SL28 was selected in the 1930s by Scott Agricultural Laboratories in Kenya, from material with Bourbon lineage, and it is one of the most successful quality selections ever made.
What it was selected for
Drought tolerance, primarily — Kenya needed varieties that could survive dry periods. Cup quality was a secondary criterion that turned out to be the enduring one.
The profile
SL28 is responsible for the acidity Kenya is famous for: blackcurrant, grapefruit, tomato, with a juicy, mouth-watering quality and a syrupy body underneath.
It is intense and immediately recognisable, and it is the reason people who love Kenyan coffee love it specifically.
Grown elsewhere
SL28 has been planted in Colombia, Panama, Ecuador and elsewhere, usually with encouraging but less dramatic results. It keeps some character and rarely reaches Kenyan intensity.
That suggests the variety is necessary but not sufficient — Kenya’s volcanic soil, unusually high in phosphoric acid, and its distinctive double-fermentation washing process are doing part of the work.
Drawbacks
Low yield and high susceptibility to coffee leaf rust and coffee berry disease. Kenya has bred resistant alternatives — Ruiru 11 and Batian — which yield better and are widely accepted as cupping less distinctively.
Many Kenyan lots now mix SL28, SL34, Ruiru 11 and Batian, and bags specifying SL28 and SL34 command a premium.
Brewing it
Filter, light roast, a clean brewer that shows acidity. Grind fine and brew hot — these are dense beans and under-extraction makes the acidity sour rather than bright, which is the commonest mistake with Kenyan coffee.