Antioxidant activity and bioactive compounds of lettuce improved by espresso coffee residues

•First report on espresso spent coffee effect in antioxidant and bioactive compounds of Lactuca sativa L.•Fresh coffee by-products potentiated antioxidant activity and phenolic fraction of lettuce extracts.•Fresh coffee by-products at least doubled major and minor carotenoids in lettuce.•Composted coffee by-products greatly improved lettuce’s photosynthetic capacity.•Espresso spent coffee is a renewable feedstock for horticulture nutritional quality improvement. The antioxidant activity and individual bioactive compounds of lettuce, cultivated with 2.5–30% (v/v) of fresh or composted espresso spent coffee grounds, were assessed. A progressive enhancement of lettuce’s antioxidant capacity, evaluated by radical scavenging effect and reducing power, was exhibited with the increment of fresh spent coffee amounts, while this pattern was not so clear with composted treatments. Total reducing capacity also improved, particularly for low spent coffee concentrations. Additionally, very significant positive correlations were observed for all carotenoids in plants from fresh spent coffee treatments, particularly for violaxanthin, evaluated by HPLC. Furthermore, chlorophyll a was a good discriminating factor between control group and all spent coffee treated samples, while vitamin E was not significantly affected. Espresso spent coffee grounds are a recognised and valuable source of bioactive compounds, proving herein, for the first time, to potentiate the antioxidant pool and quality of the vegetables produced.