PH26620A - Method for decaffeinating coffee with a supercritical fluid - Google Patents

Description

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METHOD FOR DECAFFEINATING COFFEE

2 WITH A SUPERCRITICAL FLUID 4 TECHNICAL FIELD

The present invention relates to a method of 6 extracting caffeine from green coffee beans with a supercritical fluid. More particularly, the invention 8 involves continuously feeding an essentially caffeine-free supercritical fluid to one end of an extraction vessel containing moist green coffee beans and continuously withdrawing a caffeine-laden supercritical 12 fluid from the opposite end. A portion of decaffeinated beans is periodically discharged while a fresh portion of 14 undecaffeinated beans is essentially simultaneously charged to the extraction vessel. Substantially all the 16 caffeine is then removed from the caffeine-laden supercritical fluid stream in a countercurrent water 18 absorber. The caffeine present in the water exiting the absorber is subjected to reverse osmosis to obtain a concentrated caffeine solution and a permeate stream containing dissolved non-caffeine solids and 22 substantially no caffeine. The permeate stream is recycled to the green coffee prior to extraction or the 24 water absorber to not only recover solids, but increase the rate of caffeine extraction from the green coffee. 26 The method of the present invention is more efficient and produces a better quality decaffeinated coffee than prior 2 art batch processes. 4 BACKGROUND ART ~ Various coffee decaffeination methods are well-known 6 in the art. The most common techniques involve first swelling the coffee beans with water and then extracting 8 the caffeine with an organic solvent or a caffeine-deficient solution of green coffee solubles which solution is then itself contacted with a solvent to remove the caffeine therefrom. In either case, at least 12 some of the solvent typically contacts the beans, leaving minute traces therein. The most useful solvents are 14 halogenated hydrocarbons, but it is becoming increasingly desirable to avoid such solvents so as to leave the 16 coffee free of any trace solvent.

One of the more promising, although costly, 18 alternative techniques is the use of a supercritical fluid, preferably supercritical carbon dioxide, to extract the caffeine from green coffee beans. Such a technique is disclosed in U.s§. Pat. No. 4,260,639 to } 22 Zosel wherein green coffee is contacted with water-moist supercritical carbon dioxide in order to extract the 24 caffeine, The caffeine may be absorbed from the @ caffeine-laden supercritical carbon dioxide by bubbling 26 the carbon dioxide through a water reservoir, said . reservoir being replaced by fresh water every 4 hours, as 28 disclosed in U.S. Pat. No. 3,806,619 to Zosel. However, such a recovery system is highly inefficient because the water reservoir fails to provide a continuous driving force for caffeine recovery and the periodic replacement 32 of the reservoir results in an undesirable discontinuity in the process. 1In still another technique,” diszlbsed in U.S.

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Patent No. 4,247,570 to Zosel, the green coffee is mixed 2 with a caffeine adsorbent prior to contact of the coffee and the supercritical fluid. Then, as the caffeine is 4 extracted by the supercritical fluid, it is adsorbed by the caffeine adsorbent, eliminating the need for a 6 separate caffeine removal step. The prior art methods are batch techniques which tend to be less efficient than 8 would be more nearly continuous methods. In addition, loss of non-caffeine solids in solid absorbents and in purging the system adversely effects the green coffee roasted flavor. 12 An advantage of the present invention is a more nearly continuous method of extracting caffeine from 14 green coffee beans with a supercritical fluid removal of the caffeine and recovery and recycle of non-caffeine 16 solids to the green coffee.

Another advantage is to produce a decaffeinated 18 coffee of improved quality by limiting the loss of non-caffeine solids during decaffeination and by decreasing substantially the residence time of green beans in the process. Cee a noe . 22

SUMMARY OF THE INVENTION . 24 It has now been found that the objects of the invention are met by a method which involves continuously 26 feeding an essentially caffeine-free supercritical fluid to one end of an extraction vessel and continuously 28 withdrawing a caffeine-laden supercritical fluid from the opposite end of the vessel. Periodically, a portion of decaffeinated coffee beans is discharged at the end of the vessel to which the caffeine-free supercritical fluid 32 is fed while a portion of undecaffeinated beans is charged to the opposite end. The caffeine-laden 34 supercritical fluid is then fed to a countercurrent liquid absorber wherein caffeine is transferred from the supercritical fluid to a polar fluid. The then 2 essentially caffeine-free supercritical fluid is recycled to the extraction vessel. The caffeine rich liquid is 4 then fed to a reverse osmosis unit to recover 98% of the caffeine which has been concentrated and an acidic 6 substantially caffeine free permeate to be recycled to either the absorber or the green coffee. 8

