Bioprocess optimization for kefiran production by lactobacillus kefiranofaciens

Abstract

Kefiran is water soluble heteropolysaccharides produced by Lactobacillus kefiranofaciens. In this work, optimization process was carried out to maximize kefiran production in both pure culture and mixed culture system using yeast strain. For pure culture studies, the cultivation medium composition was first optimized using both one factor at a time (OFAT) and statistical method. The optimal medium for kefiran production using OFAT composed of (in g L-1): lactose, 50; yeast extract, 12; phosphate, 0.25; Triton X-100, 1; and medium osmotic pressure 550 mOsmol kg-1. On the other hand, the optimal medium composition using statistical method composed of (in g L-1): lactose, 58.74; yeast extract, 11.09; phosphate, 0.52; Triton X-100, 0.85, and medium osmotic pressure of 630 mOsmol kg-1. The maximal kefiran production of 1.51 g L-1 was obtained in OFAT optimized medium which was approximately 8.61% higher than those obtained in the statistically optimized medium. After this step, the process was optimized in 16-L stirred tank bioreactor. The maximal kefiran production reached 2.32 g L-1 and 1.87 g L-1 in bioreactor with and without pH control, respectively. Aeration rate of 1.0 v/v/min was the best for kefiran production (2.32 g L-1). Furthermore, a series of fed-batch cultivations were carried out to determine factors that limit cell growth and kefiran production. A combination of constant complete medium feeding coupled with dissolved oxygen control was found to be the best strategy for highest kefiran production (4.73 g L-1). Among different yeasts tested, S. cerevisiae was found to be the most suitable strain for mixed culture system. The mixed culture of L. kefiranofaciens and S. cerevisiae enhanced kefiran production from 0.31 g L-1 in pure culture up to 0.39 g L-1 in the mixed culture. The cultivation medium was then optimized using OFAT method reaching a maximum kefiran production of 2.12 g L-1. The optimal medium for kefiran production by the mixed culture using OFAT method composed of (in g L-1): lactose, 50; yeast extract, 12; phosphate, 0.5; Triton X-100, 1.25 g L-1, and osmotic pressure of 600 mOsmol kg-1. Batch cultivations of mixed culture in 16-L stirred tank bioreactor with and without pH control produced kefiran with concentration of 3.01 g L-1 and 2.40 g L-1, respectively. Aeration rate at 1.0 v/v/min produced maximal kefiran production of 4.42 g L-1. Further improvement in kefiran production was achieved by using constant lactose feeding which was the best strategy to increase the production up to 5.51 g L-1.