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. 2026 Aug 19;91(8):e71384. doi: 10.1111/1750-3841.71384

Hydrocolloid‐Assisted Hydrothermal Modification of Intact Rice Kernels for Reduced Digestibility and Cooked‐Rice Quality

Dong‐Hwa Cho 1, Mingyo Ha 1, HyunHo Bae 2, Ju Hun Lee 3, Hyun‐Jung Chung 1,✉
PMCID: PMC13490283  PMID: 42618755

ABSTRACT

Cooked rice is a high‐glycemic‐index staple, and reducing its starch digestibility is therefore important. This study evaluated hydrocolloid‐assisted hydrothermal treatment of intact kernels for lowering cooked‐rice starch digestibility while preserving texture and appearance. The kernels were mildly heated near the starch onset gelatinization temperature with κ‐carrageenan, locust bean gum, xanthan gum, or guar gum (GG), each at 0.3%, then dried under humidity‐controlled conditions (50°C, 80% relative humidity). GG was further examined at 0.1%–0.5%. Hydrothermal treatment alone lowered the estimated glycemic index (eGI) from 91.3 to 86.4 without significantly changing hardness, although adhesiveness increased. Adding a hydrocolloid during mild heating altered digestibility and texture: xanthan gum lowered eGI but deformed the kernels and raised hardness and adhesiveness, whereas GG gave the lowest eGI, raised adhesiveness but not hardness, and better‐preserved kernel integrity. Increasing the hydrocolloid concentration reduced digestibility: at 0.3%, GG lowered eGI to 80.8. Raising it to 0.5%, however, lowered eGI only slightly further (to 79.2) while markedly increasing adhesiveness and causing kernel agglomeration. Thus, 0.3% GG most effectively reduced digestibility while preserving cooked‐rice quality. Differential scanning calorimetry indicated that the reduced digestibility did not arise primarily from internal starch reorganization: the thermal profile of the treated kernels became more native‐like even as digestibility fell. It was therefore more likely associated with a GG‐rich surface layer that restricted kernel swelling. Overall, hydrothermal treatment with 0.3% GG is a simple, commercially viable strategy for lowering the starch digestibility of cooked rice while retaining texture and appearance comparable to native rice.

Keywords: cooked rice, cooked‐rice texture, estimated glycemic index, hydrothermal treatment, resistant starch, starch–hydrocolloid interaction

1. Introduction

Rice is a major staple food, particularly in Asia, and is commonly consumed as cooked milled rice (Srikaeo and Paphonyanyong 2020). Starch is the main component of milled rice (about 90% on a dry basis), and its physicochemical properties largely determine the eating quality of cooked rice. During cooking, starch gelatinization produces the characteristic soft, sticky, and elastic texture of rice kernels but also increases starch digestibility, thereby contributing to the relatively high glycemic response of cooked white rice (Goñi et al. 1997). Thus, reducing starch digestibility without compromising eating quality is an important processing goal, particularly because high‐glycemic diets are associated with metabolic disorders (Jenkins et al. 2021; Reynolds et al. 2019; Livesey et al. 2019).

Physical modification can alter starch functionality and digestibility without the use of chemical reagents. Among such approaches, hydrothermal treatments use controlled combinations of temperature and moisture to modify starch properties while limiting complete gelatinization. Depending on the treatment conditions, they can alter starch swelling, pasting, and enzymatic susceptibility without markedly disrupting granular structure (da Rosa Zavareze and Dias 2011; BeMiller and Huber 2015; Iuga and Mironeasa 2020). Mild heating near the onset gelatinization temperature under excess‐water conditions may induce limited starch swelling and leaching without complete gelatinization (C. Zhang et al. 2018, 2019). In contrast to this near‐onset treatment under excess‐water conditions, heat–moisture treatment limits gelatinization primarily through restricted moisture, whereas annealing is conducted in excess water below the gelatinization temperature. Starch released during mild heating may subsequently reassociate during drying and modify the outer kernel region, although this mechanism has been examined mainly in isolated starch systems (C. Zhang et al. 2019).

Hydrocolloids are widely used food biopolymers that can modify texture, water mobility, and the physicochemical behavior of starch‐based foods (BeMiller 2011; Iuga and Mironeasa 2020). Even at low concentrations, hydrocolloids can alter starch swelling, pasting behavior, viscosity, and retrogradation, although their effects vary with hydrocolloid type and processing conditions (Heyman et al. 2014; Funami et al. 2005). Hydrocolloids can also reduce starch digestibility by restricting the diffusion and accessibility of digestive enzymes through increased viscosity and by interacting with starch or starch‐containing matrices (BeMiller 2011; Sasaki 2018). In the present study, κ‐carrageenan (KC), xanthan gum (XG), locust bean gum (LBG), and guar gum (GG) were selected because they have been reported to affect starch swelling, pasting behavior, and enzymatic hydrolysis in starch‐based systems (Tester and Sommerville 2003; Brennan et al. 2008; Sasaki 2018; He et al. 2020; He et al. 2021). These hydrocolloids also represent contrasting structural and rheological classes: KC and XG are anionic polysaccharides, whereas LBG and GG are neutral galactomannans. In addition, KC is primarily a gelling hydrocolloid, whereas XG is a highly viscous, non‐gelling thickener. The two galactomannans also differ in their degree of galactose substitution (Morris et al. 2008; Mudgil et al. 2014). These structural and rheological differences were expected to produce distinct associations with starch leached from the rice‐kernel surface and, consequently, different effects on starch digestibility and cooked‐rice texture.

Hydrothermal treatments of intact cereal kernels have been investigated in relation to starch digestibility and physicochemical properties (Cheng et al. 2019; Chung et al. 2012). However, these treatments have generally been conducted below To. Meanwhile, hydrocolloid studies have largely focused on isolated starches, cereal flours, or direct addition during cooking (Chung et al. 2007; Sasaki 2018; Srikaeo and Paphonyanyong 2020). Hydrothermal treatment of intact kernels is challenging because kernels are prone to fissuring during treatment and drying (Srisang et al. 2011; Truong et al. 2012). In contrast, direct hydrocolloid addition during cooking may primarily affect the kernel surface rather than induce structural modification (Chung et al. 2007; Fuwa et al. 2016). Starch–hydrocolloid associations formed during hydrothermal treatment and subsequent drying could influence both digestibility and eating quality after cooking.

Because these hydrocolloids are large polymers with limited diffusion into intact kernels (Morris et al. 2008), they are expected to interact mainly with starch leached from the outer kernel region during mild heating. Such associations may contribute to the formation of a hydrocolloid–starch layer on the kernel surface during drying, which may subsequently restrict starch hydrolysis (Slaughter et al. 2002; He et al. 2020). To address this gap, intact white rice kernels were hydrothermally treated with hydrocolloids near T o to induce limited starch swelling and leaching. The treated kernels were subsequently dried under controlled humidity to limit fissure formation. Unlike previous approaches that applied hydrocolloids to isolated starches or cereal flours, or added them during cooking, the present intact‐kernel approach applied them during hydrothermal treatment, before cooking.

Therefore, this study investigated whether hydrocolloid‐assisted mild hydrothermal treatment could modify the surface structure of intact rice kernels and reduce starch digestibility without compromising cooked‐rice texture. The effects of KC, XG, LBG, and GG on kernel morphology, in vitro starch digestibility, cooked‐rice texture, and thermal properties were compared, and the concentration‐dependent effects of GG were further evaluated.

2. Materials and Methods

2.1. Materials

Paddy rice (Oryza sativa L. subsp. japonica cv. ‘Samgwang’) was grown at the experimental farm of the National Institute of Crop Science (Rural Development Administration, Suwon, Republic of Korea) and harvested on October 11, 2023. Milled white rice (10% degree of milling) was supplied for research use and stored at 4°C until analysis. All hydrocolloids used in this study were obtained as commercially available food ingredients: KC (Marcel Trade Corporation, Philippines), GG (Lotus Gums and Chemicals, India), LBG (Cesalpinia Food S.P.A., Italy), and XG (NutraSweet Kelco Co., USA). Pancreatin (from porcine pancreas; P7545), amyloglucosidase (from Aspergillus niger; A9913), and pepsin (from porcine gastric mucosa; P7000) were purchased from Sigma–Aldrich (St. Louis, MO, USA). All chemicals and solvents used in this study were of analytical grade.

