Effect of natural selenium-enriched rice flour addition on product properties of fresh extruded rice-shaped kernels (FER) based on naked oat (Avena nuda L.)

Abstract

Naked oat was used to develop a kind of fresh extruded rice-shaped kernels (FER) by extrusion technology, and natural selenium-enriched rice flour was added to solve the problems of irregular shape, dark color and hardness of extruding pure naked oat. Results showed that the addition of rice flour had significantly improved the product properties of FER including shape, color, transparency and texture. Compared to the control (CK), the contents of protein, fat and total dietary fiber (TDF) in FER decreased obviously (p < 0.05) with rice flour addition. The oat-rice FER's shape turned to plump and appearance was acceptable, and had higher water uptake ratio (WUR), elongation ratio and solid loss (SL) than CK. The hardness of FER significantly decreased (p < 0.05) after adding rice flour, and FER with 35% rice flour ratio (35% FER) had the highest sensory evaluation score (7.31 out of 9). Then 35% FER was selected for further vitro activity study, and result showed that bile acid binding rates of 35% FER were stronger than normal natural rice.

Introduction

As a modern grain processing technology, extrusion is common used for varieties of food manufacture, such as breakfast cereals, baby foods, flat breads, snacks, and modified starch (Sayanjali et al., 2019; Su and Kong, 2007). The advantage of extrusion technology is high-efficient, low cost and multi-function such as the combination of mixing, cooking and forming (Gopirajah and Muthukumarappan, 2018). Generally speaking, with the appropriate mold and cutter, extruder has the ability of convert the raw material into any desired shaped product, including rice shaped product. This kind of product was defined as fresh extruded rice-shaped kernels (FER). East Asians prefer rice, and about half of people live on rice as their staple food. Therefore, the emergence of FER would enrich people's choice of staple food.

The main raw material of FER was varieties of substances rich in starch, such as oat, rye, barley, sorghum, and so on. Among them, oat is considered a healthy cereal, which generally can be divided into two categories: hulled oat (Avena sativa L.) and naked oat (Avena nuda L.) around the world, and China mainly grows the latter (Hu et al., 2014). Oat is a good food material being rich of nutrients, such as protein, unsaturated fatty acids and dietary fiber (DF). Oat also has specific function for human health, especially in hypolipidemic effect. Peng et al. (2013) had indicated that oat effectively reduced both the serum and hepatic lipids, and reduced LDL-C for adults with overweight and obesity. However, the composition of raw material significantly affected extruded product properties in processing (Guy, 1993). Because of high dietary fiber content, oat has poor gas retention capacity during extrusion cooking. High fat and protein levels exacerbate the problem. The lubrication of lipid reduce the specific mechanical energy and melting temperature, thus reducing extrudate expansion (Sandrin et al., 2019). As a result, extruding pure oat flour result in shriveled appearance, dark color, hard texture, discordant flavor and poor consumer acceptance of the products, which is typically the main limiting factor for FER based on pure oat. In order to solve these problems, other ingredient must be added to change raw material rheology properties and improve FER quality. For example, Liu et al. (2000) adopted corn flour as expansion agent to prepare oat-corn puffed snack presenting acceptable sensory properties.

In this study, natural selenium-enriched rice from Enshi of Hubei province (China) was selected as quality improvement ingredient. Rice is easy to digest, white color, mild flavor, nonallergenic, good processability, and lower protein, lipid and total dietary fiber (TDF) contents (Sandrin et al., 2019), therefore, there were many studies of producing gluten-free bread, cakes, biscuits, and noodles using rice (Kang et al., 2015; Lang et al., 2020; Eyenga et al., 2020; Geng et al., 2020). In addition, rice is also used to improve quality of novel foods such as tortillas, beverages, processed meats, puddings, ready-to-eat flour and salad dressing (Kadan et al., 2008; Sandrin et al., 2019). Compared to other types of rice, natural selenium-enriched rice contains higher amounts of safe selenium (Se). Se is an essential trace element for human health, which has various important biological functions such as antioxidant, hormonal and anti-carcinogen (Wang et al., 2013; Wu et al., 2019). It has also been deemed as one of the important factors for lipid metabolism, and relevant studies have shown that consumption of Se-enriched pork significantly reduced total cholesterol levels (Mrazova et al., 2020). However, few information is available about FER making with naked oat and natural selenium-enriched rice flour, and the rice flour ratio on the quality of FER.

Therefore, the aim of this study was (1) to obtain a novel product of FER based on naked oat; (2) to get rid of the unacceptable quality properties of FER by adding natural selenium-enriched rice flour; (3) to evaluate vitro bile acid binding capacity of FER with the best product properties. The research provides insights for the development of a novel FER with high quality according to extruded processing technique, which has the potential function of reducing the cholesterol level.

