Understanding the difference between “good” and “bad” bitterness in coffee can quickly become complicated. We’ve already discussed causes of unpleasant bitterness including Robusta beans, coffee defects and dark roast. Now we will look beyond bitterness as a stand-alone flavour and look at the balance between bitterness and coffee acidity. (We will discuss sweetness next.) After all, it’s the balance of these three flavour components that truly defines the most delicious coffees.
When we talk about acidity in coffee, know that we’re not referring to the pH level of the drink. Instead, acidity is a measure of the flavours imparted by various acids present in coffee. Acidity levels in coffee are important both for their own flavour contributions and for their role in balancing out bitterness. Optimal acidity, in harmony with sweet and bitter notes, is one of the key factors that makes specialty coffee so superior to commodity coffee.
WHAT IS ACIDITY IN COFFEE BEANS?
Green, or unroasted coffee beans, have varying levels of different acids depending upon many factors. The varietal, growing conditions (climate, soil, elevation etc.) and processing techniques all influence the acidic components of coffee beans. Some acids lend coffee positive characteristics, such as brightness. Others, such as those causing astringency, are best avoided. In coffee lingo, though, acidity is generally considered a good thing.
Coffee growers seek to develop beans with “good” acidity. This means growing beans with high levels of the acids that lend flavours we perceive as bright, crisp or sharp. Those flavours are key to balancing out bitterness. In contrast, low levels of those acids result in coffee that tastes dull or flat – characteristics that tend to let bitterness become more prominent. Low acidity is common cause of unpleasant bitterness in commodity coffee.
ACIDITY AND ROASTING
Equally important in determining the acidity of coffee beans is roasting. More accurately, roasting is key to creating the right balance between acidity, sweetness, and bitterness. Some acids are diminished by roasting, whilst others are enhanced. Generally, most of the “good” acids in coffee diminish with longer roasting times. Therefore light roasts have more pronounced acidity whereas dark roasts lose almost all their acidity. We find that the optimal balance between acidity, caramelized sugar flavours and pleasant bitter notes is at the medium-roast level.
One notable acid – quinic acid – is enhanced by longer roasting times. Quinic acid is responsible for astringent, chemical-like flavours in coffee, so this is not considered a positive characteristic. An expert roaster is able to find the ideal level of roasting to bring out the best flavours associated with acidity and diminish the undesirable ones. Doing so also decreases a coffee’s perceived bitterness. More often than not, this “sweet spot” is achieved with a medium roast.