Coffee supercritical carbon dioxide

FIG. 22-22 Schematic diagram of the Kraft process for producing decaffeinated coffee using supercritical carbon dioxide (McHugh and Ktukonis, op. cit.). 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

The dense fluid that exists above the critical temperature and pressure of a substance is called a supercritical fluid. It may be so dense that, although it is formally a gas, it is as dense as a liquid phase and can act as a solvent for liquids and solids. Supercritical carbon dioxide, for instance, can dissolve organic compounds. It is used to remove caffeine from coffee beans, to separate drugs from biological fluids for later analysis, and to extract perfumes from flowers and phytochemicals from herbs. The use of supercritical carbon dioxide avoids contamination with potentially harmful solvents and allows rapid extraction on account of the high mobility of the molecules through the fluid. Supercritical hydrocarbons are used to dissolve coal and separate it from ash, and they have been proposed for extracting oil from oil-rich tar sands. 

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. 

Methylene chloride is probably the most generally used solvent for decaffeination processes, but others, some of which are already found in small amounts in coffee beans, are coming into use. For example, ethyl acetate,8 formaldehyde-dimethylacetal, ethanol, methanol, acetone,9 propane,10 benzyl alcohol,11 carbon dioxide,12 and supercritical carbon dioxide with an acid13 are used. Generally the pressure and temperature of the system are adjusted to keep the solvent in the liquid state. Coffee oil itself is even described for this use in one patent.14... 

Solvent-assisted decaffeination of coffee can result in residues of solvent reaching the consumer.208 The use of chlorinated hydrocarbon solvents such as chloroform,209 methylene chloride, trichloroethylene,208 and difluoromonochloromethane (Freon),210 will probably be replaced by compounds already found in roasted coffee. The use of an ethyl acetate and 2-butanone mixture leaves a 26-ppm residue in green coffee, but zero residue in roasted coffee.211 Other solvent compounds used or suggested for coffee improvement or decaffeination include propane, butane,212 carbon dioxide,213 214 acetone215 dimethyl succinate,2161,1-dimethoxymethane, and 1,1-dimethoxyethane.217 Of all these, supercritical carbon dioxide, ethyl acetate, and methylene chloride are the solvents most used currently in decaffeination processes. 

Another challenge is to develop methods to replace the volatile organic solvents that are used in many industrial procedures. One choice is water as a solvent it is easily repurified, and has a harmless vapor. Another choice is supercritical carbon dioxide, a good solvent for many organic substances. It is not as innocuous as is water, but carbon dioxide can be easily recovered and reused. It is currently used to remove caffeine from coffee, and is being developed as a dry-cleaning solvent to replace organic solvents (Chapter 9). 

Figure 5.8 Coffee is decaffeinated by constantly irrigating the ground beans with supercritical carbon dioxide schematic representation of a Soxhlet apparatus for removing caffeine from coffee...

Supercritical fluids (SCFs) are best known through their use for the decaffeination of coffee, which employs supercritical carbon dioxide (scCC ). In this chapter, we will demonstrate that SCFs also have many properties that make them interesting and useful reaction media. Firstly, the physical properties of SCFs will be explained, then the specialist equipment needed for carrying out reactions under high temperatures and pressures will be described. Finally, we will discuss issues relevant to the use of SCFs as solvents for reactions. 

You won t find this happening in your fireplace, though. It doesn t have enough heat or high enough pressure. The cores of stars have supercritical fluids, and the planet Jupiter has some gaseous layers that are supercritical and denser than water. Most decaffeinated coffee has its caffeine removed using supercritical carbon dioxide. 

Coffee andteaaTe decaffeinated using cacbon dioxide in a fourth phase of matter known as a supercritical fluid. This phase behaves like a gaseous liquid, which is attained by adding lots of pressure and heat. Supercritical carbon dioxide is relatively easy to produce. To get water to form a supercritical fluid, however, requires pressures in excess of 217 atmospheres and a temperature of 374°C. Supercritical water is very corrosive. Also, so much oxygen can dissolve in supercritical water that flames can burn within this medium, which is ideal for the destruction of toxic wastes. 

