Brix is a measurement of the percentage of solids in the fresh sap or juice of plants. Solids include sucrose, fructose, vitamins, minerals, amino acids, proteins, hormones, and other solids. Dr. Carey Reams was the first to use Brix levels as a measurement of crop quality. He asserted that plants with high Brix values taste better, are more nutritious, and are resistant to insects and disease. A refractometer is used to obtain Brix readings from the sap of plants - the juices of ripe fruit or, if there are no ripe fruit, the leaves.
In this interview, Rex Harrill answers questions about Brix:
Refractometers are inexpensive, easy to use devices that can be used by home gardeners. Both digital and analog versions are available. Used, high quality analog refractometers can be purchased on ebay. One high quality brand is Atago. Refractometers are used in a number of different applications including medicine, wine making, home brewing, gemology, beekeeping, and marine aquarium maintenance in different ranges. A range of 0-32 is appropriate for measuring Brix in fruit and vegetables.
To use an analog refractometer, place a few drops of juice from ripe fruit or vegetables on the glass surface of the instrument. The Brix reading can be seen by looking through the device at a scale. If the fruit or vegetables aren't yet ripe, sap from the most recent mature leaves that have had full sunlight for at least 2 hours can be used. A mortar and pestle can be used to obtain sap from leaves or roots.