Elsevier

Fluid Phase Equilibria

Volume 313, 15 January 2012, Pages 182-189
Fluid Phase Equilibria

Determination of the crystallization thermodynamics and kinetics of l-tryptophan in alcohols–water system

https://doi.org/10.1016/j.fluid.2011.09.028Get rights and content

Abstract

Aimed at exploring new solvents for the crystallization of l-tryptophan, the solubility and metastable zone of l-tryptophan in binary solvents of methanol–water, ethanol–water and 1-propanol–water were measured using a laser method. It is demonstrated that the dielectric constant of the solvent significantly affects the solubility of l-tryptophan. Furthermore, results indicate that 1-propanol–water is a better solvent system from which regular and perfect crystals can be obtained. The induction time of l-tryptophan was measured to analyze the nucleation and growth mechanism. It is found that homogeneous nucleation is inclined to occur at high supersaturation while heterogeneous nucleation would be dominant at low supersaturation. The growth mechanism of l-tryptophan follows the continuous mode.

Highlights

► The solubility, metastable zone and induction time of l-tryptophan were measured. ► The dielectric constant of the solvent plays an important role in the solubility. ► 1-propanol–water is a better solvent system to crystallize l-tryptophan. ► The growth mechanism of l-tryptophan follows the continuous mode.

Introduction

l-tryptophan is one of the essential amino acids for human and animals since it participates in the protein synthesis and metabolic regulation of bodies. l-tryptophan is widely used in the fields of medicine, food and feed additives [1]. For the applications in medicine, l-tryptophan can be converted to hormones, such as 5-hydroxy tryptophan which has the function of antidepressant, relieving nervous tension, improving sleep and so on. In the food industry, l-tryptophan can be used as food additives to strengthen the protein utilization of body. Moreover, l-tryptophan has the function of oxidation resistance to prevent the decomposition of fish and powder milk. In the feed industry, l-tryptophan is very effective to accelerate the rate of liveweight growth and enhance the ability of disease resistance of animal body.

Due to the good market prospects, many investigators endeavor to study the production of l-tryptophan. In the cost of l-tryptophan production, the separation and purification hold a large proportion and has become a bottleneck problem [2]. As is well known, crystallization is an attractive separation technique for manufacturing because of the advantages of low energy consumption, high efficiency and low pollution [3]. However, to design an optimized process of crystallization, it is necessary to study the thermodynamics and kinetics of the crystallization first. In this paper, we measured the solubility, metastable zone and induction time of l-tryptophan to supply the valuable thermodynamic and kinetic data for crystallization.

Section snippets

Materials

l-tryptophan with mass fraction purity higher than 99.0% was purchased from Sigma–Aldrich Corporation. The methanol, ethanol and 1-propanol (supplied by Tianjin Jiangtian Chemical Technique Co., Ltd., China) were of analytical reagent grade and had mass fraction purity of 99.5%, 99.7% and 99.8%, respectively. The three solvents and l-tryptophan were used without further purification. In addition, all the solutions for experiments were prepared with deionized water.

Apparatus

A laser method [4] was applied

Solubility of l-tryptophan in binary solvents of alcohols and water

Since l-tryptophan is much soluble in alkaline or acidic solvents, we have ever tried to crystallize l-tryptophan by neutralization method. In those experiments, l-tryptophan was dissolved under high or low pH, and then crystallized by adjusting the pH to 5.89 at which l-tryptophan had its lowest solubility. However, sometimes the color of products is yellow that probably arises from the decomposition or oxidation of l-tryptophan. Therefore, new solvents need to be explored.

In this paper, the

Conclusions

Solubility, metastable zone and induction time are the fundamental data for a crystallization process. In this paper, the solubility and metastable zone of l-tryptophan in the binary solvents of methanol–water, ethanol–water and 1-propanol–water were firstly measured. As expected, the solubility of l-tryptophan increases with temperature and the metastable zone is wider at lower concentrations. However, the solubility has a maximum value at constant temperature. Our study demonstrates that the

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