Studies on Milk Clotting Enzymes Produced by Basidiomycetes

Abstract

In the course of screening tests of Basidiomycete proteolytic enzymes, it was observed that some strains produced milk clotting enzymes with fairly weak proteolytic activities.

When sucrose-polypeptone and sucrose-corn steep liquor media were used, only 6 strains out of 44 strains tested showed weak milk clotting activities. Cheddar cheese making with culture filtrates of these 6 strains revealed that the culture filtrates of 2 strains, Irpex lacteus Fr. and Fomitopsis pinicola (Fr.) Karst., were able to produce Cheddar cheese of good quality.

On the other hand, when sucrose-distillers solubles media were used, a lot of strains showed high proteolytic activity in addition to high milk clotting activity. The ratio of milk clotting to proteolytic activities (MCA/PA) was assumed to be an important index for the selection of organism, and F. pinicola and Coriolus consors (Berk.) Imaz. were selected as the strain with high MCA/PA ratio.

As the investigation on culture conditions of 3 strains mentioned above showed that F. pinicola and I. lacteus, were richly productive of milk clotting enzymes, the 2 strains except C. consors were used for further studies on cheese making.

Cheddar cheese making with crude enzymes revealed that cheese products produced by the enzyme of F. pinicola had a slightly bitter taste after 5 months’ ripening but that those produced by the enzyme of I. lacteus had good quality.

Abstract

In the previous paper it was reported that three strains of Basidiomycetes, Coriolus consors (Berk.) Imaz., Fomitopsis pinicola (Fr.) Karst. and Irpex lacteus Fr., produced milk clotting enzymes into culture media.

These three strains were cultivated in a liquid medium and crude enzymes were obtained from the culture filtrates. Some properties of the crude enzymes thus obtained were investigated.

These enzymes are a sort of acid proteases with optimum pH of 2.5 for casein digestion. Different from a calf rennet, the proteolytic activity of the enzymes increased along the prolongation of reaction time.

The clotting activity of the enzyme of F. pinicola was much more influenced by pH of milk and Ca ion concentrations in milk than that of the calf rennet. But the enzyme of I. lacteus is very like the calf rennet in these properties.

Optimum temperatures of the enzymes lie at 55~60°C.

When the ratio of milk clotting activity to proteolytic activity (MCA/PA) is measured, the ratio of the calf rennet is remarkably higher than those of microbial rennets tested. Among microbial rennets, Mucor rennet and the enzyme of I. lacteus have rather higher values than others.