Microbial Population on Freshly-Harvested Rice and Factors Affecting Prevalence

Abstract. During on-farm drying and storage of rice, weather conditions may not allow complete drying of rice in a timely manner leading to growth of molds, yeast, and bacteria in the grain mass. Under some stressful conditions of temperature, relative humidity and grain moisture content (MC), certain molds occurring on the grain multiply and may produce mycotoxins. Some mycotoxins, such as aflatoxin, are very toxic and highly carcinogenic, and pose health hazards to consumers. Therefore, it is important to determine the initial levels of microorganisms and the factors that influence these populations on freshly-harvested rice so that effective drying and storage strategies can be implemented to avoid mycotoxin contamination. In this study, a survey was conducted to determine the effect of various factors including rice cultivar, growing geographic locations, and harvest time on the prevalence of microorganisms on rough rice. Specifically, the total (ground sample) and surface aerobic plate count (APC) and mold count on freshly-harvested, long-grain hybrid (XL723 and XL753), long-grain pureline (CL152 and RoyJ) and medium-grain (Caffey and Jupiter) rice cultivars grown at four Arkansas locations (Stuttgart, Rohwer, Colt and Keiser) were studied. The average water activity and MC of all the rough rice samples at harvest were 0.914±0.008 and 22.26±3.02 (%, w.b.), respectively. The average total APCs on rice (total APCs) and APCs on the rice surface (surface APCs) were 7.21 and 6.29 log colony forming units per gram (log cfu/g), respectively. The average total mold counts on rice and the surface mold counts on rice surface were 5.53 and 5.05 log cfu/g, respectively. The APCs and mold counts significantly depended on the rice cultivars and harvest locations. Long-grain hybrid cultivars had the least mean total APC (7.1 log cfu/g) followed by medium-grain (7.2 log cfu/g) and long-grain pureline (7.2 log cfu/g) rice cultivars. The long-grain pureline cultivar CL152 had the greatest average APCs (7.31 log (cfu)/g). Long-grain hybrid rice cultivars had the least mean total mold count (5.2 log cfu/g) followed by long-grain pureline (5.4 log cfu/g) and medium-grain (5.6 log cfu/g) rice cultivars. The medium-grain cultivar (Caffey) had the greatest mean total mold count (5.6 log cfu/g). The geographic location where rice was grown had a significant effect on the level of microbial contamination on the rice kernels at harvest. As much as 1.5- and 3.2-fold total APC and mold count were observed in rice harvested at Rohwer compared to Keiser. Samples of rice harvested at Keiser tended to have the least microbial loads followed by those from Colt; samples from Stuttgart and Rohwer, the southern-most locations, had the greatest microbial loads. These findings could benefit the rice industry by helping to develop effective strategies for drying, storage and/or decontamination of rice, thereby helping to prevent mycotoxin development.