Science in the Neighborhood: how to make a really good coffee

Science in the Neighborhood: how to make a really good coffee: "
Sitting at the end of the long wooden bar, I watch with curiosity as Richie begins his pour. He starts the stopwatch on his cell phone and proceeds to pour steaming hot water over the coffee grounds in a precise choreographed motion. The water hits the grounds and starts to seep through while small bubbles rise up to form a layer of foam in its place. Richie adds the water in successive streams spiraling outward from the center of the grounds, an ounce at a time. The water level rises and then falls. At 2 minutes and 31.48 seconds, there is a mound of dissolved grounds left in the filter and a rich, caramel-colored liquid fills the glass below. For the first time in my life, I find myself eager to taste a cup of plain coffee.
Rich Nieto, or Richie as I know him, is a co-owner of my local coffee bar, Sweetleaf, which is a stone’s throw from my apartment in Queens. Since its opening in the spring of 2008, Sweetleaf has become a staple both in the neighborhood and in my daily life. I stop in each morning for a mocha to satisfy my sweet tooth and for a chat with the baristas who have come to know me as their “local astrophysicist.” Most of the time, we engage in small talk about the day ahead, the weather, or the weekend, but occasionally the conversations wander deeper into what I’m teaching, what they’re reading, or what’s new in the neighborhood. With Richie, these conversations often veer into science.

When Richie decided to open Sweetleaf, he knew very little about the world of specialty coffee. But he threw himself into learning all there was to know about its production and consumption. He identified the variables he had to play with as a barista and started experimenting. On more than one occasion, I’ve stopped in to find Richie enlisting the staff and even a customer or two in blind and double blind taste tests as he played with the settings on the coffee machines. His co-owner, Alfred Arundel tells me, "Richie has always been like this, he’ll spend hours just studying one thing, thinking about what the variables are and how they interact."

His systematic approach impresses me, and we’ve often discussed the importance of th scientific method. While I’ve never stopped to think about the variables he is working with, his baristas are all too familiar with his tinkering. Georgia Sanford has been working in coffee shops most of her life, but was only ever trained to get the same tasting product each time, and not to question whether the taste itself could be better. "Richie started asking why not play with the process and see what happens to the taste? Now not only are all our baristas trained to consistently deliver the best tasting results, but we understand more about the variables and how they affect the outcome."

It was a slow Wednesday afternoon when I discovered three electronic scales sitting neatly in a row at the end of the bar. Seeing my curiosity, Richie walked over to explain they were for a new drink he wanted to offer called a "pour over." Essentially a single serving of coffee brewed to order, a pour over is the coffee world’s return to basics.

Coffee trees grow primarily in a belt around the equator in remote regions at moderate altitudes with mild climates. The beans themselves are actually the seeds inside the fruit of the coffee tree, known as the “cherry.” Unfortunately, coffee cherries don’t all ripen at the same time between trees or even between branches. They must be individually inspected and hand-picked. The cherries are put then through one of two processes, dry versus wet, to separate the beans from the pulp of the fruit. Once dry, the last protective layer around the bean, the husk, is removed and the beans are sorted, packaged, and shipped to commercial customers around the world. Upon reaching local distributors, the beans are then roasted and delivered to retail clients such as coffee bars and cafes where they are finally transformed into your beverage of choice.

Coffee drinks generally take two forms: espresso based or brewed. A shot of espresso is ‘pulled’ by forcing hot water through coffee grounds at 135 pounds per square inch. From beginning to end, water is in contact with the grounds for all of thirty seconds. Contrast this with brewed coffee, which requires upwards of five minutes depending on the number of cups being brewed at a time. The grounds are poured into a filter and the requisite amount of water is added. Then you wait. Drip by drip the water makes its way through the grounds under the force of gravity alone, dissolving the grounds in its path. Cafes tend to brew up to a dozen cups at a time in advance so they add a large amount of water all at once to use the weight of the water to speed the process. However, neither of these methods allow the barista full control over the extraction process.

Extraction is the process of dissolving the coffee grounds in water. When the water hits the grounds, acids are the first chemical components to be dissolved, then sugars and finally, the bitter components. The more grounds that dissolve, the higher the “extraction.” But more is not necessarily better. Too much and the bitter flavor is overwhelming; too little and the coffee is on the sour side. But in between 19%-22% is the “sweet spot” – literally. Proper extraction brings out complexities within a coffee that are imparted during its production but often get lost in more commercial consumption. The percent extraction for a given cup of coffee can vary based on several factors, the most important of which are: the size of the grounds, the time that the water is in contact with the grounds, and the distribution of the water among the grounds.

Brewed coffee is often characterized by uneven extraction. The automated system of passing water through a conical mound of coffee grounds inhibits even extraction. The grounds in the center get doused with the most water and as a result, the coffee from these grounds is over- extracted. Other grounds around the perimeter barely interact with the water at all producing coffee that is under-extracted. And unfortunately the law of averages is no help as the average extraction is often dragged upwards towards 22% into borderline bitter territory.

