In this post I am going to look at coffee oils in both green and roasted coffee, but before I talk just about coffee I want to cover the different types of lipids in coffee and give some explanation of what they are.
Lipids is a term used to cover a range of hydro-carbon organic compounds with certain characteristics – they are soluble in non-polar solvents, and hence not soluble in water. This means they are also able to dissolve non-polar substances. Their functions in nature vary from being used for energy storage as well as structure and in some cases are hormonal.
Fat and oil are both general terms used to describe lipids – fat is simply lipids in a solid state, and when at a temperature that turns it to a liquid is described as an oil. Now for our different types of oils:
This group is the one that gets the most press and attention from the media, mostly in relation to heart disease. They are drawn in a way that resembles a bizarre capital E. They have a backbone (of glycerol) and three chains of fatty acids bonded to it. (So the name should make sense now)
Their main role is in energy storage. The two groups we hear most about are saturated and unsaturated. Saturated cause the human heart more troubles because the solidify into fat at a lower temperature than unsaturated. The picture above looks very neat. Stacking up that particular triglyceride would be very easy due to its uniform shape. Compare it to this unsaturated triglyceride:
This would be much harder to stack neatly because of the strange shapes of the fatty acids. These kinks occur around the double bonds we see in this picture because not every carbon in the chain has as many hydrogens as it could so it uses its spare bond to link to another carbon in the chain causing this shape. For this reason they have a much lower melting point and remain liquid inside our bodies, not clogging up our hearts.
In the food industry health is often ranked after sensation, and both are ranked behind convenience. Top of their agenda is the fact that saturated fats are less susceptable to rancidity than unsaturated fats. The saturated fats are also better for things like butter substitute spreads and for bakery products so the natural oils undergo hydrogenation where hydrogen molecules are added to try and get rid of all the double bonds. It is not always carried through to completion, instead until a desired melting point has been reached. There are now laws about packaging foods that contain hydrogenated or partially hydrogenated fats.
Free Fatty Acids
The fatty acids that make up the chains do not have to be bonded to anything and can exist as free molecules. The body can manufacture all but two of the fatty acids it needs – linoleic and alpha-linolenic. These are widely available to us in plants and fish.
Some fatty acids:
These are another varied class of hydrocarbons. In nature they are a main component of resins, and also make up a great deal of essential oils (so are excellent carriers of volatile aromatics). Structurally they are built from isoprene units:
Multiple units of isoprene can exist in chains or sometimes may form rings.
A group of lipids, derived from steroids, that are have both hydrophobic and hydrophilic sections (know as amphipathic). I don’t want to dwell on these too much, mostly because it is difficult to find something interesting to say about them!
Now onto the interesting stuff:
Lipids in green and roasted Coffee
The oils in coffee are written about far more than really they ought to be. They seem to have been given for an explanation for almost every aspect of espresso coffee, from crema to mouthfeel to finish. Undoubtedly they play a role in all these but, as I hope I’ve vaguely explained in my crema article, it isn’t necessarily the one we think.
The breakdown of lipids in coffee comes from Maier’s research published in 1981. Whilst farming practises may have changed and processing may have evolved at the moment science seems reluctant to retread old ground when there is little indication they will find something new. Hence most data in coffee is some years old.
Esters of diterpenes 18.5%
Diterpene alcohols 0.4%
Esters of sterols 3.2%
Tryptamine derivatives 0.6-1.0%
Hopefully this table should explain the weighting of explanation above…
In terms of coffea arabica vs coffea canephora the total lipid fraction is higher in arabica, at approx 15% by weight with robusta in at around 10%. The way this is usually presented implies that within this fraction the ratios are the same, and it isn’t that robustas just have less triglycerides.
The free fatty acids are dominated by two – palmitic and linoleic. There are fair quantities of oleic and stearic too, with most others found in trace amounts. So good for our essential fatty acid intake.
Theiterpenes, mostly diterpenes, seem to have generated some media interest lately – specifically cafestol, kahweol and 16-O-methylcafestol which have been linked to heart disease. Interestingly 16-O-methylcafestol is a good indicator for the presence of robusta should you been running tests on a blend. Apparently arabica has more terpenes than robusta.
The sterols in coffee are similar to those found in any oil. The breakdown of differente sterols between arabica and robusta is pretty similar, and thankfully for us there are minimal quantities (0.2-0.3%) of cholesterol.
Whilst there isn’t the myriad of changes and reactions that one sees in the carbohydrates and proteins in the roasting process, there are still some interesting changes that would make for a short article in itself.
I am aware this article is a little dry and heavy on the chemistry, but the point of it is to create some ground work.
What I am want to look at in future articles is the role oils play in how we enjoy our coffee, in mouthfeel, in extraction, and I thought I ought to make sure all the terms I was using had been defined somewhere. So if you’ve waded through this far then I shall do my utmost to justify it in future.