2.2. Preparation of Hydrothermally Treated Rice Kernels With Hydrocolloids

Hydrocolloid solutions were prepared by dissolving each hydrocolloid (0.45 g, corresponding to an addition level of 0.3%, w/w, relative to rice dry solids) in distilled water (180 mL) at 58°C under mechanical stirring until fully hydrated. The concentration of 0.3% used to compare the four hydrocolloids was determined from preliminary trials; at higher concentrations (e.g., 0.5%), XG became excessively viscous and did not hydrate completely, making it difficult to soak the intact kernels uniformly and to drain the solution afterward. Thus, 0.3% was the highest concentration at which all four hydrocolloids formed homogeneous dispersions at 58°C. The apparent viscosity and turbidity measured at 58°C (Table S1) characterize the hydration state of each hydrocolloid dispersion. Milled white rice (150.0 g, dry solids basis) was added to the hydrocolloid solution, and the mixture underwent mild heating during soaking at 58°C in a shaking water bath (50 rpm) for 1 h. The soaking temperature (58°C) was selected to facilitate hydrocolloid hydration and to match the onset gelatinization temperature (T o) of starch from native rice determined by DSC (Section 2.5). This condition was intended to promote controlled starch swelling/leaching without complete gelatinization (C. Zhang et al. 2019), which may facilitate associations between leached starch components and hydrocolloids. After soaking, the liquid phase was drained using a sieve, and the wet kernels were immediately transferred to a sealed glass container to minimize moisture loss. Tempering was performed at 80°C for 1 h. The treated kernels were subsequently dried in a humidity‐controlled chamber at 50°C and 80% relative humidity for 24 h. Dried samples were sealed in polyethylene zipper bags and stored at 4°C until analysis. A schematic overview of the procedure is shown in Figure 1. The effects of hydrocolloid type on starch digestibility and textural properties after conventional cooking were evaluated using rice kernels hydrothermally treated with solutions containing KC, GG, LBG, or XG at an initial level of 0.3% (w/w; rice dry‐solids basis). The resulting samples were designated HT‐KC0.3, HT‐GG0.3, HT‐LBG0.3, and HT‐XG0.3. These sample codes denote the hydrocolloid addition level rather than the amount retained in the kernels. Based on these comparisons, GG was further evaluated at 0.1%–0.5% (w/w; 0.1%, 0.3%, and 0.5%) to assess dose–response behavior and to determine an appropriate addition level (HT‐GG0.1, HT‐GG0.3, and HT‐GG0.5). GG hydrated fully and formed homogeneous dispersions across this range (0.1%–0.5%) at 58°C. Native rice without hydrothermal treatment (Native) and hydrothermally treated rice without hydrocolloid (HT) were prepared as controls.

FIGURE 1.

FIGURE 1

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Schematic overview of hydrothermal treatment of rice kernels in the absence or presence of hydrocolloids. Kernels were mildly heated during soaking at 58°C for 1 h, drained, tempered at 80°C for 1 h, and then dried at 50°C and 80% relative humidity for 24 h.

2.3. Conventional Cooking of Rice Samples

The hydrothermally treated rice samples (60.0 g dry basis) were soaked in distilled water (90 mL; rice:water = 1:1.5, w/v) at room temperature for 40 min, and then cooked in an electric pressure cooker (HOB0310FS, Cuckoo, Seoul, South Korea) for 23 min and kept warm for 30 min in the cooker. For subsequent analyses, 20 g of cooked rice was collected from the central portion of the cooked mass, cooled in a water bath maintained at 30°C for 30 min, and used immediately. Native (untreated) rice was cooked under the same cooking conditions.

2.4. Appearance and Surface Morphology of Rice Kernels

The overall appearance of rice kernels before and after cooking was recorded using a digital camera (Canon M50, Canon, Tokyo, Japan). Kernels were placed in a Petri dish and photographed on a lightbox for transillumination (i.e., transmitted‐light imaging; Figures 2 and 3). Images were taken at ISO 100, shutter speed 1/60 s, and aperture f/6.3, with a camera‐to‐sample distance fixed at 18 cm. The surface morphology of cooked rice kernels was examined by scanning electron microscopy (SEM; SU‐70, Hitachi, Tokyo, Japan). Cooked kernels were quenched in liquid nitrogen immediately after removal from the cooker, freeze‐dried, mounted on aluminum stubs with double‐sided adhesive carbon tape, and sputter‐coated with gold. SEM images were acquired at an accelerating voltage of 20.0 kV (Figure 4).

FIGURE 2.

FIGURE 2

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Digital transmitted‐light (transillumination) photographs of rice kernels before (a) and after (b) conventional cooking: untreated white rice (Native), hydrothermally treated rice without hydrocolloid (HT), and hydrothermally treated rice prepared in the presence of guar gum (GG), κ‐carrageenan (KC), locust bean gum (LBG), or xanthan gum (XG) at 0.3% (w/w, rice dry‐solids basis). Scale bar = 1 mm.

FIGURE 3.

FIGURE 3

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Digital transmitted‐light (transillumination) photographs of rice kernels before (a) and after (b) conventional cooking: untreated white rice (Native), hydrothermally treated rice without hydrocolloid (HT), and hydrothermally treated rice prepared in the presence of guar gum at 0.1%, 0.3%, or 0.5% (w/w, rice dry‐solids basis). Scale bar = 1 mm.

FIGURE 4.

FIGURE 4

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SEM images showing the surface morphology of cooked rice kernels: untreated white rice (Native), hydrothermally treated rice without hydrocolloid (HT), and hydrothermally treated rice prepared in the presence of guar gum at 0.1%, 0.3%, or 0.5% (w/w, rice dry‐solids basis). Scale bar = 1 mm.

2.5. Thermal Transition Behavior

Thermal transition properties of native rice kernels and hydrothermally treated rice kernels prepared in the absence or presence of various hydrocolloids were determined using a differential scanning calorimeter (DSC 6100; Seiko Instruments Inc., Chiba, Japan). For DSC analysis, rice samples were ground to a powder and passed through a 100‐mesh sieve. Rice powder (3.0 mg, dry basis) was weighed into an aluminum DSC pan, and distilled water (7.0 mg) was added. The pan was immediately sealed and equilibrated at 4°C for 3 h. Thermal scans were performed from 20°C to 150°C at a heating rate of 5°C/min, using an empty sealed pan as a reference. The onset temperature (T o), peak temperature (T p), conclusion temperature (T c), and gelatinization enthalpy (ΔH gel) were calculated using the analysis software (EXSTAR‐6000, Seiko, Chiba, Japan). The onset gelatinization temperature (T o) of starch from native rice determined by DSC was used to set the mild heating temperature (58°C) employed during sample preparation (Figure 1; Section 2.2).

