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I sometimes drink tea with or without milk. What I have noticed is that when I dip the bag in water the tea spreads out evenly and I have a cup of tea in like 5 minutes.

But when I do the same with milk in it, the tea takes forever to dissolve into the milk and even after a long time half of the milk is absorbed by the tea bag for some reason.

Could someone explain me why this is happening?

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Milk is an emulsion containing small bubbles of oil/fat. Whenever you boil milk or do something drastic, this fat tends to separate. You can do this nicely, and get cream or butter, preserving the bubbles, only part of the water content is separated, or can do more drastically, when these bubbles collapse to a big oily blob.

When you put your filter in the milk, most probably some of the oil/ fat bubbles hit the surface and cover it with an oily film. This film prevents the easy dissolution of caffeine, etc from the tea leaves in water.

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    $\begingroup$ Small correction: milk direct from the cow will automatically separate, with nothing more drastic than leaving it sit for a while. Store-bought milk is usually homogenized, where they force it through a tiny opening to make the milkfat globules smaller, so Brownian motion effects are enough to keep them in suspension. Also, if the milkfat-coating explanation is correct, you should see a difference in behavior between milks with different milkfat percentages (e.g. whole vs. skim). $\endgroup$ – R.M. Sep 25 '15 at 15:58
  • $\begingroup$ @R.M. Thanks for the correction. I am not sure on the second, though: it is essentially a nonlinear process once an oil film is produced. The thickness of this film would only mildly influence the dissolution, only its completeness which matters much. $\endgroup$ – Greg Sep 26 '15 at 0:58
  • $\begingroup$ What is the technical term for milk spreading out within and combining with the tea? Does the milk dissolve in the tea? Does it combine with the tea? Does the tea dissolve in the milk? Or is it something else? $\endgroup$ – dwkns Dec 7 '15 at 11:28
  • $\begingroup$ As I said, milk is basically a mixture of small oil bubbles and water. Tea is a water based solution. So tea would mix with the water part of milk freely. It is not even dissolution as both are mainly water based solutions. $\endgroup$ – Greg Dec 7 '15 at 12:33

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