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration showing an extraction vessel, 12 Figure 2 is a schematic illustration showing a system for decaffeinating green coffee in an extraction vessel 14 and recovering caffeine from the caffeine solvent in a liquid absorber. 16 Figure 3 is a schematic illustration showing a system for recovering nearly pure concentrated caffeine from the 18 caffeine solvent and recycling the solvent to either the extractor or absorber with acidic non-caffeine coffee solids. 22 DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, caffeine is 24 extracted from the green coffee beans with a - supercritical fluid. A supercritical fluid is a fluid, 26 typically one which is gaseous at ambient conditions, - which is maintained at a temperature above its critical . 28 temperature and at a pressure above its critical pressure, Suitable supercritical fluids for use in the present invention include carbon dioxide, nitrogen, nitrous oxide, methane, ethylene, propane and propylene. 32 Carbon dioxide, having a critical temperature of 31°C and a critical pressure of 72.8 atmospheres, is particularly 34 preferred. Carbon dioxide is abundantly available, relatively inexpensive, non-explosive and thoroughly safe

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for use in food processing. The supercritical fluids may 2 be used either individually or in combinations, as mixed supercritical solvents. 4 In addition, a so-called enhancer may be added to the supercritical fluid to improve the solvent characteris- 6 tics of the supercritical fluid. The most useful enhancers are the low to medium boiling alcohols and 8 esters. Typical enhancers include methanol, ethanol, ethyl acetate and the like. The enhancers may be added to the essentially caffeinefree supercritical fluids at proportions of between about 0.1% and 20.0% by weight. 12 The enhancers contemplated for use herein are most typically not supercritical fluids at the disclosed 14 operating conditions but rather, the enhancers are simply dissolved in the supercritical fluid, improving its ! 16 solvent properties.

In one embodiment the chosen enhancer is combined 18 with the essentially caffeine-free supercritical fluid at the described proportions prior to feeding the super- critical fluid to the extraction vessel. Alternatively, the essentially caffeine-free supercritical fluid is fed 22 to the extraction vessel without the enhancer. The

Co enhancer is then introduced into the extraction vessel 24 and thereby combined with the supercritical fluid at a point at which the supercritical fluid has progressed 26 through between one-quarter and one-third of the length of the column. Operation in this manner provides for 28 some washing of the beans with enhancer-free supercritical fluid so as to remove any residue of the enhancer from the coffee beans.

The extraction vessels intended for use herein 32 include those which provide for efficient contact of the green coffee beans and the supercritical fluid, and which 34 are capable of withstanding the necessarily elevated pressures involved with the use of supercritical fluids.

The preferred extraction vessel is an elongated column, 2 having a length between four and ten times the diameter, so that the green coffee beans are maintained as a bed as 4 the supercritical fluid passes therethrough. The extraction vessel, particularly an elongated column, is 6 most typically situated vertically so as to take advantage of gravity in providing the movement of the 8 beans through the vessel.

Inasmuch as the supercritical fluid extraction method is countercurrent, the end of the vessel from which the decaffeinated coffee beans are discharged is also the end 12. to which the essentially caffeine-free supercritical fluid is fed, and the end of the vessel to which the 14 undecaffeinated green coffee is charged is also the end from which the caffeine-laden fluid is withdrawn. For i 16 the vertical elongated vessel, it is preferable to discharge the portion of decaffeinated coffee from the 18 bottom of the vessel so as to best use gravity in assisting the movement of the green coffee through the column. The progression of the green coffee bed through the vessel arises from the periodic discharging and 22 charging of the portions of green coffee. When the portion of decaffeinated green coffee is periodically 24 discharged, the weight of the coffee bed causes said bed to shift downward, with the void created at the top of 26 the column being filled by the portion of undecaffeinated coffee which is simultaneously charged to the vessel. 28 The net effect is the progression of the green coffee charged to the extraction vessel downward through the column whereupon the decaffeinated coffee is eventually discharged. Of course, it is not necessary to situate 32 the column vertically nor to discharge the decaffeinated green coffee from the bottom of the vessel, but such a 34 scheme is the most convenient, particularly with respect to charging and discharging of the green coffee beans. . Cee

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In view of the high pressures involved, the periodic 2 charging and discharging of the coffee is most easily accomplished through the use of intermediate pressure 4 vessels known as blow cases. Blow cases are merely smaller pressure vessels of about the same volume as the 6 portions of coffee that are periodically charged and discharged, and which are isolated on both ends by 8 valves, typically ball valves. A blow case is situated both immediately above and below the extraction vessel and each connects therewith through one of the valves.

