Brown millet (BM) refers to millet that has only been hulled after harvest, without any chemical processing or other refining processes. Millet bran (MB), which comprises 9–12% of the weight of BM, is a by-product of millet refining process. It contains essential nutrients, such as fat, protein, starch, dietary fiber, phytosterols, γ-glutenin, polyphenol compounds and squalene (Ji, Liu, Ge, Zhang, & Wang, 2019; Liang, Yang, & Ma, 2010), and the concentration of nutrients in MB is much higher than that of refined millet. However, due to its unpalatable taste and short shelf life, BM is often processed into refined millet by removing nutritious bran layer, and MB is usually used as animal feed in China, resulting in huge waste. The retention of MB can not only avoid such waste, but also improve nutritional value of millet and meet the growing consumer demand for whole grain foods. Moreover, similar to rice bran, MB can be used to produce millet bran oil and develop as a food-grade ingredient (Xiao, Du, Tang, Yang, & Yang, 2017). BM retains all vital nutritional components of MB, making it as an abundant source of nutrients. Additionally, BM contains carotenoids that other grains typically lack, with concentration ranging from 1.10 to 19.55 mg/kg. The yellow pigment in BM is a type of carotene that not only possess antioxidant properties but also enhance human immunity and protect cardiovascular system (Wang, Yin, Yang, & Chen, 2004). Phenolic, carotenoids, and dietary fiber components within BM can promote heat production in adipose tissue and are critical in developing whole grain diets and functional products to regulate obesity (Soujanya & Jayadeep, 2022). Moreover, adopting a whole grain diet can promote better sleep quality (Zeng et al., 2014) and reduce the risk of chronic diseases (Cho, Qi, Fahey Jr., & Klurfeld, 2013). As a limited gluten-free raw material, BM also plays an important role in developing low-allergenic products.

However, the stability of BM and MB is poor, rancidity deterioration can easily occur during storage especially at inappropriate storage conditions, as well as insects and fungi contamination, which significantly reducing its value and affecting further applications. During the storage of BM, endogenous enzyme activity would cause hydrolytic and oxidative rancidity of lipids. The processing of BM can result in hydrolysis of lipids due to the presence of endogenous lipases (LA), which can produce free fatty acids, thereby increasing fatty acid value (FAV). Furthermore, under the action of peroxidase (POD), peroxides are generated, leading to oxidative rancidity, which can negatively affect storage stability of BM and MB (Kumar et al., 2021). Thus, endogenous enzymes, mainly LA and POD, play a dominant role for the rancidity deterioration of BM and MB. Exploration an effective stabilization method to inactivate LA and POD activities is a key prerequisite and crucial for both long-term storage of BM and value-added utilization of MB.

Several methods have been explored and applied for stabilization of BM and MB, including ohmic heating (Dias-Martins et al., 2019), microwave heating (Barbhai & Hymavathi, 2022) and dry heat (Rashid, Liu, Han, & Jatoi, 2022; Sun, Gong, Li, & Xiong, 2014), etc. Among various stabilization techniques, radio frequency (RF) heating has gained significant attention lately. This technique uses electromagnetic waves with frequency ranging from 3 kHz to 300 MHz to heat dielectric materials. Under high-speed alternating electric field, charged ions in the sample move quickly, and polar water molecules rotate rapidly, thus generating heat inside dielectric materials by molecular friction (Chen, Yen, & Chen, 2021). RF heating can quickly and volumetrically increase sample temperature, which substantially reducing heating time in comparison to conventional heating methods. Unlike ohmic heating, RF heating does not require direct contact with product. Moreover, compared to microwave heating, RF heating has a deeper penetration depth and more uniform electric field distribution, which makes it more easily applicable for large scale industrial processing (Marra, Zhang, & Lyng, 2009; Piyasena, Dussault, Koutchma, Ramaswamy, & Awuah, 2003).

In recent years, scholars have explored the potential of using hot air-assisted radio frequency (HA-RF) heating for stabilization of rice bran (Liao et al., 2020; Ling, Lyng, & Wang, 2018), wheat germ (Ling, Ouyang, & Wang, 2019), and millet (Yarrakula, Mummaleti, Pare, Vincent, & Saravanan, 2022). Based on these existing studies, we could make a hypothesis that HA-RF technique could also be used for stabilization of BM and MB which is more susceptible for rancidity and off-flavor. Currently, only a few investigations on HA-RF stabilization of refined millet have been reported (Yarrakula et al., 2022), there is no systematic study on HA-RF stabilization of BM and MB. Therefore, this study aims to investigate HA-RF treatment inactivation effects on LA and POD activity of BM and MB, and evaluate the impact of HA-RF treatment on physicochemical properties, microstructure, nutritional quality and storage stability of BM and MB. This is crucial important for developing whole grain foods based on millet and enhancing the added value of MB.