Relation between textural attributes and surface leachate structural and compositional characteristics of cooked rice

Abstract

The objective of this study was to evaluate the leachate and textural characteristics of cooked rice, and the correlations between the leachate properties and texture attributes were also investigated. Cooked waxy rice had much higher total solids and amylopectin amount in leachate than the normal and high-amylose rice. For all varieties, the amylopectin chain length of the leachate was similar, excluding Dodam cultivar. The rheological characteristics of the leachate solutions were highly dependent on the amylopectin amount of the leachate. Regarding the textural characteristics, Dodam had the highest hardness and the lowest adhesiveness. The principal component analysis showed substantial differences in leachate and textural characteristics of Korean cooked rice according to its amylose content. The adhesiveness was positively and negatively correlated with amylopectin amount of leachate and the proportion of long amylopectin chains, respectively. These results indicated that the leachate characteristics of cooked rice significantly influenced its textural attributes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-023-01446-3.

Introduction

Rice is one of the most widely consumed staples worldwide and is a major source of energy for human due to its high starch content (Li et al., 2019a; 2019b). Rice varieties are divided into short-grain, medium-grain, and long-grain according to the shape of the grains. Rice varieties are also classified into the following groups according to amylose content: 0–5% (waxy), 5–12% (very low), 12–20% (low), 20–25% (intermediate), and over 25% (high) (Li and Gilbert, 2018). Rice is usually consumed as whole grains after cooking, and the textural characteristics of the cooked grains directly affects the eating quality of cooked rice (Xu et al., 2020).

The textural characteristics of cooked rice are affected by various factors, such as amylose content, degree of milling, and cooking method (Leelayuthsoontorn and Thipayarat, 2006). The composition of starch is one of the most fundamental factors affecting the eating quality of cooked rice (Tao et al., 2019). Starch constitutes about 90% of the dry weight of rice and consists of amylose and amylopectin. Native starch consists of crystalline lamellae containing both amorphous and crystalline regions (Bertoft, 2017). Amylose mainly exists in the amorphous regions, while some long amylose chains associate with amylopectin in the crystalline regions. Due to the structural characteristics of amylose, the amylose has been positively linked to the hardness of cooked rice which was regarded as a factor causing the low palatability (Kumar et al., 1976). For this reason, the amylose content of raw rice was used to evaluate and predict the palatability of cooked rice in Korea. The proximate compositions and physicochemical characteristics of raw rice was used as an indicator for determining the quality of cooked rice. However, there seems to have a limit for explaining the difference in eating quality among varieties with similar amylose and protein contents. Furthermore, because rice is mostly consumed after cooking, the evaluation of raw rice could be difficult to predict the eating quality of cooked rice. Therefore, recent studies have focused on the leached materials from rice during the cooking process (Li et al., 2019a; 2019b).

Starch in raw rice is gradually gelatinized during cooking, and the disrupted starch molecules leach out from the grain during cooking (Li et al., 2019a; 2019b). Our previous study (Ha et al., 2022) showed that the characteristics of leached materials highly depended on the cooking method and affected sensory palatability. The amylopectin amount and short amylopectin chains in the leachate strongly influence the adhesiveness of cooked rice which is the most important factor for determining consumer preference (Li et al., 2019a; 2019b).

In Korea, various cultivars of rice have been developed in consideration of consumer acceptability, taste, and plant resistance (Cho et al., 2020). Waxy and intermediate rice are widely consumed in the form of whole grains; however, these rice varieties cause a rapid increase in the blood glucose level after consumption (Lehmann and Robin, 2007). With the increased interest in the glycemic index, consumers are considering nutritional value in addition to eating quality for selecting a rice variety (Mestres et al., 2019). For this reason, high-amylose rice varieties that can slowly raise the blood glucose level have been developed. However, the high-amylose rice varieties have a low eating quality because of the negative correlation between the amylose content and textural characteristics of cooked rice (Hori et al., 2016). The amylose content of rice is not the only possible explanation for the difference in eating quality and consumer preference, thus it is necessary to analyze the properties of the materials leached out during the cooking process. Therefore, the objective of this study was to investigate the leachate and textural characteristics of cooked rice for varieties with different amylose contents commonly consumed in Korea. Furthermore, the correlations between the leachate characteristics and textural attributes were also examined. Our findings may be helpful in providing insight into efficiently evaluating the textural quality of cooked rice according to cultivars with different amylose contents.

