Quality evaluation and postprandial glycemic response of gluten-free rice bread made from two types of rice flour with different amylose contents

Abstract

Considering the growing global incidence of gluten-related disorders, we investigated the quality of, and postprandial glycemic response to, gluten-free rice bread made from high-amylose Koshinokaori rice flour (KK), using Koshihikari rice flour (DK) as a control. The specific loaf volume of KK bread was the same as that of DK bread. Upon measuring its physical properties, KK bread showed higher values of hardness and adhesiveness, and lower cohesiveness than DK bread. Fresh DK bread, with a better texture, was preferred to KK bread. However, the sensory evaluation score of toasted KK bread was closer to that of DK bread. In eight healthy subjects, the glycemic response to KK bread was significantly lower than to DK bread. These results indicate that amylose content affects bread quality and the postprandial glycemic response to gluten-free rice bread.

Introduction

Cereal crop growth and consumption play a vital role in mankind's history. Three important crops—corn, rice, and wheat—account for approximately 90% of total cereal production and are the most widely grown and consumed staple foods, globally. In recent years, gluten ingestion has been linked to several clinical disorders includes celiac disease, wheat allergy, and non-celiac gluten sensitivity (Elli et al., 2015; Cabanillas et al., 2020). Gluten-related disorder are estimated to have a global prevalence of approximately 5% (Taraghikhah et al., 2020).

Gluten is mainly found in the Western diet, and consumption thereof is increasing due to the growing replacement of rice by wheat in many countries, based on progressive Westernization (Rai et al., 2018). Consequently, the incidence of gluten-related disorders is increasing. In Japan, rice consumption has halved since the '60s, whereas annual wheat consumption per person increased from 33.5 kg in 1960, to 41.4 kg in 2019ⁱ⁾. Celiac disease is still rare in Japan; however, wheat allergy occupied 11.7% of food allergies next to egg (39.0%) and milk (21.8%)ⁱⁱ⁾. The only effective treatment for patients afflicted by celiac disease or wheat allergy, is a strictly gluten-free diet. Nowadays, the rising incidence of gluten-related disorders, promotes worldwide interest in identifying various appealing gluten-free products (Lee et al., 2018; Rai et al., 2018).

Several alternative flours with special attributes, including corn, rice, and sorghum, are available to replace or minimize the use of wheat. Rice flour is particularly popular for developing hypoallergenic gluten-free foods. Rice starches have enormous potential in the formulation of gluten-free baked products and are commercially available worldwide. As required in special diets, rice lacks gluten and has a low sodium content, with high levels of easily digested carbohydrates (Rai et al., 2018). However, when baking without gluten—the key ingredient providing structural and quality characteristics to bread—a liquid batter is derived, which produces bread with an inferior, crumbly texture, and poor color and quality after baking. Therefore, additives such as hydrocolloids, proteins, and enzymes have been applied in gluten-free bread-making to improve the rheological properties and final quality of the bread (Yano, 2019; Rai et al., 2018). Hydrocolloids are often used as thickening agents that bind water and increase dough viscosity, to improve the volume, texture, and final quality of bread (Mir et al., 2016). Many reports are available regarding the selection and optimization of hydrocolloids in formulating gluten-free products (Hanger and Arendt, 2013; Mir et al., 2016; Rai et al., 2018). Hydroxypropyl methylcellulose (HPMC) and xanthan gum are the most widely used hydrocolloids for this purpose, because of their capacity to improve product quality (Hanger and Arendt, 2013). In our previous study, we reported the preparation of gluten-free rice flour bread without major food allergens—egg, milk, and wheat flour—achieving success by using 0.8% HPMC (Osaki et al., 2009).

