Influence of solid loading on enzymatic hydrolysis of steam exploded or liquid hot water pretreated olive tree biomass

Abstract

Olive tree pruning biomass, pretreated by either liquid hot water or steam explosion under selected conditions, was used as a substrate for enzymatic hydrolysis. The pretreated material was further submitted to alkaline delignification, the objective being to improve hydrolysis yields as well as increasing cellulose content in the pretreated feedstock. The enzymatic hydrolysis of pretreated residues was performed using a commercial cellulase mixture supplemented with β-glucosidase, using a solid loading range from 2 to 30% (w/v). The influence of substrate concentration on the enzymatic hydrolysis yield and on glucose concentration was studied. Comparative results with and without a delignification step are presented. Enzymatic hydrolysis at high substrate concentration (≥20%) is possible, yielding a concentrated glucose solution (>50 g/L). Nevertheless, a cellulose fraction of the pretreated residue remains unaltered.

Introduction

Lignocellulosic resources such as agricultural residues represent a renewable, low cost and largely available source of energy [1]. Their utilization in an ethanol production process solves, on the one side, environmental and disposal concerns; on the other side, it is a promising way to decrease greenhouse gas emissions and alleviate the shortage of fossil fuel energy [2]. Ethanol is produced from lignocellulose by an integrated process involving basically three steps: pretreatment, hydrolysis and fermentation. The final objective of both the pretreatment and hydrolysis steps is to break down carbohydrate polymers present in plant cell walls into low-molecular-weight sugars so that microorganisms can ferment them to ethanol. Pretreatment is specially targeted to improve the digestibility of the cellulose fraction in the hydrolysis step, by removing lignin and hemicellulose. Hydrothermal pretreatments result in substantial breakdown of lignocellulosic structure with no need of chemicals. Among them, liquid hot water (LHW) and steam explosion (SE) pretreatments have been applied to a wide range of agricultural residues [3], [4], [5], [6]. To improve the digestibility of the cellulosic residue, an alkaline peroxide delignification step has been explored on several types of biomass [7], [8].

Hydrolysis is the key step in the bioconversion process because (i) cellulose is the most abundant sugar polymer in lignocellulosic residues and (ii) microorganisms performance on glucose-to-ethanol conversion is easier than on any other simple sugar. Hydrolysis may be performed either by acids or by enzyme action. Enzymatic hydrolysis present several advantages when comparing to acid hydrolysis, including lower equipment costs (as hydrolysis is conducted at mild conditions) and higher glucose yields without sugar-degradation products, which may affect ulterior fermentation. On the other hand, enzyme purchase is a costly part of ethanol production [9], moreover the presence of residual lignin, not removed during pretreatment, has been shown to play an important role in limiting the efficiency of enzymatic hydrolysis of pretreated lignocellulosic materials [10], [11] as well as contributing to generation of degradation products [12], [13].

Regarding the overall process economy, it is important to get a glucose solution as concentrated as possible from the enzymatic hydrolysis step, from which a concentrated ethanol solution may be obtained as a result of fermentation, thus reducing separation costs [14]. One way for obtaining concentrated glucose solutions is to use as much substrate for the enzymatic hydrolysis as possible, although problems related to material agitation and inhibition by glucose may arise [15].

In olive tree cultivation, pruning is necessary to eliminate old branches and to regenerate trees. A typical olive tree pruning lot includes 70% thin branches (by weight, with approximately one third of leaves) and 30% of wood (big branches, diameter >5 cm approximately). Olive tree wood is usually separated and put to domestic use as firewood or burnt in small, local industries; it has been also considered as feedstock for ethanol production [16], [17]. The remaining residue, left in the fields, is eliminated by burning, with associated economical costs and environmental concerns. As an alternative, this work deals with the use of this agricultural residue as raw material for the production of fermentable sugars in the frame of a biomass-to-ethanol conversion process.

In this work olive tree pruning biomass other than wood is used as a raw material, the objective being to study the influence of solid loading, or consistency, on the enzymatic hydrolysis of the pretreated material in terms of both glucose concentration and hydrolysis yields. As pretreatments, liquid hot water (LHW) and steam explosion (SE) were used. We also investigated how results are affected by a delignification step performed on pretreated materials.