November Rain: How Does It Affect Coffee Cherries?

 
It has been raining continuously for many days. The coffee farming community can only sit and watch with deep frustration, unable to intervene at this moment.

What situation are the people on the mountain facing? Do the people below know?

During the harvest season, coffee plants do not require large amounts of rain to ensure the smooth ripening cycle of the coffee cherries and allow farmers to dry their coffee. But when water keeps coming and the air is extremely humid, it not only makes work difficult, but the cherries also suffer damage that is hard to fix.

Lets understand this from a scientific perspective.

The Mechanism of Damage Caused by Heavy Rain During the Harvest Month

1. Fruit Drop (Premature Abscission) Heavy rain causes ripe coffee cherries to fall from the tree, both from the force of the rain impact and an additional factor: the stem attachment (pedicel) being softer than usual. The reason the stem softens is that when there is a lot of water and high humidity, the roots and the stem receive less oxygen due to hypoxic conditions. The hormone ethylene is produced and rapidly accumulates, causing the tissue at the stem base (the abscission layer) to soften and fall easily, even if the fruit is not truly ripe. Some farms may lose 10-30% of their yield if the rain continues for a long time.
2. Fruit Cracking This is due to Excessive Water Uptake & Internal Pressure. When it rains heavily for several consecutive days, the soil becomes extremely saturated, allowing the coffee roots to continuously absorb water into the fruit without losing water through transpiration because the air is humid and cloudy. This creates an abnormally high turgor pressure inside the cherry, especially in fruits that are "changing color" or "beginning to ripen," where the cell walls are already softening due to natural ripening processes (cell wall degradation). Fruits with very soft cell walls cannot withstand the pressure, resulting in skin cracking on the surface. The mechanism occurring at the cherry level: When water is absorbed rapidly, the sugars and solutes inside the fruit are diluted. The pressure in the pulp cells increases, leading to micro-cracks on the skin. As the cracks expand, mucilage leaks out. Cracked fruits like this will "dry prematurely" and the internal bean will be lighter and "pale in color." Beans with these characteristics significantly reduce the cup score.
3. Microbial Attack and Off-Flavors When the fruit cracks, it provides an opportunity for undesirable microorganisms to attack, causing fruit rot or creating off-flavors from uncontrolled fermentation. The growth of bacteria and mold during the pre-harvest period and extending into the processing stage drastically reduces coffee quality. Fermentation in the mucilage along the crack wounds often leads to off-flavor characteristics.
4. Rapid Drying and Sunburn Cracked fruits allow water to evaporate rapidly when weather conditions switch from humid to dry. After the rain stops and the sun is intense, the fruit surface temperature increases quickly while the inner pulp still retains a lot of water. This creates a difference in the shrinkage rate between the skin and the pulp (differential shrinkage). Fruits that are already cracked will experience "crack expansion" or "tearing deep into the seed tissue," leading to wider cracks. Water in the fruit will rapidly evaporate until dry, easily becoming a dry-on-tree fruit. Eventually, we get dry, shriveled, and immature fruits that remain on the tree.
5. Tree Stress After the rain stops and the weather turns intensely hot, the plant's transpiration increases suddenly. Such a large change in conditions causes the plant to experience stress. In some cases, leaves will wilt even though the soil is still wet.
6. Post-Harvest Quality Impact When the fruit pulp cracks and is directly exposed to the air, oxidation of polyphenol and anthocyanin occurs (catalyzed by the enzyme PPO Polyphenol oxidase). Strong sun and high temperatures accelerate this reaction, causing the fruit pulp to turn dark brown and develop fermented / sour taint smells and sharp acidity.

It is difficult to achieve good flavor results when fermenting coffee that has been damaged by heavy rain before harvest.

Overall Summary:

A storm/heavy rain during the coffee harvest month causes: premature fruit drop = direct yield loss; cracking and rapid drying after the rain stops, leading to sunburn; problems with mold and bacterial contamination; shriveled, dry, and immature beans, etc. All of which make quality management extremely challenging.

How Can We Solve the Problem or Reduce the Impact?

 
Unseasonal rain has become a regular guest during the coffee season. The question is, how can we prepare to cope with the rain to reduce its impact?

Here are some approaches I can see at the moment:

Shade Grown System:

Planting coffee under large shade trees helps to reduce the severity of the wind and the impact force of raindrops hitting the coffee cherries.

Strengthen the Coffee Tree:

Specifically, strengthen the cellular structure to better withstand water pressure. To achieve this, focus on nutrients that support this strength, such as Ca, B, Mg, K, Si.

Implement a Good Drainage System:

In the plantation to quickly remove accumulated soil moisture and reduce the problem of waterlogged roots.

Sort Damaged Fruits:

Separate cracked and spoiled fruits before processing to prevent contamination in the fermentation tank. Increase the cleaning of cherries before fermentation or drying to reduce the accumulation of mold or bacteria.

Avoid Immediate Sun Drying:

Do not rush to dry cracked fruits under intense sun immediately. We may air-dry them in the shade for a period first. In my opinion, wet-milling these coffees and processing them as washed or semi-washed without intense sun drying might still help preserve quality and prevent problems like over-fermentation or off-flavor.

Reduce Fermentation Time:

And control the temperature to stay within an appropriate zone; avoid hot fermentation.
Use Efficient Drying Facilities: If drying is necessary during continuous rain, an effective drying structure is essential. Using an LTLH (Low Temperature Low Humidity Drying) system would be highly efficient.

Arrange for Backup Power:

Reserve a backup power system at the farm.

Another major problem is landslides and soil erosion resulting from the soil retaining too much water until it can no longer hold it, which has affected many coffee-growing areas this year. We need strong knowledge and management systems to effectively prepare for such situations.

Therefore, it might be beneficial to develop new know-how, both for immediate use in the area and through supportive networks that provide manpower, knowledge, and capital.

I wish all my fellow farmers strength and perseverance!


References

Phromsakha, P., & Klanrit, P. (2023). Impact of seasonal weather variations on physical properties of Robusta green coffee beans in Southern Thailand. Indian Journal of Agricultural Research, 57(6), 757-763. ARCC Journals.
Winkler, A., Knoche, M., & Peschel, S. (2016). Rain cracking in sweet cherries is not due to excess water uptake but to localized skin phenomena. Horticultural Research, 3, 16018. Nature Research. https://doi.org/10.1038/hortres.2016.18
Perfect Daily Grind. (2016, December 2). How harvest rains destroy coffee crops. Perfect Daily Grind. Retrieved from

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Thank you for the illustration from the Facebook page กาแฟโกลฮาคี-Klohaki Coffee Roasters.