Materials and methods

Materials

Six rice cultivars, including two waxy types (Dongjinchal, Baekokchal), three intermediate types (Samgwang, Ungwang, Haedam), and one high-amylose type (Dodam) were purchased by the local market (Yeonmu-Nonghyup, Nonsan, Korea) and stored at 4 °C to maintain the quality before experiments.

Water absorption of raw rice

Water absorption was carried out according to the method reported by Hu et al. (2020). Fifty grains were weighed (W₀) and soaked in 25 mL of distilled water for up to 60 min. The soaked rice was drained, surface dried on the sieve for 2 min, and then weighed (Wt). The water absorption was calculated by the following equation.

$$\mathrm{Water}\mathrm{absorption}(\%)=\frac{W_t-W_0}{W_0}\times 100$$

Rice cooking

Rice (250 g) was washed with distilled water three times and the water was added to rice to give a rice/water ratio of 1:1.2 (w/w), followed by soaking for 30 min. The soaked rice was cooked by an electronic pressure cooker (HOB0310FS, Cuckoo, Seoul, Korea) for 20 min and the followed warming up for 10 min. The cooked rice was cooled for 30 min at room temperature before the experiments.

Water and lipid contents of cooked rice

Water content was obtained by drying about 1g of cooked rice at 105 °C until it reached a constant weight. Lipid content was analyzed using 20 g of cooked rice according to the method of Vasanthan and Hoover (1992). The solvent for extraction the surface and bound lipid from cooked rice was used chloroform/methanol (2:1 v/v) and n-propanol/water (3:1 v/v), respectively.

Extraction of leached materials

Cooked rice (20 g) was placed in a beaker and rinsed with 100 mL of hot distilled water (ca. 95 °C) with stirring for 10 s using a glass rod, followed by passing through a 250 μm sieve. The collected leachate solution was freeze-dried for further analysis.

Compositional and structural analysis of the leachate

The freeze-dried leachate from cooked rice was weighed to measure the total solids. The protein content of the leachate was determined using a Pierce BCA protein assay kit (Thermo Fisher Scientific, Middlesex County, MA, USA). The total starch content and the amylose content of the leachate were measured using a Megazyme assay kit (Megazyme International Ireland Ltd., Bray, Ireland) and a colorimetric method described by Williams et al. (1970), respectively. The amylopectin chain length distribution in the leachate was analyzed using a high-performance anion exchange chromatography system (HPAEC, Dionex ICS-5000, Dionex Corp., Sunnyvale, CA, USA) equipped with a pulse amperometric detector (PAD) according to the method of Ha et al. (2022). Briefly, leachate was dissolved in dimethyl sulfoxide (DMSO) and heated in a boiling water bath with stirring. The ethanol was added to solution and centrifuged. The precipitate was dispersed in sodium acetate buffer and incubated for 20 h after adding isoamylase (E-ISAMY, Megazyme International Ireland Itd., Bray, Ireland). The debranched solution was diluted with 150 mM NaOH before the injection.

Rheological properties of leachate solution

Dynamic oscillatory measurements of the leachate solution were performed using a Discovery HR-1 rheometer (TA Instruments, New Castle, DE, USA) equipped with a 40 mm parallel plate geometry. Each leachate solution (5 mL) obtained in extraction of leached materials was carefully placed on the plate, and the shear rate was increased from 0.1 to 300 1/s at 30 °C to measure the flow curves of apparent viscosity as a function of the shear rate.

Textural properties of cooked rice

The textural properties of cooked rice were analyzed using a texture analyzer (TA-XT + , Stable Micro Systems, Surrey, UK) in texture profile analysis (TPA) mode. Ten intact kernels of cooked rice were randomly chosen and compressed to 50% of the original sample height using a 20 mm cylindrical plunger (diameter) at 1.00 mm/s test speed. All textural measurements were replicated at least 15 times for each sample using different rice kernels.