Materials and Methods

Raw materials    Naked oat grain was purchased from the Zhangjiakou Lvfeng cereal processing Co. Ltd. (Hebei, China), with the moisture content of 13.65%, protein content of 12.14%, fat content of 5.82%, total starch content of 59.44%, dietary fiber content of 4.92%. Natural selenium-enriched polished rice was purchased from Enshi Selenium agricultural technology Co. Ltd. (Hubei, China), and the moisture, protein, fat, total starch and dietary fiber content were 12.83%, 7.80%, 0.62%, 77.51% and 0.71% respectively. Normal natural rice was purchased from the local market of Tianjin, China.

Just before extrusion processing, the naked oat grain was treated with 95 °C at 20% relative humidity for 10 min to inactivate lipid enzyme (Sandrin et al., 2019). Then the naked oat grain and natural selenium-enriched rice were ground at 25000 rpm for 5 min using a high-speed grinder (XL-10B, XuLang Machinery, GuangZhou, China). Both of the oat flour and rice flour were passed through a 60-mesh sieve. Then they were vacuum packaging (Aode, Co. Ltd., Beijing) respectively and stored at 4 °C.

Extrusion process    Preliminary experiments showed that the shape of oat-rice FER was not good if rice flour ratio was less than 20% (figure not shown), so the naked oat flour and rice flour were blended with the ratios of 75:25, 70:30, 65:35, 60:40 and 55:45 (w/w), respectively. Extruded pure naked oat flour was used as the control (CK). The ratio of the mixed powders and water was 100: 16 (w/w) and treated with an industrial mixer (HWT 20, Jinan Sanguan Food Machinery Co., Ltd., China) for 10 min to homogeneity. The blends were then left at room temperature for 1 h to achieve moisture equilibration. Next, extrusion processing was applied using a double screw extruder (DSE32-I, Jinan Shengrun Technology Development Co., Ltd., China) with 1.25 mm wide rice-shaped die. The extruder screw was divided into three zones with the temperature of 60 °C (zone 1), 80 °C (zone 2) and 110 °C (zone 3), respectively. The screw speed was 18 Hz, feeding speed was 8 Hz, and cutting speed was 35 Hz. Finally, the FERs were dried at 45 °C for 5 h until moisture content to 13.0%–14.5%, then vacuum packaging and stored at 4 °C.

Analysis of raw FER Centesimal composition of FER

Protein, crude fat and total dietary fiber (TDF) was detected according to Improved Kjeldahl Method in AACC 46-11A (2002), Soxhlet extraction Method in AACC method 30-25 (2002) and AACC method 32-05 (2002), respectively. The analysis was replicated 3 times for each sample.

Expansion ratio, color and transparency of FER    Expansion ratio of the FER was measured as the ratio between the cross-sectional diameter of FER and that of the die (Alvarez-Martinezet et al., 1988). Vernier caliper was used to measure the diameter of FER.

The expansion ratio of FER was calculated by Eq. 1:   

The analysis was replicated 3 times for each sample (n=3)

The color of FER was detected using colorimeter (WR-18, Shenzhen Wave Photoelectric Technology Co., Ltd., China), and Satake MM1D rice meter (brown rice model) was used to measure transparency of FER (Liu et al., 2018). The analysis were replicated 3 times for each sample.

Microstructure of FER    Scanning electron microscopy (SEM) imaging was utilized for microstructure observation of FER and the analysis method was according to Liu et al. (2018). Briefly, extruded rice grains were broke transversely and mounted on 12 mm specimen stubs. Then the samples were sprayed with gold-palladium (BioRad Polaron sputter coater) and imaged with a SEM (S-2500 Hitachi) at an accelerating voltage of 20 kV (Quanta 200, FEI). The cross-section of samples were observed at 700 times magnification.

Analysis of cooked FER Cooking properties of FER

Accurate 5.0 g FER samples were cooked in excess distilled water with a known weight met al colander, which was soaked in a boiling water at 98 °C for 10 min. Then the water uptake ratio (WUR), the elongation ratio and the solid loss (SL) of FER were analyzed according to the method of Singh et al. (2005) and Shen et al. (2019) respectively, and the calculated equations were as follows:

The WUR of FER was calculated by Eq. 2:   

The analysis was replicated 3 times for each sample (n=3).

The longitudinal elongation ratio of FER was calculated by Eq. 3:   

The analysis was replicated 3 times for each sample (n=3).

The residual cooking water were transferred to 100 mL volumetric flasks with several washings and filled to volume with pure water. Then 10 mL aliquot were placed in pre-weighed glass plates and air-dried in an oven at 105 °C until a constant weight.

The SL of FER was calculated by Eq. 4:   

The analysis was replicated 3 times for each sample (n=3).

Texture profile analysis (TPA) of FER    Just before TPA, 200 g FERs were soaked in 300 g distilled water for 10 min. Then an automated electric rice cooker was used for FERs cooking (about 20 min). Next, the cooked FER was maintained warm for 10 min.

A texture analyzer, TA. XT plus Texture Analyzer (Stable Micro System Corp., UK) was used to perform the TPA. The tests were performed using the P/36R probe (36 mm diameter) and two-cycle compression tests. Three grains of cooked FERs were selected from the center of the inner container of electric rice cooker. These grains were carefully placed side by side and then were compressed to 70% at 1.0 mm/s. The time between chews was 5 s. Textural parameters recorded from the test curves included hardness, springiness, gumminess, chewiness and resilience (Singh et al., 2005). The texture profile analysis was repeated 10 times (n=10).

