pH Land, Suddenly Strange and Stranger
Outside the Scuola di Espresso, the smell of freshly finished and brick-oven browning baguettes mingles with the smokey, golden morning. A vineyard across the flinty valley is rusty-green. And I am wondering if I can expose my personal confusion about: WHAT IN THE H. . . IS pH, by quizzing il Professore. He is not a morning type guy, ya know, but I stumble onward.
"So, you are asking a simple question", starts the Professore over his
latest, another adventure-plus blend he is working on, "for which there
is nothing short of the most complex answer."
I take from him the cup; it is brilliant acid, designed for the morning shot, then cream to caramel, very rich with butter and nut aftertaste--fleeting but intense.
"This one is to point the way to an acid front end that complicates itself with the smoothness of elements, offering texture and roasted tastes, in this case a blend of Gimme's Iskander, surreal darkness of Sumatra, a middle of Rwanda, the fondly smooth Epiphanie, and then the front opening note of Costa Rica, a charmingly bright-eyed La Union. Now is the time to turn to the question of acid, of pH.
"You see, pH is commonly understood as the relative measure of the strength of dissolved acids in a solution. But the lack of comprehension starts with the scale itself, for on one end, say theoretical 0, or maximum-total, is acidic, declining up the scale to, for instance 4, as mildly acid, to 7, which is then neither acid or alkaline (salty), the other side of the scale. Now understand that acid increases and the numbers go from 7 down!
"Why this reverse number scale, you ask me. I can say that acids dissolved in water (in coffee, in wine) tend to separate in two: a hydrogen (H+) ion and a negative, an anion (A-), this one being different for each acid. The H+'s are 'dissociated' ions, those capable of transmitting an electrical charge, so measurable. The pH is the measure, say power for 'p', of this activity. Since you asked, here is more complexity: '. . . it is the logarithm of the reciprocal of the hydronium ion concentration. Water is very weakly dissociated, about 1 in 10,000,000 atoms are dissociated. This can be written as a concentration of 10 to the -7 power. The reciprocal of the logarithm of that number is 7. A pH of 6 would be 10 times as acidic as pH 7; pH 5 would be 100 times as acidic as pH 7. Grape wines will have pH levels between 3 and 4; hence are 1,000 to 10,000 times as acidic as water'. I'm referring to 'The Complete Handbook of Winemaking', by the American Wine Society, at 88.
"Therefore, like measuring the earthquake, a step difference is a large difference. But the question is how we perceive it?
The pH of coffee is said to be in the range of 4 to 5. Earlier, in Part II of 'PULLING SHOTS, Chemistry', I mentioned during roasting there is a drop in pH from 5.7-6.0 to 4.9-5.5. In general, arabicas have a pH of 4.85-5.15 in the brew and robustas have a range of 5.25-5.40. Citric acid is the highest contributor to perceived acidity; chlorogenic and quinic acids are less so.
"From our test blend, we found balanced acid, probably still within this range; but, if we change one element of the blend with a thoughtful choice, a powerhouse, such as the Hartmann Honey of Panama, we are struck down with an acid slam at the first sip; the acid is heightened by the combination.
"But here is the confusion of it: the pH is likely the same for each. Our perception is, though, different. And why, you say?
"We answer by looking at wine, but briefly now: interesting to see that there is a composite of acidity in solution so that in the case of wine the pH part is only 1% of all acidity, only 1% dissociated molecules. The rest, in the range of .6 to .85% of the whole bottle, is weak acids, lacking pH activity. This part is like vinegar, ascetic acid, weakly acid, but you know you can taste it for sure.
"Surprisingly and to add confusion, the composite is what you taste. But it's the weak acids that dominate our taste perception. The complexity is that our response to the H+ ions is 10 times stronger than to the A- weaks, but the weak ions exceed the others by about 100 times. It may be similar with coffee, though I haven't a reference guide for you, except this comparison with wine.
"Our perception of acids is a blend, after all. Of course, acid is necessary. Without acidity, the cup is plain, bland; without acid, wine is flat, it will not age. It will deteriorate from clear, ruby red color to frothy brown; it will re-ferment from latent bacteria to awful sulfur-sweet. Enough of this, yes?"
I sip another beaut shot, with a bite at the front of "dissociated" and "weak" acids, then to the texture, to nutty finish, as il Professore pulls another and concludes: ". . , and pH measures a part of the complexity of the cup and is itself a complex piece of chemistry, but overall we are perceiving a range of subtle and strong acids, like a classy red, chateau Bordeaux, is it not beautiful?"
