The Mystery of the Most Beautiful Flavor in Coffee

Call it the Solai taste, the Blackcurrant taste, or the Kenyan Highlands Taste, my personal favorite flavor in coffee is an enigma.

A few weeks ago, I was at a coffee event and someone kindly brought me a cup of coffee to drink. As they handed it to me, a beautiful aroma wafted from the cup- and I recognized its origin instantly. I inquired, and later found out it was Fruktig Sommerkaffe (Fruity Summer Coffee) from the great roaster Solberg and Hansen of Norway. And, just as I suspected, the coffee in the cup originated from Thunguri, Kenya. Its distinct aroma gave it away, but that very scent is a mystery. Here’s the story.

Although Kenya adjoins Ethiopia, the indigenous origin of Coffea arabica, Kenyan coffee agriculture is relatively new. Coffee was first planted there in 1893, and it took quite a while to get popular- and that was the doing of the colonial British Empire. In its early years, coffee in Kenya was almost completely a colonial project- if you want a glimpse, check out the movie (or book) “Out of Africa”, a romantic tale of a colonial coffee farming enterprise.

As expanding coffee farms pushed their way up the foothills of Mount Kenya north of Nairobi, farms got less productive and riskier to manage. This is because high altitudes put extra stress on coffee trees, reducing their production. However, this same phenomenon does something else- it makes the coffee more delicious. This is the reason that coffee experts obsess about the altitude of coffee farms: all things being equal, altitude is a key predictor of coffee flavor.

Nowhere is this truer than in Kenya, where coffee from the mountains has an amazing intensity of flavor. And sometimes, in coffee from these highlands, a particular flavor emerges that exists nowhere else in the world. And nobody knows quite why.

Let me try to explain this flavor to you. You smell it first, when the coffee is grinding, and it hits you like a brick. It accompanies the familiar sweet-brown smell of coffee but is totally different. It’s definitely fruity, in a tangy, pointed way. You feel the tang in your nose and it makes your mouth instantly water. While brewing the coffee, the fruity aroma persists and seems to get riper and rounder. By the time the cup of coffee is ready, the flavor has totally permeated the coffee, your kitchen, and your mind. And each sip has it. It’s glorious.

Now let’s be clear about this: coffee descriptions can be imaginative, and often evoke fruit terms. That’s because coffee often has hints of fruity flavors. I say “hints” because that’s what they usually are- gentle wisps of citrus, or peach, or cherry. But the Kenyan highland coffee taste is a completely different animal- in the best coffees it demands notice: it is THERE and it wants you to TASTE IT.

And what do we call this flavor? An early Kenyan agricultural science paper shared by my friend Mario called it the “Solai taste”, after the highlands coffee-growing town of the same name. The paper explains that it is an “up country” flavor, known only to coffees from the mountains of central Kenya. I was introduced to this same flavor by British coffee exporters who called it “the blackcurrant taste”, their way of describing the flavor’s intense purplish fruitiness. (That’s a quintessentially British way of describing flavor- since the introduction of Ribena blackcurrant syrup during World War II, the British have something of an obsession with blackcurrant: in the UK you can buy blackcurrant gum, blackcurrant gelato, and the purple Skittles in Britain are blackcurrant instead of grape.) Americans generally lack any familiarity with blackcurrants, so to us, the flavor resembles something more like blackberry or boysenberry. Cranberry, maybe. But really, any of these fruit metaphors are inadequate: this flavor is its own thing, and it can only be found in coffees from the Kenyan highlands. It’s kind of magic.

“But there is no such thing as magic, Peter”, you might say. “Everything real is discoverable by science.” This may be true, but science hasn’t figured this one out yet. People have theories, however: and here are a few of them.

It’s the variety. Like any agricultural product, coffee farmers develop specific genetic strains of plants through selection and breeding. In wine we call these varietals, in everything else we call them varieties, and it’s what makes a Purple Cherokee tomato different from a Sungold tomato. Different varieties can drive an immense difference in flavor and appearance. Kenya has, over the years, developed several coffee varieties that are unique to the country, and two are often associated with the Kenya Highland Taste. These two varieties have odd names: SL-28 and SL-34.

