Exploring the Full SL Catalog Beyond SL-28 and SL-34

In the world of specialty coffee, names like SL-28 and SL-34 carry the same weight as legendary grape varietals do in winemaking. They are the architects of the "Kenyan profile"—that unmistakable, electrifying explosion of blackcurrant, juicy acidity, and syrupy body. But here is a secret that most coffee lovers don't know: these two icons are just a tiny fraction of a massive, 90-year-old genetic project.

During the 1930s, the Scott Agricultural Laboratories in Nairobi were the epicenter of an agricultural revolution. Researchers meticulously selected and numbered dozens of coffee trees, creating a catalog that stretched far beyond the two superstars we brew today.

Today, we’re going "under the hood" of the Scott Labs catalog. We’ll look at the history, the forgotten cousins like SL-14 and SL-6, and why understanding this full lineage is essential for the future of coffee in 2025.

The Mission of Scott Laboratories (1931–1939)

Established by the British colonial government in 1922, Scott Laboratories (now the National Agricultural Laboratories) wasn't looking for "tasting notes." They were looking for survival.

Kenya’s young coffee industry was being ravaged by drought and disease. Between 1935 and 1939, senior coffee officer A.D. Trench and his team embarked on a massive selection program. They didn't just "invent" new beans; they acted as talent scouts, scouring estates across East Africa for individual trees that showed exceptional yield and resilience.

Each selected tree was assigned an "SL" prefix followed by a serial number. Out of roughly 42 major selections, only a few achieved global fame. But the "failures" and the "obscurities" in that catalog hold vital genetic data that breeders are returning to today.

The Icons: A Quick Refresher

Before we explore the hidden gems, we must understand the baseline.

• SL-28 (The Flavor King): Selected in 1931 from a "Tanganyika Drought Resistant" tree. It is genetically related to the Bourbon group. It’s famous for its high quality at high altitudes and its intense, phosphoric acidity.

• SL-34 (The Adaptable Queen): Selected from a single tree on the Loresho Estate labeled "French Mission." While long thought to be Bourbon, recent DNA testing reveals it is actually a Typica relative. It thrives in high-rainfall areas and produces bigger, heavier beans.

Exploring the "Other" SLs: The Forgotten Catalog

While SL-28 and SL-34 dominate 80% of Kenya’s exports, other varieties in the catalog offer fascinating insights into drought resistance and unique terroir expressions.

1. SL-14: The Drought-Resistant Workhorse

If SL-28 is the luxury car, SL-14 is the off-road 4x4. Selected in 1936 from a tree labeled "Drought Resistant II," SL-14 is economically vital in both Kenya and Uganda.

• The Profile: At high altitudes, it produces a clean, sweet cup with a medium body.

• Why It Matters: It is significantly more cold- and drought-tolerant than its more famous siblings. In 2025, as climate change makes weather patterns more erratic, SL-14 is seeing a resurgence in planting in marginal regions where SL-28 might wither.

2. SL-6: The Cold-Tolerant Mystery

SL-6 was selected for its ability to produce good yields in cooler, higher-latitude climates. While it rarely appears as a single-variety lot on specialty shelves, its genetic material is often found in older "field blends" in East Africa. It has a slightly more "herbal" and "savory" profile compared to the fruit-forward SL-28.

3. SL-19 and SL-24: The Yield Experiments

Many numbers in the catalog, such as SL-19, were prioritized purely for their cherry-to-bean outturn. They were productive but often lacked the "sparkle" required for the specialty market. However, these varieties are the backbone of many "Kenya AB" commercial lots that provide the consistent, chocolatey base for roaster blends worldwide.

The Genetic Mystery: Bourbon vs. Typica

One of the biggest controversies in coffee science occurred when researchers finally ran DNA tests on the Scott Labs collection.

Variety	Historical Label	Genetic Reality	Flavor Character
SL-28	Bourbon-related	Bourbon group	High acidity, blackcurrant
SL-34	French Mission	Typica group	Heavy body, savory/sweet
SL-14	D.R. II	Typica group	Balanced, sweet, nutty

This discovery was a shock to the industry. It proved that the Scott Labs researchers had inadvertently preserved a massive amount of Typica diversity in a region otherwise dominated by Bourbon. This genetic diversity is the secret reason why Kenyan coffee has such a broad spectrum of flavor—it's a massive, multi-generational conversation between two different sides of the Arabica family tree.

The 2025 Perspective: Why We Need the Full Catalog

Today, the specialty coffee industry is facing a crisis. SL-28 and SL-34 are tragically susceptible to Coffee Leaf Rust (CLR) and Coffee Berry Disease (CBD).

Breeders are now looking back at the "other" SL numbers—those drought-resistant trees that were ignored in the 50s because they didn't taste like "Pop Rocks." By crossing the flavor of SL-28 with the hardiness of lesser-known SL varieties, we are seeing a new generation of hybrids like Batian and Ruiru 11.

The Modern Trend: We are seeing a rise in "Variety Micro-Lots." Instead of just "Kenya AA," roasters are now seeking out "100% SL-14" or "Single Farm SL-34." This allows drinkers to taste the specific genetic nuances that A.D. Trench first noticed nearly a century ago.

How to Taste the Difference

If you want to explore the Scott Labs catalog through your palate, look for these clues on your coffee bag:

1. High Altitude + SL-28: Expect intense fruit and high acidity (Malic and Phosphoric).

2. Shade Grown + SL-34: Expect a more velvety, chocolate-leaning cup with deep sweetness.

3. Ugandan Origin + SL-14: Expect a rounder, nuttier, and highly approachable cup that excels in milk-based drinks.

A Legacy in Every Sip

The SL catalog is more than just a list of numbers; it is a map of human ingenuity and biological resilience. When we drink an SL-variety coffee, we are participating in a research project that has survived world wars, colonial shifts, and climate crises.

As we move forward, the "icons" of Scott Labs will likely be joined by their forgotten cousins, as farmers and roasters realize that in a changing world, genetic diversity is the only true way to protect the perfect cup.