First of all, not all coffee is bitter coffee, but there are three places before the cup is placed to the mouth that it can rear its ugly head... in roasting, brewing and the finished brew. Bitterness is a good thing to have in coffee as long as it is kept in balance with other tastes. When it overpowers everything, it is sensed on the back of the tongue (circumvallate papillae).
Coffee is complex in that there are over 400 organic and inorganic ingredients (some place it now at 800 and higher) present in at least trace amounts. All Arabica coffee beans have components present for sweet, salty, bitter and sour tastes. During the roasting process, chemical reactions take place to form many more .
That is the reason roasting is so important in the process. The balanced taste that a roaster tries to achieve comes in the form of timing the roast, achieving a certain temperature rise in that time and then ending the roast where the desired aroma/taste is attained. The intensity of the compounds associated with all four tastes will increase and diminish at different times with the constant chemical reactions. It is the roaster's responsibility to end at a time when the unique aromas/tastes with a specific bean are at the forefront; before the sourness and bitterness begin to overpower.
Generally, in roasting, the lighter roasts are the sweetest because the sugars have not caramelized As a roast progresses, the sugars caramelize, then fatty oils begin to surface on the beans and, if the roast is left to continue, everything eventually turns to carbon... just before catching fire. Bitterness is more noticeable in the latter stages (darker roasts).
In brewing, the grind needs to match the brewing method. Soluble compounds are extracted in the brew cycle and, if there is over-extraction, the evidence will be bitter coffee.
In a French Coffee Press, a coarse grind is required for proper extraction in the 3 minute time allotted. If the grind changes, the extraction time and taste also change. If the grind is correct, a bitterness may still occur if the water is left in the grounds too long.
In a Pour-over brewer, if a medium or drip grind is used, there should be no issues with bitterness.
In a drip coffee maker, a medium grind is needed to obtain the correct extraction. A fine grind or a slow flow over the grounds will cause over-extraction and thus, bitter coffee.
Espresso requires a fine grind because the coffee is compressed and water is pressured through the grounds quickly.
After the coffee is brewed in a drip maker, percolator or other heating device, it usually sits in a decanter on a warming plate for extended availability. Heat constantly applied to brewed coffee keeps chemical reactions active, so a bitter coffee taste is more apparent within 30-40 minutes, which turns into stale coffee later.
It is best, if the availability of hot coffee is needed for a good while after brewing, to keep it in an airpot or thermos; the coffee will stay hot keeping the freshness for a longer period.
Some of the chemical compounds already present or formed in coffee roasting associated with bitterness are:
The amount is not such that bitterness would overpower the sweet, salty or sour tastes on their own, but the chemical reactions taking place during the roasting process enhances some aromas while diminishing others. As long as the roast is progressing, aromas/fragrances keep changing as well.
Bitterness is thought to tame sourness in coffee. Sweetness also helps to overcome bitterness...
Maybe that is why so many people put sugar in the canned store coffee...(just a thought).
Acid and salt also help to overcome bitterness.
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