Effects of hot air-assisted radio frequency stabilization of brown millet and millet bran: Enzyme inactivation, microstructure, nutritional quality, and storage stability

Brown millet (BM) and millet bran (MB) are rich sources of nutrients, but their storage is affected by rapid rancidity deterioration, which primarily due to the activity of lipase (LA) and peroxidase (POD). High moisture content and water activity also have adverse effects on rancidity. In order to inactivate enzyme activities and extend storage stability of BM and MB, hot air-assisted radio frequency (HA-RF) heating was applied in this study. BM was treated at 95 °C and 105 °C while MB was heated to 105 °C and 115 °C with various holding times. The effects on enzyme inactivation, microstructure, nutritional quality, and storage stability of the samples were investigated. The results showed that HA-RF treatment effectively reduced LA and POD activities in both BM (<13%) and MB (<10%); Low-temperature (95 °C for BM and 105 °C for MB) HA-RF treatment caused less damage to the structure of the samples and maintained the structural integrity of most starch and proteins, although the treatment significantly (p < 0.05) reduced free phenolic and flavonoid contents of the samples; Low-temperature HA-RF treatment had less impacts on main nutrients of BM and MB when compared to the high-temperature treatment (105 °C for BM and 115 °C for MB). HA-RF stabilization effectively improved the storage stability, and slowed down the increase in fatty acid value (FAV) and malondialdehyde (MDA) content during storage.