Coffee beans are full of caffeine but not everyone wants this particular bioactive in their brew. It is linked to a number of health issues but it is also a mainstay component of energy drinks and one of the reasons coffee is so popular as a legal stimulant. caffeine is also an extremely important bittering component and its presence has considerable impact on flavour. Reducing caffeine content of coffee beans generally speaking helps in reaching a safe limit of 150mg caffeine per day so that at least 2 or 3 cups of coffee can be drunk without exceeding that limit. Most drinkers find decaffeinated coffee lacking in flavour impact.
The two main species of coffee are C. arabica and C. robusta and they contain 1% (range: 0.9-1.4%) and 2% (range: 1.5-2.6%) by weight respectively of caffeine (Clarke & Macrae, 1985). A typical cup of coffee should contain on average 85mg of caffeine but the range varies between 50 and 150mg according to the way it is prepared, the variety and blend of coffee and the cup size. A cup of instant coffee probably contains about half that amount and its range is from 40 to 110mg caffeine per cup.
Caffeine has a molecular density of 1.23 g/cm3.
History
Removing caffeine from coffee was invented by Dr Ludwig Roselius and Dr Karl Wimmer at the Kaffee-Handels-Aktien-Gesellschaft in Bremen, Germany. It was encapsulated in a German patent of 1905 (German Patent, 1905; US Patent, 1908). The process was scaled-up and a caffeine-free coffee became available under the name of Kaffee Hag.
In 1912, Roselius developed his business in the USA with a caffeine extraction plant in new Brunswick, New Jersey. During the first world war, the American Kaffee Hag business was taken over by the USA government and ownership passed to Mr Gund who moved the factory to Cleveland, Ohio. This business then sold to the Kellogg Company.
The Process of Coffee Decaffeination
There are many processing methods to remove caffeine and all are conducted when the coffee is in its green bean phase before roasting. All processes require a solvent of some sort to dissolve the caffeine and allow for its transport out of the bean where it can then be dealt with.
One of the main issues is that caffeine is bound to chlorogenic acid. To obtain caffeine means separating it from this other molecule, then enabling the solvent to be able to access caffeine without encouraging rebinding and then allowing for it to diffuse out of the coffee bean. To allow this to happen means that the bean’s moisture content has to be increased and quite considerable. A coffee decaffeination process usually means green coffee beans are either treated with super-heated steam or soaked with water followed by steaming (Menthe, 1985).
Unfortunately, decaffeination not only removes the caffeine but also a number of flavour compounds. The process though has been developed to minimise these losses in flavour by returning as much of the flavour and aroma back to the green beans prior to roasting. However, the losses are noticeable to many experts, professional or amateur.
The decaffeination process also makes roasting much more difficult by adjusting the compound composition and amount. The sugars are removed for example which are key components in melanoidin formation.
Processes For Decaffeination
Overview:
The most common methods still in use rely on chemical solvents although supercritical carbon dioxide extraction is also employed. Another major approach is to soak the coffee beans in water, remove the caffeine and then recontact the beans with that water containing the flavour chemicals so they are then returned to the bean.
Two methods using solvents are used for decaffeination. The direct solvent method involves literally soaking coffee beans in a solvent to remove the caffeine. The indirect solvent method relies on soaking the beans in water to obtain a caffeine rich aqueous extract which is then treated with solvent to remove the caffeine. The beans always swell in any process as they pick up solvent.
The two solvents most commonly used are methylene chloride and ethyl acetate. Their volatility is so high that any solvents retained in the decaffeinated beans are always removed during roasting and then brewing. The main issue with any solvent method is that removal of the solvent is expensive.
Other less common but still effective solvents include methanol, acetone, ethanol, benzene, chloroform, ether, carbon tetrachloride, ammonium hydroxide and acetonitrile or their aqueous suspensions. Many of these solvents are too toxic to use but they have been employed in processing at a pilot-scale in a number of instances. Up until the mid-1970s, all decaffeination was conducted using solvent extraction methods until supercritical CO2 came along.
Direct Solvent Method
The beans are briefly steamed for 30 minutes to increase their water content. The beans are then steeped in the solvent for over 10 hours. The solvent is removed. The beans are steamed one more time to remove any remaining solvent and also excess moisture.
