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L-Glu HCl crystals

L-Monosodium L-glutamate

Figure 7. Flow diagram of production of MSG from the fermentation broth.

Crystallization of l-Glutamic Acid

In the fermentation broth, L-glutamic acid exists as an ammonium salt in the vicinity of pH 7. For obtaining L-MSG directly from the fermentation broth, it is necessary to exchange ammonium ion for sodium ions. This process is rather complicated, taking into account an environmental approach. To avoid this complexity, as a first step, crystallization of the a-form of L-glutamic acid crystals, which show lower solubility as compared to L-MSG and fairly good sedimentation as compared to /¡-form crystals, is employed in actual industrial production. The presence of salts can affect solubility of L-glutamic acid in solution. For example, the chloride-related inorganic substances NaCl, CaCl2, NH4Cl, and KCl increase the solubility of L-glutamic acid.

When crystallized under ordinary conditions, pyramidtype a-form crystals are obtained by the effect of the impurities contained in the fermentation broth.

Pyramid Type

In the growing of crystals, only those faces having the lowest translation velocities survive. The face of [001] of the a-form are fast-growing (high translation velocities) and this face tends to disappear, as overlapped by slower [111] faces, and finally results in the pyramid type.

If Ostwald's step theory is obeyed, as a-form crystals are unstable from the thermodynamic point ofview, a-form crystals first appear and then gradually transform to / -form. But this theory is not based on the thermodynamics, but on the kinetics, so exceptions frequently occur.

The conditions to obtain a-form crystals are summarized as follows (36):

1. When neutralized, 35% hydrochloric acid is added as quickly as possible to the fermentation broth and then cooled to below 10 °C instantly with vigorous agitation.

2. A small amount of a third substance is added to affect the stability of a-form, such as a peptide, active surfactant, protein hydrolyzate, or amino acid, especially L-phenylalanine and L-cysteine.

Centrifugation

The a-form of L-glutamic acid crystals are separated by a centrifuge. Residual mother liquor after centrifugation amounts to 5 to 15% of the weight of the crystals, depending on the viscosity of the mother liquor and the degree of uniformity of crystal size. It is more effective to wash the crystals on the centrifuge with fresh water to reduce impurities than recrystallization. In this process, the bacterial cells remain effectively in the mother liquor, and without another bacterial cell separation process, L-glutamic acid crystals remain effectively separated from the bacterial cells.

Transformation of «-Crystals to /-Crystals

When the a-form crystals are poured into the water and the temperature is elevated to 60 to 80 °C, the transformation of a-form to /-form occurs within about 1 h. During this solvent-mediated transformation, a large amount of adhering impurities are rejected out of the crystals, and higher purified / -form miceous plate-shape crystals are obtained (37). This transformation process has so marked an efficiency for removing impurities, especially color-related substances, that when higher purity is required, a small amount of activated carbon is used to obtain a transparent MSG solution.

For another purification of L-glutamic acid solution, the application of UF membrane or NF membrane is investigated: When these membranes are applied to permeate the fermentation broth, the permeate solution obtained is neutralized followed by cooling. Purified /-form crystals are obtained directly, without a-form crystallization.

Crystallization of l-MSG

Neutralized L-glutamic acid solution (L-MSG solution) is applied to the activated carbon column to obtain decolonized solution, which is fed to the continuous evaporator to concentrate until the saturation of L-MSG (50% at 60 °C). To meet the requirements for l-MSG monohydrate crystals—crystal size, crystal shape (aspect ratio), and size distribution—it is necessary to strictly control the operational conditions of the crystallization process, in which nucleation and crystal growth occur simultaneously. The result of the combined processes on the mentioned requirement for crystals depends on the relative rates of nuclea-tion and growth. To obtain a more uniform product, nucle-ation is controlled by adding the desired number of crystals, usually crushed, to the crystallizer when the solution is either saturated or supersaturated. In a continuous crystallizer, the number of nuclei formed per unit time will be continuous and will equal the number of crystals withdrawn from the crystallizer, so that all nuclei can receive the same time of growth. Otherwise, there must be a classifying action in the crystallizer that will retain the small crystals under treatment until they have grown to the proper size, when they can be removed from the crys-tallizer.

For practical control, it is necessary to reduce as much as possible the local variation in the temperature or concentration of the solution that causes excess nucleation. Usually, the objective of the crystallizer operator is to achieve the maximum growth rate, consistent with low nu-cleation rate. More importantly, impurities in solution may inhibit the formation of new nuclei or retard crystal growth. The degree of the effect of a given impurity cannot be predicted; in general, higher molecular weight materials seem to be more effective inhibitors.

