By Harold McGee
Citrus fruits are among the most important of all tree fruits. From their birthplace in southern China, northern India, and Southeast Asia they spread throughout the subtropics and mild temperate zones of the world. Ancient trade took the citron to western Asia and the Middle East before 500 BCE, and medieval crusaders brought sour oranges back with them to Europe; Genoese and Portuguese traders introduced sweet oranges around 1500, and Spanish explorers carried them to the Americas. Today, Brazil and the United States produce most of the world’s oranges. Barely a century ago, oranges were special holiday treats; now much of the Western world starts its day with orange juice.