A slimy, brown head of lettuce is a clear clue that it’s no longer suitable for your salad — but sometimes, bagged or boxed lettuce or heads you’ve hung onto past what seems like their expiration still appear OK to eat. Know when lettuce has gone bad so you can prevent a potential food-borne illness or, less catastrophically, an inedible salad.

Lettuce that’s brown, black or gray has likely spoiled. Fresh lettuce is typically brightly colored — most often green, but even reddish varieties have a vibrant hue when fresh. The smell of spoiled lettuce will be similarly unpleasant and unnatural.

Limp, soggy leaves are an indication that the lettuce is no longer good to eat. An overly moist environment can hasten this development and make lettuce spoil prior to its packaging expiration date. Sometimes, prewashed bagged or boxed lettuce will develop only a few slimy leaves, which really spoil the entire lot.

Mass production of vegetables, including lettuce, puts them at greater risk of harboring dangerous pathogens. Consumer Reports tested bagged, triple-washed lettuce in 2010 and found the presence of coliforms and enterococcus, which indicate poor sanitation and possible fecal contamination. These bacteria may cause stomach upset and loose stool.

A head of iceberg lettuce or romaine typically lasts for about seven days in the refrigerator after purchase. More delicate lettuces spoil quicker:

  • Butter lettuce: three to five days
  • Green leaf: five to seven days
  • Bagged, loose leaf: three to five days.

Proper storage discourages lettuce from spoiling prematurely. Any full heads you purchase should be washed right away when you get home from the grocery store. Dry the leaves thoroughly and place them with a few paper towels to absorb excess moisture in a zip-top baggie or sealed box. Prewashed lettuce also benefits from this treatment; an extra washing does it no harm and helps clear off residual bacteria and pesticides. It also gives you the opportunity to store the lettuce in a more sustainable way — in a sealed container with a few dry paper towels.