Sensory evaluation of FER    Sensory evaluation was done by using a 9-point hedonic scale. A sensory panel from our research team was made up of 10 trained panelists (four males and six females). Approximately 20 g of cooked FERs which coded with random three-digit numbers were prepared. For each test, six samples were serviced for panelists in a random order on a tray with a glass of drinking water. All samples were kept warm and the panelists were asked to rinse their mouth with water before testing the next sample (Srisawas and Jindal, 2007). The panelists were requested to express their overall acceptability by taking appearance, color, odor and taste into account. Sensory scores were based on a nine-point hedonic scale, with one being extreme dislike and nine being extreme liking (Kumar et al., 2018).

Vitro bile acid binding capacity of FER    The bile acid binding assay had been slightly modified according to the previously described procedure (Panith et al., 2016). In this study, sodium taurocholate (ST) and sodium glycocholate (SG) were used as the bile acids model to test cholate separately. ST and SG were dissolved in 0.1 mol/L phosphate buffer (pH 6.9) to prepare 7 mmol/L cholate solution. Cooked FER with the highest sensory evaluation score was chopped (simulated chewing), then accurate 5.0 g sample was stirred with 200 mL of NaCl (0.9%), and the mixture was adjusted to pH 2.0 with 0.1 mol/L HCl, and then 160 mg of pepsin was added (simulated gastric digestion). The mixture was incubated at 37 °C water bath for 1 h with shaking at 150 rpm. After 1 h, using 0.5 mol/L NaHCO₃ adjusted the mixture to pH 6.9. Next, 7.0 mL of 7 mmol/L cholate solution and 5.0 mL of porcine pancreatin solution (10 mg/mL in 0.1M phosphate buffer, pH 6.9) was added to the sample, and kept incubating at 37 °C water bath with shaking at 150 rpm. After 15, 30, 60, 90, 120 min, 10 mL resulting mixture were centrifuged at 5000 g for 20 min to obtain the supernatant. Bile acids in the supernatant were measured as unbound ST and SG using spectrophotometry at 387 nm. The contents of unbound ST and SG were obtained from the standard curve of the two pure cholates. Bile acid binding rate of FER was calculated by Eq-5. Cooked normal natural rice was used as the control, and the cooking method was the similar as the description of 2.4.2. 200 g normal natural rice was soaked in 300 g distilled water for 10 min. Then an automated electric rice cooker was used for cooking. Next, the cooked rice was maintained warm for 10 min.

  

Where “c₀” is the initial concentration of ST or SG; “c” is the concentration of unbound ST or SG (mmol/L)

Statistical analysis    The one factor analysis of variance (ANOVA) and Pearson correlation coefficients (r) were calculated using SPSS 22 (SPSS Inc., Chicago, IL) statistical software. Pair-wise comparison of mean values adopted Fisher's least significant difference (LSD) method at p < 0.05. The results are shown as the mean and ± SD (standard deviation).

Results and Discussion

Analysis of raw FER Changes in centesimal composition of FER    The effect of different natural selenium-enriched rice flour ratios on protein, fat and TDF content of FER were showed in Table 1. As expected, the contents of protein and fat in FER decreased obviously (p < 0.05) with the increasing rice flour ratios of 25% ∼ 45%. The protein content of CK was 12.14%, whereas those of 25% to 45% rice flour added samples decreased from 11.43% to 9.51%, respectively. The fat content of CK was 5.82%, after 25% ∼ 45% of rice flour ratio adding, it decreased from 4.12% to 2.57%. Similarly, the content of TDF in FER also significantly decreased (ranged from 3.87% to 3.01%) compared with CK (4.92%).

Table 1. Effect of natural selenium-enriched rice flour ratios on centesimal composition and physical properties of FER.

RFR (%)	Protein (%)	Fat (%)	TDF (%)	Expansion ratio	Color			Transparency
					L*	a*	b*
CK	12.14±0.17e	5.82±0.24f	4.92±0.15f	1.17±0.011a	19.20±0.52a	4.79±0.08f	20.38±0.14f	0.64±0.09a
25	11.43±0.19d	4.12±0.32e	3.87±0.14e	1.32±0.008b	27.62±0.39b	3.49±0.02e	18.88±0.02e	1.61±0.02b
30	10.98±0.08d	3.64±0.16d	3.45±0.10d	1.38±0.015c	31.30±0.27c	2.97±0.06d	18.39±0.06d	2.18±0.21c
35	10.52±0.11c	3.22±0.10c	3.12±0.04b	1.56±0.012d	38.18±0.18d	2.18±0.01c	17.24±0.08c	2.97±0.06d
40	10.01±0.09b	2.83±0.09b	3.22±0.03c	1.61±0.020f	46.68±0.46e	1.61±0.02b	16.43±0.09b	3.49±0.42e
45	9.51±0.08a	2.57±0.08a	3.01±0.04a	1.53±0.017e	58.82±0.61f	0.64±0.01a	13.87±0.11a	4.79±0.08f

Data was presented as means ± SD (n=3). Means with different letter (a,b,c,d) in a column represent that significant differences between groups at the p < 0.05 level.

