Citric acid, Table

The procedure towards an environmentally benign process starts with the selection of raw materials in addition to conventional raw materials from the petrochemical industry based on low- to medium-boiling aliphatic and aromatic hydrocarbons, replenishable raw materials from nature are increasingly available nowadays. In the simplest cases, these can be carbon sources, such as glucose and sucrose for fermentation, but also more complex molecules, frequently obtained from the chiral pool or through inexpensive fermentation from carbon sources, such as glutamic acid or citric acid. Table 20.2 lists a selection of raw materials from the chiral pool, with their estimated costs per kilogram. 

Thermodynamic Properties of Aqueous Solutions of Citric Acid Table 2.13 (continued)... 

Gibbs free energy, enthalpy and entropy in kJ mor as a function of temperature in the first step of dissociation of citric acid (Table 3.1)... 

Table 2 Hsts examples of compounds with taste and their associated sensory quaUties. Sour taste is primarily produced by the presence of hydrogen ion slightly modified by the types of anions present in the solution, eg, acetic acid is more sour than citric acid at the same pH or molar concentration (43). Saltiness is due to the salts of alkaU metals, the most common of which is sodium chloride. However, salts such as cesium chloride and potassium iodide are bitter potassium bromide has a mixed taste, ie, salty and bitter (44). Thus saltiness, like sourness, is modified by the presence of different anions but is a direct result of a small number of cations.

Although the values caimot be considered absolute, approximate magnitude of taste sensitivity has been measured (Table 1). Certain taste interrelationships should be considered in the evaluation of taste magnitude. The apparent sourness of citric acid is depressed by both sucrose and sodium... 

Mahc acid is a relatively strong acid. Its dissociation constants are given in Table 1. The pH of a 0.001% aqueous solution is 3.80, that of 0.1% solution is 2.80, and that of a 1.0% solution is 2.34. Many of its physical properties are similar to those of citric acid (qv). Solubihty characteristics are shown in Figure 1 and Table 1, densities of aqueous solutions are hsted in Table 2, and pH values vs concentration are shown in Figure 2. 

Inorganic heavy metals are usually removed from aqueous waste streams by chemical precipitation in various forms (carbonates, hydroxides, sulfide) at different pH values. The solubiUty curves for various metal hydroxides, when they are present alone, are shown in Figure 7. The presence of other metals and complexing agents (ammonia, citric acid, EDTA, etc) strongly affects these solubiUty curves and requires careful evaluation to determine the residual concentration values after treatment (see Table 9) (38,39). 

Acidulants. Acidulants give the beverage a tart or sour flavor, adjust pH to faciUtate the function of ben2oate as a preservative, reduce microbiological susceptibiUty, and act as a catalyst for the hydrolytic inversion process in sucrose sweetened beverages. The primary carbonated beverage acidulants are phosphoric acid [7664-38-2] and citric acid [77-92-9]. Other acidulants include ascorbic, tartaric, malic, and adipic acid (Table 2). 

Citric acid, anhydrous, crystallizes from hot aqueous solutions as colorless translucent crystals or white crystalline powder. Its crystal form is monoclinic holohedra. Citric acid is dehquescent in moist air. Some physical properties are given in Table 1 (1 3). The solubiUty of citric acid in water and some organic solvents is given in Table 2. The pH and specific gravity of aqueous solutions of citric acid are shown in Table 3. 

Table 1. Physical Properties of Citric Acid, Anhydrous...

Table 3. pH and Specific Gravity of Aqueous Citric Acid Solutions... 

Aqueous solutions of citric acid make excellent buffer systems when partially neutralized because citric acid is a weak acid and has three carboxyl groups, hence three p-K s. At 20°C pifj = 3.14, pi 2 4.77, and = 6.39 (2). The buffer range for citrate solutions is pH 2.5 to 6.5. Buffer systems can be made using a solution of citric acid and sodium citrate or by neutralizing a solution of citric acid with a base such as sodium hydroxide. In Table 4 stock solutions of 0.1 Af (0.33 N) citric acid are combined with 0.1 Af (0.33 N) sodium citrate to make a typical buffer solution. 

The mono- and disodium citrate salts are made by limiting the amount of sodium available by using only one mole of base for each mole of citric acid for the monosodium citrate and two moles for the disodium citrate. The result is primarily the mono or disalt with small amounts of the other forms and citric acid being present. Other salts that have been offered commercially are shown in Table 5. 

Citric acid occurs widely in the plant and animal kingdoms (12). It is found most abundantiy in the fmits of the citms species, but is also present as the free acid or as a salt in the fmit, seeds, or juices of a wide variety of flowers and plants. The citrate ion occurs in all animal tissues and fluids (12). The total ckculating citric acid in the semm of humans is approximately 1 mg/kg body weight. Normal daily excretion in human urine is 0.2—1.0 g. This natural occurrence of citric acid is described in Table 7. 

Recommended materials of constmction for pipes, tanks, and pumps handling citric acid solutions are 316 stainless steel, fiber glass-reinforced-polyester, polyethylene, polypropylene, and poly(vinyl chloride). At elevated temperatures, 304 stainless steel is not recommended (Table 8). 

It was estimated that 1990 U.S. citric acid and citrate salt consumption was 152,000 t. Citric acid represents approximately 90% of this volume. This citric acid/citrate use and its historical distribution in various markets is described in Table 9. From Table 9 it can be seen that although citric acid usage in the United States has shown steady growth at an average aimual rate of 4.4% from 1986—1990, the end use patterns have been quite stable. 

Ammonium salts of citric acid are made by adding either aqueous or anhydrous ammonia to citric acid dissolved in water. They are usually used in the hquid form rather than isolated as a dry product. Citric acid salts are Hsted in Table 5. SolubiUty data is as follows (1). 

The properties of citric acid esters are described in Table 10. 

Complete the following table for citric acid, C6H807> the acid found in ... 

Humans can synthesize 12 of the 20 common amino acids from the amphiboHc intermediates of glycolysis and of the citric acid cycle (Table 28-1). While nutritionally nonessenrial, these 12 amino acids are not nonessential. AH 20 amino acids are biologically essential. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Identification of the twelve amino acids that humans can synthesize rested primarily on data derived from feeding diets in which purified amino acids replaced protein. This chapter considers only the biosynthesis of the twelve amino acids that are synthesized in human tissues, not the other eight that are synthesized by plants. 

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