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. 2025 Aug 27;30:102962. doi: 10.1016/j.fochx.2025.102962

Novel techno-functional properties and starch digestibility of VD20 rice under different processes: Effect of polishing and a newly developed germination process

Le Thi Kim Loan a,⁎, Pham Do Trang Minh a,⁎, Pham Thi Minh Hoang a, Bach Long Giang b, Chaiyut Mansamut c
PMCID: PMC12418993  PMID: 40934034

Abstract

VD20 rice, a specialty cultivar from Tien Giang province, Vietnam, holds nutritional potential but remains underexplored in terms of its functional properties. This study aimed to investigate the effects of polishing and germination, including a novel germination method using “Cẩm” leaf polyphenol extract (PES) as soaking solution, on the techno-functional, antioxidant, and digestibility characteristics of VD20 rice. White and brown rice samples were compared with traditionally germinated and PES-enriched germinated rice. The results showed that germination significantly enhanced antioxidant levels, especially in the PES-enriched sample, which exhibited a 4.3-fold increase in total polyphenol content compared to white rice. Pasting viscosity decreased after germination, while setback and final viscosity increased, indicating stronger starch retrogradation. Most notably, the estimated glycemic index dropped from 70.34 in white rice to 58.23 in the PES-treated sample, highlighting its potential as a low glycemic index ingredient. This study demonstrates that combining germination with natural polyphenol enrichment can improve the nutritional and functional quality of rice flour. The findings support the application of VD20 rice in the development of value-added, health-oriented food products, particularly those targeted at glycemic control.

Keywords: Germinated rice, Polyphenol enrichment, Antioxidant capacity, Starch digestibility, Glycemic index

Highlights

  • •

    Techno-functional properties of VD20 rice were reported for the first time.

  • •

    Germination improved antioxidant activity and reduced starch digestibility.

  • •

    Polyphenol enrichment increased phenolics and lowered glycemic index.

  • •

    PES-treated germinated rice flour is a promising low-GI functional ingredient.

1. Introduction

Rice, an essential global staple, comprises two primary species: Oryza sativa (Asian rice) and Oryza glaberrima (African rice), both belonging to the Poaceae family (Loko et al., 2021). Oryza sativa is esteemed for its flexibility across many environments and its essential function in nourishing more than half of the global population. The Red River Delta and the Mekong Delta in Vietnam have the most extensive farmed lands. Rice cultivars exhibit a spectrum of hues, such as white, brown, and crimson, with carbohydrates constituting 70–80 % of the grain's makeup (Yuen et al., 2021). One of them is VD20 rice, which is considered a local specialty rice variety. The VD20 rice variety is a short-term fragrant rice that matures in 100–115 days and can be cultivated multiple times annually. It is known for resisting pests and diseases and can thrive in silt-rich mangrove soils (Loan et al., 2024). VD20 rice from Go Cong, Vietnam, is produced organically under European standards and is recognized for its small, milky grains and natural aroma. Despite its favorable agronomic traits and consumer appeal, the nutritional and functional characteristics of VD20 rice, especially in its flour form, have not been well documented.

Post-harvest treatments such as polishing and germination significantly influence the physicochemical and nutritional composition of rice. While polishing enhances appearance and palatability, it also removes the bran layer and resulting in the loss of dietary fiber, lipids, proteins, and bioactive compounds (Singh, Rehal, Kaur, & Jyot, 2015). The germination process has recently emerged as a cost-effective and efficient approach for enhancing grain quality. The germination process induces structural alterations and the creation of novel bioactive chemicals, enhancing the nutritional value and stability of grains (Chinma, Anuonye, Simon, Ohiare, & Danbaba, 2015). Germinated rice flour may confer health benefits (Helland, Wicklund, & Narvhus, 2002). Germinated rice flour from the VD20 rice type is absent from the market, despite the advantages linked to the consumption of germinated cereals; also, literature lacks information on this flour's physicochemical and antioxidant properties. The physicochemical properties of rice flour may reflect its market value, applications, and consumer preferences for rice cultivars. Germination involves complex biochemical-physical changes in the rice grain (Bhavadharani & Gurumoorthi, 2025; Loan et al., 2024). During germination, hydrolytic enzymes such as α-amylase, protease, and lipase are activated, leading to the breakdown of starch, proteins, and lipids into smaller, more bioavailable molecules. Starch granules undergo partial hydrolysis, resulting in shorter chains of dextrins and oligosaccharides, while protein degradation improves amino acid availability (Helland et al., 2002). These processes not only improve nutritional quality but also significantly influence techno-functional properties such as gelatinization behavior, swelling power, and viscoelasticity of flour (Bhavadharani & Gurumoorthi, 2025).

