Yusaku Fujio

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 milk, 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.

Degradation Profiles of Potato Starch Melts Through a Capillary Tube Viscometer

The effects of time-temperature and strain history on potato starch melts at 150 °C were investigated by use of a capillary tube viscometer. Reciprocating and single extrusions were performed in this study. Shear stress at single extrusion decreased gently as initial heating time increased, while shear stress at reciprocating extrusion decreased rapidly as the number of extrusion strokes increased. A high degree of cold water solubility was obtained by reciprocal extrusion at lower moisture content. From the results of gel filtration it could be concluded that starch molecules were depolymerized by high and reciprocal shear stress. According to these results time-temperature history was more effective on the depolymerization of starch molecules and degradation of starch granules at higher moisture content, while strain history was more effective at lower moisture content.