APPRECIATING GAS BUBBLES IN FERMENTATION

(PICTURE TAKEN FROM WWW.MASTERMEDIA.ORG)
One of the most striking features in any fermentation process is the observation of gas bubbles in the fermentation process. Aerobic fermenters exhibit gas bubbles produced from the sparger at the bottom of the fermenter. Anaerobic fermenters such as anaerobic digesters produce bubbles which are predominantly methane gas.

It is very mesmerizing to see the behavior and the dance of the bubbles in any fermentor (even fish aquariums produce such beautiful train of bubbles!). But bubbles are more than just a beautiful graphic or artistic representations in fermentors. Bubbles do have very important roles to play in any fermentation process.

Bubbles are basically 'carriers' of gases. Gases richer in concentration such as oxygen in the bubbles will diffuse out into the broth medium powered by the differences in the concentration gradients. Gases or volatiles which occur in higher concentrations in the broth medium will diffuse inwards from the environment into the bubbles and released into the headspace

To a certain extent especially for large gas bubbles, their movement or generation do contribute to the mixing and even circulations within the fermentation broth

There are various parameters which affect the size, shape, buoyancy velocity or how fast it rise and stability of the bubbles.

For our discussion on bubbles in fermentors we will be concentrating on air bubbles generated in aerobic fermentation system.

The bubbles in the aerobic fermentation system are result of forced air which escaped through the nozzles at the bottom of the fermentor. Initially the bubbles generated are small at the moment it leaves the nozzles. It immediately increase in size as it rises upwards through the fermentation broth. As the bubbles rise it increase in size rapidly and rate of ascension.

We can divide the bubbles formed during the fermentation process into two types:
1) Small tiny and microscopic bubbles
2) Large bubbles

Both types of bubbles show different behaviour in the fermentors

TINY BUBBLES
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Tiny bubbles are very small and rises very slowly upwards. The slow rise in tiny air bubbles to the surface is attributed to the increase drag forces experienced by the bubble with the surrounding fluid

Due to their very small size tiny bubbles have very high surface area to volume ratio, thus optimizing mass transfer reactions to occur between the gas bubbles and the surrounding environment.These tiny bubbles are easily recirculated again and again the fermentation broth especially carried by fluid circulations caused by the impeller movements. Tiny bubbles do have the tendency to coalesce among themselves and reformed into large bubbles.

LARGE BUBBLES
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Big bubbles are generally generated in nozzles with large orifice. These large bubbles have lower surface area to volume ratio thus making them not so efficient in mass transfers between the gas bubbles and the environment.

Large gas bubbles have a rapid lift and rises to the surface very fast. There is lower drag forces acting upon large bubbles. In terms of the bubble pathway, the large bubbles do not show circulation by the impeller mixing

ARE BIGGER BUBBLES BETTER?
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One most common illusion confronting a person doing fermentation is that the sight of large air bubbles in the fermentation broth means that the fermentor is supplied with enough oxygen. In reality this is not so. Whether there are enough oxygen or air is not indicated by the sight of large air bubbles bubbling aggressively in the fermentor. To indicate the real status of the dissolved oxygen values in the fermentor, one has to measure it using a dissolved oxygen meter.

Large air bubbles need not mean there will be sufficient dissolved oxygen in the fermentor. Due to the large size of the air bubbles and the rapid rise of the air bubbles to the surface and short distance the bubble takes to rise will mean there will not be enough oxygen mass transfer occurring between the bubbles and the surrounding medium








BUBBLES EXPLOSION AT HEAD SPACE
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All bubbles on reaching the surface or the liquid/air interphase in the headspace of the fermentors will collapse or explode due to sudden release in the pressure of the bubbles