Optimization of Alkaline Pretreatment of Coffee Pulp for Production of Bioethanol Evandro G. T. Menezes, Juliana R. do Carmo, Jos e Guilherme L. F. Alves, Aline G. T. Menezes, Isabela C. Guimar~ aes, Fabiana Queiroz, and Carlos J. Pimenta Dept. of Food Science, Federal University of Lavras, Lavras, Minas Gerais, Brazil DOI 10.1002/btpr.1856 Published online December 20, 2013 in Wiley Online Library (wileyonlinelibrary.com) The use of lignocellulosic raw materials in bioethanol production has been intensively investigated in recent years. However, for efficient conversion to ethanol, many pretreatment steps are required prior to hydrolysis and fermentation. Coffee stands out as the most impor- tant agricultural product in Brazil and wastes such as pulp and coffee husk are generated during the wet and dry processing to obtain green grains, respectively. This work focused on the optimization of alkaline pretreatment of coffee pulp with the aim of making its use in the alcoholic fermentation. A central composite rotatable design was used with three inde- pendent variables: sodium hydroxide and calcium hydroxide concentrations and alkaline pretreatment time, totaling 17 experiments. After alkaline pretreatment the concentration of cellulose, hemicellulose, and lignin remaining in the material, the subsequent hydrolysis of the cellulose component and its fermentation of substrate were evaluated. The results indicated that pretreatment using 4% (w/v) sodium hydroxide solution, with no calcium hydroxide, and 25 min treatment time gave the best results (69.18% cellulose remaining, 44.15% hemicelluloses remaining, 25.19% lignin remaining, 38.13 g/L of reducing sugars, and 27.02 g/L of glucose) and produced 13.66 g/L of ethanol with a yield of 0.4 g ethanol/g glucose. V C 2013 American Institute of Chemical Engineers Biotechnol. Prog., 30:451–462, 2014 Keywords: coffee pulp, pretreatment, fermentable sugars, bioethanol Introduction The increasingly high global demand for energy, the rapid depletion of fossil fuels, and concerns about global climate change have stimulated a resurgence of interest in renewable energy. 1 Lignocellulosic biomass is an extremely promising feedstock because of its wide availability and low cost; how- ever, large-scale commercial production of ethanol fuel from lignocellulosic material has not yet been implemented. 2 The main problem in transforming lignocellulosic raw materials into liquid fuel is the economic viability of the process. 3 The conversion of lignocellulosic biomass to ethanol is more difficult than when using corn or molasses because of the complex structure of the plant cell wall. A pretreatment process is necessary in order to change the chemical and structural compositions of the biomass, facilitating rapid and efficient hydrolysis of carbohydrates to fermentable sugars. 4 In recent years, studies have been carried out on the pro- duction of bioethanol from various raw materials, including the agricultural by-products corn straw, rice straw, bagasse from sugar cane, and cotton residues. The use of nonedible agricultural products is interesting for having no impact on the food crops (sugar, corn, maize, and sorghum) prices. One source that has received little attention is the residue generated during coffee processing. 5–8 Coffee stands out as the most important agricultural prod- uct for Brazil, in particular for the state of Minas Gerais located in the southeast region. According to Vilela et al., 9 there are three methods to process mature coffee fruits to obtain green grains: dry, semidry, and wet processing. Dur- ing the coffee production, waste products such as pulp and coffee husk, are generated during the wet and dry processing to obtain green grains, respectively. According to Pandey et al., 10 these residues have been used as raw materials in the production of mushrooms, enzymes, organic acids, fla- vorings, and animal feed, as well as being employed as a source of compost. Another alternative would be to use the lignocellulosic fraction of the waste to produce bioethanol. Lignocellulosic material mainly consists of cellulose, hemi- cellulose, and lignin, in varying amounts. Cellulose is a poly- saccharide that can be converted to glucose by enzymatic or chemical hydrolysis. Hemicellulose is also a polysaccharide, and under hydrolysis produces xylose and other monosaccha- rides such as arabinose, mannose, glucose, and galactose. Lig- nin is a heterogeneous aromatic polymer formed by the polymerization of coumarilyl, coniferyl, and sinapyl alcohols. The cellulose present in wood and agricultural residues is highly resistant to enzymatic depolymerization. This is caused by the presence of a lignin–hemicellulose cover surrounding the cellulose, which prevents enzymatic attack. 11 Factors such as low porosity (low surface area), cellulose fiber crystallinity, and the hemicellulose and lignin concen- tration of the material also affect the enzymatic hydrolysis, Correspondence concerning this article should be addressed to E. G. T. Menezes at evandrogtmenezes@gmail.com. V C 2013 American Institute of Chemical Engineers 451