Coffees prepared with some standard home machines (e.g., non-paper-filtered methods like moka, Turkish, or even some espresso machines) have something on the surface. It looks like oil from the beans.

Filter-based brewing techniques like drip or Aeropress do not result in this oil film on the surface, in my observation. The filter is probably the reason, as it can block the oil and just let water get through.

This oil, if confirmed, is what contains most flavour compounds, so I guess we want to keep it. On the other hand, it is aesthetically nicer to have a clean surface for the coffee drink, which means to act such that this oil is excluded at brewing time.

I am not sure there is a tradeoff here, as "clean" surface coffees are very good (if prepared well), which means the flavour compounds did get down to the drink. So perhaps the surface oil is just that does not participate to the flavour. Any information on this topic?

  • I’ve never had this problem before, oil floating on top my coffee is not my idea of appleaing. It tasted awful as well. So I bought another new coffee maker and there’s no oil. Coffee tastes good to. Could there have been a flaw in the other maker? As for the water ,it’s super soft so I know it’s not hard water. Mar 3 '19 at 21:41

Nice question. A similar myth has been arisen for Turkish coffee pot; such as it should be cleaned only with water to keep the greasy surface made with coffee oil.

Partly correct, partly not. According to my "Barista's manual" from Lavazza, this sedimented oil may acetify. Thus, end up bad flavor.

Again, (I cannot remember the source) as of my knowledge, paper filters are very good filtering everything; including those valuable oils. (That's why classical French press is still very common to prepare filter coffee, I think.) Paper soak in most of the flavor itself if it does not have proper fibers. This seems like the main problem of any drip coffee machine, V60, Chemex, etc.

Final words: keep it clean in the long run. You may not necessarily clean it until acetifying; which is you may keep it as it is up to one-two days.

Same rule for Turkish coffee pots. :)

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    Thanks. Interesting relation to Turkish coffee. What do you mean by myth? Mar 3 '16 at 6:37
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    The myth is not to use any detergent or soap to clean the Turkish coffe pot "cezve". It should be cleaned every day or every other day, though. However, it may be beneficial to keep the grease on the cezve during the day, while brewing a series of cups. Then, the coffee won't contact with the pot directly, but with the remains of coffee oil. This helps to get rid of possible metallic taste from pot and finer brewed coffee.
    – MTSan
    Mar 3 '16 at 10:32

The amount of oil in your coffee correlates to what is called the "body"? This is the "fullness" that you feel of the coffee in your mouth. Like the difference between a light cake and fudgey brownie. More oil means more body. You get more body from brewing methods which, like you said, do not use a filter. This includes things like french press. The amount of oil/body is also influenced by the beans you use, with darker roasted coffees generally having more oil/body.

Most people probably wouldn't say a cup of coffee without oil looks any nicer than one with, because there's nothing wrong with it, just a different variety.

  • Thanks for the explanation around the body. As for the oil "foil", I have met several "veterans" striving to avoid it, and most of the breweries I visit just throw away coffees with foil, without serving. I do not really pay attention well, though, and I may just notice this pattern in people from the same "school". Not sure yet if there is any definitive answer anyway. The question may be too subjective. Mar 8 '16 at 5:38
