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I usually hot brew coffee as it's the most convenient method. I occasionally cold brew coffee for a less bitter coffee, which I prefer, but is a bit of a pain in the a***.

I'm assuming the bitterness is caused by the breakdown of a protein in the coffee bean at higher temperatures. Are there any coffee experts or chemists out there that know what's going on here? I'm wondering if I can use a thermal circulator and water bath to minimise brewing time and bitterness while maximising flavour and caffeine extraction from the beans.

Has anyone tried this or does anyone know the science and theory behind it?

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  • 1
    Normally we don't migrate questions which are OK on the original site, but Seasoned Advice and Coffee.SE are exceptions, we pass the questions on coffee preparation (but not using coffee in cooking) to them, as they need the volume and are better prepared to answer anyway. – rumtscho Jul 20 '16 at 18:49
  • Keep in mind that caffeine itself is a bitter tasting compound. So the more you extract, the more bitter your brew. – Suspended User Jul 21 '16 at 17:29
  • Keep in mind that caffeine extracts very early and at very low temperatures. So, you don't really worry about extracting caffeine itself. But the flavor generally builds up with many of the others: lipids and hydrocarbons. Those are what you're looking for. The aromatics. They're hard to gather. And a note: what makes hot brew bitter is, generally, the breakdown of the chlogenic acid and the caffeic acid. – MTSan Jul 21 '16 at 21:00
  • While thinking about another post today, I realized that both yours and the other may have some commons. So, I read a few articles. This article mentions the Japanese brewing method which instantly cools down the hot brewed coffee and explains some of the author's experience on dissolved aromatics and flavor. – MTSan Jul 23 '16 at 15:43
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Well I'm a chemist, but I can't say I'm an expert at scientific coffee extraction; take what I've got to say with a grain of salt.

Whenever you are extracting something into water, the temperature plays a couple different roles. First, higher temperatures generally increase the solubility of most compounds. Higher temperature water generally means more of everything gets dissolved, good and bad.Using cold water doesn't entirely solve the problem, since the lower solubility can leave a lot of great flavors behind.

Higher temperature can cause the breakdown of proteins, but I would doubt this is a significant source.After all, the beans were already roasted at high temperature before you ever got to the brewing part and the majority of the breakdown probably happened there.

I think you've got the right idea with circulating water at some intermediate temperature; you'd probably get the best of both cold and hot brew after you play with the temperatures a bit to find the best one for your tastes. The lower the temperature, the more flavors left behind and the longer you may have to brew to get to an adequate strength.

If you do decide to try it, let us know how it works!

  • Good point about the protein & roasting! – Will Calderwood Jul 21 '16 at 5:58
  • Correct. Beans are generally roasted at 400F+. That's much higher than boiling water even ~220F. – Suspended User Jul 21 '16 at 17:28
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There are a few principles involved here (a chemical engineer's perspective on leaching - essentially coffee brewing is just that).

As Eli mentioned, solubility in water for most materials increases with temperature. In this case of leaving a batch of coffee with a batch of water (assume pure water for simplicity), materials (different components) will diffuse out of the coffee driven by concentration differences for each materials in water and in the ground coffee. There is a constant exchange of these materials going from the ground coffee into water and going from what has gone into water back to the ground coffee. Each material has its own diffusion rate and solubility in water for a given temperature. Eventually, there will be an equilibrium. Thus, assuming no chemical reaction takes place for now, no matter how long you leave that brew, temperature alone will dictate how much of each material in the coffee stays in the water and how much stays in the ground coffee. Without facts, qualitatively, I imagine an overnight brew in the fridge will give you more than enough time to reach that equilibrium and brewing it for another day or two will not change anything.

However, many organic components will hydrolyse to some degree when exposed to water especially when there is a pH shift. I do not know enough to speculate on which component in the coffee will face what kind of hydrolysis at what temperature, but generally the hydrolysis reaction rate increases with temperature. I am speculating that undesirable flavours are about both products of hydrolysis and increased solubility of "bad" components. There is also the likelihood of other reactions at higher temperatures say between acids and lipids or proteins and sugars or perhaps the breakdown of polysaccharides into shorter sugars and oxidation of sugars to acids and so on. These are much more likely to produce flavours than denaturing of proteins which I think (not certain) tends to have little to no impact.

In order to systematically visualise this, I think one would go through a matrix of temperature, time and grind size and put the "extract" through an HPLC to quantify the concentration of each component. Ultimately, a sensitive nose/tongue is the real test.

