Part 1: Fermentation

Traditionally, coffee fermentation occurs during wet processing to facilitate the removal of the mucilage layer of the coffee cherry, which helps accelerate the drying process.  Essentially, fermentation uses exogenous pectolytic microbial enzymes[1]  to break down the coffee's mucilage layer, cleaning the outer layer of the parchment.  In this discussion, we will focus on the modern interpretation of coffee fermentation, which involves creating and bestowing the coffee with additional flavours and aromas, in order to enhance its value and appeal.

Before associating different forms of fermentation with specific aromas and flavours, which will be done in future articles, it is important to understand what is going on in the fermentation tank.  To clarify what is going on we will deconstruct green coffee fermentation into three parallel processes; fermentation, diffusion and “pathways”[2], over a trilogy of articles.  

We will start by addressing the function of fermentation, which is the process's namesake.  Although fermentation technically refers to the microbial metabolic conversion of organic matter, this article will highlight the unique role of fermentation in coffee processing by likening it to winemaking.

Coffee cherries and grapes are seed containing fruits.  This means that while their composition may be different, they share a similar anatomy.  In both instances it is also preferable to harvest the fruit after it has ripened completely.  

As the fruits ripen, the sugar content in their flesh, known as the mesocarp, increases. This draws water into the flesh through an osmosis-regulatory process, making the fruit succulent and irresistible to both humans and microbes alike, i.e. when you want to eat the fruit, so do the microbes.

After grapes are harvested, they are designated for the production of either red or white wine [3]. These distinctions are based not only on the grape variety but also on the processing method used.  In contrast, in coffee processing, the cherries may be sorted based on their ripeness, revealed by their colour, but this sorting does not determine the subsequent processing steps.

In white wine processing, the extraction of red pigments is unwanted, so the skin and seeds are removed before fermentation.  On the other hand, in red wine fermentation, the skins, fruity flesh of the mesocarp, and seeds are included with the juice.  The alcohol [4] produced during fermentation aids in extracting the red pigments.  After the skins have fulfilled their role, the flavourful microbial byproducts are separated from the seeds through a physical extraction process.  Grape seeds mainly play a passive role as spectators in wine’s fermentation process.

This is in stark contrast to the seed’s perspective in coffee fermentation, where the seeds take centre stage as the final product.  This role reversal means that fermentation plays a supportive function to the two complementary processes, diffusion and the alternative pathways.  The cooccurrence of these processes is likely the origin of why fermentation is treated and viewed as an art within the coffee community, rather than a science.  Within science as well as research-development divisions of the food industry these processes would be separated, where possible, studied and subsequently optimized to assess the cost-benefit of each.

The optional and secondary role fermentation plays in coffee quality technically classifies these microbial metabolites as “food additives” according to European and North American food regulatory norms.  This is much to the chagrin of those within the coffee industry that rely on the fermentation process to elevate their coffee’s quality, as it implies an intentional doctoring of the coffee’s flavour profile.  Depending upon the potency of these characters they may mask the genuine character, the terroir of the bean.

This classification is unsurprising as while this topic is new to the coffee community the generation of natural flavours and aromas [5] through the exploitation of microbial metabolic pathways is common in the broader food industry - flavour and fragrance companies have been employing this technology for several decades.  

Consequently, the major feature that distinguishes coffee fermentation from wine or other fermented products, is not the microbes [6] that frequently overlap between products, but rather the substrate [7], i.e. coffee’s mesocarp, which can be thought of as the food fed to the microbes.  This novel feature has led to the filing of several patents on cascara fermentation, all of which mention the generation of aroma compounds.

The capacity to divorce coffee mucilage fermentation from the other processes, diffusion and alternative pathways, also allows us to study it under more controlled conditions, i.e. using microbial media simulating coffee mucilage [8] in a controlled environment.  By having complete control over the microbes, diet and environment scientists can study their preferences: 

• What temperature do they prefer?  

• Do they enjoy sunlight?

• What are their favourite foods?  

• Which aromas and flavours do they form under these conditions?  

• At what rate do they produce these products?  

• Do they co-produce anything that we don’t want?

• Is there anything they don’t like? Perhaps oxygen?

These questions may remind you of a first date or an interview, because they sort of are.  The scientist is trying to “get to know” the microbe.  This will enable them, or those at origin whom they will convey the information to, to implement conditions that promote the growth of desirable microbes while suppressing the growth of undesirable ones.

It would be remiss not to mention that laboratory conditions are often not directly applicable or scalable to real-world situations.  Therefore, it is important to establish practical expectations based on the available resources, as achieving ideal conditions and identical outcomes may not always be feasible, especially in open field situations.  This holds true not only for coffee but for all fermented products.  

When fermenting coffees at origin one optimally wants to encourage the growth of beneficial microbes native to the environment [10], by adjusting the environmental conditions of the product to the growing preference of the desired microbe.  However, this is not always possible, especially at lower altitudes where the microbial population is large and the growth is unrelentingly fierce due to the warmer humid climates.  

One way of forcing the odds in favour of the desired outcome, in these competitive environments, is to deploy an army of microbes, better known as a starter culture.  Starter cultures are known microbial strains that can be commercially purchased.  When you bake bread you buy baker’s yeast, when you ferment coffee you buy coffee yeast.  The aim is to outnumber, and out-compete, the unwanted microflora [11], in essence “winning” the upper hand within the microbial battlefield of the fermentation tank by sheer numbers [12].  

These approaches - while potentially effective at increasing coffee quality - require farmers, who may not be earning a liveable income, to invest further into their facilities, supplies, training [13], time as well as risking their harvest without a guarantee of an equitable remuneration for their efforts.  Furthermore, market demand for these coffees may change, exposing these farmers’ to an additional unknown financial risk.  

Taking these auxiliary factors into consideration one might question why coffee fermentation has not been optimized to minimize these risks and create an additional revenue stream for farmers through value-added fermented cascara products.  The truth is many consider the optimization of rural practices a futile pursuit.  However, others acknowledge that development is the key to progress and have already successfully implemented these practices, e.g. Biofortunes Group in Honduras.  Groups like the one mentioned realize that coffee production, that may involve biotechnology, should and can be run like any other modern operation.  Making excuses why it is not possible is as they say making the most expensive mistake in business and saying that, “We’ve always done it that way.” 

To enhance the quality and financial benefits of coffee fermentation, it is essential to consider it as three distinct processes: fermentation, diffusion, and "pathways".  By recognizing that the creation of aroma and flavour precursors through the fermentation of coffee's mucilage is a separate process, we can isolate it from other activities in the fermentation tank.  This separation enables a more precise study of the microbial condition to determine the optimal conditions for generating the most desirable aromas and flavour precursors with maximum yield.  Implementing these conditions can then improve operational efficiency.  

We will discuss how these aroma and flavour precursors can be reintegrated with the coffee bean in one of the upcoming articles.