Effect of cooking conditions on postprandial glycemic response and eating qualities of high-amylose rice “Koshinokaori”

Abstract

High-amylose rice elicits a mild glycemic response; however, the effect of cooking condition on the glycemic response remains unclear. We aimed to investigate the effect of pressure-cooker cooking on postprandial glycemic response and eating qualities of the high-amylose rice “Koshinokaori”; and rice cooked in an electric rice cooker (KKE) was compared with that cooked in a pressure cooker (KKP). The intermediate-amylose rice “Koshihikari”, cooked in an electric rice cooker (KHE), was the reference. In ten healthy subjects, blood glucose variation was significantly lower after consumption of KKP (45 to 120 min) and KKE (60 to 120 min) compared to KHE. Incremental area under the curve after KKE and KKP intake was significantly lower than that after KHE intake. While the hardness of KKP was the highest, strength and stickiness were similar for all three samples. The sensory evaluation score of KKP was closer to that of KHE than KKE, indicating that KKP is a better choice for glycemic control.

Introduction

Rice is a staple food in Japan and many other Asian countries. It accounts for approximately 30% of the total energy intake and half of the carbohydrate intake among Japanese people. However, epidemiological studies have reported that ingesting white rice raises the risk of type 2 diabetes (Nanri et al., 2010; Hu et al., 2012). In 2016, 20 million people in Japan were reported to suffer from diabetes (Ministry of Health, Labour and Welfare, 2017). There are many varieties of rice in the world that vary considerably in the postprandial glycemic responses that they elicit. The postprandial glycemic response induced by rice is strongly influenced by the composition of starch, especially amylose content. In general, Japanese rice has an amylose content of 20% or less (intermediate-amylose rice); however, there are also varieties with high amylose content (high-amylose rice; amylose 25% or more). Higher amounts of amylose are more resistant to digestion (Fitzgerald et al., 2011). Studies have shown that high-amylose rice induces a lower glycemic response than low- or intermediate-amylose rice (Mori et al., 2018; Zenel and Stewart, 2015; Trinidad et al., 2013; Goddard et al., 1984).

Several high-amylose, short-grain rice varieties have been developed in recent years that are similar to Japanese domestic rice; Koshinokaori is one such variety. It was developed by incorporating the high-amylose properties of an Indian rice variety, Surjamukhi, into a short-grain Japanese rice, Kinuhikari (Sasahara et al., 2013). While it solved problems related to polishing suitability of high-amylose rice, it led to major disadvantages, such as undesirable texture and rapid retrogradation following cooking. Our previous study reported that the optimal cooking conditions for Koshinokaori, which involves a high water-to-rice ratio, produces more desirable physical properties and favorable sensory evaluation (Enoki et al., 2018). In addition, a lower postprandial glycemic response after intake of Koshinokaori, cooked with a high water-to-rice ratio in an electric cooker, was observed in healthy subjects (Yamaguchi et al., 2019).

Amylose content has a great influence on the glycemic response induced by cooked rice; however, the physicochemical (gelatinization) properties also largely influence the digestibility, and hence, the glycemic response (Panlasigui et al., 1991). The degree of gelatinization of cooked rice differs depending on the cooking conditions (Ranawana et al., 2009; He et al., 2018), and therefore, it is possible that the same rice elicits different glycemic responses depending on the cooking method.

The pressure cooker has been in use since the 1960s in Japan, and has recently grown in popularity. Sawada et al. (2018) reported in a survey conducted in 2012 that 43.8% of households in the Kansai area of Japan own a pressure cooker. In the case of people aged 30 years and above, the number of people using a pressure cooker rose to more than 45%. Pressure cookers have been used frequently for boiled dishes, such as Nagasaki-style braised pork, braised meat and potatoes, and simmered radish. There have been many studies concerning pressure cooking, including temperature properties (Otsubo and Miyagawa, 1985) and the quality of rice (Shibukawa, 1976; Seki and Kainuma, 1976; Kainuma and Seki, 1980; Shoji et al., 1986; Onishi et al., 1988; Khatoon and Prakash, 2006), soybeans (Odachi et al., 1980; Chung et al., 2014; Kim et al., 2015), potatoes (Shibukawa and Suzuki, 1985), and sweet potatoes (Odachi et al., 1980). Pressure cooking is characterized by high temperature and short cooking time. It softens the food quickly, although rice prepared in a pressure cooker becomes very sticky compared to that prepared in a conventional rice cooker (Shibukawa, 1976; Seki and Kainuma, 1976; Shoji et al., 1986). According to a report on energy consumption in household rice cooking (Leelayuthsoontorn and Thipayarat, 2006; Lakshimi et al., 2007), pressure cooking generally requires less cooking time and hence less fuel. Since energy conservation is important for achieving the Sustainable Development Goals (SDGs), pressure cooking could be a more eco-friendly cooking method.

