What is the SL-28 variety?

SL-28 is a Kenyan Arabica variety selected in the 1930s by the Scott Laboratories in Nairobi from a drought-tolerant Tanzanian Bourbon strain. Famous for its wine-blackcurrant-citrus profile and intense tartaric acidity, it has become one of the sensory signatures of Kenyan coffee and one of the most sought-after Arabica varieties in the world.

SL-28's story begins in 1931, when the Scott Agricultural Laboratories in Nairobi, under A. D. Trench, received seeds of a drought-tolerant Tanzanian variety descended from French Bourbon stock. Out of 42 selections tested and labelled SL-1 to SL-42, SL-28 stood out for its acceptable yields and, above all, for the exceptional cup quality it delivered at altitude. A lesser-known sibling often planted alongside it, SL-34, came from a farm near Nairobi (Loresho Estate) and is more productive but slightly less complex. For nearly a century, these two varieties have made up the bulk of Kenya's high-altitude plantings, alongside Ruiru 11 (1985) and Batian (2010).

The cup profile of a well-processed Kenyan SL-28 (Kenyan-wash double fermentation, altitude 1,700-2,000 m, phosphorus-rich volcanic soils) is instantly recognisable: very intense tartaric acidity, occasionally almost 'wine-like', dominant notes of blackcurrant, ripe tomato, pink grapefruit, cranberry, rhubarb, with a dense, syrupy body and a long retro-olfaction. Top lots from Nyeri, Kirinyaga or Kiambu reach 88 to 92 SCA points. That signature has given SL-28 near-iconic status in the third wave: on roasters' menus in London, Copenhagen or Berlin, a 'Nyeri SL-28 AA washed' is often one of the most scrutinised listings.

The variety remains relatively low-yielding, susceptible to leaf rust and to coffee berry disease (CBD). Since the 1990s, Kenya's Coffee Research Institute has pushed growers to partially replace it with Ruiru 11 and Batian, which are hardier but carry less prestige. SL-28 has also travelled: it is now grown in Uganda, Rwanda, Burundi, and in Central America (Costa Rica, Panama, El Salvador) under the 'SL-28' label or sometimes relabelled locally. For Belgian specialty roasters — Brussels, Ghent, Antwerp, Liège — a Nyeri SL-28 remains a flagship on the single-origin menu, best served as filter to preserve its acid structure, far from the chocolatey profile the Belgian speculoos-and-filter tradition has favoured.

SL-28 — variety sheet

SL28: Kenya's Crown Jewel and the Most Celebrated Variety in East Africa

SL28 is the variety that most comprehensively defines what Kenyan coffee tastes like at its best — the characteristic blackcurrant, phosphoric acidity, and winey intensity that make Kenya one of the most distinctive producing origins on earth. Developed by Scott Agricultural Laboratories (the 'SL' in the name) in the 1930s during the British colonial period, SL28 was selected from 42 varieties trialed for yield, size, and adaptation to dry conditions — drought tolerance was the primary breeding objective, not cup quality. The variety traces its ancestry to a Tanganyika (now Tanzania) drought-resistant selection, which in turn is believed to derive from a Yemeni Mocha coffee stock brought to East Africa by Arab traders, giving SL28 a genetic lineage that connects it directly to coffee's earliest commercial heritage.

The cup profile of SL28 is unlike any other variety's and difficult to fully prepare a first-time taster for. The blackcurrant note — caused by specific sulfur-containing compounds called thiols — is so pronounced in peak-condition SL28 from Nyeri or Kirinyaga that it registers as an almost wine-like fruitiness, occasionally prompting comparisons to Ribena cordial or Cabernet Sauvignon. The phosphoric acidity layer — that mineral, electric 'zing' that makes the cup feel almost effervescent — adds a structural dimension that deepens the fruit rather than competing with it. Body tends toward medium-full, and the finish is long and clean with a berry-chocolate transition that satisfies the palate in a way that few coffees match. These characteristics have made SL28 the benchmark against which other East African varieties are measured.

Practical Recommendations

Getting the best from SL28 requires careful brewing. The variety is sensitive to over-extraction — push it too hard and the thiols that produce the blackcurrant note shift from pleasant berry to something closer to cat urine, a genuinely unpleasant transformation. Stay in the 18 to 20% extraction yield range rather than pushing toward the upper specialty range. Water temperature of 91 to 93°C is the sweet spot; hotter water accelerates the extraction of bitter-adjacent compounds that undercut the berry brightness. If you have access to a refractometer, use it: SL28 is one of the varieties where the difference between 19% and 22% extraction yield is most perceptible in the cup. Serve it in a pre-warmed ceramic mug — the thermal stability matters for maintaining the aromatic peak during the drinking window.