Indirect Solvent Method
The green coffee beans are soaked in hot water which draws caffeine to the surface of the beans. The method is indeterminate by removing not only caffeine and other flavours and aromas.
The water is removed from the beans and solvent added to this aqueous extract to remove the caffeine. The caffeine solvent mixture is heated and the volatile solvent and caffeine evaporated off. The beans are added to the remaining mixture to reabsorb the remaining aroma chemicals and restore some of the flavour. Using this method is not an issue when considering residual solvent in the beans.
The Use of Methylene Chloride
The FDA states that methylene chloride is safe to be used in coffee decaffeination. It is known as the KVW method throughout Europe. It allows for up to 10 parts per million (ppm) of residual solvent but in practice the levels are much lower. The process means 96 and 97 per cent of the caffeine is removed.
To obtain decaffeinated coffee beans, they are first soaked in hot water which removes most of the caffeine. The beans are dried whilst the caffeine extract is treated with methylene chloride which binds to the caffeine. The bound caffeine is removed as a top layer leaving an aqueous layer to which the beans are returned to be rehydrated.
One study showed that using this type of technology, a coffee bean of 1.45%w/w caffeine was reduced to 0.01-0.5% caffeine after decaffeination (Bichsel et al., 1976).
Residual methylene chloride levels in coffee after decaffeination is very low. The boiling point of the solvent is 40°C and coffee passes through temperatures around 70 ° C for solvent volatilization. Green coffee beans should not contain any significant amounts of solvents. Additionally, roasting temperatures (commonly 210–230°C) are high enough to allow volatilization of any remaining amount of dichloromethane in coffee. The US FDA (Food and Drug Administration) allows up to 10 ppm in roasted coffee, while the European Union allows up to 3 ppm (Farah, 2009).
Use of Ethyl Acetate
The solvent of choice. It is detected in coffee aroma and occurs in other fruits too so is regarded as a natural product. There is no set limit on the residue for this solvent. Morrison and Phillips (1987) characterised a continuous method for the accelerated decaffeination of green coffee beans involving wetting of the beans followed by countercurrent extraction using ethyl acetate for 3 to 5 h. The residual solvent is removed by steam stripping.
The Use of Fats In Extraction
Water-immiscible liquid fats and lipids have also been used as solvents. These are fatty acid esters such as glycerol esters and the process is described as the triglyceride process. Others have tried coffee oil, soy bean oil and safflower oil. They have the benefits of being edible.
As with any hot water extraction process, green coffee beans are soaked in water to draw caffeine to the surface. The green coffee beans are wetted to between 40 and 60% moisture content before treated with the fatty extractants. The most efficient temperature of operation is between 90 and 120ºC. The beans are moved into another vessel and soaked in coffee oil obtained from spent coffee grounds. The coffee oil or some other oil relies on its triglycerides to extract the caffieine but the flavours remain. the beans are separated from the oil and dried. The caffeine is removed from the oil and these are then reused for further decaffeination.
Liquid green coffee extracts are obtained from a multistage countercurrent extraction using a high oil to extract ratio and an operating temperature of 30ºC. The fatty acids extractant is recovered using liquid-liquid extraction with water.
Caffeine Recovery From Activated Carbon
Activated charcoal or carbon is commonly used to remove caffeine from process strams. It is the principal removal process in the Swiss Water Process for example. Activated charcoal only appears to remove caffeine and allow the coffee flavour and oils to pass through. As has been often stated, the coffee beans initially decaffeinated are discarded and the caffeine-free water then used to further decaffeinate new coffee beans. As this water is concentrated with flavour and oils, the new coffee beans only discard caffeine but retain their oils and aromas.
The activated carbon or charcoal used in the Swiss Water Process as in other processes is derived from a variety of resources including coconut shells. The reason is that activated charcoal is becoming more expensive and it is difficult to regenerate. A lot of interest is being expended on greener solutions such as agricultural biomass waste, clay minerals, soils and sediments and various compounds. Other types of adsorbent have been tried such as zeolites and silica gels
Caffeine is adsorbed onto activated carbon but has always proven difficult to get it off. Many of the processes are patented. A US Patent 2,508,545 by General Foods showed that activated carbon and other adsorbents could be used to removed impurities from caffeine solutions but in the early 50s it was not clear how to recover the caffeine.
