The first step or stage of the downstream processing is called Solid-Liquid Separation. Solid-Liquid
Separation includes the breaking of the cells and elements that are insoluble from the fermentation broth.
If the task is to produce an intracellular metabolite, then the intracellular metabolite has to be released
from the cells before the Solid-Liquid Separation process.
Some biologist instead of using the term Solid-Liquid Separation that refers to the separation of insoluble
elements from the fermentation broth use the harvesting term of the microbial cells.
There are several methods that are used in Stage I of the downstream processing, and those methods are:
When we introduce a particular gas into the liquid broth, the gas produces bubbles in the liquid broth.
Solid particles and the cells in the liquid broth get adsorbed on the gas bubbles. The gas bubbles that form
in the liquid broth rise to the foam layer, and when they do so, they can be then removed and collected.
The foam, from which the gas bubbles are collected, that forms in the liquid broth, is facilitated by the
presence of certain substances known as collector substances.
The usual collector substances that facilitate the foam formation are long chain fatty acids and amines.
During the flocculation phase of Solid-Liquid Separation, the cells form large accumulations to settle
down for the easy removal.
Flocculation phase strictly depends on the cell nature and the ionic constitution of the medium.
The third subphase of the downstream processing process of Solid-Liquid Separation is Filtration.
Filtration is used for separating the biomass and filtrate.
Centrifugation is the fourth and the last process of the first stage of downstream processing called Solid-
Liquid Separation. Centrifugation technique is used for the separation of solid particles from the liquid