How is Fermentation Detected in Homebrew and Industrial Processes?

December 1, 2025 •

4 min read

According to food scientists, monitoring fermentation is essential for controlling product quality and consistency. Whether you are a homebrewer creating your next batch or working in industrial biotechnology, understanding how is fermentation detected is crucial for achieving your desired outcome.

Quick Summary

Fermentation can be detected through simple visual cues like bubbling and krausen formation, or by using tools such as hydrometers to measure specific gravity changes. Advanced methods involve monitoring pH, temperature, and using biosensors in industrial settings.

Key Points

Visual Cues: Bubbles in an airlock, a layer of foam (krausen), and a cloudy liquid are all signs of active fermentation.
Hydrometer Reading: A decreasing specific gravity reading over time is the most accurate way for homebrewers to track fermentation progress and know when it's complete.
pH Monitoring: The production of organic acids causes the pH to drop, which can be monitored with a pH meter or pH-sensitive indicator dyes.
Advanced Sensors: Industrial processes rely on sophisticated in-line sensors for real-time monitoring of parameters like dissolved oxygen, CO2, and biomass concentration.
Gas Production: A buildup of CO2, visible as bubbling in an airlock or trapped gas in a Durham tube, is a direct indicator of fermentative activity.
End of Fermentation: A stable specific gravity reading over several days, combined with a decrease in visual activity, confirms that fermentation has finished.

Visual and Sensory Methods

For the home enthusiast, visual and sensory clues are often the first indicators of fermentation activity. After pitching yeast into a sugary liquid, such as wort or must, the onset of fermentation typically occurs within 12 to 48 hours.

Observing the Signs of Activity

Bubbling: The most common sign is the release of carbon dioxide (CO2) gas. In a closed fermenter with an airlock, you will see the airlock bubble as pressure builds inside.
Krausen Formation: During the active phase of fermentation, yeast produces a thick, foamy layer on the surface called krausen. The presence of this layer is a clear sign that yeast is at work.
Clarity Changes: The liquid, which may start out clear or translucent, often becomes cloudy as yeast multiplies and becomes suspended in the solution.
Smell: As yeast and bacteria metabolize sugars, they produce a range of aromatic compounds. The smell can change from sweet to yeasty, and may even develop complex, fruity, or spicy notes, depending on the ingredients and microbe strain.
Taste: As sugars are converted, the sweetness of the liquid will decrease. For lacto-fermented foods like sauerkraut or kimchi, a sour taste indicates lactic acid production.

Common Equipment-Based Methods

Beyond simple observation, more accurate and reliable methods are available for detecting and tracking fermentation, particularly for those who need more precise control over their final product.

The Hydrometer: Measuring Specific Gravity

The most accurate tool for homebrewers to monitor fermentation progress is the hydrometer. It measures the specific gravity (SG) of a liquid, which is its density relative to water. Sugar is denser than water, so a high concentration of sugar results in a high SG. As yeast consumes the sugar and converts it into less-dense alcohol and CO2, the SG of the liquid decreases.

How to Use a Hydrometer:

Take an Initial Reading: Measure the wort or must's SG before adding yeast. This is the Original Gravity (OG).
Monitor Progress: Take readings periodically throughout the fermentation process. A continuous drop in the numbers indicates ongoing activity.
Confirm Completion: Take a reading, wait 24-48 hours, and take another. If the reading remains stable, fermentation is likely complete. The difference between the OG and the final gravity can be used to calculate the final alcohol by volume (ABV).

The pH Meter: Tracking Acidity

Another reliable measurement is pH. As microorganisms ferment carbohydrates, they often produce organic acids (like lactic acid), which causes the pH to drop. A handheld digital pH meter offers accurate and repeatable results. Monitoring a decreasing pH over time provides clear evidence of fermentation, especially for lactic acid fermentations.

Phenol Red Fermentation Test

In a laboratory setting, specific tests are used to determine a microorganism's ability to ferment a particular carbohydrate. The Phenol Red fermentation test uses a nutrient broth with a specific sugar and a pH indicator, phenol red.

Acid Production: If the organism ferments the carbohydrate, acidic byproducts are released, and the phenol red indicator turns the broth from its neutral reddish-orange to yellow.
Gas Production: A small, inverted Durham tube placed in the broth captures any gas (CO2) produced during fermentation, confirming gas formation.

