In the fractionation of a given crude extract, the pH of the medium is very important in effecting isolation of the desired component from other components. Low pH (acidic) of an aqueous medium is needed to solubilize a component that can act as a base to form the component's respective salt  that is more soluble in aqueous medium compared to its original state. N-based components like amino acids, proteins and alkaloids can act as base which in the presence of acid in the medium will form their respective salts. These salts are rendered more soluble in the aqueous medium than the original component amino acids, proteins, and alkaloids. Once in the medium, the other undesired components that are not soluble can be easily taken out as residue after centrifugation / filtration.

I agree with Dr. Evangelina S Ella - it depends on the nature of substances.

At the same time, I would say there is no logic in acidification of the crude extract during its fractionation when the target components are not formed in salts and moreover vice versa could be destroyed by acidification.

Regards)

If you are interested to isolate compounds having basic nature like Alkaloid (SALT COMPOUND) in that case from different pH variation you can isolate different type of compound in a salt form. So may try it if you are interested to isolate Alkaloids group of compounds.

Regards. 

Thanks to all..