How Did Kenya Coffee Varieties SL28 and SL34 Take Shape?

SL28 and SL34, bred by Scott Laboratories in the 1930s, are the varieties behind Kenya coffee's bright fruit acidity and juicy body, with flavors often described as dark plum, citrus and stone fruit. Released in the 1950s, they spread nationwide and largely replaced the earlier Bourbon plantings, becoming the benchmark style of Kenyan washed coffee.

Their weakness is susceptibility to leaf rust and coffee berry disease, which drove later breeding of Ruiru 11 and Batian. Ruiru is indeed a coffee variety, developed for disease resistance rather than for the cup profile that made SL28 and SL34 famous. Scott Laboratories recruited an entomologist, a mycologist and a plant breeder, and collected tree stock in Tanzania to screen for drought and pest tolerance.

Kenyan coffee is renowned worldwide for its bright, uplifting fruit acidity and full, juicy body, and SL28 and SL34 are the core varieties that shaped this classic style. These two varieties, bred by the Scott Laboratories, have become stars of the specialty coffee world thanks to their complex flavors of dark plum, citrus, and stone fruit. However, their vulnerability to leaf rust and coffee berry disease led to the later development of the more disease-resistant Ruiru 11 and Batian. This article will systematically trace the evolution of Kenyan coffee varieties—from their origins, flavor characteristics, and cultivation performance to the current market situation—and answer the question of whether Ruiru is a coffee variety.

When it comes to Kenyan coffee, the first impression that comes to most people's minds is its vivid, lively fruit acidity and thick, juicy mouthfeel—a distinctive flavor that countless coffee lovers find unforgettable. The Kenyan-style washed processing method is widely sought after, and the SL series varieties, as the country's signature varieties, enjoy a global reputation.

SL is the abbreviation for Scott Laboratories, the former name of the Kenya Coffee Research Center, which first bred the SL series varieties in the 1930s. Among the 40 experimental varieties at the time, SL28 and SL34 were two of them, and they are now the most famous and most popular varieties in the specialty coffee circle.

Scott Laboratories was established in 1922 by the colonial government and has now been renamed NARL (National Agricultural Laboratories). The laboratory hired an entomologist, a mycologist, and a plant breeding expert at the time, mainly to provide farmers with technical advice and cultivation knowledge, while also conducting research on yield trials, grafting experiments, pruning, shade cultivation, and root mulching.

Because Kenyan agriculture had long suffered from threats such as drought, researchers at Scott Laboratories traveled to neighboring Tanzania to collect local coffee tree species and then selected varieties with potential for drought resistance and pest and disease resistance. After nearly three decades of breeding work, the laboratory finally released the highest-quality SL28 and SL34 in the 1950s and promoted their cultivation nationwide. Soon after, these two new varieties gradually replaced the previously planted Bourbon coffee trees and became Kenya's main coffee varieties in the second half of the 20th century.

Among them, SL28's lineage combines several varieties, including French Mission, Mocha, and Yemen Typica. It has a tall tree shape, emerald-green leaf tips, relatively high yield, larger beans, and strong drought resistance, but it is susceptible to coffee leaf rust, coffee berry disease (CBD), and damage from soil nematodes. In terms of flavor, SL28 carries distinct notes of dark plum and citrus, presenting an intense, deep stone-fruit acidity, with a mellow, sweet mouthfeel and a wine-like texture.

By contrast, SL34 has dark bronze leaf tips and a higher nutrient demand during growth, with relatively lower yield. Its beans are also large, and when grown at higher altitudes it can show excellent quality potential, with good fruit acidity and aromas of nuts and spices. Its tree characteristics are similar to those of SL28, and it is likewise highly sensitive to CBD, leaf rust, and pests.

It is precisely these two varieties, SL28 and SL34, that forged the classic flavor of Kenyan coffee known around the world, enchanting many lovers of acidic coffee. Front Street's own-roasted Kenya Asalia coffee beans use exactly SL28 and SL34, and the brewed black coffee presents rich aromatic notes of cherry tomato, black plum, snow pear, and berries.

Front Street Coffee: Kenya Asalia Coffee Beans

Region: Thika region, Kenya

Altitude: 1,550–1,750 meters

Varieties: SL28, SL34

Grade: AA

Processing method: Washed

Flavor: Dark plum, snow pear, yellow sugar, cherry tomato, plum

However, because SL28 and SL34 lack tolerance to common pests and diseases in East Africa, the Coffee Research Institute (CRI) of Kenya released a new variety, Ruiru-11, in 1985, and later released Batian in 2010.

These two new varieties, Ruiru 11 and Batian, offer high yield and resistance to leaf rust and coffee berry disease, helping Kenyan coffee farms remain viable in environments where pests and diseases are frequent. As a result, more and more Kenyan coffee farms are turning to planting Ruiru 11 and Batian when dealing with pest problems.

According to information reviewed by Front Street, Ruiru 11 was bred by crossing 11 sibling varieties. The paternal line comes from the original SL28, SL34, Bourbon, and Rume Sudan, as well as Tanzanian selections K7 and N39, while the maternal line comes from Catimor from East Timor, Indonesia. It is the earliest F1 in the coffee world with hybrid vigor. Although Ruiru 11 has high yield and strong pest and disease resistance, its drawbacks are a relatively shallow root system, difficulty absorbing nutrients, high sensitivity to drought, and inferior flavor compared with SL28. Therefore, it is often criticized in coffee cupping, with its quality regarded as mediocre.

Another emerging variety, Batian, was developed by the Kenya Coffee Research Center after summarizing the shortcomings of Ruiru 11 by backcrossing it with SL28 and SL34. It can be understood as an "upgraded version of Ruiru 11," and it is named after Batian, Kenya's highest peak. Batian combines the advantages of high yield and resistance to coffee leaf rust and coffee berry disease, requires no pesticide spraying, and has an annual planting cost 30% lower than the SL series. In his book The Fourth Wave of Specialty Coffee, Mr. Han Huaizong describes Batian as having a clean, sweet sugarcane aroma, citrus acidity, a rich berry character, and relatively high flavor distinctiveness.

However, single-variety coffee bean products of the above varieties are still rarely seen on the market. This is because traditional Kenyan coffee is mostly grown by smallholders, and single-variety lots are extremely rare. Similar to Ethiopia's coffee production model, farmers pick coffee cherries and then sell them centrally to various processing stations, which mix them and carry out washed processing, and finally they enter the consumer market under the name of the processing station. For this reason, the Kenyan coffee we buy today is more often a blended lot "containing" Batian or Ruiru 11.

The above content is compiled by CoffeeHunters, a coffee news website.