In a previous post, we told you all about the latest research on impacts that coffee has on health. With the start of our latest series, MicroEducation, we’re focusing on the role that microbes play in your everyday life. As an avid coffee drinker, I decided to bring my favorite beverage back into the spotlight and write a short post about the role that microbes play in the process of producing coffee beans. I’m going to discuss three main ideas in this post - which microbes are used to make coffee; what role they play in coffee bean production; and could we have coffee without them?
What role do microbes play in coffee bean production and which microbes are there?
First, a bit of background on how coffee goes from the plant to the luscious aromatic beans we use for brewing. Coffee beans are the processed and roasted seeds of the coffee plant. They are obtained by taking the fruit produced by the plant - called the coffee cherry - and removing the flesh from around the seed. This removal is completed in one of three ways called - wet, dry, and semidry treatment. While the specific steps involved in these methods vary, they all depend to some extent on a microbially driven process called fermentation. This is the microbial breakdown of complex molecules into simpler molecules. No matter which treatment method is used to remove the pulp of the coffee cherry, roasters are all faced with the same problem - the removal of the sticky covering that remains around the seed called the mucilage. If this covering (made of pectin, cellulose, and starch) was left on - many of the seeds would rot before they could be turned into a delicious cup of joe. Luckily for us, certain microorganisms such as yeasts and bacteria that are found naturally in many environments can degrade this covering (Haile and Kang 2019).
Researchers have identified over 50 types of microbes involved in the coffee fermentation process, and are still working to figure out which ones play the most important roles in mucilage degradation and flavor development. For example, one study found that a single-celled fungus called Saccharomyces cerevisiae - more commonly known as Brewer’s Yeast (yep, you can thank yeast for making two of your favorite beverages ;)) in combination with two other species of yeasts were particularly good at breaking down the mucilage of the coffee seed and produced beans that had aroma of caramel, herbs, and fruits. While another combination of yeasts in the fermentation process resulted in coffee that smelled fruity, buttery, and had a fermented aroma (Halie and Kang 2019).
The research on using ‘starter cultures’ - pre-made batches of microbes that can be added during the fermentation process to make it faster - for coffee processing has really just begun. One study reported that adding a mixture of 3 different types of yeasts did indeed speed up the breakdown of the mucilage layer. However, another similar study that used different microbes, did not find that the microbes they added made any difference in the fermentation time or the flavor of the coffee (Reviewed in Halie and Kang 2019). These contrasting studies highlight how important understanding microbes can be in the food industry. It is important to know the taxonomic profile and functional potential of the microbial community, in other words - what microbes are there and what they can do!
We mentioned that all three coffee treatment types (wet, dry, semidry) depend on microbes to remove the mucilage from the coffee seed. What is important for you to understand is that in each of these treatments the microbes are exposed to the seed for different lengths of time. It turns out that time is a vital component to the coffee making process.
Microbes and Time in coffee production - a double edged sword.
Roasters have to carefully balance the fermentation time given to the microbes and the quality of their beans. A variety of factors have been found to change the aroma and taste of your coffee. Things like where the coffee plant was grown and how the beans were roasted are all relatively obvious - you usually find these on the package of your beans. However, processing -wet vs dry- and microbial fermentation also impact the taste of your coffee.
Researchers have identified that wet-processed coffee can have more complex aromas than dry-processed because the microbes involved in wet-processing have more time with the beans. Meaning that they can produce more molecules that contribute to the taste/smell of the coffee. However, this process can cause your beans to go from small batch/artisanal to gas-station coffee real quick.
Overfermention is when non-beneficial microbes take over the fermentation process and spoil the coffee seeds. An uncontrolled fermentation often results in molecules (e.g. propionic and butyric acids) being made that can give your coffee an onion-like taste. Conversely seeds can be underfermented meaning that the mucilage covering isn’t removed all the way, which opens the door for spoilage microbes to settle in and nom away. All of these unwanted microbes will cause your beans to have an ‘off-flavor’. In short, the microbes involved in the coffee making process are very powerful - if used properly they make one of the most popular drinks in the world. But if left unchecked they can ruin it.
Conclusion - Can we have coffee without microbes?
Microbes are a vital part of coffee production. They help remove the sticky surrounding called the mucilage from around the coffee seed, as well as help different beans develop various aromas and flavors. Without them, we’d never know the wide diversity of tastes and smells that different coffee beans have to to offer.
Edited by Alexey Vorobev
Glossary:
Fermentation - microbial breakdown of complex molecules - here pectin or starch in the mucilage- into smaller simpler molecules
Starter culture - pre-made batch of microbes that can aide in the fermentation process
Uncontrolled fermentation - in the food industry - fermentation that isn’t being monitored for time or microbial community
Overfermentation - the microbes have been breaking down molecules too long, conferring off-flavors to the coffee
Taxonomic profile - what types of microbes are they?
Functional potential - what can these microbes do?
References:
Mesfin Haile & Won Hee Kang, "The Role of Microbes in Coffee Fermentation and Their Impact on Coffee Quality", Journal of Food Quality, vol. 2019, Article ID 4836709, 6 pages, 2019. https://doi.org/10.1155/2019/4836709
https://www.ncausa.org/About-Coffee/10-Steps-from-Seed-to-Cup
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