Foods made from fine rice flour are easily digested in the gastrointestinal tract and rapidly increase postprandial blood glucose levels. Diabetes is increasing worldwide, and in Japan, the number of people strongly suspected of having diabetes, has increased from 6.9 million to 10 million between 1997 and 2016ⁱⁱⁱ⁾. Dietary control of blood glucose levels plays a key role in managing diabetes. The postprandial glycemic response induced by rice is strongly influenced by the composition of its starch content. In this regard, amylose—higher amounts of which are more resistant to digestion (Fitzgerald et al., 2011)—is especially important. Studies have shown that high-amylose rice induces a lower glycemic response than low- or intermediate-amylose rice (Goddard et al., 1984; Trinidad et al., 2013; Zenel and Stewart, 2015; Mori et al., 2018). Several high-amylose, short-grain rice varieties that are similar to Japanese domestic rice, have been developed in recent years; Koshinokaori is one such variety. It was developed by incorporating the high-amylose properties of an Indian rice variety (Surjamukhi), with the short-grain Japanese rice, Kinuhikari (Sasahara et al., 2013). Our previous study reported observation of a lower postprandial glycemic response in healthy subjects, after ingestion of Koshinokaori that had been cooked at a high water-to-rice ratio, in an electric rice cooker (Yamaguchi et al., 2019). Moreover, both typical rice preparation methods, using either an electric rice cooker or a pressure cooker, elicited a lower glycemic response to Koshinokaori than Koshihikari rice (Yamaguchi et al., 2021). These results suggested that Koshinokaori consumption could attenuate the postprandial glycemic response, even under altered cooking conditions. Noro et al. (2016) reported the prevention of an abrupt increase in postprandial blood glucose levels, by rice bread made from the novel rice cultivars, Konayukinomai and Koshinokaori; however, their investigations were conducted using gluten-containing products. Aoki et al. (2012) evaluated the bread-making quality of 26 varieties of rice with different amylose contents and amylopectin structures, but this study was also verified under gluten-affected conditions. Accordingly, the effect of amylose content on the quality of gluten-free rice bread, and the postprandial glycemic response after ingestion thereof, remain unclear.

The purpose of this study was to evaluate the quality of gluten-free rice bread made from two types of rice flour with different amylose contents: intermediate-amylose Koshihikari and high-amylose Koshinokaori rice flours. In addition, the postprandial glycemic response to ingestion of gluten-free rice bread was investigated.

Materials and Methods

Materials    Two types of rice flour, intermediate-amylose Koshihikari rice flour (DK) and high-amylose Koshinokaori rice flour (KK), were used in this study. DK was purchased from Niigata Seifun Co., Ltd. (Niigata, Japan) and KK was obtained from Katayama Seifun Co., Ltd. (Osaka, Japan). The properties of these rice flours are listed in Table 1. The amylose content was measured at the Kenoh-Labo (Niigata, Japan), using the iodine colorimetric method. Water content was calculated as a measure of weight loss after drying at 135 °C for 2 h. The degree of starch damage was measured using a starch damage kit (Megazyme, Wicklow, Ireland), according to the manufacturer's instructions. The particle size and distribution of the rice flour were measured using a Laser Micron Sizer (LMS-2000e, Seishin Enterprise Co., Ltd., Tokyo, Japan). HPMC (SFE-4000) was obtained from Shin-Etsu Chemical Co., Ltd. (Niigata, Japan), whereas instant dried yeast (Super Camellia, Nisshin Foods Inc., Tokyo, Japan) and other ingredients were purchased from a local supermarket.

Table 1. Properties of two types of rice flour.

	DK	KK
Amylose content (%)	17.5	32.3
Water content (%)	13.7 ± 0.4	12.7 ± 0.1
Starch damage degree (%)	3.3 ± 0.1	2.4 ± 0.1
Median diameter (µm)	53.7 ± 3.4	56.1 ± 2.5
Average diameter (µm)	64.9 ± 7.2	61.8 ± 2.3

Data represents the mean ± SD for three to six indicators.

Preparation of rice bread samples    Two hundred and fifty grams of rice flour was mixed with 5 g salt, 20 g sugar, 2 g HPMC, and 12.5 g olive oil. Hereto, 225 g warm water containing 3.75 g dried yeast was added and the ingredients mixed for 3 min using a handy mixer. After the first fermentation at 25 °C for 50 min, the batter was mixed for 1 min using a handy mixer, to remove gas formed during fermentation. Thereafter, it was molded into a bread case and allowed to proof at 36 °C and 90% humidity, for approximately 60 min, before being baked in an oven at 180 °C for 45 min. Each bread was removed, placed on gauze, and cooled at 25 °C for 3 h, and used for loaf volume and color measurements. The breads were then stored in plastic bag at 25 °C until the next day for other measurements. Bread-making was repeated three times for each flour type. Nutrient compositions of the test foods are shown in Table 2.

Table 2. Nutritional data of rice bread used as test food (94g).

	DK bread	KK bread
Energy (kcal)	250	253
Water (g)	34.7	34.0
Protein (g)	3.6	4.3
Lipid (g)	3.6	3.6
Carbohydrate (g)	50.9	50.9
Sugar (g)	50.2	49.8
Dietary fiber (g)	0.7	1.1
Ash (g)	1.3	1.2
Sodium (mg)	477	484
Salt (g)	1.2	1.2

Measurement of specific loaf volume    The loaf volume of each bread was measured using the rapeseed replacement method and specific loaf volume calculated as volume per weight (mL/g).

