Lettuce

The lactose connection

The genus name Lactuca comes from the Latin lac, meaning “milk” — a reference to the white milky sap that oozes from cut stems. The sap contains lactucin and lactucopicrin, mildly bitter compounds that turn old or bolted lettuce sour. Wild lettuce was actually used as a mild sedative by the ancient Egyptians and Romans for its sap content; cultivated forms have been bred to minimize bitterness.

Bolting

When days lengthen and temperatures rise, lettuce switches gears from leaf production to flowering — the plant bolts, sending up a tall central stalk. The leaves on bolted plants are bitter and tough; the bitter compounds protect the seed-producing flowers from herbivores. Heat-tolerant varieties bolt later, but no lettuce thrives in midsummer.

Iceberg’s reputation

Iceberg lettuce gets a bad rap in food circles for its mild, watery flavor — but it has unique virtues. Its tight crunch holds up under heavy dressings. It’s the only lettuce that gives a “wedge salad” its structure. And cold iceberg with blue cheese and bacon remains a steakhouse classic precisely because nothing else has its texture.

Why it goes from crisp to limp

Lettuce stays crisp because of turgor pressure — water inside cells pushing against rigid cell walls. As leaves age and water leaves the cells, the walls relax and the leaf wilts. Reviving slightly wilted lettuce in ice water for 15 minutes pumps water back into the cells and restores some crispness — a cook’s quick trick.