Effect of organic nitrogen sources on raw starch-digesting glucoamylase production of Rhizopus sp MKU 40

In a liquid cultivation of Rhizopus sp. MKU 40, supplementation of the medium with 1.5% (w/v) organic nitrogen sources (neopeptone, casein from mill;, and meat extract) had a slightly positive effect on glucoamylase (GA) (EC 3.2.1.3) activity compared with the medium lacking organic nitrogen sources. The addition of organic nitrogen sources induced production of protease. Supplementation of the medium with 1.5% (w/v) organic nitrogen sources resulted in an acid and neutral protease activity of 11-25 U/mL and 12-20 U/mL. respectively. The co-existence of GA-I [a highly raw starch-digesting glucoamylase (RSDG)] and protease in the same medium leads to the production of Ga-II (an extremely weak RSDG) from GA-I. As a result the RSDG activity in the medium decreases. Raw starch adsorption rates of a medium without organic nitrogen sources were 100%, because the medium contained only GA-I. In contrast, the media supplemented with organic nitrogen sources had low starch adsorption rates because the media contained both GA-I and GA-II. The results presented in this paper indicate that supplementation of the culture medium of Rhizopus strains with organic nitrogen sources negatively affects GA-I production.