RFR: rice flour ratio; TDF: total dietary fiber.

Expansion ratio, color and transparency of FER    In the minds of consumers, most of natural rice including extruded rice should have relatively plump shape. Expansion ratio was adopted to reflect the fullness of FER. As can be seen in Table 1, expansion ratio of FER were found to increase significantly (p < 0.05) after adding rice flour than CK. Expansion ratio of CK was 1.17, whereas those of 25% to 45% rice flour added samples were 1.32, 1.38, 1.56, 1.61 and 1.53 respectively, which mean the addition of natural selenium-enriched rice flour could improve the shape of FER. This may be because of the decrease of TDF and fat contents (Table 1). Due to the rapid release of pressure after leaving the die, many tiny steam bubbles were generated, thus forming an expanded structure in extruded food (Yagei and Gogus, 2008). The binding of dietary fiber and water is stronger than starch, which could inhibit the water loss and reduce its expansion ability (Tsokolar-Tsikopoulos et al., 2015). This is supported by the earlier findings that wheat fiber could cause rupture of gas cells in advance and thus reduce expansion and porosity of extruded cornstarch snack (Yanniotis et al., 2007). Fat would provide lubricating effect to starch, reducing gelatinization degree and mechanical energy loss, and thus reduced the expansion during the extrusion process (Mercier et al., 1980). Therefore, the expansion ratio of FER would increase if the dietary fiber and fat contents decreased, and result validated this.

The instrumental color measurements included measurements of parameters L* ((L*=0 [black] and L*=100 [white])), a* (amount of red tone (+a*) or green tone (−a*)) and b* (amount of yellow (+b*) or blue tone (−b*)). Compared to CK, lightness (L* value) of FER gradually increased with increase in the rice flour ratio significantly (p < 0.05) (Table 1 and Fig. 1). Lightness of CK was 19.20, while those of 25% to 45% rice-added samples were 27.62, 31.30, 38.18, 46.68 and 58.82 respectively. Rice flour addition caused a significant reduction in a* and b* values of FER compared to that of CK, which mean the decrease in redness and yellowness of the product. Meanwhile, the transparency of FER was also significantly increased (ranged from 1.61 to 4.79) compared with CK (0.64).

Fig. 1.

Photographs and transverse section SEM images (Magnification is 700×) of FER with the addition of different natural selenium-enriched rice flour ratios.

Appearance and microstructure of FER    Fig. 1 showed the appearance of FER with different rice flour ratios. The shape of CK was shrivelled and irregular, and the color was relative dark without gloss. Compared with CK, the appearance of FERs turned to plump and translucent. When selecting natural selenium-enriched rice as quality improver, with the rice flour ratio increased from 25% to 45%, the color of FERs became more and more lighter and changed from yellowish-brown to off-white, which was in agreement with previous change of L*, a* and b* values. Appearance is an important physical property of foods that affects consumer acceptance (Sayanjali et al., 2019). As shown in Fig. 1, the FERs with 35%, 40% and 45% rice flour ratio presented more desirable appearance because of more plump shape, higher lightness and better transparency.

Fig. 1 also showed the transverse section SEM images of FERs with different rice flour ratios. As shown in figure, natural selenium-enriched rice flour ratio had considerable effect on inner structure of FER, which became more and more looser with the rice flour ratio increased from 25% to 45%. The inner structure of CK and 25% rice-added sample were compactness and dense, however, increasing the rice flour ratio from 30% to 45% resulted in a structure where the pores appeared more numerous and the walls thinner, and cellular structure appeared more disorganized. The addition of rice flour reduced the contents of lipid and dietary fiber, which promoted the moderate puffing at the high temperature, high pressure and high shear environment during FER processing.

Analysis of cooked FER Cooking properties of FER

Cooking properties are normally used as a criterion for consumer acceptability (Duangkhamchanet et al., 2017). The effect of different natural selenium-enriched rice flour ratio on cooking properties of FER including water uptake ratio (WUR), elongation ratio and solid loss (SL) was shown in Table 2. High WUR was considered as good feature of rice (Sanjiva et al., 1952). WUR of CK was 215%, comparing with CK, WUR of FERs were significantly increased (p < 0.05) from 226% to 245% by increasing the rice flour ratio from 25% to 45%. On one hand, a disorganized and loose cellular structure promotes the increase of water absorption during cooking, resulting in softer cooked grain (Lisle et al., 2000). On the other hand, the increase of WUR resulted from the reduction of lipid content with increasing rice flour ratio. Previous investigators showed starch and lipid could form starch-lipid complexes, which could prevent starch from binding to water (De et al., 2012; Derycke et al., 2005). The elongation ratio and SL in treated groups of the increase of rice flour ratio also significantly increased (p < 0.05) (ranged from 1.36 to 2.37, and 13.87 to 32.92 mg/g respectively) compared with CK (1.13 and 11.86 mg/g respectively). It is postulated that the relative looser structure lead to higher SL. However, excessive SL was undesirable because of a large number loss of nutrients such as protein and microelements (Liu et al., 2018).

