Fermentation technology, though mature, remains challenged by the accumulation of potentially inhibitory products and by-products. A semi-partition bioreactor resolves this limitation, allowing extraction with greater ease than the in-situ extractive fermentation methods currently used.

Rather than linking the extraction unit externally, a semi-partition bioreactor combines a fermentation and an extraction unit in one vessel, using a mixer-settler principle, according to a recent paper.

As Robert W.M. Pott, PhD, at Stellenbosch University in South Africa and one of the paper’s authors, tells GEN, “Part of the reactor is well-mixed, while a protected zone has little mixing. This allows for settling of separate phases,” and a second, extracting phase in the fermentation process. “The semi-partition reactor then allows you to continuously remove the extracting phase (and the product it contains) from the system. An external separation allows you to recover your product and then recycle the extractant phase back into the bioreactor.”

Proof of concept can also apply to other systems

When lactic acid concentrations increase in the bioreactors, pH drops and yields decline. In experiments, Pott and colleagues increased lactic acid production from 0.6 grams of product formed per gram of substrate consumed (g.g-1) to 0.75 g.g-1. “The increased productivity is good,” he says. What may be most important is that this proof of concept also can apply other systems for which bioreactor control is more challenging.

While this approach reduces stress on the micro-organism caused by recirculating fermentation media or time beyond the bioreactor control parameters, “The key benefit to a semi-partition bioreactor is that in situ extraction is much easier,” Pott says. The reactor configuration is simple, so stirred tank reactors can be converted easily to include a settling zone.

A semi-partition bioreactor isn’t ideal for all fermentations. It is, however, suitable when product inhibition is an issue. “Acetone-butanol-ethanol fermentation or organic acid fermentation are examples,” Pott says, as are “products where the organism’s productivity is limited by the fermentation product concentration.”

Even in ideal circumstances, however, there are certain tradeoffs. Because part of the reactor is taken up by the extractant, volumetric productivity is reduced. Additionally, the need to remove solvent, recover product and recycle the solvent adds operational complexity and increases the risk of contamination.

For some products, however, “these limitations could be significantly superseded by the benefits of in situ production and extraction.” It could operate continuously, rather than in batch mode, he points out.

Going forward, Pott and colleagues plan to test other candidate bioproducts, and are encouraging other researchers to test semi-partition bioreactors for their own projects.

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