Altitude & Antioxidants

If you have come across CQAs in coffee literature you are likely to have heard the praise bestowed on them as antioxidants.  You may have also come across the widely disseminated fact that robustas are richer in CQAs than arabicas, which is frequently held responsible for their disagreeable bitterness found in the former’s roast coffee quality (which you will learn more about in another section).  Unfortunately, this does not make sense as why would coffee, e.g. robusta, grown at lower altitudes be richer in antioxidants than coffees, e.g. arabica, grown at higher elevations, exposed to a greater oxidative stress?

Literature on CQAs primarily surround their ability to deescalate the threat posed by radicals, otherwise known as their antioxidant capacity.  These compounds are known as antioxidants, as radicals are commonly formed when oxygen, in its excited singlet (reactive) state, interact with another molecule, e.g. protein, lipids or carbohydrates.  The resulting radicals have unpredictable biological outcomes that are frequently harmful to the organism.  In other words one can think of antioxidants as the police of the chemical world, trying to keep the peace [1].  Consequently, antioxidants are commercially valued for their capacity to prevent these detrimental outcomes [2].  If CQAs act as antioxidants within coffee beans, than would one not expect them to be richest in the most oxidative environment?  

Despite the temperature and oxygen levels found at sea level being greater than those found at high altitudes, these differences are eclipsed by the magnitude with which UV radiation increases with elevation, refer to figure above.  Increasing UV radiation is a recipe for radicals as it effortlessly excites oxygen into its singlet state promoting oxidation in these environments.  This begs the question why aren’t CQAs greatest at altitude?  

There is the occasional study that has observed an increase of CQAs with altitude, however these studies are distinct in that they involve robusta varieties ().  Research investigating the influence of altitude on CQA levels in arabica varieties consistently demonstrate that they decrease or stay the same as the elevation increases ().  This contradiction is noteworthy as it speaks to a potential fundamental difference in the behavioural adaption of different coffee species to oxidative stress.

The good news is that CQAs are antioxidants, the bad news (I am going to suggest), is that they are not powerful enough to deal with the oxidative stress experienced at higher altitudes.  Robustas may up-regulate CQAs, unable to shift into the next gear, that may be accessible to arabicas.  In other words, CQAs may not be acting alone.  Anchored, by their root systems, it is interesting to consider how plants cope with elevated levels of stress.  The truth is that very little attention has been given to this topic within coffee research, but a recent review and perusal of plant science literature provides us with some insights on why this topic should be of interest – not only in the context of climate change, but quality!  So what is potentially happening?

The increase in radiation (UVA [3]) may be enhancing the photosynthetic efficiency, which is responsible for the generation of sugars within the plant.  When the coffee cherries are green they actually participate in photosynthesis [4] (Cannell, 1985; Ságio et al., 2013; Esquivel et al., 2020), possibly contributing to the accumulation of sugars during this stage of bean development (Joët et al., 2009).  Therefore increasing the plants’ exposure to radiation, e.g. at altitude, potentially promotes the accumulation of sugars, as seen in literature (Worku et al., 2018).  

During the development of the coffee cherry’s exocarp (skin) UV protective pigments, e.g. anthocyanin and carotenoids, will accumulate.  These pigments are better known for their role as a visual aid during coffee’s ripening process, where the green chlorophyll pigments degrade slowly revealing the underlying yellow and red hues [5] (Esquivel et al., 2020; Li et al., 2023).  This makes the colour change observed during coffee development not unlike the ones seen in Canada’s boreal forests during the fall.  Nevertheless, the question remains what does this have to do with the seed’s response to environmental oxidative stress?  

If you are interested in coffee quality than you should pay attention to the fat content of green beans.  Most aromas [6] naturally present in green beans and generated during roasting are lipid soluble.  Meaning that lipids contribute significantly to roast coffee aroma retention, with even some claiming that lipid content and cup quality are directly related ().  

The purported relationship between lipid content and coffee quality would not surprise me.  In my cupping experience I have noticed that some coffees lose their fragrance upon adding water, resulting in these coffees receiving a significantly lower score than those that retain their quality throughout preparation.  In other words, in the latter case, the expectation set by the dry coffee aroma is met upon brewing.  I have noticed that those coffees that retain their aroma are also frequently accompanied by round mouthfeel - not quite creamy – which is suggestive of a lipid emulsion.  These observations have led me to the hypothesis that coffees that allow the aroma to escape upon brewing lack lipids.  Unfortunately, I have yet to come across a study that corroborates or contradicts this suspicion.  

Nevertheless, fundamental science suggests that if you want coffee’s quality to reach the consumer, lipids are needed.  Luckily lipid content is one of the few traits that influence roast coffee quality, that can be measured [7] in the green bean, as lipids are not appreciably lost [8] during roasting.

We last left off that an increase in lipid content contributes to an increase in coffee quality, by promoting aroma retention, but how do lipids help coffee’s antioxidant capacity -  don’t lipids oxidize?  Yes, they do, but not all and the occurrence of the ones that do are fairly predictable in crops.  

Lipids are primary components of cell membranes, in every organism, including coffee.  Cell membranes function as flexible barriers between cell compartments.  This flexibility allows them to adapt to changing conditions, e.g. moisture.  Without this flexibility the membranes can rupture killing the cell – not good.  In other words cell membranes are vital structures.  Plants maintain membrane fluidity by metabolically modifying the fatty acids, refer to figure below, to the plants’ growing environment.  For instance in cooler climates, like those at higher altitudes, coffee plants will integrate more unsaturated fatty acids (UFAs) into their lipids (Joët et al., 2010). UFAs contain a kink within their structures disrupting how closely the lipids can pack within the membrane.  This reduces the rigidity [9], allowing the membrane to remain flexible at lower temperatures.  Unfortunately, while UFAs are enriched at higher elevations and cooler climates, they are also more vulnerable to oxidation!  

UFAs are vulnerable to oxidation, but this is why plants protect them with several layers of defences.  First one needs to remember that the coffee’s husk is an oxygen, moisture and UV barrier during the bean’s development.  This shields the bean and allows UFAs to be used and accumulate within the beans’ endosperm in preparation for the environmental stress awaiting the bean upon separation from the plant and removal of the protective husk upon germination.  

As CQAs are lipid soluble, a greater lipid content leads to a greater dispersal of these protective antioxidative compounds within the cell.  The distribution of CQAs is not the only role lipid droplets (LDs) serve in defending the cell against oxidative damage.  LDs are thought to serve as lipid reservoirs that are used in the repair of cell membranes when oxidative damage occurs.  Part of this repair mechanism is thought to involve separate organs known as peroxisomes. These organs are laden with “oxidative [10]” enzymes that facilitate in the repair of damaged cellular structures, e.g. by reactive oxygen species such as radicals.  In other words coffee beans cultivated under oxidative stress, e.g. high elevations, may be repairing the oxidative damage at a greater rate of efficiency than can be inflicted upon them.  This repair mechanism preserves the bean’s viability in these environments, decreasing the need to accumulate antioxidants such as CQAs.