Measurement of crust and crumb color    The color parameters, L*, a*, and b*, were measured using a colorimeter (CR-200b, Minolta Co., Ltd, Osaka, Japan) based on the CIELAB color system. Parameters are described as follows: L* = 0 (black) and L* = 100 (white), -a* = greenness and +a* = redness, -b* = blueness and +b = yellowness.

Measurement of the degree of gelatinization    Baked bread samples were frozen in liquid nitrogen, freeze-dried, and pulverized. The degree of gelatinization was measured using the beta-amylase-pullulanase method (Kainuma et al., 1981).

Measurement of physical properties    Texture analysis was performed using a texture analyzer (TPU-2CL, Yamaden, Tokyo, Japan) at a constant speed of 5.0 mm/s, with a strain rate of 75%, using an 8 mm cylindrical plunger. Bread crumb was prepared as 20 × 60 × 20 mm slabs. Hardness, cohesiveness, and adhesiveness were measured using texture profile analysis at 25 °C.

Measurement of glycemic response    In total, 10 subjects were screened, and 8 healthy subjects (two males and six females, Japanese participants, aged 20.6 ± 1.2 years, BMI 19.4 ± 1.1 kg/m²) were recruited in this study. Two subjects were excluded from screening owing to noncompliance or high fasting blood glucose levels (≧110 mg/dL). This study was approved by the research ethics committee of Niigata University (approval number: 2019-2-010) and conducted in accordance with the Declaration of Helsinki.

Koshihikari rice cooked in an electric rice cooker (NP-BC10; Zojirushi Corporation, Osaka, Japan) at a water-to-rice ratio of 1.4:1 was prepared as a reference (KH) food. DK and KK breads were prepared the day before measurement (baked approximately 20 h before) and served without toasting. The three prepared foods (KH, DK bread, and KK bread) were offered to eight subjects under a single-blind crossover design, with a washout period of at least 1 week. After 11 h of fasting, participants consumed a reference or test food containing 50 g of available carbohydrates. All foods were consumed over 10 min with 200 mL of water. Finger-prick blood samples were collected using a lancing device (Medisafe Finetouch™ II, Terumo Corporation, Tokyo, Japan) at 0 (fasting), 15, 30, 45, 60, 90, and 120 min after starting the meal. Blood glucose levels were measured using a self-testing blood glucose meter (Medisafe FIT™, Terumo Corporation). The incremental area under the curve (IAUC) was calculated based on the trapezoidal rule from 0 to 120 min for glucose, ignoring the area below the baseline value. The maximum blood glucose concentration (Cmax) was determined based on the observed data.

Sensory evaluation    Sensory evaluation was performed by 41 panelists using a 7-point scale (in which 3 = extreme like and −3 = extreme dislike). Two types of rice bread (DK and KK) and two variations of each (fresh and toasted) were presented to the panelists. The evaluated attributes of fresh bread were crust and crumb color, fine appearance, elasticity, scent, softness, chewiness, moistness, sweetness, taste, and comprehensive evaluation. Toasted bread was assessed in terms of crust and crumb color, scent, texture, chewiness, taste, and comprehensive evaluation.

Statistical analysis    Glycemic response (blood glucose levels, IAUC, and Cmax) is presented as mean ± SE, and other data are presented as mean ± SD. All statistical analyses were performed using GraphPad Prism 7.04 (GraphPad Software Inc., San Diego, CA, USA), with p < 0.05 considered to be significant. The data on bread quality were analyzed using the Student's t-test. The effects of food, time, and their interaction on blood glucose levels were analyzed using two-way repeated measures analysis of variance (ANOVA). The other measures (e.g., IAUC) were analyzed using one-way repeated measures ANOVA. Post-hoc analysis was performed using Tukey's multiple comparison test.

Results

Properties of the two types of rice flour    Table 1 shows the properties of the two types of rice flour, DK and KK. The amylose content of DK and KK was 17.5% and 32.3%, respectively. KK rice flour showed very similar properties to DK, except for its amylose content.

Properties of rice bread    The nutritional composition and properties of DK and KK breads are shown in Tables 2 and 3. Specific loaf volume of DK and KK bread were 3.60 and 3.54 mL/g, respectively, showing no significant difference. The degree of gelatinization of DK and KK bread were 58.8% and 17.1%, respectively; KK bread had a considerably lower degree of gelatinization than DK bread (p < 0.01).

