What's this smell?
July 10, 2019 7:32 AM   Subscribe

What causes a washcloth or towel to smell "sour" after being wet and dried out?

No other variables -- no soap, no washer-dryer, etc. Just wet it and hang it over a towel rack. Within a day or so it takes on a distinctive smell, which I think most people would call "sour." So what is it?
posted by LonnieK to Home & Garden (21 answers total) 3 users marked this as a favorite
I'm not certain, but I'd guess that while the washcloth is wet, microorganisms have a chance to grow in it, and they produce some substance (maybe lactic acid?) that has the odor you are detecting.
posted by alex1965 at 7:35 AM on July 10 [3 favorites]

Mildew? If you're somewhere humid and it's not fully drying, it will get mostly-invisible mildew that will make it smell this way. Putting it out in the sun will help, as will a wash with some bleach (and a thorough drying)
posted by jessamyn at 7:45 AM on July 10 [12 favorites]

Hi. Houstonian here. This is a constant battle here in the summer, because it's so humid.

One solution people other than us use is to toss the towels in the dryer for a bit after usage. This is popular with folks who have laundry machines close to, or in, the master suite, which unfortunately doesn't describe our house.

We just end up washing them frequently.
posted by uberchet at 7:54 AM on July 10 [2 favorites]

It's bacteria. To me it smells like wet dog, or possibly peas...

For the most part, drying things out thoroughly soon after use is the key thing. And putting them through the laundry cycle every few days. For a small washcloth, microwaving it while damp once a day (just for 1 minute, until it's steaming) will help, but you still have to dry it thoroughly. If the weather's good, hanging things outside for a while has the double benefit of drying and UV light.
posted by pipeski at 8:13 AM on July 10

I read somewhere that it is caused in large part by not thoroughly rinsing it after using it with soap.
posted by swlabr at 8:25 AM on July 10 [1 favorite]

I get the same thing. Towels from the sauna, which haven't been used for anything soap related but get damp from the humidity, will smell sour from mildew if they dont dry quickly enough. In the winter when I can hang them over the woodstove, or in summer when I can hang them outside in the sun theres no problem. But cloudy or damp days mean those towels have to get put in the washer asap.
posted by ananci at 8:55 AM on July 10

I recently did some washcloth shopping, and while I eventually purchased from a chain store, I shopped online and discovered the solution that a lot of people have decided on: buy lots of washcloths. 7- and 10-packs were typical, and instead of hanging it up, you just check it in the dirty laundry (or chucked right in the washer itself) where it can be renewed on your next laundry day.

But for reasons of bulk and expense, this doesn't scale up to towels, I'd imagine. Still, as half-measures go, it's a decent and affordable one to do with washcloths.
posted by Sunburnt at 9:39 AM on July 10 [2 favorites]

I've found sunlight a good solution to a whole range of dampness related pongs.

Hang things out on a washing line, over a balcony or even infant of an open sunny window.
posted by tomp at 11:04 AM on July 10 [1 favorite]

Also make sure to thoroughly clean the towel rack you're hanging it on, because even if you get a nice clean washcloth the bacteria will be lying in wait on the rack, all ready to jump on there and reproduce enthusiastically.
posted by exceptinsects at 11:15 AM on July 10

Taking a step back to when the washcloth last went through the washing machine, the culprit might be the products used there. If you use either a little too much laundry detergent, or a fabric softener or any sort, this can lightly coat the fibers of the terrycloth and stop them from being fully rinsed clean during the spin cycle. They will seem fine when they emerge from the washing and drying cycle, but wetting them outside of this process allows the old not-rinsed-off ick and bacteria to 'reactivate' and start to be smelly again.

To strip off this coating and get them thoroughly cleaned, wash them once on the hottest cycle possible with a VERY SMALL amount of detergent and at least a full cup of white vinegar. Then going forward, when washing the towel load, use hot water, much less detergent than you think you need, and leave the fabric softener out of the batch entirely.

Of course, if you go through these steps and your washcloth still smells sour, then this wasn't the issue! I'd speculate in that case that someone's bodywash or soap is oily and is not rinsing out of the washcloth fully before it's hung up, attracting bacteria.
posted by DSime at 11:16 AM on July 10 [1 favorite]

Oh man, I had some bath towels that just would not stop smelling... weird. I gave myself ONE LAST SHOT to clean them and make them not weird before replacing all my towels. My girlfriend this morning said they were totally fixed.

How did I do that? I didn't have any white vinegar to wash them in which is normally step 1... but I just washed them normally on hot and then laid them on my porch to sun dry in the hot, direct sunlight. Flipped them once. THEY SMELL SO NEW.

