Wine Diamonds: Is My Wine Flawed?
Tartrate crystals, how and why they form, and whether you can taste the difference
A couple of weeks ago, I took a roadtrip out to Mendocino to enjoy the cool, coastal air, a small respite from the warmer inland temperatures we’ve been experiencing in Northern California. On the way, I made a couple of pit stops to taste (responsibly) and acquire wine. At one particular winery, I recall the wine educator letting me know the wine, a 2023 Chardonnay, wasn’t fined, filtered, or cold stabilized. An idea popped into my head.
You may have noticed that some wines, especially higher acid whites, develop crystalline solids over time. These solids are mostly made up of potassium bitartrate (KC4H5O6), also known as tartrate crystals, a precipitate that forms when the tartaric acid in the wine reacts with potassium, typically at cooler temperatures. They’re also known as wine diamonds, marketed to consumers as an indication of high quality, evidence that the wines haven’t been over-manipulated or heavily filtered. They’re odorless and non-toxic and said not to have any impact on the flavor of the wine.
Many larger volume, bulk producers will use a process called cold stabilization to remove the tartrate crystals from the wine prior to bottling. During this process, wine is brought to near-freezing temperatures and held there for several days to promote the development of tartrate crystals, which are later filtered out before bottling. Cold-stabilized wines are therefore less likely to develop new tartrate crystals once the consumer has brought the wine home—reducing the likelihood of a perceived fault that could increase returns.
I wanted to put the claim to the test, so I bought two bottles of the same 2023 Chardonnay. When I returned home, I tucked one safely into my wine fridge, held at a relatively constant 55°F and the other bottle was placed into the lowest shelf of my refrigerator, which I had just set to 37°F, its lowest setting, for this experiment. After three weeks, the bottle in the refrigerator had developed tartrate crystals while the bottle in my wine fridge had not. I placed the bottle without tartrate crystals into the fridge for two hours so both bottles would be at the same starting temperature.
Next, I had a friend Coravin exactly 50g into 3 identical glasses, two pours from one bottle, and one pour from the other. Each glass was labeled with a 1” piece of opaque blue masking tape placed under the base, identifying the bottle it came from without my knowledge. I let the glasses sit for ~15 minutes (ambient temperature 70°F) until the surface of the liquid read 45°F, then began tasting.
The color of all three wines was identical to my eye. The nose more or less as well. All three wines showed lovely aromas of tart green apple, lemon, and crunchy yellow peaches, dried herbs, almost medicinal in nature, and slight flinty, reductive notes that blew off after a few minutes. The palate offered no significant differences. I’m not the best blind taster, but to my palate, all three wines were identical—citrus, tart stone fruit, and a beautiful spine of acidity that gives way to a beautiful saline finish.
I let the wines sit for another ten minutes and tried them again, slightly warmer. No difference, at least to me. It didn’t make sense. In one bottle, a precipitate had formed, removing some compounds from the wine—I thought surely I’d be able to tell the difference. In the end I guessed that the second glass was the odd one out, but it ended up being one of the two alike glasses, which I later found out was from the bottle with the tartrate crystals.
In practice, my nose and palate weren’t able to discern a meaningful difference between the two wines. I washed out the glasses and we repeated the experiment two more times. By chance, I did call the outlier glass on my third attempt, but that was pure luck. We repeated the experiment three more times with my friend doing the tasting. He came to the same conclusion I did: the wines smelled and tasted identical.
Baffled by this outcome, as any good engineer does, I went straight to the math.
We finished the bottle with the tartrate crystals being careful not to pour out any of them. Before the bottle was empty, I swirled the remaining wine to collect as many of the crystals into the liquid as possible and poured them into a coffee filter. After allowing the wine to drain and fully evaporate, I weighed the crystals at approximately 0.9g. This is an upper bound because evaporation won’t remove sugars and other nonvolatile compounds.
Potassium bitartrate (KC4H5O6) has a molecular weight of ~188.18g/mol while tartaric acid (C4H6O6) comes in around 150.09g/mol. 1g of tartaric acid will produce at most 1.25g of potassium bitartrate. 0.9g of crystals means 0.72g of tartaric acid precipitated out of the wine (750mL), or roughly 0.96g/L. A rough winemaking rule of thumb is that pH drops 0.1 units for every 1g/L increase in titratable or total acidity (TA), but the inverse isn’t true. Wine is what’s known as a buffered solution, which means that changes to TA, don’t necessarily cause a corresponding change in pH. In fact, the two measures aren’t related at all1.
Instead we have to look at the chemistry of cold stabilization2, which presents a counterintuitive concept. When the starting pH is above 3.65, the TA decreases and the pH increases. This is fairly intuitive. We’re precipitating out acid, so we would expect the TA to decrease and the overall solution to increase in pH. However, when the starting pH is below 3.65, not only will the TA decrease, but the pH also decreases. The specific chemistry involved is complex and outside the scope of this piece, however those that are curious should seek out Margalit’s Concepts in Wine Chemistry, Third Edition.
Since the experiment wine is listed at a pH of 3.3, it’s safe to assume that the precipitation of tartrate crystals slightly decreased both its pH and TA, so while chemically, the two wines were different, they were not different enough to be perceptible on the palate, at least not to mine.
The next time you open a bottle and see tartrate crystals, just remember that they’re non-toxic and the small amount of precipitation is unlikely to have substantially changed the wine’s TA or pH to the point of impacting what’s in your glass. So enjoy the wine with confidence and share what you learned about TA, pH, and cold stabilization with your fellow imbibers, and save yourself the trip back to the wine shop to return the bottle.
Margalit, Yair. Concepts in Wine Chemistry, Third Edition. 2012. pp 352-360