☕ Key takeaways
- One laboratory, two selections, two different genetic backgrounds: SL28 sits in the Bourbon group and SL34 in the Typica group, despite the French Mission label that gave SL34 its origin story.
- Neither variety resists disease. Coffee leaf rust, coffee berry disease and nematodes hit both. The cup quality is paid for in husbandry, in treatments and in yield.
- What the trade calls the Kenyan character is only half varietal. The double fermentation wash used by Kenyan stations is a process, not an inherited trait, and it is what half the tasting notes are actually describing.
SL28 & SL34 Coffee Guide: Kenyan Varieties, Bright Acidity, Blackcurrant
3 key takeaways
- SL28 was selected in 1935 from a single tree in the Tanganyika Drought Resistant population; 1931 is the year of the collecting tour, not of the selection.
- SL28 carries a Bourbon-type background and SL34 a Typica-type one, the reverse of what the French Mission nickname attached to SL34 suggests.
- Both are susceptible to coffee leaf rust, coffee berry disease and nematodes; SL28 compensates with drought tolerance and an unusually long productive life.
If you have ever tasted a great Kenyan coffee and been struck by that vivid burst of blackcurrant, that juicy grapefruit zing, that full body coating the palate, you were almost certainly drinking SL28 or SL34. The two names travel together, always. They came out of the same laboratory a few years apart, and the industry has been treating them as twins ever since. They are not twins. They do not share a genetic background, they do not share an origin story, and a good part of what gets credited to them belongs to somebody else entirely.
Where did SL28 and SL34 actually come from?
The Scott Agricultural Laboratories opened in 1922 at Kabete, near Nairobi, when the British colonial government set up an agricultural research service staffed with an entomologist, a mycologist and a plant breeder. The buildings were older than the institution: put up in 1913 as a sanatorium, used as a war hospital, and already named after Dr Henry Scott, a Church of Scotland missionary. The coffee section only moved in during 1934, with twenty-four acres given over to the crop. Single-tree selections made there between 1935 and 1939 carried the SL prefix. Forty-two trees of various origins were put under observation for yield, cup quality, drought tolerance and disease resistance.
SL28 was picked out in 1935, from one tree in a population called Tanganyika Drought Resistant. The widely repeated date of 1931 refers to something else entirely: that is the year A. D. Trench, the laboratory's senior coffee officer, toured Tanganyika and spotted a coffee tree in the Moduli district that seemed to shrug off drought, disease and pests. Seed came back to Kabete, where the drought tolerance was confirmed. Four years sit between the collecting trip and the selection, and collapsing the two dates erases the observation work done in between. Recent genetic testing places SL28 in the Bourbon group.
SL34 came from a different isolated tree, on Loresho Estate, also at Kabete. That tree was labelled French Mission, a name tied to the Bourbon seed brought from Réunion by French missionaries who settled in Kenya at the end of the nineteenth century. Genetics tells a different story: SL34 belongs to the Typica group, which makes the origin label doubtful. It is SL28, not SL34, that carries the Bourbon-type background. The confusion has held because the baptismal name outlived the laboratory verdict, and because a missionary lineage reads better than a mislabelled tree.
How do SL28 and SL34 differ once they are in the cup?
Both share the same Kenyan ground and the same aromatic family, which is why they are quoted in one breath. That does not make them interchangeable:
- SL28. Large beans, high cherry to green bean outturn, low yield. The acidity is sharp, with blackcurrant, grapefruit and sometimes dried tomato or tamarind. Full, almost chewy body. The more sought-after of the two by specialty roasters, and the stingiest per hectare.
- SL34. Same aromatic family, rounder and less cutting, with more black fruit (blackberry, blueberry) and dark chocolate. Also full-bodied. It is what fills the lot when SL28 underdelivers at harvest.
In practice, many Kenyan coffees blend both varieties within a single plot, picked and processed together. Micro-lots separating pure SL28 from pure SL34 exist, but they remain the exception. What the two share most solidly is a weakness: coffee leaf rust, coffee berry disease and nematodes hit both, and neither carries resistance to any of them. SL28 earns its keep elsewhere, on drought and on time. Trees sixty to eighty years old are still producing in Kenya, and a plot abandoned for years can be brought back into production. Very few arabica varieties take neglect that calmly.
What does phosphoric acidity taste like?
"Phosphoric acidity" is a cupping descriptor, not a laboratory measurement. It names an acidity that is bright and clean, one that wells up in the mid-palate rather than hitting on the attack, without the pointed lemon edge of other origins. Tasters reach for pomegranate juice or gooseberry as comparisons. Phosphoric acid does exist in arabica, alongside malic, citric and acetic acid. But pinning the sensation on an unusually high concentration of that one acid is a convenient hypothesis, not an established fact. What is observable is the sensation, and it turns up reliably enough on washed Kenyans to have become shorthand on the cupping table.
Which Kenyan regions grow SL28 and SL34?
