Climate

Through the roots, the terroir speaks,

Imparting strength to the plant's physique.

Disease resistance, a gift from the land,

Nurtured by nature's guiding hand.

In the cup, the essence blooms,

A symphony of flavours, rich perfumes.

Terroir's influence, a journey to savour,

In each sip, a taste of its labour.

ChatGPT, 2024

In the previous article, it was proposed that the secondary metabolites, which are synthesized by the plant for self-defence, also may be playing a significant role in determining the final cup quality.  The article also suggested that the extent to which these compounds impact the coffee quality are influenced by the processing methods used.  These practices, much like the expression of the secondary metabolites, depend upon the characteristics [1] of the region where the coffee is grown and how it is processed.  The current article aims to discuss how our understanding of regional differences not only influence the composition of secondary metabolites, but also the processing methods employed inevitably leading to the introduction of the concept of terroir or the association of flavour profiles with specific regions.  

To enhance the inclusion of secondary metabolites, such as glycosylated bound volatiles (GBV), in green coffee, the previous article proposed that the coffee cherries should be allowed to, at least partially, dry on the plant – reminiscent of traditional Yemeni practices.  However, in some coffee-growing regions, the climate is prohibitive to this practice.

“[T]he Coffee in the West India islands cannot easily be dried in a proper manner, from the great moisture of the air.”

A citation from Dr. Fothergill’s Letter to J. Ellis, Esquire, F. R. S. Agent for Dominica

As found in “An Historical Account of Coffee” written John Ellis (1774)

In humid coffee growing regions the excess water can cause the coffee cherries to ferment and rot on the plant if not promptly harvested.  The microbial contamination compromises the integrity of the coffee cherry by attacking its tissues, triggering the release of GBVs in a defensive response.  Moreover, the excess water absorbed increases the cell pressure and encourages enzymatic activity, which weakens and ruptures the cell walls.  This results in the unification of the glucosidase with the GBVs, releasing the aromatic volatiles.  These events result in the depletion of GBVs, draining the aroma pool’s potential of contributing to the coffee’s cup quality, suggesting that harvesting coffee cherries beyond their optimum ripeness in humid climates would negate the intended purpose of leaving the coffee cherries on the plant to begin with [2]. 

Furthermore, allowing the cherries to ferment on the plant can also be detrimental to the plants’ health as the microbial attack can spread to other tissues and structures, weakening the plants overall health.  This is also why almost every book on coffee agriculture emphasizes the importance of pruning for air-flow [3] in the plant’s structure, as localized accumulation of warm moist conditions are optimal for microbial growth and in turn make the plant vulnerable to disease.  

The intrusion of microbes is not unlike that of insects, e.g. coffee berry borer, as they are both foreign organisms that are violating the coffee cherry’s integrity.  The trauma imposed by foreign organisms is known within literature as biotic stress.  The coffee cherry’s defensive response will remain similar between organisms [4], but the amplitude may be significantly different, i.e. proportionate to the damage inflicted.  

It is interesting to consider that the release of these volatiles is not solely meant to repel the invading species, but also may be intended [5] to attract their predators.  As coffee is not indigenous to Asia or the Americas its defence mechanism may be like a North American calling 911 while in Europe or India where the emergency telephone number is actually 112.  At best there is no response, but at worst, calling for help attracts aggressors rather than dispels them, increasing the magnitude of the trauma inflicted.  This negative feedback would result in the demise of these plants and seeds, disincentivizing the genetic preservation of these traits.  In other words the plant would no longer teach its offspring to call 911 in case of emergency.  Unable to dial 911, it is worth reflecting on whether a plant can learn to teach its offspring to call 112?  Then there is the consideration that if 911 is a delicious aroma, is 112 equally tasty?  Project Origin’s website suggests that disease resistance may be able to exist in tandem with quality in some instances,

“The producers can see that any variety can make it to this top tier. Most of the varietals in the top 20 of the auction this year were resistant to leaf rust, which is also a great sign.”

https://projectorigin.coffee/best-of-honduras-recap/

Would the cultivation of the new variety be dissuaded if it were of poor quality?  History suggests not.  Nevertheless, these questions currently remain outstanding.  Coffee’s capacity to inherit new traits will be discussed in a separate section on coffee genetics.  