Table 2. Effect of natural selenium-enriched rice flour ratios on cooking properties of FER.

RFR (%)	WUR (%)	Elongation ratio	SL (mg/g)
CK	215±2.66a	1.13±0.015a	11.86±0.32a
25	226±1.04b	1.36±0.013b	13.87±0.16b
30	229±1.15c	1.67±0.012c	14.67±1.39b
35	234±1.53d	1.83±0.015d	19.60±0.76c
40	238±1.37e	1.96±0.020e	21.23±0.13d
45	245±1.00f	2.37±0.015f	32.92±0.77e

Data was represented as means ± SD (n=3). Means with different letter (a,b,c,d) in a column represent that there were significant differences between groups at the p < 0.05 level.

RFR: rice flour ratio; WUR: water uptake ratio; SL: solid loss.

Texture profile analysis (TPA) of FER    Texture property of cooked rice plays an important role in consumers' preferences (Duangkhamchan et al., 2017), and most of east Asians like rice with relatively soft and sticky mouthfeel. As shown in Table 3, the textural properties of the cooked FERs with different natural selenium-enriched rice flour ratio added were analyzed using TPA. An increase rice flour ratio of 25% ∼ 45% resulted in a significant decrease (p < 0.05) of hardness, gumminess and chewiness, probably because of the decrease of protein and TDF contents (Table 1). Comparing with CK (1 864 g), the hardness of FERs was significantly decreased from 1 621 g to 1 025 g by increasing the rice flour ratio from 25% to 45%. Onate et al. (1964) conducted that cooked rice with higher protein presented harder texture than rice with low protein. Merca et al. (1981) also reported a similar result that protein content positively correlated with cooked rice hardness. As for dietary fiber, it was reported that the addition of wheat fiber reduced the radial expansion and increased the hardness of extruded corn starch (Yanniotis et al., 2007). Extrudates with a high fiber and protein content usually have compact, tough, not crisp and undesirable texture (Lue et al., 1991). As a result, under the same processing conditions, reducing the protein and TDF contents could reduce the hardness of FER. These conclusions were confirmed by the correlation between centesimal composition and textural parameters of FER (Table 4). Hardness had significant positive correlation with protein content (r = 0.935, p < 0.01) and TDF content (r = 0.926, p < 0.01). Springness had significant negative correlation with protein content (r = −0.982, p < 0.01) and TDF content (r = −0.984, p < 0.01). Moreover, there were significant correlation among all textural parameters. Hardness had a very strong positive correlation with gumminess and chewiness (r = 0.915, p < 0.01; r = 0.930, p < 0.01) while showed an inverse relation with springiness and resilience (r = −0.951, p < 0.01; r = −0.887, p < 0.01), which consistent with former studies on rice (Singh et al., 2005).

Table 3. Effect of natural selenium-enriched rice flour ratios on textural parameters of FER.

RFR (%)	Hardness (g)	Springiness	Gumminess (g)	Chewiness (g)	Resilience
CK	1864±20.11f	0.38±0.0104a	2213±40.78f	1985±29.09f	0.16±0.0049a
25	1621±70.86e	0.66±0.0141b	1239±10.06e	1098±72.00e	0.33±0.0140b
30	1463±40.15d	0.76±0.0078c	1122±23.45d	949±26.85d	0.35±0.0193c
35	1224±32.44c	0.83±0.0120d	934±17.78c	787±42.85c	0.36±0.0160c
40	1123±44.09b	0.89±0.0080e	840±12.34b	637±28.86b	0.38±0.0084d
45	1025±32.45a	0.92±0.0053f	721±31.66a	497±23.52a	0.43±0.0143e

Data was represented as means ± SD (n=10). Means with different letter (a, b, c, d) in a column represent that there were significant differences between groups at the p < 0.05 level.

RFR: rice flour ratio.

Table 4. Pearson correlation coefficients for the relationship between protein, TDF content and textural parameters of FER.

	Protein	TDF	Hardness	Springiness	Gumminess	Chewiness	Resilience
Protein	1
TDF	0.994**	1
Hardness	0.935**	0.926**	1
Springiness	−0.982**	−0.984**	−0.951**	1
Gumminess	0.987**	0.993**	0.915**	−0.988**	1
Chewiness	0.994**	0.995**	0.930**	−0.992**	0.995**	1
Resilience	−0.980**	−0.986**	−0.887**	0.968**	−0.980**	−0.982**	1

*  p < 0.05

**  p < 0.01

TDF: total dietary fiber

Sensory Evaluation of FER    Ten panelists were invited to evaluate the cooked FER of 6 treatments and the result was showed in Table 5. The panelists were requested to express their overall acceptability by taking appearance, color, odor and taste into account. The effect of natural selenium-enriched rice flour ratio on the sensory evaluation was varied in different formulas. The overall acceptability of FERs initially significantly increased from 25% to 35% rice flour ratio and then decreased from 35% to 45% rice flour ratio. The mean scores of overall acceptability of FER with different ratios of rice flour were between 2.69 to 7.31. In all the formulas, the extruded rice with 35% rice flour ratio was generally preferred (7.31 out of 9) because it had the highest evaluation scores of appearance, odor and taste among all treatments.

