Bioethanol production from mandarin (Citrus unshiu) peel waste using popping pretreatment
Highlights
► Popping pretreatment designed for bioethanol production using mandarin (Citrus unshiu) peel waste. ► d-limonene concentration (fermentation inhibitor) was reduced from 0.21% to 0.01%. ► Particle size of popping-pretreated MP also reduced to less than 1 mm. ► The enzymatic hydrolysis on pretreated MP has produced 95.6% saccharification rate. ► Ethanol productivity and yield efficiency by Saccharomyces cerevisiae were remarked to 3.85 g/L h and 90.6%.
Introduction
Bioethanol is a promising alternative to fossil fuels because it is a renewable resource. It is a liquid fuel that can be adapted to existing fuel supply systems and can replace fossil fuels in the transportation sector [1], [2], [3], [4]. However, starch-based bioethanol production is criticized because starch is generally derived from food. Thus, the use of starch as a source of bioethanol can reduce food supplies and increase food price. The use of starch- or sugar-rich feedstock has directly increased food and bioethanol prices due to the high price of raw materials, which account for 40–75% of the total expenses for ethanol production [5]. Therefore, alternative biomass sources are required for bioethanol production, such as agricultural byproducts, forest residues, or energy crops (lignocellulosic biomass) [6], [7], [8], [9], [10], [11], [12], [13], [14].
Citrus peel waste is a suitable candidate for bioethanol production because it contains low lignin and high soluble sugar concentrations [15], [16]. Citrus peel waste is rich in fermentable sugars such as glucose, fructose and sucrose, along with insoluble polysaccharides such as cellulose and hemicellulose [16]. According to the USDA [17], world citrus production was estimated at 49.9 million tons during the 2009/2010 growing season. After juice extraction, citrus peel waste (containing peel and saccharides) accounts for approximately 44% of the wet fruit mass [18]. Mandarin peel (MP) waste is also considered an important source for bioethanol production because the soluble sugar and carbohydrate contents are similar to citrus peel waste [15]. In Korea, the average production of fresh mandarin fruits and mandarin waste per year are approximately 700,000 and 70,000 tons, respectively. Discarded mandarin fresh fruit and waste will be likely increased over time due to the increase in imported citrus fruit from the USA. Approximately 30% of mandarin waste is currently used in medicinal herbs and the remaining 70% is disposed into the ocean. However, ocean disposal causes environmental problems and is forbidden by the London Convention and London Protocol of 2013 in Korea.
Citrus peel waste contains 0.8–1.6% d-limonene, which is an inhibitor of yeast fermentation [19]. When using of citrus peel waste for bioethanol production, acidic steam explosion pretreatment is required to lower the limonene content to below 0.05% because d-limonene inhibit microbial growth [19], [20], [21]. Steam explosion pretreatment has been used to produce bioethanol from MP [15].
Pretreatment is required to improve the accessibility of substrates to cellulose-degrading enzymes [22]. We have designed a biomass pretreatment system with a fired burner and horizontal cylinder, which rotates on an axis, called “popping pretreatment” (there is a “pop out” as biomass is produced from the pretreatment). Previously, we applied popping pretreatment to rapeseed straw and obtained a high glucose yield after enzymatic hydrolysis [23].
In this study, we investigated bioethanol production using MP after popping pretreatment and established an efficient bioethanol production process base. We investigated the effect of popping pretreatment on MP composition, limonene concentration, and particle size. We also studied the efficiency of enzyme cocktails on hydrolysis and ethanol yields after individually optimizing the hydrolysis and fermentation of the enzymatic hydrolysate of pretreated MP.
Section snippets
Mandarin peel wastes and popping pretreatment
Fresh mandarin (Citrus unshiu) peel wastes (MP), with a dry matter content of 17.8% (wet basis), was obtained from Jeju Island. Raw MP was ground in a homogenizer, frozen (−20 °C), lyophilized (−50 °C), and stored at −20 °C until further use. The pretreatment experiment after defreeze, 1 kg of fresh MP matter without any earlier cut or chemical pretreatment was performed by modified popping equipment. The main structure of the reactor, with an inner horizontal cylinder of 300 mm from left to right,
Effect of popping pretreatment on mandarin peel composition
Popping pretreatment uses whole MP without chemical treatments or mechanical cuts. This procedure simplifies the MP pretreatment compared to other methods such as steam explosion. Generally, steam explosion requires diluted chemicals (sulfuric acid or sodium carbonate) or sample cutting to produce ethanol from citrus species [15], [18].
We evaluated the observed structural and chemical changes through the popping pretreatment on MP. The d-limonene concentration was also analyzed before and after
Conclusion
Mandarin (C. unshiu) peel waste is an attractive source of biomass for bioethanol production and has several advantages over lignocellulosic biomass rich in hemicellulose and lignin (Table. 1). Popping pretreatment ruptures MP cell walls, producing particles with diameters less than 1 mm and also reduces d-limonene concentrations to less than 0.01%. The pretreated MP does not yield higher sugar concentrations in the hydrolysate after enzymatic treatment but significantly reduces d-limonene
Acknowledgements
This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (2010T100100573) grant funded by the Korea government Ministry of Knowledge Economy and by Priority Research Centers Program (2011-0018393), and WCU (World Class University) project (R31-2009-000-20025-0) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology to H.-J. Bae. I.S. Choi is grateful for the BK21 program
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