The low to moderate humidity in drier climates makes coffee cherries less desirable to organisms seeking to establish a home.  Some may mistakenly believe that in the absence of another organism the cherry does not experience stress, but it does from its environment.  When the environment triggers a plant’s defences, this is known as abiotic stress in scientific literature.  The influence of abiotic stress on secondary metabolic pathways is an emerging field of study, that has yet to be thoroughly explored within coffee research.  Therefore, the following discussion will attempt to translate the information available into a coffee relevant context.

Coffee cherries are climacteric fruits which means that they emit and respond to the phytohormone ethylene during ripening.  The emitted phytohormone stimulates its own metabolic pathway increasing ethylene production.  This in turn also increases the cherry’s respiration rate – yes the cherries breathe - accelerating [6] the process until the resources within the fruit are exhausted.  Remember coffee cherries are their own entity when fully ripe and therefore have finite resources.  We know from post-harvest research in numerous other climacteric fruits that the production of this hormone and the fruits’ respiration rate can increase with temperature, accelerating the ageing process.  Most people instinctively know this as they have left a climacteric fruit like an apple or a banana out in a warm environment [7] and observed that it aged more rapidly than had it been stored at a cooler temperature [8].  Therefore one can expect coffee cherries to demonstrate similar behaviour [9].

Coffee’s climacteric nature reveals two potential clues to how the secondary metabolism may respond to environmental stress,

1. 1. 1. 1. Temperature may increase the emissions of other compounds, e.g. terpenes and terpenoids, not just ethylene.

         2. Ethylene may be increasing the generation of secondary metabolites, not just its own.

The ability to stimulate gene expression is a characteristic behaviour of a plant hormone. Therefore, I initially assumed there was a connection between the genes being expressed and the hormone ethylene, which is typically emitted in climacteric fruits like coffee, especially if temperature enhances its release as mentioned above.  However, upon further investigation, I found no evidence to support this.  The hormone’s effect on these pathways can be isolated by spraying ethylene on the plant or fruit in the absence of any stress.  Had ethylene played a role in stimulating the production of free and bound volatiles, an increase would have been observed when the fruit or plant was treated with the hormone.  However, no reports were found that supported this hypothesis [14].  Ethylene exposure instead stimulates the production of the phytohormone abscisic acid [15] (ABA).  ABA in turn serves as a precursor of ethylene production and plays a role in breaking down polysaccharides into their sugars, as well as in the degradation of chlorophyll and the formation of pigments like anthocyanins, but not the generation of free or bound volatiles, like terpenes.  

We can experience the limitations of ethylene in fully activating secondary pathways that enhance the quality of climacteric fruits during ripening when we compare the flavour profiles of tropical fruits bought in Northern climates to those purchased in the tropics.  Tropical fruits harvested for consumption in the global North are often picked early and shipped while unripe [16].  They are then artificially ripened using ethylene or an ethylene analogue to prepare them for the marketplace.  While this artificially induced ripening process does change the colour and texture of the fruit as expected, it does not fully develop the natural flavour that would occur if the fruit were allowed to naturally ripen on its own in its native environment.  In other words while ethylene contributes significantly to the quality characteristics of ripe produce, it does not explain the creation of the entire aroma profile, allowing us to conclude that ethylene is not working alone.  Since multiple hormones are likely involved in creating the aroma profile characteristic of ripened fruit, it is possible that one of these hormones is also responsible for enhancing the activation of secondary metabolic pathways when the fruit is under environmental stress.

