Streptomyces sp. spoiling rough rice with a musty odor after drying and storage

Abstract

The spoilage of rough rice with a musty odor occurred after harvest, drying and storage in an area of Niigata prefecture in 2020. Both surface sterilized and unsterilized husked rice and the rice hull were incubated on four kinds of agar media in darkness at 25℃ for 10-15 days to discover the microbes causing the odor. Although many fungi and several bacteria were isolated, they did not have a musty odor. In contrast, an actinomycete that ranged in color from pale yellow to white and with a powdery texture and a strong musty odor grew on unsterilized rice grains and hull pieces on water agar. The spoilage was reproduced by an inoculation with a representative isolate of the actinomycete on milled rice grains. The isolate grew up to 10 mm in diameter after being cultured on nutrient agar in darkness at 25℃ for 50 days. The surface side of the colony was conical and pale-yellow in color with radial wrinkles and a white frilled margin. The isolate produced abundant spiral spore chains from aerial hyphae. The cause of the musty odor of the rice was identified as a species of Streptomyces based on the properties of colonies, microscopic morphology, and the DNA sequence of the 16S rRNA gene. The spoilage in 2020 was assumed to be caused by rice plants often falling over before the harvest, and consequently the musty Streptomyces sp. appeared to contaminate the ears and propagate on rough rice.