How Does pH Affect Fermentation?


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pH plays a significant role in fermentation, influencing enzyme activity, microbial growth, and product spectrum. During fermentation, as the pH drops and becomes more acidic, the rate of fermentation increases. This is because the organisms that allow glucose to ferment have adapted to low-pH environments. 

Fermentation is a widely used process in various industries ranging from food and beverage production like beer and kombucha, to renewable fuels and pharmaceuticals. The intricate relationship between pH levels and fermentation is a critical factor that influences the quality and efficiency of the process. 

pH And Enzyme Activity During Fermentation

pH significantly affects the shape and function of proteins, including enzymes responsible for metabolic processes in fermentation.

Enzymes are proteins that act as organic catalysts, accelerating chemical reactions without being consumed in the process. In the case of fermentation, enzymes are responsible for breaking down complex molecules, such as glucose, into simpler compounds like ethanol and carbon dioxide (CO2).

Amino acids, the building blocks of proteins, can be acidic or basic, depending on the side chain. A polymer is formed as the amino acids bond together. They cross-link between the R-groups which makes them a unique shape. 

When the pH level increases, the bonds within the proteins are disrupted, altering their three-dimensional shape and, consequently, their function; as the pH increases, the proteins bend too far out of shape and become denatured. When the pH is lower during fermentation, the proteins typically bend to the right shape so that fermentation can happen. This is a form of natural selection of enzymes that are used today in yeast. 

An optimal pH level is essential for enzymes to work efficiently and maintain their shape. For example, in alcoholic fermentation, the enzymes responsible for converting pyruvic acid into ethanol and CO2 function best in slightly acidic conditions. This is because these enzymes have evolved to work effectively under the acidic conditions created by the fermentation process.

Microbial Growth And pH

Microorganisms, such as yeast and bacteria, play a crucial role in fermentation. The growth and multiplication of these organisms are also influenced by pH levels.

Yeast grows and multiplies more rapidly at a pH higher than 5.0, with an optimal growth rate at a pH of 5.5. However, fermentation is more efficient if the pH is below 5.0. Maintaining suitable pH levels is vital for the overall success of the fermentation process.

pH Regulation In Mixed Culture Fermentation

By utilizing mixed culture fermentation (MCF), various products can be produced from glucose. Nonetheless, the product spectrum is usually not in line with thermodynamic expectations owing to the lack of understanding of the effect of operational factors on the microbial community/functionality.

Therefore, research has been conducted to manipulate MCF to enhance the production of particular compounds like ethanol, organic acids, and hydrogen, with diverse results. The results demonstrate the importance of pH in determining the product spectrum, with different pH values found to be dominant for the production of these compounds.

Microbial Community Structure And pH

The pH of an MCF system can have a substantial influence on the microbial community structure. 

This is because organisms of varying types thrive in different pH levels, thus resulting in a change in the overall product range. Glucose can be fermented by a vast array of microorganisms, and the diversity is further boosted when more complex feedstocks are employed.

Consequently, there is an anticipated variance in the yield and spectrum of products.

pH Regulation Modes In Fermentation

To assess the impact of pH regulation mode on the product spectrum, two modes can be applied: progressive and reset pH regulation. In progressive pH regulation, the pH is adjusted from 4.5 to 8.5 in progressive steps of 0.5. In reset pH regulation, the pH covers the same range but is reset to 5.5 before each change.

Effects On Product Spectrum And Microbial Communities

It has been noted that acetate, butyrate, and ethanol production can take place in a wide range of pH values in both regulation methods. When the pH is below 6.5, butyrate is the primary product; if the pH is above 6.5, ethanol is the primary product.

Additionally, there is a consistent shift from hydrogen to formate as the zpH increases. Microbial analysis indicates that progressive pH regulation results in Klebsiella dominance, while reset pH regulation provides an advantage for the bacteria Clostridium spp. when the pH is low. Reset pH regulation is more responsive to any changes in pH, and the experiments conducted come closer to thermodynamic equilibrium, particularly concerning the formate/hydrogen balance. This implies that resetting the pH periodically may be per thermodynamic expectations.

pH And Fermentation Types

Fermentation can be divided into three categories: butyrate-type, propionate-type, and ethanol-type. Acetate is a VFA which is always created, and carbon dioxide and hydrogen are released as carbon and electrons respectively. 

Butyrate-type fermentation produces mainly butyrate and acetate, with hydrogen and carbon dioxide as side effects. Propionate-type fermentation generates propionate and acetate in sizable amounts, with valerate in smaller amounts and no considerable gas production. 

Ethanol-type fermentation results in ethanol and acetate as the main products, and hydrogen and CO2 as by-products.

pH Regulation Of Fermentation Pathways

The impact of pH on these pathways is widely understood, however, there are discrepancies. It is still uncertain whether the product range is influenced by phylogenetic (bacterial population) or physiological (chemical) factors. 

When the pH is adjusted incrementally, the culture adjusts over time to the new pH, causing a gradual alteration in the product mix as pH changes. On the other hand, abrupt pH regulation leads to substantial transitions in the product spectrum as pH levels modify.

The Importance Of pH Monitoring In Brewing

To make top-quality, consistent beer, it is vital to keep a close eye on and maintain the ideal pH level throughout the brewing process. The source water commonly found in the United States is usually more alkaline than the desired mash pH range of 5.2 to 5.8. If the pH reading of the beer drops below 4.0, it can be a sign of spoilage caused by acid-producing bacteria, and if it is higher than the ideal range, it can produce an unpleasant, soapy, or metallic flavor.

Taking precise pH readings at each stage of brewing is essential to attain the desired outcome. This includes source water, mash in, boil, knock out, and fermentation. With a pH that falls within the optimal range, the beer will have a balanced taste, be more stable, and have a longer shelf-life.

How To Adjust The pH In Brewing?

The majority of water systems utilized for brewing have a pH that is too high for the desired result. The brewer has to guarantee that all steps of the process adhere to the specific standards, which can include adjusting the acidity of the water. Food-grade lactic and phosphoric acid can be used to reduce the pH of the source water to an acceptable level for brewing.

To determine the correct amount of additives needed, accurate and reliable pH meters are an indispensable tool for brewers. Knowing how to precisely adjust the pH is of great importance during the fermentation process. The wort must remain sanitized, aerated, and held within an exact temperature and pH range for the brewer’s yeast to thrive.

pH And Fermentation Efficiency

Maintaining the pH level during the fermentation process is essential for sustaining the well-being of the yeast, as well as harvesting them and giving the beer its unique flavor. If the final pH reading is lower than anticipated, it may be due to bacterial contamination or poor yeast health.

Final Product pH In Beer

To produce a great-tasting beer, the pH level should be lower when it’s finished than it was at the pre-fermentation wort stage, and should be in the anticipated ranges. If the pH reading is too low, this could mean a sour-tasting beer that appears cloudy or hazy

When the pH reading is too high, this could be a signal of a misstep made early on or a possible contaminant. Experienced brewers understand just how important it is to have an accurate pH reading at every step of the beer brewing process, and that’s why they rely on reliable pH meters.


Understanding the relationship between pH and fermentation is crucial for optimizing product spectrum, microbial communities, and overall process efficiency. Maintaining optimal pH levels at each stage of the fermentation process, whether in mixed culture fermentation or brewing, ensures the production of high-quality, consistent products.

By closely monitoring and adjusting pH levels, you can harness the full potential of fermentation and create products that meet desired specifications and consumer expectations.

If you have any questions regarding fermentation or which pH meter to use, do not hesitate to contact the world-class team at Atlas Scientific.

pH Probes & Sensors

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