Coffee is one the most important agricultural commodities in the world, but also the one with higher amounts of waste-products of low added value (90% of the crop), representing a major ambient threat in producing countries. It is estimated that more than 5 million tones/year of coffee wastes are generated worldwide. Several strategies to reconvert both coffee pulp and husk have been attempted in producing countries (fuel pellets, silage, biogas production). Most of them are not technologically efficient, cause secondary pollution, or are economically unviable. Due to their high organic matter and nitrogen contents, attempts to reuse for animal feed have also been described but the presence of tannins, caffeine, and phenols, induces anti-nutritional effects. Solid-state cultivation of mushrooms was also described, with positive outcomes. Nevertheless, plant germination can be affected by the high phenol amounts and one reference details the incorporation of caffeine, with consequences for human consumption. Importing countries, Portugal included, are co-responsible for these wastes, but only deal with the spent roasted coffee after beverage extraction. Nevertheless, it is a small but still representative reality: 36.000 tons last year in Portugal, a medium consuming country. According with official data from AICC, about 70% of our coffee is consumed outside home. Despite being mandatory to separate the spent coffee, imposed for traceability purposes, spent grounds are mixed with common garbage and send for sanitary land fill or incineration, among other options. In opposition to the residues from the instant coffee industries, these spent grounds are not exhausted, retaining important coffee components, caffeine included (about 20% remains after espresso coffee preparation), and antioxidant activity, highlighting its richness in biologically active compounds. References to the benefits of coffee grounds in domestic cultures are common but very little scientific information focused on the feasibility of its utilization. A detailed study on the influence of spent coffee in agriculture (cultures, soil and ambient) is inexistent.
Therefore, the objective of this study is to determine the use of spent coffee for horticultural purposes without compromising this triangle and, if possible, with benefits for one or more fractions. How much coffee can be used without compromising the plant, its nutritional properties, the soil and environment? Can spent coffee be used as is or it must be stabilized by aerobic or anaerobic composting? These are issues that must be fully studied and understand: only after that a safe use of this waste can be implemented and spread.