Prior to the time for the periodic charging and 12 discharging, the upper blow case (for the embodiment of a vertical elongated vessel) is filled with the desired 14 volume of beans, which blow case is then isolated. The remaining void space in the blow case is then filled with i 16 the supercritical fluid So as to increase the pressure to that maintained in the extraction vessel. The lower blow 18 case is pressurized with the supercritical fluid. When it is time for the periodic charging and discharging, the valve connecting the lower, pressurized blow case with the extraction vessel is opened. Similarly, the valve ’ 22 connecting the upper blow case and the extraction vessel is opened, charging the undecaffeinated coffee beans to 24 the vessel. Both valves are then shut. The upper blow case is essentially empty but for a small ‘amount of 26 supercritical fluid. The lower blow case contains the decaffeinated coffee and some supercritical fluid. The 28 supercritical fluid in the lower blow case may be vented to a holding vessel or the upper blow case prior to emptying the beans therefrom so as to conserve the costly fluid. Alternatively, rotary locks of the sort known for 32 use on pressure vessels may be used to provide smoother, more easily automated operation. However, such rotary 34 locks tend to be more mechanically complex, costing more initially and generally requiring more maintenance.

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The discharging of the portion of decaffeinated green 2 coffee beans and charging of the portion of undecaf- feinated beans is carried out periodically, after a 4 period of time established as hereinbelow described. The portion of decaffeinated beans periodically discharged 6 most preferably ranges between 5% and 33% of the volume of the green coffee contained in the extraction vessel. 8 Similarly, the portion of undecaffeinated coffee beans periodically charged to the vessel is also measured as against the volume of the coffee bed. A height about equal to the portion of discharged decaffeinated beans is 12 simultaneously charged to the opposite end, usually the top, of the elongated vessel. For instance, if 15% of 14 the volume of the green coffee bed is discharged the equivalent 15% of the volume is then simultaneously 16 charged to the vessel as undecaffeinated green coffee beans. 18 Particular operating conditions are obviously related to the configuration of a given system, but the invention is most preferably operated so as to maximize product- ivity while providing sufficient decaffeination of the 22 green beans, from which it is typically desired to extract at least 97% of the caffeine initially present. 24 Two of the more important operating conditions are the weight ratio of supercritical fluid to coffee and the 26 frequency of the periodic discharging and charging of the coffee beans. There are competing aims in choosing the 28 optimal weight ratio. It is, of course, preferable to use the least possible amount of the supercritical fluid so as to minimize operating expense. However, use of an . insufficient amount of the fluid impairs productivity and 32 raises the caffeine concentration of the caffeine-laden supercritical fluid to its maximum obtainable level prior 34 to reaching the desired level of decaffeination, thereby eliminating the overall driving force for the extraction of caffeine from the green coffee beans.

It has been 2 found that the weight ratio of supercritical fluid to coffee is most preferably between 30 and 100 kg. 4 supercritical fluid/kg. coffee processed through the vessel. 6 . The frequency of the periodic charging and discharging is also a significant operating condition 8 related to decaffeination efficiency.

It is desirable to maximize productivity but it is also important to extract the desired amount of caffeine from the beans and so the frequency of the discharging and charging must be 12 balanced between the two objects.

The most preferable frequency will depend on a given system, but it has been 14 found that the portions of substantially decaffeinated coffee beans are conveniently discharged between about 16 every 10 and 120 minutes.

Considering that the charging of the portion of undecaffeinated green coffee beans is 18 most preferably concurrent with the discharging of the beans, the frequency of the charging of the portions of undecaffeinated beans is also between about every 10 and 120 minutes.

The total residence time of the green 22 coffee beans in the extraction vessel is established by the frequency of the periodic discharging and charging in 24 addition to the size of the portion periodically discharged and charged.

Thus, if 15% of the volume of an 26 elongated column is discharged (and the corresponding portion charged) every 54 minutes, the total residence 28 time of the beans in the vessel is § hours.

According to the limits hereinbefore set, the total residence time of : 30 the green coffee beans in the elongated vessel is between about 2 and 13 hours. 32 In addition, the temperature and pressure maintained in the extraction vessel are also significant operating 34 variables because both temperature and pressure must be above the critical constants so as to give the super critical fluid. Although there is no corresponding upper 2 limit on the temperature or pressure, the temperature should not be so high as to damage the quality of the 4 beans nor the pressure so high as to require excessively expensive equipment. The green beans are sensitive to 6 the effects of temperature with different types of beans having varying degrees of tolerance for increased 8 temperature. A temperature in excess of about 100°C may tend to degrade the flavor of some green bean types. The rate of decaffeination, though, is favored by a relative- ly high temperature and so it is not desirable to feed 12 the supercritical fluid to the vessel precisely at the critical temperature. It is preferable to maintain the 14 temperature in the extraction vessel between about 70°C and 140°C, preferably 80-140°C and more preferable to . 16 maintain the temperature between about 80°C and 100°C, preferably for arabica, or 100°C to 120°C for Robusta 18 depending on the green bean tolerance to temperature.

