Secondary Metabolites

The jasmine odour scents the morning air,

The busy bees are ransacking the flowers;-

This is the first reward of all our care.

May Heaven protect us now from the thunder-showers

To dash the blossom, leaving branches bare-

Marring our hopes; may better luck be ours,

And weather neither over-dry nor wet,

But softly moist, to make the blossom set!

Aliquis, 1861

The relationship between the passage above and the quality of roasted coffee may only be immediately clear to a small number of individuals within the coffee community.  This is because topics like the one we are about to discuss are often seen as too abstract for research.  However, understanding the relationship between the scent of a coffee blossom and the aroma in roasted coffee is no more abstract than grasping how a person's childhood experiences can shape their adult behaviour [1].  While other scientific fields readily accept these relationships, some within the coffee science community fail to recognize the importance of understanding them and are often hesitant to explore such links.  This reluctance means that this field of research remains rich with opportunities for further exploration, growth and as an emerging area of study and innovation.   Allow me to convince you of its potential.

The delightful scent of jasmine that fragrances the coffee fields as they blossom are a part of a group of compounds known as secondary plant metabolites.  We will primarily concern ourselves with those that are volatile.  Secondary metabolites do not contribute to the essential functions of the plant, e.g. photosynthesis, respiration or cell division amongst others (Bhatla, 2018).  Instead secondary metabolites play critical roles in a (coffee) plant’s reproductive success and overall longevity by,

• • • Attracting bees to pollinate coffee flowers, resulting in the conception of our beloved coffee cherries.  

    • Enticing animals to disperse the seeds of ripe coffee cherries.

    • Protecting coffee seeds, beans, from insects and pathogens [2]. 

Secondary metabolites, for instance monoterpenes, which attract us with their scent, are toxic to a variety of organisms, including certain insects, microbes and even humans at the right dose [3].  This is likely why these compounds are naturally engineered to be volatile, allowing them to escape quickly from the plant’s tissue without causing it harm, as they are often also phytotoxic – toxic to the plant.  

Despite these inherent roles, plants, including coffee cherries, do not need to be in a constant state of alert, trying to entice bees to pollinate their flowers or disseminate their seeds.  Instead, research suggests that plants can regulate the production and release of these compounds in response to both living (organisms) and non-living (environmental) stressors.  The impact of climate stress will be briefly discussed in a later section, exploring how its potential to uniquely influence cup characteristics may be contributing to the concept of terroir within the coffee industry.  

Regulating the production of these secondary metabolites is key as trying to attract herbivores or birds to spread coffee seeds while the cherries are green would be nonsensical, as the beans have yet to mature to a point where they can survive on their own, away from the plant.  Hence, recent findings () indicating that these compounds are produced and stored during the ripening of the fruit’s mesocarp, align with their biological role [4].  In other words as the coffee cherry matures its need for self-defence and ability to attract animals to disseminate its grain grows, enabling it to gain its independence.

The storage strategy of these toxic volatile compounds is quite clever and aligns with the biological processes we know occur during fruit ripening.  As the plant hormone ethylene orchestrates the transformation of the  complex carbohydrates, within the mesocarp, into sugars, boosting the fruit's Brix value, some of these sugar molecules are being tethered to the volatile secondary metabolites being simultaneously synthesized [5] - this is sometimes seen as a decrease in the compounds’ emission during ripening as they are no longer volatile.  The new molecules formed are known as "bound volatiles" or "glycosidically bound volatiles (GBV)" in scientific literature [6].  

Adding a sugar molecule serves as an anchor, keeping the volatile secondary metabolite in the plant's tissue, similar to how one would secure a helium balloon to prevent it from floating away.  This process also reduces the molecules' phytotoxicity, as GBVs are not considered harmful to the plants.  Although these compounds are no longer volatile and do not impact the aroma, they still have the potential to influence the taste of green coffee.

For instance, researchers have identified the diterpene glycoside mozambioside [7] in green C. pseudozanguebariae and C. arabica coffee whose bitterness threshold (60 μmol/L) is more than ten times lower than that of caffeine (680 μmol/L) (Prewo et al., 1990; Lang et al., 2015).  In other words, if you have one gram of mozambioside, you would need around 11.3 grams of caffeine to match the perceived bitterness, according to Lang and colleagues’ (2015) study.  As mozambioside decreases [8] during roasting, the incomplete breakdown [9] of this compound may contribute to the characteristic bitter taste found in underdeveloped coffees, a roast defect.  As there are few studies covering the sensory contribution of GBV’s [10], to this author’s knowledge, it is unsurprising that the connection between scientific outcome and practical application remains to be substantiated.  

If we want to produce coffee with enticing aromas, such as blueberry and blackcurrant, then we need to be alert to why these terpenes are there and do what we can to encourage their presence in the beans.

Terpenes are secondary metabolites that are naturally formed within the maternal tissue as part of the plant's defence mechanism against living and environmental stress.  To be effective for defence, they have to be volatile. But to survive through to the roasted bean they need to be attached to one or more sugar molecules. So we need a balance of build-up of defence chemicals (the plant should be aware of potential threats) without the need for those chemicals to be released due to a real threat.

The Release article helps you to understand this subtle balance, so that your knowledge of your plants and their environment might allow you to more reliably add these delicious notes to your roasted beans.