Table 3. Properties of rice bread.

	DK bread	KK bread
Specific loaf volume (mL/g)	3.60 ± 0.05	3.54 ± 0.04
Degree of gelatinization (%)	58.8 ± 5.7	17.1 ± 2.6 **

Data represents the mean ± SD for three indicators.

**  p < 0.01

Color of rice bread    Table 4 shows the crust and crumb color of the breads. All indicators showed significant differences between DK and KK bread. The L* value of both the crust and crumb of KK bread was lower than that of DK bread. The a* value of both the crust and crumb of KK bread was higher than that of DK bread, whereas the b* value of the KK bread's crust was higher but that of the crumb was lower, than that of DK bread.

Table 4. Crust and crumb color of rice bread.

	Crust			Crumb
	DK bread	KK bread		DK bread	KK bread
L*	54.1 ± 0.5	51.7 ± 0.2	**	76.7 ± 1.4	74.3 ± 0.2	*
a*	9.9 ± 0.2	12.4 ± 1.5	*	−1.8 ± 0.1	−1.5 ± 0.1	**
b*	30.8 ± 0.3	38.8 ± 0.2	**	5.1 ± 0.2	3.3 ± 0.2	**

Data represents the mean ± SD for three indicators.

**  p < 0.01

*  p < 0.05

Physical properties of rice bread    The physical properties of DK and KK bread are shown in Table 5. KK bread showed higher hardness and adhesiveness values than DK bread. In contrast, the cohesiveness value of KK bread was lower than that of DK bread.

Table 5. Physical properties of rice bread.

	DK bread	KK bread
Hardness (×103 Pa)	23.1 ± 3.0	59.8 ± 6.3	**
Cohesiveness	0.74 ± 0.02	0.24 ± 0.01	**
Adhesiveness (×102 J/m3)	7.7 ± 0.9	13.6 ± 1.1	**

Data represents the mean ± SD for three indicators.

**  p < 0.01

Glycemic response    Participants' blood glucose levels after ingestion of the test foods, are shown in Fig. 1. A significant main effect of the test food was observed in blood glucose levels (p < 0.05), with KK bread causing lower blood glucose than DK bread (p < 0.05). No interaction effect of time and food on blood glucose levels (p = 0.1689) was observed. Glucose IAUC and Cmax values are shown in Table 6. The IAUC was significantly lower for KK bread than for DK bread. There were no significant differences in Cmax values between the test foods.

Fig. 1.

Blood glucose levels after ingestion of test foods.

○KH: Koshihikari cooked rice; ●DK: Koshihikari rice bread; ●KK: Koshinokaori rice bread. Data represents the mean ± SE of eight subjects.

Table 6. Glucose IAUC and Cmax of each test food.

	KH	DK	KK
Glucose IAUC (mg·min/dL)	4991 ± 670 ab	5478 ± 842 a	3801 ± 544 b
Cmax (mg/dL)	167 ± 10 a	170 ± 10 a	155 ± 9 a

KH: Koshihikari cooked rice; DK: Koshihikari rice bread; KK: Koshinokaori rice bread. Data represents the mean ± SE of eight subjects. Different letters indicate significant differences among the test foods (p < 0.05).

Sensory evaluation    Figures 2 and 3 reflect the sensory evaluation of DK and KK breads. Significant differences were observed between the two bread types, in terms of fine appearance, softness, chewiness, moistness, and comprehensiveevaluation of fresh bread (Fig. 2). However, the sensory evaluation score of toasted KK rice bread was closer to that of DK bread, and no attributes were significantly different between the two breads (Fig. 3).

Fig. 2.

Sensory evaluation of fresh rice bread.

●DK: Koshihikari rice bread; ●KK: Koshinokaori rice bread. n = 41. **p < 0.01, * p < 0.05

Fig. 3.

Sensory evaluation of toasted rice bread.

●DK: Koshihikari rice bread; ●KK: Koshinokaori rice bread. After baking and cooling, bread was warmed up in a toaster for 2 min. n = 41.

