Ultrasound in coffee fermentation: Can it control microbial activity and bean flavor?

In coffee fermentation, producers typically control time, temperature, pH, water volume, and oxygen conditions. A new research direction introduces an additional variable into the process: ultrasonic energy. Could this physical impact help microorganisms function more efficiently, alter bean structure, and generate new flavor precursors?

Why introduce ultrasound into the fermentation process?

Coffee fermentation is a complex biological process. Microorganisms present in the coffee cherry, mucilage, and fermentation environment utilize available substrates for growth and metabolism. During this process, sugars, organic acids, pectin compounds, and other metabolites may change.

These changes affect the environment surrounding the bean and can influence the precursors that form aroma during roasting.

The issue is that metabolic processes depend not only on the microbial strain. Substrate accessibility, diffusion of solutes, and environmental conditions also play crucial roles.

Ultrasound is studied as a physical treatment method that can affect material surfaces, mass transfer, and microbial activity. However, its impact is not always beneficial: excessive intensity can cause cell damage or undesirably alter the microbial community.

How can cavitation affect the fermentation system?

At appropriate levels, cavitation can generate fluid motion and mechanical impact at the material surface. This may facilitate the release or transport of certain solutes into the fermentation environment.

However, as ultrasound intensity increases, its impact on microbial cells can also become stronger. Depending on conditions, ultrasound may support growth, increase membrane permeability, or cause cell damage. Therefore, it cannot be assumed that ultrasound always stimulates microorganisms.

New study on Robusta coffee

A study published in Ultrasonics Sonochemistry in October 2026 investigated the combination of ultrasound and co-fermentation using two yeast species, Pichia fermentans and Kluyveromyces marxianus, on Robusta coffee.

The research team examined ultrasound power levels of 100, 200, and 300 W. The published results showed that treatment at 200 W produced the most pronounced promotion of yeast growth among the tested conditions. Ultrasound also altered the microscopic surface structure of green coffee beans.

The study recorded changes in chemical composition after combined fermentation treatment, including an increase in total polyphenol and flavonoid content, and a decrease in caffeine and chlorogenic acid compared to the control sample, as reported by the authors.

This is a noteworthy research direction because it combines three factors: physical ultrasound treatment, yeast activity, and chemical transformation of the coffee bean.

Does ultrasound create new flavors?

It is necessary to distinguish between changing chemical composition and creating perceivable flavors in the cup.

Green coffee beans contain many precursors involved in reactions during roasting. Reducing sugars, amino acids, and nitrogen-containing compounds can participate in Maillard reactions and other thermal reactions, generating numerous volatile compounds associated with aroma.

If ultrasound alters substrate accessibility or microbial metabolic activity, the precursor composition may change. However, this is insufficient to conclude that specific flavor notes such as fruit, wine, honey, or chocolate will appear.

To demonstrate flavor changes, post-roast volatile compound analysis using appropriate methods must be combined with controlled sensory cupping. Measuring polyphenols or caffeine alone cannot replace flavor evaluation.

If conducting trials on Vietnamese Robusta, what conditions need to be controlled?

For Robusta, a valuable experiment should begin with standardizing the raw material. Coffee cherries must have relatively uniform ripeness, be sorted, and have their initial condition recorded before fermentation.

Sample groups should be designed to distinguish the impact of each factor:

• Fermentation sample without ultrasound.
• Sample with ultrasound but without the addition of the test microbial strain.
• Sample with the added microbial strain but without ultrasound treatment.
• Sample combining ultrasound and the microbial strain.

Throughout the process, monitor temperature, pH, time, substrate quantity, microbial density, and oxygen conditions. After fermentation, dry to an appropriate storage moisture content and evaluate post-roast quality.

Such a control design helps answer a critical question: do the observed changes stem from ultrasound, from microbes, or from the combination of both?

A potential technology, but not yet a ready-made fermentation recipe.

Ultrasound could become a tool to support coffee fermentation control. However, practical application requires identifying an appropriate operating window where energy is sufficient to produce the desired effect without damaging the microbial system or causing unintended changes to the raw material.

For producers, a reasonable approach is to start with small-scale trials, including control samples, monitoring fermentation data, and evaluating post-roast results. Do not introduce ultrasound into commercial processes based solely on a single power parameter or an isolated analytical result.

References

1. Ni, J. et al. (2026). Synergistic effects of ultrasound and yeast co-fermentation on microstructural, metabolic, and sensory properties of Robusta coffee. Ultrasonics Sonochemistry, 133, 108030. DOI: 10.1016/j.ultsonch.2026.108030
2. Regarding the mechanism of cavitation and the application of ultrasound in food processing: distinguish between mechanical, thermal, and chemical effects; do not generalize results across different raw materials or equipment.