The measurement of both ‘a’ as well ‘K_L‘ within the fermentation process is quite difficult and for this reason, the two terms are normally combined to form the term K_La. This K_La is called the coefficient of volumetric mass transfer. The measurement of K_La helps in how much Oxygen is being transferred in the bioreactor. To successfully carry out aerobic cultivation a proper oxygen transfer is important. Efficiently calculating the K_La that is the coefficient of volumetric oxygen transfer along with the OTR helps an individual to optimize the overall bioprocess within the bioreactor. This in turn helps in ensuring a successful process of scale-up.

Factors influencing K_La as well as OTR

It can be clearly stated that OTR is essentially defined K_La as well as the difference of oxygen concentration within the medium and the concentration of oxygen within the introduced gas. The force that drives this is the concentration gradient. The OTR can be considerably elevated as different factors can influence the concentration of saturation within the media. Either the Oxygen content can be increased in the gassing, or the pressure can be increased. Theoretically, it can also be achieved by decreasing the temperature of the overall process. In addition, these OTR can be increased by improving the value of K_La. The K_La essentially consists of two variables. These have been described in the following.

• Here ‘a’ denotes the area of gas-liquid exchange for each unit of liquid volume.

• Here ‘K_L’ denotes the transport gas and liquid within the liquid phase.

Normally it is seen that the value of K_La rises due to the accelerated speed of stirring due to the input of energy of the stirrer shredding and disturbing the bubbles thereby making them smaller. In comparison to larger bubbles, smaller bubbles are seen to have two main advantages while transporting Oxygen. Firstly, a longer residence time within the medium can be seen which allows a greater amount of oxygen to be transferred within the liquid. Secondly, it facilitates a greater gas-liquid interface for each unit of the volume of the liquid. Moreover, KLa also increases when the rate of gassing is increased. This happens as more Oxygen is introduced within the medium.

Methods to estimate K_La

There are several methods such as Dynamic Integral Gassing-Out, Oxygen Balance, and chemical methods that help in estimating K_La within bioreactors These have been described in the following.

Dynamic Integral Gassing-Out: The method Differential Gassing-Out has been used widely, but it incorporates several weak points. To overcome these particular weak points, Dynamic Integral Gassing-Out was developed. The experimental process is the same as the previous method. However, there is a slight difference between the two.

Oxygen Balance Method: Many researchers argue that the method of Oxygen balance over the entire system is the most appropriate method for evaluating KLa within the fermenters. This is because no assumptions are necessary concerning viscosity, cell effects, and surface-active agents. Based on the concept of Oxygen balance, a linear mathematical correlation was developed between the concentration of DO and oxygen proportion in exit and inlet fermenter from which the determination of K_La can be done.

Chemical Method: This method is also known as the method of sulfide oxidation. It specifically involves the determination of the maximum oxidation rate of sodium sulfide concerning sodium sulfate with CuSO₄ or CoSO₄ as the catalyst. Here, the dissolved Oxygen is giving no back pressure to the overall medium. Further, the reaction does not depend on the concentration of sulfide within the range of 0.02 M to 0.8 M. For the determination of KLa usually a pH of 7.5 – 7.8 with a concentration of 0.5 to 0.8 M of sodium sulfite is taken.

Difference K_La OUR and OTR

Oxygen transfer Rate or OTR and Coefficient of Volumetric Mass transfer (K_La) show how Oxygen has efficiently transferred the bubbles of gas into the medium in which the bioreaction is occurring. This means the amount of oxygen that is present in the cultivated biomass. On the contrary, the specific rate at which the available Oxygen is absorbed by the biomass is described by utilizing the Oxygen Uptake Rate or (OUR).

Conclusion