Spent coffee grounds reveal a route to biodiesel and other bio-based products
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Coffee grounds that usually end up being thrown away can have a second life as a raw material for producing biofuels and other high-value-added products. A study by the Universitat Rovira i Virgili (URV) has evaluated how to extract oil from coffee grounds efficiently while preserving the rest of the plant material so that it can also be used in other processes.
The research, published in Biomass and Bioenergy, focuses on spent coffee grounds, which are a very abundant waste product. According to the article, global coffee bean production stands at around 10 million metric tons per year, only a small proportion of which actually ends up in coffee after the drink is brewed. The remainder becomes solid waste in the form of coffee grounds, which contain approximately 15% lipids, or fats that can serve as a basis for producing biodiesel.
The research team, comprising Jorge F. Romero, Alberto Tampieri, Daniel Montané, Magdalena Constantí and Francesc Medina, all from the URV's Department of Chemical Engineering, studied how three key factors influence the extraction of this oil: temperature, processing time and the solvent-to-coffee-grounds ratio. To do this, they used n-hexane, a solvent commonly used in fat extraction, and applied an experimental design that allowed them to analyze the combined effects of all these variables.
Balancing yield with cleaner oil
"We have found that the optimal conditions are at 45°C for 60 minutes with a ratio of 35 milliliters of hexane per gram of dry residue," explained Constantí, one of the study's authors. With these parameters, the process can recover approximately 90% of the amount of oil that can be obtained with Soxhlet, a laboratory technique widely used as a point of reference because it offers high yields but requires more time and energy and is less suitable for industrial applications.
In addition to the amount of oil recovered, the study emphasizes the quality of the extract. The optimized process yielded an oil with a very low impurity content of 0.3%, in contrast to Soxhlet, where this figure is 3.9%. Analysis of the fat composition showed that the fatty acid profile remained stable under different test conditions and was dominated by linoleic and palmitic acids, two components that indicate this oil's potential as a raw material for biodiesel production.
"In our study, we also demonstrate that extracting the oil does not mean that the rest of the material cannot be used for something else," pointed out Medina, a researcher at the Department of Chemical Engineering who participated in the research.
Keeping the residue useful
In fact, one of the objectives of the research was to preserve the so-called lignocellulosic matrix of the residues, which is made up of components such as cellulose, hemicellulose and lignin. These ingredients can be used to obtain other products, such as bioethanol, lactic acid, polyhydroxyalkanoates or precursors for sustainable aviation fuels and phenolic compounds.
The extraction process not only recovers oils but also acts as a pretreatment. The fats present in the residue can prevent solvents or catalysts from accessing the rest of the biomass. By removing this barrier without significantly altering the material's structure, the fat-free residue is left in a better state for subsequent use.
Moderate conditions beat faster alternatives
The research team also compared its method with techniques such as ultrasound- or microwave-assisted extraction. Although these alternatives can accelerate the initial extraction, the study concluded that they do not offer a sufficient advantage in terms of oil quality, overall efficiency, energy demand and scalability. For this reason, the batch process with n-hexane under moderate conditions is likely to be a better, more balanced option for integration into a biorefining strategy.
The scientists' work forms part of efforts to develop techniques for a circular economy and addresses the need to develop renewable fuels for hard-to-electrify sectors, such as heavy transport.
By using every part of the coffee grounds, the researchers are able "to transform a typically underused waste product into various energy vectors and bio-based chemical products, and to reduce the environmental impact associated with its accumulation," explained Montané, a researcher in the same department who also participated in the research. This, in turn, paves the way for the sustainable production of biofuels.
More information
Jorge F. Romero et al, Systematic evaluation of batch hexane extraction as a scalable pretreatment for the comprehensive valorization of spent coffee grounds, Biomass and Bioenergy (2026). DOI: 10.1016/j.biombioe.2026.109467
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Citation: Spent coffee grounds reveal a route to biodiesel and other bio-based products (2026, July 23) retrieved 23 July 2026 from https://phys.org/news/2026-07-spent-coffee-grounds-reveal-route.html
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