Brix: How Do We Measure It?
Optical refractometer

Now we have a definition of brix: the weight of dissolved sugar expressed as a percentage of the weight of the entire solution.  The next question is how we measure brix.

The Refractometer

As its name suggests, a refractometer operates on the principle of refraction.  When a beam of light passes through a liquid solution, it bends, or refracts.  The more solids (e.g., sugars) that are suspended in the solution, the more the light refracts.  A refractometer is simply a device used to measure refraction.

Two types of refractometers are commonly sold:

  • Analog or optical.
  • Digital.

An analog refractometer uses a prism and an external light source to operate.  A few drops of solution are placed on the prism, the refractometer is held toward a light, and the results are read on a scale.

A digital refractometer works on the same principle, but shines its own light on the prism from an LED.  A sensor takes the measurement, a computer calculates the results, and a screen displays the brix reading.

Note that refractometers specifically sold for testing honey are made to display the inverse of a brix reading—that is, they display the moisture content of the sample and not the percentage of dissolved solids.

Shortcomings of the Refractometer

The reading displayed on the refractometer is actually not a true measurement of the sugar dissolved in the substance being tested.  Any dissolved solids can cause light to refract; therefore all dissolved solids are included in the refractometer results.  This includes minerals, proteins, and carbohydrates other than sugar, as well as other non-nutrient solids.  Adding fertilizer to a glass of water, for instance, will change the brix reading according to the refractometer.

Of course, this may still help us reach our goals in measuring brix in the first place.  After all, if we are measuring the brix of fruits and vegetables, whether from the store or our own backyard garden, we are using brix as a gauge of overall nutrient content.  If there are more minerals in the produce, so much the better.

However, the main disadvantage of using the refractometer is that it cannot tell us exactly what is changing the reading.  The only way to know for sure what sugars, minerals, and other solids are in our food or forage is to do a complete nutrient analysis—feasible in a livestock business, but a little too costly for everyday kitchen use.

Other factors that may influence refractometer results include:

  • Sample preparation.
  • Sample settling.
  • Plant part tested.
  • Ambient temperature.

Digital refractometers are programmed to compensate for some of these variables (particularly temperature).  To ensure the most accurate results, however, try to test the same part of the plant at the same time of day in every test.  Testing in similar weather conditions is also preferable.

Next week: How do we use brix?