Statistical analysis

All the tests were performed at least three times, and the statistical significance was measured using SPSS 12.0 software (SPSS Inc., Cary, NC, USA). As the data followed a normal distribution, statistical analysis was performed with a one-way analysis of variance (ANOVA) and Duncan's multiple range test and p < 0.05 was accepted as the significance level. The principal component analysis (PCA) plot was constructed using SIMCA software (Sartorius, Gottingen, Germany). Pearson’s correlation coefficient (r) between the leachate and textural properties was determined using SPSS 12.0 software (SPSS Inc., Cary, NC, USA).

Results and discussion

Water absorption and water content of rice

Water soaking before cooking is a process to improve the taste of rice by supplying moisture necessary for the gelatinization of the starch in the rice (Choi et al., 2021). Water absorption during the soaking process is one test method used to measure the quality of cooked rice (Choi et al., 2021). The water absorption changes of the six rice varieties during soaking are shown in Fig. 1. The water absorption characteristics were significantly different among the varieties. The water absorption rates of Dongjinchal and Baekokchal increased rapidly and peaked at 40 min, indicating that the water absorption equilibrium has been reached. The high-amylose rice Dodam slowly reached its maximum absorption level compared to the normal and waxy varieties (Fig. 1). The time to reach the final water absorption equilibrium depended on the rice variety. The maximum water absorption of Dongjinchal and Baekokchal was much higher than those of the other samples as 45.6% and 46.7%, respectively. This result could be attributed to their relatively higher amylopectin content because amylopectin has stronger water binding and holding properties than amylose. Hu et al. (2021) also reported that the water absorption and the expansion ratio during the absorption process of waxy rice were higher than those of normal rice. Consequently, the amylopectin-amylose ratio in rice might be a major factor determining water absorption, and the degree of water absorption of rice could influence the textural quality of cooked rice (Zhang et al., 2020).

Fig. 1.

The changes in water absorption of the six different rice varieties during soaking

The water content of the cooked rice is shown in Table 1, and the proximate composition of raw rice is shown in Supplementary Table S1. The water content of cooked rice ranged from 53.1 to 55.2% with the lowest value in Dodam and the highest value in Samgwang. The high-amylose content of Dodam limited water migration into the rice starch granules, resulting in the low water content of the cooked rice. Zhu et al. (2021) measured the water interaction with starch in rice by the proton mobility of low-field nuclear magnetic resonance (LF-NMR) and reported that the T₂ value, representing the degree of intragranular proton exchange, increased at a cooking temperature of 30–70 °C because of the water migration into the starch granules driven by the heat and moisture gradients across the grains and influenced by the degree of gelatinization during cooking of rice. High-amylose rice has a high gelatinization temperature because of a restricted hydration of the amorphous regions due to a large amount of amylose (Jeong et al., 2021). For this reason, Dodam could require more water and a higher temperature for gelatinization than other rice cultivars. However, because all six samples were cooked under the same condition, the relatively low water content of Dodam after cooking could be because of insufficient water amount and cooking time for complete gelatinization.

Table 1.

Water and lipid contents of cooked rice with six different varieties

Cooked rice	Water content (%)	Surface lipid (%)	Bound lipid (%)
Dongjinchal	53.9 ± 0.3b	0.13 ± 0.07a	0.40 ± 0.05a
Baekokchal	54.8 ± 0.5a	0.03 ± 0.01c	0.19 ± 0.01c
Samgwang	55.2 ± 0.5a	0.08 ± 0.02b	0.17 ± 0.02c
Ungwang	54.8 ± 0.6a	0.04 ± 0.02c	0.29 ± 0.02b
Haedam	54.5 ± 0.3ab	0.08 ± 0.01bc	0.21 ± 0.02bc
Dodam	53.1 ± 0.7c	0.11 ± 0.01ab	0.39 ± 0.06a

Values followed by the different superscripts in the same column are significantly different (p < 0.05)

Lipid content of cooked rice

The lipids in rice affect the gloss of freshly cooked rice and rancidity during its storage. The surface and bound lipid contents of cooked rice ranged from 0.03 to 0.13% and from 0.17 to 0.40%, respectively (Table 1). The crude lipid content of raw milled rice was in the range of 1.53 ~ 4.21% (Supplementary Table S1). The difference in surface and bound lipid contents of cooked rice might vary depending on the lipids in raw rice. Rice lipids could be divided into non-starch lipids and starch lipids (Zhou et al., 2002). The non-starch lipids refer to the lipids associated with non-starch materials, which, in the present study, were extracted from the rice with chloroform/methanol (2:1, v/v) as solvent at room temperature. The starch lipids refer to the lipids bound with starch, which, in the present study, were extracted by propanol/water as solvent at 90–100 °C. Considering the determination methods in our study, the surface and bound lipid could be regarded as non-starch lipids and starch lipids, respectively. The significantly higher contents of surface and bound lipids in cooked Dongjinchal and Dodam could be attributed to their greater amounts of non-starch and starch lipids.