In contrast to the decaffeination of coffee, which is primarily executed with green coffee, black tea has to be extracted from the fermented aromatic material. Vitzthum and Hubert have described a procedure for the production of caffeine-free tea in the German patent application, 2127642 [11]. The decaffeination runs in multi-stages. First, the tea will be clarified of aroma by dried supercritical carbon dioxide at 250 bar and 50°C. After decaffeination with wet CO2 the moist leaf-material will be dried in vacuum at 50°C and finally re-aromatized with the aroma extract, removed in the first step. Therefore, the aroma-loaded supercritical CO2 of 300 bar and 40°C will be expanded into the extractor filled with decaffeinated tea. The procedure also suits the production of caffeine-free instant tea, in which the freeze-dried watery extract of decaffeinated tea will be impregnated with the aromas extracted before. 

Supercritical carbon dioxide is considered a "green" solvent because it is made of nothing more than carbon dioxide, a substance naturally present on Earth. Currently, supercritical carbon dioxide is used to decaffeinate coffee beans, extract new scents for perfume, and clean clothes. 

The liquid-gas equilibrium line terminates at a point known as the critical point. The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure. For example, nitrous oxide has a critical temperature of 36°C and a critical pressure of 72.45 bar (1051 psi). When the temperature and pressure exceed these critical values, the system becomes a supercritical fluid. Supercritical fluids have the flow properties of gases but densities similar to liquids, and supercritical fluids have no surface tension. Therefore, supercritical fluids are terrific solvents. For example, supercritical carbon dioxide is an excellent solvent for extracting caffeine from coffee without resorting to more toxic organic solvents like dichloromethane. 

Removal of stimulants like caffeine from (green) coffee beans and black tealeafs is practised since the beginning of the century because of potential health hazards to consumers Since about 15 years supercritical Carbon dioxide is used as solvent. 

Plants and plant extracts have been used as medicine, culinary spice, dye and general cosmetic since ancient times. Plant extracts are seen as a way of meeting the demanding requirements of the modem industry. In the past two decades, much attention has been directed to the use of near critical and supercritical carbon dioxide solvent, particularly in the food pharmaceutical and perfume industries. CO2 is an ideal solvent because it is non-toxic, non-explosive, readily available and easily removed from the extracted products. At present the major industrial-scale applications of supercritical fluid extraction (SFE) are hop extraction, decaffeination of coffee and tea, and isolation of flavours, fragrances and other components from spices, herbs and medicinal plants [1-4]. 

By the late 1980s, technologies for the removal of cholesterol with supercritical carbon dioxide were offered by a number of companies (38). Commercialization was never attempted by any major food company for removal of cholesterol. Successful scale-up and commercialization was achieved by the General Foods Corporation for removal of caffeine from coffee (45). The primary disadvantages for the dairy industry were the low yields, low cholesterol removal, and the very high capital and operating costs of the equipment. 

Since the first large-scale supercritical extraction process was commercialized for the decaffeination of green coffee with carbon dioxide a decade ago, scientists and engineers in the food industry have been paying considerable attention to this technique for similar separations, i.e., removal of cholesterol from butter, removal of cocoa butter from cocoa beans, and extraction of hops, spices, and... 

Water has been shown to be an effective solvent in some chemical reactions such as free radical bromination. Supercritical fluids such as liquified carbon dioxide are already commonly used in coffee decaffeination and hops extraction. However, supercritical carbon dioxide can also be used as a replacement for organic solvents in polymerization reactions and surfactant production. Future work may involve solventless or neat reactions such as molten-state reactions, dry grind reactions, plasma-supported reactions, or solid materials-based reactions that use clay or zeolites as carriers. 

The attractiveness of supercritical carbon dioxide extraction is shown by the already existing industrial applications of hop extraction, decaffeination of tea and coffee, defatting of cocoa powder, and extraction of herbs and spices and is also demonstrated by the large number of patent applications and scientific publications in recent years. 

Certain important industrial processes are based on the high solubility of organic species in supercritical carbon dioxide. For example, this medium has been employed to extract caffeine from coffee beans to produce decaffeinated coffee and to extract nicotine from cigarette tobacco. 

