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Wheat bran is a by-product rich in bioactive compounds, but possess a strong polyphenol oxidase (PPO) activity, which can act undesirably on product quality. This study aimed to optimize the particle size (500, 255, and 10 µm) of wheat bran and time and amplitude of high-intensity ultrasound (HIU) treatment on the PPO activity, total phenolics, antioxidant activity, and water swelling using response surface methodology. The PPO activity decreased depending on the particle size and HIU treatment time. Phenolic content, antioxidant activity, and swelling were dependent on bran particle size. Temperature rise, depending on the HIU time and amplitude, affected PPO, phenolics, and antioxidants. In optimized conditions (particle size 10 µm, HIU 15 min), 93% of bran PPO was inactivated while phenolic content increased for 11%. Wheat flour combined with micronized and ultrasonically treated bran (at 20% wt/wt) had improved gluten formation and pasting properties compared with flour containing native bran. Practical applications Consumers show a growing awareness of health benefits of whole wheat and highfiber foods. Bran is a by-product of milling industry with a great potential as a functional food ingredient. However, the enzymatic browning of bran containing products that occur during processing worsens their sensory properties, nutritional value, and consumers' acceptance. Currently, new technologies such as micronization and ultrasound treatment are getting more attention in cereal processing. This study showed that the combined micronization and high-intensity ultrasound treatment can successfully inactivate the polyphenol oxidase (up to 93%) from wheat bran, and thus, prevent the enzymatic browning. Also, treated bran can be successfully incorporated up to 20% to wheat flour with lesser disruption of the gluten formation and pasting properties when compared with the native bran. The findings of this study contribute to better future usages of wheat bran in food applications adding the sustainability of cereal chain. How to cite this article: Habuš M, Novotni D, Gregov M, et al. Influence of particle size reduction and high-intensity ultrasound on polyphenol oxidase, phenolics, and technological properties of wheat bran.

3D printing is an emerging technology that offers the ability to produce tailor-made foods. This work addresses the physical properties of 3D-printed snacks enriched with wheat bran as a function of flour type (oat, barley), addition of acidity regulators (citric acid, sodium bicarbonate), printing temperature (20 °C, 30 °C, 40 °C), and bran pre-processing (high-intensity ultrasound, vacuum microwave and pulsed light). Polyphenol oxidase activity, total phenolic content, antioxidant activity of bran, the viscosity profile of the flour-bran blend, the precision of 3D printing and browning kinetics of the physical properties of the dough and of baked snacks were investigated. During 1 h required to print ten pieces, the dough became very distinctly darker. Adjusting the printing temperature to 20 °C and adding sodium bicarbonate resulted in a dough, which changed colour less, but still very distinctly. Bran pre-processing inactivated polyphenol oxidase activity by 77-92%, which stopped browning of the dough within 50 min without affecting the printing precision. The use of ultrasound, vacuum microwave and pulsed light could be extended to other food components to achieve a greater inactivation of undesirable enzymes. Preprocessing techniques resulted in minor differences in the baked snack, so their future choice depends mainly on the amount of water that can be added to the recipe.

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Millet is an unexploited cereal with potential in the food industry due to its nutritional value and resistance to harsh climate conditions. Nutritious millet byproducts have a potential application in the development of functional cereal products, but require processing in order to improve their physical and nutritional quality. Therefore, we investigated high intensity ultrasound as a pretreatment to increase the amount of freely available bioactives from proso millet bran. We also analysed the effect of high intensity ultrasound on enzymatic browning, water retention and protein digestibility, which are crucial for the utilization in the bakery and pasta industry. A 15 % millet bran suspension in water was treated with 400-W ultrasound probe for 5, 12.5 or 20 min, with the 60, 80 or 100 % amplitude. High intensity ultrasound treatment with 80 % amplitude for 12.5 min improved most significantly the nutritive value; the antioxidant activity measured by FRAP test increased by 15 % (p<0.05), and total phenolic content by 16 % (p<0.05). Still, the impact on the increase of water-soluble and ethanol-insoluble dietary fibre by 38 % was evident after the treatment for 20 min at 100 % amplitude. High intensity ultrasound treatment at 100 % amplitude for 5 min caused the largest improvements in water retention and limited browning of the sample. High intensity ultrasound treatment activated polyphenol oxidase, regardless of the applied heating of the sample. Due to its ambiguous impact on proso millet bran characteristics, the treatment required an optimization, which showed that the optimal pretreatment of a 15 % millet bran suspension in water is at 100 % amplitude for 9.3 min.

Summary Buckwheat hulls (BWH) are a lignocellulosic material with valuable nutrient contents, especially rutin and dietary fibre, which are worth exploring from a food industry perspective. BWH are also a voluminous material that is difficult to grind to very fine particle sizes, which can be achieved under cryogenic conditions. Therefore, we investigated the effect of fractionation by cryo‐grinding and size separation (<56, 56–180 and >180 µm) on the dietary fibre content and bioactive profile of BWH. While we hypothesised that micronisation would increase fibre solubility and release of bioactive compounds, we found that BWH must be fractionated to a size <56 µm to show a significant increase in soluble fibre content (106%), rutin (66%), total free phenolic compounds (94%) and antioxidant capacity (45–63%). Moreover, we have shown that near‐infrared spectroscopy combined with partial least squares regression can be easily used for rapid scanning of BWH to select the most suitable pre‐processing conditions for maximum nutritional value of this food industry by‐product.

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