Downstream processing - Cell harvesting, Whole-broth Treatment

1. Cell harvesting

The first step in the downstream processing of suspended cultures is a solid–liquid separation to remove the cells from the spent medium. Each fraction can then undergo further processing, depending on whether the product is intracellularly located, or has been secreted into the periplasmic space or the medium. Choice of solid–liquid separation method is influenced by the size and morphology of the microorganism (single cells, aggregates or mycelia), and the specific gravity, viscosity and rheology of the spent fermentation medium. These factors can influence the transfer of the liquid through pumps and pipes.

 

1.1  Broth conditioning

Broth conditioning techniques are mostly used in association with sedimentation and centrifugation for the separation of cells from liquid media. They alter or exploit some property of a microorganism, or other suspended material, such that it flocculates and usually precipitates. However, in certain cases it may be used to promote floatation. This uses the ability of some cells to adsorb to the gas–liquid interfaces of gas bubbles and float to the surface for collection, which occurs naturally in traditional ale and baker’s yeast fermentations.

 Certain floc precipitation methods are also used at the end of many traditional beer and wine fermentation processes, where the addition of finings (egg albumen, isinglass, etc.) may be employed to precipitate yeast cells.

Major advantages of these techniques are their low cost and ability to separate microbial cells from large volumes of medium.

Some organisms naturally flocculate, which can be enhanced by chemical, physical and biological treatments. Such treatments can also be effective with cells that would not otherwise form flocs.

Coagulation, the formation of small flocs from dispersed colloids, cells or other suspended material, can be promoted using coagulating agents (simple electrolytes, acids, bases, salts, multivalent ions and polyelectrolytes).

Subsequent accumulation of these smaller flocs into larger settleable particles, called flocculation, is done by adding inorganic salts (e.g. calcium chloride) or polyelectrolytes. These are high molecular weight, water soluble, anionic, cationic or non-ionic organic compounds, such as polyacrylamide and polystyrene sulphate.

 

Whole-broth Treatment

In some fermentations such as the acetone-butanol fermentation, the whole unseparated broth is stripped of its content of the required product. In the antibiotic fermentations, resins are used to directly absorb the antibiotics streptomycin (using cationic-exchange resin) and novobiocin (anionic resin.) The antibiotics are eluted from the resins and then crystallized. This process saves the capital and recurrent expense of the initial separation of solids from the broth.