Yeast Fermentation

Yeast fermentation in coffee processing emerged in the 2010s as producers sought more control over the fermentation stage. While spontaneous fermentation (using wild microorganisms) has always occurred in coffee processing, the deliberate inoculation of specific yeast strains was inspired by winemaking and brewing. Researchers in Colombia, Costa Rica, and Brazil began studying which yeast strains produced desirable flavors in coffee. Commercial yeast products (Saccharomyces cerevisiae strains, Torulaspora delbrueckii, and others) were adapted for coffee. The technique has spread throughout the specialty coffee world, with producers using both commercial wine and beer yeasts and coffee-specific yeast products developed by research institutions.

Yeast fermentation involves the deliberate addition of specific yeast strains to coffee during the fermentation stage. The primary yeast used is Saccharomyces cerevisiae, the same species used in winemaking and brewing. Different strains produce different metabolic byproducts. Yeasts consume sugars from the cherry mucilage and produce ethanol, carbon dioxide, and a wide range of aromatic compounds including esters (fruity aromas), higher alcohols, and volatile fatty acids. By selecting specific yeast strains, producers can target specific flavor compounds. For example, some strains produce more estery, fruity aromas, while others produce more spicy or savory notes. The controlled environment allows for reproducible results, unlike wild fermentation where the microbial population varies. Yeast fermentation can be done aerobically or anaerobically, with anaerobic producing more intense results.

1. Selective Harvesting: Ripe cherries are hand-picked, as sugar content is important for yeast fermentation. 2. Sorting: Cherries are sorted for uniform ripeness and quality. 3. Optional Depulping: Coffee may be depulped before fermentation (for washed-style yeast fermentation) or left as whole cherries (for anaerobic-style yeast fermentation). 4. Tank Loading: Coffee is loaded into fermentation tanks (open or sealed, depending on the desired process). 5. Yeast Preparation: Selected yeast strain is rehydrated and activated in a sugar solution. 6. Yeast Inoculation: The activated yeast is added to the fermentation tank and mixed thoroughly with the coffee. 7. Fermentation: Coffee ferments for 24 to 96 hours. Temperature and pH are monitored. The yeast consumes sugars and produces ethanol, CO2, and aromatic compounds. 8. Monitoring: pH, temperature, and Brix (sugar content) are tracked. Target pH drop is to approximately 4.0. 9. Washing or Drying: After fermentation, the coffee is washed (if depulped) or dried (if whole cherry). The secondary processing follows traditional methods. 10. Drying: Coffee is dried to 10 to 12% moisture on raised beds or in mechanical dryers. 11. Resting and Hulling: Coffee rests in parchment for 30 to 60 days, then is hulled and sorted.

Yeast fermentation is defined by the deliberate inoculation of specific yeast strains. The most common yeast is Saccharomyces cerevisiae, available in many commercial strains originally developed for wine or beer. Other yeasts like Torulaspora delbrueckii and Pichia kluyveri are also used. The yeast is rehydrated, activated in a sugar solution, and added to the fermentation tank. The yeast consumes sugars from the cherry mucilage, producing ethanol, CO2, esters, higher alcohols, and volatile acids. Different strains produce different aromatic profiles, giving producers control over the final flavor. Fermentation lasts 24 to 96 hours at controlled temperature (20 to 30 degrees Celsius). The pH drops from approximately 5.5 to 4.0. The controlled, reproducible nature of yeast fermentation is its primary advantage over wild fermentation.

After yeast fermentation, the coffee is processed and dried using traditional methods. If depulped, the coffee is washed and dried on raised beds for 10 to 20 days. If whole cherry, it is dried as a natural. Drying to 10 to 12% moisture is critical to preserve the aromatic compounds developed during fermentation. The coffee may be more delicate after yeast fermentation and requires careful drying. Target moisture is 10 to 12%.

Yeast-fermented coffees should be stored in breathable bags during the parchment stage, then in GrainPro or hermetic bags for green coffee. The aromatic compounds developed during yeast fermentation may be more volatile than those in traditional processing, so these coffees are best consumed fresh. Store at 15 to 25 degrees Celsius with 50 to 60% relative humidity.

• •Produces consistent, reproducible flavor profiles through controlled yeast strains
• •Allows producers to target specific aromatic outcomes (fruity, floral, spicy)
• •Reduces the risk of spoilage from unwanted wild microorganisms
• •Can be combined with other processing methods (washed, natural, anaerobic)
• •Faster and more predictable fermentation than wild methods
• •Enables producers to differentiate their product with unique flavor signatures

• •Requires purchasing commercial yeast strains, adding cost
• •Requires skill and knowledge to select and manage yeast strains
• •Can produce uniform flavors that some find less interesting than wild fermentation
• •Requires additional equipment (rehydration containers, monitoring tools)
• •Risk of yeast contamination if equipment is not properly sanitized
• •Some consumers are skeptical of 'engineered' flavors in coffee

Yeast fermentation produces clean, consistent, and well-defined flavor profiles. The specific flavors depend on the yeast strain used. Common descriptors include: enhanced fruit notes (berry, citrus, stone fruit), floral aromatics, clean and balanced sweetness, and sometimes spicy or savory notes. The body is typically medium, and acidity is clean and well-integrated. Compared to wild anaerobic fermentation, yeast-fermented coffees tend to be cleaner, more balanced, and more reproducible. The flavors enhance rather than mask the coffee's inherent terroir.

• •Yeast contamination from improperly sanitized equipment
• •Over-fermentation if the yeast is too active or fermentation runs too long
• •Off-flavors from inappropriate yeast strain selection
• •Inconsistent results if yeast activation is not done properly
• •Loss of coffee's inherent character if yeast flavors are too dominant

Yeast fermentation has a similar environmental footprint to other processing methods. The yeast strains used are food-grade and commercially produced. The process does not produce additional wastewater beyond what the secondary processing generates. The main consideration is the energy used for yeast production and transport. Overall, yeast fermentation is environmentally comparable to traditional processing methods.

• •Coffee-specific yeast strains developed by research institutions for optimal flavor
• •Combination of multiple yeast strains in sequence for complex flavor layering
• •Use of non-Saccharomyces yeasts (Torulaspora, Pichia) for unique aromatics
• •Integration with anaerobic or carbonic maceration for compounded flavor effects
• •Yeast fermentation with specific temperature profiling for targeted aroma production
• •Development of yeast strains that also suppress spoilage organisms

• •Chapter 2: Processing Methods
• •Chapter 5: Extraction Science

• •Specialty Coffee Association
• •Coffee Science Foundation
• •World Coffee Research
• •Cenicafe