Acidity, Bitterness and Sourness are often confused when describing coffee flavour.
We know that volatile compounds are responsible for aroma, whereas flavor, aftertaste, and body usually come from the carbohydrates (sugars), proteins, melanoidins (the large molecules that give roast coffee its brown color and texture), lipids (oils), etc.
When discussing coffee, the terms which keep cropping up are Acidity, Bitterness, Sourness, Astringency and Sweetness.
Acidity is measured on a pH scale of
0 for the Strongest Acid to 14 for the strongest Base/Alkali (least acidic)
Perceived acidity of coffee results from protons being donated to receptors on the tongue.
Acidity (as opposed to bitterness) is a valued quality of coffee
Acidity is a revered but contentious attribute in specialty coffee. Loved by third wave consumers and prized by competition judges, but it is also a cause of confusion.
Acidity isn’t easy to define mainly because it takes so many different forms. A coffee which is more acidic affects the flavor and the aroma, and takes on the characteristics of stone fruits such as sweet nectarines or juicy apples. You can define acidity by the "sharpness" the coffee leaves in one’s mouth. No sharpness: no acidity ... or very low acidity.
Without acidity coffee will taste ‘flat.’
According to cofferesarch.org, some 43 acids can be found in coffee.
Of these, 13 are considered volatile meaning those acids can be converted into a gaseous form so they evaporate off over time giving coffee much of its aroma; 18 are considered non-volatile and these remain stable within the coffee.
We also know that volatile compounds are responsible for aroma, whereas flavor, aftertaste, and body usually come from the carbohydrates (sugars), proteins, melanoidins (the large molecules that give roast coffee its brown color and texture), lipids (oils), etc.
High acidity found in coffee beans is associated to:
beans grown at high altitudes
beans grown in mineral-rich soils (e.g. volcanic soil)
washed coffees often have higher acidity than dry processed beans.
Brewed coffee acidity is dependent upon a variety of things such as the darkness of the coffee roast, the type of roaster(roast method), the roast "profile" and the brewing or extraction method. Even the storage of the beans pre-brewing will affect the acidity of the cup.
Acidity can be measured by the variation in the composition of organic acids such as citric, malic, lactic, tartaric, to name the main ones, and large molecules called polyphenols, in particular chlorogenic acids (CGAs) and tannins which are the primary sources of astringency in brewed coffee.
The majority of astringent polyphenols found in brewed coffee occur when channeling occurs during extraction (in both espresso and percolation brewing)
The best coffees in the world still have a hint of bitterness! This can come across as flavors of dark chocolate, spice, or woodiness!
These lower flavour notes are actually good as they can round-out the flavor and bring a balance to the sharper acids to provide a more pleasant overall flavour in a full mug of brew.
Bean Origin and variety affect the acidity.
Malic acid is more prevalent in Kenyan coffees, while Citric acid is more common in Colombian coffees, so this means more apple-like notes from Kenya and more citrus fruit notes from Colombia.
Arabica species, tend to have less chlorogenic acids which decreases its perceived acidity.
Climate and elevation also affect acidity.
More coveted coffee beans are typically grown at higher elevations. Coffee grown at cooler temperatures in the higher altitudes tends to ripen slower, which encourages development of more complex flavors. When brewed, these tend to be more acidic and aromatic than coffees lower down in warmer climates.
Sourness is an extreme form of acidity and is considered a coffee defect.
A sour flavour hits you quickly and aggressively and creates an immediate reaction like puckering of the lips or it might feel sharp on the sides of the tongue. Sourness is undesirable and distracting.
Bitterness
When we talk about bitterness, we mean "in-your-face" bitterness.
It doesn't add anything to the depth or complexity of other flavors and frequently overshadows those other flavours.
As noted above, this kind of bitterness is caused by extracting bitter chemicals from the grounds. These chemicals, other than caffeine, typically extract after everything else and this is why over extracted coffee is often unpleasantly bitter.
Coffees which are roasted very dark also tend to have more of these bitter chemicals from the outset, so, beans from a roasting company who favour really dark roasts may make coffee taste over extracted!
Over extracted bitterness can sometimes be so overpowering that it completely kills the other flavors, leaving the brew to seem lifeless, dull, and bland.
Under extracted coffee can also have a thin, dull flavor, but in that case, an intense sourness will be present because flavors haven’t been sufficiently pulled out of the grounds to balance the overall taste.
On the other hand, Sweetness is one of the universally appreciated characteristics of a good cup of coffee. Whether its a bright, acid-forward, light roast or darker, full-bodied coffees, developing and highlighting sweetness is key to achieving a balanced, enjoyable brew.
When it comes to sweet flavors in coffee, terminology generally speaks of the sugar-browning category: nutty, caramelly, and chocolatey.
Proteins and carbohydrates in green coffee break down during the roast process into caramelized sugars and amino acid complexes. These are responsible for giving coffee its sweetness.
Sweetness is an essential part of coffees' flavour balance and roundness.
High altitude coffee cherries which ripen more slowly, generally develop more sugars.
Honey processed beans typically result in sweeter coffees than washed processing.
Most mature, healthy, and defect-free Arabica beans have some degree of sweetness.
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