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In the title I am refering to silver spoons, which, when used to consume milk products (like yogurt), get a weird taste. It could be described as umami/meaty, a little bit like onions. I came to the conclusion that it must be the silver, because this taste is never observed when using steel spoons.

My guess was: the silver (or maybe its oxide?) reacts with an ingredient of the milk product.

My question is: Is this documented/known process/reaction or is this some kind of misperception on my part? If the former is true, what exactly is happening? (By)-products?

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You're right: the silver is reacting with sulfur compounds in the food to form a tarnish of silver sulfide. This is most commonly observed, in my experience, using silver teaspoons with boiled eggs, which are pretty rich in sulfur.

There are a number of reactions that can take place depending on the sulfur-containing species - the abstract from this paper from the Journal of Chemical Education (unfortunately pay-walled) has a nice example:

Silver and silver-plated objects react with sulfur and sulfur compounds to produce silver sulfide $\ce{Ag2S}$ or tarnish. Contact with materials that contain sulfur compounds, such as hard boiled eggs, mayonnaise, mustard, and rubber bands can cause tarnish. In the air, a silver object can tarnish owing to the reaction of silver with hydrogen sulfide ($\ce{H2S}$). This is a gas found in the air as a result of some industrial processes and the decomposition of dead plants and animals. The reaction of silver with hydrogen sulfide to form tarnish is as follows:

$\ce{2 Ag(s) + H2S(g) -> Ag2S(s) + H2(g)}$

In Sterling silver specifically, you'll often also get copper sulfide formed.

The paper also mentions that you can remove the tarnish abrasively, or through reacting the tarnish with aluminium (via solution, electrochemically):

$\ce{3 Ag2S(s) + 2 Al(s) -> 6 Ag(s) + Al2S3(s)}$

But a more common approach used by many commercial products is to dissolve off the tarnish, usually in sodium thiosulfate or thiourea. This means you end up with slightly less silver on your item, but is otherwise pretty effective and very mild (it's commonly used for museum pieces).

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    $\begingroup$ I'm not totally convinced by this answer: OK it does explain the appearance of tarnish, but is there some evidence that tarnish (or silver sulfide) might have a specific taste, as observed in the initial question? The taste should then be apparent for any foodstuff getting in contact with the tarnish deposit. I would think of some other reaction being involved (but maybe starting from silver sulfide). $\endgroup$
    – PLD
    Commented May 24, 2012 at 20:16
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    $\begingroup$ Actually, the mentioned boiled eggs also "produce" this taste, I forgot to mention that, so this sounds plausible. What I'd like to know if ingesting silver sulphide is "safe". $\endgroup$
    – Evgeni
    Commented May 24, 2012 at 20:30
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    $\begingroup$ Silver sulfide's solubility in water is very low ($8.5 10^{-15}$ g/L), so it should have no taste (and probably low toxicity, though I can't find detailed data on that). Also, it would mean that the spoon would continue to taste that way after the tarnish has formed, not only when eating the milk (because tarnish really forms on the surface and stays there… that's why it's annoying to get rid of it). $\endgroup$
    – F'x
    Commented May 24, 2012 at 20:43
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    $\begingroup$ And also: milk doesn't actually contain that much sulfur, unless it's condensed milk. $\endgroup$
    – F'x
    Commented May 24, 2012 at 20:44
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    $\begingroup$ I'm not entirely convinced by this answer either, and humbly suggest that what might be going on is some kind of catalytic decomposition of something in the dairy on the spoon surface. That's just wild guessing though. $\endgroup$
    – Richard Terrett
    Commented May 25, 2012 at 14:01
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Yoghurt is an acid product, as it contains lactic acid that originates from the fermentation of lactose by Lactobacillus. This acid may react slightly with silver sulfide and the resulting salt produced the weird taste you mentioned.

Incidentally, silver used to manufacture objects such as a spoon or a piece of jewelry is not pure, soft silver; 7.5% w/w copper is added to it to improve its strength, thus copper might also have a role in the unpleasant taste you felt.

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