Oligosaccharides and sugars production from olive stones by autohydrolysis and enzymatic hydrolysis

• Olive stone (OS) is a potential lignocellulose material for biofuels production.
• The goal was to establish the pretreatment conditions that maximize the sugar yield.
• Severity factors = 3.7 allow complete hemicellulose solubilization in autohydrolysis.
• OS autohydrolysis changes the surface porosity, improving the enzymatic attack.
• Highest yield of d-glucose (12.6 kg/100 kg OS) was found at a severity factor = 4.80.

Olive stones (OS) are an abundant industrial by-product in countries of the Mediterranean Basin. In this work, olive stones were pretreated with hot water (autohydrolysis) at temperatures ranging from 150 °C to 225 °C for 0–600 s, and results were analyzed by using the severity factor (log R0). A log R0 value equal to 3.7 guaranteed the total solubilization of hemicelluloses, largely preserving cellulose and lignin in solid residue. In the prehydrolyzate, high oligosaccharide yield (14.7 kg/100 kg OS) and low acetic acid yield (0.24 kg/100 kg OS) were achieved at 190 °C and 300 s (log R0 = 3.59). The solid residues were hydrolyzed by cellulases to assess the enzymatic digestibility. Results showed that enzymatic saccharification of pretreated solids at 225 °C for 600 s resulted in d-glucose yield of 54.3% (12.6 kg/100 kg OS), which improved 28-fold the yield obtained by using solids pretreated at 150 °C for 600 s. Mercury porosimetry illustrated that this increase could be explained by changes in average pore diameters rather than by changes in specific surface areas. The results demonstrated that olive stones are a potential raw material for the production of fermentable sugars. In the best operating conditions tested, autohydrolysis and subsequent enzymatic process have allowed the achievement of very high yields in oligosaccharides and d-glucose, respectively. Considering the autohydrolysis and enzymatic process, the total production of fermentable sugars leads to a total yield of 27.3 kg/100 kg OS.

Variation of hemicellulose fraction and solubilized solids percentages, and acetic acid yield with severity factor after autohydrolysis process.Figure optionsDownload as PowerPoint slide