Caffeine is a compound that is known worldwide due to its presence in many, widely consumed, foods and drinks; it has many natural sources, including coffee beans, cocoa beans and tealeaves. It has the systematic name 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione, with the chemical formula C8H10N4O2.
The effects of caffeine on the human body are well known. Caffeine in low doses is a stimulant; allowing the drinker increased concentration and reduced tiredness, however, the consumption of higher doses of caffeine can result in insomnia, nervousness, hostility and mood swings – among other side effects. Commonly, the biological effects of caffeine are explained by it mimicking adenosine (look at their similar structures). In the body, adenosine acts on a receptor that causes a 'slowing-down' of the body, whereas when caffeine acts on the same receptor it has the opposite effect. So, because of its similar shape, caffeine competes with adenosine and reversibly blocks the action of adenosine on its receptor.
Is the decaffeination process safe, and are coffee’s healthy antioxidants stripped from the beans along with the caffeine? The short answer: Health experts say go ahead, keep drinking decaf if.
The negative effects of caffeine have led to an increasing demand for decaffeinated brews. In order to provide decaffeinated coffee, various methods of removing caffeine from the beans have been developed. We will look at two of the most common methods.
Supercritical CO2
The first extraction method uses supercritical carbon dioxide (CO2). Carbon dioxide, at standard temperature and pressure, is a gas and, when frozen, carbon dioxide forms a solid (called dry ice). It is also possible for carbon dioxide to exist in a fluid state but, for this to occur, it has to be held at its critical pressure and critical temperature.
The critical point is the combined critical temperature and critical pressure where, at or above which, the molecule is no longer able to be classed as a liquid or gas, but is instead known as a supercritical fluid. Above its critical point, there are no clear liquid or gas phases, and the carbon dioxide can take on both liquid and gas properties. Carbon dioxide in this state is called supercritical CO2 (abbreviated sCO2) and has the ability to act as a solvent and dissolve other materials.
Supercritical CO2 has been used in industry to extract caffeine from coffee beans since the 1970's. Caffeine has a low solubility in supercritical carbon dioxide and, for this reason, a co-solvent (either water or ethanol) is used which triples the caffeine solubility, hence accelerating the extraction. It is a widely used method of decaffeination, as it has proved to be more efficient, as well as being better for the environment, than other decaffeination methods. On top of this, it is also an easily removable, non-flammable and non-toxic solvent. These beneficial properties have helped ensure its application in other extraction areas, such as teas and cocoa beans.
Large-scale decaffeination takes place on green coffee beans (before roasting). The green beans are soaked in hot water (until they have about 50% moisture content) and put into an extraction vessel, which is then sealed. Soaking the beans in water allows the pores of the coffee beans to open as the beans swell, thus allowing the caffeine (which converts into a mobile form) to diffuse out of the beans. At a pressure of about 300 atmospheres, the supercritical CO2 is pumped into the sealed vessel, where the CO2 solvent dissolves the caffeine compound and draws it out of the coffee beans. Any larger odour molecules are not dissolved and remain inside the beans. During the decaffeination process, the carbon dioxide moves through an extractor and a scrubber, on a loop, for approximately 8-12 hours. The scrubber has a high water to CO2 ratio and the water carries away any dissolved caffeine before the carbon dioxide recirculates to extract more caffeine.
The CO2 and caffeine mixture is then moved into a strong tank, where the pressure on the container is then removed. As the pressure lowers, the supercritical CO2 returns to the gaseous phase – when the pressure is lowered, it falls below the critical value. The gaseous CO2 evaporates, leaving the caffeine behind in the container. Any remaining caffeine is removed from the gaseous CO2 by charcoal filters, allowing the clean CO2 to be pumped back into a high-pressure vessel so it can be recycled and used again and again.
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Extraction using supercritical carbon dioxide is a highly selective process. It only extracts the caffeine compound and leaves the other flavour precursors (such as carbohydrates and peptides) in the bean. However, there are two disadvantages to using this decaffeination method; firstly, precise processing parameters need to be specified and secondly, the process is very capital intensive meaning that this process is only commercially viable if the plant makes over 3000 tons of decaffeinated coffee a year.
What happens to the caffeine that is extracted?
Caffeine obtained from the decaffeination is mainly used by pharmaceutical, cola-type soft drinks and cosmetic industries (it is a common ingredient in cellulite creams), and, more recently, in energy drinks. Although decaf coffee amounts to around 10% of the global coffee market (in 2012), the need for caffeine by these industries is much bigger than is produced, hence caffeine is prepared in the lab (the first reported synthesis was back in 1895). Following on from the discussion in week 1, it is interesting that ‘natural’ caffeine (mainly from coffee beans) is generally assumed to be healthier than energiser drinks (or beauty products) containing ‘synthetic’ caffeine.
You may also like to read about another process that can be used to remove caffeine from coffee beans, the so-called Swiss water method (see below).
Why not try this ‘stimulating’ practical?