Important to many current processes is the ability of gasses at pressure above the critical to have increased solubility capability. A common example is the decaffei-nation of coffee beans by supercritical carbon dioxide. In this case, it is assumed that species 1 is the solute dissolved in species 2, which is therefore the solvent. It is assumed that the solvent is insoluble in the solute. Thus, the equilibrium can be written as follows. 

Supercritical fluid extraction offers several advantages over conventional extraction processes. The extraction is carried out at high pressures and then the extract is usually recovered by lowering the pressure, as the solubility is a strong function of fluid pressure. The compositions of the extracts are different from those from the liquid extraction. Supercritical fluid extraction has been well accepted for coffee decaffeination and is being applied in other food, cosmetics, and pharmaceutical applications. Supercritical carbon dioxide is an environmentally benign nonflammable fluid. 

Supercritical carbon dioxide has been industrially used in a variety of processes, including coffee decaffeination, tea decaffeination, and extraction of fatty acids from spent barley, pyrethrum, hops, spices, flavors, fragrances, com oil, and color from red peppers. Other applications include polymerization, polymer fractionation, particle formation for pharmaceutical and military use, textile dyeing, and cleaning of machine and electronic parts. 

Among the practical applications of supercritical extraction is the use of supercritical carbon dioxide as the solvent in a number of processes. Carbon dioxide has several favorable properties as a supercritical solvent. It is nontoxic, low-cost, and noncorrosive. Its critical temperature is 304.2 K, which is near ambient. These properties are especially desirable in food processing, for the extraction of food components that must not be exposed to high temperatures. Examples are the removal of caffeine from coffee and the extraction of oil from beans and corn. 

The term supercritical fluid is used to describe any substance above its critical temperature and pressure. The discovery of the supercritical phase is attributed to Baron Cagniard de la Tour in 1822 [3], He observed that the boundary between a gas and a liquid disappeared for certain substances when the temperature was increased in a sealed glass container. While some further work was carried out on supercritical fluids, the subject remained essentially dormant until 1964 when a patent was filed for using supercritical carbon dioxide to decaffeinate coffee. Subsequent major developments by food manufacturers have led to the commercialization of this approach in coffee production. The use of supercritical fluids in the laboratory was initially focused on their use in chromatography, particularly capillary supercritical fluid chromatography (SFC). However, it was not until the mid-1980s that the use of SFE for extraction was commercialized. 

In the food industry, supercritical carbon dioxide is used for the decaffeination of coffee, deodorization of plant oils, and extraction of hops and spices. In the tobacco industry, supercritical carbon dioxide is utilized in denicotinizing tobacco. Studies have also been made for the refining of coal by supercritical extraction. 

In the process depicted in figure 1.2, coffee beans charged to the extractor, 10, via line (18) are decaffeinated with supercritical carbon dioxide. The caffeine-laden CO2 stream leaving the extractor is passed into the bottom of a water-wash column where the caffeine is removed from the CO2. (Chapter 8 describes the thermodynamic equilibrium situation for the water-wash column.) The caffeine-free CO2 is recycled to the coffee bean column, and decaffeinated coffee beans leave the extractor via line 20. Let s now follow the water stream leaving the bottom of the CO2 scrubbing tower. The caffeine-... 

Each patent has somewhat different features and claims. We select one patent for more detailed discussion to highlight certain technical facets of the process. First we explain the (often misunderstood) effect of water on the extractability of caffeine by selective supercritical carbon dioxide. A number of references report that dry carbon dioxide cannot extract caffeine from dry coffee, either green or roasted, but moist carbon dioxide can. The inability of dry carbon dioxide to extract caffeine from coffee should not be misconstrued to mean that dry carbon dioxide cannot dissolve neat caffeine. This same moist-versus-dry effect is experienced if, for example, methylene chloride is used to extract caffeine from coffee. Dry methylene chloride cannot decaffein-ate dry coffee but moistened coffee can be decaffeinated. It is thought that the caffeine is chemically bound in a chlorogenic acid structure present in the coffee bean. Thus, water somehow acts as a chemical agent it frees caffeine from its bound form in the coffee matrix in both the carbon dioxide and the methylene chloride processes. 

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