Valorization of spent coffee grounds into metal–phenolic coatings for producing spray-dried microbial biostimulants for sustainable agriculture

Dry microbial biostimulants offer a convenient and sustainable approach to enhancing crop health and yield. However, their efficacy is limited by the challenge of stabilizing microbes into dry products because scalable dehydration methods such as spray drying impose substantial thermal and osmotic stresses that often result in substantial viability losses. In this work, metal-phenolic network (MPN) coatings were applied to Klebsiella michiganensis M5al, a Gram-negative, plant-beneficial bacteria, prior to spray drying to enable their survival during the rapid thermal dehydration process. Uncoated cells lost 5.0 ± 0.21 log viability during spray drying, whereas tannic-acid-iron coatings reduced this loss to 3.2 ± 0.34 log with a streamlined, single-coat process, and 2.1 ± 0.47 log with a two-coat process. Furthermore, phenolics present in water extracts of spent coffee grounds (SCG) were shown to form MPNs with protective performance comparable to those formed from purified tannic acid, reducing viability loss during spray drying to 3.5 ± 0.19 log. Nano-scale infrared spectroscopy confirmed the presence of metal-phenolic coatings on cell surfaces, while atomic force microscopy indicated improved structural preservation during spray-drying. These findings show that SCG-derived phenolics can serve as sustainable feedstocks for protective MPN coatings and demonstrate the feasibility of using waste-derived phenolics as functional substitutes for purified reagents in MPN coatings. This approach simultaneously enables scalable production of dry microbial biostimulants and valorization of abundant, phenolic-rich agro-industrial residues.