Finally, for those of you keen to do some further practical work and have access to a laboratory (including a fumehood and equipment such as a thermometer) - we have added an extension activity on the extraction of caffeine from tea and/or coffee (see below). This gives you the opportunity to extract a reasonably pure sample of caffeine from tea leaves and/or filter coffee – as always, ensure you read all of the instructions and risk assessment before starting (the experiment involves heating an appropriate cooking oil, such as corn oil, to around 250 °C).
We would love to see photos of your caffeine crystals so please post them on our open Padlet or the Twitter hashtag #FLchemistry!
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'First, some background. Coffee is the second most popular beverage in the world, after tea. Historians believed the use of coffee as a stimulant originated in ancient Abyssinia (Ethiopia). Caffeine is the component of coffee that is responsible for its mild stimulatory effect on the central nervous system. A six-ounce cup of coffee typically contains approximately 50 to 75 milligrams of caffeine, although the amount varies considerably depending on the method of preparation and the type of coffee; Robusta coffee contains nearly twice as much caffeine as Arabica, for instance. For people who are sensitive to caffeine, even 10 milligrams can cause discomfort. That is why nearly all decaffeinated coffees contain less than 10 milligrams of caffeine (typically two to five milligrams) per serving. Today decaffeinated coffee accounts for approximately 12 percent of total worldwide coffee consumption, or nearly 1 billion pounds per year.
'The first process for decaffeinating coffee was invented by Ludwig Roselius in 1905. Roselius's method used benzene, a potentially toxic hydrocarbon, to remove caffeine from premoistened, green coffee beans. Modern decaffeination processes are much more gentle; many make that point by claiming to be 'naturally decaffeinated.'
'There are three main decaffeination processes currently in use. They have some basic similarities. In all three approaches, the green or roasted beans are first moistened, making the caffeine soluble so that it can be drawn out. Also, they all decaffeinate green coffee at moderate temperatures, typically ranging from 70 to 100 degrees Celsius (160 to 210 degrees Fahrenheit).
'One method is water processing. As you might expect, this process employs water as the solvent to remove caffeine from the green coffee beans. Typically a battery extraction process using eight to 12 vessels is employed; each vessel contains green coffee at a different stage of decaffeination.
'A mixture of water and green-coffee extract that has already been reduced in caffeine is circulated around the coffee beans within the extraction battery (oils in the coffee extract aid in the decaffeination process). After a predetermined time, the vessel that has been exposed to the low-caffeine extract is isolated and emptied. The decaffeinated coffee beans are then rinsed and dried, and a vessel containing fresh green coffee is put on stream. The caffeine-rich extract that was drawn off from the vessel containing the fresh, green coffee is passed through a bed of activated charcoal, which absorbs the caffeine. This charcoal has been pretreated with a carbohydrate, typically sucrose, that helps it absorb caffeine without removing other compounds that contribute to the flavor of the coffee. The sucrose blocks carbon sites that would normally absorb sugars from the liquid, green-coffee extract. The caffeine-reduced extract can then be reused to begin the process anew. The water process is natural (that is, it does not involve any chemicals), but it is not very specific for caffeine; it removes 94 to 96 percent of the caffeine.
'A second decaffeination method is the direct solvent method. These days this technique usually employs methylene chloride (used predominately in Europe), coffee oil or ethyl acetate to dissolve the caffeine and extract it from the coffee. Ethyl acetate is an ester that is found naturally in fruits and vegetables such as bananas, apples and coffee. The liquid solvent is circulated through a bed of moist, green coffee beans, removing some of the caffeine; the solvent is then recaptured in an evaporator, and the beans are washed with water. Residues of the solvent are removed from the coffee to trace levels by steaming the beans. Often this process utilizes batch processing--that is, solvent is added to the vessel, circulated and emptied several times until the coffee has been decaffeinated to the desired level. Solvents are used because they are generally more precisely targeted to caffeine than is charcoal, leaving behind nearly all the noncaffeine solids. The more caffeine-specific solvents, such as methylene chlorides, can extract 96 to 97 percent of the caffeine.
'The third approach, supercritical carbon dioxide decaffeination, is very similar to the direct solvent methods, except that in this case the solvent is carbon dioxide. High-pressure vessels (operating at roughly 250 to 300 times atmospheric pressure) are employed to circulate the carbon dioxide through a bed of premoistened, green coffee beans. At such pressures, carbon dioxide takes on unique, 'supercritical' properties that enhance its usefulness as a solvent. Supercritical carbon dioxide has a density like that of a liquid, but its viscosity and diffusivity are similar to those of a gas. These attributes significantly lower its pumping costs. Carbon dioxide is a popular solvent because it has a relatively low pressure critical point, and it is naturally abundant. The caffeine-rich carbon dioxide exiting the extraction vessel is either channeled through a bed of activated charcoal or through a water 'bath' tower to absorb the caffeine. The carbon dioxide is then recirculated back to the extraction vessel. Supercritical carbon dioxide decaffeination is capital-cost intensive, but it offers very good yields. It typically can extract 96 to 98 percent of the caffeine originally present in the beans.'