อัปเดตเมื่อ 21 ก.ค. 2019
I love cold brew coffee. I also really enjoy hot brewed coffee, but there is something about cold brew that lends itself to drinking copious amounts, especially here in hot hot Thailand. Wanting to discover why my taste buds crave this smooth coffee, I went on a journey to discover the secrets behind great cold brew coffee.
The Basics of Coffee Chemistry
Okay, so the chemistry of coffee is actually quite complex, which is why there seem to be innumerable variations on the drink…I mean it has been around for hundreds of years and we are still coming up with novel ways to brew and prepare those magical beans. And all the ways to prepare coffee involve water and coffee beans. Regardless of the method, water will always extract the different flavor compounds in this order: fats and acids, then sugars, and finally the plant fibers. Let’s look at the first key ingredient, water.
Water is an effective solvent, able to dissolve other substances well because of its molecular makeup of two hydrogen atoms and one oxygen atom. The two positively charged hydrogen atoms and the one negatively charged oxygen atom makes the water molecule very attractive to other molecules. This attraction is so strong that it can pull apart the bonds of other molecules and dissolve into the water. Heating water makes the water molecules move around with more energy and will increase the solvent effect of the water. That’s why hot brewing takes so much less time than cold brewing. When you mix ground coffee with water the water will extract or dissolve the flavor compounds of the coffee. Because our coffee drinks are made up of mostly water you should always use the best tasting water you have available. What are the compounds in coffee?
What is Coffee?
Coffee is, of course, a plant, but did you know there are over 120 species of coffea? The two most popular are coffea arabica ("Arabica") which accounts for 75–80 percent of the world's coffee production, and coffea canephora ("Robusta") accounting for about 20 percent. But what is it about the chemical makeup of coffee that holds such flavor and antisleep properties?
Coffee beans are the roasted seeds of the coffee fruit, often called the cherry. Think about this, these beautiful coffee cherries hope to one day become a coffee bush and have little coffee cherries of their own. So, the seeds are packed full of the building blocks for potential growth: complex sugars, fats, and acids. And it’s this potential that we extract with water to brew the coffee that gets the world up and going.
Roasting - Making the Coffee Usable
Whether you are hot brewing or cold brewing or something in between you are probably using roasted coffee. Roasting the coffee is an important step to unlock the wonderful qualities of the coffee. When we roast coffee, it evaporates moisture that is in the seed which makes it dry and brittle. The heat of the roaster will also break down the complex sugars into simple sugars which are easier for us to taste. As the roast progresses those sugars will caramelize which releases nutty or caramel scents. If the roast continues then these sugars will burn, and you will get smoky or burnt aromas.
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The acids undergo changes as well. As the heat increases, many of the acids in the coffee will also break down. If a roast is stopped too early, there will be more acids in the coffee. If a roast goes too long, the acids will eventually break down completely and the coffee will taste bland and burnt. Most coffee roasters try to balance the levels of acidity, so the brewed coffee will be neither intensely sour or incredibly bland. Quality roasters will be able to control their roasts in order to achieve coffee beans that have a careful balance of oils, fruit acids, simple sugars, and caramelized sugars.
How Cold Brew Is Different
We have our water that is a great solvent to extract all that pent-up coffee goodness and our wonderfully roasted coffee to make all those acids and sugars ready for us to brew. Now, comes the difference between hot brewing and cold brewing. Where hot brewing extracts all that coffee flavor very quickly bringing out that intoxicating coffee aroma cold brew does not. Hot brewing will extract all those acids and will highlight a lot of the more complex flavors of the coffee. Hot brewed coffee is described as having a fuller body.
The downside to hot brewing is something every coffee drinker has experienced, it goes bad very fast. Let your carefully prepared pour over coffee sit for ten minutes and then take a sip, the flavor has already deteriorated. And heaven forbid you leave hot brewed coffee out for thirty minutes or more, that’s the kind of coffee to cringe at. Hot brewing methods extract more oils from the coffee which oxidize very quickly and that is why the flavor starts to taste flat and sour. Acids also degrade, the most troublesome of which is chlorogenic acid into quinic and caffeic acid, causing coffee to taste bitter. So hot coffee is amazingly delicious with complex acids and oils and mouthfeel but get it while it's hot and fresh!
Where cold brewed coffee lacks in speed and aroma it more than makes up for by being able to produce an extremely smooth, low-acidic, non-bitter, clean cup of coffee that holds up deliciously when iced or mixed with milk. Cold brew doesn’t merely taste like hot brew without the bitterness. I love the cold brew method because I can achieve a completely different flavor profile that can’t be found with hot brews. Going back to the idea of solubility, not all flavor compounds of coffee solubles are equally soluble. A good majority of the coffee solubles are still able to leach out of the grounds, even in colder water. The compounds that don’t dissolve are the ones often attributed to unfavorable flavors: these stay in the grounds that are subsequently tossed away. Consequently, cold brews take on a much sweeter, floral profile.
Either way you brew, if you under extract or over extract you are going to get very different flavors. And achieving that sweet spot is what we are all striving for, well in terms of brewing good coffee.