<p>You're actually asking two different questions. </p>

<p>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 <em>fresh</em> coffee to that cup, yes, you are essentially adding a minute amount of an external (and somewhat <em>off</em>) flavor component to your brew.</p>

<p>Now whether that flavor component contributes to any any <strong><em>perceptible</em></strong> 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 <em>"100% fresh"</em> and <em>"Christian's 0.083% dust-like coffee residuals."</em></p>

<p>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 <em>soap</em> on the container. Soap is completely foreign to the coffee-tasking experience so it is easily <em>tasted</em> if it is present. But the residuals you are leaving behind in your unwashed-coffee-cup study are still <em>essentially</em> coffee (somewhat <em>off</em> coffee, but coffee nonetheless). So mixing the equivalent of "a few drops" of <em>off</em> coffee in a hot cup of fresh I suspect will have no <em>perceptible</em> effect on taste. </p>

<p>I also suspect that your coffee cup experiment is <em>not</em> 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 <em>stainless</em>, so you are not likely building up any patina&hellip; 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.</p>

CC BY-SA 3.0
2015-03-04T15:47:28.433Z
That's why I wasn't so much concentrating on *perceptible* things only. But thanks for the insights.
Christian Rau
CC BY-SA 3.0
2015-03-04T17:01:23.803Z
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
CC BY-SA 3.0
2015-03-04T15:43:48.310Z
2015-03-04T22:02:35.267Z
2015-03-04T22:02:35.267Z
Robert Cartaino
<p>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.</p>

<p>I am however wondering <strong>if those residuals actually have any kind of influence on the fresh coffee drank from this cup</strong>. 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 <strong>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</strong>.</p>

CC BY-SA 3.0
2015-03-04T14:04:28.363Z
Of course, if anyone has more insight into the tagging process, feel free to add relevant tags.
Christian Rau
<flavor>
What influence do the dry residuals of previous coffee in the cup have on the drinking experience?
Christian Rau
# What influence do the dry residuals of previous coffee in the cup have on the drinking experience?

(Asked by: username_0 on 2015-03-04)

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**.

**Tags:** flavor

> **Comments:**
> **username_0**: Of course, if anyone has more insight into the tagging process, feel free to add relevant tags.  

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## 1 answers

### Answer by username_1 on 2015-03-04. Score: 9 | Accepted answer

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.

> **Comments:**
> **username_0**: That's why I wasn't so much concentrating on *perceptible* things only. But thanks for the insights.  
> **username_1**: 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.  

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