Reading soluble solids with a refractometer
Updated
What it does
A refractometer reads dissolved solids directly rather than inferring them from heat, which is why it is the instrument the standard is effectively written around. It answers the one question that decides what the product may be called.
It measures how much light bends passing through a drop, which varies with dissolved solids. For a sugar solution that reading is reported as Brix, and for these products Brix and soluble solids are close enough that the instrument answers the regulatory question directly.
The steps
The reading takes seconds and the two things that spoil it are both about temperature: a hot sample and a warm instrument read differently from a settled one, and neither tells you it is doing so.
You need
- A refractometer reading to at least 85 Brix
- Its calibration standard, or distilled water
- A clean cloth
- Calibrate before the season, on a bench at room temperature, with the standard that came with it.
- Take a small sample and let it cool to the instrument's working temperature. A hot sample reads low.
- Place a drop on the prism, close the plate and read.
- Clean the prism between readings.
Why calibration is not optional
An uncalibrated instrument gives a confident, precise, wrong number, which is worse than no number because you act on it. Calibration takes a drop of distilled water and a minute, at the start of each production day. Write the calibration in the batch log beside the readings it produced, so a reading you later doubt can be traced to the state the instrument was in.
An uncalibrated refractometer does not fail visibly. It gives a clean, confident, precise reading that is wrong, and it will be acted on because it looks like data. That is worse than not owning one.
When it is worth buying
At the point you are selling, because that is the point the 65 percent figure stops being about texture and becomes about whether the label is accurate. Below that the three published tests are enough.