What can you surmise about coffee beans that have oil on them at the time of purchase?

Coffee beans have a natural oil in them. Often beans that have been roasted longer will have more visible oil on the surface. Not really an indicator of quality, though, but a longer roast will be darker, have a stronger flavor, and (paradoxically to some) less caffeine.

(At least, this is what I learned when slinging Cappuccinos at a Canadian coffee chain during my college years a long time ago. Someone correct me if I'm mistaken.)

Argh. The oil is not the caffeine. The oil is not (alone, or even predominantly) what makes the flavor. Also, beans that look oily are not oilier than beans that look dry. A bean that's roasted dark enough to produce an oily surface has had many of its lower viscosity oils volatilized, leaving only heavier oils. A more lightly roasted bean has not lost these lighter molecular weight oils in the lower heat (say, less than 435 degree bean temperature), so more oil is actually retained. Brewing methods that don't use paper filters will deliver these oils to the cup.

Personally, aside from not liking dark roasts in general, I don't like brewing dark roasts using methods that get the oil to the cup because I don't like the flavor that comes from a balance of oils that lacks the lighter weight oils. I do, however, very much enjoy a lighter roast brewed to yield the oils in the cup, because to my palate the balance of oils is better with the lighter weight oils not lost in the roast.

Beans stale relatively fast. Lighter roasts stale more slowly, and indeed are best a week or so after roasting. Darker roasts are best in about half that time. Oxygen and warmth (the latter affects the former's rate of activity in the bean) are enemies no matter what.

I think someone's being mischievous here, because claiming that regional distinctives don't matter, or are not what makes coffee good, borders on the insane. Charitably presuming that the poster is not insane, I conclude to mischief. Dittos for the idea that if there's not much oil visible on the exterior, there must not be any left inside either. That's backwards thinking, since oils are burned off and migrate to the surface only on darker roasts. How could less oil be present in a lighter roast that hasn't had those oils migrate to the surface and be burned off yet? Argh.

just as a point of personal experience: I spent some time in Ethopia and had fantastic coffee in many places. The beans were ALL non-oily and downright dry looking. I was really surprised as my assumption was the same as yours.

I've noticed some of the Starbucks beans are so oily I have wondered at times if they add oil for that effect.

To my mind, a fine sheen of oil is good, but a noticeably oily surface means the beans are overroasted.

I roast beans at home, and when there is oil on the surface after several days, it always means the inside of the bean is more carbonized than I like.

I generally roast beans to the point of "second crack", which produces coffee that is dark, but not so dark that all varietal character is lost. On the excellent Sweet Maria's website, this stage is called a "Full City+" roast. Beans are typically at their best between 3 and 10 days after roasting (though it varies with the kind of bean) and with an overroasted bean I may see excess oil from the third day onward.

There are 800 flavor compound combinations in coffee and most of that comes from the oils inside of the bean. Roasting the bean brings those oils to the surface (due to the pressure of the roasting) and depending on the bean and the roast this can be normal.

Oilier = darker roast, although that only applies if the beans are relatively fresh. The beans will usually reabsorb the oil as they stale. In other words, the oiliness doesn't really tell you much that you can't already figure out from the appearance and smell (and roasting date) of the beans.

Recently I was in Arles and the beans used in this one establishment were completely dry. They seemed to be very happy to announce to me this result. The taste of the coffee was as excellent as I have generally known in France, Spain, Switzerland, Italy, the U.S, Argentina (though not as good as in Costa Rica). I noticed one thing. Unlike when drinking the oily coffees as in the U.S. for instance, I had no abdominal problems with the dry beans. I had it everyday for one month and was well. Any oily beans always produce immediate stomach upset. So there must be actual oil in those beans that is either removed in some types of roasting or never has been there because of the type of bean that it is, in the Arles case. The oil does not seem to be responsible for the taste at all, but is heavy on the stomach. It may also taste 'heavier'. Both oily and non-oily can taste excellent.

Lighter roast beans, who have contact with the lower heated surfaces will taste less burnt, and less like an ashtray. That's what my taste buddies tell me. The oil on darker roasted beans is something shiny. It distracts me from the intricate differences in flavors of the light roast.

Oils are not in and of themselves bearers of "bad" qualities, and can provide a preferred character for some beaners: mainly, they give us information about time, heat and flavor of that little roasted guy.

Imagine hanging out at the beach..you roast lightly and are in a good mood for the party that night, butyou stay all day rolling over in the sun - letting your skin burn, you will be an rosy red grump at the party, and oozy and crispier as the days go by.

Oil on the surface of the bean goes rancid very quickly giving the coffee a bitter taste. Oily coffee is bad coffee.

If they're not oily, it means they're dry. The oil is what makes the flavour. If there is no oil on the outside, there is a good chance there is not much oil left on the inside.

The reason why you're supposed to grind you beans right before you brew your coffee is that if the bean is closed, it retains the flavour longer.