Kenyan coffee geography fits into a handful of districts, mostly between 1,400 and 1,800 m, on red volcanic soils:
- Central Province. Kiambu, Murang'a, Kirinyaga: the historical heartland, the oldest plantings and the steadiest demand.
- Nyeri. The region specialty buyers chase hardest, on volcanic soils at high elevation.
- Embu and Meru. The slopes of Mount Kenya offer high-altitude microclimates that favour marked acidity and aromatic complexity.
- Machakos. Drier, slightly lower, earthier and sweeter in the cup.
Those conditions matter, but they do not finish the explanation. The second half happens after picking, inside the washing station, and that is exactly where the commercial vocabulary goes off the rails.
Is the Kenyan double wash part of the variety or part of the mill?
Part of the mill. Double fermentation washing, often called double-soaked washed, is a post-harvest process, not an inherited trait of SL28 or SL34. After pulping, the beans ferment dry for roughly twenty-four hours, then sit in clean water for a second twelve to twenty-four hour phase. Mucilage breaks down more completely and the cup gains clarity. Not every Kenyan washing station works this way, but it is the reference standard for specialty micro-lots. The consequence is blunt: when a roaster writes "the SL28 profile", they are usually describing an SL28 that went through a Kenyan station practising the double wash. The same tree, pulped and dried as a natural, does not give that cup. Keeping the two apart is not pedantry, it is what explains why the same variety planted elsewhere disappoints everyone who came for blackcurrant.
Do SL28 and SL34 grow outside Kenya?
They do, and the idea of a Kenyan enclosure is simply wrong. SL28 is one of the better travelled African varieties:
- Uganda is the main home of SL28 outside Kenya, where it matters in the arabica-growing areas.
- Malawi and Zimbabwe grow it too.
- It reached Latin America more recently, where the resulting profiles do not reproduce the Kenyan style.
What remains true is that the reputation was built in Kenya, and that the cup style labelled Kenyan owes as much to altitude, soil and the double wash as it does to genetics. Outside that arrangement SL28 gives a different coffee: not worse, not better, simply different. That is why "Kenyan specialty coffee" and "SL28/SL34" became near synonyms internationally, even though the variety itself has travelled.
Variety Comparison: SL28, SL34 and Bright Acidity Varieties
| Variety | Origin | Dominant Profile | Acidity (type) | Body | Characteristic Notes | Relative Price |
|---|---|---|---|---|---|---|
| SL28 | Kenya, 1935 selection | Blackcurrant, grapefruit, juicy | Phosphoric, very bright | Full, juicy | Bourbon background, disease susceptible, low yield | High to very high |
| SL34 | Kenya, late 1930s selection | Blackberry, blueberry, dark chocolate | Bright, less sharp | Full | Typica background, disease susceptible | High |
| Ruiru 11 | Kenya, composite hybrid released 1985 | Generic fruity, less complex | Moderate | Medium | Rust and coffee berry disease resistant, dwarf, very high yield | Medium |
| Batian | Kenya, composite variety released 2010 | Fruity-acidic, more balanced | Bright | Medium | Coffee berry disease resistant, intermediate rust resistance | Medium to high |
| Bourbon (Rwanda) | Bourbon introduced to Africa | Red fruit, floral, chocolate | Bright, malic | Medium | Softer profile than SL28 | High |
| Typica (Kenya) | Introduced Typica | Floral, soft, stone fruit | Moderate | Silky | Less distinctive on Kenyan terroir | Medium |
Why does Kenyan SL28 cost so much?
Three mechanisms stack up, and none of them is decorative. Yield first: SL28 is rated low on that criterion, lower than Caturra or Bourbon, which means fewer kilos per hectare for at least as much work. Crop protection second: with no resistance to leaf rust or coffee berry disease, the grower pays in treatments and losses what a resistant hybrid would have spared them. Route to market third: a large share of Kenyan coffee moves through an auction system, which passes demand straight through to the best scoring lots. Seasonality finishes the picture, with two harvests a year in Kenya, the main one from October to December and a lighter second one from June to August. Lots reach European roasters several months behind those cycles.
Which brewing method gets the most out of SL28 and SL34?
These varieties shine in brewing methods that foreground acidity and clarity:
- V60 or Chemex. The reference. Paper filtration preserves brightness and aromatic intensity. Temperature 92 to 94 °C, ratio 1:15.
- Comparative tasting. A Kenyan SL28 alongside a Rwandan Bourbon or an Ethiopian Yirgacheffe is an excellent exercise for understanding the role of variety in the final profile.
- Espresso. Possible but demanding: the acidity can turn aggressive if extraction is too short or the roast too light. A well-executed SL28 espresso delivers a juicy, intense shot comparable to concentrated blackcurrant juice.
- Avoid long total immersion. Extended French Press or Aeropress immersion dilutes the finesse of the acidity in a thicker texture.
SL28 shows better than any other variety that a cup profile is never a purely inherited trait. What people think they are reading in a variety name is often the work of a washing station, an altitude and a soil. Take it out of that arrangement and SL28 is still SL28, but the blackcurrant leaves.