Brewminating: The Science behind the Perfect Cup of Coffee

Photo by Jmacarthur0417, Wikimedia Commons

Its chemical composition, flavor and shelf life, as well as the uses for coffee grounds after brewing.

By Dr. Gabriel Keith Harris
Associate Professor
Food, Bioprocessing, and Nutrition Sciences
North Carolina State University


Coffee is a popular beverage around the world. About half of consumers drink coffee brewed from grounds. The other half drink instant coffee. In the US, the average coffee consumer drinks over three cups a day – that’s 450 million cups daily in the US alone! Coffee has a long and intriguing history stretching back over 1,400 years to its discovery in Sub- Saharan Africa. It was cultivated and traded on the Arabian Peninsula, then spread to tropical and sub-tropical regions around the world.

Over time, coffee has played an important and sometimes controversial role, at times being promoted and, at other times, being restricted or banned. “The fact that one little bean represents so much historical, economic and cultural change, not to mention microbiology, biochemistry, nutrition, engineering and deliciousness is amazing,” says Dr Keith Harris, a food scientist at NC State University in the US. “The best things do come in tiny packages!”

The Journey to the Perfect Brew

Coffea robusta flowers / Photo by Okkisafire, Wikimedia Commons

Coffee originates from two coffee plant species, Coffea arabica and Coffea canephora, commonly known as Arabica and Robusta. “Arabica beans are less acidic, higher in fat and sugar, and lower in caffeine than Robusta beans,” explains Keith. “Robusta beans, on the other hand, contain more acid and caffeine and also come from hardier plants, as the name Robusta implies.”

Arabica beans are preferred (and therefore more expensive), due to the milder flavoured coffee beverages they produce. Plant genetics, soil quality, soil microbes, weather and altitude all affect the growth of coffee plants. Fermentation, drying, transport, roasting and storage of the beans all affect the flavour of the final brew. With so many processes involved, a lot can go wrong!

From Bean to Cup

Unroasted coffee beans / Photo by Isai Symens, Wikimedia Commons

A coffee plant grows in the tropical sun, its fruit turning a deep red. This plant could be growing in Brazil or Mexico or Ethiopia or Vietnam or any number of places near the equator. While some coffee is grown on large plantations, much of the world’s coffee is grown on small farms. Each coffee plant’s fruit, called a cherry, contains two coffee beans – its seeds. Ripe, red coffee cherries are harvested and the green beans inside are quickly removed before the fruit spoils. The beans are then fermented, dried, hulled, sorted, placed into plastic and burlap bags, then loaded onto ships to be exported.

Roasting turns green coffee beans into the brittle, brown, aromatic beans we are familiar with. Roasted beans are ground and brewed in a variety of ways. Instant coffee is made by freeze drying brewed coffee. Any variation in any stage in this process will affect the end product, by altering the very chemicals that give coffee its distinctive aroma and taste.

The Chemistry behind the Flavor

Skeletal formula of a caffeine molecule / Image by Benjah-bmm27, Wikimedia Commons

Controlling the flavor of coffee and espresso is something that’s caught the attention of food scientists and engineers for a long time. The latest technology of espresso machines have systematic ways of manipulating flavor by controlling the water temperature and pressure applied to the coffee grinds.

The Maillard reaction is the key to both flavour and colour development in coffee. “This is a reaction between sugars and proteins that occurs quickly at high temperatures,” says Keith. “It’s the same process that makes toast so wonderfully different from untoasted bread.”

Over time, roasted coffee beans can lose their flavour and even form objectionable flavours. This process is known as ‘fade’. Improper storage can accelerate this process, which is why it is important to store coffee in a cool, dry place away from direct sunlight. Keith’s team has been involved in uncovering the optimum conditions for extending shelf life, but this doesn’t explain what actually drives the process of fade itself. “The interesting thing about flavour fade is that, although everyone in the coffee industry knows about it, nobody has ever been able to explain it fully,” says Keith. “That’s what I love about science! There’s always a new mystery to tackle.”

Lessons in Efficiency

When composted properly, coffee can be an excellent fertilizer. / Photo by Tien Huynh, The Conversation

In 2018, the US sent six million tons of ‘spent’ coffee grounds (left over after brewing) to the landfill. “There are many known uses for spent coffee, including compost, the creation of biodiesel, pressing grounds into logs for firewood, and extracting caffeine for sale,” says Keith.

His team has investigated two particularly intriguing usages: removing pollutants from wastewater and replacing flour in brownies.

Keith’s team has also looked into more efficient ways of producing cold brew coffee. They found that it takes about twice the amount of coffee to create a similar flavour intensity in cold brewed versus hot water brewed coffee. His lab is working on a process to produce cold brew coffee using less coffee grounds, which could result in savings for the coffee industry.

Health Effects of Caffeine

Depending on the media source, caffeine (coffee’s primary stimulant) could appear to be a miracle cure or a gateway to an early grave. This paradox arises because many factors affect caffeine’s stimulant properties. “Positive or negative effects of caffeine are related to the dose, the timing of the dose, and how capable our bodies are of using and getting rid of caffeine,” says Keith.

Acute (short-term) positive effects of high doses include sharpened focus on repetitive tasks. Acute negative effects of high doses may include shakiness, palpitations, difficulty sleeping and sometimes a laxative effect.

Chronic (long-term) effects of moderate doses, involving weeks to years, are less well understood, but may include reduced risk of diseases like Alzheimer’s and Type II diabetes. What constitutes a high dose depends on the person, but according the US Food and Drug Administration, 400 milligrams or four to five cups of coffee per day is considered the limit of a “healthy dose range” for most adults. From its health effects to sustainability to chemistry, who would have thought that a simple coffee bean could teach us so much?

Originally published by Futurum under the terms of a Creative Commons Attribution-NonCommercial 4.0 International license.



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