Extraction kinetics of rice bran oil and its minor bioactive compounds from rice bran by subcritical propane

Abstract:Rice bran, the main by-product of rice process, contains oil and lots of bioactive compounds, such as γ-oryzanol and α-tocopherols. Preliminary study shows that the active ingredient in rice bran could be well preserved during the subcritical fluid extraction. In order to reveal the extraction kinetics of rice bran oil (RBO) and its soluble active ingredients during subcritical propane extraction process, rice bran oil and active ingredient (γ-oryzanol and α-tocopherols) were extracted from fresh rice bran using subcritical propane as medium at different temperature and time. We developed a two-dimensional transitory mathematical model by using Baümler model to simulate the kinetics of RBO and γ-oryzanol, α-tocopherols during subcritical propane extraction process. The diffusion model of Fick's law, which fit for the system of suspending cylindrical particles in a dilute solution, was used to describe the diffusion process of oil and its minor compounds. The results showed that, Yt/Y∞ (the mass ratio of substance that has diffused at time t and infinity time) increased with extraction time. Moreover, the free lipid on rice bran surface was extracted more easily than the cell bound lipid inside. As a result, the increasing rate of Yt/Y∞ and the effective diffusion coefficient of RBO at initial stage were both faster than that of second stage during the extraction process. Similarly, the trends of γ-oryzanol and α-tocopherols were similar. However, due to the higher energy barrier of γ-oryzanol than rice bran oil, the rate of Yt/Y∞ increase was slower than that of rice bran oil in the same extraction condition. In general, as temperature increases, the vapor pressure of the extracts increase, resulting the increase of diffusion coefficient and mass transfer coefficient. This can be used to explain the result that higher temperatures enhanced oil and minor compounds extraction from rice bran as the Yt/Y∞ and effective diffusion coefficient increased. The De value of γ-oryzanol was lower than that of rice bran oil. Only when the extraction temperature was 55℃, the De value of γ-oryzanol (3.415×10-8 m2/s) and RBO（3.461×10-8 m2/s）be similar. It indicated that the extraction of γ-oryzanol was more dependent on the temperature. The α-tocopherols were extracted more difficult because of their amphiphile structure. The effective diffusion coefficient was positively correlated with the temperature. The results also showed that, the extraction of α-tocopherols was significantly influenced by the temperature. The Baümler model fit well the extraction kinetics of rice bran oil, γ-oryzanol and α-tocopherols （R2>0.95）during the subcritical propane extraction process. The temperature dependence of the diffusion coefficient was represented by an Arrhenius-type equation for target extract, and the extraction of γ-oryzanol depended on temperature mostly. Activation energy of γ-oryzanol (9.11 kJ/mol) was higher than that of oil (5.23 kJ/mol) and α-tocopherols (7.05 kJ/mol). Based on the extraction kinetics model obtained, high predicted precision of rice bran oil, γ-oryzanol and α-tocopherols from rice bran extracted by subcritical propane, which were 95.8%, 94.7% and 94.4%, respectively. In conclusion, based on Baümler model, the extraction kinetics of rice bran oil and its concomitant subcritical can be better revealed, and the model can predicted the Yt/Y∞ of rice bran oil and its trace active components accurately to guide the subcritical extraction process of rice bran oil and its trace active components.