Yeast inoculation is a relatively new coffee processing technique where specific strains of yeast are purposefully introduced to the coffee during the fermentation stage.
With well known producers like Wilton Benitez jumping onto yeast inoculation, it's a processing technique that's currently enjoying a surge in popularity.
Today, we'll take a deeper dive on exactly how that works, what effect it has on the coffee, and what flavours and characteristics you can expect to see from yeast inoculated coffees.
But first, how does yeast inoculation actually work?
Well, to answer that we need to first do a quick recap on traditional coffee processing techniques.
Until recently, we really only had 2 (or maybe 3 including honey) processing methods that almost all coffee went through. Washed and Natural.
In washed coffee processing, the ripe coffee cherries are pulped and then quickly washed to remove all fruit from the coffee seed. The seeds are then dried.
In natural coffee processing, the coffee cherries are left in the sun to dry, fully intact, often for several weeks. They are then sent to the mill to be hulled and dry milled.
The role of fermentation in coffee processing
Fermentation happens in all coffee processing techniques.
It's the process of natural yeasts and bacteria in the atmosphere reacting on the sugars in the fruit. They consume the sugars in the fruit and produce important biproducts like acids. Some of those acids then work their way into the seed, where roasters like us can turn them into all of those tasty flavour notes we all enjoy.
Yeast inoculation then, goes one step further than this...
In yeast inoculation, the producer introduces specific strains of yeast at the fermentation process, in order to more directly control the outputs, and therefore the flavours.
It's a controlled fermentation approach.
By introducing specific strains of yeast, producers have a much more reliable understanding of the compounds that will be produced as a result of the fermentation, and therefore they can push flavour development in a particular direction rather than relying on the more unpredictable natural yeasts and bacteria in the environment.
Advantages of yeast inoculation
As with many new and experimental coffee processing techniques, the main advantage of yeast inoculation is that it allows producers to take a coffee that otherwise would be just OK, and turn it into something much more special (and valuable!).
In one study by Food Research International, they found that "Yeast inoculation modified the sensorial profile and increased the coffee beverage scores by up to 5 points." - source here.
In specialty coffee, that's a LOT.
And to many producers, 5 points can easily be the difference between qualifying as specialty coffee, and being forced to sell on commodity markets where prices are all too often below the actual production costs.
Besides improving quality and enabling producers to make a living, yeast inoculation also has another advantage.
It also helps to improve consistency.
Producers have a better understanding of what the outputs will be because they control the inputs.
Take a look at this video from Roaster Kat, for example. There's a clip starting at around 4:00 where Lucia Solis describes the effects of yeast inoculation processing on different qualities of cherry from the same harvest. She says that with yeast inoculation, you can take very overripe cherries and very under ripe cherries, and get to the exact same flavour profile. Here's the clip:
We'll leave it up to you to decide whether that's a good thing or not for you.
There is one huge plus side though, which everyone can agree on no matter where you fall on that...
The consistency and reliability of yeast inoculation as a processing method does really help farmers who have varying or lower quality harvests to elevate their lots and therefore achieve better prices for their coffees.
How does yeast inoculation effect flavour profile?
Just like a lot of modern or experimental processing techniques, yeast inoculation tends to produce coffees that are firmly on the wilder side.
Elevated perceived acidity is a common theme for yeast inoculated coffees, particularly citric acidity. Malic acidity is also common, represented by notes like apple.
Depending on the strain of yeast being used and how the process is applied, there's a good chance you'll also see more tropical and floral notes on yeast inoculated coffees.
Many yeast inoculated coffees have complexity, with layered flavour profiles.
What yeasts are used in yeast inoculated coffee?
There's a handful of specific yeast strains that are commonly used in yeast inoculation.
Most of which have been traditionally used in wine making, and are now gradually working their way into the coffee industry.
Cima is one popular strain. Others include Lalcafe's Oro and Intenso strains.
But there's still lots of work and innovation happening here, with new strains and processing techniques being experimented with all the time.
