Introduction:

In batch microbial fermentation, oxygen transfer is critical for the growth and metabolic activities of microorganisms. However, when dissolved oxygen concentration (DOC) remains near zero during fermentation, accurately estimating the oxygen transfer rate (OTR) becomes challenging. Several methods are available for determining OTR, each with its advantages depending on the fermentation conditions.

In this article, we’ll explore the most suitable method for estimating oxygen transfer rate in a batch fermentation process where the DOC remains almost zero, and explain the working principles behind each method.

Oxygen Transfer Rate in Microbial Fermentation:

Oxygen transfer rate is a key parameter in fermentation, particularly in aerobic fermentations where microorganisms require oxygen for growth and product formation. The oxygen transfer rate (OTR) indicates how effectively oxygen is transferred from the gas phase (air or oxygen supply) into the liquid phase (culture medium). If DOC is nearly zero during fermentation, it implies that oxygen consumption by microorganisms is very high or oxygen transfer is limited.

Methods for Estimating Oxygen Transfer Rate (OTR):

1. Static Gassing Out Method:

The static gassing out method is commonly used to determine the dissolved oxygen transfer rate under controlled conditions. In this method, the fermentation medium is sparged with oxygen or air for a fixed period, and the change in the dissolved oxygen concentration is observed over time. However, since this method assumes equilibrium conditions (which may not be suitable for fermentations where oxygen is consumed rapidly), it is not ideal for situations where DOC is maintained at almost zero.

2. Dynamic Gassing Out Method:

The dynamic gassing out method is more appropriate for situations where the oxygen concentration fluctuates over time, such as in batch fermentation. In this method, the fermentation vessel is sparged with oxygen or air, and the time required for the oxygen concentration to decrease is monitored. This method provides a real-time assessment of oxygen consumption and is particularly suitable for estimating oxygen transfer rates in systems with low DOC. It accounts for variations in oxygen transfer rates during the fermentation process.

3. Oxygen Balance Method:

The oxygen balance method involves applying the mass balance principle to the system. In this approach, the input of oxygen to the system (through sparging or aeration) is measured, and the consumption rate by the microorganisms is calculated by subtracting the dissolved oxygen concentration from the total oxygen supplied. This method is effective for estimating OTR when DOC is near zero, as it gives a comprehensive picture of the oxygen dynamics in the system.

4. Sulfide Oxidation Method:

The sulfide oxidation method measures the oxygen transfer rate indirectly by oxidizing a known amount of sulfide in the culture medium and measuring the oxygen consumed during the oxidation process. While this method can provide valuable information about oxygen consumption, it is generally more applicable to certain types of microbial processes, and its use in fermentation with low DOC may be limited.

Answer:

For estimating the oxygen transfer rate (OTR) during a batch microbial fermentation process where the dissolved oxygen concentration remains almost zero, the most appropriate method is:

3. Oxygen balance method.

This method accurately accounts for oxygen input and consumption, making it suitable for fermentation processes where DOC remains low.