Optimal Pretreatment of Eucalyptus globulus by Hydrothermolysis and Alkaline Extraction for Microbial Production of Ethanol and Xylitol

Abstract

Fractionation of Eucalyptus globulus wood chips is mandatory for developing a biorefinery process. In order to achieve this, products released from hydrothermolysis pretreatment were studied within the severity (log R 0 ) range 3.3−4.5. The highest solubilized xylose yield was 56% (DWB) at log R 0 3.9. Values higher than log R 0 4.1 have led to high xylose dehydration rates. Detoxification steps applied to the liquor removed acetic acid (55%), furfural (94%), and hydroxymethylfurfural (HMF) (31%). Fermentation of liquor with Candida guilliermondii resulted in a xylitol yield of 0.37 g of xylitol produced per gram of xylose consumed (equivalent to 71% of the control with pure xylose). Pulp-1 (from hydrothermolysis pretreatment) was further pretreated with NaOH for lignin removal (obtaining Pulp-2). Cellulose composition in Pulp-1 and Pulp-2 remains almost unaltered; lignin content in Pulp-2 was 50% lower than in Pulp-1. Simultaneous saccharification and fermentation (SSF) experiments for both pulps reached similar ethanol yields of 0.76 and 0.70 g of ethanol produced per gram of ethanol potentially produced from wood, respectively. Recovery of hemicellulose at the beginning of the process is a key step for getting pulps with high cellulose composition available for developing a biorefinery process of E. globulus.

Xylitol production from cashew apple bagasse by Kluyveromyces marxianus CCA510

“…However, the concentration of the acetic and formic acids increased approximately 2.4 and 4.8 times (Table 1), respectively, which is consistent with the data cited by other authors [10,29]. It is emphasized that no concentrations of furfural and 5-HMF it were detected in CABH.…”

“…In turn, the presence of acetic acid below 1 g L −1 favored xylitol production by a strain of C. guilliermondii, but when above 3 g L −1 it becomes an inhibitory compound [23]. Castro et al [29] also evaluated xylitol production by a strain of C. guilliermondii and noted that the strain was able to metabolize 62% of the acetic acid present in eucalyptus hydrolysate medium, which corresponded to a concentration of 5 g L −1 of the acid.…”

Research on the Dissolution of Pentosans during Eucalyptus Hydrolysate Pretreatment

“…With proper control of holding time (less than 60 min), the absolute quantity of saccharide was improved, which could be favorable to its utilization once suitable strain and process without detoxification were chosen (Kelbert et al 2016;Romani et al 2014) or when a proper detoxification process was employed (Castro et al 2013). Since saccharides already contained in the HP medium, its conversion to furfural and HMF during the pretreatment, would increase furfural and HMF content, and make it more feasible to recover them as side-products.…”

Structural Characteristics and Distribution of Lignin in Eucalyptus Globulus Pulps Obtained by a Combined Autohydrolsis/Alkaline Extraction Process for Enzymatic Saccharification of Cellulose

“…Autohydrolysis (also called hydrothermal pretreatment) uses water at relatively high temperatures (140-200°C) under mild acidic conditions to partially fractionate LCB in a water-soluble fraction (with hemicelluloses and soluble lignin) and a water-insoluble fraction (mainly cellulose and insoluble lignin) 21,22 . The hemicelluloses are degraded to mono-and oligosaccharides leaving a pulp with the cellulose more susceptible to enzymatic hydrolysis 23,24 .…”