Stimulatory effects of rhamnolipid on corncob residues ethanol production via high-solids simultaneous saccharification and fermentation

•High-solids SSF of unwashed CRs with rhamnolipid was investigated.•The use of rhamnolipid enhanced the SSF of unwashed CRs even without pretreatment.•Addition of 0.2 g/L rhamnolipid enhanced ethanol yield of unwashed CRs by 22.39%.•SSF with 0.2 g/L rhamnolipid addition showed 82.38% cellulose conversion rate.•Rhamnolipid addition efficiently reduced the adsorption of cellulase onto lignin. In this study, the feasibility of rhamnolipid-assisted high-solids simultaneous saccharification and fermentation (SSF) of unwashed corncob residues and the process of rhamnolipid action was investigated. The effects of enzyme dosage (5–15 FPU/g-cellulose), inoculation amount (4–12 g/L), and rhamnolipid concentration (0.2–4 g/L) on ethanol production were evaluated. The ethanol yield at 20% (w/w) solids loading and 10 FPU/g-cellulose enzyme dosage increased to 61.99% by adding 0.2 g/L rhamnolipid, which was 22.39% higher than that produced from the control sample with 10 FPU/g-cellulose, and was close to the ethanol yield of the control sample (61.32%) with 15 FPU/g-cellulose. However, adding rhamnolipid at a high dosage (>2 g/L) led to sugar accumulation and negative effects on yeast growth. A schematic showing how rhamnolipid enhancement influences the high-solids SSF revealed that: i) Adding rhamnolipid to the fermentation medium reduced the surface tension of the SSF system; ii) Rhamnolipid accumulated at the interface between the two phases and reduced the level of deactivation of cellulase; and iii) X-ray photoelectron spectroscopy spectra analyses revealed that rhamnolipid addition efficiently reduced the adsorption of the enzyme on fermentation residues. The addition of rhamnolipid is an effective strategy to enhance ethanol production from corncob residues, even without pretreatment.