A combined intervention using fermented ethanol and supercritical carbon dioxide to control Bacillus cereus and Bacillus subtilis in rice

Although the contamination of rice by Bacillus spp. is a great concern to the rice industry, very few studies to date have been performed on the control of Bacillus spp. in rice. The elimination of microorganisms in rice is quite difficult due to its solid phase. In this study, a new method of intervention for the inactivation of Bacillus cereus and Bacillus subtilis in rice was developed using fermented ethanol (FE) and supercritical carbon dioxide (SC-CO₂). Raw rice samples inoculated with Bacillus spp. were treated with 1. FE only (concentrations: 0, 10, 30, 50, and 70%; treatment duration: 5, 10, 20, 30, and 60 min), 2. SC-CO₂ only (temperatures: 36, 40, and 44 °C; pressure: 100, 150, and 200 bar; treatment duration: 10, 20, and 30 min) or 3. with both FE (30% for 30 and 60 min) and SC-CO₂ (36 °C, 100 bar, 10 min and 44 °C, 200 bar, 30 min). The survival of Bacillus spp. populations was assayed, as were any changes in the hardness and color of the treated rice. When rice samples were treated with the combined treatment of FE (30%, 60 min) and SC-CO₂ (200 bar, 44 °C, 30 min), Bacillus spp. were maximally reduced by approximately 5 log CFU/g. Sufficient log reduction of B. cereus (4.76 log CFU/g) and B. subtilis (4.66 log CFU/g) were also obtained with mild treatment (FE: 30%, 30 min and SC-CO₂: 100 bar, 36 °C, 10 min). The combined treatment led to beneficial changes including decrease in the hardness of cooked rice and whitening of rice (increase of lightness and decrease of yellowness) compared to that of untreated rice, resulting in an increase in perceived quality. The changes of hardness and lightness were mainly affected by FE and SC-CO₂, respectively. Our results indicate that a combined treatment with FE and SC-CO₂ can be successfully used for the inactivation of Bacillus spp. in rice, with simultaneous improvements in rice.