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Sugar beet molasses is a well-known, inexpensive and available carbon source for microbial cell growth. Its sugar components are used to produce energy for microbial growth and non-sugar components, especially nitrogen components, have important roles in improvement of cell growth. On the other hand, immobilization of whole cell is establishment and physical limitation of intact cells in specific space that keeps their catalytic activity and provides the possibility of reuse of the cells. This technique allows continuous and accelerated biological processes. It also improves production efficiency and quality and simplifies recycling of product. Immobilized living cells, as controlled catalysts, are able to perform one-step enzymatic reaction and continuous fermentative processes. In this research, E.coli cells were immobilized in calcium alginate hydrogels and using sugar beet molasses as carbon source, were applied for tryptophan production reaction in the presence of its precursors, serine and indole. In comparison between free biocatalysts and immobilized bacterial cells that entrapped in alginate gels, indicated that larger amounts of amino acids (about 42/9%) can produce in calcium alginate. Also the production reaction was followed up for 9 sequential cycles, and results showed that the cells could produce tryptophan amino acid under above conditions. Use of sugar beet molasses (by-product of agriculture industries) for growth of microbial cells and tryptophan production, causes decrease in production cost and more economical production of tryptophan by immobilized E. coli.

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In this research, response surface methodology based on 3-factor 5-level central composite design was used to determine the effect of refining pretreatment variables on quality indexes of beet molasses. Hence, the effect of bentonite concentration (2-10 g/L), gelatin concentration (0.01-0.1 g/L) and pH (3-7) on turbidity, color, concentration of colloids (CCs) and ash of beet molasses were studied and well fitted by a quadratic polynomial model (R² > 0. 85). The results showed that the pH was the main factor affecting the purification indexes as it had a significant effect on all responses. Bentonite and gelatin as two common clarifying agents improved the quality of molasses, although the ability of bentonite was much higher. The optimum condition for purification pretreatment was found as 8.3 g/L for bentonite, 0.08 g/L for gelatin and 4.03 for pH which were determined on minimization of the turbidity, color, concentration of colloids (CCs) and ash. At this condition, the turbidity, color, ash and concentration of colloids were calculated as 12.1 NTU, 2573 IU, 2.7, 0.9 g/L respectively.