The purpose of this post is to try and explain the difference between those two terms, as they are often confused.
Acidity is perhaps the most often confused term. This is because coffee roasters speaking of acidity actually mean a desirable attribute in a coffee's flavour. To a roaster acidity describes the pleasant brightness a coffee on the taster's tongue. Acidity is similar to the idea of carbonation in a soda. We would not want a flat coffee just as we would not want a flat soda. Similarly, we would not want too much brightness in a coffee; we would rather have something that makes the coffee lively without an excessive bite.
The average coffee drinker however, tends to equate the term acidity to an acidic feeling in the stomach opposed to something you taste. Instead of a desirable taste, acidity is associated with a sour feeling occurring after the coffee is consumed. The latter of these acids are a result of the organic compounds of coffee being overexposed to heat and water causing quinic acidulation. This can occur when a pot of coffee sits too long on a heated surface, allowing the brew to 'cook'. This is a common experience at countless restaurants with a very poor understanding of coffee, who serve a brew that has been sitting on a hot plate for long periods of time. Think filter coffee jugs... Airpots and thermal carafes do a better job of eliminating quinic acidulation for quality-conscious restaurateurs.
Quinic acids can also be produced during roasting in machines that have poor heat transference. Green coffee beans contain about 12% water by weight. This water must be dried off before the bean can be effectively roasted. Many poorly designed roasting machines are inefficient at driving off this moisture and they allow the still-wet beans to be exposed to high temperatures for lengthy periods. In a good quality roaster, water is driven off the beans within the first minutes of the roast, thereby sharply reducing quinic acids from the roasted coffee.
Bitterness is easier to describe perhaps because it is one of the four primary taste sensations that our tongue perceives. The other three: sweet, salty, and sour are also experienced in coffee. As with all these characteristics there are both desirable and undesirable elements to the same taste. A coffee with too much of a salty characteristic would be undesirable, while a coffee that has some saltiness is often described as a soft or neutral coffee and can be quite pleasing. One too sour causes us to pucker up while the right type of sour sensations can give a coffee a wine-like quality. The correct stimulation of the bitter taste buds creates a pleasant bite to the coffee and can be most desirable.
More often than not, excessive bitterness is a result of improper grinding and brewing of coffee. When there is a lingering bitter characteristic from coffee it is an indicator that the coffee was too finely ground or too much coffee was used in the filter basket. Overly fine grind or too much coffee does not allow the water to flow properly and makes for a bitter tasting brew.
Finally, the brew rate of a coffee maker has a direct effect on bitterness and acidity. An ideal brewing time is less than four minutes. Press pots, vacuum pots and pour-over filter carafes produce superior coffee since the water is heated in mass before brewing. Most electric home brewers don’t cut it because of inadequate water heating elements that lengthen the brew time.
Great coffee is a product. But it is also a process that begins when you buy fresh, expertly roasted coffee. What you end up drinking is, however a combination of the right storage, proper grinding for your brewing device, and the right dosing. Enjoy...