The Colour Blue

When I first read that natural and washed coffees were often depicted in literature as having different colours, I wanted to know why weren’t they the same?  Imagine that you harvest coffee from the same plot of land, mix them all together, so you know the beans are chemically identical, homogenous, before separating them for post-harvest treatment.  Half you spread out on a raised bed and allow them to dry naturally before hulling, resulting in yellow-green beans, while the other half you husk, wash in tanks before drying on raised beds, leading to beans developing a blue-green hue.  What happened during the process that caused the colour to change?   Having yet to encounter an explanation on why this colour change takes place, I thought I would fill in this gap by providing an explanation that complements the chemistry behind both the sensory and compositional differences encountered in life and literature for natural and washed coffees.  As the pigment and sensory relevant chemistry will be dealt with separately, this explanation will focus on what is occurring within the bean during processing that results in the coffee pigment adopting a different character.

Despite the apparent inanimate nature of raw coffee beans, they are living organisms, the offspring of their parental plants.  This means that when cherries are harvested, they are alive, until they either lose their viability – no longer germinate - or are roasted . 

We will assume for the purposes of this conversation that the coffee is ripe upon harvesting.  Coffee cherry development and degradation of quality over coffee’s shelf-life will be covered in a separate section.

Until they sprout and start acquiring their own nutrients, coffee beans rely on the finite amount of resources they received from their parent plant to survive.  As the nutrient reserves within the seed are meant to last until it has become self-reliant, i.e. a plant, this means that until a suitable growing environment is found, beans must practice self-restraint to avoid prematurely consuming what their parents have given them.  Luckily plants do not need to rely on willpower, but instead they contain a hormone, abscisic acid (ABA), that suppresses the urge to germinate, i.e. initiate the bean’s metabolism.  Detected throughout the coffee cherry ABA has been found to only be genetically expressed, i.e. produced, in the pericarp (husk).  This means that ABA’s presence in other tissues is dependent on the presence of the husk and its transportation into the neighbouring tissues, i.e. the bean or endosperm.  Therefore, as long as the husk is present, e.g. dry processing, and the cherry is moderately hydrated, e.g. 9-13% w.b., the beans’ metabolism is suppressed, keeping its composition relatively constant.  Natural coffees which are dried in their husk preserve the original composition of the bean - provided they do not become contaminated by insects or microbes which is not infrequently the case when dried in the open air for days on end.

The beans composition and consequent sensory character are the outcome of communication between ABA and the beans’ cells, but what language are they using to convey the message?  The colour of the bean, I believe, is related to the language used, rather than the message being sent.  Much like how one would learn another language to understand and communicate with different cultures, scientists study how seeds speak to understand and control their behaviour, e.g. ensure long-term storage.  The current discussion will apply knowledge gained from studying other seeds, as I have yet to find research investigating this topic in coffee literature.

Oxygen within the coffee community is often perceived as detrimental to coffee quality, e.g. enzymatic browning or lipid oxidation.  These unfavourable outcomes reflect the role of oxygen when the bean’s integrity has been compromised, either by excessive:

When the bean is healthy oxygen plays quite a different role, as its levels are modulated by several means:

The beans’ preoccupation in controlling oxygen uptake originates from the molecule’s role in giving the orders to germinate.  Oxygen is transformed into reactive oxygen species (ROSs) through a variety of enzymatic and non-enzymatic pathways.  ROSs progressively increase the oxidative state of the cells, with those of the embryo being of particular importance, activating genes and metabolic processes responsible for germination.  In essence oxygen orchestrates the activation of all germination functions, much like how a project manager would coordinate the delivery of a project.  As, “germination is an all-or-nothing event for each seed,” (Finkelstein et al., 2008) once started it will continue to completion, where completion refers to either the development of the seed into a plant, or seed death.  The outcome in either case is due to oxidative pathways initiated during germination.

Coffee’s main pigment is a redox (reduction-oxidation) indicator, meaning that its colour changes depending upon the environment that it is in, refer to Figure below. 

In natural coffees that retain all protective measures against oxidation of the seed, the bean has a “reductive” internal environment and consequently appears greenish-yellow.  Whereas washed coffee’s germination is triggered, increasing the oxidative state within the beans’ cells resulting in the pigment turning a blue-green colour.

Germination is initiated in washed coffees by removing the husk containing ABA, before submerging the beans in water.  One may initially think that plunging the beans in water is depriving them of oxygen, but the objective of washing coffee is not to suffocate the seeds, but rather to control the microbial activity, which will be discussed in a separate section on germination.  Depriving the seed of oxygen is thought to interfere with the internal detoxification process[1] for ROSs, responsible for keeping the beans dormant.  Therefore washing coffee actually results in the counterintuitive accumulation of ROSs within the seed, increasing its internal oxidative state, that cascades into ROSs that participate in the initial stages of germination.

Probably not.  This is likely the first publication connecting the bean’s pigment chemistry to our emerging understanding of the role of redox reactions in germination, so we cannot know for sure if every bean will have the capacity to turn blue as research in this field is currently lacking.  Nevertheless, judging from the literature that is currently available, the pathways responsible for controlling the redox status of the bean are likely dependent upon oxidative enzymes, and we do know that:

So depending upon how tightly the redox status (homeostasis) is controlled within the bean, some may not have the capacity to turn blue.   This would also align with green being the standard colour associated with raw coffee beans.

Yes.  Actually, before investigating coffee’s pigment chemistry my experience in Maillard reaction research and cupping coffee led me to the hypothesis that redox status was an essential component of coffee’s quality chemistry.  However, this information is completely useless in practical applications if there is not a convenient way of assessing the redox status of the raw bean.  It was really the repetitive individual observations within coffee literature that bean colour and quality were related, that spurred me to suspect that coffee’s pigment was what we in chemistry would call an indicator.  It would only be upon further investigation that I would uncover that it was a redox indicator and that it complemented my earlier flavour observations, which will be discussed further in other articles.