3.2. Effects of LHW and DSA pretreatments on lignin, hemicelluloses
and cellulose
Olive stones were subjected to liquid hot water and dilute sulphuric acid pretreatments at temperatures in the range 190e230 C to extract the hemicellulosic fraction and increase the accessibility of enzymes to cellulose. While sulphuric acid was the main responsible (together with temperature) for olive stones hydrolysis in DSA pretreatments, the continuously released hemicellulosic acetyl groups provided the acidic medium to carry out the hydrolysis in hydrothermal pretreatments. The pH of the hydrolysates obtained after LHW pretreatments ranged between 3.20 (190 C) and 3.03 (230 C).Therefore, higher temperatures led to higher release of acetyl groups, thus reducing pH of the resulting hydrolysate.
By contrast, pH of hydrolysates after DSA pretreatments was found to be 1.65 ± 0.04. This fact is logical since the pH of these hydrolysates was strongly influenced by the addition of 0.025 M H2SO4. Fig. 2A shows the percentage of material that remained in solid phase at the end of each pretreatment. As expected, the percentage of solid recovery decreased when temperature and acid concentration increased. Thus, total solid recovery ranged between 59.8% and 94.2% for LHW and between 44.7 and 73.5% for DSA assays. These values were consistent with other authors' data, confirming total gravimetric recoveries between 66% and 95% for hydrothermal pretreatment of Eucalyptus globulus wood [28] and
between 44% and 83% for dilute sulphuric acid hydrolysis of Cardoon [29].