Refractometer: a device used to measure the bending of light through a solution

Brix: calibration of the bending of light through a solution to solid content of the solution if the solution contains 100% sucrose

Using a refractometer to check the solids is a quick way to get a good idea of where you are in the cooking process. The definition of Brix is the amount of sugar dissolved, and the refractometer is calibrated for a 100% sugar solution.  The dissolved solids is directly proportional to the “Brix” reading for a specific formula.  

A refractometer is a useful tool since it is much more precise to cook to a brix reading than a temperature or to cook for a certain amount of time.  Once you have all the solids dissolved in a mixture (in a certain formula), the brix will not change unless you evaporate water.  For example, when adding crystalline sugar, and all of it gets dissolved, that brix reading will stay the same.  Sugar is only soluble up to 67% and that is not stable for a candy, so the mass must be cooked, to evaporate the water and increase the solids (and increase the brix reading).  You always want to cook to a certain finished solids.  Cooking to the refractometer reading ensures you will end with a stable piece of candy.  After cooking gelatin and water, or color, flavor and acid can be added.  Make sure all additives are added above 150F for food safety reasons.  

After all ingredients are added, the refractometer should again be checked to make sure everything looks okay and to understand the deposit solids.  The deposit solids will govern the finished texture, as well as, how long it needs to “cure” or set.  Ideal deposit solids will depend on the type of product and the depositing equipment but depositing typically happens in the 70-80% solids range.