Date Published: July 7, 2016
Publisher: Springer Berlin Heidelberg
Author(s): J. Sharon Mano Pappu, Sathyanarayana N. Gummadi.
In this study, the optimization of different process variables—pH (4–6), aeration rate (200–550 rpm) and agitation rate (0.6–1.8 vvm) were investigated using rotating simplex method and uniform design method to enhance xylitol production from xylose by D. nepalensis in a batch stirred tank bioreactor. Maximum xylitol productivity (0.576 g L−1 h−1) was obtained at pH 4.0, agitation 300 rpm and aeration 1.5 vvm by rotating simplex method. Individual optimum values of pH, agitation and aeration are 4.2, 370 rpm and 1.2 vvm, respectively, for productivity, 4.3, 350 rpm and 1.0 vvm, respectively for xylitol concentration and 4.4, 360 rpm and 0.8 vvm, respectively for yield. Using generalized distance approach, the simultaneous optimal values were found to be—pH 4.3, 370 rpm and 0.9 vvm. After multi-response analysis, batch fermentation at optimal operating conditions resulted in enhanced productivity (0.76 g L−1 h−1), xylitol concentration (59.4 g L−1) and yield (0.58 g g−1) with an increase of 76.74 % of xylitol productivity.
Xylitol is a naturally occurring non-fermentable sugar alcohol with one third calories lesser than sucrose (Granström et al. 2007). Being a low caloric sweetener, it is used as a suitable sugar substitute for diabetic patients, parenteral nutrition (Ladefoged et al. 1982), odontological preparations (Maguire and Rugg-Gunn 2003) and also known to improve health and biomechanical properties of the bone (Mattila et al. 2002). In recent years, interest in xylitol has increased considerably, mainly due to many commercial applications in several industrial sectors like food, dental and pharmaceuticals. Increasing interest in xylitol has led to a strong demand for the product in global market. In 2013, global consumption of xylitol was estimated to be 160 thousand metric tons equating to approximately 670 million USD in value and is expected to reach 1 billion USD in 2020 (Hou-Rui 2012). To meet the world’s increasing demand, it is indispensable to produce xylitol in large scale.
Optimizing the fermentation conditions would be more economic for enhanced production on an industrial scale. The dependency of process parameters such as pH, agitation intensity and aeration rate on productivity, xylitol concentration and yield was investigated using rotating simplex method and uniform design method. UD was proved to be a good experimental design as the number of experimental runs were reduced and specifically used in conducting bioreactor studies. Individual optimum values of pH, agitation and aeration were 4.2, 370 rpm and 1.2 vvm, respectively, for productivity, 4.3, 350 rpm and 1.0 vvm, respectively, for xylitol concentration and 4.4, 360 rpm and 0.8 vvm, respectively, for yield. The process parameters were optimized simultaneously using generalized distance approach. The simultaneous optimal values were found to be—pH 4.3, 370 rpm and 0.9 vvm. Experiments at simultaneous optimal conditions resulted in enhanced production of xylitol. In this work, analysis of experimental run was carried out by regression, which has lesser prediction accuracy when compared to neural network modelling. Optimization of parameters can also be done by artificial intelligence based methods to check further enhancement in production of xylitol.