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Decaffeination of coffee
The aim of decaffeination is to produce a coffee which retains its aroma and taste despite the processes which are necessary to remove the caffeine. As most flavour components develop during roasting, coffee is almost always decaffeinated in the green bean form, before roasting. Decaffeinated beans, roast and ground and instant coffees are now widely available.
The first successful extraction of caffeine from coffee beans was achieved by a German chemist, Runge, in 1820. His friend, the poet Goethe, had suggested that Runge analyse the constituents of coffee to discover the cause of his insomnia - and the history of decaffeinated coffee began. However, the real technical breakthrough did not come until the turn of the century, when Ludwig Roselius decided to pre-treat the coffee beans with steam before bringing them into contact with the caffeine-removing solvent. Steaming swells the beans, increasing their surface area and making the caffeine easier to remove. Roselius' discovery made it possible to produce decaffeinated coffee on a commercial scale for the first time; he founded Kaffee HAG in Bremen in 1906.
Danish Coffee Hag advertising from the 1920s. Similar advertising was used all over Europe, claiming that the coffee 'cleaned' the heart
Pre-treatment with steam is still the first stage of many modern decaffeination processes, but significant changes have taken place in the technology and in the solvents used.
Methods of decaffeinating coffee
Decaffeination processes basically involve treating the green coffee beans with a solvent, then removing the caffeine-laden solvent from the beans. The three types of decaffeination method in commercial use today involve either:
Chemical solvent decaffeination
Firstly, the green beans are treated with steam, under pressure. This treatment swells the beans, increasing their surface area and making the caffeine easier to remove. The next stage is extraction of the caffeine by a solvent, again under pressure, at a temperature close to the boiling point of the solvent. Ideally, the solvent should remove caffeine selectively, without affecting the coffee in any other way. After decaffeination only minute traces of the solvent are left in the coffee. Nevertheless, the chemical used must be safe, so that these traces do not affect the health of anyone drinking decaffeinated coffee. The safety of solvents used in decaffeination is tested in animal and human studies and reviewed by government scientific authorities. Solvents in current use which pass these stringent criteria include methylene chloride (dichloromethane) and ethyl acetate.
Methylene chloride has the advantage of a relatively low boiling point (40° C) and can therefore be used at lower temperatures. After stringent investigations, the United States Food and Drug Administration reaffirmed its approval of methylene chloride use in decaffeination in December 1985.
Ethyl acetate is included on the FDA list of chemicals "Generally Recognised As Safe" for use as flavouring agents in foods. It occurs naturally in many fruits at levels higher than the traces found in decaffeinated coffee.
The caffeine which is removed from the solvent by distillation has many commercial applications, for example in pharmaceuticals and as a flavouring agent. Traces of solvent still adhering to the beans are forced out by further steaming and the coffee is dried.
Decaffeination by supercritical gas
At temperatures above their 'critical point' under pressure, gases behave rather like liquids and can be used as solvents. Supercritical carbon dioxide is used as a selective solvent for caffeine. It is applied to previously steamed green coffee at temperatures about 70° C and at high pressure. The caffeine is separated from the gas by rinsing or by adsorption and the gas recirculated. In this method the wax layer of the coffee bean is retained and nothing but the caffeine is removed.
Methods using water and caffeine-free extracts
Various processes have been devised in which caffeine is removed, not from the bean, but from an extract of water-soluble substances produced by steeping the coffee in hot water. If the caffeine is removed by a solvent, this is known as an 'indirect solvent' method; otherwise, the caffeine may be separated from the extract by adsorption onto a substance such as activated carbon (charcoal). The caffeine-free extract is then used to decaffeinate the green coffee, as caffeine passes readily from the beans into the extract. However, these methods also result in the loss of some other water-soluble components of coffee such as carbohydrates and chlorogenic acids.
In the 'Swiss water process', the green beans are immersed in water and the resulting extract passed over activated carbon to remove the caffeine, as above. The caffeine-free mixture is then added to the partially dried coffee beans before they are fully dried and roasted.
Regulations concerning decaffeinated coffee
EU regulations state that the caffeine content of decaffeinated coffee should not exceed 0.1% in the case of green beans or 0.3% in the case of coffee extracts (instant coffee) on dry matter basis.
All decaffeination methods in use today remove at least 97% of the caffeine naturally present in the coffee bean. A cup of decaffeinated coffee contains about 1-5 mg caffeine, depending on the strength of the brew.
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