Chemistry and coffee

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rpavlis

#1: Post by rpavlis »

Several years ago when I was in charge of a polymer chemistry laboratory I had the class perform a somewhat lengthy experiment that involved using mercaptans to regulate polymer molecular weight. Mercaptans are sulphur analogues of alcohols. They are notorious for having bad smells, usually like skunks, burning tyres, or rotten vegetables. I decided that a good mercaptan to use would be 2-furfurylthiol. I chose this because it is famous for being one of the primary odours of coffee. The image below shows the molecule,




Red represents oxygen, cyan carbon, white hydrogen, and yellow sulphur.

It is interesting that when traces of the compound escaped from the fume hoods the laboratory was filled with an intense odour of coffee! However when more than traces of it got into the air the smell became increasingly like skunks with hints of burning tyres!

Coffee just is not coffee without extremely small amounts of this compound! It really is the definitive essence of coffee--but NOT too much of it. If I be not mistaken, roasted coffee beans have about 1 part per million of this. One gram per ton!

There are at least two other compounds of sulphur with strong odours in coffee. One is 3-mercapto-3-methylbutyl formate. This is the formate ester of the primary odour compound in cat urine. Compounds related to this also occur in some wines. Chemical analysis of roasted coffee beans indicates that it is present to perhaps 0.1 ppm.

Another is 3-methybut-2-ene-1-thiol. There are only a few parts per billion (1e9) of this compound in coffee. I think that fortunate. I have used related compounds in the laboratory, they smelled strongly like a flock of skunks.

Mercaptans (or thiols as they are also called) are weak acids, between acids like acetic acid and ethanol in acid strength. They are generally fairly insoluble in neutral water, but because of their acidity they are soluble in strong base, and their solubility is also increased, especially at high temperature, by weak bases like carbonate and bicarbonate. (However note that solubility of neutral organic compounds in water is decreased by ionic impurities.)

I recently did an espresso machine experiment. Many years ago a pharmacist gave me an old prescription balance with the old apothecary weights--scruples, drams, etc. I could not find my standard weights so I tried to mimic the amount of bicarbonate in moderately hard water by weighing out ℈s of sodium carbonate (one half scruple, or about 650mg) and adding it to 4 litres of distilled water.

One would expect enhanced extraction of the three mercaptans mentioned above. Because sodium and bicarbonate are both singly charged ions and their concentration is low other compounds in the coffee beans should show little solubility change, except for other acids which should also dissolve more, and perhaps weakly soluble weak bases which should dissolve somewhat less.

In preliminary experiments coffee made with the bicarbonate containing water seem to have a noticibly stonger "coffee" taste, and they also seem to leave a stronger after taste. Some morning I am going to start both of my La Pavonis and make one cup from pure water, and another with bicarbonate containing water. I will mark the bottom of the cups, shuffle them around, and see if I can identify each cup.

(Coffee beans contains massive amounts of potassium, it also contains quite a bit of Mg, some Ca, and traces of Na.)

2StrokeBloke

#2: Post by 2StrokeBloke »

This post is why I love this forum.
It's not just the nuts and bolts discussions of various machines and techniques for extracting espresso.
It's the 'above my head' science that members bring to the table where you learn so much more about what we're playing with.
I think my wife would be surprised to know just how much science there is to coffee!

chang00

#3: Post by chang00 »

Excellent post!

In a previous experiment with espresso, there was difference in calcium vs sodium containing water. The coffee cakes were examined with microscopy; brewed with sodium containing water, the fibers swell more and therefore the taste difference, compared with calcium.

If you don't mind, can you add another set with calcium carbonate instead?

A Mypressi will probably also work for this. The difficult part is always to have double blinding and randomization in the home setting.

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catweasel

#4: Post by catweasel »

Interesting post!
Glad to see others taking their work home with them too.
I agree that thiols are notoriously nasty and have been exposed to more than my share. I have unfortunately not experienced any that smell like coffee (not coming from my cup that is). Perhaps the concentrations in my case were simply too high or just the wrong chemicals. We do have a wonderful on-board detector capable of detecting 0.001 ppm in some cases. Do you really think that amount of bicarbonate will influence the thiol extraction?

//Kurt

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TomC
Team HB

#5: Post by TomC »

chang00 wrote:Excellent post!

In a previous experiment with espresso, there was difference in calcium vs sodium containing water. The coffee cakes were examined with microscopy; brewed with sodium containing water, the fibers swell more and therefore the taste difference, compared with calcium.

If you don't mind, can you add another set with calcium carbonate instead?

A Mypressi will probably also work for this. The difficult part is always to have double blinding and randomization in the home setting.

+1

I played around with pickle crisp which is calcium chloride, but it would be neat to play with calcium carbonate too.

I found the calcium chloride, in almost any minuscule concentration, really made for stronger, but soapy and more bitter tasting brews.

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yakster
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#6: Post by yakster »

Very interesting post, I've been trying to understand water's impact on coffee lately, and to that end I enjoy these posts. I'm also looking forward to an eBook on coffee that Colonna And Smalls is working on.

It's interesting that an increase to the mercaptans leads to skunky, burning tires smells, something associated with Robusta coffees.
-Chris

LMWDP # 272

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rpavlis

#7: Post by rpavlis »

I did not include the image showing the structure of the compound that is a derivative of the compound in cat urine. The colours indicate the same atoms as in the original post:



(These images were generated by the openbabel program, and displayed with the University of Illinois Department of Biochemistry program "vmd.")

It would be very interesting to experiment with different concentrations of bicarbonate and various cations that occur in ground water. The quantity of potassium already present in coffee is extremely high. Potassium is very very much required for life. We have about 2 grams of it per kg of body weight. It is also radioactive, the third most radioactive element with stable isotopes. Those 150 or so grams of K we have within our bodies results in about 4500 decays per second! Dry coffee contains much more K than we do, over 10 grams/kg. It is very effectively extracted into prepared coffee so that the primary cation in coffee is potassium, even when made with the hardest water.

happycat

#8: Post by happycat »

yakster wrote:Very interesting post, I've been trying to understand water's impact on coffee lately, and to that end I enjoy these posts. I'm also looking forward to an eBook on coffee that Colonna And Smalls is working on.

It's interesting that an increase to the mercaptans leads to skunky, burning tires smells, something associated with Robusta coffees.
...so would changing the brewing water improve the flavour?
LMWDP #603

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yakster
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#9: Post by yakster »

That's the real question, something that Colonna And Smalls are trying to answer. I have a hard time causally determining a change in taste with pure water and pure water mixed with filtered hard tap water, but I haven't tried blind tasting under controlled conditions.

The flip side is that some roasters tailor their roast to suit their local water, and their roasts may not taste quite the same using other water, similarly to how Londinium Espresso changed their roast profile to better suite using a lever.
-Chris

LMWDP # 272

MWJB

#10: Post by MWJB »

My tap water is OK for quick, drip brewing, but foul for steeps of any length...it's far from a subtle difference. It's not the water in my area, just my block as my work's (1/2 mile from home) tap water is fine for steeping in a French press, as is my parent's tap water (2 miles away). Add to that 1 of my kettles noticably taints the brew water if allowed to boil. If I use the 2 good kettles & bottled water I can bring to boil and steep as much as I need.

Water makes up 90-99% of your cup. It makes a difference.