Caffeine, solubility

FIGURE 2.3 Caffeine solubility in dioxane-water binary solvents. The curve is an interpolation between calculated values. (Reproduced from Adjei, A., J. Newburger, andA. Martin. D9P i.arm. Sci69 659-661. Copyright 1980, American Pharmacists Association. Reprinted with permission from Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc.)... 

Ebeling, H., and Franck, E.U., (1984), Spectroscopic determination of caffeine solubility in supercritical carbon dioxide, Ber. Bunsenges. Phys. Chem. 88, 862-865. 

Liquid CO2 as extractant. A second modification to the processes described in section 5.4.2 which has been proposed is to use liquid, rather than supercritical, CO2 as the extractant. By operating at pressures not substantially in excess of the vapour pressure, substantially lower pressures can be used than in the supercritical case, though the caffeine solubilities are also lower. As pointed out in chapter 10, this will result in a higher solvent throughput for a given amount of product. A process of this type is described in European... 

C7HgN402. Occurs to a small extent in tea, but is chiefly prepared synthetically. Like caffeine, it is a very weak base which forms water-soluble compounds with alkalis. It has a similar pharmacological mechanism to that of caffeine and is used, in combination with ethy-lenediamine. as a diuretic and a bron-chodilator. 

Chicory is harvested as fleshy roots which are dried, cut to uniform size, and roasted. Chicory contains no caffeine, and on roasting develops an aroma compatible with that of coffee. It gives a high yield, about 70%, of water-soluble soHds with boiling water and can also be extracted and dried in an instant form. Chicory extract has a darker color than does normal coffee brew (55). 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

In some cases, the solids themselves are subjected to extraction by a solvent. For example, in one process used to decaffeinate coffee, the coffee beans are mixed with activated charcoal and a high-pressure stream of supercritical carbon dioxide (carbon dioxide at high pressure and above its critical temperature) is passed over them at approximately 90°C. A supercritical solvent is a highly mobile fluid with a very low viscosity. The carbon dioxide removes the soluble caffeine preferentially without extracting the flavoring agents and evaporates without leaving a harmful residue. 

Chlorogenic acid forms a 1 1 complex with caffeine, which can be crystallized from aqueous alcohol and yields very little free caffeine on extraction with chloroform. Other compounds with which caffeine will complex in this way include isoeugenol, coumarin, indole-acetic acid, and anthocyanidin. The basis for this selection was the requirement for a substituted aromatic ring and a conjugated double bond in forming such a complex. This kind of complex does modify the physiological effects of caffeine.14 Complex formation will also increase the apparent aqueous solubility of caffeine in the presence of alkali benzoates, cinnamates, citrates, and salicylates.9... 

Similar methods with modifications such as the one by Schutz et al.8 have been in use for over 20 years. In 1968, Ferren and Shane9 published a paper on the differential spectrometric determination of caffeine in soluble coffee and drug combinations. It had the advantage of eliminating a preliminary separation that was required by the earlier method. While the method was successful for coffee, it was not as successful in the determination of caffeine in acetaminophen/phenacetin/caffeine tablets. They proposed that phenacetin was a limiting factor. The official AOAC methods for these methylxanthines in coffee and tea still involve similar methods.10... 

Ferren, W.P. and Shane, N.A., Differential spectrophotometric determination of caffeine in soluble coffee and drug combinations, JAOAC, 51,573,1968. 

A great deal of effort has been put into methods for removing only the caffeine from the extracting solvent, and somehow returning all of the other components to the coffee beans for reabsorption. The principle of the method most generally seen involves exposure of the extract-laden solvent to a caffeine-specific adsorbent. Once the solvent has been treated in this way, it is returned to remove more caffeine. Flowever, the solvent is already saturated with the other solvent-soluble components and does not extract them from the second and subsequent batches of steamed green coffee beans. Adsorbants used for this purpose include activated char-... 

One of the most significant differences between Arabica and Robusta coffees is in the caffeine content. Robusta coffees contain almost twice the caffeine found in Arabica coffees. There are some other differences recognized thus far Robustas contain almost no sucrose and only very small amounts of the kaurane and furokaurane-type diterpenes they contain higher proportions of phenols, complex carbohydrates (both soluble and hydrolyzable), volatile fatty acids on roasting, and sulfur compounds, all in comparison with Arabicas. References to these distinctions can be found in Chapter 6 of this book. 

