For some time now, there has been contention over the pros and cons of utilizing dry processing methods (natural processing) over that of wet processing methods (washed). In this post, I will attempt to dissect this debate. The first critical component to understanding this issue is to understand how green coffee is processed.
The first step in green coffee production is the picking of the ripe fruit from the coffee plant. The coffee fruit, commonly referred to as a “coffee cherry”, contains the coffee beans (the 1 or 2 seeds residing within the fruit), the mucilage (the pulpy fruit), the pergamino (the parchment-like coating around the bean), and the skin.
The second step in green coffee production is to remove the several layers between the outer skin of the fruit and the beans inside. Underneath the outer layer of the fruit is the mucilage, and beneath the mucilage is a papery parchment-like layer. The process of removing the skin, mucilage, and paper is where the difference between natural processed versus washed comes into play! Often, the processing of the bean is dependent on the region and farm where it was grown; certain areas are known for their variations in green processing techniques.
Washed processing is a method by which the mucilage of the fruit is removed through wet fermentation. The outer layer of skin and fruit is stripped away by a pulp processing machine, leaving behind remnants of pulp. The beans are then soaked in water for times ranging from several hours to two days. During this period, bacteria and fungi loosen the pulp from the coffee beans inside. The soaking beans are then washed down channels to remove the remaining pulp. After the removal of the mucilage, they are allowed to dry. At this point it is referred to as green coffee.
Natural processing is a method by which the entire fruit is allowed to sun-dry on the bean over the course of two to three weeks on raised beds, which may be covered or uncovered. Allowing the fruit to dry on the coffee bean imparts distinct and special flavor notes that are sought after by a small (but growing) percentage of the coffee drinking population. Yet a known problem in the production of natural processed coffees is that the presence of mucilage and lack of control during fermentation can lead to rot and spoilage – a great risk for the farmers in coffee producing countries. Thus, because of the special flavor-attributes and the risks during production, natural processed coffees often sell at a price premium similar to peaberry coffees. Furthermore, natural processing was traditionally only used in dry climates, where the risk of mold or spoilage was comparatively lower; yet, the price premium and recent increase in market demand has given rise to the “new naturals” – natural processed coffees made in wetter areas.
The microbial processes that occur during the loosening of the mucilage in washed versus natural processing are very different. First, it is necessary to address where these microbes are coming from and why that information is important. Local bacteria and fungi reside on the surface of the coffee fruit and in the environment. During both washed and natural processing, these are the microbes doing the fermentation. And due to the different conditions under which the fermentation occurs, it can have very different effects.
In washed coffee, the local microbial flora also secretes pectinase enzymes to loosen and "digest" the mucilage. However, this process is much more controlled and happens much faster (can be a matter of hours). The fermentation is also significantly simpler because the only substrate on which to ferment is the surface of the mucilage.
In natural processed coffee, the fermentation happens significantly slower and in a significantly drier environment. This is going to select for different microbes that will produce different secondary metabolites and will create different flavors – particularly the fermented fruit notes that natural processed coffees are known for. The microbial flora that has intrinsically inoculated the fruit releases pectinase enzymes that will break down the mucilage. Through the conversion of pectinaceous sugars, the microbes will produce many by-products such as ethanol, acetic acid, lactic acid, and butyric acid. In natural processed coffee, the matrix of fermentation is much more complex due to the presence of the outer layer. So the likelihood for a farmer or producer to lose their entire harvest when they natural process their coffee greatly increases.
These by-products, when left too long in the fruit, can leach into the coffee beans inside, rendering them spoiled or sour. If dry fermentation goes too long, it can even lead to the production of mycotoxins.
Therein lies the primary reason many consumers and coffee roasters shy away from natural processed coffee – they would rather not engage in the sale and production of natural processed coffee in order to lessen the demand and economic risk that some producers take, leaving them financially vulnerable. Washed coffee production methods are far less risky and generally yield more consistent results. However, natural processed coffee is easier and cheaper to produce in terms of hardware (wetmills), and fetches a price premium. In addition, the risk of losing a crop (often due to too wet conditions) is significantly low in some of the drier microclimates that natural coffee is produced in. Natural processing also cuts down on clean water usage, which can be a limiting factor in some of the drier coffee-producing areas.
Analytical Flavor Systems has thousands of artisan-roasted coffee reviews in our databases. We have compiled the objective flavor profile of coffees we believe are representative of the average roasted African or Central American coffee, which can be thought of as “smart” average where many factors are taken into account from each review.