News - Monitoring of pH Levels in the Bio Pharmaceutical Fermentation Process

The pH electrode plays a critical role in the fermentation process, primarily serving to monitor and regulate the acidity and alkalinity of the fermentation broth. By continuously measuring the pH value, the electrode enables precise control over the fermentation environment. A typical pH electrode consists of a sensing electrode and a reference electrode, operating on the principle of the Nernst equation, which governs the conversion of chemical energy into electrical signals. The electrode potential is directly related to the activity of hydrogen ions in the solution. The pH value is determined by comparing the measured voltage difference with that of a standard buffer solution, allowing for accurate and reliable calibration. This measurement approach ensures stable pH regulation throughout the fermentation process, thereby supporting optimal microbial or cellular activity and ensuring product quality.

Proper use of pH electrodes requires several preparatory steps, including electrode activation—typically achieved by immersing the electrode in distilled water or a pH 4 buffer solution—to ensure optimal responsiveness and measurement accuracy. To meet the stringent demands of the biopharmaceutical fermentation industry, pH electrodes must exhibit rapid response times, high precision, and robustness under rigorous sterilization conditions such as high-temperature steam sterilization (SIP). These characteristics enable reliable performance in sterile environments. For example, in glutamic acid production, precise pH monitoring is essential for controlling key parameters such as temperature, dissolved oxygen, agitation speed, and pH itself. Accurate regulation of these variables directly influences both the yield and quality of the final product. Certain advanced pH electrodes, featuring high-temperature-resistant glass membranes and pre-pressurized polymer gel reference systems, demonstrate exceptional stability under extreme temperature and pressure conditions, making them particularly suitable for SIP applications in biological and food fermentation processes. Furthermore, their strong anti-fouling capabilities allow for consistent performance across diverse fermentation broths. Shanghai Boqu Instrument Co., Ltd. offers various electrode connector options, enhancing user convenience and system integration flexibility.

Why is pH monitoring necessary during the fermentation process of biopharmaceuticals?

In biopharmaceutical fermentation, real-time monitoring and control of pH are essential for successful production and for maximizing the yield and quality of target products such as antibiotics, vaccines, monoclonal antibodies, and enzymes. In essence, pH control creates an optimal physiological environment for microbial or mammalian cells—functioning as "living factories"—to grow and synthesize therapeutic compounds, analogous to how farmers adjust soil pH according to crop requirements.

1. Maintain optimal cellular activity
Fermentation relies on living cells (e.g., CHO cells) to produce complex biomolecules. Cellular metabolism is highly sensitive to environmental pH. Enzymes, which catalyze all intracellular biochemical reactions, have narrow pH optima; deviations from this range can significantly reduce enzymatic activity or cause denaturation, impairing metabolic function. Additionally, nutrient uptake through the cell membrane—such as glucose, amino acids, and inorganic salts—is pH-dependent. Suboptimal pH levels may hinder nutrient absorption, leading to suboptimal growth or metabolic imbalance. Moreover, extreme pH values can compromise membrane integrity, resulting in cytoplasmic leakage or cell lysis.

2. Minimize by-product formation and substrate waste
During fermentation, cellular metabolism generates acidic or basic metabolites. For instance, many microorganisms produce organic acids (e.g., lactic acid, acetic acid) during glucose catabolism, causing a drop in pH. If uncorrected, low pH inhibits cell growth and may shift metabolic flux toward non-productive pathways, increasing by-product accumulation. These by-products consume valuable carbon and energy resources that would otherwise support target product synthesis, thereby reducing overall yield. Effective pH control helps maintain desired metabolic routes and improves process efficiency.

3. Ensure product stability and prevent degradation
Many biopharmaceutical products, especially proteins such as monoclonal antibodies and peptide hormones, are susceptible to pH-induced structural changes. Outside their stable pH range, these molecules may undergo denaturation, aggregation, or inactivation, potentially forming harmful precipitates. Additionally, certain products are prone to chemical hydrolysis or enzymatic degradation under acidic or alkaline conditions. Maintaining appropriate pH minimizes product degradation during manufacturing, preserving potency and safety.

