Whole slurry saccharification of mild oxalic acid-pretreated oil palm trunk biomass improves succinic acid production

Highlights

• Mild oxalic acid is promising in solubilising the hemicellulose of oil palm trunk.

• Two different routes were compared for enzymatic saccharification.

• Actinobacillus succinogenes exhibited high tolerance towards acetic acid.

• Succinic acid substantially improved using the whole slurry saccharification.

Abstract

The present work compared two different routes in preparing oil palm trunk biomass (OPTB) hydrolysates for producing industrially-valuable succinic acid (SA). OPTB was initially pretreated with mild oxalic acid (1% w/v) under different reaction time to investigate its effect on the total sugar and inhibitor yields during pretreatment and subsequent enzymatic saccharification. Substantial solubilisation of hemicellulose (56–59 %) was achieved after pretreatment for 3−4 h. After enzymatic saccharification, high cellulose digestibility of 86–87 %, with 22.1–22.2 g sugar per 100 g of raw OPTB from washed pretreated solids (Route 1), and 71–74 %, with 43.2–44.9 g sugar per 100 g of raw OPTB from whole slurry (Route 2) saccharification, respectively was achieved. Fermentation of the hydrolysates derived from Route 2 by Actinobacillus succinogenes 130Z afforded a twofold increase in the SA production compared to that from Route 1. Mass balance calculations showed that 175 g of SA could be produced per kg of OPTB via Route 2 vs. 153 g via Route 1. The proposed approach in this study offers pioneering insights into the greatest improvement in SA yield achievable via efficient biomass utilisation.

Introduction

Pollution secondary to the combustion of fossil fuels and the accumulation of non-degradable synthetic products have raised environmental concerns among authorities and stakeholders, leading to their endeavours to minimise non-sustainable industrial practices and waste mismanagement (Poulsen and Lema, 2017). By contrast, biological processes are relatively environmentally-friendly and have garnered interest worldwide, especially from the chemical industry. One such biological process is fermentation-based routes with carbon-neutral feedstock, which have presented commercial opportunities to palm oil-producing countries, given their abundance of locally-produced lignocellulosic biomasses (Luthfi et al., 2017).

Oil palm trunk biomass (OPTB) is recognised as one of the promising lignocellulosic resources generated from oil palm plantations (Bukhari et al., 2019a). In Malaysia, approximately 21.8 million tonnes of OPT (on a dry-weight basis) were resulted from 5.85 million ha of oil palm plantations in 2019 (Parveez et al., 2020). At present, such OPTB is underutilised, as reflected by the diminutive portion used for plywood manufacturing and the sizable amounts left unused at plantation sites (Bukhari et al., 2019b). As with other lignocellulosic biomasses, OPTB comprises polysaccharides such as cellulose (mainly C6 sugar polymers), hemicellulose (mainly C5 sugar polymers), and lignin (aromatic polymers). Given its lignocellulosic nature, OPTB is highly recalcitrant to enzymatic degradation for saccharification and fermentation. Hence, pretreatment is warranted to effectively deconstruct its lignocellulosic components for microbial susceptibility.

Various pretreatment methods have been explored for the bio-conversion of OPTB. Excellent sugar yields have been reported from lignocellulosic materials that can affordably serve as a fermentative substrate for manufacturing bioproducts (e.g. bioethanol, biobutanol, and lactic acid) (Eom et al., 2015a, 2015b; Komonkiat and Cheirsilp, 2013; Rattanaporn et al., 2018). Furthermore, OPTB has been explored for the production of succinic acid (SA) (Bukhari et al., 2020), a bio-based platform chemical valued for its potential in numerous applications. In the food industry, SA offers practical utility as an acidity regulator, flavouring agent, bread-softening agent, flavour and oil microencapsulator, and sweetener. In the pharmaceutical industry, it serves as an excipient and intermediary compound for the production of sedatives, anti-phlegm medications, antibacterial agents, and disinfectants (Jusoh et al., 2020). Conventional industrial applications of SA include the production of polybutylene succinate-terephthalate, polyester polyols, resins, coatings, and pigments (Pateraki et al., 2016). The utilisation of OPTB in SA production would further expand the range of products that can be developed from the palm oil industry.

Previously, only the cellulosic components of OPTB after pretreatment were used whereas the hemicellulosic sugar liquids (or prehydrolysates) were discarded. In most studies, solids containing mainly glucose from OPTB hydrolysates (pretreated via washing) are commonly chosen as the fermentative substrate; such a choice avoids the accumulation of inhibitory compounds during pretreatment and catabolite repression induced by C5 sugars during fermentation (Menon et al., 2010). For efficient conversion of biomasses (OPTB) into the targeted product (SA), the use of unrecovered prehydrolysates containing hemicellulosic components is essential to increase the initial concentration of fermentable sugars. Concurring to its natural habitat, Actinobacillus succinogenes, a versatile wild-type bacterial host having capability to metabolise a variety of carbon sources (Dessie et al., 2018), is of advantageous to harness both the C5 and C6 sugars from OPTB to ensure optimal bio-conversion into SA (Bukhari et al., 2020).

The hemicellulosic prehydrolysates of OPTB containing structural carbohydrates can be recovered via dilute acid pretreatment (DAP). Following DAP, two streams, one containing xylose-enriched liquids and the other cellulose-enriched solids, are produced. DAP with oxalic acid (OA) has been reported to be desirable, given its catalytic performance at low concentrations in efficiently solubilising hemicellulose into the monomeric xylose (Bukhari et al., 2020). Although a relatively strong organic acid, OA is non-mutagenic and not hazardously corrosive (Zhang et al., 2013). OA has been examined in pretreating various lignocellulosic biomasses, including yellow poplar (Kundu and Lee, 2015), sisal pulp (Lacerda et al., 2015), corncob (Qing et al., 2015), sugarcane bagasse (Yan et al., 2018) and oil palm empty fruit bunch (Anita et al., 2020). These studies have collectively demonstrated that the mild and environmentally-friendly pretreatment with dilute OA produced not only high xylose yields in prehydrolysates, but also highly-digestible solids for subsequent enzymatic saccharification. Little literature, however, has hitherto focused on OPTB (Bukhari et al., 2020; Rattanaporn et al., 2018).

In our earlier work concerning DAP of OPTB (Bukhari et al., 2020), OA at 5% (w/v) has been verified as an effective catalyst in solubilising the hemicellulose, and thereby attaining high xylose yields alongside only trace concentration of by-products, i.e. furfural and 5-(hydroxymethyl) furfural (HMF). However, pretreatment with OA at such high concentrations has been unfavourable for SA fermentation by A. succinogenes, for which optimal deployable conditions would be examined in the present study. We aimed to improve SA fermentation by reducing OA concentration while manipulating the reaction time to achieve highly-solubilised hemicellulose and satisfactorily-digestible cellulose for subsequent enzymatic saccharification. We investigated the applicability of enzymatic saccharification of the whole slurry (WS) to achieve higher initial sugar concentration and compared it with the commonly-employed route with the washed pretreated solid (PS). Moreover, we evaluated the ability of A. succinogenes 130Z to metabolise both C5 and C6 sugars recovered from OPTB as well as its tolerance towards an inhibitor i.e., acetic acid released during pretreatment with mild OA for SA production.