2.6. In Vitro Starch Digestibility

In vitro starch digestibility of cooked rice samples was determined according to Englyst et al. (1992), with slight modifications. Porcine pancreatin (5.0 g, activity 8 × USP/g) was dispersed in 33.0 mL of water with vigorous stirring and centrifuged at 1500 × g for 10 min. The supernatant was then mixed with amyloglucosidase (1.3 mL, 5000–8000 U/mL) to prepare the enzyme solution, which was freshly prepared for each digestion analysis. Prior to the in vitro starch digestibility assay, cooked rice samples were passed through a hand mincer to simulate mastication. The in vitro digestion was carried out in two stages, comprising a gastric phase followed by an intestinal phase. For the gastric phase, each minced sample (0.5 g, dry basis) was incubated with 0.05 M HCl (10 mL), pepsin (0.1 g; P7000, Sigma‐Aldrich), GG (0.1 g), and seven glass balls (10 mm diameter) in a shaking water bath at 37°C and 170 rpm for 30 min. For the subsequent intestinal phase, sodium acetate buffer (10 mL, 0.5 M, pH 5.2) and the enzyme solution (5 mL) were added, and the mixture was incubated at 37°C and 170 rpm for 3 h. GG is a standard component of the digestion medium in the modified Englyst assay, added to provide a uniform viscosity that keeps the cooked rice samples in suspension during incubation (Englyst et al. 1992; Chung et al. 2006). Because it was present in the digestion medium of every sample, this GG formed a constant assay background and is distinct from the GG incorporated into the kernels during hydrothermal treatment, which was the experimental variable. Aliquots (200 µL) of the hydrolysates were periodically collected and mixed with 80% ethanol (4 mL) to terminate enzymatic hydrolysis. Glucose content was determined using a glucose assay kit (GOPOD format, Megazyme Ltd., Wicklow, Ireland). Digestibility‐based starch fractions were classified according to Englyst et al. (1992), with hydrolysis times counted from the start of the intestinal phase, as rapidly digestible starch (RDS; hydrolyzed within 20 min), slowly digestible starch (SDS; hydrolyzed between 20 and 120 min), and resistant starch (RS; remaining unhydrolyzed after 120 min).

The estimated glycemic index (eGI) of the cooked rice samples was determined using an in vitro digestion kinetic method (Goñi et al. 1997). The kinetics of starch hydrolysis were described using a nonlinear first‐order equation (C = C ∞ (1 – e– kt )), where C is the percentage of starch hydrolyzed at time (t), C ∞ is the equilibrium hydrolysis percentage at the final time (180 min), and k is the kinetic constant at time t (min). The hydrolysis index (HI) was calculated as the ratio of the area under the hydrolysis curve (AUC) of each sample to that of fresh white bread, used as the reference sample. AUC was calculated using the equation: AUC = C ∞(t f − t 0) − (C ∞/k)[1 – exp(–k(tf − t 0))], where t 0 is the initial time (0 min) and t f is the final time (180 min). The eGI was then calculated as eGI = 8.198 + 0.862 × HI, as described by Granfeldt et al. (1992).

2.7. Texture Analysis

Texture profile analysis (TPA) of cooked rice samples was performed using a texture analyzer (TA‐XT2, Stable Micro Systems, Surrey, UK) equipped with a cylindrical stainless‐steel probe (20 mm diameter). Cooked rice (10 g) was molded into a cylindrical block (40.5 mm diameter × 15.0 mm height) using a cylindrical container and analyzed by a two‐cycle compression test to 40% deformation at a speed of 0.5 mm/s. Textural parameters, including hardness, springiness, cohesiveness, chewiness, and adhesiveness, were obtained from the TPA data using XTRA Dimension software (version 3.7G, Stable Micro Systems Ltd., Surrey, UK). The TPA was repeated six times for each sample.

2.8. Statistical Analysis

All samples were prepared and analyzed in triplicate, and the results are expressed as mean ± standard deviation (SD). Statistical significance was determined by one‐way analysis of variance (ANOVA) followed by Duncan's multiple range test (p < 0.05), using SAS software (version 9.4; SAS Institute Inc., Cary, NC, USA).

3. Results and Discussion

3.1. Comparative Effects of Hydrocolloid Type During Hydrothermal Treatment

3.1.1. Kernel Appearance Before Cooking

Kernel images of untreated white rice (Native) and hydrothermally treated rice samples (with and without hydrocolloids) before cooking are shown in Figure 2a. Native showed uniform, intact kernels with no visible fissures. In contrast, hydrothermally treated rice prepared without hydrocolloid (HT) showed visible fissures and occasional kernel breakage. Similar cracking has been reported in thermally treated rice and is often attributed to moisture gradients and the tensile stresses induced during posttreatment drying (Srisang et al. 2011). To mitigate rapid moisture migration, the hydrothermal process in this study incorporated a tempering step followed by humidity‐controlled drying, which has been reported to alleviate cracking and reduce kernel breakage compared with conventional hot‐air drying (Jeong et al. 2023; Truong et al. 2012). Accordingly, kernels dried by tempering followed by humidity‐controlled drying exhibited fewer cracks and reduced kernel breakage than kernels dried by conventional hot‐air drying (Figure S1). However, visible fissures were still observed in hydrothermally treated kernels, indicating that the current processing conditions improved kernel integrity but did not yield intact, fissure‐free kernels comparable to native rice.

The addition of hydrocolloids (0.3% w/w, rice dry‐solids basis) during the mild‐heating step altered the visual appearance of the treated kernels (Figure 2a). In particular, XG markedly increased kernel opacity, consistent with the reduced transparency values measured for rice kernels hydrothermally treated in the presence of 0.3% XG (Table S2). This pronounced change in appearance might be due to greater retention of XG on the kernel surface or within surface fissures/defects during draining and subsequent drying, potentially increasing light scattering. Among the hydrocolloids tested, XG exhibited the highest apparent viscosity (Table S1), which may enhance its retention on the kernels during draining. The greater apparent retention of XG may also reflect its anionic nature and relatively high viscoelasticity, which may promote its association with starch surfaces (Chaisawang and Suphantharika 2006; Kim et al. 2009).

Compared with XG, KC and LBG induced only minor changes in kernel morphology and opacity at the same addition level (0.3%, w/w; Figure 2a; Table S2). This limited change in opacity may be related to the relatively low apparent viscosities of KC and LBG solutions (Table S1), which could reduce their adhesion to kernel surfaces during the draining step after soaking and thus have a minimal impact on kernel opacity. GG, which showed a higher apparent viscosity than KC and LBG but lower than XG (Table S1), produced a clear but smaller increase in kernel opacity when applied at 0.3% during hydrothermal treatment (Figure 2a; Table S2). This pattern could be related to differences in hydrocolloid adhesion to kernel surfaces during soaking and draining, with GG showing an intermediate effect between KC/LBG and XG. Similar observations were reported by Chaisawang and Suphantharika (2006), who showed by SEM that XG coated starch granules more extensively than GG. This difference was attributed to polymer characteristics and interfacial interactions. Notably, the hydrothermally treated sample without hydrocolloid (HT) showed higher transparency than native rice (Table S2). This change may be related to the leaching of chromophoric components (e.g., pigments and other soluble constituents) from the kernels during the mild heating and draining step, which could contribute to the observed increase in transparency. Whiteness differed among the samples depending on hydrocolloid type (40.2–45.2; Table S2). HT‐XG0.3 showed lower whiteness (40.2) than the other samples, consistent with its markedly increased opacity, whereas HT‐GG0.3 (43.7) was comparable to native rice (44.2). Kernel integrity of cooked rice is an important quality attribute in this study, and it cannot be inferred from the head rice ratio of raw milled rice because cooking alters kernel shape and integrity. As cooked‐rice kernel integrity is difficult to quantify reliably, we assessed it qualitatively from cooked‐rice images (Figures 2b and 3b, Figure S4), which show kernel shape retention and inter‐kernel separation after cooking.

3.1.2. Kernel Appearance After Cooking

Kernel images of the cooked rice samples are shown in Figure 2b. After cooking, native rice kernels showed pronounced swelling and deformation, consistent with extensive starch gelatinization and the release of starch leachate that can promote inter‐kernel adhesion. Despite the visible fissures present in HT kernels before cooking (Figure 2a), HT kernels exhibited reduced swelling and largely retained an ellipsoidal kernel shape after cooking, suggesting enhanced structural integrity and lower susceptibility to cooking‐induced disruption, with less apparent starch leachate exudation than in native rice (Figure 2b). This behavior may reflect changes at both the kernel surface and within the starch during hydrothermal treatment. Mild heating near the onset of gelatinization temperature promotes limited starch leaching (C. Zhang et al. 2018, 2019), and surface‐associated leachate may reassociate during tempering and drying to form a partial barrier that could restrict water uptake during cooking, consistent with the barrier effect reported for hydrothermally treated starch (Hormdok and Noomhorm 2007). In addition, mild heating may promote starch‐chain reorganization within the kernel, which could further reduce water uptake and swelling (Cham and Suwannaporn 2010; Mapengo et al. 2019). As these surface and microstructural changes were not measured in this study, this interpretation remains speculative.

