Lactic acid production by immobilized Rhizopus Sp. through simultaneous saccharification and fermentation process using banana peel and beet pulp

Abstract

The industrial demand for lactic acid (LA) has increased significantly due to their functional properties in different fields. LA production by Rhizopus sp. using various substrates have been studied. But, the problem of low production yield is still unsolved due to the following issues: finding a suitable low-cost substrate and controlling the fungal morphology during fermentation to maintain high oxygen concentration. Therefore, this project’s main aims are to improve the LA yield by selecting a better substrate between banana peel (BaP) and beet pulp (BeP) for higher LA yield; to optimize the Rhizopus sp. immobilization process for a maximum weight of fungal attachment on the loofah using potato dextrose, and to increase the LA yield through semi-continuous simultaneous saccharification and fermentation (SSF) in air pulsed airlift reactor (ALR) (using the selected substrate with the optimum immobilization condition). For the first part of this research, two fermentation experiments with different BaP and BeP concentrations (40 to 100 g/L) in a shake flask were carried out to determine the best substrate for higher LA yield. The results showed that the produced LA yield from BaP (0.044 g/g) was 3-fold higher than that of BeP (0.0146 g/g). For the second part of the study, optimization of Rhizopus sp. immobilization conditions (potato dextrose (0.02 to 1.5 M), agitation speed (0 to 150 rpm), and incubation time (24 to 72 h)) onto a loofah sponge were carried based on a face-centered central composite design (FCCCD) by Design-Expert software v11.1.2.0 with the attached biomass weight as the response for each parameter. Then, a scanning electron microscopy (SEM) test was conducted to evaluate the fungal attachment and loofah structure after the immobilization. Based on the parameters proposed by FCCCD, the maximum biomass weight of 0.252 g was achieved onto 0.3 g of loofah at dextrose concentration, agitation speed, and incubation time of 0.45 M, 100 rpm, and 24 h, respectively. SEM results showed a good fungal attachment with no degradation to the loofah fibers. In the third part of the research, three SSF experiments with different air pulsation frequencies (0.0384 to 0.1667 s-1) were conducted in loofah immobilized ALR using 60 g/L BaP for 8 days. After that, the immobilized loofah from each experiment was examined using SEM to elucidate the effect of different pulsation frequencies on the fungal morphology. From SSF results, the 0.0384 s-1 was the best frequency to increase the LA yield to 0.091 g/g after 3 days of fermentation. It is 2.1-fold higher in comparison with the free-cell non-pulsed shake flask culture in the previous study. Unlike the frequency at 0.0384 s-1 the SEM images showed that at 0.1667 s-1 frequency the airlift flow was restricted by the irregular formation and the trapped BaP particles between hypha may lead to oxygen transfer limitation in the ALR. The results may improve LA productions by proposing an easy, fast, and efficient method for fungal immobilization on loofah using potato dextrose. In addition, the uncontrolled fungal growth problem inside submerged cultures can be resolved by applying a simple air pulsing system. Further studies on the fermentation conditions for BaP and BeP for better fungal utilization and LA production, the substrate consumption rate and culture composition during the SSF process are also recommended for future research.