The caffeine is recovered from the activated charcoal using 0.1N NaOH and hot water. In both elution conditions the recovery of caffeine is only about 30% (Danish, 2020).
An alternative is monorillonite (MMT), clay mineral which has been used for caffeine absorption from tea extracts (Shiono et al., 2017).
Non-Solvent Processes
Jean Maclang in 1934 decided that water would be a good solution to remove caffeine from coffee and do away with chemical solvents. The process was developed in Switzerland.
The Swiss Water Process (SWP) and the Royal’s Select Water Process
The SWP process was established in the 1930s but it became cost effective in the 1980s. The business is Canadian based in Vancouver, British Columbia. It is the only organic process. It operates primarily by diffusion processes.
Green coffee beans are soaked in hot water until all the caffeine and flavour compounds are removed (Sturdivant, 1990). The first batch of beans are discarded but further beans are then used for further processing. The aqueous extraction is passed through an aqueous activated carbon-charcoal filter to remove the caffeine leaving behind the flavour compounds. The pore size is such that the larger caffeine molecule is trapped in the filter and the smaller molecules pass through. The remaining water contains flavour. Any new beans following discarding of the first batch of beans are then soaked with the flavour rich but caffeine free water. An osmotic diffusion process removes caffeine from the beans until its concentration is equally partitioned between bean and water extract. The flavour and aroma compounds are then equally concentrated between water and bean. Labelling is permitted where it is labelled as Swiss Water Decaf.
This is termed a green coffee extract (GCE).
The Royal’s Select Water Processing method is also chemical free. Here the green coffee is pre-soaked in water before being introduced to a solution of concentrated coffee solubles that removes the caffeine without leaving an undesirable bitterness. After the soaking the green coffee is re-dried. The idea is to remove the caffeine and at the same time enrich the beans with flavour. Not that often applied.
The Mountain Water Process (MWP) Decaf Process
The MWP is a method of indirect decaffeination which relies on water extraction to remove caffeine from green coffee beans. The beans are steamed before water extraction. The caffeine is removed using filtration processes. It is almost identical to the Swiss Water Process and is named as much for effect. The choice of water is important as it allows for artistic license in using mountain spring water etc.
Aqueous solutions of caffeine are cleaned up using membrane filtration. One example used PES ultrafiltration membranes (Adamczak et al., 2018).
French Water Decaffeination
The ‘french water’ process was developed in 1992 (Sturdivant, 1992). It was developed by Ed. Wakeham who is a trader for Cofinco. It is a process where the coffee beans are soaked in hot boiling water for 24 hours. the water leaches out the caffeine and various coffee solids. the water is drawn off, and the beans are dried.
The caffeine rich water is purified through a natural filter to absorb the caffeine without any chemical reactions with the water. Once the caffeine is removed, the water is added back to the coffee beans. These beans behave like a sponge and reabsorb the water to recover their flavour. The caffeine removal is 99.9% . One of the benefits of a double drying process of beans is they contain less residual water and absorption of coffee solids is higher.
Nanofiltration
Largely undeveloped as a technology for coffee extraction but possibly having a degree of success in a conceptual sense. Membranes have largely been overlooked as a unit operation but may find value in the recovery of caffeine from adsorbents.
Studies indicate with a hollow fibre membrane system that caffeine in the feed stock is recovered in the permeate. If other molecules, especially polymers are present then the recovery of caffeine and the permeate flux drop off with increasing levels of material in the feedstream. Increasing temperature means the permeate flux rises. It is possible to recover caffeine from roasted coffee operated at 55ºC and decaffeinate this coffee removing 25% (Ong et al., 2015).
Supercritical CO2 Extraction
The use of supercritical CO2 extraction of caffeine is regarded as a gentle process with little impact on the processed material (Diaz-Reinoso et al., 2006). One of the main benefits is the absence of solvent residues in the extracted beans. It is also one of the most specific extraction methods available and doesn’t extract flavour. The caffeine is also recovered in high quality and relative purity and is one of the main methods for production of this alkaloid.
Caffeine has relatively low solubility in supercritical carbon dioxide because of its apolar character but the addition of polar solvents such as water or ethanol helps with solubilising the caffeine. Cosolvent effects are due to specific chemical (hydrogen bonds and acid–base interactions) or physical interactions (dipole–dipole or dipole-induced dipole) between the cosolvent and solute.
