I don't often think about fat in my coffee (unless you like to add cream to yours - I prefer not to dilute my coffee.) If you consider that a coffee bean is just a seed for the coffee tree, it's not so surprising. The growing plant needs food and fat is a pretty good source of calories. You can see for yourself what the fat content in coffee is: the USDA has an online database you can search for nutritional info. Do a search for coffee - I chose the listing for:
Coffee, brewed from grounds, prepared with tap waterFor a one cup serving, there is about 50 mg of fat, most of which is listed as "18:1 c" which means there are 18 carbons and one double bond in the fatty acids present, with a cis double bond. The common name for this fatty acid is oleic acid.
By way of Greg Ladens blog, I came across a paper describing how Biodiesel can be obtained from coffee grounds. Greg's post discusses some of the more practical aspects - and possible problems - in coffee being an economically useful source of Biodiesel. I want to talk about the chemistry involved.
Spent Coffee Grounds as a Versatile Source of Green Energy describes how Biodiesel can be obtained from used coffee grounds. Specifically they got their used grounds from Starbucks, probably a good source. According to the paper, the used coffee grounds yielded about 15% oil which compares favorably with the yields from soybean oil and palm oil.
So just how to you get biodiesel from coffee grounds? Surprisingly you start the same way you would if you were brewing coffee. However instead of using water, they "brewed" the used grounds with an organic solvent which would dissolve any oils remaining in the grounds. They tried three different solvents: hexane, dichloromethane and ether. In addition to the oils, they also obtained Free Fatty Acids (FFA's), which had to be removed before they could convert the oil to biodiesel. Hexane extracted the least amount of FFA's, so that was the solvent they chose to use.
The grounds were separated from the solvent-and-oil mixture by filtering it - again, just like brewing coffee. The solvent was evaporated and collected for re-use and the FFA's removed from the crude oil by extracting with water and base. This effectively converts the FFA to soap which is water soluble and can be separated from the non-water-soluble oils. The FFA's have to be removed for two reasons.
- FAA's are typically solids, or become solids at temperatures you might experience while driving. Oleic acid has a melting point of 13-15 degrees C, that's about 57 degrees F. Having your fuel become a solid in the fuel line or engine would probably be inconvenient.
- The method they use to convert the crude oil into biodiesel would not work on FFA's.
You could use the straight oil to run a diesel engine without doing anything else. In fact, Rudolf Diesel did experiment with vegetable oils in his engine. Usually, the straight vegetable oil is converted into the methyl (or ethyl) ester and this is what is meant by "biodiesel." The main reason for this is that the methyl ester is less viscous and stays liquid at a lower temperature than the straight vegetable oil does.
Biological fats and oils are triglycerides, which consist of a molecule of glycerol and three fatty acid molecules joined together to form a single molecule that is a triple-ester. In making biodiesel, a transesterification reaction replaces the glycerol triple-ester with three molecules of methyl ester.
As any organic chemistry student could tell you, the transesterification reaction needs a catalyst and both acid and base will work as the catalyst. If they had used an acid as their catalyst, they would probably not have needed to remove the FAA's, because under acidic conditions the FFA's would also have been converted into the corresponding methyl esters. Under basic conditions the FAA's become soap instead, which is even less useful in your engine than the straight FFA.
So why did they use base as the catalyst? The reaction is faster with the basic catalyst than with the acid catalyst, see this paper for a comparison of the two catalysts. There might be other practical reasons, too. KOH is a solid and not volatile. Although the KOH dust is corrosive, it is probably more convenient to handle and spills could be swept up. Sulfuric acid would be handled as a liquid, it produces corrosive vapors and any spills would much more trouble to clean up.
Cool Google tip: Google will calculate unit conversions for you. I got the melting point for oleic acid from Wikipedia, which gave the mp in celcius. While I use celsius all the time in the lab, I'm not so familiar with thinking about celsius in terms of weather forcasts, so I converted it to Fahrenheit. And I didn't need to look up the tedious equation since Google can do the work for me. In the google search box type: "14 degrees C in F" without the quotes and hit return. Google will reply with: "14 degrees Celsius = 57.2 degrees Fahrenheit"
Narasimharao Kondamudi, Susanta K. Mohapatra, Mano Misra (2008). Spent Coffee Grounds as a Versatile Source of Green Energy Journal of Agricultural and Food Chemistry, 56 (24), 11757-11760 DOI: 10.1021/jf802487s