MIXING AND SHEARING- THE GOOD, THE BAD AND THE UGLY

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(PICTURE TAKEN FROM DRAGLIST.COM)

Everyone dealing with fermentors knows that mixing is one of the most important parameters that affect the fermentation process. Put it this way, in order to support the high growth of microorganisms and the production of high concentration of fermentation products, mixing is necessary.

Mixing is essential in order to provide homogenous condition throughout the fermentor required the optimum growth of the microorganisms. Homogenous conditions within the fermentor will allow efficient mass transfers of nutrients, heat and oxygen throughout the fermentor. It will also eliminate formation of gradients and built up of toxic products

However, even though mixing is crucial to the fermentation process, it needs to be properly executed as there are the good, bad and ugly sides of mixing which have impact on the fermentation process.

One of the sensitive sides of mixing is the effect of shearing. In fermentor as the impeller rotates it creates a flow. Shearing forces will be generated where there are velocity gradients between two points in the liquid flow. Differences in velocity gradients in shearing could occur between the same or different phases in the broth such as between liquid and liquid, liquid and gases and liquid and solids.

Under non turbulent or laminar flow, shearing still occur but would not be significant in fermentors as high rate mixing is often the order of the day.

In a simple model of visualization we could see shearing forces as forces that physically stretch, tear or wear out surfaces. Thus, in this context if uncontrolled shearing forces are damaging to the microorganisms and the products formed. The intensity of shearing are often related to the amount of physical force or mixing power exerted into the system. Higher turbulence would relatively lead to increased or more powerful turbulence.

There are many areas or zones in the fermentor which are strongly affected by the shearing forces generated by mixings. Most intense shearing is known to occur in the mixing zones of the impellers, and especially at the tip of rotating impellers. Shearing forces could also occur in the region of the baffles or where strong air sparging occurs.


THE GOOD- THINNING OUT BOUNDARY LAYER

Shearing may result in the reduction of the boundary layer. This will increase the rate of mass transfers across the boundary layer between the microbial cell and the environment

THE BAD -EFFECT OF SHEARING ON MICROORGANISMS IN FERMENTORS

Shearing can result in damages to the microorganisms and directly will affect the efficiency of the fermentation process. The effect of shearing on the microorganisms much depends on the morphology and growth forms of the microorganisms. Filamentous fungi will be easily tangled and damaged during mixing and shearing.

In the case of unicellular microbial cells the effect of shearing is not so much on the damage to individual cells but to the microbiological aggregates. Rarely do microorganisms occur as independent single cells in fermentors. Most, if not all occur as microbial aggregates such as microbial flocs, microbial mats.

Under these conditions the formation of the microbial aggregates is often the physiological response of the microorganisms to conditions of stress. These microorganisms will produce high amount of EPS around them in a protective response against damage.

Flocs are however not strong structures and studies in activated sludge system showed that larger flocs are easily broken down to smaller flocs under turbulence. Most of the damage by shearing occurs at the impeller tips.





UGLY SIDE-CONSEQUENCES ON DOWNSTREAM PROCESSING

Damages by shearing of the microbial cells will result in the release of metabolites which will complicate the fermentation process by increase in foaming and difficulty of recovery of fermentation products








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