Biotechnological route for sustainable succinate production utilizing oil palm frond and kenaf as potential carbon sources

Biotechnological enhancement of lactic acid conversion from pretreated palm kernel cake hydrolysate by Actinobacillus succinogenes 130Z

The aim of this study was to establish an improved pretreatment and fermentation method i.e. immobilized cells for high recovery of fermentable sugars from palm kernel cake (PKC) and its effects on fermentability performance by Actinobacillus succinogenes 130Z in the conversion of the fermentable sugar to lactic acid. The effects of oxalic acid concentrations (1-6% w/v) and residence times (1-5 h) on the sugar recovery were initially investigated and it was found that the highest mannose concentration was 25.1 g/L at the optimum hydrolysis conditions of 4 h and 3% (w/v) oxalic acid. The subsequent enzymatic saccharification of the pretreated PKC afforded the highest enzymatic digestibility with the recovered sugars amounting to 25.18 g/L and 9.14 g/L of mannose and glucose, respectively. Subsequently, the fermentability performance of PKC hydrolysate was evaluated and compared in terms of cultivation phases (i.e. mono and dual-phases), carbonate loadings (i.e. magnesium and sodium carbonates), and types of sugars (i.e. glucose and mannose). The highest titer of 19.4 g/L lactic acid was obtained from the fermentation involving A. succinogenes 130Z in dual-phase cultivation supplemented with 30 g/L of magnesium carbonate. Lactic acid production was further enhanced by using immobilized cells with coconut shell-activated carbon (CSAC) of different sizes (A, B, C, and D) in the repeated batch cultivation of dual-phase fermentation producing 31.64 g/L of lactic acid. This work sheds light on the possibilities to enhance the utilization of PKC for lactic acid production via immobilized A. succinogenes 130Z.

Uncoupling growth and succinic acid production in an industrial Saccharomyces cerevisiae strain

This study explores the relation between biomass‐specific succinic acid (SA) production rate and specific growth rate of an engineered industrial strain of Saccharomyces cerevisiae, with the aim to investigate the extent to which growth and product formation can be uncoupled. Ammonium‐limited aerobic chemostat and retentostat cultures were grown at different specific growth rates under industrially relevant conditions, that is, at a culture pH of 3 and with sparging of a 1:1 CO₂–air mixture. Biomass‐specific SA production rates decreased asymptotically with decreasing growth rate. At near‐zero growth rates, the engineered strain maintained a stable biomass‐specific SA production rate for over 500 h, with a SA yield on glucose of 0.61 mol mol⁻¹. These results demonstrate that uncoupling of growth and SA production could indeed be achieved. A linear relation between the biomass‐specific SA production rate and glucose consumption rate indicated the coupling of SA production rate and the flux through primary metabolism. The low culture pH resulted in an increased death rate, which was lowest at near‐zero growth rates. Nevertheless, a significant amount of non‐viable biomass accumulated in the retentostat cultures, thus underlining the importance of improving low‐pH tolerance in further strain development for industrial SA production with S. cerevisiae.

An improvement in fermentability of acid-hydrolysed hemicellulose from kenaf stem for xylitol production

In recent years, there has been a growing interest in the use of agricultural biomass for fermentation purposes; however, efficient strategies to counter lignocellulose inhibition are warranted to enhance xylitol production performance. Dilute-acid hydrolysis has been studied to selectively release a significant portion of xylose from hemicellulose, while leaving cellulose and lignin intact. The formation of inhibitory compounds, however, could jeopardise the overall performance during fermentation to produce xylitol. In this study, the fermentability of nitric acid-hydrolysed kenaf stem was substantially improved, through either adaptive evolution of the recombinant Escherichia coli BL21 (DE3) or removal of fermentation inhibitors by detoxification with activated carbon. Both methods were compared to evaluate the superiority in fermentative performance. In the fermentation with detoxified hemicellulosic hydrolysate, the non-adapted strain produced the highest xylitol concentration of up to 6.8 g/L, with 61.5% xylose consumption. The yields of xylitol production involving detoxification were successfully enhanced by 22.6% and by 35.7% compared to those involving adaptive evolution and raw hydrolysate, respectively. The results reported herein suggest that the utilization of detoxified kenaf stem hydrolysate could be advantageous.

Multiple crystallization as a potential strategy for efficient recovery of succinic acid following fermentation with immobilized cells

This study aimed to enhance the crystallizability of bio-based succinic acid for its efficient recovery while maintaining the end product at the highest purity. Immobilization of Actinobacillus succinogenes was initially evaluated based on three different carriers: volcanic glass, clay pebbles, and silica particles. The adsorption capacity of metabolites with a low concentration (10 g/L) and a high concentration (40 g/L) was investigated. It was demonstrated that clay pebbles adsorbed the least succinic acid (< 11 mg/g clay pebbles). The repeated batch-fermentation trials with immobilized cells highlighted that succinic acid with an average concentration of up to 36.3 g/L with a metabolite-production ratio of 3:1 (succinic acid to by-products) could be attained within 130 h. Subsequently, the purification of succinic acid through crystallization was assessed in terms of pH, temperature, crystallization time, initial succinic acid concentration and multiple recrystallization processes. Increasing the crystallization time from 6 h to 9 h afforded an improvement of 17% in the recovery of succinic acid crystals. Moreover, a fourfold concentration coefficient of the broth yielded the highest purity percentage (99.9%). The crystallization in three consecutive stages at 9 h (with a fourfold concentration coefficient) successfully improved the total recovery percentage of succinic acid from 55.0 to 84.8%.

Preparation of kenaf stem hemicellulosic hydrolysate and its fermentability in microbial production of xylitol by Escherichia coli BL21

Kenaf (Hibiscus cannabinus L.), a potential fibre crop with a desirably high growth rate, could serve as a sustainable feedstock in the production of xylitol. In this work, the extraction of soluble products of kenaf through dilute nitric-acid hydrolysis was elucidated with respect to three parameters, namely temperature, residence time, and acid concentration. The study will assist in evaluating the performance in terms of xylose recovery. The result point out that the maximum xylose yield of 30.7 g per 100 g of dry kenaf was attained from 2% (v/v) HNO₃ at 130 °C for 60 min. The detoxified hydrolysate was incorporated as the primary carbon source for subsequent fermentation by recombinant Escherichia coli and the performance of strain on five different semi-synthetic media on xylitol production were evaluated herein. Among these media, batch cultivation in a basal salt medium (BSM) afforded the highest xylitol yield of 0.35 g/g based on xylose consumption, which corresponded to 92.8% substrate utilization after 38 h. Subsequently, fermentation by E. coli in the xylose-based kenaf hydrolysate supplemented with BSM resulting in 6.8 g/L xylitol which corresponding to xylitol yield of 0.38 g/g. These findings suggested that the use of kenaf as the fermentation feedstock could be advantageous for the development of sustainable xylitol production.

Potential use of coconut shell activated carbon as an immobilisation carrier for high conversion of succinic acid from oil palm frond hydrolysate

Coconut shell activated carbon (CSAC) presented excellent physicochemical characteristics for efficient conversion of oil palm frond (OPF) into succinic acid.