Kenyan coffee has a reputation for bright, juicy acidity, and a big part of that comes from two varieties picked out in the 1930s at Scott Agricultural Laboratories, a research station in Kenya. SL28 and SL34 (the SL stands for Scott Labs) are still widely grown in Kenya nearly a century later.
SL28: The Drought Survivor
In 1931 A. D. Trench, a senior coffee officer, toured Tanganyika, now mainland Tanzania, and found drought-tolerant coffee in the Moduli district. He brought seed back to Kenya. In 1935, researchers at Scott Labs selected a single tree from that “Tanganyika Drought Resistant” population, and it became SL28. It was part of a larger effort between 1935 and 1939, when 42 trees of different origins were studied for yield, quality and resistance to drought and disease.
SL28 can produce exceptional coffee at high altitude, and it’s tough. Some Kenyan trees are 60 to 80 years old and still productive, and it can be left untended for long stretches and come back. Genetic tests place it in the Bourbon family, alongside the Bourbon and Typica lines behind so much of the world’s arabica.
SL34: A Surprise in the Genes
SL34 came from a single tree on the Loresho Estate in Kabete, near Nairobi, labeled “French Mission,” and was selected in the late 1930s. The label pointed to the Bourbon seed French missionaries brought to Kenya from Réunion in 1893, so SL34 was long assumed to be a Bourbon. Recent genetic tests found it’s related to Typica instead. It does best above about 1,200 meters with plenty of rain, and it produces large beans with exceptional cup quality at altitude.
The Weak Spot
Both varieties have little resistance to coffee leaf rust and suffer from coffee berry disease, and SL34 is highly susceptible to it. That’s a serious problem in Kenya, and it’s why breeders developed disease-resistant alternatives, the best known being Ruiru 11 and Batian. The SLs’ cup quality keeps them popular with specialty buyers, which is part of why farmers still grow them despite the risk.
How Kenya Sells It
Kenyan coffee is almost entirely washed, which suits the SLs’ bright acidity, and it’s graded by bean size: AA for the largest beans, AB for the next size down and PB for peaberries. SL28 has spread beyond Kenya too, to Uganda and other African arabica regions, and more recently to Latin America.
A washed Kenyan SL28 makes a vivid pour-over and a bold espresso. In a cortado, the milk rounds its acidity into something fruity and soft. It’s worth trying at least once, especially if you think you don’t like acidic coffee.