This work evaluated the effect of different treatment process (hot air heating and microwave heating) on the physico-chemical parameters and functional properties of treated wheat bran.

Wheat bran from the variety PS Bertold (BB) and PS 215 (WB) were observed from Research and Breeding Station, Vígľaš Pstruša, Slovakia and Research Institute of Plant Production Piešťany, Slovakia. Wheat bran samples were treated using hot-air and microwave heating acording to method described by Lauková et al. (2019).

Chemical composition of wheat bran included determination of: moisture (AACC Method 44-19.01), ash (AACC Method 08-01.01), protein (AACC Method 46-13.01) and crude fat (AACC Method 30-25.01) (AACC, 2000).

TDF, IDF and SDF content was determined by enzymatic-gravimetric method (AOAC, 2003).

Phytic acid content was measured using colorimetric method according to McKie and McCleary (2016).

Antioxidant activity of wheat bran was determined according to the method of Cai et al. (2014) by measuring free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity. Wheat bran (0.1 g) was extracted with 1 cm³ of pure methanol at 25 °C for 2 h with continuous shaking under a dark environment and centrifuged at 1,200 × g for 10 min. The extract (0.05 cm³) was reacted with 1 cm³ of 0.1 mM DPPH solution at 25 °C for 30 min, and absorbance was measured at 517 nm. Antioxidant activity was calculated as percent discoloration of DPPH = [1 – (A₁/A₀)] × 100, where A1 is the absorbance of wheat bran extract at the end of the reaction (t = 30 min) and A0 is the absorbance of the pure methanol control at the beginning of the reaction (t = 0). The data were reported as percentage of discoloration.

Swelling capacity, water absorption and water retention capacity were determined according to method described by Lauková et al. (2018). Oil holding capacity was evaluated using method presented by Mora et al. (2013).

Solvent retention capacity (SRC) tests were performed according to the method from authors Xiao et al. (2006) using 5% lactic acid, 5% sodium carbonate, 50% sucrose, and distilled water. Sample (5g) was added into a 50 cm³ centrifuge tube with a screw cap. Then 25.0 cm³ of an appropriate solvent was added and the mixture was vortexed vigorously to suspend the flour for 5 sec. The mixture was allowed to set and swell for 20 min and was vortexed for 5 s each at 5, 10, 15, and 20 min. After centrifugation at 1,000 x g for 15 min (not including time to achieve speed), the supernatant was decanted and the tube was drained at a 90° angle for 10 min on a paper towel. Finally, SRC value (14% mb) was calculated for each solvent as:

S R C ( % ) = ( g e l   w e i g h t − f l o u r   w e i g h t f l o u r   w e i g h t ) × 100

The color parameters L (lightness), a (redness/greenness), and b (yellowness/blueness) of the samples were measured by spectrophotometer Cary 300 UV-Vis (Agilent Technologies, Santa Clara, USA) with DRA-CA-30I sphere accessory. The spectrophotometer was calibrated with a white calibration tile. The Cary WinUV software with “Color” application was used for recalculation the Chroma (C) and hue angle (H). Color coordinates were determined five times per bran sample.

All determinations were carried out in triplicate unless otherwise state. The results were expressed as mean ± standard deviation. The significant differences between mean values of raw and treated bran were establish using a Student`s test at p <0.05. The XLSTAT statistic software was used for data evaluation.

Composition of wheat bran is purely based on the variety, cultivation conditions and the methods employed for its separation, which determines the amount of starch attached to the aleurone layer after the separation (Babu et al., 2018). Chemical composition of treated and raw wheat bran is summarized in Table 1. Results presented that moisture content of bran decreased after thermal treatment, which was caused by loss of water as a result of heating (Dong et al., 2019). Furthermore, it was observed that treatment of wheat bran using microwave and hot air heating had no significant effect on ash and fat content.

Wheat bran contains more than 15% high quality proteins, but most of them are enclosed within a matrix of cell wall polysaccharides and so they are poorly digested. Wheat bran proteins have also been explored as a source of amino acids and bioactive peptides or as inhibitors of enzymes of industrial interest (Balandrán-Quintana, Mercado-Ruiz and Mendoza-Wilson, 2015). From the results concluded that protein content of raw bran varies from 15.46% (WB) to 15.92% (BB). These results were in agreement with those obtained by Ferreira, Chang and Steel (2011) (15.30%) and Noort et al. (2010) (15.90%). After microwave treatment, the protein content increased up to 16.56% (WB) and 17.20% (BB).

