Studies on the Browning of Rice-Koji

Part 3. An Ethyl Ether soluble precursor for browning substance

Abstract

The precursor for browning substances in rice-koji was studied by comparing two kinds of koji made by culturing two strains of A. oryzae in steamed rice: one is strain RIB 128 which is always koji brown, and other is strain RIB 647 which is never brown.
When the koji cultured with R I B 128 was steeped in water at 40°C and filtered, both the filtrate and the residue gradually became brown and finally black in the presence of air, which shows that the precursor for browing substances of rice koji is soluble in water. Studies were then carried out by using the filtrate of koji and the results are as follows.
1. The eluate by acetic acid from alumina column was subjected to Dowex 50-X8 (H) chromatography, followed by successive elution with water and IN HCl. The eluate by water from this column was extracted with ethyl ether. Ethyl ether fraction was condensed by rotary evaporator and purified by polyamide column chromatography.
Comparing the R_f of paper chromatography and the maximum UV-absorption of the ether soluble substance with those of authentic protocatechuic acid, ether soluble substance was proved to be protocatechuic acid.
2. Protocatechuic acid was not found in the rice koji made by culturing RIB 647 and was regarded as one substance of the precursor for browning substances.