A towel I thought was dry smells sour

A towel's dense loops of thread hold a lot of water. The surface may look dry quickly, but the folded or bunched-up inside gets no air and stays damp for a long time.

That spot also holds sebum and skin flakes from drying off, plus detergent that wasn't fully rinsed out. For bacteria, that is water and food all in one place, so their numbers grow as time passes. That's why the same towel starts to smell much sooner when left bunched up than when it's spread out.

A common misconception is that one wash and dry gets rid of the smell. When a Japanese research team studied laundry odor in 2012, a bacterium called Moraxella stood out as the main producer of the sour smell. It holds up fairly well to drying and sunlight, so it stays in the towel and starts making the smell again once it gets wet.

That's how a towel that seemed fine when dry can smell the moment you dry off with it. Strongly scented fabric softener only covers the smell for a while, and using a lot of it can actually make towels less absorbent.

The surest fix is to cut down the time a towel stays wet. When a wash finishes, don't leave it sitting in the machine; take it out right away and hang it spread out. Hang a used towel to dry before it goes in the basket, too.

A towel that already smells often improves if you wash it in hot water or use an oxygen bleach. Thick towels dry more slowly inside, so thinner ones are easier to keep fresh for everyday use.

If the smell returns soon after washing, it's worth checking the washing machine too. When water pools in the rubber door seal or the detergent drawer, the same bacteria can grow there and spread to your laundry.

Leaving the door open for a while after a wash so the inside dries, and cleaning the drum now and then, both help. More detergent doesn't mean cleaner laundry, either. Unrinsed detergent can become food for bacteria, so it's better to stick to the recommended amount.