Foaming During Fermentation

by Greg Benz, P.E.

In aerobic fermentation, a frequent issue is the tendency for an aerated liquid to generate foam. In extreme cases, the foam can be so severe as to render the fermenter inoperable when the foam builds up and starts coming out of the exhaust vent.  One way to prevent such extreme foam is to install a foam breaker above the aerated liquid level.  However, mechanical foam breakers only break foam within their swept circle, leaving foaming in the annulus. Moreover, if operator error causes the aerated liquid level to submerge the foam breaker, a massive overload and possible equipment breakage will occur. (The foam breaker is designed to operate in foam only. Foam is less than 20 kg per cubic meter, whereas aerated broth usually has a density of about 800 kg/cubic meter. Power draw is proportional to density, so operating in aerated liquid will cause the foam breaker to draw at least 40 times its design power).

A second possibility is to add chemical antifoam. With the correct antifoam, the foam produced can be reduced drastically. However, when antifoam is used, it increases the production cost and, in many situations, becomes the second largest material cost in fermentation after the media cost. Moreover, antifoam reduces the mass transfer coefficient, which may reduce productivity or require more power to achieve the same result.

There is a third way: to reduce the foam generation in the first place. This can be done by placing the proper impeller, size, and power in the optimum position below the liquid surface. This technique has been successfully applied by agitation experts in bacterial and yeast fermentation, and clients normally report about a 50% reduction in antifoam usage.

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