For those readers who are not familiar with the article referenced, I talk about how it was likely a reader’s malolactic fermentation would pick back up again when the weather warmed up again in the spring (he wanted to over-winter his wine undergoing MLF outside in order to help it get cold stable). It sounds like you also decided to take advantage of a cold spell in an attempt to get your wine cold stable and are hoping your malolactic fermentation will reignite now that you’ve brought it inside . . . and you have a few questions about cold stability in addition. I like that you’ve added some ML bacteria nutrients (Opti-Malo Plus is a good choice), which are especially important as ML bacteria are notoriously fastidious, in that to fully function they have very specific nutritional needs.
A little background on cold stability: Grape juice naturally contains tartaric acid and potassium, which can combine to form a salt called potassium hydrogen tartrate. Also known as KHT, potassium bitartrate, or cream of tartar. This salt can exist in a soluble form, wherein it is completely dissolved into grape juice, must, or wine. As alcohol levels increase during fermentation, however, and especially if temperatures drop during storage, these salts can precipitate out as a solid. If you’ve ever seen clear, sandy crystals at the bottom of your wine bottles or carboys, it’s a good chance you’re looking at sediment that contains a lot of KHT crystals.
Winemakers often like to force this precipitation to happen before bottling so the crystals don’t turn up in the finished wine bottles. Commercial wineries almost always cold stabilize their white and rosé wines because they don’t want to scare consumers by thinking there are glass shards in their bottles of wine. By getting the wine cold (and sometimes also adding additional solid KHT as “seed crystals”), we can encourage the formation and precipitation of KHT crystals during barrel or carboy aging. The resulting wine (as long as it’s racked or filtered off the crystals, see below) will be more “cold stable”, i.e., it can be subjected to cold temperatures in the future and fewer (or no) crystals will emerge. There are different ways of assessing a wine’s cold stability and the whole topic of the tests, their interpretations, and how to get a wine cold stable has been covered elsewhere in the magazine (and would take way too much space and time to address fully here). So, I’ll just get right to your specific questions.
Should I have racked the wine?
• After a wine is chilled down and KHT crystals have precipitated out, it’s important to either rack or filter the wine off of them so that they don’t re-dissolve into the wine after it warms up. A wine is only stable for the temperature it was filtered or racked at, i.e., if your wines outside got down to 32 °F (0 °C) but then when they came back into your garage they warmed up to 45 °F (7 °C) before you got around to separating the wine from the solids, the wine is only considered “cold stable” to 45 °F (7 °C), not 32 °F (0 °C).
Should I be concerned that some of the tartrate crystals that have fallen out will be mixed back into the wine or will they remain as crystals never to dissolve back into the solution/wine again?
• See earlier for some nuance. If the wine warms up there’s a good chance some of the KHT crystals will re-dissolve into the wine as potassium and tartaric acid, making those same components available for precipitation in the future. For this reason, it’s important to either rack or filter off of the precipitate.
How concerned should I be about subjecting the MLF bacteria to atmosphere? I can purge the carboys with CO2 if you think that would help.
• Protecting your wine from air (a mix of mostly nitrogen and oxygen) is always a good thing, even when it’s going through MLF. The small amount of carbon dioxide gas produced by ML bacteria isn’t enough to completely protect the wine so it’s a good idea to supplement here. Similarly, even though the wine is technically fermenting, it’s a
good idea to maintain it in as topped up of a situation as you can.
So, to conclude, that’s great you’ve pushed your wine closer to being cold stable, but it is important to separate the precipitated potassium bitartrate crystals from the wine by racking or filtration, to keep oxygen excluded, and to keep containers topped up.
An alternative to traditional cold stability, where we chill wine down, seed it with KHT crystals, then rack or filter, is to use carboxymethylcellulose (also abbreviated to “CMC”). Solutions are available through most commercial winemaking supply houses. CMC is clear, aromatically and flavor-neutral, and basically acts as a physical deterrent to crystal formation. Essentially, it disrupts the crystallization process and prevents KHT crystals from happening in the first place. This is a rather newer way of aiding cold stabilization and only works 100% of the time if one’s wine is not “too unstable.” To determine if your wine is a candidate for CMC, send a sample to ETS Labs in St. Helena, California for a test called the “Degree of Instability —Tartrates” or “DIT.” In the panel of results you’ll get back, if the “rate of electrodialysis” is <24%, then your wine is a candidate for CMC addition, i.e., it’s stable enough and the CMC will push it over the edge and make it stable. CMC (a common product is “Celstab,” sold by Laffort USA) is added at a rate of 1 mL per liter wine, right after final filtration and just before bottling.