Chris Smith:Tell us about how coffee, said to power many scientists and mathematicians, could also power cars.
Ben Valsler: I love this story. Researchers have found that oil extracted from used coffee grounds makes very viable biodiesel, and currently with the amount of coffee we drink we could supply up to 350 million gallons of biofuel, and they’ve put this report in the Journal of Agricultural and Food Chemistry. You can make biodiesel from many different plant oils such as soya bean or palm oil and even cooking oil recycled from restaurants which gives your exhaust a very distinctive chip shop smell. However, with the world’s population rising, we must strike a balance between the land we use to grow food and the land set aside to grow crops to meet the demand for biodiesel.
Chris Smith: How do we actually do it though? How does one get all the used coffee grounds and turn that into something that you can put into your car?
Ben Valsler: Well, Mano Misra and colleagues at the University of Nevada observed that spent coffee grounds contain between 11 and 20 per cent oil by weight, which is actually a similar amount to the more traditional biodiesel crops like rapeseeds. So they managed to grab lots and lots of used grounds from a multinational chain of coffee shops which would have just been sold off for compost.
So what they did was they dried out the grounds and mixed them up with solvents like ether and hexane, the oils dissolve into the solvents, and then they filter out all the solid stuff which can still go on to be compost. The solvents evaporate off leaving just the oil, which they then have to put through a stage called transesterification. They kept it going until there was no trace of oil left which showed the whole thing had been converted to biodiesel. And interestingly the fuel itself actually smells of coffee - which might be quite a lot nicer coming out of your exhaust than chip shop smells.
Chris Smith: Certainly give any caffeine addicts a hankering for a cup of coffee wouldn’t it, but is this actually beneficial to the environment? Because with all those steps and all those other chemicals being slung into the equation does it still make economic and scientific sense to do this?
Ben Valsler: Well simply put we are producing lots of biodiesel anyway, so all of the chemicals that go into the process as it expands those chemicals are going to be used. By using something like used coffee grounds, we’re actually taking a source that’s already being produced, it's already going to waste, and when they tested it, they found it's of a better quality and more cost-effective than other waste sources of oil such as used chip shop oil. So actually it does look like it may be a very, very good source of biodiesel with a slightly lower environmental impact than some of the others.
Chris Smith: We'll just have to wait and see I think.
- This discussion was originally broadcast on Breaking Science on BBC Radio Five Live in December 2008. You can listen to the whole episode on OpenLearn.
Update: April 2014
In 2013, researchers at the University of Cincinatti presented similar results from experiments using leftover grounds from Starbucks.
And Nestle is using coffee waste to fuel some of its coffee plants.