The presence of hydrocolloids during the mild‐heating step strongly influenced the morphology and integrity of cooked HT kernels (Figure 2b). At an addition level of 0.3% (w/w; rice dry‐solids basis), kernels treated with KC, LBG, or XG exhibited greater cooking‐induced deformation and loss of kernel integrity than the hydrothermally treated control without hydrocolloid (Figure 2b). These observations suggest that hydrocolloids can modulate surface‐layer development during hydrothermal treatments and affect hydration and swelling during cooking, potentially through interactions with starch leachate released during mild heating. Among the hydrocolloids tested, XG produced the most distinctive macroscopic appearance after cooking. The cooked rice from the XG‐treated sample appeared surrounded by a thick, highly hydrated, gel‐like matrix, making it difficult to separate individual grains (Figure S2). This observation coincided with the marked kernel deformation and fragmentation observed in the corresponding kernel images (Figure 2b). This behavior is consistent with the strong thickening and adhesive properties of XG, which can form highly hydrated structures with starch and persist on cooked‐rice surfaces (Kim et al. 2009; Chaisawang and Suphantharika 2005, 2006; Fuwa et al. 2016).

In contrast to LBG, KC, and XG, GG produced cooked kernels with a more uniform appearance and good retention of an ellipsoidal shape (Figure 2b, Figure S2). As a nonionic galactomannan, GG can interact with starch leachate to form starch–hydrocolloid networks (Ahmad and Williams 2001; Funami et al. 2005; Weber et al. 2009), possibly forming a more stable surface layer that limited cooking‐induced swelling. Consistent with this, GG‐treated kernels retained their shape after cooking. Collectively, these observations suggest that hydrocolloid type influences surface‐layer formation and kernel swelling during cooking, which may contribute to the starch digestibility and textural properties discussed in the following sections. The proposed mechanisms, involving formation of a surface barrier layer and starch–hydrocolloid interactions, were not directly examined here and would require confirmation by structural and molecular‐level analysis.

3.1.3. In Vitro Starch Digestibility and Estimated Glycemic Index

In vitro starch hydrolysis kinetics of cooked rice samples are plotted in Figure S3, and digestibility fractions (RDS, SDS, and RS) and eGI values are summarized in Table 1.

TABLE 1.

In vitro starch digestibility fractions and estimated glycemic index of cooked rice prepared from native rice and from rice hydrothermally treated in the absence or presence of hydrocolloids.

Samples RDS (%) SDS (%) RS (%) eGI
Native 59.20 ± 0.50c 37.22 ± 1.04f 3.59 ± 0.79a 91.34 ± 0.33e
HT 54.38 ± 0.66b 32.42 ± 1.15e 13.21 ± 0.56b 86.37 ± 0.17c
HT‐GG0.3 47.23 ± 1.12a 29.35 ± 1.25d 23.42 ± 1.84d 80.76 ± 0.95a
HT‐KC0.3 67.54 ± 0.31d 19.75 ± 0.22b 12.71 ± 0.14b 88.60 ± 0.10d
HT‐LBG0.3 58.90 ± 0.30c 24.32 ± 0.18c 16.78 ± 0.46c 85.57 ± 0.25c
HT‐XG0.3 63.67 ± 1.59e 12.08 ± 2.23a 24.25 ± 1.06d 82.76 ± 0.47b
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Note: HT‐GG0.3, HT‐KC0.3, HT‐LBG0.3, and HT‐XG0.3 denote samples hydrothermally treated with 0.3% (w/w, rice dry‐solids basis) guar gum, κ‐carrageenan, locust bean gum, and xanthan gum, respectively. Values are mean ± SD (n = 3). Different superscript letters within a column indicate significant differences at p < 0.05 according to Duncan's multiple range test.

Abbreviations: eGI, estimated glycemic index; HT, hydrothermally treated rice without hydrocolloid; Native, untreated white rice; RDS, rapidly digestible starch; RS, resistant starch; SDS, slowly digestible starch.

3.1.3.1. Effect of hydrothermal treatment

Across the hydrothermally treated samples, eGI decreased and RS increased relative to Native, irrespective of hydrocolloid addition. Hydrothermal treatment alone reduced eGI to 86.37, accompanied by an increase in RS from 3.59% to 13.21% and a concomitant decrease in RDS. The reduced digestibility likely reflects combined surface and internal modifications induced by the sequential hydrothermal steps. Mild heating near the onset gelatinization temperature (T o) may promote release of amylose‐rich starch leachate without full gelatinization (C. Zhang et al. 2018, 2019). During subsequent tempering and humidity‐controlled drying, starch leachate present at or near the kernel surface may reassociate and retrograde into an enzyme‐resistant layer (Chang et al. 2021; Birt et al. 2013; Putseys et al. 2010). Hydrothermal treatment may also promote starch‐chain reorganization within the kernel, possibly accompanied by a modest increase in molecular order, further reducing susceptibility to enzymatic hydrolysis (Cham and Suwannaporn 2010; Mapengo et al. 2019). Consistent with this, Chung et al. (2012) reported that hydrothermal treatment decreases starch digestibility in germinated brown rice.

3.1.3.2. Effect of hydrocolloid type

Hydrocolloid addition during the mild‐heating markedly altered the in vitro starch digestibility of the cooked hydrothermally treated rice samples (Figure S3; Table 1). Relative to the hydrothermally treated control without hydrocolloid (HT), XG and GG further reduced eGI, whereas KC and LBG had eGI values similar to HT (Table 1). Notably, XG and GG produced distinct shifts among the RDS, SDS, and RS fractions. At 0.3%, the XG‐treated sample showed the highest RS content (6.8‐fold vs. Native and 1.8‐fold vs. HT) and a concurrent increase in RDS (from 54.38% to 63.67%), indicating a shift from SDS toward both RDS and RS. In contrast, GG reduced RDS (from 54.38% to 47.23%) and increased RS (from 13.21% to 23.42%) relative to HT, resulting in a lower eGI than both Native and HT (80.76 vs. 91.34 and 86.37, respectively). Such contrasting fraction profiles suggest that, while both hydrocolloids can reduce overall starch digestibility, their dominant contributions likely differ. After cooking, the XG‐treated sample was surrounded by a thick, highly hydrated, gel‐like matrix that bridged grains (Figure S2). This behavior is consistent with the high apparent viscosity of the XG hydrocolloid solution, which could promote the formation and persistence of a hydrated continuous phase around the grains. Such a viscous phase at the kernel surface may limit water and enzyme transport, thereby contributing to the increased RS fraction. Consistent with this, Sasaki and Kohyama (2012) reported a significant relationship between starch digestibility and apparent viscosity (at low shear) in starch–polysaccharide suspensions. However, HT‐XG0.3 also showed pronounced loss of kernel integrity after cooking, with marked deformation and fragmentation (Figure 2b). Structural collapse and fragmentation of cooked rice kernels can increase exposure of gelatinized starch and thereby promote rapid enzymatic hydrolysis, which may explain the higher RDS fraction. Tamura et al. (2016) reported that the structural and histological characteristics of cooked rice strongly influence starch digestion.

