Origins and Development at Scott Agricultural Laboratories
SL28 was selected in 1935 from a single tree in a population called Tanganyika Drought Resistant. The development of Kenya’s legendary SL varieties began during British colonial rule, when Scott Laboratories, which is the name of the Kenyan research centre that first developed these varieties in the 1930s , was established to improve agricultural practices throughout the region.
Individual tree selections made at the Scott Laboratories during the 1935-1939 period were prefixed SL. The research program was methodical and ambitious: Together with the Lyamungo Research Station in Tanganyika (now Tanzania), they selected 42 different coffee trees to study. Their mission was clear—to develop varieties that could thrive in Kenya’s challenging climate while delivering exceptional cup quality.
SL34 was originally selected in Kenya the late 1930s at the Scott Agricultural Laboratories… SL34 was selected from a single tree on the Loresho Estate in Kabete, Kenya since research at the Scott Laboratories was often conducted in cooperation with local, private estate owners. This tree was labeled “French Mission,” tracing its lineage to French missionaries known as Spiritans established a mission in 1893 at Bura (Taita Hills, Kenya), in which Bourbon coffee seeds originating from La Réunion island were planted. The seedlings from Bura were brought to another French Mission in Saint Austin (near Nairobi) in 1899 .
Genetic Heritage and Physical Characteristics
Recent genetic tests have confirmed that SL28 is related to the Bourbon genetic group. Both varieties showcase their Bourbon ancestry through their morphological traits and cup characteristics. SL28 was developed by Scott Agricultural Laboratories, an institution that later became part of Kenya’s national agricultural research system. The variety is genetically linked to the Bourbon lineage, with some influence from Typica-related stock.
The physical characteristics of these varieties are distinctive. SL28 trees grow tall and produce large cherries. They are known for excellent cup quality and strong drought tolerance, making them suitable for high-altitude environments.
For Arabica reference, Caturra = Average, SL28 = Large in terms of bean size, reflecting their robust genetic makeup.
SL28 is notable for its rusticity—a quality meaning that it can be left untended for years or even decades at a time, and then return to successful production. There are SL28 trees in many parts of Kenya that are 60-80 years old and still productive. This remarkable longevity speaks to the variety’s genetic resilience and adaptation to Kenyan terroir.
Distinctive Flavor Profiles and Cup Characteristics
In the cup, SL28 is famous for its distinct fruit-forward acidity, pronounced sweetness, and long, elegant finish. The flavor profile that has made these varieties legendary is unmistakable. Highlighting such characteristics surrounding taste, when describing the SL28, you’ll be using fruity descriptors for the most part, commonly finding blackcurrant, berries and citrusy notes used as descriptors.
In tastings, the SL28 variety typically stands out for its bright acidity (sometimes citrusy, other times more like currant), a silky texture, and a clarity of flavor that makes it unmistakable. Its fruity profile can range from mango and orange to red fruits, and it is often a favorite among those seeking expressive coffees with character.
Flavor-wise, SL28 is complex and unique. It delivers dazzling acidity reminiscent of black currant or fresh summer tomatoes. The juicy body and super sweet, tropical tastes make SL28 a favorite among specialty coffee aficionados.
SL34 presents a complementary but distinct profile. The SL34 variety, on the other hand, offers a somewhat rounder and more comforting profile: fuller body, caramel sweetness, and a less sharp, though equally vibrant, acidity, with notes of tropical fruits, currants, and black tea.
Its flavor profile resembles SL28 but is sometimes described as slightly softer in intensity, while still delivering complex sweetness, acidity, aroma, and lingering finish.
Agricultural Characteristics and Growing Requirements
The variety is suited for medium to high altitudes and shows resistance to drought, but is susceptible to the major diseases of coffee. This duality defines the cultivation challenge these varieties present—exceptional drought tolerance coupled with disease vulnerability.
Today, it is estimated that SL-series varieties comprise as much as 80% of all exported Kenyan coffee. Their dominance stems from practical advantages: An advantage of the SL varieties over other, newer coffees is that even after years of neglect, the coffee still continues producing (although at lower volumes)… Furthermore, once you get back to caring for the SL, it can return to producing as though it was never forgotten. The trees do not demand a lot of attention or fertilisers, but still still give good returns .
However, these varieties face significant challenges. However, they also have drawbacks, including low yields and poor disease resistance.
One of the biggest obstacles facing SL28 and SL34 is their susceptibility to diseases, particularly Coffee Leaf Rust and Coffee Berry Disease. This vulnerability has driven the development of newer, disease-resistant varieties like Ruiru 11 and Batian in Kenya.
Global Expansion and Modern Cultivation
It has consequently spread from Kenya, where it was originally selected in the 1930s, to other parts of Africa (it is important in Arabica-growing regions of Uganda, in particular) and now to Latin America. The global journey of SL varieties represents one of coffee’s most successful varietal migrations.
Coffee growers in Latin American countries, attracted by the varieties’ unique flavor profiles and adaptability, began experimenting with these beans. Today, SL28 and SL34 can be found in countries like Colombia, Panama, and Costa Rica.
In Colombia, SL28 is becoming increasingly popular, especially in high-altitude regions like Huila and Nariño, where the climate and terrain are ideal for coffee development. SL34, with its good disease resistance, has also been successfully tested in some areas of Brazil and Peru.
Several countries such as Honduras, Costa Rica, and Guatemala, have embraced SL28. They are now producing exceptional coffees from this variety. The adaptation to Central American terroir has yielded fascinating results, with The varietal maintains its inherent qualities like bright acidity and complex flavour profiles but adopts a new personality influenced by Costa Rica’s unique terroir. Notes of tropical fruits like mango and pineapple have emerged, blending seamlessly with the varietal’s traditional characteristics.
Processing Traditions and Roasting Considerations
This is because washed processing highlights the bright acidity of SL varieties, while complementing their dark berry and wine-like flavour notes. It also helps deliver the cup clarity typically associated with SL28 and SL34. The Kenyan washed method has become synonymous with these varieties, employing an additional 24 hours of soaking and fermenting after thorough agitation that enhances their characteristic brightness.
For roasters working with these varieties, understanding their density is crucial. The highest grade of coffee in Kenya (AA) are typically large, dense beans grown at high altitudes. As such, most roasters will need to use slightly more heat when roasting them to break down the structure of the bean. They may also require slightly longer roast time to ensure the bean is fully developed.
SL28 and SL34 not only redefined Kenyan coffee: they laid the foundation for a new way of thinking about the quality and value of African coffee. They helped position Kenya as one of the most highly regarded regions in the world of specialty coffee, and they remain a benchmark for tasters, roasters, and coffee lovers worldwide.
Legacy and Future Impact
As specialty markets matured, SL28 and SL34 became synonymous with currant, berry, citrus, and winey structure, often scoring among the most memorable lots on cupping tables worldwide. Their influence extends far beyond their original Kenyan borders, inspiring breeding programs and quality standards globally.
SL28 and SL34 are more than just varieties: they are part of the history of modern coffee and an invitation to discover what happens when science, agriculture, and a passion for flavor come together. Nearly 90 years after their development, these varieties continue to represent the pinnacle of what systematic agricultural research can achieve—combining practical cultivation benefits with extraordinary sensory experiences that define specialty coffee excellence.