Industrial and Advanced Monitoring Techniques

Large-scale industrial fermentations demand precise, real-time monitoring to optimize yield and consistency. This requires sophisticated sensor technology and data analysis.

Real-time Sensors

Optical Density (OD) Sensors: Measure the turbidity of the fermentation broth by passing light through it. The amount of light scattered or blocked is proportional to the microbial biomass, providing a real-time measure of cell growth.
Off-gas and Dissolved Gas Analyzers: Mass spectrometers and other gas analyzers monitor the concentration of dissolved gases like O2 and CO2, as well as the composition of the gases exiting the fermenter. The ratio of CO2 produced to O2 consumed (Respiratory Quotient or RQ) is a key indicator of metabolic activity and can be controlled to optimize production.
Biosensors: These use biological components to detect specific compounds in the fermentation broth. They are highly specific and can measure metabolites, substrates (like glucose), or other products in real time.
E-Noses and E-Tongues: Electronic sensor arrays are used to profile the overall aroma (e-nose) and taste (e-tongue) of the ferment. Combined with machine learning, these can track fermentation progress and standardize flavor development. For more details on smart fermentation technologies, refer to the academic review, "Smart Fermentation Technologies: Microbial Process Control and Optimization for Traditional Fermented Foods".

Comparison of Fermentation Detection Methods


Method	Relative Cost	Accuracy	Best for...
Visual/Sensory Cues	Very Low	Low	Hobbyists and small-scale batches, initial activity detection.
Airlock Bubbling	Low	Low	Simple confirmation of CO2 production, not for precision or completion.
Hydrometer Readings	Low	High	Homebrewers and small batches needing ABV calculation and reliable completion data.
pH Meter	Medium	Medium-High	Any scale where acidity changes indicate progress, especially lacto-fermentation.
Lab Indicators (Phenol Red)	Medium	High	Microbiological identification and quality control in lab settings.
Advanced Sensors (OD, Gas Analyzers)	Very High	Very High	Industrial-scale production requiring real-time, high-precision monitoring.

Conclusion

Fermentation can be detected using a wide spectrum of methods, ranging from simple sensory observation to high-tech, real-time sensor arrays. For homebrewers, visual cues and a hydrometer provide reliable and accurate ways to track progress and determine when fermentation is complete. In industrial settings, the need for scalability, consistency, and optimized production drives the use of advanced techniques like spectroscopy, biosensors, and automated gas analyzers. The appropriate detection method ultimately depends on the scale of the operation, the required level of precision, and the specific characteristics of the fermentation process being monitored.

Frequently Asked Questions

The first visual signs of fermentation include fine bubbles appearing on the surface of the liquid, which can develop into a thick layer of foam known as krausen. For closed containers, bubbles moving through an airlock indicate gas release.

To use a hydrometer, take an initial specific gravity (SG) reading before pitching yeast. As fermentation progresses and sugar is converted to alcohol, the SG will decrease. You can confirm completion by taking readings on consecutive days; if the SG is stable, fermentation is finished.

A drop in pH during fermentation indicates the production of organic acids by microorganisms. This is a crucial indicator, especially for lactic acid fermentation in products like yogurt and kimchi.

Industrial processes use advanced sensor technology for real-time monitoring. This includes in-line sensors for pH, temperature, and dissolved oxygen, as well as gas analyzers using techniques like mass spectrometry for off-gas analysis.

No, while airlock bubbling indicates activity, it may not reliably signal when fermentation is complete. Bubbling can stop prematurely due to temperature changes. A stable hydrometer reading is the most reliable way to confirm completion.

In lab settings, a phenol red fermentation test uses a pH-sensitive dye that turns yellow from red or orange when acid is produced from carbohydrate fermentation. An inverted Durham tube also traps any gas produced.

For bread dough, visible signs of fermentation include the dough rising and becoming lighter and airier. A poke test can also confirm: if an indentation made with your finger remains, the fermentation is likely done.

Monitoring allows for precise control of the process, which is critical for ensuring consistent product quality, flavor, and alcohol content. It also helps detect potential issues like stalled or contaminated batches before they ruin a product.