Compositional analysis of leachate

The composition of leachate obtained from the cooked rice with various cultivars are presented in Table 2. The total solids varied substantially among the cultivars, showing the lowest and highest values in the leachate from Dodam and Baekokchal, respectively. The total amount of leachate decreased as the amylose content in cooked rice increased (Table 2). Li et al. (2019a; 2019b) reported that the total amount of leachate in cooked rice was the highest in glutinous rice and the lowest in high-amylose rice, suggesting that amylopectin was more easily leached out than amylose during cooking. Li et al. (2019a; 2019b) and Li and Gilbert (2018) proposed that amylose might infiltrate the crystalline-amorphous lamella by forming a crystalline structure with amylopectin chains. These structures could cause heat resistance and limit starch leaching during the cooking of rice. Moreover, amylose molecules form complexes with lipids, inhibiting starch swelling and gelatinization. As the starch in Dongjinchal and Baekokchal is mainly composed of amylopectin, these cultivars might produce more leachate during cooking than the other cultivars. Conversely, the extremely low amount of leachate from Dodam might be attributed to the incomplete gelatinization of starch due to insufficient water as a result of the substantially high amylose content (Supplementary Table S1). High-amylose rice varieties tend to require more water for cooking than glutinous and intermediate rice varieties (Li et al., 2019a; 2019b; Tao et al., 2019).

Table 2.

Composition and amylopectin branch chain length distribution of leachate obtained from cooked rice with six different varieties

Cooked rice	Total solids (mg/g)	Protein content (%)	Total starch (%)	Amylose content (%)	Amylopectin amount (mg/g)	Average chain length	Amylopectin chain length distribution (%)
							DP 6–12	DP 13–24	DP 25–36	DP ≥ 37
Dongjinchal	40.3 ± 1.9b	3.9 ± 0.1c	84.5 ± 1.9a	7.1 ± 0.5c	30.6 ± 1.1b	20.0 ± 0.1b	29.7 ± 0.3b	47.7 ± 0.2d	10.4 ± 0.0e	12.3 ± 0.3b
Baekokchal	51.0 ± 0.5a	3.8 ± 0.1c	85.0 ± 1.2a	6.4 ± 0.1c	39.9 ± 0.6a	19.4 ± 0.1c	30.4 ± 0.1a	48.5 ± 0.0c	11.3 ± 0.1b	9.7 ± 0.2e
Samgwang	28.8 ± 2.2c	4.6 ± 0.1b	84.0 ± 1.0a	18.6 ± 0.0b	18.3 ± 0.5d	19.9 ± 0.1b	29.2 ± 0.1c	48.8 ± 0.0b	11.4 ± 0.1b	10.4 ± 0.0d
Ungwang	26.1 ± 0.3c	4.6 ± 0.4b	86.1 ± 1.1a	18.0 ± 1.3b	22.9 ± 0.1c	19.5 ± 0.0c	29.5 ± 0.1bc	49.4 ± 0.2a	10.9 ± 0.2c	9.3 ± 0.1f
Haedam	26.5 ± 1.6c	4.6 ± 0.1b	85.2 ± 0.2a	17.8 ± 0.5b	18.5 ± 0.1d	19.9 ± 0.1b	29.7 ± 0.3b	48.2 ± 0.0c	10.7 ± 0.1d	11.2 ± 0.1c
Dodam	7.4 ± 0.1d	8.2 ± 0.4a	48.3 ± 0.8b	24.3 ± 0.4a	1.7 ± 0.1e	25.6 ± 0.1a	17.0 ± 0.0d	44.7 ± 0.0e	14.1 ± 0.0a	23.7 ± 0.0a

Values followed by the different superscripts in the same column are significantly different (p < 0.05)

