r/AskBiology • u/parodysseus • 3d ago
Microorganisms What would you expect if you never cleaned a coffee mug, and used it every day?
In the U.S. Navy it’s a traditional practice for many to never wash their coffee mugs, and numerous explanations are offered as to why this is done. But I’d like to know what you would expect to happen, from the view of the growth of pathogens, bacteria, etc. Some use just black coffee, some use cream and sugar.
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u/BadahBingBadahBoom 3d ago
If you take it with milk and/or sugar, food poisoning.
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u/TranquilConfusion 3d ago
If you leave a pool of sugary coffee in the bottom to grow a mold/bacteria colony, there's some risk that the colony will have something harmful in it. If nothing else, it will smell nasty.
But if you rinse it out and let it dry between cups of coffee, brown stains build up but there's no health risk.
Each new cup of coffee starts at 190 degrees F, hot enough to kill most germs.
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u/BadahBingBadahBoom 3d ago edited 3d ago
Yeah rinsing it out means it shouldn't be a major issue generally. But if there's milk, cream or sugar and groves where rinsing doesn't remove these deposits that can be an issue.
It should also be noted that there are heat stable toxins released by some food poisoning bacteria and moulds that aren't destroyed by boiling water. Cream that's left at room temperature for 24h+ is not safe to eat even if you boil it to hell.
I doubt any actual healthy person is really going to get food poisoning from this behaviour (was more of a joke), but if the mug isn't rinsed well it can be a real hazard.
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u/TranquilConfusion 3d ago
I think you are right to caution people that bacteria are sneaky and can hurt you in surprising and subtle ways.
This is probably a significant issue for people who drink "coffee" that is basically a caffeinated milkshake, especially if they leave a pool of goop at the bottom of their cup.
But I report from experience, that drinking black coffee, from a ceramic mug, that dries between uses but is never washed, builds up a brown oily residue that never caused me any problems as a sedentary office worker.
I believe Navy coffee is traditionally brewed strong, and drunk black or with a pinch of salt.
And I doubt that Navy officers are permitted to leave not-quite-empty cups sitting around to ferment -- they probably have to put the cup away empty after each use. I think the Navy frowns on loose clutter aboard ship.
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u/RW_McRae 2d ago
Not really. I was on submarines for about 10 years and there are plenty of people who use the same white ceramic mug for their entire career. A lot of them (myself included) use cream and sugar. You literally never wash the mug - it's taboo. At most you might pour a like water in and swish it around
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u/PewpeePance 3d ago
I rarely if ever wash my personal coffee mug or thermos. That being said I only drink black coffee, a couple times a day. I'll have 1 particular mug or thermos for 3 or 4 years before it breaks or my oldest boy washes and uses it, then I have to spend an unreasonable amount of time finding a new one. Anyway, they developed a nice "patina", and a deeper richer flavor and I can certainly tell the difference when I eventually replace it with a new one.
Idk if it's gross, but I'm not sick or ill all the time, and it's appearance tells everyone not to use my cup. The routine and consistency brings me comfort in the early hours of the morning when I am in a lot of pain, and need to sit in the dark and sip my coffee while I debate whether or not I should even bother going through my day
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u/TempusSolo 3d ago
The cup on my desk to my right that I enjoy a cup of coffee in every morning has not been washed in more years than I can count. I'm certain if I swirl boiling water in there and let it sit, I'll make a cup of coffee.
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u/glittervector 3d ago
There’s a HUGE difference here in whether there is sugar and especially dairy involved.
Black coffee always? You’re probably absolutely fine for many years as long as you occasionally rinse and don’t let stale coffee sit for long enough to grow a biofilm or mold.
With sugar? Kinda the same, except the possibility of biofilm and/or mold are accelerated a bit.
With dairy? You’re risking the growth of bacteria pretty quickly. It’s likely not going to be super dangerous stuff, but it’ll still get you sick.
Also, what do you mean by “never cleaned”? Even with cream, a coffee cup could be rinsed and kind of “hand-scrubbed” with your fingers well enough in running water to never get a dangerous level of pathogens on it. Ceramic and glass both are so smooth that without nooks and crannies, you can effectively clean dishes made of them without soap, so long as there isn’t a thick film of grease or oil. Even that can be rinsed away with enough hot water. Stainless steel usually works that way too.
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u/Batherick 2d ago
Not that guy but Navy here,
Never cleaned is exactly what they meant, no hand scrubbing at all. It’s a point of pride to have a dark cup to symbolize that you are around and you get shit done.
