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At work I usually drink coffee (multiple times daily) from a metallic cup which I actually never ever wash on the inside. This leads to the building of interesting dust-like coffee residuals. While this started out of mere laziness, I meanwhile deliberately refrain from washing it as the result is visually quite appealing and an interesting long-term endeavour to see how that coffee cup evolves over the years.

I am however wondering if those residuals actually have any kind of influence on the fresh coffee drank from this cup. Now of course there isn't anything moulding, it's an entirely dry thin dust layer, and I don't really make out any taste difference to a clean cup (as the coffee isn't too much of a high quality product in the first place and I'm not a big tasting expert either). So this might be a stupid question. But for example I heard somewhere (though, that's entirely anecdotal hearsay, too) that you shouldn't wash a tea can with dishwasher but only with water because of some flavoury reasons. So maybe there is actual proper or even scientifically grounded reasoning that the dry residuals of previous coffee in the cup have any kind of influence on the coffee drinking experience, however small and imperceptible, be that flavour-wise or regarding other effects, advantageous or defective.

  • Of course, if anyone has more insight into the tagging process, feel free to add relevant tags. – Chris says Reinstate Monica Mar 4 '15 at 14:04
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You're actually asking two different questions.

The minute amount of coffee left to evaporate in the bottom of a cup will have completely oxidized and lost virtually all of its volatile compounds (i.e. It probably wouldn't taste very good if you reconstituted it on its own). So when you add more fresh coffee to that cup, yes, you are essentially adding a minute amount of an external (and somewhat off) flavor component to your brew.

Now whether that flavor component contributes to any any perceptible change in flavor is highly debatable. Line up 100 coffee drinkers in a blind tasting, and I suspect there would be no statistically significant detection between "100% fresh" and "Christian's 0.083% dust-like coffee residuals."

Your analogy to washing tea cans in a dishwasher is a bit off. Washing containers in a dishwasher is generally known to leave a perceptible taste of soap on the container. Soap is completely foreign to the coffee-tasking experience so it is easily tasted if it is present. But the residuals you are leaving behind in your unwashed-coffee-cup study are still essentially coffee (somewhat off coffee, but coffee nonetheless). So mixing the equivalent of "a few drops" of off coffee in a hot cup of fresh I suspect will have no perceptible effect on taste.

I also suspect that your coffee cup experiment is not evolving over the years. The metallic cup is likely stainless steel or perhaps aluminum (and most coffee cups are made of ceramic). In either case, the cup is largely stainless, so you are not likely building up any patina… so you are essentially "washing away" the previous day's experiment each time you add a fresh cup of hot coffee. There should be no substantial, cumulative effect.

  • That's why I wasn't so much concentrating on perceptible things only. But thanks for the insights. – Chris says Reinstate Monica Mar 4 '15 at 15:47
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    A back-of-the napkin calculation would estimate the ratio of old coffee to fresh coffee to be about 1200:1, or about 0.5 drops per ounce. – Robert Cartaino Mar 4 '15 at 17:01

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