Discussion

This study revealed that differences in the amylose content of rice affect the quality of, and glycemic response to, gluten-free rice bread. Limited reports exist on studies involving gluten-free rice bread; of those that are available, many evaluate the quality of rice bread made from various rice flours with different amylose contents from different rice cultivars but include wheat flour or wheat gluten (Araki et al., 2009; Nakamura et al., 2009; Takahashi et al., 2009; Aoki et al., 2010; Takahashi et al., 2011; Aoki et al., 2012; Araki et al., 2016; Homma et al., 2016). It is known that the specific loaf volume of rice bread is related to properties of the rice flour, such as the degree of starch damage, particle size, and amylose content. It has been reported that a lower degree of starch damage and smaller particle size induce a higher specific loaf volume (Takahashi et al., 2011; Araki et al., 2016; Homma et al., 2016). In addition, Takahashi et al. (2009) reported that rice flour with low amylose content (6–10%) induced low specific loaf volume and caving in rice bread. Rice flour with medium (16–22%) and high (30–36%) levels of amylose content resulted in a relatively high specific loaf volume of rice bread. The highest specific loaf volume was obtained from rice flour with approximately 25% amylose content. Of the rice flour used in this study, the starch damage degree of KK was slightly lower than that of DK, but the other properties excluding amylose content were very similar between the two flours (Table 1). No significant difference was observed in specific loaf volume between KK and DK bread (Table 3); therefore, the amylose content of the rice flours evaluated herein, had no effect on the specific loaf volume of the gluten-free rice bread.

Concerning color measurement, all indicators showed significant differences between DK and KK bread (Table 4). The lower L* value of bread was due to Maillard browning, which is influenced by the reaction between reducing sugars and amino acids (Kent and Evans, 1994). The higher protein content of KK bread compared with DK bread could have induced a lower L* value of the crust (Tables 2 and 4).

We observed significantly higher values for hardness and adhesiveness, and lower cohesiveness, in KK bread than in DK bread. Amylose content is positively correlated with the hardness of rice bread that has been made with wheat gluten (Takahashi et al., 2009; Aoki et al., 2010; Aoki et al., 2012). Aoki et al. (2012) reported that the hardness of bread made from Koshinokaori rice flour was the highest among 26 rice cultivars with different amylose contents, and it was 5.7 times higher than that of Koshihikari rice bread. A similar hardness trend was observed in our study. Evaluation of the physical properties of KK bread revealed undesirable lower outcomes in terms of texture—such as softness, chewiness, and moistness—and comprehensive sensory evaluation of fresh rice bread (Fig. 2). The fine appearance of KK bread was evaluated to significantly surpass that of DK bread. Moreover, all evaluated attributes of toasted KK bread were closer to those of DK bread (Figs. 2 and 3), indicating that toasting improves the undesirable texture of KK bread. Several studies have been conducted to improve the quality of gluten-free rice bread by the addition of pectin, guar gum, glucomannan, trehalose, glutathione, β-amylase, or cyclodextrin glycosyl transferase (Nakamura et al., 2016; Higuchi and Koyama 2011; Yano, 2010; Ichikawa, 2017; Gujral et al., 2003). Further investigation is required to improve the texture and taste of fresh gluten-free rice bread with a high amylose content.

We found that high-amylose Koshinokaori rice elicited a significantly lower glycemic response than Koshihikari, even after ingestion of gluten-free rice flour bread (Fig. 1 and Table 6). Although particle size and the degree of starch damage in rice flour affect its digestibility, no clear differences in this regard were observed between DK and KK, in this study. As mentioned, amylose is a key component affecting the quality and digestibility of rice bread. The amount of apparent amylose has been used as a predictor of the glycemic response, and the relationship between the long-chain (LC) amylopectin that makes up apparent amylose and the glycemic response has been reported (Guzman et al., 2017). There are two types of high-amylose rice varieties: varieties with high true amylose (TA) and varieties with high LC. Using the estimated formula by Nakamura et al. (2015), the LC content of Koshinokaori is 13.1%, suggesting that Koshinokaori is a high TA type variety. A previous study by Noro et al. (2016) examined three types of rice flour: high-LC Konayukinomai rice, high-amylose Koshinokaori rice, and conventional Koshihikari rice. Konayukinomai and Koshinokaori bread prevented an abrupt increase in blood glucose and showed a low glycemic index value compared to Koshihikari bread, likely due to the high amount of slowly digestible and resistant starch (RS). Although the RS content was not measured in this study, it is reportedly higher in Koshinokaori than in Koshihikari rice (Enoki et al., 2020, Nakayoshi et al., 2015). Moreover, it was confirmed that the dietary fiber content of KK bread was slightly higher than that of DK bread. As shown in Table 3, the degree of gelatinization of KK bread (17.1%) was lower than that of DK (58.8%), whereas that of cooked Koshihikari rice (KH) was proven to be as high as 92.9%, in our previous study (Yamaguchi et al., 2021). Gelatinization leads to swelling of starch granules, increasing accessibility for digestive enzymes (Tester and Sommerville, 2003). Apart from the amounts of amylose, RS, and dietary fiber, the low degree of gelatinization may contribute to the low glycemic response to KK bread. Nakayoshi et al. (2015) reported that the gelatinization and retrogradation characteristics of Koshinokaori, which had the highest RS content, were significantly different from those of other high-amylose rice cultivars, Hoshiyutaka and Yumetoiro. Therefore, further analysis of starch structure and characteristics in detail is expected to improve our understanding of starch digestibility and the blood glucose response.