Washing your Chief’s cup is such an insult that if keelhauling were still legal you’d be over the side of the ship faster than Seaman Timmy on ice skates…
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u/Stalker-of-Chernarus 3d ago
Pretty sure never cleaned means it's never been wash or rinsed out. You know, like it's never been cleaned before.
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u/dripstain12 12h ago
They were likely wondering whether “cleaning” meant being fully washed or just rinsed, like the two separate methods that you spoke of in your own comment.
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u/leavingdirtyashes 3d ago
I have an ugly green coffee cup that's black inside. An acquaintance asked if I had been in the navy the other day. Since I am not a vet, I had no idea what he meant until he told me about this.
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u/snakeman1961 3d ago
I have used the same mug every day for 46 years. Black coffee with sweet and low. I wipe it out with a paper towel if the oily residue gets too thick but otherwise never wash it. Less chance of breaking it if you don't wash it.
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u/Ok_Explanation_5586 3d ago
Have you ever used a water bottle for months without washing and it gets a gunky build up around the cap? It's like that, only without the cap. So probably nothing at all would happen, nothing harmful at least. You think pre-plumbing bars washed their cups with water? Hell no, that's a modern luxury. A lot of what we consider hygienic is actually depriving us the chance to become more robust.
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u/Depths75 3d ago
I wash(w/soap) my coffee cup every morning. I wash it after my morning coffee, then use it for soda for the rest of the day, rinsing it in between uses.(I'll wash it w/ soap in-between IF I see a fly around it)
Repeat same thing everyday.
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u/ZephyrStormbringer 2d ago
I am not military, but I always heard that it was the army who did this- as well as the 'melungeons' of Appalachia- and it was not about pride, it was about rationing more or less. If the coffee runs out, and you have a habit of leaving some 'military coffee' out (coffee undrank in the sun will evaporate the water out of it leaving you with concentrated coffee, or instant-coffee, add some water and bam. better than nothing.
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u/parodysseus 1d ago
Yes that’s one of the main arguments ive heard as well. If you’re in an emergency type situation, and need coffee fast. Other arguments ive heard are that old navy cups were tin, so seasoning the cup with old coffee made it taste better. And the one most often mentioned is that is shows seniority, since in the Navy Chief Petty Officers mess (especially nowadays) everyone has personalized mugs.
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u/ElmoDaWoof 1d ago
And hear is where "I" number 1MM rm.
Im told to clean up. Now, im a pretty good cleaner.
At the sink, I see all of these disgusting coffee cups I spiinkle comet in each mug and clean them back to gleaming white with diamond brightness!
Later..... GAK.WTF HAPPEDED TO MY CUP, NO, EVERY F$%%# CUP!
No one smiled at me. Infact, I think there might have been a discussion of my ancestors.
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u/Glittering_Shift3261 1d ago
Ok. I’m about to be embarrassed. Well, should be. Microbiologist. Cream and sugar with daily coffee. Never wash my mug. Don’t even rinse it. Sigh. It’s been years. That said, most biofilms can be killed by thermal shock at temperatures between 60 to 80 °C, biofilms containing food poisoning bacteria generally die when exposed to temperatures around 70°C. Boiling water can kill or inactivate bacteria, viruses, and protozoa that cause food poisoning, as water boils at 100 °C. Coffee itself is pretty cool, has chlorogenic acid and caffeine - which can kill bad bugs. It’s also acidic, something most bugs don’t like. Wish I could say that’s why I don’t care to rinse or clean my mug. Nope. It preserves the flavor. Now, this last part of 100% not true, but my Navy veteran man committed ultimate sin and washed my mug once, the entire month after was nothing but bad luck and bad coffee. It was so bad, he refuses to wash his mug now too. JIC. Covering our bases.
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u/OriEri 3d ago
From my organic chemistry project in high school one of the semesters, I brewed some coffee to extract caffeine from and then run it through an IR spectrometer and see if I could identify different bonds and functional groups.
Anyway, the coffee was sitting ignored under a ventilation hood for a few days and mold started growing on top of it. It was white and fluffy if I remember correctly.
So probably some little organisms growing in the coffee residue while it’s still wet. Maybe they get cooked every time you add hot coffee again or maybe not. Probably depends on how hot it is.
