Coffee sublimation

C8H10N4O2. An alkaloid occurring in tea, coffee and guarana, from which it may be prepared by extraction, It is also manufactured by the methylation of theobromine and by the condensation of cyanoacetic acid with urea. Crystallizes with H2O or anhydrous from organic solvents. M.p. (anhydrous) 235"C, sublimes at 176 C. Odourless, and with a very bitter taste. Caffeine acts as a stimulant and diuretic, and is a constituent of cola drinks, tea and coffee. 

Typical lengths of cycles for food stuffs are 5-10 hr, for bacterial pellets 2-20 hr, and for biological fluids 20-50 hr. A production unit with capacity of 500 L may have 75 kW for refrigeration and 50 kW for heating. Conditions for the preparation of freeze dried coffee are preparation of an extract with 20-25% solids, freezing at —25—43°C, sublimation at approx. 200Torr to a final final moisture content of 1-3%, total batch processing time of 6-8 hrs. 

Caffeine forms white hexagonal crystals by sublimation. Caffeine has a melting point of 238 Celsius, but the crystals begin to sublime when heated to 178 Celsius. Caffeine is only moderately soluble in water, but more soluble in hot water. The crystals are also moderately soluble in alcohol, acetone, but are much more soluble in methylene chloride, chloroform, and practically insoluble in ether. Caffeine is capable of forming a hydrate, which looses it water of hydration when heated to 80 Celsius. Caffeine is a widely used stimulant, ingested by millions in the form of coffee, tea, ect.,... 

Cuffeine is extracted from tea dust and during coffee roasting, caffeine sublimes. Caffeine has above all a centrally stimulant (analeptic) effect on the cerebrum, produces tachycardia and has a diuretic effect. 

The fruit is, when ripe, a red drupe containing white seeds, the seed coat of which has a thin parchment-hke membrane, which is called silver membrane. The seed consists mainly of endosperm. The coffee-bean consists of the peeled seed, which is roasted at a temperature of 200-250°C, when the caffeine is partly sublimated, and the aromatic substances are formed. 

Figure 7-18, p. 82, is a diagram of a simple entrainer sublimator. If the tube that collects the sublimate is cooled to different temperatures at successive regions along the length, then it is possible to fractionally sublime a mixture of compounds. The separation in this case is usually not clean, but it can be used in some situations. Commercially, coffee is decaffeinated by entrainer sublimation. 

The separation of caffeine from coffee by entrainer sublimation... 

Your instructor has been given a new and simple entrainer sublimator that it is supposed to work quite well. You are to test it for him by separating caffeine from coffee. First you are to determine if anything can be sublimed from coffee. If there is any sublimate, you will try to identify it. 

About 2 to 3 g of coffee are placed in the sublimator, the pressure is reduced to 50 to 60 torr, and the temperature of a slow flow of entrainer air is raised to 170 to 180 "C. The entrainer gas is heated because, if cold gas passes over a hot sample, the caffeine will condense as a fog. Many of these particles then will be pumped through the system and not collected, thus causing low results. The caffeine will separate as white crystals in the cooled downwind side. These crystals are dissolved in chloroform and verified as being caffeine by infrared spectroscopy. 

The inside of the collection tube usually will be white from end to end. With some coffee a small amount of a yellow compound also will sublime and this will be close to the sample. 

Caffeine, mp 263°C, sublimes on heating and is more soluble than dimethylxanthines. Caffeine has a stimulating effect on the central nervous system. It is extracted from green coffee beans with liquid CO2 [151]. Caffeine is synthesized by methylation of xanthine, theophylline or theobromine with methyl iodide or dimethyl sulphate [149]. 

Depending on temperature and pressure, all materials in the solid and liquid state have a vapour pressure, that is, some of their surface atoms or molecules exist as a vapour or gas above the material. Thus under certain conditions a liquid evaporates and some solids sublimate. This is most familiar to us as we smell coffee or perfumes the fragrant molecules leave the liquid and enter the surrounding atmosphere. Mothballs are an example of the tendency of solids, like naphthalene, to enter the gaseous state. Volatility is the meas-... 

Freeze drying is a special case of drying under vacuum at a temperature below the triple point of water here water (ice) sublimes directly into water vapor. Although the heat required for sublimation is severalfold lower than for evaporation, vacuum operation is expensive. Freeze drying of coffee, for example, costs two to three times more than spray drying. On the other hand, the product quality and flavor retention are better. 

Alkaloid contained in coffee, tea and cocoa. Cryst. + IHgO from BLO. M.p. anhjd. 235°. Sol to 2% in HgO, z% in EtOH, 0-3% in EtjO, 12 5% in UHClj. Sublimes at 178°. Deccnnp. by hot alkalis —> caffeidine. Weak base, forming unstable salts BMAuClf m.p.,248 ... 

Nakamura et al. studied the effect of freezing conditions upon the sublimation rate of coffee extract [27]. The study was hindered by the... 

Tea yields better-quality caffeine than that obtained from coffee. Caffeine from tea is relatively colorless, whereas the caffeine extracted from coffee is highly colored. About 25 mg of caffeine is isolated in either case. Sublimation removes much of the color from the tea and coffee samples. 

The caffeine obtained from tea or coffee can be purified by sublimation using the procedure described in Experiment 13A. At your instructor s option, you may combine your sample with another student s sample for sublimation. After sublimation, determine the weight of caffeine recovered and calculate the weight percentage recovery of the caffeine. Compare this value to the amount of crude sample obtained. At the instructor s option, determine the melting point of the purified caffeine. The melting point of pure caffeine is 236°C however, the observed melting point will be lower. Submit the sample to the instructor in a labeled vial unless it is to be used for infrared spectroscopy (recommended) or mass spectroscopy (also recommended). 

Attach your infrared spectra to your report and label the major peaks. If you determined the mass spectrum, try to identify the important fragment ion peaks (Technique 28). Include the melting point, if it was required. Report the weight percentage of the caffeine recovered from the tea or coffee sample before and after sublimation. 

Sublimation can be used to purify solids that readily vaporize. Figure 11.3 shows a simple way to purify iodine by sublimation. Impure iodine is heated in a beaker so that it vaporizes, leaving nonvolatile impurities behind. The vapor crystallizes on the bottom surface of a dish containing ice that rests on top of the beaker. Freeze-drying of foods is a commercial application of sublimation. Brewed coffee, for example, is frozen and placed in a vacuum to remove water vapor. The ice continues to sublime until it is all gone, leaving freeze-dried coffee. Most freeze-dried foods are easily reconstituted by adding water. 

Suppose a solid is warmed at a pressure below the pressure at the triple point. In a phase diagram, this corresponds to moving along a horizontal line below the triple point. You can see from Figure 11.11 that such a line will intersect curve AD, the vapor-pressure curve for the solid. Thus, the solid will pass directly into the gas that is, the solid will sublime. Freeze-drying of a food (or brewed coffee) is accomplished by placing the frozen food in a vacuum (below 0.00603 atm) so that the ice in it sublimes. Because the food can be dried at a lower temperature than if heat-dried, it retains more flavor and can often be reconstituted by simply adding water. 

Fig. 5.5 (a) Design space forthe primary drying stage of a coffee extract freeze-drying process calculated for different values of dried cake thickness (b) Values ofthe sublimation flux as a function ofthe operating conditions when Lj/... 

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