Bench methods
Cold stability, by freezer or conductivity
Whether the wine will drop tartrate crystals when chilled. They're harmless, but customers mistake them for glass. You either chill a sample and look at it, or seed it and watch the readings, and the verdict is what you see. Which test to run depends on the equipment you have.
These are the standard bench procedures in plain words, not a copy of any one published protocol. If your reagent kit comes with its own procedure, follow the kit. It was written for the normality in that bottle. The end of each method explains which of its figures come from the chemistry and which are standard practice.
Wear eye protection and gloves for every one of these methods. Sodium hydroxide is caustic, iodine stains and irritates, and the acids and peroxide burn. The chromatography solvent is flammable, so use it in a hood, not on an open bench. Each reagent's safety data sheet is the authority on handling and storage. These steps don't replace it.
- Produces
- A verdict, stable or unstable, recorded as a stability trial on the lot along with how the sample was held.
- Takes
- The freezer test: five minutes, then three days cold. Conductivity: half an hour with a meter.
Which one to run
| Freezer test | Conductivity (mini-contact) | |
|---|---|---|
| Answers | Does it drop crystals when held cold? | Does seeded tartrate keep growing? |
| Kit | A freezer and a clear bottle | A conductivity meter, a chilled bath, KHT seed |
| Takes | Days | Half an hour |
| Result | Crystals seen or not | A percentage drop in conductivity |
| How strict | Very strict, colder than any shop | Closer to what a bottle will face |
Most cellars have what they need for the freezer test, and it's harsher than real conditions on purpose. A wine that passes won't drop crystals anywhere it's sold. A wine that fails may still be fine on a shelf, and that's where the conductivity test is worth its meter.
What you need
Freezer test
- A clear bottle of the wine, filtered bright and sealed
- A freezer that holds about −4 °C (25 °F): a household one set warmer, checked with a thermometer
- A strong light and a dark background to check the bottom of the bottle
Conductivity
- A conductivity meter that reads in µS/cm
- A bath that holds the sample at 0 °C (32 °F), stirred
- Finely ground potassium bitartrate to seed with, about four grams per liter
What this test is for
This is a TEST, not a treatment. Chilling a tank for two weeks, seeding it, and dosing CMC or metatartaric are cellar work done to the wine, recorded with finishing. This test tells you whether the wine needs any of them.
The freezer test
Fill a clear bottle with bright, filtered wine, seal it, and put it in the freezer at about −4 °C (25 °F).
Filter it first, because tartrate crystals grow on anything suspended in the wine. A hazy sample can throw crystals a clear one wouldn't, and then you'd be testing the haze, not the wine.
Hold it for three days. Enter the temperature and the hours on the trial.
Seventy-two hours is the usual hold. Some wineries run six days at 0 °C instead. Run it the same way every time and record how, so you can compare results.
Let it warm slowly to room temperature, then hold the bottom of the bottle up to a strong light against a dark card.
Crystals that formed in the cold can dissolve again as the wine warms, so look before it's fully warm and again after. Anything glittering at the bottom means the wine dropped crystals.
Record the verdict, and describe what you saw on the trial: a few crystals, a carpet of them, or nothing.
Conductivity
Chill the sample to 0 °C (32 °F) in a stirred bath and read its conductivity.
Add finely ground potassium bitartrate, about four grams per liter, keep stirring, and read the conductivity at intervals for twenty to thirty minutes.
Seeding gives the wine's own tartrate a surface to grow on. If the wine is supersaturated, potassium and bitartrate come out of solution onto the seed, and the conductivity falls. If the wine is stable, the reading hardly changes.
Work out the drop as a percentage of the starting reading, and record the verdict.
A drop of about 5% or more is commonly read as unstable, and under about 3% as stable. In between is a judgment call. Like the freezer test's three days, this is common practice, and the verdict is your call.
Where it goes wrong
Passing the freezer test is a safe sign. Failing it may not matter.
The freezer is colder than any shop, so a wine that passes is safe anywhere. A wine that fails has faced a harder test than any bottle will, and the conductivity test is better for judging whether that failure matters.
Blending, acid additions and pH changes mean testing again.
Tartrate stability is a balance between potassium, tartaric acid, pH and alcohol, and changing any of them changes it. That's why each verdict is dated. A stable verdict on a wine that has since been blended describes a wine that no longer exists.
Calcium tartrate is a different crystal, and these tests barely detect it.
It forms slowly, sometimes months after bottling, and neither the freezer nor a bitartrate seed brings it on. A wine high in calcium, from fining or from deacidifying with calcium carbonate, can pass both tests and still throw a deposit. That's something to know about the wine, not a failure of the test.
Like the heat test, everything here is convention, not chemistry. The hold, the seed rate and the percentages a conductivity drop is judged against are common practice found across winery lab manuals, not figures from any regulation or tech sheet. Fermata records the verdict you reach using your own. Nothing here comes from an equivalent weight, so there's no factor to print.