Backstory:
So something died behind our refrigerator recently, and as it rotted the whole fridge was filled with an unspeakably awful smell. Within a few days, it quickly got to the point that opening the door was physically nauseating and the smell would fill the entire house. We emptied the refrigerator and cleaned the entire inside with soap and water while wearing improvised masks (which didn’t work all that well). We removed various drawers and washed them, we took out various pieces that the online manual says should be cleaned to get rid of a bad odor. Nothing worked - the smell just continued to get worse.
We called up the refrigerator company to get a technician to help. He came in, and as soon as we told him the problem, he said without blinking an eye, “Grind up a lot of coffee, put it in a bunch of bowls uncovered, and let it sit in there. In one or two days, the smell will be gone.”
We found this hard to believe, but he swore up and down it would work. We paid the outrageous service fee ($100 to be told to put coffee in the fridge!) and followed his instructions. And amazingly, it worked. It worked fast, too - the smell was halfway gone in six hours, and in two days was entirely gone.
The Question:
How does this work? Why do coffee grounds get rid of bad odors? You don’t even need to smear them around or disturb them often - just leaving them out in the air gets rid of the smell.
I’ve used the coffee trick, too. We rarely drink it at home, so there’s usually some around. It really works a treat on smelly cars and microwaves that someone tried to melt soap pieces in using a plastic container and hit 20 minutes instead of 2 minutes then went and played on the internet until this unbearable cooked burning melted stench of soap combining with plastic in an unholy coupling from the depths of hell came stealing out merrily from the kitchen.
So he solved a problem that was making your fridge something you feared opening and now you bitch about $100 service call. The Geek squad would charge you more to plug your computer back in.
When you have been cutting garlic or onion and your hands stink, the best way to get rid of the smell is to rub you hands with coffee grounds. USED coffee grounds. There’s no need to go around wasting perfectly good coffee.
When I was a kid, my mom kept a sock full of coffee grounds in the cooler that we used for camping (one of those big, hard sided ones, and this was the old days before fancy things like “wheels” and “pull handles” so that was a heavy cooler when full that us kids couldn’t carry, only dad).
She only kept the sock in there while the cooler was stored in the garage; the sock stayed home when we went camping. The cooler didn’t stink, either, although it did have this weird kind of off-coffee smell that even now I sort of associate with camping.
I’m guessing something about the grounds being acidic? I.e., baking soda is alkaline and gets rid of the acid smells but sometimes you need the opposite.
I’ve never studied coffee grounds under a microscope, but I if I had to make a WAG, this probably works for the same reason that activated carbon does. Odor-causing molecules that are dissolved in air will tend to cling to any surfaces they touch. I would assume this is because of cohesion and adhesion (the same reason that water clings to the surface of a glass). If these molecules are stuck to a surface somewhere, then you aren’t inhaling them.
Activated carbon is fairly inert, chemically speaking. It’s “activated” because of its surface area. Draw a straight line from point A to point B. Now draw a wavy line between them. The wavy line is longer, right? And the wavier you make it, the longer it is. This is the classic “length of the shoreline” problem from fractal geometry. The shoreline problem works in higher dimensions, too. A flat surface has less area than a convoluted one, just like your straight line has less length than your wavy line.
Activated carbon is “fluffy” at a microscopic level. It has a very convoluted surface, and therefore has an enormous surface area relative to its volume. Math geeks would say that although the surface has a topological dimension of 2, its fractal dimension is somewhere between 2 and 3.
Long story short, that enormous surface area allows lots and lots of molecules to cling to it, so it’s very handy for getting rid of odors. They use “carbon scrubbers” in a lot of industrial applications where strong odors need to be reduced/eliminated. The process is called adsorption (clinging to the surface), as opposed to absorption (diffusing through the surface). I would suspect that coffee grounds have a similarly high surface area, though probably not as high as activated carbon. So on one hand, it’s probably not as adsorptive (is that a word?) as activated carbon, but it’s probably more absorbent.
As **Stathol **said, I suspect it’s something to do with the fact that coffee grounds are roasted, thus turning them into a form of activated carbon, which allows them to have a huge surface area to adsorb gaseous compounds. In fact, in some cases, it can be more effective than commercial activated carbon.
The surface areas are truly stupendous, as Wiki notes: “A gram of activated carbon can have a surface area in excess of 500 m², with 1500 m² being readily achievable.”
This goes along with my $1000 snowball. Early in my IT career, I supported wireless networking links, T3 speeds (45Mbps) and higher.
One snowy March morning in Boston, we got a call for service on a link that had gone down. We could find nothing wrong with the equipment, but it was getting very low, to near zero signal to the receiver. On the roof, the antenna, a large drum looking device, was covered in snow. It doesn’t take much to degrade high frequency signals.
The antenna was too high to reach from the base of the tower (on a 40+ story building’s roof), and while waiting for maintenance staff to bring a very tall ladder (20’, to get from the roof to the drum), I speculated that a snowball might bounce the cover material enough to clear the snow. My co-worker had no objection to trying it, and I threw the snowball well, and hit it dead center. Snow fell off, the signal bounced up to full strength, and we went home.
2 men, onsite for 2 hours, plus 2 hours drive time, (8 man hours at $125/hr) = $1000 for a snowball fix. They weren’t paying for the snowball, but they did pay for my expertise in knowing HOW to throw the snowball to hit in the right place. Makes for a great interview answer, though I don’t support that sort of product any more.
</hijack>
As to the OP, I’m going with it being a porous substance, which absorbs the odor, additionally covering up lingering scents not yet absorbed with the much more pleasant aroma of coffee.
I read the wiki too… And I just don’t understand how roasting something could make it “activated”… Looks to me like activation is simply increasing the surface area dramatically (like baking powder, used for the same reason). But if coffee grounds and baking soda work great for it… Why not… oh… flour or powdered milk?
So it was not that much of a stretch for beverly hills cop when eddy murphys character mentioned the drugs being surrounded by coffee grounds. Instead of the drug pooches sniffing the drugs, the area would have a more neutral or an absence of aroma ? Or would the dogs just smell the coffee.
They weren’t paying for the snowball, but they did pay for my expertise in knowing HOW to throw the snowball to hit in the right place. Makes for a great interview answer, though I don’t support that sort of product any more.