Solid-state fermentation: a review of its opportunities and challenges in the framework of circular bioeconomy

Solid-state fermentation (SSF) is on the way to become an attractive alternative for the valorisation of a wide range of organic waste. In biological terms, SSF can be defined as the cultivation process in which microorganisms grow on solid materials without the presence of free water. When these solid materials are organic waste and the objective is to produce one or several bioproducts of added value, SSF is a clear opportunity for circular bioeconomy, with a new paradigm "from waste to resource". The development of SSF started a couple of decades ago, with biomolecules that only could be produced through biological systems, such as enzymes or antibiotics. However, a new generation of SSF bioproducts, which have a "twin" produced by chemical pathways, has emerged in the last years, with the advantages of having lower environmental impacts (for instance, no need of water and less energy requirements) and using waste as substrate. Some of these compounds are highly relevant in the field of chemical engineering: biosurfactants, biopesticides, aromas, bioplastics, pigments and bioflocculants, among others. This review explores the new advances in SSF: from the organic waste used as substrate to the main challenges SSF is facing, that is, mass and heat transfer limitations, bioproducts downstream and scale-up.