The protein content of the leachate from cooked rice varied significantly among the tested cultivars (Table 2). Waxy-type rice varieties, Dongjinchal and Baekokchal, had the lowest value for this parameter, and Dodam had the highest, 8.2%. The protein content of intermediate amylose type was between them (Table 2). However, it was interesting to observe the inconsistency between the original and leached protein contents (Supplementary Table S1). This result could be attributed to the morphological characteristics of starch and protein in rice. Zhu et al. (2020) observed that the degree of protein leaching during rice cooking was highly dependent on the cooking temperature, and protein leaching from the in rice was related to the gelatinization of starch in the rice. In our previous study (Ha et al., 2022), confocal laser microscopy images of the inner morphology of cooked rice revealed that gelatinized starch particles compressed the proteins in the inner cell wall after cooking. Zhu et al. (2020) similarly observed a certain gap between the protein bodies and the starch granules at 50 °C, and as the cooking temperature increased, the protein bodies were covered by gelatinized starch. The protein molecules might be restricted from leaching out during the cooking of rice due to their compression by the gelatinized starch in the rice. Waxy rice cultivars are mainly composed of amylopectin and have a lower gelatinization temperature than intermediate- or high-amylose rice cultivars, leading to greater protein compression by gelatinized starch, which could explain the significantly lower protein content of the leachate from cooked waxy rice. Meanwhile, the comparatively high amount of protein leached from the Dodam cultivar could be because the starch granules were not completely swollen and gelatinized, resulting in a relatively insufficient compression of the protein by swollen starch granules.

The total starch in the leachate was around 85%, without significant difference among the cooked rice varieties, except for Dodam (Table 1), while the raw rice varieties contained more than 80% starch (Supplementary Table S1). Due to its substantially high amylose content, high gelatinization temperature and the aggregated helical amylose structures in the amorphous regions, complete gelatinization of Dodam was not achieved during the cooking process, thereby limiting starch leaching from the cooked rice.

The amylose content and amylopectin amount in leachate from cooked rice differed significantly among the tested rice cultivars (Table 2) and were highly dependent on the original contents in raw rice (Table 2 versus Supplementary Table S1). For instance, the amylopectin amount of leachate was the highest in glutinous rice (Dongjinchal and Baekokchal) and the lowest in Dodam. Tao et al. (2019) reported that the leachate of cooked rice with high amylose had a high amount of amylose. Interestingly, the leachate from glutinous rice had a higher amylose content than the raw counterpart rice, whereas the leachates from intermediate-amylose rice and the Dodam cultivar had less amylose than their raw counterparts (Table 2 versus Supplementary Table S1). During the cooking of glutinous rice, the relatively linear amylopectin could easily leach out of the starch granule compared the branched amylopectin, which could contribute to the increased amylose content of the leachate because the somewhat linear amylopectin could complex with iodine. On the contrary, the substantially reduced amylose content of the leachate from the Dodam cultivar suggests limited amylose leaching during cooking due to incomplete gelatinization, as explained above.

Amylopectin chain length distribution of leachate

The normalized HPAEC-PAD chromatogram of the branch chain length distribution of amylopectin in the leachate from cooked rice varieties with different amylose contents is shown in Supplementary Fig. S1. According to Ha et al. (2022), amylopectin chain length distributions could be grouped into chain types of DP 6–12 (A chains), DP 13–24 (B1 chains), DP 25–36 (B2 chains), and DP ≥ 37 (B3 + chains), and the results are presented in Table 2. The amylopectin chain length distribution of the leachate from cooked rice varied significantly among the rice varieties. This could be attributed to the specific structure of amylopectin in each starch according to the varieties. Wu et al. (2019) also reported that the distribution of amylopectin chain length in the native and leached starches from waxy rice differed depending on the cultivar.

The leachate from cooked waxy rice (Dongjinchal and Baekokchal) and normal-type rice (Samgwang, Ungwang, and Haedam) showed a marginal difference in the amylopectin branch chain length distribution, whereas the leachate from Dodam had substantially higher proportions of B2 chains (DP 25–36) and B3 + chains (DP ≥ 37) than the other varieties (Table 2). Many researchers have reported a high proportion of long amylopectin chains in high-amylose rice varieties (Jeong et al., 2021; Tao et al., 2019).

