Crystals, Tartrate or Tartaric Acid
Tartrate crystals and their effect on wine
If you see yellowish white crystals forming in your finished and bottled wine, that are slightly gritty
but can be crushed easily with your fingers, they are quite likely to be tartrate, or tartaric acid,
crystals coming out of solution. You can tell immediately whether they are tartrates or sugar
crystals by collecting and tasting some of them. Sugar will taste sweet, while tartrates will taste
neutral or faintly tart, and they’ll also be crunchy, since they don’t dissolve in saliva.
The first thing to realize is that these crystals are not harmful to your wine – in fact, all wines
contain the compounds that form them. And surprising as it may be, tartrate precipitation actually
has a beneficial effect on wine quality, even if it looks untidy. When the crystals drop out, the wine
becomes slightly smoother and crisper tasting. The pH also drops, which increases the wine’s
resistance to oxidation, browning and spoilage, and the fruitiness is enhanced.
The only negative change is the physical appearance of the crystals themselves, which is easy to
associate with deficient quality. So you can take steps to either try to reduce crystal formation or to
filter them out. For example, storing your bottles at room temperature should prevent formation
altogether. Or if you do have a bottle that contains crystals, decant it prior to serving, so the
crystals remain in the decanter.
How tartrate crystals form
These crystals form from tartaric acid, the predominant fruit acid in grapes, by combining with a
potassium source. That source is usually nitrogen that the grapes picked up from the soil in which
they grew. When these compounds combine, they form a crystal of potassium bitartrate, which may
become large enough to see with the naked eye, and fall out of suspension to the bottom of the
bottle.
All wines contain these compounds, but most never form crystals. However, if there’s an imbalance
of tartaric acid, and the finished wine is stored at cooler than room temperature, that’s when
crystals may form. And one of the problems for all winemakers is that you can’t accurately predict
which wines will develop tartrate crystals and which won’t.
In recent years, wine made from Winexpert kits has started experiencing the same crystal formation
that can affect commercial wines. The reason actually stems from a very good thing: we’ve
consistently increased the amount and quality of fresh juice we’ve been adding to our most
premium kits. This has taken the kits to the upper limit of microbiological stability and shelf-life, and
has translated into the potential for more tartrate crystals in the finished product.
What Winexpert does about tartrate crystals
Winexpert does take steps that prevent much of the crystal formation, by cold stabilising all of our
raw materials at temperatures of 2C/36F, which produces the maximum density of water before ice
forms. This speeds up tartrate drop-out, so that after three months, most crystals are left behind in
our tanks. At that point, we start using the clear juice in the kits.
Yet even this doesn’t completely remove the potential for crystal formation. Some tartrates simply
don’t become liberated until after fermentation, even when the juices we purchase have been put
through an efficient tartrate bioreactor. This is because high levels of sugar block crystal growth. If
you have juice made from grapes with a very high brix (that is, sugar by weight), this will slow
tartrate precipitation, even when the product is held below freezing for a few months. Once the
sugars are converted by the yeast to achieve the desired alcohol content in the wine, tartrate
crystals can form again, especially if the wine is exposed to cold conditions. So the more top-quality
fresh juice a kit contains, the more potential there is for tartrate crystals.
What to do – and not do - about tartrate crystals
The first, best way to avoid having any crystals precipitate is to make the kits precisely to the
instructions. Changes in volume, timing and handling can alter the eventual profile of the kit.
Secondly, since the tartrate crystals can only come out during low temperature storage (below
10C/50F), try to ensure that your storage cellar doesn’t get colder than that, throughout the year.
And don't store wine in the refrigerator. On the other hand, if you have a large enough, reliably cold
refrigerator, you might hold your carboy there for a month to mimic the professional cold-stabilising
process, allowing crystals to fall out of suspension so you can bottle the cleared wine afterward.
One thing that is sometimes suggested is the use of a product called metatartaric acid. However, we
would caution against using it. It does prevent tartrate dropout, but its effect diminishes over time.
And once its effectiveness ceases, it triggers a cascade reaction, dropping all of the tartrates out of
your wine at once, within a 24 hour period. Unless you plan to drink your wine fairly soon, or you
are fond of the ‘snow globe’ look in your bottles, it’s better not to use this product.
Whether you take the above steps and prevent crystals, or end up with bottles that contain them,
there is one final sure-fire way to deal successfully with them. You can simply ignore them, and
decant every bottle for a while before you serve the wine, leaving the crystals behind in the
decanter.