Harvest

Small pin-head clusters we can now discover;

The fruit has set; these are nascent berries.

The intervening stages pass we over; -

First like green peas, then like olives, then ripe cherries,

O’er which you gloat as fondly as a lover,

Whose love, like yours, with expectations varies.

Arrange your gangs the several fields to suit;

Pick row by row, and only the ripe fruit;

Aliquis, 1861

Many contemporary descriptions of coffee harvesting lack the captivating charm present in Aliquis' prose.  Instead, the process is often communicated in a brief monotonous manner, serving as a transitory topic to the more colourful one of post-harvest processing.  These short texts often convey a practical simplicity that could be effectively captured in a diagram [1].

With the connection to coffee quality often distilled down to the cherry’s [2] colour, where red [3] indicates ripeness - signalling the field’s [4] harvest.  In selective harvesting, only ripe cherries are picked, while in strip picking [5], cherries are harvested without regard for colour or maturity and sorted later. 

What these texts fail to capture is the relationship between harvesting - physically separating the seed from the plant - and its implications on coffee quality, which will be the focus of the current discussion series.  Where we will initially discuss the physical implications and then in subsequent articles the chemical.

If you have ever been apple or cherry picking, you might have noticed a clear anatomical separation between the fruit's stem and tree, or stem and fruit, revealing where the ripe fruit naturally detaches during harvesting.  This natural anatomical division is known as an abscission zone and is also observed in tomatoes, strawberries, as well as a variety of other fruits and vegetables.  During ripening a biochemical process triggers the separation of the tissues, allowing the anatomical parts, like the stem and fruit, to part company without causing either structural harm.  

Unlike other fruits [6], coffee cherries do not have an abscission zone (Tongumpai, 1993; Brandão et al., 2016).  Instead, as coffee cherries ripen, enzymes break down the carbohydrates in the cherry's flesh.  This process increases the concentration of soluble solids, often measured as Brix [7], which then osmotically draws water into the fruit's flesh from both the plant and the cherry's surface [8].  The absorption of this water increases the coffee cherry’s weight.  This, when combined with the swelling flesh and ongoing enzymatic activity, weakens the cell walls, eventually causing the coffee tissue to rupture and detach from the plant, on its own or with minimal effort.

1. Coffee cherries should be processed promptly after harvesting to prevent potential infections that could lead to unwanted fermentations.  In other words treat the wound before an infection sets in.

2. Cherries that have fallen on the ground should not be mixed with freshly harvested ones from the tree. The idea is that cherries on the ground may be heavily contaminated with microbes, especially undesirable ones, which could potentially contaminate the other cherries and depreciate the value of the harvest.  Not unlike how people with contagious infections are quarantined in a hospital setting, especially from those who are immunocompromised, to prevent the disease from spreading – the same general concept is used in fruit harvesting.

With the common recommendation to prevent infections being to reduce the time between harvesting and processing, I came across an alternative interesting approach in literature: using beneficial bacteria, such as lactic acid bacteria (LAB), on cherries to prevent unwanted fermentation from establishing.  Under aerobic conditions, yeasts and fungi flourish, so by introducing a more benign, but competitive microbial species into the environment, e.g. LAB, one might be able to prevent undesirable fermentation [9].  It's worth noting that despite the comparisons between coffee fermentation and the wine industry, chemicals like sulphite [10], commonly used in winemaking to control microbial fermentation, are not typically mentioned in post-harvest coffee processing.

The wound created upon harvesting coffee is why many within the coffee community believe that fermentation is ubiquitous in coffee.  This would also mean that there is always a possibility that microbial metabolites contribute to coffee’s aroma.  So is it possible to harvest coffee without injuring the cherry?  

One possible approach is to relocate the point of injury away from the fruit by cutting the stem, also known as the peduncle.  By relocating the injury away from the main fruit structure, we may decrease the size of the injury and shift it to a different type of tissue (the stem instead of the fruit). Essentially, this action gives the coffee plant some time if infection occurs, as the infection would need to travel up the stem to reach the fruit.  Harvesting by cutting the fruit's stem is common practice in the cocoa industry and is used when harvesting premature fruits and vegetables like oranges and tomatoes [11] for transportation to regions in the global North, where ripening is then induced with a phytohormone, e.g. ethylene.  However, this approach is ill suited for coffee harvesting due to the short length of coffee peduncles and the close clustering of cherries, making it challenging and impractical to harvest in this way.  

While separating the cherry at the point where its stem meets the branch may seem like an obvious alternative solution, it is not recommended.  This is because the location on the branch would scar over, hindering the formation of flowers and coffee cherries from developing there in the future (Gomez, 1894).  So, what other options does one have?

Not only do these traditional practices likely largely preserve the integrity of the coffee cherry, but they also make the harvesting process easier.  Drying out the coffee stems causes them to become brittle and prone to breaking when the coffee tree is shaken.  This means that,  

“When [you] have a Mind to gather the Coffee, [you] lay some Pieces of Cloth under the Trees, which [you] afterwards shake and all the ripe Coffee falls down easily; these are put in Sacks to be carry'd elsewhere . . .”

La Roque, 1732

Optimal harvesting of coffee is made difficult by the fact that coffee does not naturally separate from the plant, meaning that a wound is formed when lush ripe fruit are harvested.  This injury is susceptible to infections, which can result in the fermentation of the coffee cherry.

Although other fruits can be cut from the stem to avoid injury, this is not an option for coffee because of the clustering and length of its stems.

For those in dry climates, following the tradition Yemeni practices of partially drying the ripe fruit on the plant before harvesting might be a good way to improve the flavour and simplify harvesting through shaking the plant. For those in semi-dry climates, if there is a way to restrict water supply towards the end of the growing season, then this option opens up with its flavour/shaking benefits and the bonus of saving water for other crops or other parts of the cycle.