Espresso, by contrast, is the product of even extraction, but comes with its own concessions. While not automated, the process of pulling a shot of espresso is extremely standardized with little room for variation. In order to have repeatable success (i.e., a consistent product), the grounds must be finer and made up of a blend of beans. The espresso grind size allows water to come into contact with more of the grounds as it is forced through them at high pressure. If the grounds were larger, the water would pass straight through without barely any extraction. Smaller, and more pressure would be required to extract anything at all. A blend of beans is used to defend against noticeable differences in an individual coffee harvest from year to year.

With the pour over, the barista can now control all aspects of the extraction process; the precise timing and distribution of the water can ensure that the grounds are evenly extracted in both time and space. The set up is roughly the same as any filter or drip coffee machine; the difference lies in scale and technique.

At the end of the bar, I watch as Richie measures out 21 grams of coffee beans from a Colombian roast. He puts the beans in the grinder and adjusts the knob to set the grind size. With the touch of a button, the machine whirls to life and the beans are transformed into a coarse powder. Richie transfers the coffee to an inverted conical ceramic cup that sits atop a glass receptacle on one of the scales. The cup has a nickel-sized hole in the bottom and is lined with custom-sized filter, which Richie has dampened with water so that the filter sticks to the wall of the cup. He fills a small kettle with twelve ounces of hot water and tilts it over the coffee grounds.

As he pours the water, he explains that he is pre-wetting the grounds, "The point of the pre-wet is to get the coffee ready to receive the water. There’s a lot of carbon dioxide trapped in the grounds so we have to de-gas them first." As the hot water hits the grounds, it forms a layer on the surface before it starts to drop and seep through them. He gives the grounds a stir to wet them evenly and the water begins to bubble and foam as more and more carbon dioxide is released.

When the water level drops to just about level with the grounds, Richie picks up the kettle again and starts his stopwatch. This time he adds a couple of ounces of water to soak the coffee again and then he begins his pour.

"I'm going to work in a circular motion, starting from the center and making bigger and bigger circles around it. By doing this, I'm controlling a few different things. One, I'm controlling the time of the extraction [the time that the water is in contact with the grinds]. And two, I’m controlling the distribution of the water. If I fill the water all the way up, I can speed it [the extraction process] up. If I only put a little bit [of water in], I can slow it down. What I don't want is for the water to drop below the coffee because any coffee that's not touching water is not extracting."

Richie adds two ounces at a time, each one in an expanding spiral pattern. Gravity draws the water down through the grounds, dissolving them along the way, and drawing their rich flavor out and into the glass below. After the third spiral pour, he puts the kettle down and picks up a teaspoon. He gives the mixture a stir, creating a small vortex. "This little whirlpool here is going to keep the coffee grounds in the center and off of the side of the walls. The most important part about the brewing process is that all the grounds be in contact with the water," he explains.

The stopwatch is still running, approaching two minutes and thirty seconds, Richie’s target time for the water to have completely filtered through the grounds and into the cup below. At only 1.48 seconds past his mark, the last of the water flows down into the glass below leaving behind a small, porous dome of dissolved grounds.

Richie can tell when he’s between 19-20% extraction now by taste alone, but when he was learning he had to rely on data – scientific data. He bought a refractometer to measure the total amount of dissolved solids in the resulting cup of coffee and he has a program that can translate this number into the percent extraction. He spent well over fifty hours perfecting his technique, but through research, conversations with other baristas, and experimentation, he began hitting the sweet spot again and again. As Richie says, "When you're converting coffee beans into a cup of coffee, you can play a major role in what that ends up tasting like."

As a novice with a sweet tooth, I’ve never strayed from my preferred mocha, fearing the bitter taste I associate with straight coffee. I admit that impression is at least a decade old and I can’t recall the last time I gave coffee a chance. So after witnessing my first pour over, I was eager to taste coffee again for the first time. The sweetness hit me immediately and the subtle layers of caramel were immediately apparent. It was so smooth and rich and complex. As clichéd as it sounds, I’d never experienced anything like it. The pour over is a drink that tells a story. It captures all that the beans have seen and reveals how each part of the process has left its fingerprint. After tasting my first pour-over, my mocha tasted like a chocolate milkshake.

Image: Coffee Pour Over, by Kate.moon at Wikia Coffe Wiki

About the Author: Summer Ash is currently a post-doc at Columbia University in the Astronomy & Astrophysics Department and an instructor for Frontiers of Science in the Core Curriculum. Her doctoral research was on the evolution of radio galaxies and active galactic nuclei. She values the power of the scientific method, the history of science and the necessity of skeptical inquiry. As a self-professed space cadet, Summer grew up dragging friends and family out at all hours of the day or night to look up at the sky. In her previous life she was a rocket scientist, but now enjoys getting paid to spread her love of space with anyone who will listen. She attempts to blog at Newtonianism for the Ladies, tweets as @Summer_Ash, and is the in-house Astrophysicist for The Rachel Maddow Show.

The views expressed are those of the author and are not necessarily those ofScientific American.