The ruby-red grapefruit is a staple of grocery stores’ produce section. In cereal-box ads promoting a “balanced breakfast,” it appears halved in a bowl and glistening. Its rosy-colored flesh is sweeter and more mellow than that of a yellow grapefruit, and has become a familiar favorite across the United States. In many fad diets of the 1970s and 1980s, pink grapefruits figured heavily, and occasionally appeared in recipe books dusted with sugar and caramelized under a broiler for a dash…