Therefore, the advantages of preparing rice in a pressure cooker include energy conservation and the potential to change the physical properties of high-amylose Koshinokaori rice to enhance its eating qualities. However, whether Koshinokaori rice cooked in a pressure cooker elicits a lower postprandial glycemic response remains unclear. The aim of this study was to investigate the effect of pressure cooking on the postprandial glycemic response and eating qualities of high-amylose Koshinokaori rice.

Materials and Methods

Rice samples    High-amylose Koshinokaori rice and intermediate-amylose Japanese standard Koshihikari rice were used for the study. Both rice varieties were cultivated and polished in Niigata, Japan. The amylose contents of Koshinokaori and Koshihikari were 26.3% and 16.6%, respectively.

Preparation of cooked rice samples    Rice was soaked in water for 30 min at 20 °C. Koshinokaori rice was cooked in both an electric rice cooker (NP-BC10, Zojirushi Corporation, Osaka, Japan), with a water-to-rice ratio of 2.2:1 (KKE), and a pressure cooker (HB-1734, Pearl Metal Co., Ltd., Niigata, Japan), with a water-to-rice ratio of 1.5:1 (KKP). Pressure-cooker cooking was conducted under conditions in which good eating qualities had been obtained in a preliminary study. Koshihikari rice was cooked in an electric rice cooker (NP-BC10, Zojirushi Corporation) with a water-to-rice ratio of 1.4:1 (KHE), as a reference. Changes in temperature during cooking were measured by data loggers (Super thermochron, KN Laboratories, Inc., Osaka, Japan).

Measurement of degree of gelatinization    The cooked rice samples were frozen with liquid nitrogen immediately after cooking, freeze-dried, and pulverized. Degree of gelatinization was measured using the beta-amylase-pullulanase (BAP) method (Kainuma et al., 1981).

Measurement of glycemic response    Ten healthy subjects (Japanese; 5 males and 5 females; aged 21.3 ± 1.3 years; BMI 20.7 ± 1.9 kg/m²) participated in this study. The study was approved by the research ethics committee of Niigata University (approval number: 2017-2-007), and was conducted in accordance with the Declaration of Helsinki. Three meals (KHE, KKE, and KKP) were given to the subjects under a single-blind crossover design with a washout period of at least 1 week. After 11 h of fasting, subjects consumed a reference or test meal containing 50 g of available carbohydrate. All meals were consumed over 10 min with 200 mL of water. Finger-prick blood samples were collected with 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 was measured using a self-testing blood glucose meter (Medisafe FIT®, Terumo Corporation). Blood glucose variation was determined by subtracting the fasting blood glucose level from the postprandial blood glucose level. Incremental area under the curve (IAUC) was calculated using the trapezoid rule (Flint et al., 2004), ignoring the area below baseline.

Sensory evaluation    Sensory evaluation (appearance, smell, taste, stickiness, hardness, and overall evaluation) was carried out using a 7-point scale (where 3 = extremely like/sticky/soft and -3 = extremely dislike/less sticky/hard) along with the measurement of blood glucose levels.

Measurement of water content    The water content of cooked rice was measured using an oven drying method. The samples (3 g) were placed in aluminum bags and dried at 135 °C for 2 h. Water content of the samples was calculated from the change in weight before and after drying.

Measurement of physical properties    The physical properties (hardness, strength, adhesiveness, and stickiness) of cooked rice were measured using a Tensipresser (TTP-50BX II, Taketomo Electric Inc., Tokyo, Japan) at 20 °C. The measurement conditions were as described previously (Enoki et al., 2018).

Statistical analysis    Glycemic response was expressed as mean ± SE. Effects of treatment, time, and their interaction on blood glucose variation were analyzed using two-way repeated-measures ANOVA. When an interaction effect was observed, significance at each time point was determined using Tukey's test. The other data were expressed as mean ± SD and analyzed using one-way ANOVA followed by Tukey's test. Significance level was set at p < 0.05. All statistical analyses were performed using Prism v7.04 (GraphPad Software Inc., San Diego, CA, USA).