In conclusion, this study showed that the characteristics of the rice flour used, have a significant impact on the quality of, and postprandial glycemic response to, the resultant gluten-free rice bread. Differences in physical properties were ascribable to the different amylose contents in Koshinokaori and Koshihikari rice flours. Improvement in the postprandial glycemic response after ingestion of Koshinokaori rice bread—from a rice variety that is high in amylose—demonstrates the health benefits of these new rice varieties. The wide prevalence of celiac disease and wheat allergy has led to a growing demand for gluten-free foods. Further research is needed on the quality and glycemic response of Koshinokaori rice flour when applied to a variety of gluten-free foods other than rice flour bread.

Acknowledgements    The authors are grateful to Dr. Wataru Noro (Food Research Center, Niigata Prefecture) for advice on rice flour properties. We also thank the students at Niigata University for their cooperation in this study.

Conflict of interest    There are no conflicts of interest to declare.

Abbreviations

DK

Koshihikari rice flour

HPMC

Hydroxypropyl methylcellulose

KH

Koshihikari rice

KK

Koshinokaori rice flour

RS

Resistant starch

References

•  Aoki,  N.,  Umemoto,  T., and  Suzuki,  Y. (2010). Properties of bread made using wheat gluten and rice flour from high-yielding cultivars. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 57, 107-113 (in Japanese).
•  Aoki,  N.,  Umemoto,  T.,  Hamada,  S.,  Suzuki,  K., and  Suzuki,  Y. (2012). The amylose content and amylopectin structure affect the shape and hardness of rice bread. J. Appl. Glycosci., 59, 75-82.
•  Araki,  E.,  Ikeda,  T.M.,  Ashida,  K.,  Takata,  K.,  Yanaka,  M., and  Iida,  S. (2009). Effects of rice flour properties on specific loaf volume of one-loaf bread made from rice flour with wheat vital gluten. Food Sci. Technol. Res., 15, 439-448.
•  Araki,  E.,  Ashida,  K.,  Aoki,  N.,  Takahashi,  M., and  Hamada,  S. (2016). Characteristics of rice flour suitable for the production of rice flour bread containing gluten and methods of reducing the cost of producing rice flour. Jpn. Agric. Res. Q., 50, 23-31.
•  Cabanillas,  B. (2020). Gluten-related disorder: Celiac disease, wheat allergy, and nonceliac gluten sensitivity. Crit. Rev. Food Sci.Nutr., 60, 2606-2621.
•  Elli,  L.,  Branchi,  F.,  Tomba,  C.,  Villalta,  D.,  Norsa,  L.,  Ferretti,  F.,  Roncoroni,  L., and  Bardella,  M.T. (2015). Diagnosis of gluten related disorders: Celiac disease, wheat allergy and non-celiac gluten sensitivity. World J. Gastroenterol., 21, 7110-7119.
•  Enoki,  Y.,  Maejima,  D.,  Kubo,  K., and  Nagashima,  Y. (2020). Evaluation of postprandial glycemic response and taste of rice salad made with high-amylose rice “Koshinokaori”. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 67, 339-344 (in Japanese).
•  Fitzgerald,  M.A.,  Rahman,  S.,  Resurreccion,  A.P.,  Concepcion,  J.,  Daygon,  V.D.,  Dipti,  S.S.,  Kabir,  K.A.,  Klingner,  B.,  Morell,  M.K., and  Bird,  A.R. (2011). Identification of a major genetic determinant of glycaemic index in rice. Rice, 4, 66-74.
•  Goddard,  M.S.,  Young,  G., and  Marcus,  R. (1984). The effect of amylose content on insulin and glucoese responses to ingested rice. Am. J. Clin. Nutr., 39, 388-392.
•  Gujral,  H.S.,  Guardiola,  I.,  Carbonell,  J.V., and  Rosell,  C.M. (2003). Effect of cyclodextrinase on dough rheology and bread quality from rice flour. J. Agric. Food Chem., 51, 3814-3818.