Rheology properties of leachate solution

The apparent viscosity-shear rate behaviors of the leachate solution obtained from cooked rice are presented in Fig. 2. For a given shear rate, the experiment was repeated three times with a freshly loaded sample. Because the relative error was always less than 20%, the rheological curves were taken as the mean values of three measurements. The apparent viscosity of the leachate decreased significantly with increasing shear rate up to 50 1/s followed by no significant change, except for Dodam. This substantial decrease in the apparent viscosity of the leachate from waxy and intermediate rice varieties indicates shear-thinning behavior due to the degradation of structure in the leachate solution (Guo et al. 2016). Chen et al. (2019) reported that when starch in cooked rice was completely gelatinized during cooking and a large amount of materials leached out, the apparent viscosity of the leachate solution increased, and the leachate solution displayed shear-thinning behavior when a shear force was applied. Ha et al. (2022) suggested that the amylopectin content in the leachate from cooked rice affected its apparent viscosity. Consequently, the high apparent viscosity of leachate from cooked waxy rice, Dongjinchal and Baekokchal, could be attributed to their large amount of amylopectin in the leachate (Table 2).

Fig. 2.

Rheological properties in leachate solution of cooked rice with six different varieties

The apparent viscosity of the leachate from cooked Dodam remained almost unchanged, with only a marginal increase observed at shear rates over 50 1/s due to shear-thickening behavior (Guo et al., 2016). This result could be because of incomplete gelatinization of the starch during cooking, subsequently limit the leaching of materials, such as proteins, lipids, and amylopectin. Similarly, Chen et al. (2019) reported that the leachate from rice cooked for 10 and 20 min showed shear-thickening behavior due to uncomplete gelatinization. Our result suggests that the rheological properties of the leachate solution from cooked rice are highly dependent on the components leached during cooking.

Textural properties of cooked rice

The textural characteristics of the cooked rice are presented in Table 3. The hardness of cooked rice was highly dependent on the amylose content of the cooked rice varieties. High-amylose variety Dodam had the highest hardness (60.6 N), whereas waxy rice variety Baekokchal had the lowest value (16.0 N). Li and Gilbert (2018) noted that the amylose molecules both in the amorphous regions and incorporated into the crystalline structure of amylopectin limited starch swelling and leaching out during cooking, thereby increasing the hardness of cooked high-amylose rice. Interestingly, the Samgwang, a variety with intermediate amylose content, had a remarkably lower hardness than Dongjinchal but was comparable to Baekokchal. This result could be attributed to the water content of cooked rice, which is one of the major factors affecting the texture of cooked rice (Li and Gilbert, 2018). Prasert and Suwannaporn (2009) observed that the hardness of rice decreased as the water content increased. In the present study, although the amylose content of Samgwang was higher than that of Dongjinchal, the high-water content of Samgwang (Table 1) could cause relatively low hardness.

Table 3.

Textural characteristics of cooked rice with six different varieties

Rice variety	Hardness (N)	Adhesiveness (N∙s)	Springiness	Cohesiveness	Chewiness (N)
Dongjinchal	16.1 ± 0.4d	1.11 ± 0.00b	0.857 ± 0.018bc	0.617 ± 0.010c	8.6 ± 0.1d
Baekokchal	16.0 ± 0.2d	1.52 ± 0.05a	0.874 ± 0.006bc	0.619 ± 0.002c	7.9 ± 0.0f
Samgwang	15.4 ± 0.3e	0.36 ± 0.03d	0.887 ± 0.002bc	0.691 ± 0.002b	7.7 ± 0.1e
Ungwang	20.1 ± 0.1c	0.44 ± 0.02c	0.875 ± 0.033c	0.687 ± 0.006b	10.2 ± 0.2c
Haedam	26.7 ± 0.0b	0.30 ± 0.00e	0.850 ± 0.009b	0.591 ± 0.007d	16.2 ± 0.0b
Dodam	60.6 ± 0.2a	0.01 ± 0.00f	0.920 ± 0.002a	0.766 ± 0.003a	44.3 ± 0.1a

Values followed by the different superscripts in the same column are significantly different (p < 0.05)

