Bioethanol is the most widely used alternative transportation fuel to petrol. Bioethanol is considered a clean, renewable, and environmentally friendly fuel that can contribute to climate change mitigation, decreased environmental pollution, and enhanced energy security. Commercial bioethanol production is based on traditional agricultural crops such as corn, sugarcane, and sugarbeet, primarily used as food and feed. In order to meet the growing demand for this fuel and decrease competition in the food and biofuel sectors for the same feedstock, other raw materials and process technologies have been intensively studied. Lignocellulosic biomass is one of the most abundant renewable resources, with it being rich in compounds that could be processed into energy, transportation fuels, various chemical compounds, and diverse materials. Bioethanol production from lignocellulosic biomass has received substantial attention in recent decades. This review gives an overview of bioethanol production steps from lignocellulosic biomass and challenges in the production process. The following aspects of bioethanol production are covered here, including pretreatment methods, process strategies, strain development, ethanol isolation and purification, and technical hurdles.

Cellulases are enzymes that are attracting worldwide attention because of their ability to degrade cellulose in the lignocellulosic biomass and transform it into highly demanded bioethanol. The enzymatic hydrolysis of cellulose by cellulases into fermentable sugars is a crucial step in biofuel production, given the complex structure of lignocellulose. Due to cellulases’ unique ability to hydrolyze the very recaltricant nature of lignocellulosic biomass, the cellulase market demand is rapidly growing. Although cellulases have been used in industrial applications for decades, constant effort is being made in the field of enzyme innovation to develop cellulase mixtures/cocktails with improved performance. Given that the main producers of cellulases are of microbial origin, there is a constant need to isolate new microorganisms as potential producers of enzymes important for biofuel production. This review provides insight into current research on improving microbial cellulase production as well as the outlook for the cellulase market with commercial cellulase preparation involved in industrial bioethanol production.

The current industrial production of the biodiesel relies mainly on vegetable oils that could result in the shortage of edible oils in food markets and increase in their prices. Microbial lipids produced by oleaginous microorganism have attracted a lot of attention in the recent years as a source of high-value polyunsaturated acids as well as alternative feedstock for the production of biodiesel. However, the production of microbial oils faces a number of problems concerning the costs of lipid extraction, carbon source and operational cost for microbial cultivation in conventional stirred tank bioreactor which makes production economically unfeasible. Non-food feedstocks, lignocellulose biomass and different waste streams containing lignocellulose, are low-cost sources of renewable carbon that could significantly reduce the production cost of microbial lipids. This review analyses the current production of microbial lipids from lignocellulose feedstocks and gives an overview of the main stages in the process of lipid production, pretreatment and hydrolysis of the feedstock and microbial cultivation. Cultivation of oleaginous microorganisms has been conducted by submerged cultivation and solid state fermentation. Three process configurations have been used in the lipid production including, separate hydrolysis and lipid production (SHLP), simultaneous saccharification and lipid production (SSLP) and consolidate bioprocessing (CBP). Implementing the biorefinery concept that includes co-production of different value-added products (polyunsaturated fatty acids, amino acids, lignin and pigments) could improve the feasibility of lipid production bioprocess.

Microbial lipids have similar fatty acid composition to plant oils, and therefore, are considered as an alternative feedstock for biodiesel production. Oleaginous yeasts accumulate considerable amounts of lipids intracellularly during growth on low-cost renewable feedstocks such as lignocellulosic biomass. In this study, we cultivated yeast Trichosporon oleaginosus on hydrolysate of alkaline pretreated corn cobs. Different process configurations were evaluated and compared, including separate hydrolysis and fermentation (SHF) with cellulase recycle and simultaneous saccharification and fermentation (SSF) in batch and fed-batch mode. At low enzyme loading, the highest lipid concentration of 26.74 g L⁻¹ was reached in fed-batch SSF fed with 2.5% (g g⁻¹) substrate. Batch SHF was conducted for four rounds with recycling the cellulase adsorbed on unhydrolyzed lignocellulosic biomass. Thirty percent of cellulase saving was achieved for rounds 2–4 without compromising productivity and lipid yield. The addition of Tween 80 to lignocellulosic slurry improved the hydrolysis rate of structural carbohydrates in pretreated lignocellulosic biomass. Furthermore, supplementing the growth medium with Tween 80 improved lipid yield and productivity without affecting yeast growth. Oleaginous yeast T. oleaginosus is a promising strain for the sustainable and efficient production of lipids from renewable lignocellulosic feedstock.

The aim of the present study was to investigate structural modifications and physicochemical properties of bacterial nanocellulose (BNC) pellicles produced statically in a beetroot peel medium as a mixture of C-sources (8% sucrose, 2% glucose, and 0.86% fructose). Characteristics of these BNC samples were compared to those produced using a chemically defined and modified Hestrin–Schramm medium with 3% (w/v) glucose, fructose, and sucrose as sole carbon sources. Compared with the beetroot peel medium, where 11.57 g/L BNC (wet weight) was obtained, glucose gave the highest BNC yield (13.07 g/L), sucrose 10.55 g/L, and fructose 7.9 g/L. FT-IR spectra showed almost identical chemical profiles assigned to cellulose I. XRD analysis revealed that the BNC produced in beetroot peel medium had a larger crystallite size than other BNC samples. The crystallinity index of beetroot peel BNC was higher than that of Hestrin–Schramm BNC. The water-holding capacity, water release rate, and iodine sorption value from the beetroot peel medium were greater than those of other BNCs. Furthermore, as there is a necessity today to find cheaper carbon sources to obtain valuable products at a lower cost, beetroot peels represent an interesting alternative to producing BNC as a new functional material for industrial applications.