In contrast, functional properties dictate food nutrient behavior throughout processing, storage, and preparation, influencing food quality and acceptability (Singh, Sharma, & Singh, 2017). Pasting profiles of starch directly influence the quality of rice-based products. Rice flour is an essential component in numerous rice-based products, prompting alterations to their physicochemical qualities to enhance the quality of the final products (Qadir & Wani, 2023; Van Ngo & Luangsakul, 2025). Modified rice flour has gained significance in processed meals due to its enhanced functional qualities that satisfy the food industry's demands (Chinma et al., 2015; Luangsakul & Van Ngo, 2025). The appropriate modification significantly enhances the functions of starch. Furthermore, polyphenols may function as antioxidants and provide numerous potential health benefits, such as reduced digestion rate (Ngo, Kusumawardani, Kunyanee, & Luangsakul, 2022). While germination alone confers substantial benefits, enriching polyphenols during germination may enhance flour quality and render it appropriate for various food applications. Polyphenols, especially anthocyanins, can interact with starch molecules via hydrogen bonding or through the formation of V-type amylose inclusion complexes (Ngo et al., 2022). These interactions can reduce starch swelling and enzyme accessibility, which helps modulate starch digestibility (Ngo et al., 2022; Wu, Liu, Jia, Zhang, & Ren, 2024).

The extract from the leaves of the “lá cẩm” plant (Peristrophe bivalvis) (PES), which is naturally rich in anthocyanins – the purple-red pigments with potent antioxidant properties. It is also traditionally used in Vietnamese cuisine to impart a vibrant purple color to foods like sticky rice, and its deep hue reflects a high anthocyanin content (Thuy, Han, Minh, & Van Tai, 2022). By soaking VD20 rice grains in PES solution, it migh allow these water-soluble anthocyanins and other polyphenols to diffuse into the rice bran and endosperm. This treatment is expected to enrich the rice with additional antioxidants and also introduce a natural coloration. Importantly, anthocyanins may interact with the rice's starch fraction in beneficial ways (Van Ngo & Luangsakul, 2025). Prior studies indicate that anthocyanin molecules can associate with starch, forming inclusion complexes, and can even inhibit starch-degrading enzymes. Such interactions suggest that anthocyanin-infused rice might have a modified starch structure or slower digestibility (Ngo et al., 2022; Wu et al., 2024).

Thus, the polyphenol-soaking step serves a dual purpose: enhancing the antioxidant content of the germinated rice and modifying the starch on a molecular level. This combined intervention leverages the bioactivity of anthocyanins alongside the intrinsic benefits of germination. Therefore, this study is aimed to fill the knowledge gap regarding the functional potential of VD20 rice by evaluating the effects of polishing, germination, and polyphenol-enriched soaking on its starch properties, antioxidant capacity, and digestibility. Specifically, it sought to determine how these treatments influence the physicochemical and nutritional quality of VD20 rice flour, and whether the combined germination and polyphenol enrichment approach could yield a low-glycemic index (GI), antioxidant-rich ingredient suitable for functional food applications. Such improvements are scientifically and practically significant. It might point to a new way of producing nutrient-enriched rice flour that could be utilized in health-oriented food products.

2. Materials and methods

2.1. Materials

Polished and unpolished VD20 rice was purchased from a local farmer in Tien Giang province (Vietnam). All enzymes were purchased from Sigma-Aldrich (MO, USA). Peristrophe bivalvis leaves were collected fresh from a local market and used for polyphenol extraction as the method of Thuy, Han, et al. (2022) and denoted as PES (Peristrophe bivalvis leaves extracted solution). Briefly, the leaves were subjected to water extraction at a ratio of 1:10.83 (w/v) using microwave assisted extraction techniques. The microwave power was consistently set at 600 W, the extraction duration was 4.39 min, and the resultant extract contained an anthocyanin concentration of 30.97 mg/g dry weight and total phenolic content of 225.9 mg GAE/g.

Pepsin and pancreatin used in this study's in vitro starch digestibility method were identical to those recommended in the protocol Van Ngo and Luangsakul (2025). Other chemicals used were analytical reagents.

2.2. Preparation of the sample

Unpolished VD20, brown VD20 rice, was separated into bran and polished rice (white VD20 rice) using a local polishing machine. The polishing rate was 89 %. For the preparation of germinated VD20 rice, brown rice was used for this experiment. The germination of VD20 rice was conducted by soaking rice grains in water with a grain-to-liquid ratio of 1:3 (w/v) for 6 h at a temperature of 25 °C in the dark. Germination conditions were controlled as the study of studied Thao, Nhi, Giang, and Phat (2024), including the germination temperature of 37 °C and germination time of 20. In comparison, the sample was soaking in the PES and denoted as germinated rice + PES sample.