The pressure in the vessel must be maintained at at least the critical pressure in order to provide for the supercritical fluid. 1It has long been known that 22 increasing pressure increases the solvent capacity of the supercritical fluid. However, a point is reached, 24 typically at around 400 atmospheres, where the increased capacity does not justify the added expense of 26 maintaining such pressures.

It may be desirable to introduce moisture into the 28 system to facilitate decaffeination. The undecaffeinated green coffee beans may be moisturized prior to charging the beans through the extraction vessel, solubilizing the caffeine contained in the beans, thereby making the 32 solubilized caffeine more easily extractable. The undecaffeinated beans are typically moisturized to 34 between about 25% and 50% by weight moisture. 1In addition, the essentially caffeine-free supercritical , Co

~ - 11 - fluid may be saturated with water prior to being fed to 2 the extraction vessel. Such saturation of a super- critical fluid is typically between about 1% and 3% by 4 weight moisture. Decaffeination efficiency is thus increased by introducing moisture into the system. 6 It has been found according to the present invention that countercurrent operation of the supercritical fluid 8 caffeine extraction step achieves an improved decaffeina- tion efficiency and allows the production of a decaffeinated coffee of improved quality over prior art systems. The contact of a supercritical fluid with 12 caffeine-containing green coffee beans results in a partitioning of caffeine between the fluid and the beans 14 regardless of the system design. It is, of course, desirable to partition as much caffeine from the beans 16 into the fluid as is possible. However, said parti- tioning is limited by the relative solubility of the 18 caffeine in the supercritical fluid versus its solubility in the green coffee bean. A partition coefficient may be calculated based on experimental measurements at a given set of conditions, said partition coefficient being 22 defined as the concentration of caffeine in the supercritical fluid divided by the concentration of 24 caffeine in the green coffee beans, at an equilibruim point, The conditions which generally effect a partition 26 coefficient include temperature, pressure, and moisture level of the green beans. For example, the partition 28 coefficient for supercritical C0, as a caffeine solvent for green coffee beans has been calculated to be 0.026 at a temperature of about 85°C, a pressure of about 250 bar, and a green bean moisture level of about 35 to 40% by 32 weight.

It has been found that the continuous countercurrent 34 system of the present invention offers a tremendous advantage over prior art batch systems because the caffeine-laden supercritical fluid, just before it exits 2 the extraction vessel, is then in contact with fresh green coffee beans having the green coffee's naturally 4 occurring caffeine level. The naturally occurring caffeine level differs depending on the type of green 6 beans being decaffeinated. For example, Robusta coffees typically have a caffeine level of about 2.0% by weight 8 whereas Colombian coffees are typically about 1.1% by weight caffeine, as is. Because the exiting super- critical fluid is in contact with fresh green beans, the oo caffeine concentration in the exiting supercritical fluid oo 12 increases to its asymptotic limit, or nearly thereto, based on the caffeine partition coefficient for the given 14 fluid. It has been found that with counter-current operation the caffeine concentration in the supercritical 16 fluid exiting the extraction column is typically at least 40% of the maximum obtainable caffeine concentration and 18 preferably at least 50% of the maximum obtainable caffeine concentration, and preferably at least 70% of the maximum obtainable caffeine concentration when decaffeinating Robusta coffee, the maximum obtainable 22 caffeine concentration being defined by the partition coefficient and the naturally occurring caffeine level in 24 the green coffee being decaffeinated. Such a high caffeine concentration is very desirable -because it 26 reflects an efficient decaffeination system and it enables efficient recovery of the caffeine from the 28 supercritical fluid as a valuable by-product.

In a batch system, however, as caffeine is parti- tioned from the green coffee beans contained therein; the maximum caffeine concentration obtainable in the super- 32 critical fluid drops dramatically. Thus, a much larger amount of supercritical fluid is necessary in a batch 34 system as compared to the countercurrent extraction system of the present invention to achieve the same oo oo Cy Co Cy degree of decaffeination. For example, to achieve 97% 2 decaffeination of green coffee with supercritical carbon dioxide, approximately 5-8 times as much carbon dioxide 4 is needed to decaffeinate the beans in a batch system as compared to the countercurrent system of the present 6 invention. Further, the caffeine concentration of the caffeine-laden supercritical carbon dioxide exiting the 8 countercurrent extraction system of the invention, said extraction system containing Milds coffee beans, is on the order of 190 ppm as compared to a batch system wherein the carbon dioxide exits at an average caffeine 12 concentration of about 35 ppm. For Robusta coffee the caffeine concentration of the caffeine-laden 14 supercritical carbon dioxide exiting the extraction system is on the order of 440 ppm as compared to a batch 16 system concentration of 60 ppm. This increased caffeine concentration achieved by the countercurrent extraction : 18 of the invention is particularly important in allowing efficient recovery of the caffeine from the supercritical fluid.

