By Harold McGee
Cactus pads, nopales, and nopalitos are all names for the flattened stem segments of the prickly-pear cactus Opuntia ficus-indica, a native of the arid regions of Mexico and the U.S. Southwest. They’re eaten raw in salads or salsas, baked, fried, pickled, and added to stews. Nopalitos are remarkable for two things: a mucilage that probably helps them retain water, and that can give them a somewhat slimy consistency (dry cooking methods minimize this), and a startling tartness thanks to their malic acid content. Cactuses, purslane, and other plants that live in hot, dry environments have developed a special form of photosynthesis in which they keep their pores closed during the day to conserve water, then open them at night to take in carbon dioxide, which they then store in the form of malic acid. During the day, they use the energy from sunlight to convert the malic acid to glucose. Pads harvested in the early morning therefore contain as much as 10 times more malic acid than pads harvested in the afternoon. The acid levels in the pads slowly drop after harvest, so the difference is less apparent after a few days.