Prufrocker: The Blog


What makes coffee taste sweet, acidic, bitter, or sour?

by  |   |  Coffee Science

Collaborative Coffee tasting event @ Prufrock

Take a look at these score sheets:

Cup of Excellence Cupping Form
WBC Sensory Score Sheet

They all need the judges to say something about three things:


Divide taste into:


We’ll talk about salts in another post.

What makes coffee taste sweet?

There are more than 70 species of Coffea. The specialty coffee world is focused on Arabica. The specialty coffee world definitely looks for sweet varieties. The indigenous variety of Arabica coffee has been shown to be native to Ethiopia and is called Typica.

Typica is still widely cultivated across the tropics and prized for its high cup quality. Typica naturally mutated into Bourbon. This happened on Reunion island in the 19th centuary and was a big turning point for coffee farming. It was sweeter than Typica.

Being sweet causes problems. Insects love sugar as much as we do. Bourbon and Typica are naturally high in sugars and are susceptible to pests. Most plants need insects to help them pollinate but Arabica is self pollinating. There are fewer insects at higher altitudes and it is no surprise that the heirloom Typica varieties found in Ethiopia are grow above 1200m in a much more insect free zone.

Most of us are new to specialty coffee and are only just getting to grips with varieties because we have recently started drinking coffee. Think about your experience with foods like apples. You would say a Russet was sweeter than a Cox. Apples grow all over the world but you never mistake a Russet for a Granny smith, or a Pink lady for a Golden Delicious whether it grew in New Zealand or Shropshire. Varieties greatly affect flavour and researching them will greatly enhance your coffee experiences. We are good with distinguishing apple varieties because we grew up with them. Varieties greatly affect flavours in and researching them will enhance your coffee experiences.

What makes coffee taste acidic?

Acids in coffee are prized. Arabica coffee averages a pH of around 5. (7 is neutral, less than 7 is acidic.) Almost all drinks are acidic. Apple juice and orange juice are about 3pH. Acidity helps make foods and drinks interesting. It makes us feel they are fresh and palate cleansing and fruity and spicy. The thing with acidity is that we tend to enjoy it more if there is sweetness there in equal proportions to acidity. When this occurs we consider a drink to have balance. This is how you can interpret the Taste Balance category on the WBC Sensory score sheet.

Extraction levels affect acidity levels. During a part-tasting exercise the first sip of an espresso tasted in sections as it pours is very acidic. The second sip (lets say at 15 seconds over a 30 second 30g pour) can be shockingly sour. This suggests acids are easy to extract from coffee and come out faster than sugars and alkaloids. Under-extracted coffee might taste more acidic than properly extracted coffee. Malic acid’s melting point is 20degC so it is likely this is one of the first things to make it into the cup.

Altitude has a big impact on acidity. Slower growth seems to give a plant longer to store sugars in the seeds. Check out this article in Roast Magazine:

‘At higher altitudes we tend to produce coffees higher in perceived acidity. Such that for every 100 meters gained in altitude we can expect a 0.60° C drop in temperature, and for every 300 meters, a 10 percent increase sugar production, namely sucrose. What does all this mean? Higher acidity!’

Cross cultivation can improve plant hardiness and flavours. The Scott Labs scientists were trying to add a phosphoric acid tingle to their coffees. The Bourbon variety is wonderfully sweet but it is not the hardiest variety. Kenya grows two main varieties that are favourites with baristas. SL28 and SL34. SL28 is meant to be the better variety. It is sweet but it is also highly acidic by coffee standards. Perhaps the most acidic of all coffee varieties. It can vividly remind you of Red Currents and Citrus fruits. The variety is not a natural mutation like bourbon, but rather a hybrid. A mix of Yemani Typica and bourbon developed in the Scottish Laboratories (SL) between 1935 and 1939 in Kenya and Tanzania. SL28 and SL34 are both drought resistant and high yielding, they have genetics from Bourbon to bring sweetness to the cup and are more suitable to prevailing East African whether conditions with the Ethiopian Typica genetics from heirloom varietals that it was developed from.

Some coffees leave your mouth feeling just like you’ve eaten an apple. Apples contain malic acid.

This is one of seven principal acids found in coffee. There are 49 acids listen on the coffee research institute’s web page. Most roasters and resources make particular mention of Malic and another 6.

Chlorogenic acid is the most abundant. In Arabica it makes up about 7% of the dry weight of grinds. In Robusta it’s 10% on average.

‘There are basically two families of these acids; mono-caffeoyl and di-caffeoyl. While mono-caffeoyl acids readily decompose during roasting, those of the di-caffeoyl family remain almost unchanged and have been reported to impart a metallic-bitter taste. It’s no surprise then that robustas, which exhibit a similar metallic taste, contain a larger concentration of these acids than their arabica counterparts.’

What causes coffee to taste bitter?

Robusta is indigenous to Rwanda and constitutes 45% of the worlds coffee production. It is not self pollinating and grows naturally at altitudes between 0-1000m. Plenty of pests at this altitude but Robusta has about half the sugars as Arabica and twice the alkaloids.

Alkaloids are bitter tasting compounds, widely found in plants with many pharmacutical applications. These are part of a plants natuaral defence against pests. Caffeine is an alkaloid, as is Chlorogenic acid. Chlorogentic acid in roasting breaks down into Quinic acid. Think quinine found in tonic water.

Over Roasting causes higher concentrations of quinic acid in the beans. Prufrock looks for an optimum roasting balance between very light roasting that may leave a chalky sourness and over roasting that will make the coffee much more bitter.

What makes coffee taste sour?

Roasting has a role to play in optimising Citric Acid levels in the cup. Citric acid tastes sour. It is a positive sensation at lower concentrations. Medium roasts contain about half as much citric acid as green beans. Unripe coffee will have higher levels of citric acid. In a fruit, citric acid converts into sugar so if coffee cherries make it into the washing station unripe, you can expect sourness in the cup. The extreme of unripe is visably blonde in colour and we refer to these as quakers. They are sour and smell of peanut. Pick them out. Coffee scoring 85% and higher shouldn’t contain more than one quaker per kg.

If beans have fermented or been allowed to dry on the tree we encounter a much larger quantity of a familiar acid. Acetic acid is found in vinegar and is very sour. This is the fourth most concentrated acid usually found in brewed coffee.There are higher proportions of acetic acid found in washed coffees and very light roasts. It moderate proportions it can add a positive winey taste to coffee but it can certainly taste like ferment and in high concentrations is a fault. Roasters are tempted to roast darker to evaporate off this acid to reduce sourness. This certainly works. We are looking for coffees that through careful picking and processing have not develope an imbalance of acetic acid.

The other principal acid in coffee is phosphoric acid. It is tasteless but creates the tingle on the tongue that is prised in coffees like Kenyan SL28. This same acid is found in almost all fizzy soft drinks. More about this in Tactile.