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

Background During the processing of spent coffee grounds (SCGs) several residues are obtained, which are mostly disposed of in landfills. There is an urgent need for a comprehensive waste management strategy for these residues. This study evaluates the potential of SCGs as a biofertilizer by assessing their effects on lettuce leaves and the release of antioxidants following in 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 in the 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 the digested fraction: applying the Ferric Reducing Antioxidant Power (FRAP) method, the addition of SCGs from 10 g kg −1 to 125 g kg −1 increased the amount of antioxidant from 43.88 ± 4.81 to 105.96 ± 29.09 μmol Trolox g −1 of dry weight ( P < 0.05). The Indigo Carmine Reducing Capacity (IC RED ) method also showed a similar trend, but in this case the highest value was 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 fermented fraction a significant increase in the antioxidant released was found with low levels of SCG( P <0.05), while lettuces fertilized with 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 acceptable visual quality.