Cooked waxy rice varieties (Dongjinchal and Baekokchal) exhibited significantly higher adhesiveness than the other varieties, whereas Dodam had an extremely low adhesiveness, 0.01 N·s (Table 3). Li et al. (2019a, 2019b, 2021) reported that the adhesiveness of cooked rice had positive correlation with the total solids of the leachate, the amylopectin amount of the leachate, and the proportion of amylopectin with DP < 37. In the present study, the total solids and amylopectin amount of leachate decreased with the increased amylose content of cooked rice (Table 2), which could explain the lowest adhesiveness value of Dodam. Furthermore, as shown in Table 2, Dodam had a significantly higher proportion of long B3 + chains (DP ≥ 37) in the leachate than the other samples, which could also contribute to its low adhesiveness. Tao et al. (2019) also reported that the proportion of medium- and long-chains in amylopectin (DP 31–92) was negatively correlated with the stickiness of cooked rice. These long amylopectin chains have less chance of interacting with the probe of the texture analyzer than short chains, explaining the low adhesiveness of cooked rice (Li and Gilbert, 2018).

The springiness and cohesiveness of cooked rice with different amylose contents differed substantially among the rice varieties (Table 3). Springiness and cohesiveness ranged from 0.850 to 0.920 and from 0.591 to 0.766, respectively. Dodam showed the highest springiness and cohesiveness among the samples. These results might be related to the degree of deformation of cooked rice. Huang et al. (2007) reported that springiness was an assessment of how much the gel structure collapses in the first compression, and generally increased as a gel structure was split into large chunks. Alvarez et al. (2002) suggested that the degree of deformation of the tissues of the tested sample considerably influenced the cohesiveness parameter. In this context, the high values of springiness and cohesiveness for Dodam could be attributed that structural collapse inside the rice kernel was not fully achieved due to insufficient gelatinization.

Correlations between rice varieties and properties of cooked rice

The PCA was used to investigate the correlations between rice varieties and the characteristics of cooked rice such as leachate compositions and textural properties. The PCA score plot (Fig. 3A) was largely divided into three groups according to the amylose content of rice (waxy, intermediate, and high amylose rice). The variation of the principal component 1 (PC1) and the principal component 2 (PC2) was 66.6% and 13.6%, respectively, resulting in a total variation of 80.2%. The PCA loading scatter plot, which presents the factors influencing the grouping, is shown in Fig. 3B. The hardness and chewiness of cooked rice, and the average chain length (ACL), DP 25–36, DP ≥ 37, and the protein content of the leachate from cooked rice were located on the right of PC1, suggesting that these factors significantly affected the grouping of high-amylose rice, Dodam. This result might be combined with the highest protein content in the leachate (Table 2), the proportion of long amylopectin chains (Table 2), hardness, and chewiness (Table 4) of Dodam among the tested varieties. The water absorption, adhesiveness, amylopectin amount, and total solids were located on the left top of PCA loading plot (positive PC2), indicating that these factors substantially affected the grouping of waxy rice (Dongjinchal and Baekokchal). From the PCA results, it could be suggested that the amylose content of rice varieties considerably affected the leachate compositions and textural properties of cooked rice. Interestingly, as shown in Fig. 3A, the gap within two waxy rice varieties was wider than that among the three intermediate rice varieties. This result could be due to the larger difference in leachate characteristics, such as total solids and amylopectin amount between Dongjinchal and Baekokchal than among the three intermediate rice varieties.

Fig. 3.

Score (A) and loading (B) plots for principal component analysis (PCA) derived from leachate compositions and textural properties cooked rice from six different varieties. Pearson correlation coefficients(C) for the relationship between compositions of leachate and textural properties of cooked rice with six different varieties. *, **, and *** mean that the correlations are significant at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively. DJC Dongjinchal, BOC Baekokchal, SG Samgwang, UG Ungwang, HD Haedam, DD Dodam

