Conceptualization of a spent coffee grounds biorefinery: A review of existing valorisation approaches

The valorisation of food waste is an increasingly practical and sustainable solution to the problem of a growing demand for chemicals, fuels and materials and the rising tonnage of municipal waste sent to landfill. Spent coffee grounds (SCG) are the end product of the coffee processing industry, generated after beverage preparation, and have been exploited as a valuable source of polysaccharides, lipids, protein, minerals and bioactive secondary metabolites including diterpenes, sterols, chlorogenic acids, flavonoids and caffeine. Within the biorefinery paradigm, where renewable resources are converted into a range of high, medium and low value products, in an analogous manner to fossil fuels in a petrochemical refinery, SCG have been established as an amenable lignocellulosic feedstock through numerous research efforts. In this critical review, we give an extensive overview for the first time of the primary and secondary product suites that can be generated from SCG, along with their potential applications. The handful of preliminary technoeconomic and lifecycle assessment of using SCG for bioenergy is discussed, highlighting the economic limitations of a single capability, phase one biorefinery operating under the current scale and logistics of SCG collection. A concluding perspective towards future SCG-based biorefineries is presented, where isolation and production of higher value bioactive products is expected to be integral to the economic feasibility of the process.