Oh, for sure, strange and stranger, and I turn, saying ciao to il Professore, to head out of pH land.
I take from him the cup; it is brilliant acid, designed for the morning shot, then cream to caramel, very rich with butter and nut aftertaste--fleeting but intense.
"This one is to point the way to an acid front end that complicates itself with the smoothness of elements, offering texture and roasted tastes, in this case a blend of Gimme's Iskander, surreal darkness of Sumatra, a middle of Rwanda, the fondly smooth Epiphanie, and then the front opening note of Costa Rica, a charmingly bright-eyed La Union. Now is the time to turn to the question of acid, of pH.
"You see, pH is commonly understood as the relative measure of the strength of dissolved acids in a solution. But the lack of comprehension starts with the scale itself, for on one end, say theoretical 0, or maximum-total, is acidic, declining up the scale to, for instance 4, as mildly acid, to 7, which is then neither acid or alkaline (salty), the other side of the scale. Now understand that acid increases and the numbers go from 7 down!
"Why this reverse number scale, you ask me. I can say that acids dissolved in water (in coffee, in wine) tend to separate in two: a hydrogen (H+) ion and a negative, an anion (A-), this one being different for each acid. The H+'s are 'dissociated' ions, those capable of transmitting an electrical charge, so measurable. The pH is the measure, say power for 'p', of this activity. Since you asked, here is more complexity: '. . . it is the logarithm of the reciprocal of the hydronium ion concentration. Water is very weakly dissociated, about 1 in 10,000,000 atoms are dissociated. This can be written as a concentration of 10 to the -7 power. The reciprocal of the logarithm of that number is 7. A pH of 6 would be 10 times as acidic as pH 7; pH 5 would be 100 times as acidic as pH 7. Grape wines will have pH levels between 3 and 4; hence are 1,000 to 10,000 times as acidic as water'. I'm referring to 'The Complete Handbook of Winemaking', by the American Wine Society, at 88.
"Therefore, like measuring the earthquake, a step difference is a large difference. But the question is how we perceive it?
The pH of coffee is said to be in the range of 4 to 5. Earlier, in Part II of 'PULLING SHOTS, Chemistry', I mentioned during roasting there is a drop in pH from 5.7-6.0 to 4.9-5.5. In general, arabicas have a pH of 4.85-5.15 in the brew and robustas have a range of 5.25-5.40. Citric acid is the highest contributor to perceived acidity; chlorogenic and quinic acids are less so.
"From our test blend, we found balanced acid, probably still within this range; but, if we change one element of the blend with a thoughtful choice, a powerhouse, such as the Hartmann Honey of Panama, we are struck down with an acid slam at the first sip; the acid is heightened by the combination.
"But here is the confusion of it: the pH is likely the same for each. Our perception is, though, different. And why, you say?
"We answer by looking at wine, but briefly now: interesting to see that there is a composite of acidity in solution so that in the case of wine the pH part is only 1% of all acidity, only 1% dissociated molecules. The rest, in the range of .6 to .85% of the whole bottle, is weak acids, lacking pH activity. This part is like vinegar, ascetic acid, weakly acid, but you know you can taste it for sure.
"Surprisingly and to add confusion, the composite is what you taste. But it's the weak acids that dominate our taste perception. The complexity is that our response to the H+ ions is 10 times stronger than to the A- weaks, but the weak ions exceed the others by about 100 times. It may be similar with coffee, though I haven't a reference guide for you, except this comparison with wine.
"Our perception of acids is a blend, after all. Of course, acid is necessary. Without acidity, the cup is plain, bland; without acid, wine is flat, it will not age. It will deteriorate from clear, ruby red color to frothy brown; it will re-ferment from latent bacteria to awful sulfur-sweet. Enough of this, yes?"
I sip another beaut shot, with a bite at the front of "dissociated" and "weak" acids, then to the texture, to nutty finish, as il Professore pulls another and concludes: ". . , and pH measures a part of the complexity of the cup and is itself a complex piece of chemistry, but overall we are perceiving a range of subtle and strong acids, like a classy red, chateau Bordeaux, is it not beautiful?"
Oh, for sure, strange and stranger, and I turn, saying ciao to il Professore, to head out of pH land.
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JK Jun 20, 2009 – 8:52 AM
Professore, would it be a turn-on for sci-geeks and armchair chemists if Gimme printed the pH of each coffee right on the label?
Bunn Coffee Aug 17, 2010 – 1:10 AM
I can see that you are an expert at your field! I am launching a website soon, and your information will be very useful for me.. Thanks for all your help and wishing you all the success.