Pearson correlations

Pearson’s correlation coefficients (r) for the relationship between the compositions of leachate and texture properties of cooked rice are presented in Fig. 3C. The total solids were positively correlated with the leachate compositions of total starch (r = 0.738, p ≤ 0.01), amylopectin amount (r = 0.961, p ≤ 0.001), DP 6–12 (r = 0.744, p ≤ 0.01), and DP 13–24 (r = 0.697, p ≤ 0.05). Patindol et al. (2010) found that the molecules of leached amylopectin were much smaller than that of native starch due to the restricted leaching of long amylopectin chains. This suggestion could help explain the negative correlation of the total solids with DP 25–36 (r = − 0.655, p ≤ 0.05), DP ≥ 37 (r = − 0.767, p ≤ 0.01), and ACL (r = − 0.776, p ≤ 0.01) in the present study. In addition, the total solids were positively correlated with adhesiveness (r = 0.889, p ≤ 0.001). Li et al. (2019a, b) also found that the total solids and stickiness of cooked rice had a positive correlation. The linear regression analysis between the total solids and adhesiveness (Supplementary Fig. S2A) also exhibited a significantly high correlation (R² = 0.790). Therefore, the high adhesiveness of cooked waxy rice varieties could be highly associated with their large amount of total leachate. This result suggests that the total solids could be an important factor influencing the eating quality of cooked rice.

The amylopectin amount of the leachate was negatively correlated with the hardness (r = − 0.819, p ≤ 0.01), cohesiveness (r = − 0.789, p ≤ 0.001), chewiness (r = − 0.815, p ≤ 0.001), but positively correlated with the adhesiveness (r = 0.940, p ≤ 0.001). Li and colleagues (Li and Gilbert, 2018; Li et al., 2019a; 2019b) found that the leached amylopectin was positively correlated with the stickiness of cooked rice. Unlike amylose, which limits the swelling of starch, amylopectin interacts well with water, thus easily leaching out during cooking (Li and Gilbert, 2018). Leached amylopectin forms a thin film on the surface of cooked rice with other leached materials, leading to a sticky texture. The linear regression analysis between the amylopectin amount and adhesiveness showed a high correlation coefficient (R² = 0.884) as shown in Supplementary Fig. S2B. The amylopectin amount had a positive correlation with the water absorption (r = 0.786, p ≤ 0.01). Lin et al. (2016) also presented that the swelling power of starch was positively correlated with the amylopectin content because of the superior water absorption and holding properties of amylopectin compared to amylose.

The hardness of cooked rice had a negative correlation with the adhesiveness (r = − 0.628, p ≤ 0.05), water content (r = − 0.780, p ≤ 0.01), DP 6–12 (r = − 0.971, p ≤ 0.001), and DP 13–24 (r =− 0.908, p ≤ 0.001), but a positive correlation with the springiness (r = 0.671, p ≤ 0.05), cohesiveness (r = 0.692, p ≤ 0.05), chewiness (r = 0.999, p ≤ 0.001), DP 25–36 (r = 0.910, p ≤ 0.001), DP ≥ 37 (r = 0.954, p ≤ 0.001), and ACL (r = 0.971, p ≤ 0.001). Li and Gilbert (2018) reported that the water content was one of the major factors influencing the hardness of cooked rice. Ha et al. (2022) also reported that the hardness of cooked rice increased as the water content decreased.

Adhesiveness is regarded as an important factor affecting the eating quality and palatability of cooked rice (Li and Gilbert, 2018; Li et al., 2019a; 2019b). The adhesiveness was positively correlated with the water absorption (r = 0.941, p ≤ 0.001), and DP 6–12 (r = 0.621, p ≤ 0.05), but negatively correlated with the cohesiveness (r = − 0.687, p ≤ 0.01), chewiness (r = − 0.626, p ≤ 0.05), and DP 25–36 (r = − 0.598, p ≤ 0.05). Likewise, Li et al. (2019a; 2019b) observed an increase in the adhesiveness of cooked rice with the increasing amylopectin amount and proportion of short chains in the leachate. The short amylopectin chains are easily leached during cooking due to the imperfect crystalline structure. These results suggest a significant correlation between the leachate and textural characteristics of cooked rice. These findings could provide useful information for evaluating the textural quality of cooked rice varieties.

In this study, the impact of leachate compositions on the textural properties of cooked rice with six different varieties was investigated. The total solids, amylopectin amount, and protein content of the leachate from cooked rice differed significantly among the tested rice varieties. In conclusion, the characteristics of the leachate from cooked rice considerably influence the textural characteristics of cooked rice. This valuable information could be a critical tool for evaluating the textural quality of cooked rice with different varieties.

Supplementary Information

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Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.