A systematic review of bioethanol production optimization from diverse feedstocks with natural additive supplementation

Growing interest in bioethanol as a renewable biofuel stems from the need for sustainable and environmentally friendly energy sources. Different feedstocks, particularly lignocellulosic biomass for second-generation bioethanol contain high levels of structural polysaccharides. However, due to low complex sugar conversion efficiency and high cost of synthetic promoters, bioethanol production from lignocellulosic feedstocks remains inefficient, necessitating further process optimization. The present systematic review explores the potential of natural additives in improving the fermentation processes from various feedstocks. Natural additives improve hydrolysis, boost sugar conversion, and reduce inhibition, offering a cost-effective, sustainable strategy to enhance bioethanol production and lignocellulosic biofuel technologies. A comprehensive literature review was conducted using keywords such as enzymatic hydrolysis, sugar conversion efficiency, bioenergy resources, lignin structural polymers, renewable energy sources, and biomass across databases including Google Scholar, ScienceDirect, SpringerLink, Web of Science, and Scopus. Of the 218 articles initially identified, 206 met the relevance criteria and were included in the study. The predefined relevance criteria comprised topical alignment with the study objectives, publication in peer-reviewed journals, English-language articles, the publication window mainly between 2020 and 2026, and accessibility of the full text. Findings indicate that natural additives enhance hydrolysis and fermentation by supplying essential enzymes such as cellulase, amylase, and pectinase, and vital nutrients such as nitrogen, potassium, and vitamins that support microbial growth and sugar conversion. Optimized cellulase supplementation of 3–12 International Unit per gram of substrate and synergistic multi-enzyme cocktails under controlled Solid-State Fermentation (SSF) conditions significantly enhance hydrolysis and polysaccharide conversion, increasing bioethanol productivity by 0.8–1.6 g/L·h, achieving the yields up to 51.05 g/L (60.65% theoretical). A clear research gap remains in the systematic application of additives across diverse biomass feedstocks, highlighting the need for experimental validation to determine optimal dosages, fermentation conditions, and synergistic effects.