Compared with XG, the GG solution showed much lower apparent viscosity at the same concentration (0.3%; 50.2 vs. 110.2 cP at 58°C; Table S1) and would therefore be expected to be less effective than XG in reducing starch digestibility. This expectation is supported by reports that galactomannans such as GG exhibit limited adhesion and can be partially removed during draining, and that higher hydrocolloid solution viscosity is associated with greater suppression of starch digestibility in simple‐addition systems (Slaughter et al. 2002; Fabek et al. 2014). Despite these expectations, HT‐GG0.3 showed the lowest eGI (80.76) and RDS fraction (47.23%) among the hydrocolloid‐treated samples. This unexpected result may be attributed to differences in hydrocolloid interactions with leached starch, predominantly amylose, during hydrothermal treatment. The contrasting effects of GG and XG may reflect their different chain conformations. GG exists predominantly as flexible coils in aqueous solution, which favors interactions with leached starch chains and promotes formation of GG–starch complexes (Y. Zhang et al. 2020); such GG–starch interactions can induce starch chain rearrangement and aggregation, altering the susceptibility of starch to enzymatic hydrolysis (He et al. 2020). By contrast, the comparatively rigid, rod‐like XG chains may be less effective at sequestering leached amylose, a difference that may contribute to the simultaneous increases in RS and RDS relative to HT. GG may additionally reduce digestibility through direct enzyme inhibition, as galactomannans have been shown to bind α‐amylase and act as noncompetitive inhibitors of α‐amylase–catalyzed starch hydrolysis (Slaughter et al. 2002; Singh and Gilhotra 2020). Beyond these proposed molecular mechanisms, HT‐GG0.3 retained kernel shape better and showed less fragmentation than HT‐XG0.3 after cooking (Figure 2b), which likely limited starch exposure and contributed to its lower RDS and eGI relative to HT‐XG0.3.

Compared with XG and GG, KC and LBG produced comparatively limited effects on starch digestibility (Table 1). Compared with HT, HT‐KC0.3 increased RDS and reduced SDS without increasing RS, whereas HT‐LBG0.3 showed an eGI similar to HT with only a limited increase in RS. These limited responses may be attributed to the lower apparent viscosity and weak adhesion of KC and LBG, which would reduce their retention on kernel surfaces and limit their ability to impede enzyme diffusion into cooked rice kernels. Previous studies have similarly reported that LBG can be less effective than XG or GG in reducing starch digestibility, depending on hydrocolloid viscosity and interfacial behavior (Fabek et al. 2014; Brennan et al. 2008). These surface‐ and molecular‐level mechanisms are offered as interpretations consistent with the digestibility and appearance data, rather than as directly demonstrated pathways. Overall, hydrothermal treatment reduced starch digestibility in cooked rice; moreover, hydrocolloid addition during the mild‐heating step further altered the RDS/SDS/RS distribution depending on the hydrocolloid used. GG showed the lowest eGI and a shift in the fraction profile from RDS toward RS without evident post‐cooking kernel disruption; therefore, GG was considered for concentration‐response testing (Section 3.2).

3.1.4. Textural Properties

Texture is widely regarded as a key determinant of the consumer acceptance of cooked rice (Okabe 1979). Accordingly, TPA was used to evaluate the textural properties of cooked rice prepared from native and hydrothermally treated rice in the absence or presence of hydrocolloids (0.3%, w/w, rice dry‐solids basis), and the results are summarized in Table 2. Hardness, adhesiveness, and their ratio (A/H) are commonly used as indicators of eating quality, particularly in East Asian markets, where a relatively higher A/H is often preferred (Park et al. 2012; Zhou et al. 2002). All parameters reported here were obtained instrumentally; sensory acceptability was not assessed and remains to be confirmed. Hydrothermal treatment alone did not significantly affect hardness relative to native (1567.95 → 1440.53 g), but it significantly increased adhesiveness (705.13 → 772.60 g·mm) and the A/H (0.45 → 0.54). Fissuring and occasional fragmentation of the kernels were observed after treatment and drying (Figure 1); such structural defects may promote water uptake and starch leaching during cooking, thereby increasing adhesiveness. Consistent with this, Srisang et al. (2010) reported lower hardness and higher stickiness in fissured germinated brown rice.

TABLE 2.

Texture properties of cooked rice prepared from native rice and from rice hydrothermally treated in the absence or presence of hydrocolloids.

Samples Hardness (g) Springiness Cohesiveness Chewiness Gumminess Adhesiveness (g·mm) A/H ratio (mm)
Native 1567.95 ± 39.43b 0.81 ± 0.01ab 0.39 ± 0.02b 493.73 ± 28.67b 608.39 ± 41.22b 705.13 ± 75.77a 0.45 ± 0.05a
HT 1440.53 ± 128.82ab 0.80 ± 0.00ab 0.36 ± 0.04a 413.33 ± 57.26a 513.71 ± 68.19a 772.60 ± 40.84b 0.54 ± 0.02b
HT‐GG0.3 1443.00 ± 60.35ab 0.80 ± 0.02a 0.41 ± 0.00b 470.29 ± 11.25ab 585.29 ± 24.10ab 898.50 ± 51.12c 0.63 ± 0.06c
HT‐KC0.3 1329.77 ± 189.70a 0.82 ± 0.03bc 0.39 ± 0.01b 420.68 ± 71.58ab 515.42 ± 73.88a 801.19 ± 68.40b 0.61 ± 0.05c
HT‐LBG0.3 1402.20 ± 46.06a 0.83 ± 0.00c 0.41 ± 0.01b 474.79 ± 10.37ab 574.84 ± 13.42ab 1090.40 ± 40.87d 0.78 ± 0.04d
HT‐XG0.3 2544.83 ± 181.41c 0.85 ± 0.01d 0.46 ± 0.02c 979.58 ± 111.75c 1157.90 ± 125.80c 1355.13 ± 20.28e 0.53 ± 0.03b
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Note: HT‐GG0.3, HT‐KC0.3, HT‐LBG0.3, and HT‐XG0.3 denote samples hydrothermally treated with 0.3% (w/w, rice dry‐solids basis) guar gum, κ‐carrageenan, locust bean gum, and xanthan gum, respectively. Values are mean ± SD (n = 6). Different superscript letters within a column indicate significant differences at p < 0.05 according to Duncan's multiple range test.

Abbreviations: A/H ratio, ratio of adhesiveness to hardness; HT, hydrothermally treated rice without hydrocolloid; Native, untreated white rice.

Hydrocolloid addition primarily affected adhesiveness and hardness, in a type‐dependent manner. Adhesiveness was highest for HT‐XG0.3, followed by HT‐LBG0.3 and HT‐GG0.3, whereas HT‐KC0.3 remained comparable to HT. Hardness did not differ significantly among HT, HT‐GG0.3, HT‐KC0.3, and HT‐LBG0.3, whereas HT‐XG0.3 showed significantly higher hardness and also increased springiness, cohesiveness, and chewiness. The A/H ratio increased in the GG‐ and KC‐treated samples and was highest for HT‐LBG0.3, whereas HT‐XG0.3 showed no improvement in A/H because hardness and adhesiveness increased concurrently. The pronounced effect of XG is consistent with its high thickening capacity. Among the hydrocolloids tested, the 0.3% XG solution had the highest apparent viscosity (Table S1), and HT‐XG0.3 was the most opaque before cooking and formed a highly hydrated, gel‐like matrix after cooking (Figure 2, Figure S2). These observations are consistent with retention of a hydrated XG layer at the kernel surface, which may have increased adhesiveness and, by limiting starch leaching and kernel softening during cooking, contributed to the higher hardness, although the layer was not directly characterized. Whether the resulting firm, sticky profile compromises acceptability cannot be established from TPA alone, but a comparable effect was reported by Fuwa et al. (2016), who found that 0.5% XG raised both hardness and adhesiveness by forming an XG sol layer on the kernel surface and lowered sensory quality.