Crystal Shape (Aspect Ratio)

When no impurity is contained in the solution the aspect ratio (referred to as l/d in crystal) is 7 to 8 which is not convenient for umami seasoning for cooking. To overcome this problem, the appropriate amount of amino acids is added. The crystal shape is determined by the change in the difference in the relative crystal growth of constituted crystal faces. For l-MSG crystals, a small amount of amino acids changes the crystal shape, the amino acids such as L-alanine, L-lysine, and L-arginine are easily adsorbed on the [110] face of the l-MSG crystal and inhibit the growth of this face, those having the lowest translation velocities survive, and the aspect ratio is 4 to 5, which satisfies the demand of users (38).

Recovery of l-Glutamic Acid from Mother Liquor

In mother liquor-separated L-glutamic acid crystals other than L-glutamic acid, many impurities derived from the raw material used as the carbon source, as well as ammonium hydrochloride and bacterial cells, are found. For recovery of L-glutamic acid from this mother liquor, the following process is proposed: The mother liquor is condensed and ammonium chloride crystallized by cooling for a day and night followed by filtration to obtain the ammonium chloride crystals that are used as fertilizer. The separated mother liquor is hydrolyzed with excess hydrochloric acid to convert the L-glutamic acid to hydrochloride and the bacterial cells and other debris to insoluble matters (so-called humic acid). The humic acid is separated by filtration. The filtrate is condensed, then cooled to crystallize the hydrochloride crystals at lower than 10 °C for more than several days. This is recycled in the process of L-glutamic acid crystallization from the fermentation broth to curtail the consumption of hydrochloric acid. The residual mother liquor combined with humic acid is used as raw material for the organic fertilizer.

USES Seasoning

MSG is widely used as a long-established seasoning or flavor enhancer to improve the palatability of foods. The effect of MSG is due to its characteristic taste, umami. The development of food science, including electrophysiology, psychophysics, and nutrition, have provided evidence that umami should be regarded as a basic taste, independent from the traditional four basic tastes—sweet, salty, sour, and bitter. Glutamate is ubiquitous in foods such as tomato, cheese, human milk, and seaweed. Other umami substances, that is, the 5'-ribonucleotides disodium 5'-inosinic acid and disodium 5'-guanylic acid, are found in meats, fish, vegetables, and mushrooms.

When glutamate and nucleotides coexist, a very strong synergistic effect occurs, and even a small amount of glutamate added to a food containing nucleotide causes umami to be dramatically enhanced—seven to eight times as much as the original umami of the food (39).

The taste threshold of MSG is about 0.03% in aqueous solution. The intensity of umami increases linearly with a logarithmic increase in the concentration of MSG. The synergistic effect of MSG with 5'-ribonucleotides is expressed by the following relation (41):

y = u + A u v where y is the intensity equivalent to the concentration of MSG alone, and u and v are the concentrations of MSG and 5'-ribonucleotides, respectively. A is a constant that is 1,200 for disodium 5'-inosinate (IMP) and 2,800 for disodium 5'-guanylate (GMP). In some commercial umami seasonings, 5'-ribonucleotides are mixed into take advantage of the synergistic effect with MSG.

Although MSG increases the palatability of food, it is not always palatable by itself. When MSG is tested in crystal form or in simple solutions, it does not cause a pleasant sensation. It gives a pleasant taste only in flavored solution or in actual foods. Another important property of MSG is the concentration dependence of its pleasantness rating. An excess of MSG causes an unpleasant sensation, and thus the amount of MSG that is added to foods should be limited (41).

The desirable usage level of MSG depends on the properties of the food, including the amount of original umami substances. Results of extensive taste studies indicate that the optimum MSG level in food ranges from 0.1 to 0.8 wt %, varying from food to food. In most cases, MSG is used in combination with salt. A relationship between the levels of MSG and salt, and palatability of foods, has been clarified. The lower the salt concentration, the higher the MSG concentration required to maintain the palatability, and vice versa. When umami substances are added to meals, the amount of salt can be reduced without a decrease in the palatability or degree of satisfaction.

Umami taste is considered to be a universal taste sensation for humankind. An ethnic comparison of taste sensitivity has shown that there is no difference in the thresholds of MSG and IMP between Japanese and European-American Caucasians (42).

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