Several caffeine removal techniques are known in the

Co } 22 art. For example, the caffeine-laden supercritical fluid may be passed through an absorbent bed, such as a bed of 24 activated carbon, to absorb the caffeine. Alternatively, the caffeine may be recovered by lowering the pressure of 26 the caffeineladen supercritical fluid so as to precipi- tate out both the caffeine and any enhancer that might be 28 used. However, it has been found that supercritical fluids are not entirely selective for caffeine, but rather typically extract both non-caffeine solids and caffeine. For example, supercritical carbon dioxide 32 typically extracts non-caffeine solids and caffeine at a weight ratio of about 1.5:1 to 3:1 non-caffeine solids to 34 caffeine. Thus, if supercritical carbon dioxide extracts caffeine from green coffee s0 as to increase its caffeine concentration to 220 ppm, said fluid will also contain 2 about 300 to 660 ppm non-caffeine solids. It has been found that the two methods described above for caffeine 4 recovery, namely absorption and depressurization, fail to selectively recover caffeine. Rather, non-caffeine 6 solids which are important to the flavor quality of coffee are lost from the supercritical fluid with the 8 caffeine during caffeine recovery.

According to the present invention, the caffeine-laden supercritical fluid removed from the caffeine extraction vessel is continuously fed to a 12 countercurrent liquid absorber. Continuous counter- current liquid absorption systems are impractical and 14 uneconomical for use in prior art supercritical fluid decaffeination systems because of the low caffeine 16 concentration in the caffeine-laden supercritical fluid exiting the batch extractor. However, not only is a 18 countercurrent absorber efficient and economical as used in the present invention, but it has additionally been found that polar fluids exhibit an excellent selectivity for caffeine when contacting caffeine-laden, non-caffeine 22 solids containing supercritical fluids. As such, as the essentially caffeine-free supercritical fluid exits the 24 absorber, it typically contains very nearly the same level of non-caffeine solids as it did upon entering the 26 absorber. Thus, if this fluid is recycled to the caffeine extraction vessel, it extracts no measurable 28 amount of non-caffeine solids from the green beans then being decaffeinated. As a result, the decaffeinated beans produced by the present invention are of a better } flavor quality. Additionally, the yield loss generally 32 associated with non-caffeine solids loss is eliminated by the process of the present invention. 34 According to the invention, the liquid absorber is operated under supercritical conditions. Typically the

I temperature and pressure within the absorber are , 2 identical, or very nearly identical, to the temperature : and pressure conditions in the extraction vessel. As ) 4 discussed hereinabove, the critical temperature and pressure will vary depending on the fluid employed. 6 Absorber design is considered to be well within the ordinary skill of one in the art. Typically, the 8 absorber is operated with a packing selected from those readily available in the art. Generally, the polar fluid is contacted with the supercritical fluid at a weight ratio of about 5:1 to 25:1, and typically about 10:1 to 12 20:1, supercritical fluid to polar £luid. Alternatively, the countercurrent absorber may be an empty column fitted te 14 with distributors for the carbon dioxide supercritical gas and water as described in co-pending application 16 Serial No.07/229,369, filed August 5, 1988 and entitled, "Caffeine Recovery from Supercritical Carbon Dioxide” 18 which is hereby incorporated by reference. Water is the preferred polar fluid for use in the continuous countercurrent absorber of the present invention. It is preferred that the polar fluid of the invention remove at 22 least 90% by weight of the caffeine contained in the caffeine-laden supercritical fluid, and more preferably © . 24 95% of the caffeine by weight. ht Caffeine and acidic non-caffeine solids are recovered 26 after extracting caffeine from a coffee material with . supercritical carbon dioxide and then continuously 28 absorbing caffeine from the carbon dioxide extractant by contact with an countercurrent water wash solution in an absorber. Wash solution from the absorber and containing caffeine is treated by reverse osmosis in a manner 32 described in co-pending application SerialNo.07/229,373 filed August 5, 1988 entitled "Method for Decaffeinating 34 Coffee Materials Including Reverse Osmosis Permeate

Recycle” which is hereby incorporated by reference to i - 16 - form a permeate stream containing acidic dissolved 2 non-caffeine solids and substantially no caffeine. In a first embodiment, at least a portion of the permeate 4 solution is recycled to the absorber and used as at least a portion of the wash solution. In a second embodiment, 6 at least a portion of the permeate solution is used to hydrate the coffee material prior to its decaffeination 8 with a carbon dioxide extractant. Such use of a permeate solution containing acidic dissolved non-caffeine solids increases the decaffeination rate of the coffee material, and the use of the permeate solution for hydration of the 12 raw coffee solids also increases the hydration rate.

