Eng. Life Sci. 2014, 00, 1–12 www.els-journal.com Juan Francisco Casta˜ n´ on- Rodr´ıguez 1 Jos´ e Manuel Dom´ınguez- Gonz´ alez 2 Benigno Ort´ız-Mu˜ niz 3 Beatriz Torrestiana-Sanchez 1 Jos´ e Alberto Ram´ırez de Le´ on 4 Mar´ıa Guadalupe Aguilar- Uscanga 1 1 Laboratorio de Bioingenier´ıa, Instituto Tecnol´ ogico de Veracruz-UNIDA, Veracruz, Veracruz, M´ exico 2 Universidad de Vigo, Departamento de Ingenier´ıa Qu´ımica, Facultad de Ciencias, Campus Ourence, Ourense, Espa˜ na 3 Instituto Tecnol´ ogico Superior de Tierra Blanca, Tierra Blanca, Veracruz, M´ exico 4 Centro de Excelencia, Direcci´ on General de Innovaci´ on Tecnol´ ogica, Universidad Aut´ onoma de Tamaulipas, Tamaulipas, M´ exico Research Article Continuous multistep versus fed-batch production of ethanol and xylitol in a simulated medium of sugarcane bagasse hydrolyzates Co-cultures for simultaneous production of ethanol and xylitol were studied un- der different operation bioreactor modes using Candida tropicalis IEC5-ITV and Saccharomyces cerevisiae ITV01-RD in a simulated medium of sugarcane bagasse hydrolyzates. Xylitol and ethanol tolerance by S. cerevisiae and C. tropicalis, re- spectively, was evaluated. The results showed that C. tropicalis was sensitive to ethanol concentrations up to 30 g/L, while xylitol had no effect on S. cerevisiae viability and metabolism. The best condition found for simultaneous culture was S. cerevisiae co-culture and C. tropicalis sequential cultivation at 24 h. Under these conditions, productivity and yield for ethanol were Q EtOH = 0.72 g L −1 h −1 and Y EtOH/s = 0.37 g/g, and for xylitol, Q XylOH = 0.10 g L −1 h −1 and Y XylOH/S = 0.31 g/g, respectively; using fed-batch culture, the results were Q EtOH = 0.87 g L −1 h −1 and Y EtOH/s = 0.44 g L −1 h −1 , and Q EtOH = 0.27 g L −1 h −1 and Y EtOH/s = 0.57 g/g, respectively. Maximum volumetric productivity in continuous multistep cultures of ethanol and xylitol was at dilution rates of 0.131 and 0.074 h −1 , respectively. Con- tinuous multistep production, Q EtOH increased up to 50% more than in fed-batch culture, even though xylitol yield remained unchanged. Keywords: Continuous culture / Ethanol / Fed-batch / Wild-type yeasts / Xylitol Received: May 5, 2014; revised: June 30, 2014; accepted: July 18, 2014 DOI: 10.1002/elsc.201400098 1 Introduction Among the various agricultural crop residues, sugarcane bagasse (SCB) is the most abundant lignocellulosic material in tropical countries. In general, 1 ton of sugarcane generates 280–300 kg of bagasse [1]. SCB is a residue produced in large quantities by the sugar and alcohol industries, and is mainly used as a fuel to power the sugar mill. Several processes and products have been reported that utilize SCB as a raw material. These include electricity generation, pulp and paper production, and products based on fermentation [2]. This residue has been explored for biotechnological applications to produce bioethanol and value- added commercial products such as xylitol, specialty enzymes, organic acids, single-cell protein, among others [3]. Correspondence: Dr. Mar´ıa Guadalupe Aguilar-Uscanga (gaguilar@itver.edu.mx), Laboratorio de Bioingenier´ıa, Insti- tuto Tecnol´ ogico de Veracruz-UNIDA, Av. Miguel A. de Quevedo 2779, Col. Formando Hogar. CP. 91860, Veracruz, Veracruz, M´ exico. Abbreviations: Q EtOH , ethanol productivity; Q XylOH , xylitol productiv- ity; RD, respiratory deficient; SCB, sugarcane bagasse; Y EtOH/s , ethanol yield; Y XilOH/s , xylitol yield Several studies focused on the utilization of glucose and xy- lose as carbon sources from lignocellulosic feedstocks for the biotechnological production of metabolites of industrial in- terest (ethanol, xylitol, lactic acid) under different operation modes. These include barley bran hydrolyzates [4], SCB [5, 6], corn cobs [7], and vine trimming wastes [8, 9]. Others have used mixed cultures to produce one or two products of inter- est. For example, Delgenes et al. [10] associated Saccharomyces cerevisiae CBS 1200 and Pichia stipitis NRRL 11545 cultures to produce ethanol and Lactobacillus reuterii with the yeast Candida guilliermondii to produce xylitol on aspen wood hy- drolyzate and wheat straw hemicellulosic hydrolyzate, respec- tively. On the other hand, Arrizon et al. [11] evaluated different hydrolyzates such as SCB, agave tequilana bagasse and coffee husks, and chose SCB hydrolyzate for the production of ethanol and xylitol using strains of the same genera. A simple alternative for the transformation of xylose into ethanol might be the con- version of this sugar into xylitol, which would become another valuable product of the process. For ethanol production, S. cerevisiae is the microorgan- ism most used for fermenting cellulose hydrolyzates from lignocellulosic biomass. This yeast ferments the hexoses but C  2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1