Extrapolating from this, I always feel that the drip method has to be the least desirable as you are constantly putting pure water to leach out materials from the coffee, always encouraging more materials to come out leaving no room for "bad" stuff to be left behind. If you taste the drip as the brewing progresses, you can definitely tell that about half way through, the coffee actually tastes foul and it gets worse. This is a leaching process designed to maximise extraction. This is exactly what happens at the fresh solvent end of an industrial counter-current extraction process to maximise concentration gradient and diffusion rate - not what you want for just taking the desirable bits out of coffee. It would be preferable to recirculate the brew through the grounds so that "strength" builds up to an equilibrium and by adjusting the temperature, you can control that equilibrium to achieve "optimal" taste.

I have been experimenting with cold brew and find that the brew tends to lack aroma while hot brew tends to have both a harsher taste and distorted aroma. Ideally, if there is a way to preserve the aroma of freshly roasted beans, that would be best. So far, I favour as fine a grind as practical, start with room temperature water, agitate and gradually raise the temperature to above 60C (have not found the preferred temperature yet but I dare not push it over 90C) nearer to the time of drinking. Once filtered, I can heat or chill the coffee as desired.


(Editor's note: Additionally, an anonymous user suggested the following content...)

Cold/warm brew is much more forgiving, relying on achieving some equilibrium and making grind size and time redundant variables. Hot brew on the other hand for everyday coffee making is mostly about guessing when to stop the leaching process abruptly.

Fine grind is sometimes falsely blamed for over-extraction bitterness. From my experience, bitterness from fine grind has mostly to do with small coffee solids staying suspended in the coffee. It is very telling if you make coffee using a fine grind (say espresso grind) in a press/cafetier, pour a third into a cup as control, another third through a filter, and double filter the remainder, you will be able to taste the difference.

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Your hypothesis sounds interesting. Try it.

Traditionally speaking though, these factors are what affect the characteristics that you are concerned about:

  1. Bitterness - affected by degree of roast. Darker roasting, more carbony and bitter flavors
  2. Sweetness/sourness - brew competitors use lowered brewing temps to increase the acidities in their coffee. But for me, grind size and exposure time, more than brew temp, allows you to tweak the flavor better in your coffee. So just grab that water off the boil and don't stress.
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I think what your idea would lead to a more bitter taste. Simply because it would be very easy to overdo the time needed. If the beans over extracted you would have lukewarm bitter coffee. You would need some sort of instrument to measure the extraction compared to a cold brew and find the same rate to get the flavor you like from the cold brew. It might be simpler to brew less bitter coffee, there are other variables to experiment with. There's only one way to find out

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Discovered a bit more about the leaching process: I am less sure about the effect of solubility and suspect there it has mostly to do with asymmetric diffusivity and diffusion rates.

Sample 1: Very warm brew (sous vide at 75 °C, 90 min in sealed bag) then left to cool with grounds in contact with liquid (still sealed) to room temperature for 24 hours.

Sample 2: Cold brew (4 °C, 10 hours, sealed bag) then left to warm with grounds in contact with liquid to room temperature for 24 hours.

They tasted very different. 1 has more edge, more acidity, more aroma and generally more lively then 2 which is sweeter, milder and less developed. Radically different equilibrium points.

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Came across this, yet to try it out:

Sous Vide Coffee Recipe

  • Thanks for this nice advice. A note: the links may disappear. It's a better idea to copy a summary of the source and then link the source. – MTSan Aug 22 '16 at 0:59
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Variations of an in-between brew you are looking for might exist with the French press and other methods where the grounds steep for multiple minutes as the water is cooling and, if desired, starting at several to many degrees less than boiling. The grounds being more coarse than they are for drip coffee is said to have an affect on the brew as well.

I do not know the temperature or method the restaurant industry machine dispensed Douwe Egberts coffee is brewed at but to me it tastes like it is brewed at a lower than boiling temperature. I don't mind the bitterness of drip coffee but I do find DE coffee to have a very nice taste. They make a very concentrated coffee that is sealed is bags without any air for machines that mix the concentrate with hot water at the time of serving the coffee. I would not expect their products made for drip or k-cup brewing to taste the same but I have not tried these. And I don't know what chains would be more likely to use them. A Shipley's Donuts near me used to use DE machines but does not anymore. I have heard of some salad buffets and other donut places having them but I have not run into one in a while.

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