Harvest (wine) of wine, winemakers would use the sugar and acid levels of the grape as a guide in determining ripeness. Early winemakers tasted the grapes to gauge ripeness. Modern winemakers use a refractometer to measure high sugar levels and °Brix or titration tests (using an indicator such as phenolphthalein) to determine the titratable acidity within the grape. In recent times there has been more of an emphasis on the "physiological" ripeness of the grape, usually in the form of tannins and other phenolics. Currently, tasting is the only way to measure tannin ripeness, which can take experience and skill to do
Wine in China "materia medica" compiled in the late Han, states that grapes could be used to produce wine. In the Three Kingdoms era (220–280 AD), Wei emperor Cao Pi noted that grape wine "is sweeter than the wine made [from cereals] using ferments and sprouted grain. One recovers from it more easily when one has taken too much." Grapes continued to be grown in the following centuries, notably in the northwestern region of Gansu, but were not used to produce wine on a large scale. Wine thus remained an exotic product known by few people. Not until the Tang dynasty (618–907) did