•  Guzman,  M.K.,  Parween,  S.,  Butardo,  V.M.,  Alhambra,  C.M.,  Anacleto,  R.,  Seiler,  C.,  Bird,  A.R.,  Chow,  C.P., and  Sreenivasulu,  N. (2017). Investigating glycemic potential of rice by unraveling compositional variations in mature grain and starch mobilization patterns during seed germination. Sci. Rep., 7, 5854.
•  Hager,  A.S. and  Arendt,  E.K. (2013). Influence of hydroxypropylmethylcellulose (HPMC), xanthan gum and their combination on loaf specific volume, crumb hardness and crumb grain characteristics of gluten-free breads based on rice, maize, teff and buckwheat. Food Hydrocolloids, 32, 195-203.
•  Higuchi,  S. and  Koyama,  K. (2011). Productive methods and textures contained guargum and trehalose of rice powder breads and its texture. J. Integr. Stud. Diet. Habits (Nihon Shokuseikatsu Gakkaishi), 21, 298-306 (in Japanese).
•  Homma,  N.,  Takahashi,  M., and  Yoshii,  Y. (2016). The influence of rice properties on flour milling and the relationship between rice flour quality and bread production. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 63, 551-560 (in Japanese).
•  Ichikawa,  T. (2017). Effects of β-amylase on the properties of gluten-free bread prepared with finely powdered rice flour. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 64, 584-590 (in Japanese).
•  Kainuma,  K.,  Matsunaga,  A.,  Itagawa,  M., and  Kobayashi,  S. (1981). New enzyme system–beta-amylase-pullulanase–to determine the degree of gelatinization and retrogradation of starch or starch products. J. Jpn. Soc. Starch Sci. (Denpun Kagaku), 28, 235-240 (in Japanese).
•  Kent,  N.L. and  Evars,  A.D. (1994) Bread made with gluten substitutes. In “Kent's technology of cereals : an introduction for students of food science and agriculture”, Oxford UK, Pergamon Press, pp.215.
•  Lee,  Y.,  Park,  J., and  Lee,  J-S. (2018). Gluten-free diet: A review focusing on wise food choices. EC Nutrition, 13, 676-681.
•  Mir,  S.A.,  Shah,  M.A.,  Naik,  H.R., and  Zargar,  I.A. (2016). Influence of hydrocolloids on dough handling and technological properties of gluten-free breads. Trends Food Sci. Technol., 51, 49-57.
•  Mori,  H.,  Mori,  M.,  Ikuta,  Y.,  Enoki,  Y.,  Sasagawa,  K.,  Tanaka,  S.,  Kida,  N.,  Nomura,  T., and  Yamori,  Y. (2018). Effect of different rice cooking conditions on glycemic response of Japanese high amylose rice variety “Koshinokaori”: a double-blind, randomized, crossover study. J. Metabol. Clin. Nutr. (Nihon Byotai Eiyo Gakkaishi), 21, 237-246 (in Japanese).
•  Nakamura,  R.,  Teshima.  Y.,  Miura,  M., and  Konishi,  F. (2016). Effect of adding glucomannan on the rheological properties, sensory characteristics and staling of gluten-free rice bread. J. Home Econ. Jpn. (Nihon kasei Gakkaishi), 67, 141-150 (in Japanese).
•  Nakamura,  S.,  Suzuki,  K., and  Ohtsubo,  K. (2009). Characteristics of bread prepared from wheat flours blended with various kinds of newly developed rice flours. J. Food Sci., 74, E121-130.
•  Nakamura,  S.,  Satoh,  H., and  Ohtsubo,  K. (2015). Development of formulae for estimating amylose content, amylopectin chain length distribution, and resistant starch content based on the iodine absorption curve of rice starch. Biosci. Biotechnol. Biochem., 79, 443-455.
•  Nakayoshi,  Y.,  Nakamura,  S.,  Kameo,  Y.,  Shiiba  D.,  Katsuragi,  Y., and  Ohtsubo,  K. (2015). Measurement of resistant starch content in cooked rice and analysis of gelatinization and retrogradation characteristics. Biosci. Biotechnol. Biochem., 79, 1860-1866.