Where the coffee material comprises raw coffee solids, 14 portions of the permeate solution containing acidic dissolved non-caffeine solids can be used both as recycle 16 to the absorber and to hydrate the coffee material. In all cases recycle of non-caffeine coffee solids increases 18 yield and overall coffee quality, particularly flavor.

Green coffee in the form of raw coffee solids is hydrated to a moisture content between 25-50% preferably about 30-45% prior to decaffeination. This is 22 accomplished by means well known in the art such as ‘ steaming or soaking. For example, green coffee beans may 24 be steamed soaked at about 100° C for up to two hours.

In another embodiment of the instant invention, an 26 aqueous reverse osmosis permeate solution substantially free of caffeine, is used to moisten the raw coffee 28 solids, The use of the permeate solution rather than city water increases the rate of hydration of the coffee material about 5-15% and increases the rate of decaffeination about 10-20%. i 32 The invention is further described by reference to the figures. Figure 1 shows a preferred embodiment of 34 the caffeine extraction vessel. At steady state conditions, the extraction vessel 5 is filled with a bed

. '. ’ : - 17 - of green coffee beans. An essentially caffeine-free 2 supercritical fluid is fed to the first end of the extraction vessel 6 and caffeine-containing supercritical 4 fluid is withdrawn from the second end of the extraction vessel 4. Green coffee is periodically admitted through 6 valve 1 into blow case 2. Valves 3 and 7 are simultaneously opened intermittently so as to charge the 8 green coffee from blow case 2 to the second end of the extraction vessel 4 and discharge a portion of substantially decaffeinated green coffee beans from the first end of the extraction vessel 6 to blow case 8. 12 Valves 3 and 7 are then closed. Valve 9 is then opened to discharge the substantially decaffeinated green coffee 14 from blow case 8. Additional green coffee is admitted through valve 1 into blow case 2 and the procedure is 16 repeated.

Figure 2 is a schematic illustration of a decaffeina- 18 tion system according to the invention wherein green coffee (12) is fed to an extraction vessel (10) and is ” removed therefrom as decaffeinated green coffee (14). An essentially caffeine-free supercritical fluid is fed } 22 countercurrently to the green beans as stream 16 ‘ihto the ” extraction vessel, and exiting as a caffeine-laden fluid 24 stream (18). The caffeine-laden stream (18) is then fed to a water absorber (20) and exits as an essentially 26 caffeine-free supercritical fluid stream (16). Counter- currently, water is fed as stream 22 to the water 28 absorber and exits as an aqueous caffeinecontaining stream (24).

Figure 3 is a schematic illustration of a preferred embodiment of a decaffeination system according to the 32 invention wherein green coffee (30) is fed to a oo moisturizer (32) wherein fresh water (34) or permeate 34 recycle (36) which is essentially free of caffeine but containing acidic non-caffeine solids or both are added

} .. , © = 18 - to hot green coffee beans to moisturize them to between 2 25-50% water prefereably 30-45%. If desired both recycle and fresh water may be added and in most cases a portion 4 of fresh makeup water must be added either to the green bean or absorber (56) or both. The moist beans are 6 discharged from the moisturizer (32) through valve (38) to blow case (40) and thereafter fed under pressure into 8 the extractor (44) through valve (42) while approximately 97% extracted coffee is discharged through valve (46) to pressurized blow case (48) and thereafter, on reducing the pressure, recovered through valve (50) dried, and 12 further processed into decaffeinated coffee.

An essentially caffeine free supercritical carbon 14 dioxide (52) is fed countercurrently to the green beans , in extraction vessel (44) and exits as a caffeine-laden . : 16 supercritical carbon dioxide (54) which is fed through a distributor (55) into an empty water absorber (contains 18 no packing, plates or the like) (56) and exits as an essentially caffeine free supercritical carbon dioxide stream (52) which is recycled to the extractor (44).

Water (60), either fresh or recycled from reverse 22 osmosis or a mixture thereof is fed countercurrently to the supercritical gas in absorber (56) through a 24 distributor (62) to contact countercurrently the : supercritical carbon dioxide and removed taffeine which 26 is passed through line (64) to storage tank (66) and then to one or more reverse osmosis units operated either in 28 series or parallel and shown collectively as (68) which concentrate the water laden caffeine from (66) some 5 to 100 times preferably 10-50 fold to produce relatively pure aqueous caffeine solution of 1 to 15% caffeine (70) 32 which can be further processed by crystallization or other recognized means to pure caffeine. The 34 permeate (72) from the reverse osmosis unit (68) which is rich in acidic, non-caffeine solids and contains

: y i - 19 - substantially no caffeine (less than 0.010%) is recycled 2 either to the water column (56) through line (60) or to the moisturizer or beans (36) or otherwise as by 4 recycling a portion of the permeate to each of the beans ‘and water column. 6 .

