The bean that could change the taste of coffee

On the fertile slopes of Mount Kenya, an extinct volcano, smallholder coffee farmer Martin Kinyua has decided against planting new crops.

The seedlings, he says, will simply die in the heat.

"We have an extended drought season," he explains. "We are used to two rainy seasons, the short rains and the long rains. Right now, you cannot say when the short rains are coming."

Mr Kinyua, a member of the Mutira Farmers Cooperative in Kenya's Kirinyaga County, adds that higher temperatures attract more pests and diseases, raising the cost of protecting his produce.

Asked if he has ever felt at risk of not making enough money to get by, he says unequivocally: "Yes, I've felt it many times."

What's happening on Martin's farm is an insight into the danger the coffee industry is in.

Arabica, the species Martin grows, accounts for the majority of coffee beans traded globally, some 70%. But it is highly sensitive to changes in both temperature and humidity. For the last two years, production has failed to meet demand.

The industry is now pinning hopes on another coffee species to sustain production - liberica. Native to western and central Africa, commercial cultivation is centred on the Philippines and currently accounts for only 2% of the global coffee bean harvest.

With a tougher bean that's tricky to process, and perceived as producing a less desirable flavour, liberica is now gaining renewed interest for its hardiness in changing climates.

In a hot and humidified greenhouse, Dr Aaron Davis parts the branches on a lanky plant to reveal a cluster of dark red berries.

This is Palm House at Royal Botanic Gardens, Kew, in London, which simulates a tropical climate. The red berries, known as coffee cherries, are what's processed and roasted to create the drink that's loved the world over.

Dr Davis' work has been carried out with increasing urgency in recent years.

Findings from his latest study suggest that if global temperatures rise 2°C, countries supplying a quarter of the world's arabica will suffer major declines in yield. A rise of 2.5°C will have this impact on 75% of supply.

"We need additional coffee crop species that are able to grow under altered conditions," he says. "And what we're seeing is that liberica coffee is more climate resilient than arabica."

On the question of what happens if the coffee market doesn't adapt, Dr Davis says soberly: "We're going to have less coffee and prices will go up, but the real hit and the real jeopardy is for farmers."

The outcome of all this could be that the product in our coffee cups is transformed.

Romulo, a Filipino café in west London, serves barako coffee - made from a blend of liberica and arabica beans. It's traditional in the Philippines.

"When we were growing up in the Philippines it was prestigious to have Nescafe, and the common man, we drank barako coffee," says owner Chris Joseph.

In front of him, Mr Joseph has scattered a mix of arabica and liberica beans onto a plate. The liberica are almost twice the size, about a centimetre long, and aesthetically more uniform than the arabica.

But what about the taste?

"For me, liberica is sweet-ish. And maybe nutty as well," says Mr Joseph.

Liberica has in the past been considered an inferior crop to arabica. Its large beans are harder to harvest and process due to their thick skin and pulp, and the taste is widely regarded as more bitter.

Like the world's second-most traded coffee bean, robusta, it is considered more of a supplementary species than the main act.

However, researchers are focusing on a subspecies of liberica with a smaller bean - called excelsa - which they believe has an improved flavour profile.

Regardless, the spread of liberica may not be a matter of popularity, but necessity.

Coffee traders, middlemen connecting farms to retailers, are under immense pressure to meet rising demand. Volcafe is one of the world's biggest traders, handling roughly 600 million kilograms of coffee a year.

"We've seen global coffee consumption generally in the last decade increasing at a rate of 2% per year," says Hannah Rizki, global head of research.