Results

Degree of gelatinization of cooked rice    Figure 1 shows the degree of gelatinization of cooked rice. Values were 93.1, 77.8, and 64.0% for KHE, KKE, and KKP, respectively. Koshinokaori (KKE and KKP) had a lower degree of gelatinization than Koshihikari (p < 0.01), with KKP showing a significantly lower degree of gelatinization than KKE (p < 0.05).

Fig. 1.

Degree of gelatinization of cooked rice.

KHE: Koshihikari rice cooked by electric cooker; KKE: Koshinokaori rice cooked by electric cooker; KKP: Koshinokaori rice cooked by pressure cooker. Data represents mean ± SD, n = 3. Different letters indicate significant differences (p < 0.05).

Glycemic response of cooked rice    The glycemic response and IAUC after intake of the three cooked rice samples are shown in Figs. 2 and 3. Blood glucose variation was significantly lower in KKP at 45 to 120 min and KKE at 60 to 120 min than in KHE (p < 0.05, Fig. 2). In addition, IAUC of KKE and KKP was significantly lower than that of KHE (p < 0.05, Fig. 3).

Fig. 2.

Blood glucose variation after intake of cooked rice.

□KHE: Koshihikari rice cooked by electric cooker; ●KKE: Koshinokaori rice cooked by electric cooker; ▲KKP: Koshinokaori rice cooked by pressure cooker. Data represents mean ± SE of 10 subjects. *: KHE vs. KKE, ^#: KHE vs. KKP (p < 0.05).

Fig. 3.

IAUC for blood glucose after intake of cooked rice.

KHE: Koshihikari rice cooked by electric cooker; KKE: Koshinokaori rice cooked by electric cooker; KKP: Koshinokaori rice cooked by pressure cooker. Data represents mean ± SE of 10 subjects. Different letters indicate significant differences (p < 0.05).

Sensory evaluation of cooked rice    Figure 4 shows the sensory evaluation of cooked rice. Significant differences were observed in all attributes of KKE compared to those of KHE (p < 0.05). However, no significant difference between KKP and KHE was observed in appearance, smell, stickiness, or hardness. The sensory evaluation score of KKP was closer to that of KHE than of KKE; therefore, KKP appears preferential to KKE in sensory evaluation.

Fig. 4.

Sensory evaluation of cooked rice.

□KHE: Koshihikari rice cooked by electric cooker; ●KKE: Koshinokaori rice cooked by electric cooker; ▲KKP: Koshinokaori rice cooked by pressure cooker. *:KHE vs. KKE, #:KHE vs. KKP, †:KKE vs. KKP (p < 0.05).

Temperature curve during cooking of rice    Figure 5 shows the temperature curve during cooking of rice. The cooking times of KHE, KKE, and KKP were 51, 57, and 28 min, respectively. The highest temperatures of KHE, KKE, and KKP were 100.7, 101.5, and 113.5 °C, respectively. The temperature curve of KKP showed high temperature and short cooking time. The temperature curve of KHE and KKE showed a gradual increase, with two steps, and the cooking time of KKE was longer than that of KHE.

Fig. 5.

Temperature curve during cooking of rice.

KHE: Koshihikari rice cooked by electric cooker; KKE: Koshinokaori rice cooked by electric cooker; KKP: Koshinokaori rice cooked by pressure cooker.

Water content of cooked rice    Water content of the three varieties of rice is shown in Fig. 6. The water content of KHE, KKE, and KKP was 59.5, 68.3, and 60.2%, respectively. While it was significantly higher in KKE (p < 0.01), those of KKP and KHE were not statistically different.

Fig. 6.

Water content of cooked rice.

KHE: Koshihikari rice cooked by electric cooker; KKE: Koshinokaori rice cooked by electric cooker; KKP: Koshinokaori rice cooked by pressure cooker. Data represents mean ± SD, n = 3. Different letters indicate significant differences (p < 0.05).

Physical properties of cooked rice    Table 1 shows the physical properties of the three varieties of cooked rice. KKE showed the highest value for adhesiveness. KKP showed the highest value for hardness and the lowest for adhesiveness. No significant differences in strength and stickiness were observed among them.

Table 1. Physical properties of cooked rice.

