Of course, that’s a pretty big simplification, but it really is a marvel not only that coffee exists, but that there’s so much going on in those beans we love so much. While there are many things that roasting “brings to the surface” from inside the seed’s potential, one of the more obvious ones is oil: Perhaps you’ve noticed that some coffee beans look shiny or slick on the outside, as though coated in melted butter. That oily residue is an amazing reminder that these beans are really seeds — just like the sunflower and sesame seeds we use to make cooking oil.
There are a few important things to know — and myths to bust —about this oil, but first, let’s talk about what it is.
Coffee seeds contain fatty or fat-like lipids in them, mostly triacylglycerols, tocopherols, and sterols — all of which are also found in vegetable oils — and these lipids can comprise up to 17 percent of the seeds’ makeup. Most of this “coffee oil,” as it’s called, is stored inside the seed itself, where it would be used as nutrients for a young plant upon germination. Green coffee isn’t oily to the eye or the touch at all: It’s only through the roasting process that those lipids start to peek out.
Why? Roasting the seed affects the cellulose, or the kind of tough, woody material that makes up the bean’s primary structure. Inside that cellulose are all the flavor compounds, alkaloid compounds, and, yes, oil, that make great coffee the way it does. As the cellulose is heated in the roasting machine, it begins to degrade and become more porous, which allows the oil to start to seep out. The darker the roast degree of the coffee, the more porous the cellulose, and, subsequently, the more oil will bead up on the surface of the coffee straight out of the heat.
For the purposes of understanding oil, it’s helpful to know about two of the later stages in the process: first crack and second crack, both named after the sound that the coffee makes as well as the actual goings-on inside the beans.
First crack happens in part because the heat in the roaster will cause the moisture inside the seeds to pressurise and become steam: Once that pressure has built up sufficiently, it will force its way out of the cellulose, causing it to pop or crackle as it escapes. If the beans continue to be exposed to heat after first crack, gases that are being produced inside the seed continue to increase in pressure, and eventually they will also burst out of the more-degraded cellulose, making a secondary, quieter “pop” sound — second crack. Second crack is also the point at which the cellulose is porous enough to allow some oil to sneak through, though all coffee will eventually release oil after being roasted, if it sits around long enough.
Now, this is where the myths come in!
Myth #1: Oily coffee is fresher.
Equating oiliness with freshness seems like it should be a no-brainer on roasted coffee, right? The beans look so glossy and sexy when they’ve got that sheen! However, it’s not a fail-safe way to tell if a coffee’s been sitting for a while, again because darker-roasted beans will naturally show more oil sooner than lighter roasted ones. (Decaf, too, tends to be a little tricky: Because of the trauma the seeds undergo in the decaffeination process, they tend to have weaker cellulose and — that’s right, you’re paying attention! — an easier structure for the oil to seep through.) Your best bet is always to find a roasted-on date on the beans, because there’s no perfect equation for how much oil might appear after a certain number of weeks or even months.
Myth #2: Coffee oils are part of what makes coffee delicious.
Walking through the coffee aisle at the grocery store and seeing the grody slime coating the inside of all the bulk bins full of heaven-knows-how-old bean? That slime was the result of oil squeezing out of the beans and building up on the plastic as they sat and sat and sat, waiting for someone to take them home.
As with any oil, prolonged exposure to light and oxygen (as well as heat) will create rancid, off flavors, so if the oil is a result of stagnant coffee sales, it’s definitely not contributing to the goodness of the cup. It also means that you should take special care with your darker-roasted faves: Storing them in a cool, dark place will be your best bet to keep them from turning before their time.
Myth #3: Oily beans make brews with a higher fat content.
Because oil isn’t soluble in water, most brewing methods will leave the lipids behind, typically in the spent grounds, especially in a filter-brewed preparation. According to Karl Speer and Isabelle Kölling-Speer’s 2006 report, “The lipid fraction of the coffee bean”: “With filtered coffee prepared in a common household coffee maker, the amount of lipids was less than 0.2 percent. In contrast, when preparing an espresso, between one to two percent of the lipids... flow from the finely ground espresso coffee into the beverage.”
Paper filters are considered significantly more effective in retaining and/or absorbing coffee oils during the brewing process, while perforated or mesh metal (such as in a portafilter basket on an espresso machine, or the screen on a French press) have larger holes and will therefore let more oil pass through to the final drink.
What that primarily means, however, is that the mouthfeel of an espresso (or a French press) is quite a bit heavier or creamier— not necessarily that the coffee itself has considerably higher calories or fat content. Filter-brewed coffee might have one to three calories per eight-ounce serving, while espresso clocks in around five calories for a two-ounce serving, largely due to the amount of oil allowed to pass through to the finished drink. (Consider how much richer the body of an espresso is from a less-concentrated filter coffee.)
No matter what you think about coffee and its lipid content, remember that if the squeaky wheel gets the grease, then the curious coffee-lover must get the espresso — not too bad a deal, if you ask us!
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