The removal of caffeine is something most coffee drinkers would scoff at, however for someone that likes to drink the beverage for the taste, and not the caffeine, finding a good decaf is a never-ending journey.
But before I answer how come it’s so hard to find a good decaf, how does the coffee become decaffeinated in the first place?
It starts with dissolving the caffeine out by some solvent, or other compound.
This can be done is range of ways and is designed to minimise the amount of flavour compounds that are dissolved with the caffeine.
Firstly, this can be done by chemical solvents that selectively dissolve caffeine. The original solvent being Benzene, a known carcinogen, which we have gladly moved on from to methylene chloride, approved by the FDA, and ethyl acetate, which occurs naturally in trace amounts in ripening fruit. Even if some of these solvents remain in the beans, they are very volatile and so when the beans are exposed to heat in the roasting process and finally the brewing, it is very unlikely these solvents will still be in the beans and therefore in your cup of coffee.
Indirectly-dissolved
The beans are soaked in hot water, and then is water is added to the solvent which bonds to the caffeine, before the mixture is heated, and the solvent and caffeine is evaporated. The remaining water is then added back to the beans and so the oils and flavouring compounds are reabsorbed the beans.
Directly-dissolved
The beans are first steamed, to open their pores for better absorption of the solvent. Then they are rinsed repeatedly for 10 hours with the solvent to remove the caffeine, before being steamed again to evaporate the remaining solvent.
The beans can also be treated by other substances that act to dissolve the caffeine that aren’t classified as “chemicals”.
Swiss-Water Process:
First the beans are soaked in hot water to dissolve the caffeine, and this water in then passed through an activated charcoal filter, which allow smaller molecules like oils and flavour compounds to pass through and blocks larger caffeine molecules.
This first batch of beans is entirely caffeine free, but also flavourless, but you are left with the water that is saturated with dissolved flavour compounds.
Another batch of coffee beans is then able to be dissolved with the previous water. Since this water is saturated with flavouring compounds, no more can dissolve, but the caffeine in the new batch of beans is able to be dissolved, and then removed via the activated carbon filter.
Supercritical Carbon Dioxide Method:
This method is the mostly recently developed and utilises carbon dioxide to selectively dissolve the caffeine in the coffee beans.
Water soaked beans are placed into an extraction vessel, which is pressurised and sealed, to create the conditions for Co2 to be in its liquid phase and dissolve the caffeine. The CO2 is then transferred to the absorption chamber where the pressure is released, and CO2 returns to its gaseous state, and the caffeine is left behind. This allows the CO2 to be recycled and decaffeinate another batch of coffee beans.
Many problems that arise from these methods is the use of heat and loss in moisture content.
As we know that heat can denature proteins and break bonds in compounds, so heat treating the coffee beans, in addition to roasting and making the coffee, can compromises the flavour compounds in the coffee.
Decaffeinating the coffee beans also reduces their moisture content, and browns them, which means they roast faster, but also lack the visual indicator of how roasted the bean is as compared to caffeinated beans which are green before the roasting process.
These factors mean it is more likely your cup of decaf is over-roasted and has that burnt flavour that we all hate, as well as less favourable than your regular cup.
Referenced from https://coffeeconfidential.org/health/decaffeination/ and Chem 120 Section One Content: application of CO2 as a supercritical fluid.
Science Scholars 2019 