Wheat bran appears as an important dietary fiber source (Ferreira, Chang and Steel, 2011). It was observed, that TDF content of raw bran samples were 46.35 and 45.99%, indicating bran as good source of dietary fiber. Similar TDF content in wheat bran was found by Ma, Lee and Baik (2018) (46.50%) and Ferreira, Chang and Steel (2011) (46.30%). Furthermore, IDF and SDF content of raw BB, 44.93% and 1.42%, respectively, were higher compared to raw WB (44.62 and 1.37%). Thermal treatments can change their physico-chemical properties of dietary fiber by altering the ratio between soluble and insoluble fiber (SDF/IDF), TDF content (Ozyurt and Ötles, 2016). It can be concluded, that treatment of bran using both methods had significant effect on TDF and IDF. The results suggested that hot air treatment resulted in higher fiber content compared to microwave treated bran. Moreover, hot air treatment significantly increased the SDF content. In general, the changes in the dietary fiber composition during thermal processing may be partly attributed to the redistribution of the insoluble and soluble components of dietary fiber, and partly to the formation of resistant starch. An increased temperature breaks week bonds between polysaccharide chains and split glycosidic linkages in the polysaccharides (Căprită, Căprită and Hărmănescu, 2012). Increase in total fiber can be also attributed to the formation of fiber-protein complexes that are resistant to heating and are quantified as dietary fiber (Dhingra et al., 2012).

Bran also contains phytic acid which is the major phosphorus storage component and comprises 80% of total phosphorus in cereal grains (Aktas-Akyildiz et al., 2017). The presence of phytate has been considered as an antinutrient in humans because of its effect on the bioavailability of iron, magnesium, zinc and calcium. While the mechanism is not entirely understood, it is suggested that phytic acid binds strongly with these mineral cations to form phytate-mineral complexes, changing their solubility, functionality absorption and digestibility. Consequently, the complex cannot be absorbed or easily hydrolysed by the human body and so there is an adverse effect on bioavailability of minerals (Stevenson et al., 2012). The phytic acid content (Figure 1) in raw WB samples was higher (51.9 mg.g^-1) than in raw BB (40.4 mg.g^-1). These values are in agreement of those reported by Noort et al. (2010) (47.9 mg.g^-1) in wheat bran. It can be stated that thermal treatment significantly decreased phytic acid content in both bran variety. After microwave treatment, the phytic acid content was significantly reduced to 22.6 mg.g^-1 (BB) and 26.4 mg.g^-1 (WB). Recently, Mosharraf, Kadivar and Shahedi (2009), and Zhao, Guo and Zhu (2017) also described a decrease in phytic acid content in treated wheat bran after steeping in acetate buffer and fermentation.

The antioxidant activity of wheat bran measured using DPPH is illustrated in Figure 2. The results showed that antioxidant activity of raw wheat bran was 24.84% and 28.85% for BB and WB, respectively. Both values were higher than those reported by Verma, Hucl and Chibbar (2008) (12.5 – 20.1%) in bran from 51 wheat cultivars. On the other hand, Cai et al. (2014) observed higher antioxidant activity (29.2 – 53.6%) in bran from American and Korean wheat varieties. Microwave and hot air heating of bran decreased its antioxidant activity. The lowest antioxidant activity values were recorded after hot air heating of bran samples (23.11 and 23.64% for BB and WB).

The technological interest and physiological effects of dietary fibre are related to their functional properties. The hydration properties of dietary fibres determine their optimal usage levels in foods because a desirable texture should be retained (Yaich et al., 2015). Wheat bran is rich in polysaccharides which can bind water on a molecular level through formation of hydrogen bridges. These mechanisms contribute to water uptake by bran in the case of unconstrained hydration. Alternatively, when bran is exposed to an external stress, only the water strongly bound in nanopores or through hydrogen bonds will govern water retention (Hemdane et al., 2016). Functional properties of untreated and treated wheat bran are summarized in Table 2.

WHC is related to the porous matrix structure formed by polysaccharide chains which can hold large amounts of water through hydrogen bonding (Du, Zhu and Xu, 2014). The WHC of raw bran samples were 2.46 g.g^-1 (BB) and 2.57 g.g^-1 (WB), which was similar to those described by Cai et al. (2014) (2.04 – 2.51 g.g^-1). From the results concluded that thermal tretment of bran resulted in increased WHC values. Moreover, hot air treated bran had higher WHC than microwave treated bran. The high WHC of modified bran suggested that this material could be used as a functional ingredient to avoid syneresis and to modify the viscosity and texture of formulated products in addition to reducing calories by the total or partial substitution of high-energy ingredients (Grigelmo-Miguel, Gorinstein and Martín-Belloso, 1999). Yan, Ye and Chen (2015) demonstrated higher WHC value of bran after extrusion.

