Development of an integrated pretreatment fractionation process for fermentable sugars and lignin: Application to almond (Prunus dulcis) shell

An environmentally friendly pretreatment process was developed to fractionate cellulose, hemicellulose and lignin from almond (Prunus dulcis) shells, consisting of hot water pretreatment (HWP) coupled with organic solvent (organosolv) pretreatment of water/ethanol (OWEP). This integrated pretreatment process proved more effective on the basis of yield of fermentable sugar and lignin separation compared with HWP alone, dilute acid pretreatment (DAP), ammonia pretreatment (AP), lime pretreatment LP, organosolv water/ethanol pretreatment (OWEP), and organosolv water/acetone pretreatment (OWAP). In the coupled hot water-organosolv process, hemicellulose sugars were recovered in the first residual liquid while varying amounts of cellulose was retained in the residual solid. The lignin fraction was obtained by simply adjusting the pH from the second liquid. The optimal two-stage process consisted of first HWP stage at 195 °C for 30 min, resulting in wglucose = 95.4% glucose recovery yield and wxylose = 92.2% xylose removal. The second organosolv OWEP stage was operated at 195 °C for 20 min, in ethanol in water mixtures of <phi>ethanol = 50% and resulted in nearly wglucose = 100% glucose recovery yield, wxylose = 90% xylose and wlignin = 61% lignin removal. After enzymatic hydrolysis, glucose yield was up to wglucose = 95%, compared to 61% yield from untreated almond. Images obtained via scanning electron microscopy (SEM) highlighted the differences in almond structure from the varying pretreatment methods during biomass fractionation.