Methods for the decaffeination of green coffee beans, mainly with solvents after a steaming, have already been described. Even with the selective adsorption techniques to remove only caffeine, it is unlikely that the full character of the starting beans can be realized in a final decaffeinated beverage the result is that Robusta coffees are generally used to prepare decaffeinated coffee. The cost is kept down and the treatment, anyway, reduces any harsh or bitter flavor that the Robusta coffee may have had. The resulting beverage will be relatively caffeine-free, but Robusta coffee will contribute more soluble carbohydrates, phenols, and volatile fatty acids, and much less of the diterpenes found in Arabica coffees. 

Cocoa butter is the fat obtained from subjecting chocolate liquor to pressure. Since the alkaloids are sparingly soluble in fat, only trace amounts of theobromine and caffeine in cocoa butter have been reported. The theobromine and caffeine content of four cocoa butter samples averaged 0.008% and 0.038%, respectively.33... 

Also, hydrates are more soluble in water-miscible solvents than are the corresponding anhydrous forms. For example, the solubility of caffeine hydrate is lower than that of anhydrous caffeine in water but higher in ethanol. The maximum concentration seen may be due to the solubility of the anhydrous crystalline phase or due to a temporary steady state in which the rate of dissolution of the metastable anhydrous form and the rate of crystallization of the stable hydrate are equal. The decreasing concentration represents crystallization of the stable hydrate from a solution supersaturated with respect to it. If the maximum concentration of the solute in the dissolution experiment corresponds to the solubility, then the initial increase in concentration follows the Noyes-Whitney equation [15]. Van t Hoff plots of log solubility versus the reciprocal of temperature give linear relationships (Fig. 16). 

Li S, Varadarajan GS and Stanley H. 1991. Solubilities of theobromine and caffeine in supercritical carbon dioxide correlation with density-based models. Fluid Phase Equilib 68 263-280. 

One of the classic examples in this series is the solubilization of p-amino-benzoic acid (PABA) by caffeine [51], for which the essential data are summarized in Fig. 9. The solubility of PABA in the absence of caffeine was reported to be 6.2 mg/ml, which could be increased to 7.7 mg/ml by the addition of at least 2 mg/ml of caffeine. The linear increase in PABA concentration as a function of caffeine concentration is consistent with the formation of a 1 1 stoichiometric complex. From the data obtained in the linear concentration region, a value of 48 L/mol was calculated for K. Further increases in the caffeine concentration up to 6 mg/ml had no effect on the PABA solubility. However, larger concentrations of caffeine led to a reduction in the dissolved... 

Fig. 9 Phase solubility diagram showing the changes in the apparent aqueous solubility of p-aminobenzoic acid (PABA) brought about by the addition of the complexing agent, caffeine, at 30°C. (The data are adapted from Ref. 51.)...

Water is a good choice of solvent in a standard kitchen percolator because it removes all the water-soluble components from the coffee - hence the flavour. Clearly, however, a different solvent is required if only the caffeine is to be removed. Such a solvent must be cheap, have a low boiling point to prevent charring of the coffee and, most importantly, should leave no toxic residues. The presence of any residue would be unsatisfactory to a customer, since it would almost certainly leave a taste and there are also health and safety implications when residues persist. 

Class 4 Steam Distillation. Isolating tars, oils, and other liquid compounds insoluble, or slightly soluble, in water at all temperatures. Usually natural products are steam distilled. They do not have to be liquids at room temperatures (e.g., caffeine, a solid, can be isolated from green tea.). 

Purified, caffeine (Figure 4.1) is a white crystalline powder with a bitter taste. While caffeine is not particularly soluble in water, it is extracted from plant material with hot water. The longer the extraction period, the greater the amount of caffeine extracted. In plants, caffeine s purpose may be to discourage consumption by predators with its bitter taste and mild nervous system effects, but with humans it clearly has the opposite effect of encouraging consumption of the plant. 

Alkaloid. A large, varied group of complex nitrogen-containing compounds, usually alkaline, that reacts with acids to form soluble salts, many of which have physiological effect on humans, e.g., nicotine and, caffeine, etc. 

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