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I noticed that most commercial beans are shiny/oily on the outside:

enter image description here

While the beans I normally use on my Espressos (blend of 80% Brazil Santos + 20% Colombia) are dull/opaque: enter image description here

Being a curious mind I found that shine is a byproduct of higher temperature (read quicker) roast.

Obviously I went googling to see how it impacts brew quality and found several conflicting answers. What's the community opinion on this? Do slow-roast beans produce a better quality or does it not matter?

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Oily surfaces are related to roast level

I don't think oily beans are related to roasting time as you say in your question. Instead, oily beans are characteristic of a darker roast. Roasting levels are primarily related to the internal temperature of the beans, the higher the internal temperature of the bean, the darker the roast.

Wikipedia has an overview of different roast levels. Three of the roast levels in their overview mention an oils on the surface of the beans (emphasis mine):

225 °C (437 °F), Full City Roast

Medium-dark brown with dry to tiny droplets or faint patches of oil, roast character is prominent. At the beginning of second crack, body is fully developed.

230 °C (446 °F), Vienna Roast

Moderate dark brown with light surface oil, more bittersweet, caramel flavor, acidity muted. In the middle of second crack. Any origin characteristics have become eclipsed by roast at this level

240 °C (464 °F), French Roast

Dark brown, shiny with oil, deep caramel undertones, acidity diminished. At the end of second crack. Roast character is dominant, little of the inherent aroma or flavors of the coffee remain.

Wikipedia doesn't list the oily surface explicitly for the darkest roast: the Italian roast. Instead, it just describes it as "nearly black and shiny". Nevertheless, the Italian roast has a very oily surface, as described by this article form Seattle Coffee Gear which compares the French and the Italian roast levels:

However, we know that there are die-hard devotees of dark roasted coffee and we were recently asked what the difference was between French Roast and Italian Roast.

They're both roasted quite darkly, so that they have an oily sheen to them after the roasting process is complete. With a French Roast, the temperature of the roast is high enough that these oils are brought to the surface and will impart a roasted flavor to the produced coffee or espresso. Aromas can vary from berry to citrus. Italian Roast is much darker and oilier than a French Roast and often preferred in Italy.

Oily vs. non-oily beans

I don't think it makes sense to say that one is better than the other. In comparing the two, I think it's better look at light vs. dark roasts because there are more factors at play. As a general rule I would say that the darker the roast, the more prominent the roast profile becomes over the origin characteristics.

Another note about oily beans is that they can leave more residue in your grinder (and other parts of the machine where the beans pass through).

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  • "I don't think it makes sense to say that one is better than the other." If the beans are as oily as in the picture it means that they were roasted well beyond the second crack. The coffee has lost all of its origin characteristics and just tastes bitter, burnt and "roasty". Roasting it this dark is usually done with sub-par and mixed batches to get a more uniform taste across batches. It does very much make sense to at least call "oily" coffee out for what it is (regardless of preference) - cheap, burnt coffee with no origin characteristics, no acidity and very little sweetness. Jun 26 at 0:06
  • @technical_difficulty basically any dark roast going past second crack can have some oily residue. I think it's a bit overgeneralising to say all of them are bad per se, but I agree that it's a common way to get rid of lesser quality beans by making them all taste similar.
    – JJJ
    Jun 26 at 5:03
  • I agree that almost all dark roasts beyond the second crack will be oily. I do not agree that it is overgeneralising the quality of such coffee. There is simply no way a coffee can retain its quality (as determined by professionals cupping it) and origin characteristics when it is roasted like that. And even if there are some exceptions to this rule, I'd expect them to so incredibly rare that they only further prove the point. Of course some people may prefer coffee with those bitter-roasty notes, but all Q-Graders, which have been trained to objectively grade coffee, would agree with me here. Jun 26 at 22:11
  • @technical_difficulty And I agree with you, I just don't think this answer is the right venue to bash dark roasts. Remember that the question is about oily beans, not about light roasts vs dark roasts.
    – JJJ
    Jun 27 at 5:23

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