Can yeast processing improve the quality of coffee?

Josephine Walbank
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April 18, 2022
yeast coffee processing

As the specialty coffee industry becomes increasingly competitive, many roasters have increasingly gone in search of ways to set their coffees apart.

These include unique branding strategies, roast profiles, and most recently, yeast fermentation. Yeast is always present during coffee fermentation, but using a specific strain in a controlled environment can unlock unique flavour characteristics.

While some are concerned that controlling fermentation with yeasts can make all coffees taste the same, this may not be the case. Many parameters influence the flavour in the cup, such as variety, terroir, and growing techniques.

To find out more about yeast processing, I spoke with innovative coffee roaster Dr Robert McKeon Aloe.

yeast coffee processing

What is yeast processing?

While it may vary in length and nature, every type of coffee goes through fermentation. It is a chemical reaction where bacteria, yeast, and microorganisms break down natural matter into sugars and other substances.

In coffee, this typically occurs when the sugar or water present in the coffee cherry reacts with the fruit and the green bean.

Yeast processing is typically done at origin and involves adding a yeast starter to green coffee in order to further ferment the beans.

“Typically, yeast has been used to process coffee cherries at the farm,” says Robert, who works as a data scientist. “This is done before further processing and drying.”

However, this process can also be done after processing. This allows roasters like Robert to buy green beans and experiment with different methods.

The most common method involves adding yeast and warm water to green beans. The mixture is covered to allow fermentation to occur, and then the beans are rinsed and dried before roasting. For dry processed coffees, this can help boost the coffee’s quality and complexity.

While relatively new in the specialty coffee scene, yeast fermentation is widely practised in the wine and beer industries.

It is the fermentation process that gives coffee many of its desirable characteristics. These include complex flavours, a rich aroma, and a significantly better cupping score – particularly when comparing wet and dry fermentation methods.

When beans are fermented using a spontaneous wet method, bacteria, yeast, and other microbes in the atmosphere take part in the process.

Each of these interacts with the coffee differently, engaging in unique reactions that produce different volatile compounds which we experience in the cup.

When this process is uncontrolled, it can either result in an outstanding coffee, or ruin the potential of a good bean. By carefully monitoring the process and doing it under specific conditions, roasters could secure consistent, high quality results.

Essentially, yeast fermentation can help create consistent, high-quality coffee. These yeast cultures help keep the fermentation process under control, and allow roasters to showcase the full breadth of coffee characteristics at their finest.

coffee in a chemex

How does yeast processing affect coffee’s characteristics?

While several optimised bacteria and organic cultures can be used‌, yeast is one of the most popular options.

This may be because yeast is easy to source, it is natural, and it captures consumers’ attention. What’s more, it is believed yeast may also enhance coffee by manipulating certain characteristics.

While typically done at origin, Robert often experiments with yeast processing to alter the profile of green beans.

“Yeast processing can reduce acidity and bitterness,” he explains. “I tend to use sweeter, more floral coffees, and I pull a very strong coffee, such as the staccato shot. The upside is the shot is very sweet, but there’s often an imbalance of acidity in the cup. With yeast processed coffees, the acidity isn’t there.”

Robert also noticed a significant impact on bitterness. “Usually, I won’t roast dark in order to avoid bitter notes. However, this has encouraged me to roast a little darker.

“It has resulted in a cup with amazing clarity. The downside is the coffee doesn’t have a traditional coffee taste. It seems very unique in that way.”

Yeast processing can give coffee an extra layer of flavour, which may resemble the crisp profile of whiskey or white rum. This has the potential to improve green bean cupping scores.

yeast processed coffee in a recyclable kraft paper bag

Should roasters offer yeast processed coffees?

Yeast processing can help innovative roasters improve their coffee in three key areas – quality, consistency, and desirability.

An additional benefit is that it can enhance the flavour notes, aromas, and complexities of different coffees. Plus, the label of “yeast processing” is sure to intrigue specialty coffee consumers.

While it is an opportunity to showcase coffee at its finest, roasters will need to invest time to learn the method and identify which is the best fit for each coffee.

However, Robert points out that there are some ethical issues with buying low-grade coffee and then using yeast processing.

“The problem goes back to money. I think coffee farms should offer yeast processed coffees, whether they process the fruit or the green beans. Processing the green beans may be more economical.”

He firmly believes it would be fairer to do this at the source. “It’s better than someone paying very little to bring in lower grade coffees, using yeast to process it, and then selling it for a premium.”

Notably, this is just one fermentation strategy roasters can use. Other methods used include experimenting with temperature, microbes, environments, and nutrients.

It is important to note that if roasters use yeast processing to improve the quality of cheaper beans to increase profit margins, it may compromise their brand.

However, if roasters use this method to experiment with flavour profiles and new notes, it can be an effective technique to add to their repertoire.

While controlled fermentation processes are not new, the coffee industry has only just begun using them. In many cases, coffee is processed with little thought on how varying fermentation can enhance the end result.

The increasing popularity of yeast processing can send the specialty coffee sector down an exciting new path. By studying fermentation, producers and roasters can identify the best conditions, microbes, and processes for different coffees.

As the industry improves its knowledge on fermentation conditions and how they can be used, consumers can expect to see a wide range of innovative and exciting coffee offerings.

To prolong the shelf life of this unique roasted coffee and ensure it arrives at the consumer in peak condition, roasters should invest in multilayer coffee bags that will help keep out oxygen, moisture, heat, and light.

At MTPak Coffee, we understand how important it is for specialty roasters to provide their customers with the best coffee possible. To help keep your roasted coffee fresh until consumption, we offer fully recyclable coffee packaging made from low-density polyethylene (LDPE), kraft paper, rice paper, and PLA.

Additionally, our low-VOC water-based inks and BPA-free degassing valves are also recyclable, helping to preserve the unique characteristics of each coffee.

For more information on our sustainable coffee packaging, contact our team.

Can yeast processing improve the quality of coffee?

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