The phenomenon of light being refracted (bent) by water has been known for a very long time. Some 150 years ago a certain Dr A. F. W. Brix discovered that if sugar is added to water the light is bent to a greater extent. Furthermore he was able to show that the relationship between the sugar content and the degree of light bending was so close that he was able to produce a set of tables which gave an instant reading to the relative density and therefore the percentage sugar in the solution. The modern comprehensive table used by the juice industry was drawn up about 60 years ago. The strength of concentration is measured using a refractometer with the units °Brix.
Fruit acids refract light in the opposite direction to sugars consequently if the juice contains high levels of acids the refractometer may indicate a lower quantity of dissolved sugars than are actually in the juice. It may sometimes be necessary, therefore, for the °Brix of a juice to be “acid corrected”.
The strength of fruit juices will vary with the variety, season, geographical area and many other variable factors. Aprotocol has been drawn up where a “norm” is accepted in order for the fruit juice industry to work to a standard. One such Code of Practice has been produced by the Association of the Industry of Juices and Nectars from Fruits and Vegetables of the European Union (A.I.J.N).