Lettuce

Leaf lettuce keeps 3–7 days refrigerated; a whole head keeps 1–2 weeks. Store it in the crisper drawer, set to high humidity if your drawer has a slider.

Lettuce fails by drying out and by rotting, and the trick is to allow neither. Wrap a head loosely in a slightly damp paper towel inside a partly open bag — enough moisture that it does not dehydrate, enough airflow that it does not sit in condensation.

Keep it away from apples, bananas, tomatoes, and avocados. Lettuce is highly sensitive to ethylene gas, which those fruits emit, and exposure produces the rust-colored spotting along the midribs that ruins a head early.

Bagged salad is a different product: it has been washed, cut, and packed in a modified atmosphere, and it should be used by its date. Once opened, the clock is 3–5 days.

Lettuce does not freeze. The cell walls rupture and it thaws to slime.

Lettuce is over 95% water held inside rigid cell walls, and everything about how it fails follows from that.

The crispness is turgor pressure — water pushing outward against the cell walls. As lettuce loses water it goes limp, and this is entirely reversible early on: a wilted head revived in ice water for 20 minutes will crisp up again, because the cells simply reabsorb water.

What is not reversible is bacterial soft rot. Once the cell walls are breached — by damage, by freezing, or by bacteria producing pectin-degrading enzymes — the cells release their contents and you get the characteristic slime. Slimy lettuce is not revivable and should be discarded.

Lettuce is also unusually sensitive to ethylene, the plant hormone that ripens fruit. Exposure triggers russet spotting, the tan-brown lesions along the ribs, which is why storing lettuce near apples is a genuine mistake.

The safety dimension is real and separate. Leafy greens are among the most common vehicles for E. coli O157:H7 and Salmonella outbreaks in the US, because they are eaten raw and grown close to soil and irrigation water. Washing reduces but does not eliminate the risk, and pre-washed bagged greens should not be re-washed — that introduces more contamination than it removes.