	Hardness	Strength	Adhesiveness	Stickiness
KHE	76.9 ± 2.7a	44.8 ± 3.3a	43.7 ± 1.1a	72.6 ± 5.9a
KKE	72.3 ± 2.3a	38.7 ± 2.9a	51.2 ± 1.7b	72.6 ± 2.9a
KKP	147.2 ± 3.3a	41.0 ± 1.4a	39.1 ± 0.8c	72.8 ± 0.8a

KHE: Koshihikari rice cooked by electric cooker; KKE: Koshinokaori rice cooked by electric cooker; KKP: Koshinokaori rice cooked by pressure cooker. Data represents mean ± SD, n = 3. Different letters within column indicate significant differences (p < 0.05).

Discussion

This study investigated the glycemic response of healthy individuals after intake of high-amylose rice cooked using different cooking methods. Both preparation methods of Koshinokaori, using an electric rice cooker and a pressure cooker, elicited a lower glycemic response than Koshihikari rice (Figs. 2 and 3). Compared to cooked Koshihikari, the glycemic response elicited by Koshinokaori cooked in an electric rice cooker was significantly lower 60 min onwards, whereas that elicited by Koshinokaori cooked in a pressure cooker was significantly lower 45 min onwards, suggesting that the differences in cooking methods likely affected the glycemic response. Panlasigui et al. (1991) reported that amylose content alone is not a good predictor of glycemic response, as it can be influenced by physicochemical (gelatinization) properties as well. In the present study, the degree of gelatinization for Koshinokaori rice cooked in a pressure cooker was significantly lower than that obtained using an electric rice cooker (64.0 and 77.8%, respectively) (Fig. 1). Gelatinization leads to swelling of the starch granules and increased accessibility for digestive enzymes. In the case of rice, a longer cooking time increases gelatinization, thus inducing a higher glycemic response (Ranawana et al., 2009; Wolever et al., 1986). In this study, the cooking time for Koshinokaori rice using a pressure cooker was shorter than that using an electric rice cooker (28 and 57 min, respectively) (Fig. 5). Therefore, gelatinization was considered insufficient, and digestibility and blood glucose levels were lower in the pressure cooker sample compared to that in the electric rice cooker sample.

The present study found the rice cooked in a pressure cooker to be closer in taste to Koshihikari than that cooked in an electric rice cooker. Previous studies reported rice cooked in a pressure cooker (i.e., under high temperature and short cooking time) to be very sticky and non-uniform in hardness, from the surface to the core of the grain, compared to that cooked in a conventional rice cooker (Shibukawa, 1976; Seki and Kainuma, 1976; Shoji et al., 1986). Moreover, the pressure-cooked rice appeared more yellowish than the rice prepared in a conventional rice cooker, and different amounts of compounds were detected by gas chromatography–mass spectrometry upon flavor analysis. These studies were performed using standard rice with normal amylose content. The present study, using high-amylose rice, showed pressure cooking to possibly improve the rice texture by increasing its stickiness, which typically does not occur in high-amylose rice such as Koshinokaori. This effect, moreover, was absent in the case of the electric rice cooker. The texture of cooked rice is an extremely important attribute for acceptance of rice variants (Yanagimoto, 2002). Upon sensory evaluation, pressure cooking seemed to improve the appearance, smell, and stickiness of high-amylose rice, and as a result, KKP was preferred over KKE.

In our previous study (Enoki et al., 2018), KKE showed sensory evaluation scores close to those of KHE; however, it scored lower than KHE in this study. Food preference is known to change with age (Kremer et al., 2007). The above-mentioned difference in scores may be attributed to the study subjects being in their 20s, as opposed to being in their 30s–60s, as in the previous study. Moreover, since KKP was more preferred compared to KKE in terms of hardness, appearance, and smell in sensory evaluation, a pressure cooker may be considered more suitable for cooking high-amylose rice than an electric rice cooker. In this study, KKP was found to be hard and less adhesive upon physical property evaluation, in contrast to that by sensory evaluation. In both sensory evaluation and physical property evaluation, samples were measured immediately after cooking; retrogradation of cooked rice may have been different due to the slight differences in time during operation and in the amount of sample used.

In conclusion, Koshinokaori consumption can attenuate the postprandial glycemic response, even if the cooking condition is altered. Moreover, preparing Koshinokaori in a pressure cooker improved its eating qualities, making the rice more palatable. Further research would be required to clarify the usefulness of pressure-cooked Koshinokaori, including studies to evaluate the blood glucose response in pre-diabetic and diabetic individuals, as well as the retrogradation characteristics of cooked rice.

Acknowledgments    This study was supported by the project for the promotion of regional and industrial development in supporting healthy eating habits, the Ministry of Agriculture, Forestry and Fisheries, Japan. The authors thank the students of Niigata University for their cooperation in this research.

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