Sunlight seriously does wonders.
posted by OnTheLastCastle at 11:34 AM on July 10 [5 favorites]

OP here. These are all great tips, but most are answering a question I didn't ask. Possibly I should have been more precise, but I'm not seeking a way to freshen up my washcloths, with all due respect.
I'm trying to understand the process behind the smell. What happens when water dries on a cotton washcloth to cause that odor?
Some of the early answers got at it -- bacteria, microorganisms, mildew. Are those the same thing? Is there a consensus on that? Are those things not present in a newly-laundered washcloth? Does water activate them? What's going on?
posted by LonnieK at 12:09 PM on July 10 [1 favorite]

Microorganisms here is a generic catch-all. What is happening is that air and water-borne bacteria and yeast (fungus) are forming a number of acids as part of their metabolic processes, and acids smell sour. This is how we get vinegar!

As long as there's any moisture and or organic matter to feed on, these processes continue. Ambient humidity can be enough to keep some of the magic happening, especially on something porous like a dishrag.

Eventually mildew/mold (another set of funguses) form to feed on the biofilm created by the previous stage. That generates more of a musty smell, so what you're noticing is probably the initial stage.
posted by aspersioncast at 12:37 PM on July 10 [6 favorites]

Mildew is a type of fungus, similar to mold. Microorganisms are a group of tiny living things that includes bacteria, fungi like mold and mildew, and other things like viruses.

The smell happens because mold spores like the wet environment and grow there. The spores are always around on normal surfaces, air, etc but need water to settle down and grow. Some types of bacteria can also grow there and might add to the smell, but the typical "wet towel" smell is mildew. Washing/drying will remove most of the things growing there but not necessarily all of them, especially if there's a lot, which is why it might still smell after you wash it.
posted by randomnity at 12:39 PM on July 10 [2 favorites]

Yes, mildew. Once you have gotten it on a particular towel or washcloth once (by letting it sit damp folded on top of itself in a way where it doesn't dry for awhile), it sticks around even through standard laundering. You are probably not killing it when you wash it -- so from then on, you'll smell it again basically as soon as you get it wet.

You need to wash with HOT water, with bleach/vinegar, plus conventional detergent not "eco-friendly" stuff that doesn't work as well, then dry thoroughly on HOT (or line dry in the sun).

Google "kill mildew laundry", or here is one link.
posted by amaire at 1:08 PM on July 10

Water will often reactivate fungus and bacteria that are still in fabric after washing/drying. Home washers and dryers are not sterilizers and many microorganisms can survive a trip through the laundry. Wetting the item again with water or sweat (in the case of clothing) gives them renewed life and they grow again.
posted by quince at 3:29 PM on July 10

I had another thought - the most common source of delayed-action 'sour' smell in our lives is yeast-based fermentation. Human bodies are rich sources of yeast - could the washcloth's user be depositing some yeast on the cloth to interact with any sugars/glycerin in your soaps?
posted by DSime at 4:22 PM on July 10

I use an electric towel drying rack similar to this on a 1hr timer. Fantastic at preventing mildew.
posted by qxntpqbbbqxl at 4:25 PM on July 10

OP isn't trying to fix the smell, but understand the source of the smell. Which--if we're talking about "sour" as it is usually used--is bacteria and yeast, already present in the air and water, generating acetic acid (and various other compounds) as part of their metabolic processes. Mildew is a generic term for a subset of fungal growths that generally feed on the biofilm formed during/after these processes. Some people may refer to any of the subsequent smells as "mildew," but there are different things happening at different points/humidities/concentrations.

Are those things not present in a newly-laundered washcloth? Does water activate them? Depending on the strength of your detergent and the heat of your dryer, most bacteria may very well be killed in the laundering process. You'd need to run everything through an autoclave to bring it above a certain threshold. But even if you were actually killing every single organism on the dishrag, the second it reaches open air it is immediately exposed - spores and bacteria exist to some concentration throughout the atmosphere (far beyond where humans can survive).

If you're really interested in this stuff, a history of bread would be a great place to start. Because this is probably part of the reason it took people a long time to figure out exactly what yeast was - you could provide the right environment and the yeasts would just literally form out of thin air.

And yes, water is the key ingredient that's allowing these little colonies to expand to the population level where you can actually smell their metabolic processes.
posted by aspersioncast at 9:45 PM on July 10 [4 favorites]

Dr. John Dean of Northumbria University says it's:

- Butyric acid (strong, rancid butter-like odor)
- Dimethyl disulfide (unpleasant, onion-like odor)
- Dimethyl trisulfide (powerful odor)
- 2-heptanone (banana-like fruity odor)
- 2-nonanone (fruity, floral, fatty, herbaceous odor)
- 2-octanone (apple-like odor)

posted by at at 11:19 PM on July 10 [5 favorites]

If you had used the washcloth before drying it, it would have picked up dead skin cells from your skin. Add water and/or humidity and it's bacteria/fungus time.

posted by whitelotus at 8:38 AM on July 11

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