Unlike XG, both GG and LBG increased adhesiveness without altering hardness, resulting in higher A/H ratios. LBG produced a greater increase in adhesiveness and A/H than GG despite its lower apparent viscosity, indicating that these effects were not governed by viscosity alone. Interactions between the galactomannans and leached surface starch, and differences in their galactose‐to‐mannose ratio and branching (BeMiller 2011; Morris et al. 2008), may contribute to these responses but were not examined here. KC produced the least change in hardness and adhesiveness, both comparable to HT, although its A/H ratio increased, possibly reflecting a lower affinity for the kernel surface. Overall, hydrocolloid type strongly influenced the textural response: XG increased both adhesiveness and hardness with loss of kernel integrity after cooking, whereas GG and LBG increased adhesiveness without increasing hardness, raising A/H. Although HT‐LBG0.3 had the highest A/H, HT‐GG0.3 was selected for concentration‐dependent evaluation because it offered the best overall balance of lower eGI, increased adhesiveness without increased hardness, and preserved kernel integrity. The effect of GG concentration is presented in Section 3.2. As these conclusions are based on instrumental measurements, sensory evaluation will be required to confirm the acceptability of the treated rice.

3.2. Concentration‐Dependent Effects of Guar Gum During Hydrothermal Treatment

3.2.1. Kernel Appearance and Surface Morphology

The appearance of rice kernels hydrothermally treated with different GG concentrations before and after cooking is shown in Figure 3, and the surface morphology of the cooked kernels is presented in Figure 4. Before cooking, all hydrothermally treated samples exhibited fissuring and partial breakage predominantly along the minor axis, regardless of GG concentration, suggesting that these defects were primarily caused by hydrothermal treatment rather than by differences in GG concentration (Figure 3a). However, the appearance of the treated kernels changed with increasing GG concentration. In particular, the kernels became progressively more opaque as GG concentration increased, with the 0.5% GG sample showing the most pronounced loss of transparency, decreasing sharply to 1.8, whereas HT, HT‐GG0.1, and HT‐GG0.3 maintained values between 3.4 and 3.8 (Table S2). Whiteness remained within a narrow range across GG concentrations (41.8–43.7; Table S2), with only a slight decrease at 0.5% GG (41.8) relative to native rice (44.2). Thus, increasing GG concentration mainly reduced transparency rather than whiteness.

After cooking, concentration‐dependent differences among the GG‐treated HT samples became more apparent (Figure 3b). HT and HT‐GG0.1 remained relatively well separated, although HT‐GG0.1 appeared slightly more compact than HT. At 0.3% GG, the cooked kernels appeared more uniformly compact and slightly smaller, suggesting that this concentration more effectively restricted kernel expansion during cooking. In contrast, the 0.5% GG sample did not show a further apparent reduction in kernel size. Instead, the cooked kernels appeared more swollen and irregular, with less distinct separation between kernels and a thick external layer. These observations suggest that the effect of GG on kernel expansion during cooking was concentration‐dependent, but did not increase proportionally with concentration. Fuwa et al. (2016) reported that, as hydrocolloid concentration increased, a progressively thicker hydrogel layer formed on the surface of cooked rice. At 0.3% GG, a GG‐rich surface layer may have limited water ingress into rice kernels and restricted kernel expansion, producing the most compact and homogeneous cooked kernels while maintaining kernel separation. By contrast, at 0.5% GG, excessive surface accumulation of GG appeared to produce a thick external layer and irregular aggregation rather than further improving the kernel uniformity achieved at 0.3% GG.

SEM images further revealed differences in surface morphology after cooking (Figure 4). Native showed a relatively smooth surface with slight wrinkles, likely reflecting extensive starch gelatinization accompanied by water uptake, kernel expansion, and leaching of starch components, primarily amylose (Funami et al. 2005; da Rosa Zavareze and Dias 2011). Compared with Native, hydrothermally treated rice kernels exhibited substantially less surface wrinkling. This may primarily reflect reduced starch leaching during cooking, likely due to greater kernel structural integrity developed during hydrothermal treatment. However, the hydrothermally treated control exhibited a rougher surface with visible cracks, particularly along the minor axis, corresponding to the fissures already evident before cooking (Figure 3a). These observations suggest that hydrothermal treatment altered surface morphology, with pre‐existing fissures still evident after cooking. As GG concentration increased, the kernel surface became progressively smoother. At 0.1% GG, the cracks visible in the hydrothermally treated control were no longer evident, although residual roughness remained at 0.1% and 0.3% GG. At 0.5% GG, the surface became markedly smoother, with treatment‐induced roughness largely absent. Importantly, comparison between Figures 3a and 4 suggests that increasing GG concentration did not prevent fissure formation during hydrothermal treatment. Instead, the fissures present before cooking became progressively less visible after cooking, likely because higher GG concentrations promoted the formation of a thicker GG‐rich layer on the kernel surface. These observations suggest that GG concentration influenced the visibility of pre‐existing fissures after cooking more strongly than the initial formation of structural defects during hydrothermal treatment and drying. Such surface changes may affect cooked‐rice texture and eating quality.

3.2.2. In Vitro Starch Digestibility and Estimated Glycemic Index

In vitro starch hydrolysis kinetics of cooked rice samples prepared from kernels hydrothermally treated with different GG concentrations are plotted in Figure S5, and digestibility fractions (RDS, SDS, and RS) and eGI values are summarized in Table 3. Relative to Native, hydrothermal treatment alone reduced starch digestibility, with decreased RDS and eGI and increased RS, as discussed in Section 3.1.3.

TABLE 3.

In vitro starch digestibility fractions and estimated glycemic index of cooked rice prepared from native rice and from rice hydrothermally treated in the absence or presence of guar gum at different levels.

Samples RDS (%) SDS (%) RS (%) eGI
Native 59.20 ± 0.50c 37.22 ± 1.04d 3.59 ± 0.79a 91.34 ± 0.33e
HT 54.38 ± 0.66b 32.42 ± 1.15c 13.21 ± 0.56b 86.37 ± 0.17d
HT‐GG0.1 58.82 ± 0.60c 22.78 ± 0.32a 18.40 ± 0.92c 84.82 ± 0.50c
HT‐GG0.3 47.23 ± 1.12a 29.35 ± 1.25b 23.42 ± 1.84d 80.76 ± 0.95b
HT‐GG0.5 48.26 ± 0.78a 24.22 ± 2.41a 27.52 ± 1.81e 79.16 ± 0.71a
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Note: HT‐GG0.1, HT‐GG0.3, and HT‐GG0.5 denote samples hydrothermally treated with guar gum at 0.1%, 0.3%, and 0.5% (w/w, rice dry‐solids basis), respectively. Values are mean ± SD (n = 3). Different superscript letters within a column indicate significant differences at p < 0.05 according to Duncan's multiple range test.

Abbreviations: eGI, estimated glycemic index; HT, hydrothermally treated rice without hydrocolloid; Native, untreated white rice; RDS, rapidly digestible starch; RS, resistant starch; SDS, slowly digestible starch.

GG addition during hydrothermal treatment decreased starch digestibility. As GG concentration increased from 0.1% to 0.5%, RS increased from 18.40% to 27.52%, whereas eGI decreased from 84.82 to 79.16 (Table 3). These results indicate that GG reduced starch digestibility in a concentration‐dependent manner. The concentration‐dependent changes in starch fractions of cooked rice were not entirely linear. At 0.1% GG, RDS remained similar to that of Native, whereas SDS decreased markedly and RS increased relative to HT, suggesting that even a low GG concentration altered enzyme access to starch within the cooked‐rice matrix. RDS was markedly lower at 0.3% and 0.5% GG than at 0.1% GG or HT, with little difference between 0.3% and 0.5% GG, whereas eGI was slightly lower at 0.5% GG. The hydrolysis curves showed a similar trend, with GG‐treated samples exhibiting lower starch hydrolysis than HT throughout the digestion period (Figure S5). These results indicate that GG altered not only overall starch hydrolysis but also the distribution of starch among the rapidly digestible, slowly digestible, and resistant fractions.