WRC is one of the major key parameter which has been studied in functional food. Most significant changes that happen during baking i.e. gelatinization of starch, denaturation of protein, flavor and color formation are due to water contents (Khan et al., 2018). WRC is related to the content of insoluble dietary fiber and the intact cell structure of bran (Zhao, Guo and Zhu, 2017). Raw BB bran had lower WRC value (1.74 g.g^-1) compared to WRC value of raw WB bran (2.16 g.g^-1). This WRC values are in agreement with result (2.18 g.g^-1) presented by Ma, Lee and Baik (2018). After hot air treatment the WRC significantly increased up to 2.38 and 2.63 g.g^-1 for BB and WB, respectively.

SC indicates how much the fiber matrix swells as water is absorbed, including loosely associated water. It is a consequence of the macromolecule relaxation during hydration, which leads to an increment in the occupied volume by the fiber product. The greater capacity to swell is the most desirable parameter for the physiological functionality of DF (Lebesi and Tzia, 2012). The SC of raw bran was higher (4.21 and 4.40 cm³.g^-1) than obtained Mora et al. (2013) (2.92 cm³.g^-1) for wheat bran. From the results concluded that thermal treatment significantly increased the SC of wheat bran. The highest SC (5.66 and 5.75 cm³.g^-1) was recorded after microwave treating of bran. The increase in SC might be attributed to a rise in the amount of short chains and the surface area of DF induced by thermal processing (Dong et al., 2019).

OAC is the capability of dietary fiber to adsorb fat. During food processing, the reduction of cholesterol level in blood is linked with higher value of OAC (Khan et al., 2018). Oil absorption of cereal derivatives, e.g., wheat bran, is related mainly to the surface properties of the bran particles but may also be related to the overall charge density and to the hydrophilic nature of the constituents (Elleuch et al., 2011). It was observed that OAC of raw BB bran was higher (1.33 g.g^-1) than WB bran (1.49 g.g^-1). These values were lower compared to values observed by Mora et al. (2013) for wheat bran (2.18 g.g^-1). Authors Ma and Mu (2016) describe that low OAC values might be due to the absence or limited presence of lignin. The results showed that treatment of bran using both methods had no significant effect on OAC values.

The SRC method has been conceived to produce a combined pattern of the four SRC values to establish a practical flour quality/functionality profile. It is clear that fibre functionality in food formulations derived from its interaction and spatial arrangement within the biopolymers system (Rosell, Santos and Collar, 2009). Wheat bran is increasingly added to mostly cereal-based food products (bread, cookies, breakfast cereals, pasta, snacks, cakes, and more) (Hemdane et al., 2016). For this reason the selected SRC of wheat bran samples and the effect of bran treatment were also evaluated. The SRC values are summarized in Table 2. Generally, lactic acid SRC is associated with glutenin characteristics, sodium carbonate SRC is related to levels of damaged starch, and sucrose SRC with pentosan characteristics (Rosell, Santos and Collar, 2009). The results showed that thermal treated bran had significantly higher lactic acid and sucrose SRC compared to raw bran. The highest values of lactic acid SRC were observed after hot air heating (174.44 and 198.11% for BB and WB, respectively). Furthermore, the results revealed that microwave treated WB bran had the highest sucrose SRC (224.56%). On the other hand, the sodium carbonate SRC decreased after thermal treatment process up to 163.25% (hot air heated WB).

Color is an important visual quality (attribute) of food products (Ferreira, Chang and Steel, 2011). The color attributes of raw and treated bran are summarized in Table 3. The lightness values of raw bran (63.33 and 65.84 for BB and WB, respectively) were similar to results described by Onipe, Beswa and Jideani (2017). Results showed that thermal treatment of bran samples decreased the lightness value of bran. Moreover, the lowest lightness values (58.60 and 59.87 for WB and BB, respectively) were recorded after hot air heating. It was also observed that after treatment of bran using both methods the redness of bran increased, while yellowness of bran decreased. Chroma (C), considered the quantitative attribute of colorfulness, is used to determine the degree of difference of a hue in comparison to a grey color with the same lightness (Minarovičová et al., 2019). The results suggested that treated bran using both methods had higher C value compared to raw bran samples. The higher the C value, the higher is the color intensity of samples perceived by humans (Minarovičová et al., 2019).