JA is formed from the unsaturated fatty acid linolenic acid (ALA), which are released from phospholipids in the cell membrane or potentially glycolipids within the chloroplasts [24].  More research is required to determine which pathway would be most relevant for coffee cherries.  Due to the unsaturated nature of the fatty acid, we can infer that coffees grown in cooler climates, for instance higher altitudes [25], are more likely to produce these hormones and aroma profiles compared to coffee grown in warmer environments.  

In order to convert ALA into JA, several enzymes and organelles oxidize and cyclize the original structure.  Afterwards, JA is transported into the cytosol, which serves as the cell’s circulatory system.  The fact that several oxidative steps are involved in the formation of JA complements numerous reports in the literature that portray JA production as a defensive response to the oxidative conditions introduced by abiotic [26] stress.  JA’s capacity to engage the plant’s natural defences towards environmental stress was shown in one study by spraying a plant with JA and exposing it to drought stress ().  The treated plant displayed an increased resilience.  Therefore if JA is being produced during abiotic stress in coffee production, it may be the hormone we are looking for that is capable of up-regulating the expression of the genes in secondary pathways [27] thus contributing to aroma and climate resistance.

Abiotic stress, such as high temperatures, solar radiation, or water deprivation, causes oxidative stress in the coffee cherry. This results in the release and emission of terpenes that work to protect the cherry from reactive oxygen species. The oxidative stress also triggers the production of jasmonic acid and its derivatives, which activate genes responsible for secondary pathways involved in terpene synthesis, thereby replenishing the supply of these molecules.

Research into coffee cherry respiration has largely been neglected, as focus has been on the seed within the cherry rather than the coffee cherry as a fruit.  However, the few studies available show that the respiration rate and ethylene production vary significantly between cultivars.  For example, Ságio and colleagues (2013) found that the respiration rate (mg CO2/kg hr) and rate of ethylene production (μL C2H4/kg h) were significantly higher in Catucaí 785-15 during the latter phase of ripening compared to the C. arabica cultivar Acauã.  Since the second phase of ripening is less pronounced and ethylene production is more modest in Acauã coffee, it is worth considering if this means that the variety remains longer in the initial phase of ripening, “system 1”, increasing the production of the coffee’s defences as well as aroma relevant free and bound terpenes.  While EMBRAPA [28] (2022) describes the Acauã cultivar as having good disease resistance, they also depict it as being of average quality.  Nevertheless, they do express the view that the quality assessment is not a reflection of the coffee’s potential, as a myriad of factors can influence a coffee’s quality.  A cursory search of the web provides some evidence that the cultivar Acauã has been associated with flavour notes reminiscent of several non-climacteric fruits including cherries and strawberries.  Naturally further research in this area should be complemented by sensory trials.  The cultivar Catucaí 785-15, on the other hand, is considered to be of a higher quality by EMBRAPA (2022), but is more susceptible to disease.  Another web search associates this cultivar with caramel, nutty and chocolaty flavours, which likely originate from the Maillard reaction, a topic we will cover in a separate article series.  

Interestingly, the study chose Catucaí 785-15 and Acauã as they represent early and late maturing cultivars, respectively.  This means that Catucaí 785-15 can be harvested several days or weeks ahead of Acauã, which introduces a financial consideration as time is money.  The early maturing coffees ripen more uniformly and quickly, likely due to their elevated ethylene emissions, allowing for an efficient and coordinated harvest that ultimately saves money.  If this classification system reflects the coffee cherries' metabolism during ripening, then favouring early ripening cultivars for financial reasons may be affecting coffee genetics, potentially compromising quality and disease resistance.  Making this an important area of research to develop.

The aroma profiles that terpenes can deliver are desired by many, but may only be possible in certain varieties and environments.

The evidence provided above suggest that coffee cultivar that take a longer time to mature, cooler environments and endure a moderate amount of environmental stress are likely to accumulate aroma relevant terpenes.  Whereas excessive moisture or heat will lead to the loss of these compounds due to the plant’s defensive response.