The reduction in starch digestibility with increasing GG concentration may be attributed to increased retention of GG at the kernel surface. Visual and SEM observations in Section 3.2.1, along with the concentration‐dependent increase in apparent viscosity of the GG dispersions (Table S1), suggest increased retention of GG at the kernel surface. The higher viscosity of the GG dispersion likely promoted retention of GG at the kernel surface during hydrothermal treatment and subsequent cooking. First, the resulting GG‐rich surface layer may have limited water uptake and starch swelling during cooking, thereby reducing subsequent enzyme diffusion into the kernel interior during in vitro digestion. Because starch hydrolysis depends on both gelatinization and enzyme accessibility within the food matrix, such a surface barrier may reduce hydrolysis (Sasaki 2018). Moreover, GG may have interacted with starch leachate released during hydrothermal treatment to form associations at the kernel surface, thereby limiting enzyme access to the leachate. Because starch leachate at the kernel surface is readily hydrolyzed, suppression of its hydrolysis may have contributed to the reduction in starch digestibility. Previous reports in starch model systems suggest that hydrocolloids can suppress starch hydrolysis through diffusion‐limiting effects of hydrocolloids retained at the surface and through hydrocolloid–starch interactions (Brennan et al. 2008; Fabek et al. 2014). Extending previous observations from starch model systems, the present results in hydrothermally treated intact rice kernels indicate that increasing GG concentration promoted greater retention at the kernel surface, which was associated with reduced starch digestibility.

Although RS increased from 0.3% to 0.5% GG, eGI decreased only slightly (80.76 to 79.16), and RDS remained similar at the two concentrations (47.23% and 48.26%). These results suggest that, once a continuous GG layer had formed on the kernel surface, further increases in GG concentration provided only limited additional reduction in starch digestibility. As shown in Section 3.2.1, HT‐GG0.5 exhibited an exceptionally smooth and heavily coated kernel surface, suggesting that a thicker surface layer at 0.5% GG intensified surface coverage but was not proportionally more effective in lowering eGI. Therefore, although 0.5% GG produced the lowest eGI, 0.3% GG appeared to reduce starch digestibility with less adverse impact on appearance and texture. At 0.3% GG, RS content increased about 6.5‐fold (from 3.59% to 23.42%) and RDS content decreased (from 59.20% to 47.23%), lowering the eGI by about 12% (from 91.34 to 80.76; Table 3). Because rice is a staple food consumed in large amounts, even this moderate reduction in eGI, together with the marked increase in RS content, may be nutritionally meaningful, particularly given that higher white‐rice consumption is associated with an increased risk of type 2 diabetes (Hu et al. 2012). It should be noted, however, that eGI is an in vitro estimate and does not necessarily predict the postprandial glycemic response; its effect therefore remains to be confirmed in vivo.

3.2.3. Textural Properties

The textural properties of cooked rice prepared from kernels treated hydrothermally with different GG concentrations are summarized in Table 4. Among the measured parameters, adhesiveness showed the most evident concentration‐dependent response to GG addition. Compared with HT, adhesiveness increased by 17.7% and 16.3% at 0.1% and 0.3% GG, respectively, with no significant difference between these two concentrations, whereas a marked increase of 84.2% was observed at 0.5% GG. By contrast, hardness did not differ significantly from HT at any GG concentration, while the value at 0.5% GG was comparable to that of Native. Accordingly, the A/H ratio was higher in all GG‐treated samples than in HT, with values of 0.68, 0.63, and 0.91 for HT‐GG0.1, HT‐GG0.3, and HT‐GG0.5, respectively. These results indicate that GG primarily increased adhesiveness, whereas hardness did not differ significantly from HT, thereby increasing the A/H ratio across the GG‐treated samples, with the highest value observed at 0.5% GG.

TABLE 4.

Texture properties of cooked rice prepared from native rice and from rice hydrothermally treated in the absence or presence of guar gum at different levels.

Samples Hardness (g) Springiness Cohesiveness Chewiness Gumminess Adhesiveness (g·mm) A/H ratio (s)
Native 1567.95 ± 39.43c 0.81 ± 0.01c 0.39 ± 0.02b 493.73 ± 28.67b 608.39 ± 41.22b 705.13 ± 75.77a 0.45 ± 0.05a
HT 1440.53 ± 128.82b 0.80 ± 0.00c 0.36 ± 0.04a 413.33 ± 57.26a 513.71 ± 68.19a 772.60 ± 40.84a 0.54 ± 0.02b
HT‐GG0.1 1344.63 ± 70.90a 0.77 ± 0.01b 0.39 ± 0.01b 401.41 ± 31.63a 522.55 ± 40.55a 909.25 ± 56.43b 0.68 ± 0.05c
HT‐GG0.3 1443.00 ± 60.35b 0.80 ± 0.02c 0.41 ± 0.00b 470.29 ± 11.25b 585.29 ± 24.10b 898.50 ± 51.12b 0.63 ± 0.06c
HT‐GG0.5 1562.92 ± 41.66c 0.75 ± 0.01a 0.41 ± 0.01b 477.62 ± 13.06b 633.66 ± 17.13b 1423.36 ± 55.21c 0.91 ± 0.02d
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Note: HT‐GG0.1, HT‐GG0.3, and HT‐GG0.5 denote samples hydrothermally treated with guar gum at 0.1%, 0.3%, and 0.5% (w/w, rice dry‐solids basis), respectively. Values are mean ± SD (n = 6). Different superscript letters within a column indicate significant differences at p < 0.05 according to Duncan's multiple range test.

Abbreviations: A/H ratio, ratio of adhesiveness to hardness; HT, hydrothermally treated rice without hydrocolloid; Native, untreated white rice.

These textural responses appeared to be related to concentration‐dependent changes in GG solution properties and kernel surface characteristics described above. As GG concentration increased, the apparent viscosity of the treatment solution increased markedly, reaching 72.3 cP at 0.5% (Table S1). Concurrently, kernel transparency remained relatively high at 0.1% and 0.3% GG, but decreased sharply at 0.5% GG (Table S2). These results suggest that higher GG concentrations promoted greater retention of GG at the kernel surface during hydrothermal treatment and subsequent cooking. At 0.1% and 0.3% GG, the surface layer may have been formed by GG associated with leached starch, which may have increased adhesiveness without substantially altering hardness. At 0.5% GG, however, the substantially higher viscosity likely restricted water mobility, kernel swelling, and starch leaching (Mudgil et al. 2014; Yoo and Chang 2018), thereby promoting the formation of a thicker GG‐rich outer layer. The thicker GG‐rich surface layer may have contributed to greater adhesiveness than in samples treated with 0.1% and 0.3% GG, while reduced swelling of the kernel interior may have contributed to greater hardness. Representative photographs of cooked rice prepared from kernels hydrothermally treated with different concentrations of GG provide visual support for this explanation, revealing clear differences in appearance among samples (Figure S4). HT‐GG0.1 and HT‐GG0.3 largely retained individual kernel shape, although they showed slightly greater surface adhesion than HT. In contrast, HT‐GG0.5 showed pronounced agglomeration with reduced kernel separability, indicating that the marked increase in adhesiveness was evident in both instrumental measurements and visual appearance. Accordingly, although 0.5% GG produced the highest adhesiveness, this pronounced surface adhesion may be detrimental to eating quality because of reduced kernel separability.

Overall, the digestibility, textural properties, and visual appearance of the cooked rice suggested that 0.3% GG was more appropriate than 0.5% GG under the present hydrothermal treatment conditions. At 0.3% GG, starch digestibility was lower than in Native and HT, whereas adhesiveness increased without a corresponding increase in hardness. By contrast, increasing GG concentration from 0.3% to 0.5% only marginally lowered eGI, whereas it substantially increased adhesiveness. This marked increase in adhesiveness was associated with pronounced agglomeration, reduced kernel separability, and discoloration after cooking. The limited additional reduction in eGI achieved at 0.5% GG was therefore insufficient to offset these undesirable quality changes. Accordingly, 0.3% GG appeared to be the more appropriate concentration for hydrothermal treatment, providing reduced starch digestibility while maintaining cooked‐rice texture and kernel integrity comparable to native rice.