SL-28 coffee in Thiriku, Kenya.

These two varieties were named after “Scott Labs”, an old Kenyan agricultural research station, and they are considered the traditional varieties of Kenya. Trouble is, they can be susceptible to disease, and a given farm is often mixed with other more disease resistant varieties like Ruiru 11, Batian, and K7. Though most experts associate the Kenyan Highland Taste with the SL varieties, it’s not so simple: it’s hard to find single-variety coffee farms in the highlands, and sometimes all-SL coffees fail to have the flavor. So…

It’s the altitude. Kenya has some of the highest-altitude coffee farms in the world, and true to its name, the Kenyan Highlands Taste almost always comes from coffees grown in the highest Kenyan mountains. It certainly seems that the high altitude somehow unlocks the flavor from the coffee trees planted there. But there are high-altitude farms all over the world, and they never seem to have that signature taste. So maybe….

It’s the soil. Drive around the mountains of central Kenya and you’ll notice that the dirt is often a brilliant shade of orangey-red. These are called laterite soils, a unique kind of soil rich in iron and aluminum oxides which give it that lovely color.

Brilliant laterite soil in Nyeri, Kenya. Photo by Ninara, CC 2.0

Laterite soils are found most often in the tropics, and they seem well suited for coffee. Could it be that Kenyan highlands have a specific kind of laterite that creates the Highland Taste? Possibly. This brings us to our next hypothesis….

It’s phosphoric acid. Kenyan coffees are noted for having a pleasing flavor tasters call “acidity”. And some studies have shown that Kenyan coffees are more likely to have phosphoric acid than coffees from other origins. Could phosphorous in Kenyan soil lead to phosphoric acid in Kenyan coffees? And could this phosphoric acid be the cause of a uniquely fruity, tangy taste in coffee? It’s a tempting explanation, but unlikely. Phosphoric acid doesn’t taste like fruits, for one thing- it’s just sour. Second, systematically measured concentrations of phosphoric acid in Kenyan coffees aren’t all that different from other origins. Third, acid concentrations and “perceived” acidity don’t necessarily have a direct relationship- it’s much more complex than that. So, perhaps it’s not about agriculture at all. Maybe…

It’s the processing. All coffee is processed before export, and in Kenya most coffees are washed, that is, cleaned of their fruit layers before drying and milling. In many places, washing includes a fermentation phase, which can create fruity flavors in coffees. In Kenya, many highlands producers take an extra step, soaking the coffees after washing in clean water, a technique that is unknown elsewhere. Could it be that this step contributes to the Highland Taste? It seems possible. But more likely…

It is all of the above. The right genetics are critical but even the best genes rely on an interaction with the environment: for example, a hydrangea’s flower color depends on the chemistry of the soil it grows in. It seems most likely that a unique intersection of variety, altitude, soil, and processing combine to create the unique Kenyan Highland Taste. This is a testable thing- agricultural and sensory science are able to figure this kind of thing out- but nobody in the scientific literature has done it yet. From a scientific perspective, the Kenyan Highlands Taste is still an unsolved mystery.

Mysteries are fun, but this one is a risk. So long as we don’t know what causes this precious flavor, we run the risk of losing it. Like everywhere coffee grows, Kenya is susceptible to changes in prices, climate, and economic stability. Though coffees with the Kenyan Highland Taste sell for high prices, it’s rare for given farm or cooperative to produce them consistently- which makes them rare gems indeed. Even worse, these rare gems seem to be getting rarer: I hardly ever encounter the Kenyan Highlands Taste any more. People I trust who have decades of experience in Kenya have observed the same thing- the flavor seems to be slowly getting harder and harder to find.

That’s why I am so grateful for that cup from Thunguri via Solberg and Hansen- it gave me a tremendous amount of joy, and reminded me that the Kenyan Highland Taste is still one of the loveliest things in the world.

*thanks to Mario Fernandez-Alduenda and Mette-Marie Hansen for their valuable and interesting thoughts on this subject- it’s a wonderful thing to learn from respected friends.