As of now, there are no arabica varieties in cultivation with caffeine content that meets international standards for what constitutes decaffeinated coffee. Thus, all decaf coffee comes from manually removing it from ordinary coffee. There are four commonly used solvents for doing this: methylene chloride, ethyl acetate, carbon dioxide, and water.
No matter which solvent is used, the beginning of the process is the same. Green coffee beans are steamed or soaked in water to make the caffeine more available to the solvents and to make it easier for the solvents to penetrate the beans. From here, two main pathways exist: direct solvent extraction or indirect extraction.
In direct extraction, where methylene chloride and ethyl acetate are used, the wet green beans are treated directly with the solvent for some eight to twelve hours. Then, the solvent is removed and the beans are steamed (to help drive off any remaining solvent) and dried before roasting. Unfortunately, these solvents don’t extract just caffeine. Thus, other compounds, which may be related to quality, may also be extracted. This is one reason why decaf has a historically bad reputation for quality (the other reason is that low quality coffees were often used: junk in, junk out).
Carbon dioxide is a terrible solvent for caffeine under normal conditions as the solubility of caffeine in it is low. This is not surprising, as carbon dioxide is a gas at room temperature! However, if carbon dioxide is taken to its supercritical state—where it has liquid and gaslike properties simultaneously—it improves, and if a bit of water is added, it becomes much better. To take carbon dioxide to its supercritical point requires special equipment to significantly increase temperature and pressure. The great benefit is that supercritical carbon dioxide seems to selectively extract caffeine and not much else.
The indirect method allows for water to be the only solvent in direct contact with the beans. Water can be used to extract the caffeine and other compounds and then the water solution is treated with a solvent or passed through a filter to remove the caffeine, pulling it away from the beans. The other compounds can then be returned to the coffee beans before drying them down.
When water is the only solvent used, a clever trick is employed to prevent compounds other than caffeine from being removed. The process begins with soaking the wet green beans with water and then removing the caffeine from the solution, as in the indirect method. Then, the beans are discarded! The solution, sans caffeine but with the other stuff, is then the solvent used to extract the caffeine from the next batch of coffee. Doing it this way means very little noncaffeine material is extracted by the solvent. Now, nothing has to be returned to the coffee and it is believed that the end result tastes better.
There will always be a place for decaf coffee, as there will always be someone who loves the taste of the coffee at all hours of the day but doesn’t want to deal with the physiological effects of the caffeine. Modern decaffeinated coffees can have excellent quality. Like all technology, the methods for removing caffeine are continuously improving. Thus, expect the quality to improve even more.
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