Microbial and chemical dynamics of brewers' spent grain during a low-input pre-vermicomposting treatment

The eco-sustainability of industrial processes relies on the proper exploitation of by-products and wastes. Recently, brewers' spent grain (BSG), the main by-product of brewing, was successfully recycled through vermicomposting to produce an organic soil conditioner. However, the pre-processing step there applied (oven-drying) resulted in high costs and the suppression of microbial species beneficial for soil fertility. To overcome these limitations, a low-input pre-processing step was here applied to better exploit BSG microbiota and to make BSG suitable for vermicomposting. During 51 days of pre-treatment, the bacterial and fungal communities of BSG were monitored by denaturing gradient gel electrophoresis (DGGE). Chemical (carbon, nitrogen, ammonium, nitrate content, dissolved organic carbon) and biochemical (dehydrogenase activity) parameters were also evaluated. Mature vermicompost obtained from pre-processed BSG was characterized considering its legal requirements (e.g., absence of pathogens and mycotoxins, lack of phytotoxicity on seeds), microbiota composition, and chemical properties. Results obtained showed that throughout the pre-process, the BSG microbiota was enriched in bacterial and fungal species of significant biotechnological and agronomic potential, including lactic acid bacteria (Weissella, Pediococcus), plant growth-promoting bacteria (Bacillus, Pseudomonas, Pseudoxhantomonas), and biostimulant yeasts (Pichia fermentans, Trichoderma reesei, Beauveria bassiana). Pre-processing increased the suitability of BSG for earthworms' activity to produce high-quality mature vermicompost.