Thursday, November 22, 2012

Coffee Processing Revisited

One of the first aspects of coffee to catch my attention was coffee processing, the steps involved in extracting the coffee seed (bean) from the fruit. I noticed that all of the coffees that I liked most at Peet's were either natural processed or wet-hulled. As I started drinking lighter roasts, my tastes turned to classically washed coffees. So clearly my original conception that the flavor of a coffee is just a result of how the coffee was grown and roasted was misguided.

However, I didn't have a good understanding of what the specific differences between different processing methods were, in large part due to ignorance about the steps involved and the plethora of synonymous terms used for processing methods. So I did some research and made a chart. Since then, I've discovered some finer-grained distinctions in processing methods, so at this point I think the information will probably be better presented in prose. All this information is available elsewhere, but I haven't found anywhere online where all of the details that I'm interested in have been collected in a concise and readable manner. If you're already familiar with the basics of coffee processing, you can skip this next bit.

Understanding the differences between processing methods requires understanding the steps involved in processing, which in turn requires understanding the anatomy of the coffee fruit. It starts out looking like this:

The outer skin of the coffee fruit is known as the pulp. The gooey flesh of the fruit inside the pulp is called mucilage. Inside the mucilage is a membrane surrounding the seed, known as parchment.  What follows is a brief description of the five stages involved in coffee processing: pulping, fermentation, washing, drying, and hulling. Note that not all processing methods utilize all of these stages, as I will discuss below.

The pulping stage involves removing the skin of the fruit, which is done mechanically. Then comes fermentation, when the de-pulped fruit, the gooey mess you see below, is let sit to allow microbes to break down the mucilage.

After fermentation, more water is used to wash off the loosened mucilage, and the result is parchment coffee, which is just the coffee seed and the innermost layer of parchment. The parchment coffee is then dried. (Dried parchment coffee shown below.)

Once dried, the parchment coffee is sent through hulling machines to remove the parchment, and the green coffee is ready to ship to the roaster.

1. Wet Process AKA Washed Coffee:

The vast majority of arabica coffee is wet processed. What characterizes all wet processing methods is the use of water to remove the mucilage. The most significant differences between different wet processing methods are: length of fermentation; whether water is added during fermentation (dry fermentation vs. wet fermentation); and moisture content at hulling. Wet processing is the most consistent method of producing defect-free coffee.

1.1 Latin American method

The coffee is pulped, then dry fermented for up to 24 hours. Water is introduced to wash off the loosened mucilage, followed by drying and hulling.

1.2 Kenyan method

The coffee is pulped, then dry fermented for up to 3 days. Water is used to wash the coffee, which is then soaked for 24 hours. The parchment coffee is then dried and hulled.

1.3 Ethiopian method

The coffee is pulped, then wet fermented for up to 3 days. More water is used to wash the coffee, which is then dried and hulled.

1.4 Wet-hulling (Indonesian method)

The coffee is pulped, then dry fermented overnight (usually; my sources conflict on this). The coffee is hand-washed, followed by a short period of drying. The parchment coffee is hulled before it has fully dried (i.e., more than 11% moisture remains), leaving the green coffee exposed to the environment while still moist and hence bacteria-friendly. This process results in increased body, lower acidity, and a distinctly rustic, earthy character.

1.5 Machine-assisted wet process

Some farms use elaborate machines called mechanical demucilagers to pulp and wash the coffees all at once, without a distinct fermentation stage. These can also be used to only partially remove the mucilage, resulting in a coffee that rests somewhere between washed and pulped natural. This type of coffee is sometimes called honey processed, though that term is also used to describe pulped naturals, a dry processing method.

A note on semi-washed. This term is the cause of much confusion in discussions of coffee processing. It is used to refer to either wet-hulled coffee or coffee that has been partially demucilaged mechanically, two distinct processes that produce dramatically different coffees. I prefer to avoid the term altogether.

2. Dry Process:

Dry processed coffees are produced without removing the mucilage prior to drying.

2.1 Natural Process

The coffee is not pulped after picking; rather, it is immediately laid out to dry in the sun, a stage which lasts for 2-4 weeks. When the coffee has fully dried, it is sent through a hulling machine, which removes the pulp, mucilage, and parchment all at once. Like wet-hulling, this process results in increased body and lower acidity, but generally has a bit less earthiness, greater complexity and can result in intense dark berry and wine-like flavors.

Most natural processed arabica comes from Yemen, Ethiopia, and Brazil. Brazilian natural and pulped natural coffee forms the base of most classic espresso blends. There is an important distinction between the way that Brazilian and Ethiopian natural processed coffee is generally harvested. In Ethiopia, the coffee is hand-picked over a period of time to ensure that all the coffee is at the same level of ripeness when picking, while in Brazil, farmers generally wait until the coffee is extremely ripe and then mechanically strip-pick the trees, which means that many of the coffee cherries have already begun to dry on the branches. Brazilian naturals tend to be nutty and mild, while Ethiopian naturals tend to be powerfully fruity.

2.2 Pulped Natural AKA Honey Process

The coffee is pulped after picking. When the coffee is dry, a hulling machine removes the mucilage and parchment. The flavor of pulped natural coffee rests between washed and natural processed coffee. Pulped natural coffee is produced around the world but is especially associated with Brazil.

Drying Methods

All dry processed coffee and most high-quality wet-processed coffee is sun-dried. Confusingly, sun-dried is sometimes used as a synonym for natural processed. African farmers usually dry their coffee on raised beds, while Latin American coffees are generally dried on cement patios.

An interview with Peter Giuliano which can be found on James Hoffman's blog (main source)