Chymosin cleaves beta-casein mainly towards its hydrophobic C-terminus. After the Phe-Met bond of kappa-casein, the Leu192-Tyr193 bond of beta-casein is the bond in the casein system most susceptible to chymosin action. Cleavage of this bond produces a large polypeptide, beta-CN (f1-192) and a short peptide (f193-209) which is hydrophobic and extremely bitter.
Hence (and strangely), the first question to ask if a cheese is bitter, is what is its salt content!
The rate of cleavage of the Leu192-Tyr193 bond is very dependent on the ionic strength of the cheese aqueous phase. Increasing ionic strength by increasing NaCl concentration results in increased hydrophobic interaction between the hydrophobic C-terminal regions of beta-casein thus tending to block the action of chymosin and reducing the production of beta-CN (f193-209) and related peptides and thus bitterness.
Hence (and strangely), the first question to ask if a cheese is bitter, is what is its salt content!
Note: "beta" and "kappa" above are usually written as Greek letters but I cannot seem to use them on this site. The photo above shows the amino acid sequence of the C-terminal region of bovine beta-casein with arrows indicating some cleavage sites of chymosin on this protein.