EXAMPLE 1 8 An elongated pressure vessel having a height about five times its diameter was loaded with 100% Colombian green coffee which was prewet to a moisture of about 30% to 40% by weight. Approximately 120 pounds of green 12 coffee were contained in the pressure vessel. To the - bottom of the pressure vessel was continuously fed 14 essentially caffeine-free supercritical carbon dioxide at so a pressure of about 250 atm. and a temperature of about . 16 130°C. The carbon dioxide extracted caffeine and non-caffeine solids from the green coffee as it moved 18 upwardly through the pressure vessel. The caffeine-laden supercritical carbon dioxide which also contained non-caffeine solids continuously exited the top of the - pressure vessel. Each nineteen minutes, approximately 22 10% of the volume of the coffee bed was discharged into a : bottom blow case while the same volume of prewvet _ 24 Colombian coffee was simultaneously charged from a oo previously loaded top blow case into the top of the 26 pressure vessel. The total residence time of the green coffee in the pressure vessel was about 3 hours. The 28 weight ratio of supercritical carbon dioxide to coffee was about 50 kg. carbon dioxide/kg. coffee.

The caffeine partition coefficient for supercritical . carbon dioxide and green coffee beans has been measured 32 to be about 0.026 at these operating conditions. The average caffeine concentration for Colombian Milds coffee 34 is about 1.22% by weight on a dry basis or about 1.08% by weight as is. Thus, the maximum obtainable caffeine

. ', » . - 20 - concentration in the supercritical carbon dioxide is 2 about 280 ppm. The caffeine-laden supercritical carbon dioxide exiting the top of the pressure vessel was found 4 to have a caffeine concentration of about 200 ppm, or about 71% of the maximum obtainable caffeine concentra- 6 tion. The caffeine-laden supercritical carbon dioxide was also found to contain about 350 ppm non-caffeine

B solids. The coffee discharged to the bottom blow case was found to be at least 97% decaffeinated by weight.

EXAMPLE 2 12 The caffeine-laden supercritical carbon dioxide from

Example 1 was continuously fed to the bottom of an 14 absorber measuring 4.3 inches in diameter, 40 feet in height, and with 32 feet packing height. The carbon } 16 dioxide was fed at a rate of 1350 lbs/hr. Water was fed i to the top of the absorber at a rate of 110 to 18 120 .1bs/hr. The absorber was operated at a pressure of abOut 250 atm. and a temperature of about 130°C. The following Table demonstrates the excellent selectivity for caffeine exhibited by the water, yielding a caffeine 22 purity of about 88% which discounting minerals from water is 93.5% purity. 24

TABLE . 26

Non-Caffeine 28 Rate Caffeine Solids {1lb/hr) Conc (PPM) Conc. (PPM).

CO, Feed To . Absorber 1350 200 348 32

CO; Exit From 34 Absorber 1350 19 332 36 Water Feed To

Absorber 110-120 0 171* 38

Water Exit From 40 Absorber 110-120 2,450 340* (169) 42 *Includes 171 ppm non-caffeine solids attributable to hardness of water.

The essentially caffeine-free supercritical carbon 2 dioxide exiting the absorber was recycled to the extraction vessel of Example 1. The decaffeinated green 4 coffee beans produced by recycling the essentially caffeine-free carbon dioxide containing non-caffeine 6 solids was used to prepare a coffee brew (A). A control coffee brew (B) Was prepared from identical beans 8 decaffeinated with supercritical carbon dioxide which was essentially free of carbon dioxide and non-caffeine solids.

This supercritical carbon dioxide stream had passed through an activated carbon bed which had adsorbed 12 caffeine and non-caffeine solods from a caffeine-laden : supercritical carbon dioxide stream generated by the 14 process of Example 1. Coffee brew A was judged by a panel of expert coffee tasters to be of superior flavor } 16 quality as compared to Coffee brew B.

The improved flavor quality of brew A was attributed to the presence 18 of non-caffeine solids in the recycled carbon dioxide which prevented the loss of valuable flavor precursor compounds from the green beans during decaffeination. 22 EXAMPLE 3 : Green Colombian coffee beans are moisturized to 41.1% 24 by contact with steam at 100° C for about 2 hours in an agitated mixer.

The moisturized coffee beans are added 26 to a 4 inch ID x 30 foot high extraction vessel by adding _ a volume of 0.2 cubic feet every 36 minutes to a 28 blowcase, pressurizing the blowcase to system pressure and dropping these beans into the extraction vessel while removing an equal volume of decaffeinated coffee from the bottom into a pressurized blow case.

