Auxins and phenols in anaerobic thermophilic digestion of coffee wastes and their synergistic effect in horticulture

During the manufacture of instant coffee, the finished product is only 33% of the raw material. The remainder, solid and liquid wastes, must be removed from the factory and dumped, at considerable cost and to the detriment of the environment. We investigated the anaerobic methanogenic thermophilic digestion (AMTD) of wastes from an instant coffee factory as a means of reducing the environmental hazards. It was found that the major product of the process, the sieved residue Capul, has commercial value as a plant-growth medium. Both Capul and whole digested slurry showed a clear promotional effect on the growth of mung bean (Phaseolus aureus cuttings in the laboratory and of ornamental plants in greenhouse experiments, which was the result of the synergistic action of auxins and phenolic compounds. Auxins (indole-3-acetic acid and indole-3-butyric acid) and phenols (predominantly chlorogenic acid) were identified and quantified in raw coffee beans, instant coffee wastes, and their AMTD slurries. In AMTD slurries, there was an increase of caffeic, coumaric, and ferulic acids. Extracts of peaks from HPLC corresponding to phenols were examined (in the presence or absence of auxins) for their growth-promoting effect on mung bean cuttings. It was found that auxins alone had a lower promotional effect than the extracted phenols mixed with auxins. This AMTD technology can protect the environment, while bringing a profit to coffee producers and to nurseries.