scaling up of simultaneous saccharification and fermentation of microwave alkali pretreated emptyfruit bunch for lactic acid production

Abstract

Oil palm empty fruit bunches (EFB), a major solid waste in the palm oil industries is a source of lignocellulosic biomass. Cellulose, which is the major component of EFB can be converted to lactic acid. Production of lactic acid is desirable because it can be utilized in industries including bioplastics, chemicals, and cosmetics. The aim of this study is to produce lactic acid on a larger scale from microwave-alkali (Mw-A) pretreated oil palm EFB using simultaneous saccharification and fermentation (SSF) process with Rhizopus oryzae fungus. The present work is divided into four different stages; pretreatment of EFB, development of practical and effective procedure for inoculum build up for lactic acid production on a pilot scale, optimization of process to improve the yield by using fed batch mode operation and scale up of lactic acid production in 150 L fermentor. The Mw-A pre-teatment proved to be an effective method for removing lignin, preserving cellulose fraction and enhancing the enzymatic hydrolysis of EFB. The composition changes on the lignin, hemicelluloses and cellulose after pretreatment was used as indicators to represent the effectiveness of the pretreatment. In order to fulfill the requirement of massive inoculum production for large scale fermentation, a study was performed to develop a protocol in preparing inoculum for lactic acid production from EFB. Multi-stage inocula were developed and their fermentation ability was assessed. The procedure performed eliminated the requirement of huge quantity of spore suspension and improved the fermentation consistency. In order to obtain the desired morphological form of Rhizopus pellets, several parameters such as concentration of spore suspension, storage time and doses of inoculum were varied. Longer storage time of spore suspension of more than three days led to the formation of free mycelia. Low inoculum concentrations of 107 spores/ml are beneficial for formation of pellet. In addition, xylose has a positive effect on pellet formation compared to glucose. To achieve a high lactic acid concentration in the broth, high solids loading was required to allow a higher rate of glucose conversion. However, a decrease in the final lactic acid concentration was observed when running SSF at a massive insoluble solids level. High osmotic pressure in the medium led to poor cellular performance and caused the Rhizopus oryzae pellets to break down, affecting the lactic acid production. The process performance was further improved using a fed-batch operation mode. The fed-batch operation was observed to facilitate higher lactic acid concentration of 12 g/L, compared with the SSF batch mode with final lactic acid concentration of 6.8 g/L. For scale-up of the lactic acid fermentation, the strategy was adopted to provide almost equivalent oxygen mass transfer coefficient (kLa) to the different-sized fermentor systems (16 L and 150 L), thus ensuring the same amount of dissolved oxygen supply in each fermentation broth. At kLa value of 0.06 s-1, final lactic acid concentration in both scales were found identical.