Role of Fungal Enzymes in the Biochemistry of Egyptian Ras Cheese during Ripening Period
Six fungal genera including thirteen species isolated from Ras cheese samples were tested for assimilation of lactose, skim and fat milk hydrolysis. All fungal strains gave negative results in lactose assimilation except A. nidulans which produced acid without gas after 24 hr and produced a soluble red pigment after 10 days of incubation. All the tested fungal strains showed positive results for protein hydrolysis on modified Czapek agar (MCA) medium with skim milk addition. The presence of casein in the cultural medium caused a strong growth comparing with control. Changing of growth resulting from milk casein addition was in the highest values in the cultures of Rhizopus stolonifer, A. flavus and A. niger being 87.81, 58.32 and 41.58 mm, respectively. The lowest values were found with A. nidulans and Geotrichum candidum being 7.09 and 13.71 mm. All strains gave positive results for lipolysis except Aspergillus nidulans on plates containing modified Czapek agar (MCA) medium by milk fat addition. The fungal growth diameter was varied from 9.17 and 30.67 mm in case of A. niger and A. alliaceus, respectively. The presence of milk fat in the cultural medium caused a changing of growth in the case of A. glaucus; A. alliaceus and A. flavus being 28.11, 27.42 and 25.19 mm, respectively. A. niger, Rhizopus stolonifer and A. flavipes achieved lowest values being 5.62, 10.80 and 11.82 mm, respectively. Moisture percentage were gradually decreased during storage period (3 months at 15.5˚C) from 31.29% in the first month to 30.25% in the third month. Fat percentage was 24.05% in fresh Ras cheese wheels, which increased to 25.89% after salting period (45 day). During ripening period the values of fat in Ras cheese wheels were 30.53%, 35.98% and 32.38% after 1st , 2nd and 3rd months, respectively. The highest values of total nitrogen, soluble nitrogen and non-protein nitrogen percentages of Ras cheese wheels were 4.18%, 43.97% and 18.98%, respectively. These values tended to increase with advanced storage