Spent coffee grounds as an alternative fertilizer: impact on bioaccessibility of antioxidants and commercial quality of lettuce

Abstract

Background: During the processing of spent coffee grounds (SCGs) several residues are obtained, which are mostly disposed ofin landfills. There is an urgent need for a comprehensive waste management strategy for these residues. This study evaluatesthe potential of SCGs as a biofertilizer by assessing their effects on lettuce leaves and the release of antioxidants followingin vitro digestion and fermentation.Results: Lettuce plants were grown with different amounts of SCGs (0–150 g kg−1) in the substrate. High SCG concentrations inthe soil generated lighter colored tissues, a decrease in the green color, less root development, and lower dry weight of leaves(P < 0.05). The SCG levels also affected the release of antioxidants by the final product. This effect was more pronounced in thedigested fraction: applying the Ferric Reducing Antioxidant Power (FRAP) method, the addition of SCGs from 10 g kg−1 to125 g kg−1 increased the amount of antioxidant from 43.88 ± 4.81 to 105.96 ± 29.09 ∼mol Trolox g−1 of dry weight(P < 0.05). The Indigo Carmine Reducing Capacity (ICRED) method also showed a similar trend, but in this case the highest valuewas obtained with 150 g kg−1 of SCGs (16.41 ± 3.93 mmol catechin g−1 of dry weight) (P < 0.05). Moreover, in the fermentedfraction a significant increase in the antioxidant released was found with low levels of SCG(P<0.05), while lettuces fertilizedwith intermediate amounts of SCGs (25 and 50 g kg−1) presented the highest amount of insoluble antioxidant (P < 0.05).Conclusion: A compromise should be found in order to achieve a product with a high antioxidant capacity and an acceptablevisual quality