Spent coffee ground and coffee silverskin: possible use as nutraceuticals and ingredients for fertilizer products.

Coffee is one of the most consumed beverages worldwide and according to the latest statistics the global consumption is more than 150 million of 60 kg coffee bags per year. Consequently, large amount of coffee residues needs to be disposed of. The main generated by-products are coffee silverskin (CS) and spent coffee grounds (SCGs): silverskin, the thin tegument that covers the coffee bean, is obtained when green coffee is roasted, whereas SCGs are mainly formed during the production of instant coffee. CS and SCG disposal represent an environmental problem but, at the same time, they are a potential source of valuable by-products. Indeed, they are a good source of nutrients and bioactive compounds such as soluble dietary fibre, protein, minerals, fat, caffeine and polyphenols [1, 2]. In our research group, the two matrices have been fully chemically characterize in terms of polyphenols, fat and fatty acid profile, volatiles, cholesterol-lowering phytosterols, and vitamins [1-4]. The content of polyphenols and phytosterols have been monitored also in all the coffee production chain, starting from green coffee, to CS, roasted coffee, espresso coffee and SCG to have a complete view of theirs evolution. From a biological point of view, this research highlights that CS and SCG extracts protect cells against H2O2-induced oxidative stress by upregulating endogenous antioxidant enzymes such as thioredoxin reductase, heme-oxygenase 1, NADPH quinone oxidoreductase, and glutathione reductase. Moreover, the hydroalcoholic and methanolic SCG and CS extracts were shown to be the most active against all selected enzymes such as tyrosinase, α-glucosidase, α-amylase and α-cholinesterases. Finally, we evaluated the effect of CS/SCG-containing fertilizers on plant growth and modulation of soil microbiome. SCG and CS application as organic amendment are known to modify soil chemical-physical properties, provide macro and micronutrients fundamental for plant growth, reducing the need for inorganic fertilizer, and stimulate soil microflora exerting a beneficial effect on soil-plant systems. In this study, a novel granulated formulation of SCG and CS was developed and assayed for its potential toxicity on seed germination and root elongation of four crop plants.