Generally, the unroasted coffee beans are mixed with water to achieve a moisture content of 50%. The beans are held in a sealed extraction vessel. The caffeine is extracted with liquid carbon dioxide at 300 atm. pressure and 150ºC to generate a caffeine level of 0.16% w/w – a process which takes between 4 and 6 hours. The liquid carbon dioxide is recirculated between an extractor and a scrubber for between 8 and 12 hours. The scrubber has a high water to carbon dioxide ratio. The caffeine is stripped from the carbon dioxide using this water in a high-pressure washing tower.
Any other remaining caffeine is removed by allowing the supercritical carbon dioxide to return to a gas form by releasing the pressure. The evaporating carbon dioxide leaves the caffeine behind in the vessel. Any remaining caffeine is removed from the gaseous carbon dioxide using charcoal filtering so it can be reused.
The caffeine as in most processes is recovered using a distillation-crystallization process.
Unroasted coffee beans turn brown after supercritical CO2 decaffeination. It is more than likely that even at the temperature of processing in this process, there are Maillard reactions occurring in this process forming melanoidins and producing the dark brown colour. The brown colour is different from the distinctive yellowish-green color of untreated raw coffee beans (Chu et al., 2009, 2011). The browning is due to Maillard reactions occurring as the temperature increases.
Distillation could be replaced using reverse osmosis membranes because distillation is a very costly process. It could also not only recover caffeine but also supply a clean water stream back to the washing tower. On of the issues found with this process is that dissolved carbon dioxide reduces membrane performance (Pietsch et al., 1998).
One of the other issues is that processing plant has a high capital cost and is said to be only commercially viable if the plant makes over 3000 tons of decaf. coffee every year.
Supercritical CO2 extraction has been discussed elsewhere.
Typical businesses involved in supercritical CO2 extraction include Separeco (Italy), de Dietrich, Extratex-SFI.
Losses of Other key Compounds
Caffeine extraction can lead to losses of chlorogenic acids (CGA) and subsequently the 1,5-γ-quinolactones (CGL) in the roasted coffee.
Other groups have obtained valuable extracts containing caffeine from spent coffee using only membrane fractionation techniques and no adsorbents or organic solvents in the process.
Coffee oil from spent coffee grounds could be added in fractions to butter as a way of extending this spread.
Extractors Of Caffeine
Businesses such as TharProcess (Pittsburgh, PA, USA) are one of the largest of the producers of supercritical CO2 plant, Accudyne Systems (Newark, DE, USA), Inc. Natex Prozesstechnologie (Ternitz, Austria) have produced supercritical CO2 plants for companies like Degussa. Check out IsolateSystems, De Dietrich Process Systems SepareCo (Macello, Italy), Supercritical Fluid Technologies, Inc. (Neward, DE, USA).
CalgonCarbon has developed granular activated carbon for caffeine extraction. General Foods (as it was), now Mondelez, Proctor & Gamble Co., Nestle have all actively examined decaffeination. Current players are:-
- Nestle SA
- Keurig Dr Pepper Inc
- The JM Smucker Co.
- The Kraft Heinz Company
- Strauss Group Ltd
- Tchibo Coffee Interantional Ltd
- Swiss Water Decaffeinated Coffee Inc;
- LifeBoost Decaf;
- Cafe Don Pablo Colombia Supremo Decaf;
- No Fun Jo Decaf; – uses Swiss Water Process
- Fresh Roasted Coffee LLC;
- Luigi Lavazza SPA
- Costa Ltd
- Caribou Coffee Operating Company Inc.
- Tim Hortons – Canadian coffee chain
- Jacobs Douwe Egberts Pro
- Peet’s Coffee
- Massimo Zanetti Beverage Group
- Illycaffe SpA
- Caffe Nero
- Bewley’s tea & Coffee
- Alois Dallmayr KG
- Volcanica Coffee Costa Rica Tarrazu Decaf. – uses Swiss Water Process
- Cameron’s Coffee and Puroast Coffee Company
- Kicking Horse Coffee Decaffeinated
- Koffee Kult Colombian Decaf
- Koa Coffee Kona Decaf
- Eight O’Clock Coffee Decaf.
References
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