SL28 and SL34 — Kenya's blackcurrant acidity
2026-07-10
SL28, SL34. As variety names they are terse codes, but you cannot talk about the vivid acidity of Kenyan coffee without these two.
Scott Laboratories' selection numbers
SL stands for Scott Laboratories. In the 1930s, in Kenya, then under British rule, this laboratory selected outstanding individual trees from existing coffee plants. The reference numbers used at the time simply became the variety names.
SL28 was selected from a line from Tanganyika (now Tanzania) said to be drought tolerant. Genetically it is said to belong to the Bourbon group. SL34 was selected from a single tree on a farm near Nairobi and was originally thought to come from a line brought by French missionaries (French Mission Bourbon). However, later genetic analysis has suggested SL34's origins may be closer to the Typica group, and the traditional account is being revised.
Family tree
The family tree of varieties (generations run top to bottom)
← Scroll sideways →
- Ethiopian landraces → Typica (via Yemen)
- Ethiopian landraces → Bourbon (via Yemen)
- Ethiopian landraces → Geisha
- Typica → Maragogipe
- Typica → Mundo Novo
- Bourbon → Mundo Novo
- Bourbon → Pacas
- Bourbon → Caturra
- Bourbon → Villa Sarchi
- Bourbon → SL28 / SL34
- Canephora → Timor Hybrid
- Maragogipe → Pacamara
- Pacas → Pacamara
- Mundo Novo → Catuai
- Caturra → Catuai
- Caturra → Catimor
- Timor Hybrid → Catimor
- Villa Sarchi → Sarchimor
- Timor Hybrid → Sarchimor
- Sarchimor → Marsellesa
- Sarchimor → Centroamericano
- Wild Ethiopian coffee → Centroamericano
- Marsellesa → Starmaya
- Centroamericano → Starmaya
Acidity described as blackcurrant
What defines both varieties is a strong, clear acidity. It is often described with words like blackcurrant, grapefruit and tomato, and when combined with a high-sugar fruitiness it leaves an impression nothing else can replace. This varietal character lies behind the unique place Kenyan coffee has built in the world.
Kenya's distinctive processing also helps this acidity stand out. Double Washed, which repeats fermentation and washing twice, and soaking in water after washing produce a clean cup with a sharp outline.
The weakness of resistance
Neither SL28 nor SL34 has resistance to leaf rust or coffee berry disease (CBD). To overcome this weakness, Kenya's research institutes developed and promoted disease-resistant varieties such as Ruiru 11 and Batian.
But in cup evaluations the SL lines still often come out on top. Every year, producers have to decide between disease risk and quality. The fact that SL28 fields keep shrinking reminds us that being able to drink good coffee is never something to take for granted.
Remaining a number
While many varieties carry the name of a place or a person, SL28 and SL34 circulate around the world as a laboratory's reference numbers. That impersonality, if anything, conveys what selection really is. Observing hundreds of trees, keeping records, assigning numbers and deciding which to keep. That patient work is what lies behind the cup.
The variety family tree puts the connections between varieties and their differences in character into a single diagram.
