Working with Refractometers

Refractometers measure soluble solids, the majority of which are sugars, giving you an indication of fruit ripeness. Most refractometers give readings in percent sucrose of degrees Brix. There are basically two types of refractometers; digital and analog. Both types of refractometers are available on the following page: Refractometers.

When choosing a refractometer it is important to consider the following points

• The scale should be easy to read.
• The demarcation line in the field of view is fine and distinct.
• It is easy to calibrate and doesn't become un-calibrated easily.

Refractometers are calibrated to be used at room temperature (68° F). Both the instrument and the fruit should be at room temperature. If it is not possible to take measurements are 68° F, then make sure that the unit is calibrated, if it is a non-ATC unit, at air temperature water.

To calibrate a refractometer:

• Place a few drops air temperature boiled or distilled water on the prism surface.
• Close the prism cover to ensure on air bubbles are trapped in the water. This step by the way is not necessary with the digital refractometers.
• Point the instrument toward a light source.
• The demarcation line between the light and dark portions should be at zero.
• If the line does not meet absolute zero it can be adjusted with a small screwdriver.

Using the refractometer:

• Select fruit the same way as described in the penetrometer section.
• Use enough juice so that the prism is completely covered and there are no air bubbles once the cover is closed.
• Measure the percent soluble solids as quickly as possible.
• When done taking measurement clean and dry the prism surface with a soft tissue BETWEEN each reading.
• When all measurements are completed for the day wash off prism surface with clean water and dry with a soft tissue.

Reprinted in part with permission from American Fruit Grower