Bioconversion of Crop Residues Using Alternative Fermentation-Based Approaches

Globally, the growing production of food commodities generates significant quantities of agroindustrial residues, most of which are untreated and disposed of as waste through either burning, dumping into the land, or unplanned landfilling, thereby causing environmental pollution, public health problems, and decreased soil organic matter and soil productivity. A literature review has been conducted on the current crop residue biomass valorization, analyzing raw material properties and the potential risks associated with its incorrect or absent management, as well as the major microbial fermentation strategies that are used for converting residual crops into valuable products. Approximately 2445.2 million tons of crop residues are produced worldwide. Microbial fermentation is an efficient way of managing residues that are rich in nutrients (e.g., nitrogen, phosphorus, and potassium) and converting them into single-cell proteins, antibiotics, enzymes, bioalcohols, polysaccharides, fine chemicals, and others, thereby supporting a circular bioeconomy. Although separate saccharification and fermentation (SHF) represent the predominant fermentation strategy, it requires considerable equipment costs and a long process time, which can lead to the formation of contaminations and inhibitors. Alternative conversion strategies, including simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF), and consolidated bioprocessing (CBP), can reduce time and production costs, contaminations, and inhibitor formation, and enhance process yields. Nevertheless, combining hydrolysis and fermentation into a single phase results in non-optimal temperature and pH. This review discusses crop residue valorization through fermentation strategies, and provides a 360-degree view of the topic. After investigating the major types of crop residues and the potential environmental risks associated with their incorrect or absent management, it analyzes the key steps in the crop residue bioconversion process, and the most common microorganisms and microbial cultures. In addition, this review reports on various examples of crop residues being converted into industrial products and analyzes the main fermentation strategies (SHF, SSF, SSCF, and CBP), highlighting their strengths and weaknesses. As a matter of fact, fermentation strategies need to be compared for their benefits and disadvantages before being implemented on a large scale. In addition, the properties and availability of the raw materials, investment, and operating costs, the skilled workforce availability, sustainability, and the return on investment all need to be evaluated. Finally, the discussion focus on future outlooks and challenges.