SL28 and SL34

SL28 and SL34 are the two varieties responsible for Kenya's extraordinary reputation in specialty coffee. Selected by Scott Laboratories (abbreviated SL) in the 1930s and 1940s from farms across Kenya, they were chosen primarily for drought resistance and high yield under colonial agricultural priorities — but they happened to produce a cup profile unlike anything else in the coffee world: intense blackcurrant, grapefruit, tomato, and phosphoric acidity that defines "Kenyan" as a flavour category in its own right.

Scott Laboratories and the Selection Process

Scott Agricultural Laboratories (later Scott Research Laboratories) was a British colonial agricultural research institution based in Kabete, near Nairobi, Kenya. Beginning in the 1930s, Scott Laboratories conducted extensive selection work on coffee plants from farms across the country, selecting individual trees with desirable characteristics and multiplying them for distribution. The selections were numbered sequentially; SL28 and SL34 emerged as the most successful for quality and performance in Kenyan conditions.

The selections were made from different genetic backgrounds:

SL28 was selected from a drought-resistant Kenyan variety possibly originating from Tanganyika (modern Tanzania), itself believed to be related to the ancient Bourbon lineage brought from Yemen centuries earlier. SL28 has particularly large leaves and beans (indicative of Bourbon genetics) and produces a cup of exceptional intensity.

SL34 was selected from a Kenyan farm from material believed to have French Mission Bourbon origin. It shows slightly more disease tolerance than SL28 and is a more vigorous grower. The cup character is similar to SL28 but typically slightly rounder and less extreme in acidity.

What Makes Kenyan Coffee Taste Kenyan

The SL28 variety in particular contains an unusually high concentration of phosphoric acid compared to other arabica varieties. Phosphoric acid produces a bright, clean, intensely citric acidity that is distinct from the softer malic acidity of Central American coffees or the tartaric character of some Ethiopian varieties. Combined with the specific blackcurrant compound (related to the terpene compounds also found in blackcurrant fruit) that both varieties produce, the result is a cup profile that is immediately recognisable:

• Blackcurrant or blackberry: The most distinctive Kenyan flavour note, often described as vivid and wine-like.
• Grapefruit, blood orange, or lemon: The phosphoric acidity registers as bright citrus.
• Tomato: Common secondary descriptor, particularly in slightly darker roasts or less precisely extracted cups.
• Caramel or toffee sweetness: The background supporting structure that balances the acidity.
• Full, syrupy body: SL varieties produce particularly dense, coating body even in lighter roasts.

Growing Conditions in Kenya

Kenya's Central Province highlands — the Kirinyaga, Nyeri, Murang'a, and Kiambu counties — grow most of the country's premium SL28 and SL34 at altitudes of 1,400–2,100 metres. The combination of altitude (producing slow cherry maturation and complex flavour development), red volcanic soil rich in minerals, distinct wet and dry seasons, and Kenya's well-developed double-washing processing technique produces the reference expression of SL28.

Kenya uses a two-stage washing process unusual in the coffee world: pulped beans ferment in water tanks, are washed, then soaked in clean water for 24–72 additional hours ("soaking phase") before drying. This extended water contact removes additional mucilage residues and produces the extraordinary clarity and brightness of Kenyan washed coffee.

Modern Challenges

SL28 is highly susceptible to coffee leaf rust, which devastated many Kenyan farms during outbreak years. The Kenyan government and research institutions have developed hybrid varieties (Ruiru 11, Batian) that offer rust resistance and higher yield — but at a significant cup quality cost compared to SL28. The transition from SL28 to Ruiru 11 on rust-affected farms has reduced the consistency of top-tier Kenyan specialty.

Premium Kenyan auction lots from reputable cooperatives and estates still predominantly feature SL28 or SL34, and these represent the full expression of what Kenya can produce. For coffee drinkers who have not experienced a well-sourced, lightly roasted SL28 from a top Kenyan producer, the encounter with its blackcurrant intensity is one of specialty coffee's most memorable moments.