Still being a relative newbie in the world of wine, I have a lot to learn, and last week I learned a LOT. We were fortunate that our trip to California happened to occur during the harvest season, so we not only got to witness the "picking," but also some crushing, destemming, and other grape preparations.
One thing I learned about was how the vintners know when the grapes are ready to be picked. There is an instrument called a refractometer that measures the sugar content in the grapes. Below is our tour guide, Leo, from Rodney Strong Winery, holding a refractometer.
Leo took a grape off the vine, and squeezed a bit of the juice from the grape onto the lens of the refractometer. He passed the instrument around and we all got to hold it up to the sundlight and look inside. Inside contained numbers and lines, kind of like a ruler. The line where the dark and light meets is called the "Brix Reading." The Brix reading is named for A.F.W. Brix, a 19th century German inventor. Each degree Brix is equivalent to 1 gram of sugar per 100 grams of grape juice. About 55% to 60% of the sugar is converted into alcohol. The estimated alcohol that a wine will produce is calculated by multiplying the Brix reading by 0.55. Therefore, a 20 Brix reading will make a wine with about 11% alcohol. The grapes for most of the wines we discussed on our trip had readings of 20 - 25. One exception was our final winery, Mumm Napa, which specializes in sparkling wines. They pick their grapes at "low sugar, high acidity," which yields a lower-alcohol beverage.