MACRO AND MICROMIXINGS IN FERMENTATION

Mixing or the stirring of fermentation broth in a CSTR is an important operating parameter in the fermentation process. The process of mixing is primarily carried out by the action of the stirrers and impellers, however secondary mixings of the fermentation broth are achieved by other modes such as sparging of the broth.

There are many factors which affect the mixing properties of a particular fermentation process such as broth rheology, power, geometry of vessel and design and configuration of impellers.

Why do we need to mix the fermentation broth? The main answer is to achieve homogenization of the broth so that the broth will show uniform composition, physically, chemically and even microbiologically throughout the active volume of the fermentor. This will avoid the formation of physical and chemical gradients which may lead to toxic pockets being formed in the fermentor

Mixing is accepted as the method to increase the efficiency of mass transfer between the microorganisms and the environment. This is achieved by reducing the boundary layer surrounding the cell and enhancing diffusion process through a thinner boundary layer

Mixings are proven to aid solubility and dispersion of gases as well as nutrients in the fermentor. This is especially important where the gas or solids show limited solubility

But what do we really see in the mixing process? In reality in the process of mixings carried out in a fermentor we can see two types of phenomena:
1 Macromixings
2 Micromixings

Macromixings are more involved at the larger scale seen at the level of the impeller and directly involved in the homogenization of the broth and solubility and dispersion of nutrients. Micromixings occur more at the molecular and nano level at the intimate proximity of the microorganisms. This is important especially at the level of the boundary layer. Although the two phenomena occur simultaneously during the mixing process but the efficiency and the operating regimes differ. The choice of the right mixing systems and operating conditions can help shift the the effect of micro or macromixing

One of the most common mistakes in operating the fermentor is fixing the mixing speed to a fixed value such as 200 rpm or whatever values without properly understanding the demand and impact of such mixings. Each type of fermentation requirements for mixing should be determined experimentally as no two fermentations are the same. The amount of mixing should be a balance between the requirements of:
1 Providing oxygen
2 Need to keep cultures in suspension
3 Reducing the damages to the microorganisms by the shear forces generated by mixing

In simple statement, mixings should be 'tailor made' for the specific type of fermentation