Bench methods
Residual sugar, by copper reduction
The sugar the yeast left behind, which decides whether a sealed bottle can start fermenting again. A hydrometer can't measure it, and it's the one bench titration whose arithmetic Fermata doesn't work out for you. Both are explained below.
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
- Residual sugar in g/L. It goes in the RS box on the lot's lab panel.
- Takes
- Clinitest: two minutes. Rebelein: about twenty minutes, most of it boiling.
Which one to run
| Clinitest | Rebelein | |
|---|---|---|
| Speed | Two minutes | Twenty minutes |
| Kit | Tablets and a test tube | Burette, hot plate, four reagents |
| Answers | Is it dry? | A number |
| Resolution | Steps on a printed chart | Under a gram per liter |
| Good for | Deciding whether a wine needs protecting from refermentation before bottling | A spec sheet, or a figure someone else will check |
Clinitest isn't a lesser version of the titration. It answers the question cellars ask most: is there enough sugar here to matter? Run it weekly, and run Rebelein when you need an actual number.
What you need
Clinitest: tablets and a color chart
- Clinitest tablets and their own color chart
- A test tube and a dropper
- Distilled water, to dilute a sweet wine
Rebelein: copper reduction, back-titrated
- The Rebelein reagents, and the factor printed with them
- Sodium thiosulfate, standardized
- Potassium iodide and acid
- 1% starch indicator
- A hot plate, a reflux condenser and a timer
First: why a hydrometer can't give you this
A hydrometer measures density, and in a finished wine density depends on alcohol as much as sugar. Ethanol is lighter than water, so a bone-dry wine reads well below 1.000, and a sweet one can too. The bench's gravity-to-sugar figure is accurate on a MUST, before there's any alcohol, and stops being accurate once the ferment starts. After that, you have to measure sugar chemically or you don't know it.
Clinitest
Bring the sample to room temperature, and dilute it if you think the wine is sweet.
Follow the tablet's own drop count exactly.
The two-drop and five-drop methods use DIFFERENT charts and cover different ranges. Reading five drops against the two-drop chart is the most common mistake.
Read the color at exactly the time the instructions give, not before and not after.
The reaction keeps going in the tube, so a color read late reads too sweet.
Rebelein
Pipette the sample into the flask with the copper reagent.
Boil for exactly the time the procedure gives.
The copper and the sugar don't react completely, so the figure is calibrated to THAT boiling time. Boiling a minute longer doesn't measure more thoroughly. It gives a different, sweeter reading.
Cool, add the iodide and the acid, and titrate the iodine that appears with thiosulfate to a starch endpoint.
Run a BLANK with no wine in it, exactly the same way, and take the difference.
This is a back-titration. It measures the copper the sugar did NOT use up, so more thiosulfate means LESS sugar, and you subtract the sample from the blank, not the other way around. Without a blank, there's no zero to measure from.
Work it out
USE THE FACTOR THAT CAME WITH YOUR KIT. This is the one bench method Fermata describes where that's true. Tartaric acid, acetic acid and sulfur dioxide each have an equivalent weight that comes from the periodic table, so their arithmetic is worked out and shown here. Copper reduction has no such number. The reaction doesn't follow clean proportions, so the link between thiosulfate and sugar is calibrated against a sugar standard, at set reagent strengths and a set boiling time. Use the factor printed with your reagents. Fermata doesn't print one, because any figure it gave would be right only for someone else's setup.
Then record it
Fermata stores residual sugar in g/L, alongside the acids. A wine under 4 g/L is usually called dry. That's also roughly where too little sugar is left for yeast to start fermenting again in a sealed bottle. The finishing step's refermentation check uses the same line.
That check currently uses the wine program's TARGET, the residual sugar someone planned to bottle at, not what's in the tank. Recording your measurement puts the real figure on the lot beside it. If the two disagree, that's worth knowing, so Fermata shows both instead of replacing one with the other.
Where it goes wrong
A dry wine doesn't read zero.
Pentoses, such as arabinose and xylose, react with copper just as glucose does, and wine yeast can't ferment them. So a bone-dry wine reads a gram or two by any copper method, and always will. An enzymatic glucose-and-fructose test reads lower, because it counts only what yeast can eat. This matters most right at the dry line, where you're deciding whether a wine needs protecting before bottling.
Cane sugar doesn't react with the copper until it inverts.
Sucrose isn't a reducing sugar, so a wine sweetened with it reads LOW at first. The reading climbs over the following weeks as the wine's acid breaks it down into glucose and fructose. A reading taken the day after sweetening won't match what's in the bottle later, so in this one case, waiting is part of the method.
Clinitest colors go up the chart and then back down.
Past the top of its range, the color passes through orange and back toward a shade lower on the chart, so a very sweet wine can read as moderately sweet. That's the dangerous direction, because it shows less sugar than there is. Dilute any wine you think might be sweet, run it again, and multiply by the dilution.
A hydrometer can't give you this number.
In a finished wine, density depends on alcohol as much as sugar, and the two push it in opposite directions, so no gravity reading can separate them. The bench's gravity-to-sugar figure is accurate only on a must. Reading it off a finished wine is how a wine with sugar in it gets called dry.
Unlike the acid and sulfur methods, the arithmetic here doesn't come straight from the chemistry. Copper reduction has no equivalent weight to work from, because the reaction doesn't go to completion and doesn't follow clean proportions. Instead, the link between titrant and sugar is EMPIRICAL: calibrated against a sugar standard at set reagent strengths and a set boiling time. So Fermata works out no factor for residual sugar and prints none. Use the one that came with your own reagents. The sample volumes, dilutions and drop counts are standard bench practice. If your kit comes with its own procedure, follow it. That matters more here than in any other bench method Fermata describes, because the factor that converts your titration comes with the kit.