4. Optimize process efficiency and ensure batch-to-batch consistency
From an industrial standpoint, pH control directly impacts productivity and economic viability. Extensive research is conducted to identify the ideal pH setpoints for different fermentation phases—such as cell growth versus product expression—which may differ significantly. Dynamic pH control allows for stage-specific optimization, maximizing biomass accumulation and product titers. Furthermore, regulatory agencies such as the FDA and EMA require strict adherence to Good Manufacturing Practices (GMP), where consistent process parameters are mandatory. pH is recognized as a Critical Process Parameter (CPP), and its continuous monitoring ensures reproducibility across batches, guaranteeing the safety, efficacy, and quality of pharmaceutical products.

5. Serve as an indicator of fermentation health
The trend of pH change provides valuable insights into the physiological state of the culture. Sudden or unexpected shifts in pH may signal contamination, sensor malfunction, nutrient depletion, or metabolic anomalies. Early detection based on pH trends enables timely operator intervention, facilitating troubleshooting and preventing costly batch failures.

How should pH sensors be selected for the fermentation process in biopharmaceuticals?

Selecting an appropriate pH sensor for biopharmaceutical fermentation is a critical engineering decision that affects process reliability, data integrity, product quality, and regulatory compliance. The selection should be approached systematically, considering not only sensor performance but also compatibility with the entire bioprocessing workflow.

1. High-temperature and pressure resistance
Biopharmaceutical processes commonly employ in-situ steam sterilization (SIP), typically at 121°C and 1–2 bar pressure for 20–60 minutes. Therefore, any pH sensor must withstand repeated exposure to such conditions without failure. Ideally, the sensor should be rated for at least 130°C and 3–4 bar to provide a safety margin. Robust sealing is essential to prevent moisture ingress, electrolyte leakage, or mechanical damage during thermal cycling.

2. Sensor type and reference system
This is a core technical consideration affecting long-term stability, maintenance needs, and fouling resistance.
Electrode configuration: Composite electrodes, integrating both measuring and reference elements in one body, are widely adopted due to ease of installation and handling.
Reference system:
• Liquid-filled reference (e.g., KCl solution): Offers fast response and high accuracy but requires periodic refilling. During SIP, electrolyte loss may occur, and porous junctions (e.g., ceramic frits) are prone to clogging by proteins or particulates, leading to drift and unreliable readings.
• Polymer gel or solid-state reference: Increasingly preferred in modern bioreactors. These systems eliminate the need for electrolyte replenishment, reduce maintenance, and feature wider liquid junctions (e.g., PTFE rings) that resist fouling. They offer superior stability and longer service life in complex, viscous fermentation media.

3. Measurement range and accuracy
The sensor should cover a broad operational range, typically pH 2–12, to accommodate varying process stages. Given the sensitivity of biological systems, measurement accuracy should be within ±0.01 to ±0.02 pH units, supported by high-resolution signal output.

4. Response time
Response time is commonly defined as t90—the time required to reach 90% of the final reading after a step change in pH. While gel-type electrodes may exhibit slightly slower response than liquid-filled ones, they generally meet the dynamic requirements of fermentation control loops, which operate on hourly timescales rather than seconds.

5. Biocompatibility
All materials in contact with the culture medium must be non-toxic, non-leaching, and inert to avoid adverse effects on cell viability or product quality. Specialized glass formulations designed for bioprocessing applications are recommended to ensure chemical resistance and biocompatibility.

6. Signal output and interface
• Analog output (mV/pH): Traditional method using analog transmission to the control system. Cost-effective but vulnerable to electromagnetic interference and signal attenuation over long distances.
• Digital output (e.g., MEMS-based or smart sensors): Incorporates onboard microelectronics to transmit digital signals (e.g., via RS485). Provides excellent noise immunity, supports long-distance communication, and enables storage of calibration history, serial numbers, and usage logs. Complies with regulatory standards such as FDA 21 CFR Part 11 regarding electronic records and signatures, making it increasingly favored in GMP environments.

7. Installation interface and protective housing
The sensor must be compatible with the designated port on the bioreactor (e.g., tri-clamp, sanitary fitting). Protective sleeves or guards are advisable to prevent mechanical damage during handling or operation and to facilitate easier replacement without compromising sterility.