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I boiled some milk and started to poured it in a silver cup by mistake. There was just some drops in it. The glass was clean and nothing was added to it.

But after an hour, I found that the milk turned bright yellow. Never seen milk in that color. What happened? Is something wrong with the silver cup or milk?

enter image description here

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  • Are you sure that the glass is actually silver metal and not another metal like aluminium?
    – bob1
    Commented Jan 27, 2020 at 8:42
  • I bought it from a "Silver shop" and it was sold based on weight as silver is sold. I have no way to verify if its actually silver. Does it turn yellow if its aluminium?
    – OkBeat9
    Commented Jan 27, 2020 at 9:22
  • Regarding the silver - I was merely asking as different metals may interact with the milk in different manners. You could do a density test to distinguish between aluminum and silver. Measure the volume of the cup (not how much it holds) by displacement (use a measuring cylinder if you have one) and then weigh it. Mass/volume = density, Al=2.7g/cm^2, silver=10.49 g/cm^2. Boiled milk often has a yellowish tinge to it, and milk that evaporates water and dries out is yellowish, so it could just be that you are seeing.
    – bob1
    Commented Jan 27, 2020 at 20:08

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tl;dr It looks like your milk has curdled. It is unlikely to be related to the silver cup.

Milk is composed of casein, albumin proteins, sugar, some amount of fat, and vitamins including riboflavin. The solution of protein and fats is opaque white. When milk is curdled- usually by exposure to acid and/or heat, the casein proteins denature and tangle up much of the fat. They separate into curds and settle out of the solution. The remaining sugar and albumin is transparent in solution. The riboflavin has a yellowish-green tint.

Whey looks exactly like the yellowish liquid in your photo.
Wikipedia whey
In your photo you can just see the wispy white of the casein proteins that have come out of solution.

Silver on its own won't curdle milk. Silver nitrate will if your cup was previously used to hold nitric acid but I sincerely hope that isn't the case. It's more likely that your milk spoiled enough to curdle or was exposed to some other acid unrelated to the silver cup. It is possible, if you haven't used this cup before, that the seller used an acidic cleaner to remove tarnish before selling it.

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