Re: how does salt effect the digestion of proteins by pepsin?

Answer: 
In general the solubility of proteins in [pure] water is limited, in order 
to improve their solubility, the preferred solution contains a [buffer] 
salt to have [a stable pH] an ionic strength of about 0.1-03. molar.
In doing so, the ionic charges of any protein is compensated and solubility 
problems do not occur.
In the case of pepsin it may even be more beneficial as at the pH values 
where this enzyme is functional [pH range 2-4] many protein substrates are 
poorly soluble [e.g. caseins], therefore this salting in effect - as it is 
called - is useful. Besides that, due to the specificity of pepsin, which 
cleaves peptides bonds next to hydrophobic amino acid residues like 
phenylalanine, tyrosine and others, the peptides formed have usually a 
rather hydrophobic character. To avoid their precipitation in a pure 
aqueous solution, salt is essential to keep these degradation products in 
solution too.

So [buffer] salts are used to maintain the optimal environment for the 
enzyme to function and to keep the enzyme, the substrate and the 
degradation products in solution.