Yeast Fermentation and Quality Trends In Coffee Processing by Matthew Dahabieh, Ph.D and Jason Hung, M.Sc
U
nlike wine, beer and other alcoholic beverages, the use of fermentation in coffee remains largely unknown to many consumers of the drink. Even many coffee processors themselves are unaware of the potential for harnessing fermentation as a tool for improving coffee’s quality and consistency. In reality, almost all coffee goes through a fermentation process, with the main methods of production being wet and dry processing. Conventionally, spontaneous fermentation of the beans is facilitated by wild microbes, e.g., bacteria, yeast and filamentous fungi1. These microbes not only assist in the removal of the mucilage layers surrounding the coffee bean, they also produce some of the flavor precursors expressed during roasting and brewing. The effect of fermentation on the flavor and aroma of coffee has measurable impacts on the cupping score of coffee2 once roasted and brewed. For example, coffee beans processed via the wet method had a superior aroma profile highlighted by fruity and floral aromas, while coffee that underwent mechanical processing (without fermentation) contained many unpleasant chemical compounds3. One study looking at adding a yeast starter during dry processing found that some yeast produced higher sensory scores, and ultimately a higher-quality coffee with aromas of caramel, herbs and fruits; others did not4. This difference is due largely to the fact that different microbes consume and produce different chemical compounds that are converted into the volatile compounds that comprise coffee’s unique sensory characteristics. This parallels the production of wine, in which it is well known that the local microbiome is an important factor in terroir5. Taken to the next level, the visible parallels between coffee processing and wine production provide a potential competitive advantage to forwardthinking coffee producers seeking to improve their consistency, quality and flavor. For millennia, fermented beverages such as wine, beer and spirits were also produced through spontaneous fermentation. Sometimes this yielded fantastic results, but in many cases the product was undrinkable, depending on what microbes took control of the fermentation.
Photo credit: Renaissance BioScience. Today, coffee processing operates mainly in this spontaneous fermentation “black box” of uncertainty. While coffee producers and scientists are fully aware of the impact of coffee fermentation, what actually goes on during the process and, more importantly, what microbes are good, bad or even important, are not fully understood. However, investing the time and effort into studying fermentation can certainly pay dividends. Since the 19th century, the wine, beer and spirits industry has been continually refining and developing its fermentation processes. Today, the use of techniques such as strain isolation and domestication, and selective breeding and adaptive evolution, have further optimized Saccharomyces yeast strains to the point that modern brewers and vintners now have at their disposal style-specific Saccharomyces strains that contain the positive attributes of several strains yet produce none of their faults. In addition, highthroughput non-GMO yeast development techniques are now being applied to quickly develop large numbers of novel beer yeast strains with improved flavor and aroma profiles, all while maintaining overall fermentation performance6. A focused effort in developing and utilizing coffee-specific Saccharomyces and other microbial strains should deliver similar quality improvements. First and foremost, using predetermined, pure and optimized microbial strains will allow the coffee farmer and producer much more control, consistency and specificity in their products. This also means they could modulate green coffee beans to suit changing consumer preferences or capitalize on current trends, without significant infrastructure changes such as planting new coffee cultivars or establishing new plantations. For larger organizations that source coffee beans from multiple farms and/ or co-ops, often from multiple locations around the world, standardization of the coffee fermentation process should produce more consistent green coffee, resulting in less variation in roasting and brewing (especially when factoring in different locations and roasters). In turn, more consistent green coffee ultimately reduces the need to store and blend coffee from multiple locations, while still meeting consumer brand expectations.
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Pulped coffee before the fermentation process begins. Photo credit: Renaissance BioScience.
Optimizing coffee fermentation will also give farmers and producers the ability to express the full range of flavor and aromas possible from their plantations and coffee trees. For example, farmers can use selected yeast(s) to highlight a specific flavour and aroma profile, or accentuate a trait already valued in their product. Furthermore, sorted batches of the same coffee harvest can be fermented with different sets of microbes to highlight specific roasting and brewing methods, e.g., coffee farmers could process green coffee beans differently if destined for drip coffee rather than espresso.
April 2019