Table 5. Effect of natural selenium-enriched rice flour ratios on sensory properties of FER.

RFR (%)	Appearance	Color	Odor	Taste	Overall Acceptability
CK	2.14±0.17a	2.02±0.14a	1.92±0.15a	2.20±0.52a	2.69±0.37a
25	4.45±0.29b	3.12±0.08b	4.27±0.12b	3.32±0.39b	3.50±0.33b
30	5.70±0.08d	3.64±0.12c	4.95±0.10d	4.30±0.29c	4.06±0.29c
35	7.72±0.21f	6.33±0.29d	7.62±0.56f	7.18±0.19f	7.31±0.28f
40	5.21±0.28c	6.92±0.10e	6.04±0.12e	5.68±0.26e	6.16±0.17e
45	7.31±0.10e	7.79±0.27f	4.52±0.07c	4.82±0.11d	5.03±0.15d

Data was presented as means ± SD (n=10). Means with different letter (a, b, c, d) in a column represent that significant differences between groups at the p < 0.05 level.

RFR: rice flour ratio.

Vitro bile acid binding capacity of FER    Vitro binding assay based on gastrointestinal conditions has been widely used to evaluate the potential bile acid-binding capacity of sample (Panith et al., 2016). Bile acids are synthesized from cholesterol in the liver, which are mainly in the form of sodium taurocholate (ST) and sodium glycocholate (SG) in human body. Majority of the bile acids is actively reabsorbed by terminal ileum after emulsification with fat in small intestine. If bile acid was bound to something of food in small intestine, conjugate was then excreted with feces and thus led to a reduction the blood cholesterol level (Zhou et al., 2019). In this study, 35% FER was selected for further study on its bile acid-binding capacity because of the best product properties. To calculate the remaining ST and SG concentration of the supernatant, calibration curves of A₁ = 0.3656C₁ + 0.005 (R² = 0.9992) and A₂ = 0.332C₂+0.0156 (R² = 0.9985) were established between absorbance (A) and concentration (C), and the two bile acids binding rate of FER were shown in Fig. 2.

Fig. 2.

In vitro bile acid binding assay of 35% FER. Time-effect relation of 35% FER by ST (A) and by SG (B). The results are represented as means ± SD (n=3). Different letters represent significant differences for each sampling time at the p < 0.05 level.

Variation with time, the bile acid binding rates of FER were stronger than those of normal natural rice. The ST and SG binding rate of FER were found to be in the range of 14.6–22.5% and of 10.2–20.6%, whereas those of normal natural rice were of 3.1–14.2% and of 2.7–13.4%, respectively. The superiority of FER as a bile acid-binding agent was more clearly evident, and higher bile acid binding rate by FER observed here is encouraging, which illustrated that FER has the better potential function of reducing the cholesterol level than normal natural rice. This may be due to the composition of FER. Food components such as β-glucan had high ability of bile acid binding and prevented the reabsorption of bile acid (Kim and Kim, 2017). Oat was rich in β-glucan (Hu et al., 2014; Peng et al., 2013), so oat-rice FER had higher bile acid binding rate than normal natural rice. Additionally, of note, no matter FER and normal natural rice, the bile acid binding rates increased with time, but mainly in the first 60 minutes.

Conclusion

In this study, a new-type FER based on naked oat flour was reported and the quality of FER was significantly improved after adding natural selenium-enriched rice flour. The FER had relatively loose structure (SEM), moderate protein, crude fat and TDF contents. The shape of FER turned to regular and the color, the flavor of extruded rice grains were acceptable, especially in 35%, 40% and 45% (w/w) rice flour level samples. In comparison with CK, FERs of adding rice flour had higher WUR, elongation ratio and SL. All treatments had a desirable lower hardness and higher springiness than CK. The overall acceptability of FER initially significantly increased (25% to 35%) and then decreased (35% to 45%) with increasing rice flour ratio. In all the formulas, 35% FER had the highest sensory evaluation score (7.31 out of 9). Moreover, the bile acid binding rates of 35% FER were stronger than those of normal natural rice. All of the above illustrated 35% FER was one of good substitute for staple food with the potential function of reducing the cholesterol level, and the actual functional activity needs to be confirmed in vivo studies.

Acknowledgements    This study was supported by a grant from the Science and Technology Cooperation Program of University and Hebei Province (2019HB0101), the National Key Research and Development Program of China (2017YFD0401305) and National Innovation Methodology Program of China (2017IM010800).

