Phosphoric Acid |
All acids for the most part impart sour characteristics when ingested but vary in degrees. The actual sourness, or known as "perceived acidity" - depend on a number of factors including the acids pKa, anion species, buffering capacity, etc. and whether the acid is organic or inorganic. Some important organic acids found in coffee include citric, acetic, malic, etc. all present to some extent in both green and roasted coffee beans. But perhaps one of the most important acids is not organic by nature but rather inorganic - one such is phosphoric acid. The acid constitutes roughly 0.54%1 of coffee's dry matter and is believed to originate due to hydrolysis of phytic acid absored from the soil.
However, unlike some of the other acids - phosphoric is by far the most potent and can easily be 100 times stronger than other acids. As such, many researchers believe that phosphoric acid is significantly responsible for coffee's perceived acidity2.
Although the argument is well backed, there is some debate over how much phosphoric acid actually contributes to acidty. Those one side say that because of phosphoric acids potency - as seen by its low pKa - has a much greater potential at contributing hydrogens in solution and affecting acidity3. Those on the other side state that phospshoric acid, though potent, is neutralized by an equal amount of potassium in coffee4. Though a 1999 study found an inverse relationship between perceived acidity and phosphate concentration5. Further studies involving the role of other carboxylic acids needs to be conducted before any conclusions can clearly be made.
Interestingly higher levels of phosphate have been documented in both dry processed robustas and arabicas. Though wet processed arabicas do tend to contain the lowest phosphate concentrations, partly due to the leeching during the fermentation process4.
Its also been documented that residual amounts of phosphoric acid survive the roasting process with estimated figures listed in Table 1 below. The increase in phosphoric acid at higher roasting levels is still not understood and may be due to artifacts produced during the analytical process - further studies are needed.
Based on preliminary sensorial cupping studies, addition of dilute phosporic acid in coffee results in an increase in sweetness and general brightness of the beverage. Interestingly the same effect is not seen when organic acids are used.
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