Abstract
A direct bioconversion of gelatinized starchy waste (GSW) to lactic acid by amylolytic lactic acid bacterium Lactobacillus plantarum S21 was investigated. Corn steep liquor (CSL) was selected as the most suitable low-cost nitrogen source for replacing yeast extract, beef extract, and peptone in De Man, Rogosa and Sharpe (MRS) medium. Plackett–Burman design results indicated that GSW and CSL were the two most nutrients that significantly influence lactic acid production, among eight medium components, including GSW, CSL, K2HPO4, CH3COONa, (NH4)2HC6H5O7, MgSO4, MnSO4, and Tween 80. A new low-cost medium containing only GSW (134.4 g/L) and CSL (187.7 g/L) was achieved as omitting other six components from the optimized medium had no effect on lactic acid yield. Batch fermentation at 37 °C both in 1 L and 10 L jar fermenters showed non-significantly different productivity. A by-product, maltose-forming α-amylase, was successfully achieved up to 96% recovery yield using an ultrafiltration unit equipped with a 50 kDa cut-off membrane. Crude lactic acid exhibited the additional benefit of antimicrobial activity against food and feed pathogens Salmonella enterica serovar Typhimurium TISTR 292, Vibrio cholerae TH-001, and also E. coli ATCC 25922. This study presents a promising bioprocess for the simultaneous production of lactic acid, and a value-added food enzyme, using only two industrial wastes, GSW and CSL, as the medium components.
Highlights
Lactic acid (2-hydroxypropanoic acid) is an important organic acid that is widely used in various industries, food and feed, pharmaceutical, and chemical industries
The results showed that lactic acid production was markedly influenced by the type of nitrogen source
Among the different nitrogen sources tested in this experiment, the highest lactic acid yield was obtained from Corn steep liquor (CSL) at 9.3 g/L, which was the closest to the lactic acid yield of the original MRS medium
Summary
Lactic acid (2-hydroxypropanoic acid) is an important organic acid that is widely used in various industries, food and feed, pharmaceutical, and chemical industries. Wastes from agricultural industries, such as starchy wastes, lignocellulosic biomass, and food wastes have been extensively investigated in recent years, as alternative and economic raw materials to reduce the cost of lactic acid production [5,8,9,10,11]. Utilizing these agricultural wastes is an effective strategy in environmental waste management [12]. CSL has been successfully used as an alternative for expensive nitrogen sources in many fermentation processes [8,9,18,19]
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