Oo

This does not have a C=O group, but instead intermediate

Figure 3. IR spectra of l-glutamic acid- related substances.

Figure 3. IR spectra of l-glutamic acid- related substances.

some between C—O and C=O. The spectra of the amino acids at the isoelectric point of pH do not show the existence of C—O, which indicates that the —COOH group is ionized. In a similar way, the ionization of the —NH2 is indicated.

The optical rotation changes with changing pH in solution at 25 °C; glutamic acid [a]D(G±; H2O) is +12.0, [a]D(G+; 5N HCl) is +31.8, [a]D(G-; NaOH) is -4.2, and [a]D(G=; NaOH) is +10.9, which corresponds to the ratio of ionic form in solution (21).

Solubility

For the production of the umami seasoning MSG, the characteristic solubility changes in L-Glu HCl, L-Glu, and L-GluNa are used to separate it from other impurities to meet the category of food additives. The change in solubility of L-Glu for a- and /¡-forms, respectively, with temperature are expressed by the following (22):

log S = 0.01741 - 0.377 (0-30 °C) for a-form log S = 0.01531 - 0.328 (30-70 °C) for a-form log S = 0.01591 - 0.481 for /-form

The solubility of the /-form is lower than that of the a-form throughout the temperature range measured. It follows that the /-form is a stable form from the aspect of thermodynamic kinetics. In Table 1, the solubility for various salts of L-Glu versus temperature are summarized (23).

The solubility changes with change in the concentration of both hydrochloric acid and sodium hydroxide in Figure 4, which corresponds to the existence of solid crystals, in

Table 1. Solubility of Various Salts of L-Glu (g/L)

Temperature (°C) l-Glu dl-Glu l-GluNa dl-GluNa l-GluHCl dl-GluHCl

G 3.41 8.55 514 158 298 471

25 8.64 2G.5 627 243 479 698

5G 21.9 49.3 765 372 769 1,G3G

75 55.3 119 933 57G 1,24G 1,54G

1GG 14G 285 1,14G 875 1,99G 2,28G

Figure 4. The change in the solubility of l-glutamic acid with change in the concentration of both hydrochloric acid and sodium hydroxide.

Figure 4. The change in the solubility of l-glutamic acid with change in the concentration of both hydrochloric acid and sodium hydroxide.

Crystal: L-Glu is a polymorph, having two crystal forms, a and ft. Both are anhydrous.

Crystals of the a-form are obtained by crystallization from the saturated solution of L-Glu at 60 to 70 °C by cooling rapidly with agitation or from the acidic or alkalic solution of L-Glu to which alkali or acid was added to rapidly neutralize it until the isoelectric point of L-Glu was reached (pH 3.2).

The crystal shape is a column or pyramid. The a-form crystals transform gradually to b-form when kept in solution for a long time at room temperature. b-form crystals are obtained from either the saturated solution of L-Glu at 80 to 90 °C, which is cooled gradually with agitation, or the relatively higher concentration of L-Glu hydrochloride or sodium salt, to which alkali or acid was added to neutralize it slowly until pH 3.2 was reached. The crystal shape is a needle or thin plate (24).

equilibrium solution, of the hydrochloride, free, and sodium salts of L-Glu, respectively (i.e., in the range of 0 to 30% for HCl and of 0 to 20% for NaOH). In the range of 0 to 7.7% of HCl, the solubility increases linearly so that L-Glu is soluble until the equivalent amount of HCl in solution in which L-Glu exists as solid crystals. Above the concentration of 7.7% HCl, with an increase in the HCl concentration, the solubility decreases steeply. It flattens in the vicinity 20% and then becomes a constant of about 1% above 25% HCl. Above 7.7% HCl, the hydrochloride is a solid crystal in solution. At the invariant point where equilibrium is attained between L-Glu and L-Glu HCl, the maximum solubility of 31.1% is obtained.

On the other hand, in the range of 0 to 9.33% of NaOH, L-Glu solvates an equal amount of NaOH in solution and exists as a solid crystal. At the invariant point (NaOH 9.33%), the solubility of L-Glu reaches the maximum of 35.38%. Above the concentration of 9.33% NaOH, L-GluNa H2O exists as solid crystals. A slight decrease in solubility is observed in the range from 9.33 to about 13%, and then increases with increasing concentration of NaOH. The minimum value of the solubility of L-Glu is obtained from the range of pH 2 to 4, in the vicinity of the pI.

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