Bench methods
Malic acid, by chromatography
Whether malolactic fermentation has finished, and, separately, how far along it is. Those are two different questions, and the test almost every cellar has answers only the first. A paper plate shows whether malic acid is there, but not how much.
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 result, malic present or gone. Record it under "Malic, by plate" on the lot's lab panel. The panel's Malic box takes a figure in g/L, which comes from a strip or an assay.
- Takes
- Chromatography: ten minutes of work, then several hours of waiting. A strip: two minutes.
Which one to run
| Paper chromatography | Reflectometric strip | Enzymatic assay | |
|---|---|---|---|
| Answers | Is malic still there? | About how much | How much |
| Resolution | Present or absent | Tenths | Hundredths |
| Kit | A jar, paper, solvent, standards | A meter and its strips | A kit, or a lab |
| Shows other acids | Yes, tartaric and lactic beside it | No | No |
| Good for | Confirming a full malolactic is done | Tracking one as it goes | A partial malolactic, and confirming an arrest |
The plate isn't second best. It answers the question cellars ask most, it shows the lactic acid appearing as the malic goes, and it costs almost nothing to run on eight lots at once. What it can't give you is a number, and two things in Fermata need one: a partial malolactic and confirming an arrest.
What you need
Paper chromatography
- Chromatography paper, and a jar deep enough to stand it in
- The solvent: butanol, formic acid and water, with bromocresol green indicator
- Known malic and lactic standards
- Capillary tubes, a pencil, and a hood to dry it in
For a number, when the plate isn't enough
- Reflectometric malic strips and their meter, read against the range on the insert
- Or an enzymatic L-malic kit, or a send-out to a lab
What a plate can and can't tell you
A plate tells you whether malic is present. For a PARTIAL malolactic, you need to know how low the malic has gone and how fast it's falling, so the lot's malolactic card draws its curve from figures, not plates. And an ARREST is confirmed by two readings that AGREE. Nothing shows directly that a wine has stopped, only that it hasn't moved, and that takes two figures a plate can't give.
Spot it
Draw a pencil origin line about 2.5 cm up from the bottom, and mark a spot for each wine and each standard.
Apply each wine in several small spots on the same mark, letting each one dry before the next.
One large wet spot spreads into a smear, and its acids overlap. Small repeated applications keep the spot tight while putting enough acid on the paper to see.
Spot known malic and lactic standards on the same paper. Every time.
How far a spot travels changes with the paper, the temperature and the age of the solvent. So you identify a spot by what it lines up WITH on the same plate. Without standards, you can only guess at a plate.
Run it
Stand the paper in about a centimeter of solvent, with the ORIGIN LINE ABOVE the surface.
This is the classic mistake, and it's easy to miss. Spots below the solvent dissolve into it and the plate comes out blank, which looks like a wine with no acids rather than a failed run.
Seal the jar and leave it until the solvent front is near the top. This usually takes several hours, often overnight.
Lift it out, mark the front, and dry it in a hood.
Read it
The paper turns blue-green and the acids show as yellow spots. From the bottom: tartaric, then malic, then lactic. If there's no malic spot level with the malic standard, the malolactic is finished. A lactic spot that wasn't there two weeks ago tells you the same thing from the other side.
Then record it
A clean plate means the malic is below what the test can detect, which is about the same as the 0.1 g/L Fermata treats as finished. That's why a clean plate counts as done. It doesn't mean zero malic, though, so don't enter 0 in the Malic box.
If a strip or an assay gave you a figure, record the figure. If you only ran a plate, record the RESULT: the lab panel has a choice for it (clear, or malic present) beside the figure. The malolactic card reads both. A clear plate counts as finished and a plate with a spot doesn't. If a plate and a figure disagree, the newer one counts, and the card tells you which it used. A partial malolactic and an arrest still need figures, because a plate can't give the number a target is measured against, or the two readings that confirm an arrest.
Where it goes wrong
A faint spot doesn't mean it's nearly done.
The test doesn't measure amounts, so how faint a spot is tells you nothing. A visible malic spot means malic is present, and that's all the plate tells you. A wine with malic and live bacteria in it can referment in the bottle, and reading a weak spot as almost finished is how that wine ends up bottled.
A clean plate doesn't mean zero malic.
It means the malic is below what paper chromatography can detect, which is roughly the level Fermata treats as finished. That's close enough to call a full malolactic done, but not close enough for the two jobs that need a real figure. A partial is judged on where the malic actually is, and an arrest on two readings that agree to a tenth. A spot can't tell you either.
Without standards on the same paper, you can't read the plate.
How far a spot travels changes with the paper, the temperature and the age of the solvent, so a spot on its own can't be identified. It's just a mark somewhere on the paper. Standards aren't optional here. They're the only way to know a spot is malic and not something else in the wine.
The origin line has to sit above the solvent.
Spots below the surface wash off into the jar and the paper comes out empty, which looks exactly like a wine that has finished. That's the dangerous direction to be wrong in, because a blank plate and a clean plate look the same, but only the clean one means the malolactic is done.
There are no figures to work out here. A plate is READ, not calculated, so nothing about it comes from the chemistry and nothing is calibrated either. When you need a number, it comes from a strip's own scale or an assay's standard curve, and Fermata doesn't work out either one. The solvent recipe, the origin height and the run are standard bench practice. The one figure quoted is the level Fermata treats as finished, the same one the malolactic card uses.