Researchers at the Universitat Rovira i Virgili (URV) have identified optimal conditions for extracting oil from spent coffee grounds, pointing to a route for turning the waste product into biodiesel feedstock and other bio-based materials.
The study, published in Biomass and Bioenergy, looked at spent coffee grounds as a high-volume waste stream. Global coffee production runs to around 10 million tonnes annually, and only a small fraction of the bean ends up in the drink itself — the rest becomes solid waste. That waste contains roughly 15% lipids, giving it potential as a biodiesel precursor.
The team — Jorge F. Romero, Alberto Tampieri, Daniel Montané, Magdalena Constantí and Francesc Medina, all from URV's Department of Chemical Engineering — tested how temperature, processing time and solvent ratio affected oil yield, using the solvent n-hexane and an experimental design that captured how the variables interacted.
The team found that processing at 45°C for 60 minutes, with a solvent ratio of 35ml of hexane per gram of dry residue, recovered around 90% of the oil yield achievable through Soxhlet extraction — a long-established reference method that delivers strong yields but is energy- and time-intensive and not well suited to industrial-scale use.
The optimised method also produced a cleaner extract, with impurity levels around eight times lower than those from Soxhlet extraction. The resulting oil's fatty acid profile remained consistent across test conditions and was dominated by linoleic and palmitic acids, both of which support its suitability for biodiesel production.
Researchers also tested ultrasound- and microwave-assisted extraction as alternatives. While these methods sped up initial extraction, the team concluded they didn't offer meaningful gains in oil quality, overall efficiency, energy use or scalability — leaving batch hexane extraction under moderate conditions as the more practical option for integration into a biorefinery process.
The study also aimed to preserve the coffee grounds' lignocellulosic structure — the cellulose, hemicellulose and lignin content — so the residue left over after oil extraction can still be used elsewhere, including for bioethanol, lactic acid, polyhydroxyalkanoates, or precursors for sustainable aviation fuel and phenolic compounds. The researchers noted that removing the fat content also functions as a pretreatment step, since fats can otherwise block solvents or catalysts from accessing the rest of the biomass.
The work sits within wider efforts to build circular-economy approaches to renewable fuel production, particularly for harder-to-electrify sectors such as heavy transport, by extracting multiple value streams from a single waste product.
Spent coffee grounds show potential as biomass feedstock
