Development of innovative pretreatments for simultaneous saccharification and citric acid production from banana pseudostem: bioprocess optimization and kinetic assessment.
Authors:   , Laltha Milesh 
Published: March 2024

The bioconversion of renewable lignocellulosic biomass into value-added products such as citric acid (CA) has become a research hotspot towards a sustainable economy. Approximately 2 million tons of CA are produced annually with widespread use in the food, chemical, pharmaceutical and textile industries. Nevertheless, the use of lignocellulosic-derived feedstocks for CA production requires expensive, energy- and resource intensive pretreatment and fermentation processes, that result in low product yields. Therefore, the present study explores the valorization of banana pseudostem (BP) for CA production using the simultaneous saccharification and fermentation (SSF) process to alleviate these challenges. A review on the potential of BP as a feedstock for microbial production of value-added products is discussed together with the pretreatment and bioprocess challenges in addition to the possibilities to overcome these bottlenecks. Subsequently, two novel pretreatments consisting of a: (1) microwave-assisted-iodized table salt (M-ITS) and (2) microwave-assistedpaper wastewater (M-PWW) were modelled and optimized using the response surface methodology (RSM) for the enhancement of sugar recovery from BP. Then, the SSF process with pretreated BP was modelled and optimized using RSM for CA production. Thereafter, the logistic and modified Gompertz models were used to assess the kinetics of Aspergillus brasiliensis growth and CA production, respectively. The pretreatment input parameters for the M-ITS model comprised of salt concentration (1- 5%, w/v), microwave power intensity (100-900 W) and pretreatment time (2-10 min). On the other hand, the M-PWW model consisted of substrate solid loading (10-30% w/v), microwave power intensity (100-900 W) and pretreatment time (2-10 min). The output responses were reducing sugar yield (RSY) and glucose yield (GY) for both models. The optimal pretreatment conditions predicted by the M-ITS model were 2.48% ITS concentration, 800 W (power intensity) and 10 minutes (pretreatment time), corresponding to an experimental RSY (0.515 g/g) and GY (0.433 g/g). Pretreatment and optimization revealed a slightly higher RSY and GY for the M-ITS strategy when compared to the M-PWW method (RSY = 0.498 g/g and GY = 0.413 g/g). The M-PWW model predicted optimal pretreatment conditions of 30% solid loading, 800 W for 8 minutes. Despite the higher sugar yields observed for the M-ITS pretreatment, the M-PWW pretreatment represents a more suitable strategy owing to its complete waste-based approach that generates three times the quantity of pretreated substrate required for subsequent fermentation. SSF optimization included nitrogen (ammonium nitrate) concentration (0.5-5 w/v), desorbent (acetone) concentration (1-5% v/v) and temperature (30-40 °C) as inputs with CA concentration as the output (g/L). The optimized SSF conditions (0.5% w/v ammonium nitrate, 1% v/v acetone and 35 °C) (SSFoptimizedFW) gave a maximum CA concentration of 14.408 g/L. For the kinetic experiments, three bioprocesses consisting of the: (1) optimized SSF process with freshwater (SSFoptimizedFW), (2) SSFoptimizedFW process conditions while substituting dairy wastewater in place of freshwater (SSFDWW), and (3) Sabouraud Dextrose Emmon’s medium modified by substituting glucose with BP (SSFSDEmodified), were assessed. While all three bioprocesses gave the same maximum specific growth rate (μmax) of 0.05 h-1, the SSFSDEmodified process resulted in the highest maximum potential CA concentration (Pm) (13.991 g/L) compared to the SSFDWW (Pm= 13.095 g/L) and SSFoptimizedFW (Pm= 12.967 g/L) systems. The developed pretreatment strategies demonstrated glucose yields >0.40 g/g, shown to be higher than previously established chemical pretreatments. Moreover, the SSFDWW process displayed a slightly lower Pm value compared to the SSFSDEmodified strategy, and interestingly a higher Pm value than the SSFoptimizedFW bioprocess. Major findings from this study paves the way for lignocellulosic bioprocesses by potentially negating the use of costly pretreatment chemicals, fermentation medium constituents and/or fresh water, while achieving a “waste treating waste” approach. Thus, contributing to the water-energy-food nexus in line with global sustainable development goals towards a circular bioeconomy.