Abstract
An efficient processing of organic solid residues will be pivotal in the development of the circular bioeconomy. Due to their composition, such residues comprise a great biochemical conversion potential through fermentations. Generally, the carbohydrates and proteins present in the organic wastes cannot be directly metabolized by microorganisms. Thus, before fermentation, enzymes are used in a hydrolysis step to release digestible sugars and nitrogen. Although enzymes can be efficiently produced from organic solid residues in solid-state fermentations (SsF), challenges in the development and scale-up of SsF technologies, especially bioreactors, have hindered a wider application of such systems. Therefore, most of the commercial enzymes are produced in submerged-liquid fermentations (SmF) from expensive simple sugars. Instead of independently evaluating SsF and SmF, the review covers the option of combining them in a sequential process in which, enzymes are firstly produced in SsF and then used for hydrolysis, yielding a suitable medium for SmF. The article reviews experimental work that has demonstrated the feasibility of the process and underlines the benefits that such combination has. Finally, a discussion is included which highlights that, unlike typically perceived, SsF should not be considered a counterpart of SmF but, in contrast, the main advantages of each type of fermentation are accentuated in a synergistic sequential SsF-SmF.
Highlights
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
The possibility of valorising unexploited waste biomass through Solid-state fermentation (SsF) is increasing the relevance of this technology
There has been a clear evolution in the type of substrates used in the process
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Solid-state fermentation (SsF) can be defined as a process in which solid particles, with an inter-particle continuous gaseous phase, act either as the substrate or as inert solid support for the growth of microorganisms, in the absence or near absence of free water [1]. SsF takes place in nature for example during the decomposition of solid organic matter, but for millennia, humans have taken advantage of SsF, adapting it for the production of food and beverages as in the case of bread, cheese, tempeh, sake, soy sauce or during cacao and coffee processing [2]
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