What is Downstream Processing of Microbial Cell Culture?

When fermentation is over, the desired microbial product is recovered from the growth medium. Then the product is purified, processed, and packed with equal efficiency and economy. Product recovery and purification are called downstream processing.

The technology associated with downstream processing is as important as technology-associated with the fermentation process itself. The operation of any fermentation production process integrates both the technologies.

The volatile products can be separated by distillation of the harvested culture without pre-treatment. Distillation is done at reduced pressure at continuous stills. At reduced pressure distillation of product directly from fermentor may be possible (as done for isolation of ethanol). The steps of downstream processing are as below:

(a) Separation of Biomass:

Usually the biomass (microbial cells) is separated from the culture medium (spent medium). If the product is biomass (single cell protein), then it is recovered for processing and spent medium is discarded.

Generally, cell mass is separated from the fermented broth by centrifugation or ultra-centrifugation. When there is no aeration and agitation, some of the microbial cells soon settle down in the fermentor.

Upon addition of flocculating agents, settling may be faster. For centrifugation process, settling of microbial cells is necessary. Otherwise, biomass separation may be affected. Ultrafiltration, continuous centrifugation or continuous filtration (e.g. rotary vacuum filtration) is an alternative to the centrifugation.

When a solution is passed through a membrane of 0.5 mm pore size, the particles having size more than the solvent are retained onto it. Using ultrafiltration you can separate microbial cells from fermented broth.

(b) Cell Disruption:

If the desired product is intracellular (e.g. vitamins, some enzymes and recombinant proteins like human insulin) the cell biomass can be disrupted so that the product should be released. The solid-liquid is separated by centrifugation or filtration and cell debris are discarded.

(c) Concentration of Broth:

The spent medium is concentrated if the product is extracellular.

(d) Initial Purification of Metabolites:

There are several methods for recovery of product from the clarified fermented broth e.g. precipitation, solvent extraction, ultra-filtration, ion-exchange chromatography, adsorption and solvent extraction.

The extraction procedure varies according to physicochemical nature of the molecules of products, and preference of the manufacturers. It shows isolation of intracellular microbial product such as human insulin.

(e) Metabolite-specific Purification:

Specific purification methods are used when the desired metabolite is purified to a very high degree.

(i) De-watering:

When a low amount of product is found in very large volume of spent medium, the volume is reduced by removing water to concentrate the product. It is done by vacuum drying or reverse osmosis. This process is called de-watering.

(ii) Polishing of Metabolites:

It is the final step of making the product to 98-100% pure. The purified product is mixed with several cheaper inert ingredients called excipients. The formulated product is packed and sent to the market for the consumers.