Spent coffee grounds were our main feedstock at Blue Marble for more than seven years. We collected them from across the state, extracted the oil, made biodiesel from it, and fermented what was left. Brewing had already pulled out enough to make a cup of coffee. A lot remained.
Coffee grounds hold on to their oil after brewing. Kondamudi and colleagues extracted 10 to 15% oil from spent grounds, depending on the coffee species, and turned it into biodiesel by transesterification. [1] Our extracted oil was almost entirely free fatty acids, so we used direct esterification. We pulled the oil out with bio-based butanol, then added hot sulfuric acid to the extract. The butanol that did the extracting became the butyl group in the resulting esters. A water wash, and we had butyl esters.
The deoiled grounds went into anaerobic fermentation with fiber-degrading organisms, which gave us alcohols and acids of several chain lengths. We paired those into more esters, and each acid and alcohol combination gave a different molecule. The fermentations also made a lot of hydrogen sulfide. Testing before and after showed vanillin appearing in the tank. Ferulic acid, which microbes can free from plant cell walls, is a well-documented microbial precursor of vanillin, [2] though we never confirmed that route in our process. We also found other aldehydes and furfurals.
Removing the oil changed the material entering fermentation, and leaving it in changed what came out. Fermentations on unextracted grounds made high levels of butyric and caproic acids. My best guess goes back to where the electrons go. Long-chain fatty acids can inhibit methanogenesis. [3] If the oil weakened that hydrogen sink, the reducing power had to land somewhere else, and chain-elongating microbes can use it, along with alcohols like ethanol, to build longer acids such as butyrate and caproate. [4] We saw the shift in products but never tested the cause.
Seven years of spent grounds taught me to look past the name of a waste stream. “Spent coffee” names what someone already took out. The name gives no account of the remainder, how much of it could be recovered, or what the organisms would do with it. Coffee kept us asking how much of that sequence could earn its place.
Sources
[1] Kondamudi, Mohapatra, and Misra (2008). Spent coffee grounds as a versatile source of green energy. Journal of Agricultural and Food Chemistry.
https://doi.org/10.1021/jf802487s
[2] Priefert, Rabenhorst, and Steinbüchel (2001). Biotechnological production of vanillin. Applied Microbiology and Biotechnology.
https://doi.org/10.1007/s002530100687
[3] Koster and Cramer (1987). Inhibition of methanogenesis from acetate in granular sludge by long-chain fatty acids. Applied and Environmental Microbiology.
https://doi.org/10.1128/aem.53.2.403-409.1987
[4] Angenent et al. (2016). Chain elongation with reactor microbiomes: open-culture biotechnology to produce biochemicals. Environmental Science & Technology.
https://doi.org/10.1021/acs.est.5b04847