3.2.4. Thermal Transition

The gelatinization onset (T o), peak (T p), and conclusion (T c) temperatures, along with gelatinization enthalpy (ΔH gel), are presented in Table 5. Compared with Native, hydrothermal treatment in the absence of GG increased T o from 57.7°C to 61.8°C and narrowed the gelatinization temperature range (T c − T o) from 22.7°C to 18.6°C, whereas T p and T c did not differ significantly. In addition, ΔH gel decreased from 9.9 to 8.4 J/g. These changes suggest that hydrothermal treatment induced simultaneous disruption and reorganization of the starch structure within rice kernels. During the mild heating, some thermally labile ordered starch regions may have been disrupted or partially gelatinized. Concurrently, the remaining ordered regions may have undergone reorganization through perfection of less stable crystalline regions and stronger intermolecular associations among starch chains. Similar structural changes involving concurrent disruption and reorganization have been reported in hydrothermally modified starches and rice flours (BeMiller and Huber 2015; C. Zhang et al. 2018; Iuga and Mironeasa 2020; Cham and Suwannaporn 2010; Chung et al. 2012).

TABLE 5.

Thermal properties of rice flour obtained from native rice and from rice hydrothermally treated in the absence or presence of guar gum at different levels.

Samples T o (°C) T p (°C) T c (°C) T c − T o (°C) ΔH gel (J/g)
Native 57.7 ± 0.9a 70.1 ± 0.3c 80.4 ± 0.2b 22.7 ± 1.0c 9.9 ± 0.7bc
HT 61.8 ± 0.4c 70.6 ± 0.3c 80.4 ± 0.4b 18.6 ± 0.3a 8.4 ± 0.2a
HT‐GG0.1 60.9 ± 0.2c 67.2 ± 0.6a 80.0 ± 0.8b 19.0 ± 0.8ab 9.3 ± 0.2b
HT‐GG0.3 59.4 ± 0.5b 68.3 ± 0.7b 80.3 ± 1.2b 20.9 ± 1.5bc 9.6 ± 0.3b
HT‐GG0.5 58.1 ± 0.2a 67.5 ± 0.2ab 78.3 ± 0.9a 20.3 ± 1.1ab 10.4 ± 0.4c
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Note: HT‐GG0.1, HT‐GG0.3, and HT‐GG0.5 denote samples hydrothermally treated with guar gum at 0.1%, 0.3%, and 0.5% (w/w, rice dry‐solids basis), respectively. Values are mean ± SD (n = 3). Different superscript letters within a column indicate significant differences at p < 0.05 according to Duncan's multiple range test.

Abbreviations: ΔH gel, gelatinization enthalpy; HT, hydrothermally treated rice without hydrocolloid; Native, untreated white rice; T c, conclusion temperature; T c − T o, transition temperature range; T o, onset temperature; T p, peak temperature.

The addition of GG progressively attenuated the hydrothermal treatment‐induced changes with increasing GG concentration. As GG concentration increased from 0.1% to 0.5%, T o progressively decreased from 60.9°C to 58.1°C and became closer to the Native value, while ΔH gel increased from 9.3 to 10.4 J/g. Compared with HT, the GG‐treated samples also showed a broader gelatinization temperature range. By contrast, T p was lower in the GG‐treated samples than in HT, whereas T c did not differ significantly from HT except in the 0.5% GG sample, where it was lower. A plausible explanation is that GG reduced water availability to starch during hydrothermal treatment (Liu et al. 2003), thereby limiting disruption and reorganization of starch structure within rice kernels. This trend became more pronounced with increasing GG concentration, such that HT‐GG0.5 showed the lowest T o and highest ΔH gel among the GG‐treated samples. These thermal data indicate that increasing GG concentration attenuated the hydrothermal treatment‐induced modification of starch within rice kernels. Notably, although HT‐GG0.5 showed the lowest eGI, it did not exhibit changes in thermal properties indicative of more extensive internal starch ordering than in HT‐GG0.3. Instead, its DSC parameters shifted toward a more native‐like profile. These findings suggest that the reduced digestibility of the GG‐treated samples was not attributable primarily to starch reorganization within rice kernels during hydrothermal treatment. Rather, it was more likely associated with restricted kernel swelling, reduced starch leaching, and development of a GG‐rich surface layer around the kernels, as discussed in Sections 3.2.1–3.2.3.

4. Conclusions

Reducing the starch digestibility of cooked rice without compromising its eating quality remains difficult; to address this, intact rice kernels were hydrothermally treated in the presence of hydrocolloids. The kernels were mildly heated near the starch onset gelatinization temperature (soaking at 58°C for 1 h) with or without KC, LBG, XG, or GG, each at 0.3%, tempered at 80°C for 1 h, and dried under humidity‐controlled conditions (50°C, 80% relative humidity, 24 h). Hydrothermal treatment alone lowered the eGI from 91.3 to 86.4 and raised the RS content from 3.6% to 13.2% without significantly altering cooked‐rice texture. This may reflect a partial surface barrier formed during hydrothermal treatment, together with some internal starch reorganization, both of which may restrict swelling during cooking, although neither change was measured here. The effects of hydrocolloid addition depended strongly on hydrocolloid type. At a concentration of 0.3%, GG produced the lowest eGI (80.8, vs. 91.3 for native rice) and increased the RS content 6.5‐fold over native rice. It likely formed a surface layer that restricted swelling, maintaining kernel integrity after cooking while raising adhesiveness without increasing hardness. By contrast, XG lowered eGI but the kernels were surrounded by a thick, highly hydrated matrix, accompanied by increases in both adhesiveness and hardness and by pronounced kernel deformation. The contrasting effects of GG and XG likely reflect their chain conformation: the flexible coils of GG may interact with leached amylose more effectively than the rigid, rod‐like chains of XG. Compared with GG and XG, KC and LBG showed little effect, with eGI values close to that of hydrothermal treatment alone. As the GG concentration increased from 0.1% to 0.5%, eGI decreased progressively. A further increase from 0.3% to 0.5%, however, lowered eGI only slightly (from 80.8 to 79.2) while markedly increasing adhesiveness and causing kernel agglomeration. Therefore, 0.3% was the most suitable concentration. Differential scanning calorimetry provided further insight into the reduced digestibility. Even the sample with the lowest eGI became more native‐like rather than showing the greater internal ordering expected of starch reorganization. This reduction was therefore not attributable primarily to such reorganization, but was more likely associated with a surface layer that restricted kernel swelling, likely formed by interaction between GG and leached starch. From an application perspective, hydrothermal treatment of intact rice kernels with 0.3% GG offers a simple, commercially viable strategy for lowering the starch digestibility of cooked rice while maintaining texture and appearance comparable to native rice. Further studies should verify the postprandial glycemic response in vivo, assess the adaptability of the treatment to rice cultivars differing in amylose content, optimize it for scale‐up, and explore compound hydrocolloid formulations. Sensory acceptability should also be evaluated, as hydrocolloid addition altered the textural properties measured by TPA.

Author Contributions

Dong‐Hwa Cho: conceptualization, data curation, investigation, writing – original draft, visualization. Mingyo Ha: formal analysis, investigation, methodology, software. HyunHo Bae: formal analysis, investigation. Ju Hun Lee: data curation, validation. Hyun‐Jung Chung: conceptualization, project administration, supervision, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting Information: jfds71384‐sup‐0001‐SuppMat.docx

JFDS-91-0-s001.docx (14.8MB, docx)

Acknowledgments

This work was carried out with the support of “Research Program for Agriculture Science and Technology Development (Project No. RS‐2025‐02215005)” Rural Development Administration, Republic of Korea.

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