References

•  Alvarez-Martinez,  L.,  Kondury,  K. P., and  Harper,  J.M. (1988). A general model for expansion of extruded products. J. Food Sci., 53, 609-615.
•  De,  P. T.,  Derossi,  A.,  Talja,  R.A.,  Jouppila,  K., and  Severini,  C. (2012). Starch-lipid complex formation during extrusion-cooking of model system (rice starch and oleic acid) and real food (rice starch and pistachio nut flour). Eur. Food Res. Technol., 234, 517-525.
•  Derycke,  V.,  Vandeputte,  G.E.,  Vermeylen,  R.,  De,  M. W.,  Goderis,  B.,  Koch,  M.H.J., and  Delcour,  J.A. (2005). Starch gelatinization and amylose-lipid interactions during rice parboiling investigated by temperature resolved wide angle X-ray scattering and differential scanning calorimetry. J. Cereal Sci., 42, 334-343.
•  Duangkhamchan,  W.,  Phomphai,  A.,  Wanna,  R.,  Wiset,  L.,  Laohavanich,  J.,  Ronsse,  F., and  Pieters,  J.G. (2017). Infrared heating as a disinfestation method against Sitophilus oryzae and its effect on textural and cooking properties of milled rice. Food Bioprocess. Technol., 10, 284-295.
•  Eyenga,  E. F.,  Tang,  E. N.,  Achu,  M. B. L.,  Boulanger,  R.,  Mbacham,  W. F., and  Ndindeng,  S. A. (2020). Physical, nutritional, and sensory quality of rice–based biscuits fortified with safou (Dacryodes edulis) fruit powder. Food Sci. Nutr., 8, 3413-3424.
•  Geng,  D.,  Zhou,  S.,  Wang,  L.,  Zhou,  X.,  Liu,  L.,  Lin,  Z.,  Qin,  W.,  Liu,  L., and  Tong,  L. (2020). Effects of slight milling combined with cellulase enzymatic treatment on the textural and nutritional properties of brown rice noodles. LWT Food Sci. Technol., 128, 1-9.
•  Gopirajah,  R. and  Muthukumarappan,  K. (2018). Effect of extrusion process conditions on the physical properties of tef–oat healthy snack extrudates. J. Food Process. Preserv., 42, 1-9.
•  Guy,  R.C.E. (1993). Raw materials for extrusion cooking processes. In “The technology of extrusion cooking,” ed. by  Frame  N. D., APV Baker Ltd., Peterborough, 52-72.
•  Hu,  X.Z.,  Zheng,  J.M.,  Li,  X.P.,  Xu,  C., and  Zhao,  Q. (2014). Chemical composition and sensory characteristics of oat flakes: A comparative study of naked oat flakes from China and hulled oat flakes from western countries. J. Cereal Sci., 60, 297-301.
•  Kadan,  R.S.,  Bryant,  R.J., and  Miller,  J.A. (2008). Effects of Milling on Functional Properties of Rice Flour. J. Food Sci., 73, 151-154.
•  Kang,  T.,  Sohn,  K. H.,  Yoon,  M.,  Lee,  J., and  Ko,  S. (2015). Effect of the shape of rice starch granules on flour characteristics and gluten-free bread quality. Int. J. Food Sci. Technol., 50, 1743-1749.
•  Kim,  H.J. and  Kim,  H.J. (2017). Physicochemical characteristics and in vitro bile acid binding and starch digestion of β-glucans extracted from different varieties of Jeju barley. Food Sci. Biotechnol., 26, 1501-1510.
•  Kumar,  R.,  Martin,  Xavier,  K.A.,  Lekshmi,  M.,  Balange,  A., and  Gudipati,  V. (2018). Fortification of extruded snacks with chitosan: Effects on techno functional and sensory quality. Carbohydr. Polym., 194, 267-273.
•  Lang,  G. H.,  Kringel,  D. H.,  Acunha,  T. D. S.,  Ferreira,  C. D.,  Dias,  Á. R. G.,  Zavareze,  E. D. R., and  Oliveira,  M. D. (2020). Cake of brown, black and red rice: Influence of transglutaminase on technological properties, in vitro starch digestibility and phenolic compounds. Food Chem., 318, 1-10.
•  Lisle,  K.J.,  Martin,  M., and  Fitzgerald,  M.A. (2000). Chalky and transulent rice grains differ in starch composition and structure and cooking properties. Cereal Chem., 77, 627-632.
•  Liu,  X.,  Zhao,  J.F.,  Zhang,  X.,  Li,  Y.,  Zhao,  J.,  Li,  T.T.,  Zhou,  B.Y.,  Yang,  H.Y., and  Qiao,  L.P. (2018). Enrichment of soybean dietary fiber and protein fortified rice grain by dry flour extrusion cooking: the physicochemical, pasting, taste, palatability, cooking and starch digestibility properties. RSC Adv., 8, 26682-26689.
•  Liu,  Y.,  Hsieh,  F.,  Heymann,  H., and  Huff,  H.E. (2000). Effect of process conditions on the physical and sensory properties of extruded oat-corn puff. J Food Sci, 65, 1253-1259.
