What is this scum in my coffee For full disclosure I have posted the questions originally here: https://coffee.stackexchange.com/questions/5150/what-is-this-scum-in-my-coffee but since I got no answers after two weeks, I would liked to post it here as well. I have my own theory of why its happening but would like a second opinion!

Cross posting is fine according to: https://meta.stackexchange.com/questions/65931/cross-posting-on-stackexchange-sites



Hello, Lately my coffee seems to have a film on the top. The picture makes it look a lot worse then it is. It is more translucent in the cup. This happens once the coffee gets cold. When its hot it looks like bubbles or just the oil from the coffee beans.

What is this? Is this just the oil from the coffee? Or do I need to clean my machine?

Coffee Details:


Black Coffee, nothing added.
Made in espresso machine.
Ground to order.
Run through several times as I "overpacked" the coffee. 
Hot water added to make Americano. 
Beans from local roaster, last week.


This is what my coffee looks like before it turns into that:
That scum looks like it could be hard water deposits interacting with the oils in the coffee, I get a similar scum on my tea when my brita needs replacing. I would suggest de-scaling your espresso maker and using filtered water.