other grapes — are high in tartaric acid, a natural fruit acid present in the grapes as they grow. In finished wines, tartaric acid helps your wine maintain its “pucker” and the refreshing “zing” that so many of us enjoy in red and white wines alike. Tartaric acid is often added by winemakers before a wine is fermented because sometimes the grapes (or tree fruit or berries) used to make other types of wines don’t have enough of their own natural acid.
Your finished Concord wine is full of natural, dissolved tartaric acid. The acid is part of the wine, much like sugar crystals that are dissolved in a cup of coffee. However, these tartaric acid crystals don’t always like to stay nice, dissolved and — most importantly — invisible. Temperature changes, additions or movements like bottling and filtering can change the delicate dynamics of a wine so that some of the dissolved acid will actually crystallize into a solid. Once the particles become large and heavy enough, they will fall to the bottom of the bottle where they will look like find sand, or if there is a lot of tartaric acid and it formed especially large crystals, shards of broken glass.
Temperature changes, especially cooling (like putting your leftover wine into the fridge overnight), seem to be the main culprit when the dissolved tartaric acid decides to fall out of solution. A technique many wineries employ, and one that you can try too, is “cold stabilization.” This consists of chilling your wine down (fridge temperature is fine) for a few days and letting any tartrate crystals that will form fall to the bottom. You then rack the wine off the sediment and bottle it, or continue with whatever you were going to do next. Many home winemakers and brewers keep a separate “beer and wine fridge” in which they store their kegged brew and carboys that they’re cold stabilizing. If you don’t have the luxury of this kind of appointment, you could try to make space in your own fridge and use smaller containers, such as one-gallon jugs, for this step.
The “tartrate fall-out” problem is a pretty benign one that winemakers have been dealing with ever since wine was first made from grapes. The tartrate crystals are tasteless, odorless and will do you no harm if you happen to swallow them. You’ll also notice that a similar sediment, comprised mainly of these same tartrate crystals, may form when a wine ages over the years. Again, this is entirely natural and expected with all properly cellared wines, especially reds. Wine is a complex chemical soup and the sediment that forms over time is simply something that all winemakers — and wine drinkers — must take in stride.
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