•  Lue,  S.,  Hsieh,  F., and  Huff,  H.E. (1991). Extrusion cooking of corn meal and sugar beet fiber: effects on expansion properties, starch gelatinization, and dietary fiber content. Cereal Chem., 68, 227-234.
•  Merca,  F.E.,  Juliano,  B.O., and  Banos,  L. (1981). Physicochemical properties of starch of intermediate–amylose and waxy rices differing in grain quality. Starch/Starke, 33, 253-260.
•  Mercier,  C.,  Charbonniere,  R., and  Grebaut,  J. (1980). Formation of amylose-lipid complexes by twin-screw extrusion cooking of manioc starch. Cereal Chem., 57, 4-9.
•  Mrazova,  J.,  Gazarova,  M.,  Kopcekova,  J.,  Kolesarova,  A.,  Bucko,  O., and  Bobcek,  B. (2020). The effect of consumption of pork enriched by organic selenium on selenium status and lipid profile in blood serum of consumers. J Environ Sci Health B, 55, 69-74.
•  Onate,  L.,  Mundo,  A.D., and  Juliano,  B.O. (1964). Relationship between protein content and eating quality of milled rice. Philipp. Agric., 47, 441-444.
•  Panith,  N.,  Wichaphon,  J.,  Lertsiri,  S., and  Niamsiri,  N. (2016). Effect of physical and physicochemical characteristics of chitosan on fat-binding capacities under in vitro gastrointestinal conditions. LWT Food Sci. Technol., 71, 25-32.
•  Peng,  C.H.,  Chang,  H.C.,  Yang,  M.Y.,  Huang,  C.N.,  Wang,  S.J., and  Wang,  C.J. (2013). Oat attenuate non-alcoholic fatty liver and obesity via inhibiting lipogenesis in high fat-fed rat. J. Funct. Foods, 5, 53-61.
•  Sandrin,  R.,  Mejía,  S.M.V.,  Caon,  T., and  de,  F.A. (2019). Optimization of extrusion process parameters for preparing fiber-rich oat flour. J. Food Process Eng, 42, 1-10.
•  Sanjiva,  R. B.,  Vasudeva,  M. A.R., and  Subrahmanya,  R.S. (1952). The amylose and the amylopectin contents of rice and their influence on the cooking quality of the cereal. Proc. Indian Acad. Sci, 36B, 70-80.
•  Sayanjali,  S.,  Sanguansri,  L.,  Ying,  D.Y.,  Buckow,  R.,  Gras,  S., and  Augustin,  M.A. (2019). Extrusion of a curcuminoid-enriched oat fiber-corn-based snack product. J. Food Sci., 84, 284-291.
•  Shen,  L.Y.,  Zhu,  Y.,  Wang,  L.,  Liu,  C.H.,  Liu,  C., and  Zheng,  X.Z. (2019). Improvement of cooking quality of germinated brown rice attributed to the fissures caused by microwave drying. J. Food Sci. Technol., 56, 2737-2749.
•  Singh,  N.,  Kaur,  L.,  Sodhi,  N.S., and  Sekhon,  K.S. (2005). Physicochemical, cooking and textural properties of milled rice from different Indian rice cultivars. Food Chem., 89, 253-259.
•  Srisawas,  W. and  Jindal,  V.K. (2007). Sensory evaluation of cooked rice in relation to water-to-rice ratio and physicochemical properties. J. Texture Stud., 38, 21-41.
•  Su,  C.W. and  Kong,  M.S. (2007). Effects of soybean oil, cellulose, and SiO2 addition on the lubrication and product properties of rice extrusion. J. Food Eng., 78, 723-729.
•  Tsokolar-Tsikopoulos,  K.C.,  Katsavou,  I.D., and  Krokida,  M.K. (2015). The effect of inulin addition on structural and textural properties of extruded products under several extrusion conditions. J. Food Sci. Technol., 52, 6170-6181.
•  Wang,  Y.,  Wang,  X., and  Wong,  Y. (2013). Generation of selenium-enriched rice with enhanced grain yield, selenium content and bioavailability through fertilisation with selenite. Food Chem., 141, 2385-2393.
•  Wu,  J.,  Li,  P.,  Shi,  Y.,  Fang,  Y.,  Zhu,  Y.,  Fan,  F.,  Pei,  F.,  Xia,  J.,  Xie,  M., and  Hu,  Q. (2019). Neuroprotective effects of two selenium-containing peptides, TSeMMM and SeMDPGQQ, derived from selenium-enriched rice protein hydrolysates on Pb²⁺- induced oxidative stress in HT22 cells. Food Chem. Toxicol.
•  Yagci,  S. and  Gogus,  F. (2008). Response surface methodology for evaluation of physical and functional properties of extruded snack foods developed from food-byproducts. J. Food Eng., 86, 122-132.
•  Yanniotis,  S.,  Petraki,  A., and  Soumpasi,  E. (2007). Effect of pectin and wheat fibers on quality attributes of extruded cornstarch. J. Food Eng., 80, 594-599.
•  Zhou,  D.,  Ma,  Z.,  Xu,  J.,  Li,  X., and  Hu,  X.Z. (2019). Resistant starch isolated from enzymatic, physical, and acid treated pea starch: Preparation, structural characteristics, and in vitro bile acid capacity. LWT Food Sci. Technol., 116, 1-7.