•  Noro,  W.,  Akaishi,  R.,  Nakamura,  S.,  Ohtsubo,  K.,  Maeda,  H., and  Yoshii,  Y. (2016). Prevention of abrupt increases in postprandial blood glucose levels by rice bread made with the novel rice cultivar “Konayukinomai”. Food Sci. Technol. Res., 22, 793-799.
•  Osaki,  A.,  Yamaguchi,  T., and  Odani,  S. (2009). Making of gluten free rice flour bread without major food allergens. Abstracts of Annual Congress of Jpn. Soc. Home Econ. 61, 130 (in Japanese).
•  Rai,  S.,  Kaur,  A., and  Chopra,  C.S. (2018). Gluten-free products for celiac susceptible people. Front Nutr., 5, 116.
•  Sasahara,  H.,  Miura,  K.,  Shimizu,  H.,  Goto,  A.,  Shigemune,  A.,  Nagaoka,  I.,  Uehara,  Y.,  Kobayashi,  A.,  Ohta,  H.,  Fukui,  K.,  Otsuki,  H.,  Yano,  M., and  Komaki,  Y. (2013). ‘Koshinokaori’, a new rice noodle cultivar. Bull. NARO. Agric. Res. Cent. (Chuo Nogyo Sogo Kenkyu Senta Kenkyu Hokoku), 19, 15-29 (in Japanese).
•  Takahashi,  K.,  Okunishi,  T.,  Suzuki,  K., and  Yukizaki,  C. (2011). Processing suitability evaluation method for rice bread and evaluation of rice flour produced in Miyazaki. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 58, 55-61 (in Japanese).
•  Takahashi,  M.,  Homma.  N.,  Morohashi,  K.,  Nakamura,  K., and  Suzuki,  Y. (2009). Effect of rice cultivar characteristics on the rice flour bread quality. J. Jpn. Soc. Food Sci. Thechnol. (Nippon Shokuhin Kagaku Kogaku Kaishi), 56, 394-402 (in Japanese).
•  Taraghikhah,  N.,  Ashtari,  S.,  Asri,  N.,  Shahbazkhani,  B.,  Al-Dulaimi,  D.,  Rostami-Nejad,  M.,  Rezaei-Tavirani,  M.,  Razzaghi,  M.R., and  Zali,  M.R. (2020). An updated overview of spectrum of gluten-related disorders: clinical and diagnostic aspects. BMC Gastroenterol., 20, 258.
•  Tester,  R.F. and  Sommerville,  M.D. (2003). The effects of non-starch polysaccharides on the extent of gelatinisation, swelling and α-amylase hydrolysis of maize and wheat starches. Food Hydrocolloids, 17, 41-54.
•  Trinidad,  T.P.,  Mallillin,  A.C.,  Encabo,  R.R.,  Sagum,  R.S.,  Felix,  A.D., and  Juliano,  B.O. (2013). The effect of apparent amylose content and dietary fibre on the glycemic response of different varieties of cooked milled and brown rice. Int. J. Food Sci. Nutr., 64, 89-93.
•  Yamaguchi,  T.,  Enoki,  Y.,  Sasagawa,  K., and  Fujimura,  S. (2019). Evaluation of postprandial glycemic response and physical properties of high-amylose rice “Koshinokaori”. J. Nutr. Sci. Vitaminol., 65, S117-S121.
•  Yamaguchi,  T.,  Kobayashi,  M.,  Mizutani,  M.,  Fujimura,  S., and  Enoki,  Y. (2021). Effect of cooking conditions on postprandial glycemic response and eating qualities of high-amylose rice “Koshinokaori”. Food Sci. Technol. Res., 27, 161-167.
•  Yano,  H. (2010). Improvements in the bread-making quality of gluten-free rice batter by glutathione. J. Agric. Food Chem., 58, 7949-7954.
•  Yano,  H. (2019). Recent practical researches in the development of gluten-free breads. NPJ Sci. Food, 3, 7.
•  Zenel,  A.M. and  Stewart,  M.L. (2015). High amylose white rice reduces post-prandial glycemic response but not appetite in humans. Nutrients, 7, 5362-5374.
• i) https://www.maff.go.jp/j/tokei/kouhyou/zyukyu/index.html (Feb. 22, 2022) (in Japanese)
• ii) https://www.jspaci.jp/allergy_2016/chap03.html (Feb. 22, 2022) (in Japanese)
• iii) https://www.mhlw.go.jp/stf/wp/hakusyo/kousei/18/backdata/01-01-02-08.html (Mar. 19, 2022) (in Japanese)