Effect of vapors from fractionated samples of propolis on microbial and oxidation damage of rice during storage Original Research Article

The efficacy of vapors from polar and non-polar sub-fractions of propolis on microbial and oxidation control during rice (Oryza sativa, hinohikari var.) storage was evaluated. The sub-fractions (absolute ethanol, methylene chloride, hexane extracts: AEPEV, MCPEV and HEPEV, respectively) were infused in synthetic adsorbents and their volatiles released during storage (6 months). HEPEV, MCPEV and AEPEV treatments inhibited molding and post-inoculation bacterial colonization (1.1, 1.1, 0.9 and 1.3, 1.2, 1.1 log10 cfu/g reductions, respectively) on brown rice. AEPEV treatment suppressed fat acidity damage of milled rice at 30 °C to conventional cold storage level (5 °C) and differential Gram staining of bacteria isolated after the treatment indicated a dominant Gram-positive bacterial distribution. The concentrations providing 50% inhibition of 2′,2′-diphenylpicrylhydrazyl free radical scavenging were 9.8, 3.2 and 2.8 μg/μl for hexane (HEPE), absolute ethanol (AEPE) and methylene chloride (MCPE) extracts, respectively. The oxidative degradation rate was lowest for AEPE (4.3 × 10−4 min−1) and highest for HEPE (1.9 × 10−3 min−1) in the β-carotene bleaching assay. Gas chromatograph mass spectrometry revealed that AEPE had the highest amount of caffeic acid and caffeic acid phenethyl ester. Ultimately, the volatiles from the propolis sub-fractions had varied potential in rice quality preservation.