Optimization of Chemical Pretreatments Using Response Surface Methodology for Second-Generation Ethanol Production from Coffee Husk Waste J. L. Morales-Martínez 1 & M. G. Aguilar-Uscanga 2 & E. Bolaños-Reynoso 1 & L. López-Zamora 1 Received: 23 June 2020 /Accepted: 23 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Within the strategies of substitution of energy from fossil fuels by renewable energies, the research is based on second-generation ethanol production (2G ethanol). One of the raw materials considered for this is residual biomass of the coffee industry, being the subject of study here. The cellulose contained in the coffee husk (coffee husk or coffee skin or coffee exocarp or pericarp) was maximized using pretreatment processes. In dilute acid hydrolysis (DAH), using a fixed 1:6 w:v solid to liquid ratio (SLR), process times (35, 45, 55 min) and H 2 SO 4 concentrations (3, 4, 5% v/v) were evaluated, achieving 53.63% hemicellulose removal. A delignification process resulted in 58.82% lignin removal, evaluating the effect of process times (30, 35, 40 h) and SLR (1:8, 1:10, 1:12 w:v) at a fixed concentration of 8% v/v H 2 O 2 . A 115.59 g/L glucose concentration was obtained with an interaction of fixed concentrations of 4–6% w/w Cellic CTec3 enzyme and 6:1 to 1:12 v:w SLR. The fermentation process considered the composition variation of the culture medium (enriched culture C1 and non-enriched culture C2), generating ethanol at 48.19 and 29.02 g/L concentrations, respectively. Fermentation efficiency (η f ) was improved from 21.99 to 81.74% with the addition of inorganic nutrients (KH 2 PO 4 , (NH 4 ) 2 SO 4 , and MgSO 4 ·7H 2 O). These results confirmed that the optimization of the pretreatments in coffee husk waste favored the cellulose production and facilitated the enzymatic process to produce a high glucose concentration, revealing these residues as a carbon source promising for second-generation ethanol production. Keywords Response surface methodology . Glucose . Enzymatic hydrolysis . Renewable source . Cellulose Introduction In the early 1970s, rising oil prices initiated an energy crisis which together with global warming and the depletion of oil reserves [1–3] aroused great interest in the study of renewable resources in order to produce alternative energy, allowing decarbonization in the environment and reducing the emis- sions of greenhouse gases by ~ 43% [1, 4, 5]. Different re- sources used to produce alternative energy have been studied. Currently, the world produces a large amount of waste daily (municipal, food, beverages, agricultural, and industrial) due to the increase in population and new consumer habits. Dumping the waste directly to the landfills may cause serious environmental problems due to the existence of organic com- pounds that demand excessive amounts of oxygen to be de- graded [6–11]. Utilization of the lignocellulosic agro-residues which are non-food sources, since they are cheap, unutilized, and locally available most of the time, has been used for the production of bioethanol, which allows it to become a renew- able fuel, capable of replacing crude oils because of its posi- tive net energy balance. Such residues, such as rice or coffee husks, are chemically made up of cellulose (35–50%), hemi- celluloses (25–35%), and lignin (10–15%) [1, 3, 7, 12–15]. Green coffee beans are deemed to be a second commodity only to petroleum in terms of currency traded worldwide. World exports reached 10.49 million 60-kg bags, being the third largest volume registered in May [16, 17]; it is estimated that 1 ton of coffee generates more than 600 kg of waste [3, 18]. The improper disposal of coffee husk can generate envi- ronmental problems such as water eutrophication, acidifica- tion and salinization of soils, and the toxic effects on some biological processes, aspects which have limited its * L. López-Zamora llopezz@orizaba.tecnm.mx 1 Graduate Studies and Research Division, National Institute of Technology of Mexico/Orizaba Institute of Technology, Oriente 9 852, 94320 Orizaba, Veracruz, Mexico 2 Food Research and Development Unit, National Institute of Technology of Mexico/Veracruz Institute of Technology, Calz. M.A. de Quevedo 2779, 91860 Veracruz, Mexico BioEnergy Research https://doi.org/10.1007/s12155-020-10197-6