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(Text: QQs: What happens to my wool sweater in the washer?)

Hank: You may have made a mistake. Your sweater was starting to smell a little funky, so you put it in the washing machine, and then when it came out, it had shrunk to insectoid proportions, or was full of holes, or both.

What's going on? Well it turns out that the damage was done before the rinse cycle even started. There are two very good reasons not to put your wool sweater in the washing machine, and they both have to do with the biochemistry of wool.

Wool comes from sheep and unlike most other familiar synthetic or plant fibers, like polyester or cotton, animal hair is made of protein. That's a problem because laundry detergents these days aren't just made of soap, they also contain some very specific enzymes.

Enzymes are biological molecules that help with chemical reactions, and the enzymes in your laundry detergent specifically act to break down other biological molecules.

Amylases break down starch, lipases break down fat and proteases break down- you guessed it, protein. This is great for getting biological materials like food stains out of your clothes, but the proteases can't tell the difference between the protein in ketchup and the protein in your clothes. So they just nom away at all the wool fibers like there's no tomorrow and when you take your sweater out of the machine... giant holes. Same goes for other protein fibers like silk.

But the structure of wool also complicates things. It has these little jagged structures called cuticles all over its surface and they like to latch onto things like other wool fibers. The cuticles don't really bother the sheep and actually help the fibers stay attached to its skin, but turn them into fabric and you have to be a lot more careful.

When you agitate the cuticles in water, the way your washing machine does, they grab on to each other, a bit like Velcro except you can't pull them apart again. The whole piece of fabric gets matted together and creates shrinkage.

The fibers themselves didn't shrink in the wash, the just stuck to other fibers that pulled the whole piece of fabric tighter together. You can actually do this on purpose, which is how they make felt, but I wouldn't recommend trying it on your nice wool sweater.

If you absolutely must wash the thing, put it in cold water with a detergent that specifies that it is OK for wool. And for heaven's sake, don't agitate it! Or, you know, just go to a dry cleaner.

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