“Cold brew is less acidic, so it’s still drinkable with an upset stomach.”
“The pH of the espresso is lower than a drip, so the espresso is more acidic.”
“This coffee is more acidic, so that it will be more acidic.”
Do you find these comments familiar? This is a valid but inaccurate interpretation because the acids in coffee, acidity, pH, and the perceived sourness of the tongue have a much more complex relationship.
Acid, sourness, titratable acidity, and pH are words you will encounter when learning about acids. Let’s get a deeper understanding of some of the most fundamental concepts with Helena coffee.
pH in coffee
pH ( potential of hydrogen ions) measures the activity index of free H+ ions in a solution.
Two factors affect pH: the strength of the acid (is the acid willing to ‘drop’ hydrogen ions out into the water?) and the consistency of the acid in a solution (the more acid in the solution, the better., the higher the number of hydrogen ions)
If the more and more active H+ ions are in a solution, then the answer is highly acidic and has a low pH.
The pH is calculated on a radix scale. A solution with pH 3 has ten times more hydrogen ions than a solution with pH 4 and 100 times more than a solution with pH 5.
Water has a pH of 7, which is a neutral solution. Detergent is highly alkaline with a pH of ~10. Stomach acid has a pH of 1, which is lower than lemons. What about coffee?
In most studies of pH in coffee, the average number is 5 . Based on 744 batches, Batali et al. (2021) measure coffee pH with a TDS of 0.8% to 1.6% for ‘light’ roasts from pH 4.9 to 4.85, ‘medium’ roasts from pH 5 to 4.95, and ‘dark’ roasts from pH 5.2 to 5.1
Rao & Fuller (2018) measured the pH in the cold brew in the range of pH 4.96 – 5.13, equivalent to hot medicine with pH 4.85 – 5.10.
Thus, regardless of the roast profile or with different brewing methods, the pH of the coffee does not change too much. Also, the pH of coffee is about the same as that of a banana—a conclusion that is perhaps quite shocking.
If the pH is relatively stable, what makes hot drip coffee taste so sourer than a cold brew? To answer this question, we need to know another concept…
Titratable acidity (TA)
TA (acid titration) is a concept to indicate how many bases (bases) are needed to neutralize acids in a solution. The most commonly used floor is NaOH.
In simple terms, if an acid increases the concentration of hydrogen ions in solution, a base decreases the concentration of these ions. The decrease in hydrogen concentration causes the pH to rise.
In the beer industry, TA refers to how many bases are needed to get the pH up to 8.2. If pH only calculates the number of free hydrogen ions (H+), TA calculates the total number of hydrogen ions (including free [H+] and hydrogen ions still in the ‘weak acid,’ like [COOH]) of the solution.
Usually, a solution with a low pH will have a high TA (= requires more bases to neutralize), but the two concepts are unrelated. Same pH, but two solutions can still have different TA and vice versa. Also, the TA scale is not radix-like pH. If the TA of solution A is twice as high as B’s, the drinker will usually find A twice as acidic as B.
TA is a prevalent concept in the beer and wine industry. pH plays a decisive role in predicting bacterial and yeast activity as grapes ferment. If the pH is not stable, many harmful bacteria can grow, affecting the taste and stability of the wine. However, for a wine to have a balanced sweet sour, TA is the number to know.
The latest research by Batali et al. (2021) reaffirms this in coffee. With TDS varying from 0.8% to 1.6%, TA increased by an average of 4 to 8 (ml NaOH/50ml coffee). The authors concluded that TA was related to whether the coffee was rated ‘sour’ or not, with the higher the TA, the ‘sour’ the coffee was. Meanwhile, pH cannot accurately predict the ‘sourness’ of coffee.
Acids and sourness
However, TA is not everything. Two solutions with equivalent TA but different acids will have distinct acidity and sour taste.
The White Labs Research Institute has compared the pH and TA of several Gose beers (sour beer) and concluded that pH and TA are not related. TA is the more critical parameter when predicting which beer has higher acidity.
However, there is an interesting detail if you look closely at this table. All beers with a sour taste higher than 2 contain acetic acid (the typical acid in vinegar). Beer D has the lowest pH but no acetic acid and a bad taste of only 1.95. Beer A does not have the highest TA but is considered the most acidic with all four acids.
So does the acid in coffee.
8 to 10% of green coffee is acids, including aliphatic acids (most notably citric, malic, lactic, malic, ascorbic, succinic, fumaric), phenolic (as caffeic acid), salicylic (as quinic acid), chlorogenic acids (CGA), and inorganic acids (= acids that do not contain carbon, such as phosphoric acid ).
Each type of acid has acidity, aftertaste, bitterness, acrid or salty taste. Depending on the variety, geographical or climatic conditions, soil quality, etc., each type of coffee has different amounts of acid. Coffea arabica generally has higher malic, citric, and quinic acids than robusta. Robusta has a much higher amount of CGA than arabica. This is one of the reasons for the entirely different tastes between these two varieties.
Roasting produces new acids (part of the malic acid is converted to maleic and fumaric acids ) or causes the acid to disappear. Sugars, the main sucrose, can also be converted to new acids. CGA can be converted into novel scent complexes such as guaiacol or phenol.
Acetic acid is more potent than tartaric and citric acid. Malic acid and lactic acid are not as sour but have a more prolonged aftertaste. Lactic acid has a certain ‘butter’ level, while phosphoric acid has a grapefruit note.
While most aliphatic acids only affect the taste, volatile acids such as acetic acid should contribute to the aroma of the coffee. Butyric acid smells like sour milk or cheese. Because smell dramatically affects the perception of taste, a cup of coffee with many volatile acids is more easily considered acidic.
Acid does not only affect the perception of sour taste. In coffee, quinic, formic, or caffeic acids can be bitter or astringent. CGA can be heartbreaking, acrid, or even metallic. Succinic acid can be both salty and bitter. Therefore, a cup of coffee with more acid is not necessarily sourer. The bitter and acrid taste from caffeic or CGA can completely overwhelm the pleasant sourness from citric or malic acid.
As I mentioned, sugar is not synonymous with sweetness. Similarly, a low pH does not mean that the coffee is more acidic; more acid does not mean that the coffee has high acidity or that the more ‘sour’ the coffee is, the larger the TA of that cup.
Knowing about pH, acid, and acidity also helps you answer some more intimate questions like “should I drink cold brew instead of drip?” (Not really, because cold pH brew is no different from other types of coffee. You need to pay attention to the type of acid in coffee, especially caffeic acid, to choose the proper coffee)
There are still some concepts this article has not mentioned, such as Total acidity, Buffer capacity, or weak acid vs. strong acid (some complex ideas but affect how the human tongue perceives coffee). However, I hope this article has helped you understand more and distinguish between acidity, acid, and pH in coffee.