Cheese from ultrafiltered milk : whey proteins and chymosin activity

The manufacture of (semi-)hard cheese from ultrafiltered milk (UF-cheese) enables the partial incorporation of whey proteins in the cheese, thereby increasing its yield. The transfer of whey proteins in curd from (UF-)milk was studied in relation to the degree of ultrafiltration of the milk and the degree of syneresis of the curd. In UF-cheese manufacture (from 5x concentrated UF-retentates, concentrated further by syneresis) approximately one-third of the whey protein fraction was enclosed.

Despite this yield increase, UF-cheese production trials in the last decennia have not been successful: the yield increase is limited and counteracted by retarded ripening. It was concluded that the latter is partly due to reduced chymosin activity in UF-cheese, as compared to traditional cheese. The lower chymosin activity was due to a reduced chymosin dosage to UF-retentates (because clotting occurs much faster at higher casein contents), and to enzyme inhibition.

The activity of chymosin in the first stage of cheese ripening was studied in cheese models, as well as in (semi-)hard Gouda type (UF-)cheese, with varying chymosin and whey protein contents. The chymosin activity was derived indirectly from the rate of release of degradation products from casein, or directly by application of a newly developed method that enables the direct estimation of the chymosin activity in gels, curd and cheese. Results of both methods corresponded very well.

Chymosin appeared to be inhibited in its activity by a component of the whey protein fraction. Because of this inhibition, the chymosin to casein ratio should be higher in whey protein-containing UF-cheese than in traditionally manufactured cheese. A higher chymosin activity in (UF-)cheese can be achieved by increasing the dosage of chymosin to the milk or retentate, as the enclosure in cheese is proportional to this dosage. The transfer of chymosin into cheese is mainly due to adsorption of chymosin onto casein, which is enhanced at lower pH. The enclosure of chymosin in cheese can thus also be increased by increasing the rate of acidification during cheese manufacture. Increasing the chymosin dosage to UF-milk and/or the rate of acidification would require drastic modifications of the manufacturing process.