temperature change rates impacting flavor

[scottlucey]

I love the questions that are so basic they almost stump you - these made me realize I didn't have as much of a scientific grasp behind our chosen cold brew method.

1) why don't you just cool regularly brewed coffee in the refrigerator (vs brewing it double strong on top of ice)
2) instead of brewing on top of ice, why not on top of cold water

Ultimately it was easy to say that to get the best flavors coffee needs the drastic and quick temperature change. I did a little digging and found this great quote from Peter G back in 2006. Sadly the link to CCC's write up isn't working.

The thing I love about the "Japanese" process is that the coffee is only hot for mere seconds after it brews: it is almost instantly cooled by the melting ice. This allows it to maintain its fragrance and acidity, and prevents the cooked "airpotty" flavor that inevitably results from letting coffee cool more slowly. It seems particularly good at preserving delicate aromatics, using a floral Yirg in this process is postively overwhelming.

Which leads me to ask,
How or why does a quick temperature change rate impact flavor the way it does? Can we all agree that sweetness/fragrance/nuance is preserved with a quick/drastic temp. change rather than a slow one?

I have brewed different versions in these other ways just for the questioners to taste their answers - and surely coffee cooled slowly tasted worst, oldest, even when served on ice.

Thoughts? Scientific explanations?

[nick]

You can break down the factors into two categories: volatiles and non-volatiles.

With the volatiles, it's pretty easy. About 90% of the volatile aroma compounds in coffee have a boiling point that's between room temperature and typical coffee brewing temperatures. That means that the longer the coffee spend in that zone, the more those compounds will leave the coffee and dissipate into the air.

With the non-volatiles, it's different. By definition, the non-volatiles are more stable, and don't float away into the air. On the other hand, they can go through changes within the brew itself. Chlorogenic acid, one of the most "important" organic acids (and one of the most significant antioxidants) in coffee, breaks down at higher temperatures into quinic acid and caffeic acid, neither of which are particularly yummy. That attribute of chlorogenic acid probably contributes to the most significant "negative flavor" character of coffee that's held for a long time or cooled slowly.

When you cool everything down really quickly, like when you brew directly onto ice, you are essentially capturing that stuff in the brew that, if you kept it at or near brewing temperatures, would dissipate or degrade.

When you put coffee in the fridge, it's still not fast enough... unless you chilled it really fast, or put it into the fridge in very small quantities. You could try that: take some brewed coffee and split up into various volumes in different containers. You'll see that the stuff that's in the smallest portions taste the best once they're cold.

Hope that helps!

[Jim Saborio]

Odd that one can have such success flash-icing coffee, but espresso...


Coffee is a wild, wild world.

[jason dominy]

I have been flash-chilling my Clever brewed Kenya, and it tastes delicious. Doesn't take on the bitterness that normal brewed coffees usually have. I don't change my brewing ratio or brew time at all, just brew like normal, pour directly over ice. This has worked very well. But I've wondered myself why you could do this with coffee, but not espresso, just like Jim asked....

[Peter G]

In terms of the explanation for why flash-chilled iced coffee tastes SO MUCH BETTER, Nick has it about right. At high temperatures, all chemical reactions happen faster, including loss of volatiles and flavor breakdown. That chlorogenic acid tang that Nick is talking about is perhaps my least favorite thing about coffee flavor breakdown- and it's totally commonplace when coffee is held in airpots (piping hot) for even a relatively short time. At low temperatures, chemical reactions slow down, "freezing" in place the perfect-flavor of just-brewed coffee.

Have you ever had a new-crop Yirgacheffe ice brewed Japanese style? Ohmygoditissodelicious. My mouth is literally watering right now.

Wow, but I guess I said all that in the original post Scott mentions. Just for you, Scott, here is a new writeup we made to try to spread the word of the Japanese iced coffee method: http://www.counterculturecoffee.com/docs/brewing-guide/CCC_Iced_Coffee_Brew_Program.pdf. It's amazingly difficult to describe the account-for-the-ice-melting method, but we do our best.

The problem, of course, with this method is that the shorter brew time (because you're using less hot water) screws up the total extraction percentage some. I have seen Japanese iced coffee makers that solve this: the coffee- right out of the filter basket- runs into a tube that coils through an ice bed. Once the coffee emerges from the tube, it is ice cold, but undiluted.

One last thing: I am holding on to the belief that the ugly iced-espresso taste has something to do with all that CO2 in a fresh espresso, perhaps turning into carbonic acid. I have been told I am wrong about this, but I am sticking with it until I hear a better explanation. Iced coffee- with less dissolved CO2- doesn't have the same problem.

Peter G

[scottlucey]

BIG thanks Nick, Peter. These are great explanations and I'm honored for the updated link.

Too kind, too kind....