Ethanol Production from Olive Stones through Liquid Hot Water Pre-Treatment, Enzymatic Hydrolysis and Fermentation. Influence of Enzyme Loading, and Pre-Treatment Temperature and Time

Manuel Cuevas,Vicente Bravo,Juan F García Martín,Sebastián Sánchez

doi:10.3390/fermentation7010025

Manuel Cuevas, Vicente Bravo + Show 2 more

Open Access

https://doi.org/10.3390/fermentation7010025

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Abstract

Olive table industry, olive mills and olive pomace oil extraction industries annually generate huge amounts of olive stones. One of their potential applications is the production of bioethanol by fractionation of their lignocellulose constituents and subsequent fermentation of the released sugars using yeasts. In this work, we studied the influence of temperature (175–225 °C) and residence time (0–5 min) in the liquid hot-water pre-treatment of olive stones as well as the initial enzyme loading (different mixtures of cellulases, hemicellulases and β–glucosidases) in the later enzymatic hydrolysis on the release of fermentable sugars. The Chrastil’s model was applied to the d-glucose data to relate the severity of pre-treatment to enzyme diffusion through the pre-treated cellulose. Finally, the hydrolysate obtained under the most suitable conditions (225 °C and 0 min for pre-treatment; 24 CE initial enzyme concentration) was fermented into ethanol using the yeast Pachysolen tannophilus ATCC 32691. Considering the overall process, 6.4 dm3 ethanol per 100 kg olive stones were produced.

Highlights

Olive stones are a lignocellulose biomass that represent around 20% of the olive weight [1]
Notwithstanding, S. cerevisiae is not able to ferment pentoses (D-xylose and L-arabinose), which is its main hindrance for ethanol production from lignocellulose. These results indicate that while the efficiency in the transformation of D-glucose into ethanol is very high, the rest of the substrates (D-xylose, L-arabinose, acetic acid, etc.) are largely metabolized towards biomass production by P. tannophilus
The main conclusions derived from the experimental work carried out are: a) The conversion of olive stones into ethanol can be performed by applying a process with the following stages: reduction in the particle size of the biomass, pre-treatment with liquid hot water under pressure (LHW), enzymatic hydrolysis of the pre-treated slurry for the fermentation with

Summary

Introduction

Olive stones are a lignocellulose biomass that represent around 20% of the olive weight [1]. Taking into account that olive stones account for approximately the same weight as the olive oil extracted from the fruit, and that 1.125 × 106 t olive oil were obtained in the Spanish olive oil mills in the 2019/2020 campaign [4], these facilities would annually process around one million tons of olive stones. The main components of olive stones are cellulose, hemicellulose and lignin. The production of ethanol from lignocellulose biomass via biochemical pathways has been the subject of numerous studies in recent decades. Most of the proposed schemes are composed of three main stages: pre-treatment

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Conclusion