Humic-like crop stimulatory activities of coffee waste induced by incorporation of phytotoxic phenols in melanoidins during coffee roasting: Linking the Maillard reaction to humification.

Here we showed that the water-soluble components of fresh green coffee beans inhibit the growth of lettuce in hydroponic systems, whereas those of roasted coffee waste facilitate it. The growth enhancement was hardly related to hydroponic parameters (i.e., pH and electric conductivity) or the nitrogen contents of the extracts. Rather, the presence of chromogenic polymeric melanoidins in the coffee waste was found to be crucial for the crop growth acceleration. The quantitative comparison of low-molecular-weight organics including phytotoxic phenolics between the extracts suggested that Maillard reactions occurring during coffee roasting transform the phenolics into polymeric melanoidin products. The identification of humic-like molecular compositions in the roasted coffee waste and the restoration of crop-stimulating activity by the addition of a phenol oxidase to the fresh coffee bean extract also supported that the low-molecular-weight phenols are oxidatively coupled during the roasting, which was consistent with the bottom-up synthesis of crop-stimulatory humic substances.