Issue 28: Refractometers

Issue 28
May/June – 1996
Story Title: Refractometers
Author: Steven Carruthers

A brief run-down on the operation, and uses, of refractometers
The refractometer is a device for refracting light through a prism, to measure the density of a liquid placed on the surface of the prism. Of course, this fact means nothing until one looks at the practical applications of this device. In the case of fruit and vegetable production, the density of the liquid being measured, represents the total sugars present in the fruit or leaf of the plant. A quantity of fluid is squeezed onto the instrument and then viewed through the eyepiece, where a reading on a scale is shown, this giving a percentage of total sugars present. ‘Total sugars’ are made up of fructose, sucrose and carbohydrates which have been manufactured by the plant during its growing cycle, so the higher the percentage, the healthier the plant can be assumed to be.

Refractometers gives a good indication of the success of the methods used by the grower. Of course, for the professional, this does not replace analysis done by more extensive equipment, but as a hand-held refractometer can be used on site, it is a useful gauge to the effectiveness of growing procedures, while one is waiting for laboratory results to be done.

A refractometer can also be used daily and, once a standard value has been established by the grower, effects such as changes to nutrient mixes, or the onset of impending disease, may be assessed quickly. It may be possible to make early corrections, while awaiting laboratory confirmation, or else it may show that no such changes are needed.

One interesting result I have noticed in the case of lettuce, is that on a hot, humid day, there is a substantial decrease in the percentage of sugars in the leaf, which reduces the shelf life of the product. Many large produce buyers use a benchtop version of the refractometer, to ascertain the quality of their suppliers, and even use this as a means of determining their contracts.