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In today’s competitive bioprocessing landscape, effective and seamless scaling up fermentation processes from bench-top to industrial production has become a pressuring need. However, scaling up is not a straightforward process and it requires overcoming certain challenges. Maintaining consistent product quality and achieving high yields, requires a deep understanding of the impact of the different variables that can make or break efficiency. One of the most critical factors is the oxygen transfer into the liquid medium.

The importance of scale-up in bioprocessing

Moving from bench-top to industrial scale is fundamental to ensure the viability of bioprocessing industries. Traditionally, scale-up approaches have involved the use of geometric properties, for example, the height to diameter ratio (H/d) or the stirrer ratio (d/D). Constant ratio of agitation power input per unit volume of fluid (P/V) is frequently used and it is considered a good indicator of culture homogenization, mixing, and sheer stress. This approach includes two; both agitation (mechanical power), and sparged gas (pneumatic power).

However, sufficient oxygen mass transfer, being a critical factor in cell culture, results in the use of k_La, with the aim of maintaining similar oxygen transfer across scales. In order to ensure adequate oxygen levels, the oxygen transfer rate (OTR) needs to be higher than the oxygen uptake rate (OUR). The calculation of k_La allows for a more efficient scale-up process, as long as this value can be kept constant, resulting in a more comparable growth and production rate.

Oxygen transfer rate and k_La

Dissolved oxygen is often the limiting substrate in fermentation. For optimal growth, it is fundamental to maintain the oxygen levels over the critical level, which will depend on the organisms used in the fermentation. To achieve this concentration, means of introducing oxygen are required, for example, through sparging with air or pure oxygen.

The rate at which oxygen is transferred into the medium follows the equation:

OTR = k_La·∆C, in which:

The ∆C is the difference between the saturation oxygen concentration in media minus the actual concentration, and k_La is the volumetric mass transfer of oxygen. However, k_La is not a single factor, but instead is the combination of k_L – the rate at which oxygen is transfer across the liquid/gas interface – and a – the interfacial area or the surface area in which oxygen can be transferred from the gas to the liquid phase. These two variables are incredibly challenging to be measured separately, but much easier to calculate when combined.

Determining the k_La helps determine the conditions in the process to ensure that the cells have an adequate supply of oxygen. The k_La value can then be utilized to optimize control variables.

Additionally, an accurate k_La measurement means better ability to tune the O₂ concentration to increase the process’s yield and ensure that growth is not hindered by inadequate oxygen levels. Proper characterization also avoids the waste of resources associated with unnecessary high oxygen concentrations.

Factors influencing k_La

Both the rate at which oxygen dissolves and the interfacial area affect the k_La values, but these are also affected by a number of physical, chemical, and biological parameters. For example, the size of gas bubbles in the medium affects the interfacial area, which can be modified by sparging and agitation. The time the gas is retained in the liquid can also affect the exposure of oxygen to the medium and, as a consequence, also increase or decrease its transfer. Additionally, k_La scales up linearly with the amount of oxygen introduced into the medium. Increasing agitation also favors the solubility of oxygen into the medium—however, increased agitation results in higher shearing forces, which can be damaging to the cells.

While sparging and agitation are commonly used to optimize k_La, pressure can be another way to improve oxygen transfer. Higher pressure systems enhance gas solubility, following Henry’s law, allowing more oxygen to dissolve into the liquid phase without the need to increase agitation. As a consequence, this reduces the shearing force that can be damaging for the cells.

Other variables that affect k_La include cells, media type, pH, salt concentration, antifoaming agents… Vessels also have a huge impact on k_La values, including both the size, and its configuration. This makes the prediction of k_La almost impossible, and requires to be performed for bioreactors individually.

Maximizing Fermentation Efficiency: The Key Role of k_La in Scale-Up

Understanding and optimizing oxygen transfer rates and k_La is fundamental to achieve a successful fermentation scale-up. Unlike traditional methods, like the ones that rely on geometry or power input ratios, k_La-based approaches aim to keep consistence oxygen transfer across different reactors. Achieving optimal k_La measurements enables fine-tuning of the process conditions resulting in higher yields, as well as optimizing the use of resources such as gas and energy. By accurately determining k_La, operators can establish the right balance between sparging, agitation, and pressure, leading to improved process control and reproducibility from lab-scale experiments to full-scale industrial production.

Scale-up processes need to account for oxygen transfer in order to maintain their efficiency and ensure scalability. Higher k_La values translate to better oxygen availability, which in turn results in enhanced cell growth and higher product yields. As we can see, k_La is a crucial parameter in cell culture systems. Whether through enhanced aeration strategies, advanced reactor design, or high-pressure operation, improving k_La improves the chances of success for the fermentation process and the overall sustainability and cost-effectiveness of biomanufacturing at scale.