Downstream processing

Downstream processing gets the product out and makes it pure. The product may be the cells themselves, something they release into the broth (extracellular), or something kept inside them (intracellular). Recovery must be quick and cheap.

1. Separate solids from liquid. Sometimes the broth is first heated, its pH changed, or flocculating agents added to clump the cells. Then:

• Filtration, the cheapest method, through canvas, fabric or glass fibre; in industry, continuous rotary filters; for very fine separation, ultrafiltration.
• Centrifugation: spinning the cells down.

2. Break the cells, if the product is intracellular, without damaging the product:

• Physically: bead mills, high-pressure homogenisers, ultrasound (sonication).
• Non-mechanically: detergents (surfactants), alkalis, solvents, osmotic shock, or enzymes that digest the cell wall.

3. Recover the product from the dilute solution:

• Liquid-liquid extraction: the product moves into a solvent (the extract); the leftover liquid is the raffinate.
• Adsorption onto a solid.

4. Purify it: by precipitation (with salts or solvents), chromatography, electrophoresis, or membranes.

Membranes separate by size:

• Microfiltration: pores of 0.1-10 µm (removes cells).
• Ultrafiltration: 0.01-0.1 µm (separates proteins).
• Reverse osmosis: below 0.001 µm. Pressure greater than the osmotic pressure pushes water out of the salty side.
• Dialysis: small molecules such as urea (molecular weight 60) diffuse out through tiny pores, leaving large ones behind.