Effects of Initial Super-saturation on the Polymorphism and Size Distribution of Glutamic Acid

The effect of initial glutamic acid(GA) super-saturation on GA crystallization was investigated by using monosodium glutamate(MSG) solution to replace GA fermentation broth under pH-shift operation mode.The results show that the super-saturation degree of the solution influences both crystal habit and crystal size distribution(CSD).The crystal habit is mainly in α-form and CSD exhibits a symmetric distribution with logarithmic form when super-saturation degree(S) is below 7.15(S≤7.15).With the increase of super-saturation degree,the β-form crystal percentage increases and medium size crystallization decreases but with a wider CSD.When S exceedes 14.17(S≥14.17),all of the crystal habit are in β-form,and CSD becomes dispersive and irregular.Based on the classical nucleation theory,the GA polymorphism nucleation rates under different super-saturation degrees and MSG concentrations are calculated,and the results demonstrates that the polymorphism nucleation rates both increase with the increases of super-saturation degree and MSG concentration.The α-form crystal nucleation rate is faster than that of β-form at the same super-saturation degree,while β-form crystal nucleation rate is faster than that of α-form at the same MSG concentration.The results suggest that the major reason accounting for CSD of glutamic acid is due to the nucleation rate increases with the increase of super-saturation degree,while the crystal habit mainly depends on the difference of the two competitive nucleation rates.A C-pH phase diagram model for α-and β-GA crystals was proposed based on the existing reports and the results obtained in this study,and this model could theoretically interpret the mechanism of super-saturation degree affecting GA crystal habit.