The size of the 32 blowcase is such as to give a 6 hour residence time of the coffee in the extractor. 34 Caffeine lean supercritical carbon dioxide with 7.1 ppm caffeine at 296.7 bar and 101.2°C is recirculated countercurrently into the bottom of the extraction vessel 2 at a flow rate of 1959 lb/hr and exits the top of the extractor at a caffeine concentration of 69.3 ppm. This 4 caffeine rich supercritical carbon dioxide is countercurrently contacted with tap water at the same 6 pressure and temperature in a 4 inch ID x 40 foot high absorber which is empty (free of packing or plates). The 8 water removes 89.8% of the caffeine from the supercritical carbon dioxide which 1s recirculated back to the extractor. Beans decaffeinated with this process have 95.06% of the caffeine removed in 6 hr which 12 corresponds to a 0.501 hr-1 decaffeination rate (assuming first order rate kinetics). 14

EXAMPLE 4 : 16 Another batch of Colombian green coffee beans (from the same lot as those above) is moisturized to 41.6% in 18 the same agitated mixer as above. They too are added to the extraction vessel every 36 min. to effect a 6 hour residence time in the extractor. . Caffeine-lean supercritical carbon dioxide with 22 6.7 ppm caffeine at 297.5 bar and 99.9°C is recirculated : . through the extractor at a flow rate of 1960 lb/hr and : 24 exits the top of the extractor at a caffeine concen- tration of 78.4 ppm. The caffeine rich supercritical 26 carbon dioxide is countercurrently contacted with an acidic reverse osmosis permeate solution at the same . 28 pressure and temperature as in the extractor. The permeate solution removes 91.4% of the caffeine from the carbon dioxide which is then recirculated back to the extractor. The permeate solution is obtained by a method 32 which is described below.

The caffeine-rich water which leaves the absorber is 34 flashed to atmospheric pressure and is then sent to a reverse osmosis unit which concentrates the caffeine from

0.12% to 4.5%. The reverse osmosis membrane used is 2 ZF-99 manufactured by Paterson Candy Incorporated.

The water which permeates through the membrane has 0.002% 4 caffeine content and a pH of 3.6. A small amount of tap water (about 4.5 1b) is added to bring the water flow 6 rate to 163 lb/hr.

This acidic solution is recycled back as feed to the water absorber. 8 Beans decaffeinated in this manner have 97.1% of the caffeine removed in 6 hrs which corresponds to a 0.588 hr-1 decaffeination rate (assuming first order rate kinetics). 12 Example 4 exhibits a first order decaffeination rate constant of 0.588 hr-1 which is 17% greater than the 14 0.501 hr~1l value exhibited by Example 3.

Claims (1)

1. 2) fot We claim: Cl

   1. a method of decaffeinating green coffee in an extraction system comprising: @ continuously feeding essentially caffeine-free supercritical carbon dioxide to the bottom portion of an extraction vessel containing moist green coffee beans for a period of time sufficient to transfer caffeine from the moist green coffee beans to the supercritical carbon dioxide, said transfer resulting in a caffeine concentration in the supercritical carbon dioxide which is at } least 40% of the maximum obtainable caffeine concentration therein, said maximum obtainable caffeine concentration being defined by tlie caffeine partition coefficient for said supercritical carbon dioxide and; Mb) withdrawing said supercritical carbon ‘ dioxide containing at least 40% of its . maximum obtainable caffeine concentration from the top portion of the extraction vessel, said moistened green coffee beans ’ in the extractor containing between 35% and 50% by weight moisture;

   (c) periodically discharging a portion of \ decaffeinated coffee beans from the bottom : end of the extraction vessel and;

   (d) periodically charging a portion of moistened undecaffeinated green coffee beans to the top end of the extraction vessel;

   (e) contacting said caffeine laden ’ supercritical carbon dioxide from step (b) countercurrently with an aqueous fluid in a substantitally unobstructed absorber to eh EE ee Br transfer substantially all caffeine ,... ., . :

   contained therein to the aquecus fluid with no appreciable transfer of ~ non-caffeine solids to the aquecdus fluid;

   £) collecting the caffeine laden aqueous fluid and subjecting said caffeine laden fluid to reverse osmosis to recover a concentrated caffeine solution and an acidic aqueous permeate substantially free of caffeine and;

   (9) adding the acidic permeate to the extraction system to reduce the green bean extraction time and improve green bean quality.

   2. The method of claim 1 wherein the permeate js added to the green coffee to moisturize the coffee.

   3. The method of claim 1 wherein the permeate is countercurrently contacted with the supercritical carbon dioxide in the absorber, 4, The method of claim 1 wherein said acidic permeate has a pH of less than 5.

   5. The method of claim 1 wherein said non-caffeine solids dissolved in said acidic permeate comprise organic acids,

   6. The method of claim 1 wherein said permeate contains not more than about 0.010% caffeine by E weight, a Saul Norman Katz Jean Ellen Spence Michael J, O'Brien Ronald H. Skiff Gerard J. Vogel Ravi Prasad