The ‘strength’ or concentration of Juices is assessed by the dissolved solids that are present, which for most fruit juices are the sugars.
Historically the Specific Gravity of pure sugar (sucrose) solutions was measured very accurately and tables of SG versus Sucrose content %w/w were drawn up. The Brix scale is one of a number using the SG/Sugar relationship and was later adopted by the Fruit Juice Industry as a means of measuring the ‘strength’ of juices from their Specific Gravity, with the results expressed as degrees Brix.
“Brix”, therefore, is the soluble sucrose equivalent solids %w/w giving a solution density/S.G. equal to that of a solution of pure sucrose.
Another property of solutions is that their Refractive Index (RI) increases with increasing dissolved solids. It is generally more convenient to measure the refractive index of a solution rather than its SG. So the RI of pure sucrose solutions was measured and tables established. The scale was then converted to Brix values and used in refractometers to give a direct readout.
So “Refractometric Solids” or RS is the soluble sucrose equivalent solids %w/w giving a solution refractive index equal to that of a solution of pure sucrose.
To calculate the w/w concentration of a juice, only the SG (or Relative Density) values of the single strength juice and its concentrate need be known. For pure sucrose, the Brix values relate precisely to the SG so the w/w concentration is simply the higher Brix divided by the lower Brix.
In the case of fruit juices, there are differing solutes present which have differing refractive index and density relationships compared to sucrose. When using a Refractometer the measured RS needs to be corrected to arrive at the appropriate density/S.G. value as read from standard sugar tables. In most cases, the major contributors to soluble solids after sugars are the acids and allowance is made for the acidity, expressed as anhydrous citric acid. The resulting value is known as Corrected Brix, which is used to determine the density or SG factor. Traditionally, the corrections were made by reference to published tables, but it is now possible to make the calculations directly simply from RS and Acidity values.
Once again the w/w degree of concentration is the higher Corrected Brix divided by the lower Corrected Brix.
Determination of the v/v degree of concentration is just the w/w value multiplied by the concentrate SG and divided by the starting juice SG.