June 25, 2019

How Does Processing Method Impact Coffee Chemistry & Flavor?


Why does a natural processed coffee taste different to a washed? And what makes honey processed coffees so sweet? Processing method has a big impact on a coffee’s flavor, aroma, and body.

Read on to learn more about the chemical changes that take place during processing and their impact on your cup of coffee.

Lee este artículo en español Cómo el Procesamiento Influye en la Química y el Sabor Del Café

Sorting coffee cherries during a Q-processing class in Minas Gerais, Brazil

Sorting coffee cherries during a Q-processing class in Minas Gerais, Brazil. Credit: Verônica Belchior

The Origin of Coffee Flavor & Aroma

Coffee flavor and aroma are developed during roasting. Aromatic volatile compounds are produced by transformations of the chemicals that naturally occur in green coffee beans.

The degradation of simple sugars and polysaccharides during roasting creates sweet and caramelized aromas. In contrast, the degradation of hydroxycinnamic acids (a type of phenolic compound) produces spicy aromas. Hydroxy-amino acids such as threonine and serine are transformed into volatile compounds called pyrazines and pyrroles, which create the characteristic roasted coffee smell.

But what does that have to do with processing? The choice of processing method influences how much of each of these chemical components is present when the bean makes it to the roaster, and therefore has an effect on which aroma precursors are created and in what amounts.

You may also like Coffee Science: Breaking Down Where Flavor Comes From

Coffee undergoing washed processing in tanks at a farm in Honduras.

Coffee being washed in tanks at a farm in Honduras. Credit: Perfect Daily Grind

How Does Processing Change Coffee Chemistry?

Coffee cherries usually contain two seeds, which are surrounded by the mesocarp and mucilage layer, better known as pulp, as well as the exocarp, or skin. The pulp of a coffee cherry contains a lot of sugars.

Processing is simply the method used to remove the coffee seeds, or beans, from the pulp and skin. There are different traditions and new innovations around the world, but coffee is generally processed by the natural method or the washed method.

During the natural method, the beans are dried entirely in their natural form with skin and pulp intact. The washed process removes all of the soft fruit residue, both skin and pulp, before the coffee is dried. Honey or pulped natural processing is a middle-ground, in which the skin of the fruit is removed before the beans are dried, but almost all of the pulp remains on the beans.

The processing method makes a difference to flavor and aroma because the sugars in the pulp left on natural or honey processed beans undergo metabolic changes that significantly alter the chemical composition of the green beans. These reactions create sweetness and body in the final cup. Let’s take a closer look at what happens.

Learn more in Washed, Natural, Honey: Coffee Processing 101

Natural processing coffee on raised beds.

Natural processed coffee on raised beds. Credit: Fernando Pocasangre

  • Germination

Coffee cherries are harvested when they are ripe, meaning that the seeds inside are about to start the germination process. Germination activates some enzymes that metabolize polysaccharides – that is, natural sugars start to break down.

Researchers have found that there is more enzyme activity during washed processing than in natural processing. This means that more of the free sugars and other compounds that are transformed into aroma-producing compounds in roasting are consumed in washed processing. The result is that washed coffees are sharper than natural coffees. They have  fewer aromatic compounds but more acidity.

  • Fermentation

Natural coffees are sweeter than washed coffees in large part because of the fermentation of sugars in the pulp. Both washed and natural coffees experience fermentation, but there is a lot less material for the enzymes to work with in washed coffees because the pulp has been removed.

During fermentation, microbes modify proteins, carbohydrates, and chlorogenic acids. More precursors to aromatic compounds are created in natural coffees. The result is not just sweetness, but fruity, floral, and caramel notes in the final cup. In comparison, washed coffees are cleaner and reveal more of the individual notes of the particular coffee.

Honey processing includes removing the cherry skin, which allows for better fermentation of the mucilage around the bean. This results in a more pronounced sweetness in honey processed coffees than in natural ones, with more buttery aromas and nut notes.

Coffee being dried on patios. Credit: Sicafe S.A de C.V

Fermentation can make a big difference to flavor, aroma, and body, but it needs to be carefully controlled. Over-fermentation can create too many acetic acids and phenolic compounds, which will make the final cup bitter or sour. And because the microflora population present in coffee is dense and diverse, fermentation can be inconsistent and unpredictable.

Beans that experience longer periods of germination and less fermentation can show worse results in the cup. Both of the transformations contribute to a good final cup.

A farmer moves coffee in fermentation tanks at a farm in Honduras. Credit: Perfect Daily Grind

The Impact of Processing Method on Roasting Technique

So we know that processing has an affect on the chemistry of green beans, but that roasting transforms these chemicals into aroma-producing compounds. It’s important to consider the processing method when roasting.

Natural processed coffee preserve more sugars, which can make them more sensitive to heat during roasting. The simple sugar molecules can quickly degrade during roasting and produce undesirable burned flavors. This means that the roaster needs to be more careful in their roast curve, especially during the initial stages.

Learn more in How to Roast Natural & Honey Processed Coffee

Freshly roasted coffee. Credit: Neil Soque

Washed coffee is one of the most common and popular types in the coffee industry. This processing method produces a clean cup, allowing you to taste all the nuances of the origin and variety. But many people enjoy natural and honey processed coffees for their sweetness and fruity notes. Whichever one you choose, now you’re a little more familiar with what actually happens during processing to create the profile you enjoy.

You may also like What Happens During Coffee Roasting: The Chemical Changes

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