Study On Cooling Crystallization Process Of Stevioside

Stevioside,a kind of non-nutritive and high-intensity natural sweetener extracted from the leaves of Stevia rebaudiana Bertoni,is an ideal sweetner with broad application prospect in food industry,known as the "third sugar source in the world".In general,high purity stevia glycoside(glycoside content?95%)must be obtained via crystallization separation and purification.Howeve,there are many problems about crystallization process of stevioside currently such as tedious process,long operation period,high solvent consumption and low yield of the total glycoside.Besides,the crystal products of stevioside have the weaknesses of imperfect crystal morphology,poor filtration performance,and caking easily during drying process.The crucial problem of crystallization process of stevioside is that it is difficult to control crystal nucleation and growth.Aiming at these problems,In this paper,we carried out the crystallization thermodynamics and kinetics research on stevioside,and developed a cooling crystallization process which was easier operation,less solvent consumption,higher recovery yield of glycosides and better product quality.Due to the lack of appropriate thermodynamic data,the crytallization solvent selection of rebaudioside A was unreasonable and supersaturation was difficult to control.To address this problem,we have measured the solubility of rebaudioside A with dynamic method in various binary mixture of water and organic solvents.The results indicated that the solubility increased with the temperature increasing.Besides,we have found that the polarity of solvent and the hydrogen bond interaction between solvent and solute are the essential factors affecting solubility.The experimental solubility data were well correlated by Van't Hoff model,the modified Apelblat model and ?h model,respectively.Based on the results of thermodynamics,we selected binary mixed solvent of methanol and water as the crystallization solvent,which can took the yield and purity of the product into account.Faced with the problem that the crystallization process was uncontrollable on account of the unclear nucleation mechanism,we determinated the metastable zone of rebaudioside A by means of laser dynamic technology in binary mixed solvents of methanol and water based on the thermodynamic data in this paper.The results showed that the width of the metastable zone of rebaudioside A broadened with the improvement of methanol content in the solvent and the increasement of cooling rate and narrowed with the enhancement of stirring rate.In addition,on the basis of corresponding metastable zone model,we calculated the nucleation parameters such as critical crystal nucleus size of rebaudioside A using the data of the metastable zone and discussed the nucleation mechanism that when the initial crystallization temperature was higher than 318.15 K,progressive nucleation was the dominant nucleation mechanism of rebaudioside A,when the initial temperature was lower than318.15 K,instantaneous nucleation was the dominant nucleation mechanism based on the apparent nucleation order obtained by calculation.Furthermore,the crystallization kinetics of rebaudioside A in cooling crystallization process was measured by intermittent dynamic method.The studies have shown that the growth rate of rebaudioside A crystals was size depended and then the nucleation and growth rate equations of rebaudioside A in cooling crystallization process were obtained by using ASL particle size-dependent growth rate model.Based on the results of the crystallization thermodynamics and kinetics,we investigated the effects of operating parameters on the seed quality,and found that the cooling crystallization operation curve was the critical factor for the preparation of seeds with uniform particle size and appropriate length-diameter ratio.Then,we focused on the influence of crystallization operating parameters such as seed and solvent composition on the cooling crystallization of stevioside in this paper.It turned out that the crystallization operation curve and solvent compasition had significant influence on the quality of crystal products.According to the above research,a new batch crystallization technology of stevioside by one-step cooling operation curve was developed,which simplified the complexity of the crystallization process and took the quality and yield of the product into account.Compared with the traditional crystallization process,the new crystallization process can not only shorten the crystallization period to 10 h,but also reduce the amount of solvent consumption to half of the traditional crystallization method.What's more,The total glycoside content of new crystal product were no less than 96.5% and the yield of it was more than 50%,respectively.Moreover,the morphology of crystal products of stevioside has been dramatically enhanced.The filtration performance of the new crystal product of stevioside was significantly improved,and did not cake during the subsequent drying process.The study in this paper has certain referential significance for the separation and refinement of complex system of saccharides.