Brix: Relative density scale used in sugar and winemaking industry, it indicates the percent of cane sugar (sucrose) by weight (grams per 100 milliliter of water) in a solution or juice of unfermented grapes in degrees Brix (°Bx). The most commonly used refractrometer scale for measuring solids dissolved in water, it corresponds directly to the refractive index scale. One °Bx equals one percent and, in winemaking, the alcohol concentration of the finished wine is estimated to be 0.55 times the °Bx of the grape juice. Named after the 19th century Austrian scientist Adolf Brix who invented a hydrometer that reads directly the percentage of sugar at a specified temperature.
Why it matters: Actually, the jury is still out on IF this one matters. Brix is most commonly used in the wine industry, but is growing in popularity with other farmers. Some growers claim that Brix is an indicator of nutrients, not just sugar, and so a high score equates to higher nutrient density. Thus, you might conceivably be able to find apples with a higher Brix score that had more nutrients compared to apples of the same variety that had a lower Brix.
What to look for: Very few products are marked, so you will probably have to speak directly with your grower and ask if they take any samples of their produce. Theoretically, the higher the Brix, the more nutrients in your tomato.