Bioethanol production from thermochemically pre-treated olive mill solid residues using the yeast Pachysolen tannophylus

Abstract

Olive oil mill solid residue (OMSR) is the solid waste generated during olive oil production process in three-phase olive mills. It consists of the remaining pulp of olive processing after the extraction of oil, as well as the cracked seeds of the olive fruits, containing thus mainly lignocellulose and residual oil. The commonly used practice for OMSR management is combustion, after having extracted the residual oil by secondary extraction using organic solvents. Other proposed ways of OMSR management are their exploitation as substrate for edible fungi production and compost, and as feedstock for biofuels generation such as methane and bioethanol. In the latter case, the complex carbohydrates (cellulose and hemicellulose) of the lignocellulose of OMSR have to be degraded towards their simple sugars and further fermented via microorganisms.The purpose of the present study was to investigate the effect of thermochemical pre-treatment of OMSR, on the final ethanol yield from the yeast Pachysolen tannophilus. Nine different types of OMSR-based substrates were tested i.e. raw OMSR, hydrolysates generated from pretreated OMSR with NaOH (0.5 %, 1.5 % w/v) and H 2 SO 4 (0.5 %, 1.5 % v/v), and pretreated OMSR with NaOH (0.5 %, 1.5 % w/v) and H 2 SO 4 (0.5 %, 1.5 % v/v) whole biomass. It was shown that in all cases pretreatment enhanced the consumption of carbohydrates as well as ethanol final yields.

Enhancement of Liquid and Gaseous Biofuels Production From Agro-Industrial Residues After Thermochemical and Enzymatic Pretreatment

“…The only exception noted was for SS pretreated with 0.5% NaOH, for which there was also a statistically significant difference with the thermally pretreated SS. Those findings are in agreement with previous studies during which alkaline pretreatment was applied to various types of lignocellulosic wastes and residues aiming at the facilitation of biofuels production (Senkevich et al, 2012;Antonopoulou and Lyberatos, 2013;Antonopoulou et al, 2015a). It is indeed reported that alkaline pretreatment breaks the intermolecular ester bonds between the lignin-carbohydrates matrix and modifies the biomass structure, resulting in the solubilisation of mainly lignin, whereas holocellulosic content is affected to a lower degree (Kumar and Sharma, 2017).…”

“…Taking into account that the holocellulosic content of SS and OS was estimated to be ∼50%, it can thus be assumed that almost 40% of the complex carbohydrates of the SS and also OS are actually hydrolyzed in the conditions that were tested, thus indicating that almost all hemicellulose of the substrates was actually solubilized. Similar results were obtained in the study of Senkevich et al (2012), in which olive mill solid residues were thermochemically pretreated with dilute H 2 SO 4 at 130 • C for 45 min. Regarding the effect of NaOH on the saccharification of SS and OS during thermochemical pretreatment, it is apparent that it was considerably milder than those of H 2 SO 4 .…”

“…production from SS, of different origin, using the C 5 sugarsconsuming yeast Pichia stipitis. Similarly, the results obtained for ethanol production of OS are in good agreement with those of the study of Senkevich et al (2012), in which ethanol production from pretreated solid olive mill residues at similar conditions was assessed.…”

“…At the same time, alkaline pretreatment results in structural changes of cellulose, such as swelling, increase of its surface area and porosity and a decrease in its crystallinity, thus making it vulnerable to enzymatic hydrolysis (Monte et al, 2018). As such, the increase of the yield of the free sugars of alkaline pretreated lignocellulose should indeed be expected during a subsequent hydrolysis, rather than directly after pretreatment, as also shown by Senkevich et al (2012) in the case of pretreated olive mill residues.…”