The process of removing caffeine from coffee beans is mainly performed using supercritical fluid extraction (SFE), a technique that typically uses carbon dioxide as a solvent. Supercritical carbon dioxide (SC-CO2) has a density close to that of a liquid and a viscosity of a gas, with good diffusion properties and low surface tension, thus providing superior extraction capabilities. In the process of removing caffeine, SC-CO2 is highly selective and is able to selectively extract caffeine without excessively affecting other components in the coffee bean, thus preserving the original flavor of the coffee.
The specific steps of supercritical fluid extraction of caffeine are as follows:
1. Pre-treatment: The coffee beans are first softened, for example, by adding water or steam heating, so that the bean surface is swollen and the contact area is increased.
2. Supercritical fluid extraction: Under the conditions of high pressure (usually more than 72.8 atmospheres) and appropriate temperature (the supercritical temperature of carbon dioxide is about 31.1 ℃), supercritical carbon dioxide fluid is passed through the coffee beans, and caffeine in the supercritical state of solubility increases, and thus be extracted.
3. Separation: Supercritical fluid extraction from which the caffeine has been extracted enters the separation tower, where the caffeine is separated by depressurization or by the addition of other substances (e.g. water).
4. Recycling: The separated carbon dioxide can be removed from the high pressure and evaporated almost completely, and can be recycled.
The advantages of supercritical fluid extraction are that it does not require the use of harmful chemical solvents and can be operated at lower temperatures, avoiding damage to the heat-sensitive components of the coffee beans. In addition, this method is highly efficient in removing caffeine and has no solvent residue problems, making it a green technology.
It is worth noting that although this method removes most of the caffeine, 2-3% of caffeine usually remains, so decaffeinated coffee is not completely caffeine-free. In addition, decaffeinated coffee produced by this method is relatively rare in the market as the technology requires special high-pressure equipment and is relatively costly.