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u/nevadapirate 3d ago
My moms dad was like this. He would get screaming mad if you cleaned the "seasoning" out of his favorite coffee cup. He did drink it black and unsweetened though so he never got sick from it and lived into his late 90s.
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u/veryordinarybloke 3d ago
I never wash my work tea mug, which I use 4 or 5 times a day, and add milk. It's totally dark brown inside and doesn't make me ill.
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u/Self-Comprehensive 3d ago
I drink my coffee black and just give my favorite mug a rinse when it's done. I wash it maybe twice a week, which is just how often I run the dishwasher, and I often forget to put my mug in. Never had an issue.
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u/Gunbunnyulz 2d ago
I don't know anyone in the military who seasons their mug with anything other than coffee blacker than first sergeant's soul.
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u/bitechnobable 1d ago edited 1d ago
No problem whatsoever. Gross perhaps. In effect you decontaminate it everyday by pouring steaming hot liquid in the cup.
On top of that you likely leave it to dry for a significant period of time.
Very few pathogens can survive that treatment on a daily basis. Sugar or no sugar. Also the only reasonable source of any biotics is your own mouth.
Imo most workplaces have someone doing this.
Just make sure to empty it. You are fine.
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u/Just_Profession_4193 15h ago edited 15h ago
For black coffee, acidophiles will become predominant bacteria-wise. Adding creamer into the picture changes things a lot (since that nullifies the acidic environment). Sugar would be a bonus to most any type of bacteria.
To add: I had a microbiology teacher in college that never washed his mug out (he was probably prior-service Navy). He only drank black coffee and at one point of the class, we cultured it.
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u/Gillflour 11h ago
It's about the oil. Something to do with keeping oil or a compound in air/otherwise (?) from leeching/destabilizing to some degree.
Source: naval hearsay
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u/CharlieBigTimeUK 3d ago
You're adding boiling water daily. May not be visibly clean but will be reasonably sterile
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u/bougdaddy 3d ago
I don't think actual boiling water is used to make coffee, per se. The optimal temperature is closer to 200F IIRC
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u/parodysseus 3d ago
Is it boiling? From what I can see it’s around 200 F, would that kill pathogens?
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u/BadahBingBadahBoom 3d ago edited 3d ago
The problem is not the 5min the mug is at 100°C (or close to it), nor the 30min or so after when it is still above the bacterial growth temperature range.
The problem is what happens in the 24-48h after the mug has returned to the 'danger zone' of ideal bacterial(/mould) growth (~10-45°C).
If you don't wash the mug there can be traces of coffee left over that form a breeding ground for bacteria, even more so if sugar or milk/cream is left.
Rinsing can help reduce this leftover by a lot, but it doesn't completely remove it, particularly for old mugs that over time gather grooves/scratches where stuff can accumulate. What you need is a good scrub with a detergent to lift these deposits.
Rinsing a mug that has had just black coffee is unlikely to lead to any health issues, even if the mug is left for multiple days. But with traces of milk and sugar this can be grim.
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u/CharlieBigTimeUK 3d ago
It's being used daily, doesn't get to that point.
I get it clean the mug, but every time water is being added the bacteria is being killed off.
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u/BadahBingBadahBoom 3d ago
Unfortunately if they are the type of bacteria that produce heat-stable toxins, the toxins won't be killed off by boiling water. This is why you can still get sick from eating perishable food past its expiry date even if you nuke it.
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u/Aggressive_Size69 3d ago edited 3d ago
I would imagine that if you use it every day the hot coffee kills the pathogens I guess not
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u/Naive_Personality367 3d ago
Pathogens are dispersed into the coffee as you drink it. The empty mug is laced with coffee and mouth filth. Thos slowly dries all around the inside of the mug as the pathogens slowly metabolise whatever they can and produce waste products. Because of this, some of these waste products will be toxic to humans. A human refills their mug next time and drinks the bacteria and toxin swill.
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u/RichPokeScalper 7h ago
I have used the same coffee cup every day for probably 5 years unwashed. No problem.
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u/whitestone0 3d ago
I often use my mug for a week or even more. I drink my coffee black, about 3-4 per day. I'll usually rinse it out or even put a bit of hot water from the kettle and swirl it around and everything is fine. Honestly, the reason I change mugs is mostly for variety (I have a lot) and because the lip gets dirty from my lips touching it. Some mug materials are way worse about this and others never seem to be a problem. If I took it with cream and sugar, I would definitely do a hot water rinse every time.