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Decaffeinated coffee

Coffee beans have a caffeine content of 1 to 4%. With the process of decaffeinating 97% of the caffeine can be removed. To obtain decaffeinated coffee (by ground as well as instant coffee) the caffeine is removed from the whole bean.

Coffee beans can be decaffeinated in three ways:

• with help of extraction solvents
• with help of water
• with help of liquid supercritical carbon dioxide (CO₂)

With help of extraction solvents

Caffeine can be removed from the coffee bean with help of some organic extraction solvents commonly. Dichloride methane (DCM) and ethyl acetate are used. In dichloride methane the solubility of caffeine is 8 gram/100 ml at 20ºC. In ethyl acetate the solubility of caffeine is 2 gram/100 ml at 20ºC. There are more solvents used as an extraction solvent, but they are less efficient or safe. The two mentioned extraction solvents are chemical inert and do not extract other components from the coffee than caffeine. They also need to have a low boiling temperature. This is important for releasing the caffeine later.

Decaffeinating coffee beans with extraction solvents takes four steps. The first step is steaming or damping the coffee bean. The water content increases to 20-40%. The surface and the cell structure of the bean open and become permeable to crystalline caffeine.

The second step is the extraction of caffeine. The steamed coffee beans are mixed with the extraction solvent in a bath, in counter current flow. The caffeine dissolves in the extraction solvent. The decaffeinated coffee beans leave the bath.
In a vacuum evaporation unit the extraction solvent with caffeine is brought to the boiling temperature. This boiling temperature is lowered to 70ºC by vacuum. The extraction solvent evaporates and the caffeine stays behind. After condensation the extraction solvent can be used again. The caffeine can be purified, by recrystallisation, and processed further for applications in the food and pharmaceutical industry.

The coffee beans are rinsed with water to remove the extraction solvent. The maximum allowed amount of dichloride methane in coffee beans is 2 mg/kg. In an optimal process the amount can be reduced to 0,3 mg/kg.

The third step is drying the decaffeinated coffee bean. Drying of the coffee bean is done by blowing hot dry air over and through the beans. This can be done for example in a centrifugal drum under vacuum or on a conveyor belt at atmospheric pressure. The decaffeinated coffee beans are now ready for further processing in the traditional process for ground coffee or instant coffee.

With help of water

Decaffeinating of the coffee beans can also be done with water. The solubility of caffeine in water is poor at a temperature of 20ºC, but at 80ºC the solubility is 35-40 gram/100 ml. Disadvantage is that other components also dissolve in the water as well. The product loss is reasonably high.

The coffee beans do not have to be prehumidified, because the process takes place in water already. The coffee beans are transported in a bath of hot water. In several compartments the caffeine is removed from the coffee beans. The process takes 8 hours.

After this the water is being evaporated on a belt dryer or in a vacuum evaporation unit. The obtained caffeine can be purified and processed further. The removal of caffeine from water can be simplified with the help of active carbon and saccharose. However the removed caffeine can not be reused.

The decaffeinated coffee beans do not have to be rinsed with water, because the whole treatment is done in water. The beans can be dried in the same way as in the extraction process with hot air.

Supercritical extraction with carbon dioxide (CO₂)

Supercritical liquid carbon dioxide is obtained by making pure carbon dioxide liquid under a pressure of 200 bars. The liquid carbon dioxide has now the fluidising properties equal to water. Supercritical carbon dioxide only removes the crystalline caffeine from the coffee beans.

First the coffee beans are humidified to a water content of 20-25%, and transported to a pressure tank. The pressure in this tank can be up to 350 bars. The supercritical liquid carbon dioxide is added. The extraction takes place in 8 to 12 hours at room temperature.

After the extraction the caffeine has to be released from the carbon dioxide. There are three methods available:

• First method is release with the help of active carbon. The carbon dioxide is transported over a bed of active carbon. The active carbon absorbs the caffeine. Disadvantage is that the caffeine can not be reused.
• Sublimation of the caffeine is the second method that separates the caffeine from the carbon dioxide. For this the carbon dioxide with the caffeine is heated in a fluid bed oven. At a temperature of 360ºC the caffeine sublimates immediately into gas. The gas is captured with water in a washing tower. This caffeine can be processed further.
• Third method is letting of the pressure. The carbon dioxide evaporates by releasing the pressure and caffeine is left behind. The carbon dioxide can be captured and put under pressure to be used again. This method is simple principle, but due to the high pressure differences it is not low in energy.
  

Last part of the process is drying of the coffee beans. The beans do not have to be rinsed with water, because the carbon dioxide simply evaporates during drying.

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