Abstract
Biorefinery, which utilizes carbon-neutral biomass as a resource, is attracting attention as a significant alternative in a modern society confronted with climate change. In this study, spent coffee grounds (SCGs) were used as the feedstock for lactic acid fermentation. In order to improve sugar conversion, alkali pretreatment was optimized by a statistical method, namely response surface methodology (RSM). The optimum conditions for the alkali pretreatment of SCGs were determined as follows: 75 °C, 3% potassium hydroxide (KOH) and a time of 2.8 h. The optimum conditions for enzymatic hydrolysis of pretreated SCGs were determined as follows: enzyme complex loading of 30-unit cellulase, 15-unit cellobiase and 50-unit mannanase per g biomass and a reaction time of 96 h. SCG hydrolysates were used as the carbon source for Lactobacillus cultivation, and the conversions of lactic acid by L. brevis ATCC 8287 and L. parabuchneri ATCC 49374 were 40.1% and 55.8%, respectively. Finally, the maximum lactic acid production by L. parabuchneri ATCC 49374 was estimated to be 101.2 g based on 1000 g of SCGs through the optimization of alkali pretreatment and enzymatic hydrolysis.
Highlights
Coffee is one of the most consumed beverages in the world
At 144 h, the enzymatic digestibility (ED) of the experimental groups, except for enzyme loading of 7.5 filter paper unit (FPU)/g biomass cellulase, 3.75 cellobiase unit (CBU)/g biomass cellobiase and 12.5 mannanase unit (MNU)/g biomass mannanase, was not significantly improved by the enzyme loading. These results indicate that enzyme loading of at least 30 FPU/g biomass cellulase, 15 CBU/g biomass cellobiase and 50 MNU/g biomass mannanase and a time of 96 h are recommended for efficient enzymatic hydrolysis of the experimental group
After 12 h, L. parabuchneri ATCC 49374 showed a similar tendency to L. brevis ATCC 8287, in which lactic acid was consumed even in the presence of sugar in the spent coffee grounds (SCGs) hydrolysates medium. These results indicated that L. parabuchneri ATCC 49374 was more suitable for lactic acid production using SCG hydrolysates than L. brevis ATCC 8287
Summary
Coffee is one of the most consumed beverages in the world. According to the International Coffee Organization (ICO), global coffee production in 2019 was estimated to be about 170 million bags of 60 kg [1]. Disposable coffee cups made of plastic have been associated with these forms of pollution [4]. Biorefinery that converts food waste into value-added materials has been attracting attention [5]. SCGs containing carbohydrates, lipids and proteins can be a potential raw material in biorefinery [6]. Various studies have been conducted to produce value-added materials such as biopolymers, biofuels and antioxidants by using the carbohydrates of SCGs as carbon sources for microbial fermentation [7,8]
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