Added-value molecules recovery and biofuels production from spent coffee grounds

Spent Coffee Grounds worldwide production is estimated at around 6 M tons only at industrial level. The abundance and the heterogeneity of this substrate make it an ideal substrate for a biorefinery approach based on the "cascade biorefinery hierarchy". Currently, the major part of spent coffee grounds is sent to incineration and landfill disposal, options which should be avoided. Instead, they could be valorised through biofuels production. All the operational parameters leading to the highest biogas (350-400L(CH4)/kg(TVS)), bioethanol (3-4%v/v) and biodiesel (over 90% of Fatty Acid Methyl Esters concentration) yields from spent coffee grounds have been discussed in this review paper. They are rich in an oil phase containing different added-value molecules (tocopherols, cafestol, kahweol along with linoleic and palmitic acids), which can be extracted and used as additives for food, cosmetic and pharmaceutical applications. Solid/liquid extraction techniques of coffee oil from spent coffee grounds such as the most common Soxhlet technique and the more innovative fluids in supercritical conditions have been discussed, with coffee oil recovery of around 5-15%w/w and 15-20%w/w, respectively. The most recent applications of the extracted coffee oil have been also presented: the added-value molecules recovery and purification after micro/ultra and nano filtrations processes and the polyhydroxyalkanoates (0.84 g/g) and biosurfactants (3.5 g/L) production. Considering the whole information, an integrated biorefinery scheme, along with the respective mass balances were proposed. The novelty of this paper lies in the integration of the state-of-the-art data, in a biorefinery concept that would allow the production of both biofuels and value-added products.