Brewing The Perfect Cup

Pour some science into that cup of joe! Chemistry, physics, and botany, along with hot water and roasted coffee beans, all combine to make the perfect cup of coffee.

DURHAM – If you're one of those folks who drink a cup of coffee everyday, relax. You're not alone. A new survey conducted by the National Coffee Association reports 61% of Americans drink a daily cup of coffee, which may not be all that surprising, considering the number of coffee shops that seem to be located in virtually every shopping center.

But the science behind what makes this drink so popular is pretty remarkable.

This is the story of one type of coffee bean. It’s called La Golondrina, which means “The Swallow” in Spanish.

The story starts high in the Andes Mountains in Cauca, Columbia. The farms are built by cutting down trees and burning out the underbrush, essentially clearing out sections of the rainforest. Each farm varies in size, and the plots lie in clusters around villages. The growing and harvesting of coffee is almost all manual. The beans are picked, washed, and then dried in the sun.

Eventually, the beans arrive at the coffee roasters. We found the La Golondrina beans at Counter Culture Coffee in Durham. The beans were packed in giant plastic bags, which are then inserted into the traditional burlap bags that were used to ship coffee for years. The plastic bags were added a few years ago, as more and more coffee was shipped farther distances. The plastic keeps the coffee better protected and fresher as it is shipped long distances around the world.

Technically, raw coffee beans are actually seeds. The raw coffee bean is very hard and has virtually no flavor or aroma. The raw bean also tastes very bitter. But that’s where the science of coffee begins.

“The natural processes of growing the coffee, such as the soil content and climate of the area, definitely affect the beans and the flavor of the coffee, but the roasting of the bean is where the science of coffee starts,” says Jeff McArthur, Head Roaster at Counter Culture Coffee. “We’re stripping away the free moisture in the coffee and we’re also developing chemical reactions, most notably carmelization and maillard reactions. In addition, we are taking sugars in the form of sucrose that’s found in the coffee bean and we’re starting to break it down.”

In chemical terms, roasting coffee develops a lot of volatile components, or chemicals, that are active and creating dozens of chemical reactions. There’s a chemical name for what is happening. It’s called a maillard reaction. In general, it’s the term for the hundreds of chemical reactions that are happening inside the bean at high temperatures.

Those reactions create new chemical compounds that are locked inside the bean. And it’s those new compounds that give coffee its flavor qualities, including the taste, aroma and body. 

Coffee roasting is a product of time and temperature. In general, the average roasting temperature is 400 degrees. The average roasting time is about 15 minutes. A really light roast doesn’t burn off as much of the water and other organic compounds. A dark roasted coffee has a higher final temperature and burns off much more of the moisture and organics.

The type of roast and the amount of material that is burned off requires McArthur to perform a lot of math in addition to chemistry while he is roasting. That’s because roasting can remove almost 20% of a coffee’s weight. So, he must calculate how much additional coffee must be roasted to fill an order, because some of the raw coffee weight will be burned off in the roasting process.

The roasting room is noisy. There are giant steel roasters, with trap doors on the front, just below dials and digital readouts showing temperatures, times, and pressures. A maze of pipes pump natural gas and hot air into the machines.

There’s a small window through which you can see the beans turning inside the roasters. Alongside the giant machine, the roaster is watching over it all, and occasionally pulling out a sample of the beans to see and smell what is happening.

The tiny cup and tray used to pull the beans out of the roaster is called, appropriately enough, a tryer. And while the look of the beans is important to a professional coffee roaster, it’s the aroma that is most important. The smell tells the roaster how well the coffee is developing, how smoothly it is developing and, ultimately, when it is finished roasting and ready to cool.

When the desired temperature is reached and the roasting process is finished, the door to the roaster is opened and the beans are poured into cooling trays. Giant mechanical arms stir the beans so they are cooled evenly. Once cooled, the beans are packed and sent off to coffee houses and stores.

And here’s where more of the science of coffee comes in.

Roasting leaves the coffee bean with hundreds of new chemicals inside that create a lot of desirable flavors that can be tapped with heat. That’s where the hot water for brewing comes in. Before that, of course, the beans are ground to increase the surface area that is exposed to the water, which makes it easier to extract, or dissolve the desired chemicals.

The water is heated because the extraction of molecules from a solid happens much faster and easier at a high temperature. Lydia Iannette, who teaches coffee brewing at Counter Culture Coffee, says coffee should be brewed with water that is about 200 degrees, or slightly off boil.

The best ratio for brewing coffee is 1.6-2 grams of coffee for every 28 grams, or one ounce, of water.

“There’s more science involved here because essentially coffee is the creation of a solution with some of the ground coffee. It’s dissolving soluble material from the coffee itself,” Iannette explains as she pours hot water over the ground coffee, which is sitting atop a drip coffee maker. The dark brown solution, in other words - the coffee, is dripping through the filter into the pot below. “The finer the grind, the more the water can extract because there is more exposed surface area, the courser the grind the water has to work a little harder.”

Ironically, for all of the amazing new chemicals that are inside the roasted coffee bean and will be extracted by the hot water, only about 30% of the actual bean is usable. And of that amount, only about 20% actually tastes good. 

Iannette says the key to making a good cup of coffee is to leave the bean and water in contact just long enough to extract the good flavors, but not long enough to pull out the bad. Acids are extracted first, while sugars and bitterness come later. In general, a finer grind, which keeps the water and coffee in contact longer, produces a stronger cup of coffee. A courser grind, which allows the water to flow faster, produces a weaker cup.

Iannette admits it’s a delicate balance. And while the technology of automatic coffee makers does a decent job, she prefers brewing everything herself; saying the art of brewing a great cup of coffee is fun.

“However you make your cup of coffee, it all comes down to controlling how much caffeine and how much flavoring material you are extracting,” adds Iannette, as she hands me a cup of coffee before sipping the product of her demonstration. The coffee smells wonderful and tastes even better, with hints of flowers, fruit and even some nuttiness. The liquid is rich and dark. 

“We want to maintain a lot of the inherent qualities in the coffee and I think we did,” Iannette adds, smiling.


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