Measuring alcohol properly

A measurement with an unusual problem: the quantity being measured is small, the matrix is difficult, and the answer has legal consequences.

Why the easy methods do not work

A hydrometer measures density and infers alcohol from the change between two readings. In a kombucha that is simultaneously losing sugar, gaining acid and gaining carbon dioxide, density is moving for several reasons at once.

The numbers involved are also small: the difference between 0.4 and 0.6 percent is far inside the error of a brewing hydrometer.

An alcolyzer, a refractometer correction or a brewing calculator has the same problem. They were designed for beer at five percent, where a tenth of a percent does not decide anything.

What the methods are

For a legally meaningful figure at this level the answer is a laboratory method: gas chromatography, or enzymatic assay, or distillation followed by density, run by a laboratory that does it routinely.

It costs per sample, which is why producers test a sampling plan rather than every batch.

In-house methods have a place for process control, watching a trend, comparing batches, and they are not the number to rely on for a threshold decision.

Measure at the right points

At the end of fermentation, which tells you what the process delivered.

At bottling, which is the figure people record.

And after storage, which is the figure that matters and the one almost nobody takes: a sample held at the temperature the product will actually see, for the shelf life it will actually have, then tested.

That last measurement is the only evidence that your product stays under the line rather than starting under it.

What to record

Batch, date, method, laboratory, sample condition and result.

Plus the storage conditions of the sample, because a result from a refrigerated sample says nothing about a bottle on a warm shelf.

Keep it indefinitely. It is the evidence that the classification of your product was established rather than assumed.

References