Decaffeination is a difficult, imperfect process.
The trick is to separate the caffeine from the bean while leaving all of the other chemicals intact in their original concentration, including proteins, sugars, cellulose and acids. If these are altered, the coffee bean won’t retain the same taste.
Every decaffeination process begins with washing or soaking the green, unroasted coffee bean. Water, though needed to open the pores of the coffee beans, can’t be used on its own because it removes any soluble substance, not just caffeine, including the sugars and proteins which are essential to retaining flavour.
Decaffeination techniques extract ‘selectively’, meaning that, for the purposes of decaffeination, only caffeine is removed. A big problem is that the most obvious and effective methods of selective extraction use solvents that are carcinogenic—for example, the first commercially viable decaf method, in 1905, was Ludwig Roselius’ process that used benzene.
This is an obstacle that all modern methods of decaffeination have to circumvent or account for one way or another. So without further ado, let’s explore how.
The MC method
Methylene chloride (MC), also known as dichloromethane, is a volatile chemical solvent with a very low boiling point of 40°C. This is far below the roasting temperature of coffee, which is a little over 200°C, meaning that the methylene chloride is vapourised well in advance so that none (or at least a negligible amount) is left in the finished product.
The MC method is an ‘indirect’ solvent method, where the unroasted beans are soaked in near-boiling water for several hours before the water is transferred to another tank with the methylene chloride. The methylene chloride bonds with the caffeine in the water and is skimmed from the solution. The caffeine-filled solvent is removed, and then the flavoursome, almost caffeine-free water is put back in the first tank where it is reabsorbed by the beans. This process is repeated numerous times until 95-97% of caffeine is removed. This is opposed to the method of steaming the beans to open their pores, then soaking them directly in solvent.
The CO2 method
The CO2 method was developed by Kurt Zosel of the Max Planck Institute only recently. Liquid CO2 (carbon dioxide) is used instead of chemical solvents. Water-soaked coffee beans are placed in a container known as an extraction vessel, the vessel is sealed and liquid CO2 is forced into the container at pressures of 1000 pounds per square inch.
At this pressure, the CO2 extracts the caffeine by dissolving it. The caffeine-rich CO2 is then transferred to another chamber, where the pressure is released, allowing the CO2 to become a gas again. This is then separated from the caffeine using charcoal filters, clean and ready to be reused.
The CO2 method is complicated and quite expensive so is ordinarily used to decaffeinate large quantities of commercial-grade coffee.
The Swiss Water method
The Swiss Water method is an organic, chemical-free option for decaffeination. It was discovered in the 1930s in Schaffhausen, Switzerland, and is commercialised by the Swiss Water Decaffeinated Coffee Company Inc, founded in 1988.
This method does not require the addition of chemicals, instead using an activated charcoal filter. After the beans are boiled in hot water, the water is passed through the filter, which captures the large caffeine molecules but lets other, smaller molecules through.
The flavourless coffee beans are useless at this point, so they are discarded. But the flavoured water is used to soak a fresh batch of coffee beans. As the water is already saturated with these flavours, the matching molecules won’t diffuse from the new coffee beans in the hot water—but the caffeine will. The flavour is retained in the beans while the caffeine is removed.
Whichever method is used, the decaffeination process is imperfect and the flavour will never be exactly the same as fully caffeinated coffee. It will also never be entirely decaffeinated, though the traces of caffeine left behind are usually negligible. Thanks to some scientific smarts and creativity, however, it’s possible to have decaf coffee that’s almost as good as the real thing.