Investigation of rebaudioside a crystallization behavior

Stevia rebaudiana Bertoni is a source of more than 11 zero calorie sweet tasting compounds called steviol glycosides. These steviol glycosides especially Stevioside and Rebaudioside A (Reb A) which are two major glycosides found in the stevia leaves in higher amounts have received special interest for use as natural sweetener. The stevia extract should contain steviol glycosides at least 95 % in order to be used as a sweetener. Food and Drug Administration (FDA) approves the use of over 97 % pure Reb A in food products with a generally regarded as safe status (GRAS). The extraction, separation and purification of steviol glycosides are therefore crucial for the production of high purity natural sweeteners from stevia leaves. Crystallization is one of the oldest and commonly used methods of industrial separation and purification. A compound of interest can be separated and purified from the extract by crystallization due to temperature and solvent dependent differences in the solubility values. Temperature and solvent dependent differences in the solubility values, the presence of supersolubility behavior and the growth of polycrystalline phases affect the separation/purification via crystallization. The aim of this thesis was therefore to determine temperature dependent solubility values and crystallization behavior of Reb A in water. Thermodynamic solubility values of Reb A in water at 40, 50, and 60 °C were determined and found to be 3.94, 5.93 and 10.65 mg RebA/g water, respectively. Reb A was found to exhibit supersolubility and recrystallization in water. Particle morphologies were therefore determined by optical microscopy. Rounded and cornered shaped Reb A crystals was found to crystallize as thin and long whisker shaped crystals in water. X-Ray diffraction analysis also confirmed the crystallization of different Reb A phase from supersaturated solutions. Solubility values of ground-water-grown-crystals were also determined at 20, 30, 40, 50 and 60 °C and found to be 0.55, 0.52, 0.70, 4.53 and 7.87 mg Reb A/g water. Nucleation tresholds of Reb A were determined upon the observation of its supersolubility behavior in water. Primary and secondary nucleation tresholds of Reb A were determined both by holding Reb A solutions in constant temperature water bath and by isothermal crystal growth experiments in the glass double jacketed reactor under stirring at constant temperature. Reb A seeds were prepared for the determination of secondary nucleation tresholds. Primary nucleation thresholds were found to be between 0.0025 and 0.004 g/mL for 25 and 40 °C and between 0.004 and 0.01 g/mL for 50 and 60 °C by statically. Primary nucleation treshold determinations in stirred reactor however yielded 0.00399 g/mL and 0.00474 g/mL for 30 and 40 °C, respectively. Secondary nucleation tresholds by static system were found to be between 0.002 and 0.003 g/mL for 20 °C and between 0.004 and 0.003 g/mL for 30 and 40 °C. It was not possible to discriminate the secondary nucleation during isothermal crystal growth experiments either by visually or by spectrophotometric absorbance measurements within the solid liquid ratios employed and amount of seed suspension used.