2.3. Determination of novel techno-functional properties

2.3.1. Antioxidant and hydration properties

The sample's total polyphenol content (TPC) was analyzed using the following method Shen et al. (2015). TPC was determined using the Folin–Ciocalteu method and expressed as mg GAE/g dry weight. Briefly, the sample extract (10 μL) was incubated with 100 μL of Folin-Ciocalteu's reagent (10-fold dilution) for 5 min. Then 1 M sodium carbonate (80 μL) was added and incubated at room temperature for 30 min. The total phenolic content was measured at absorbance 760 nm.

Antioxidant activity of rice samples was also followed by the method used in the study Shen et al. (2015). Rice flour extract (100 μl) was mixed with methanol (3.9 ml) and 1.0 ml of DPPH solution (1 M in methanol). Antioxidant capacity was assessed via DPPH radical scavenging activity and reported as % inhibition.

The assessment of the hydration characteristics of rice flour, including swelling power and solubility at 85 °C, according to the methodology outlined by Kraithong et al. (2018). Briefly, the sample (0.25 g) was heated in 10 mL distilled water in a water bath at 85 °C for 30 min with constant mixing and then cooled to room temperature. The samples were centrifuged at 2000 rpm for 20 min. The precipitated and the supernatant part were dried at 105 °C and weighed to calculate rice flour's swelling power and solubility.

2.3.2. Pasting properties

Pasting properties were assessed with a Rapid Visco Analyzer (RVA) (model 4800, Newport Scientific, Australia) following the standard technique for rice flour applicable to the equipment. Rice flour (3 g) and distilled water (25 g) were combined in an RVA cup and swirled at 160 rpm for 10 s to achieve a uniform dispersion. The mixture was subsequently heated to 50 °C and continued for 1 min. The sample was elevated from 50 to 95 °C for 3.42 min, kept at 95 °C for 2.3 min, subsequently cooled to 50 °C, and ultimately held at 50 °C for 3 min. The pasting temperature (PT), pasting viscosity (PV), breakdown (BD), and final viscosity (FV) have been collected and analyzed with Thermocline for Windows.

2.3.3. Rheological properties

The paste was obtained from RVA equipment and used to assess the rheological properties. All tests used an MCR 301 rheometer (Anton-Paar, Austria) with a parallel plate geometry (diameter = 50 mm). A frequency sweep was conducted at 25 °C across a frequency range of 1–100 rad/s with a strain of 0.1, which fell within the established linear viscoelastic regime (Van Ngo & Luangsakul, 2025). The storage modulus (G′), loss modulus (G″), and complex viscosity (η*) were measured as functions of frequency (ω).

2.3.4. Thermal properties

A rice flour sample (about 3.0 mg) was precisely measured in an aluminum DSC ((Differential Scanning Calorimetry) pan, to which 9 μL of deionized water was added. The sample pans were sealed and equilibrated at ambient temperature for 24 h prior to analysis using DSC equipment (Mettler Toledo, Switzerland), and subsequently heated according to a specified temperature protocol (25–95 °C, 10 °C/min) with an empty pan serving as the reference (Van Ngo, Kunyanee, & Luangsakul, 2024). The onset temperature (To), peak temperature (Tp), conclusion temperature (Tc), and gelatinization enthalpy (ΔH) were derived from the heating run curves.

2.3.5. In vitro simulated gastrointestinal starch digestibility

In vitro simulated gastrointestinal starch digestibility was conducted in triplicate using the described method of Van Ngo et al. (2024). Pepsin from porcine gastric mucosa (≥ 250 units/mg solid), pancreatin from porcine pancreas (4xUSP) were purchased from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Amyloglucosidase (from Aspergillus niger, Sigma-Aldrich, 3260 U/mL) and d-glucose assay kit [glucose oxidase/peroxide, Glucose Oxidase–Peroxidase (GOPOD) format] were purchased from Megazyme International Ireland Co. Ltd. (Wicklow, Bray, Ireland). Briefly, rice flour samples (100 mg starch equivalent) were incubated with porcine pepsin (1 mg/mL) in HCl solution (5 mL, 0.02 M) in a water bath at 37 °C for 30 min. The digesta were then neutralized with 5 mL NaOH (0.02 M) and mixed with 25 mL sodium acetate buffer (pH 6, 0.2 M, containing 200 mM CaCl2 and 0.49 mM MgCl2). Pancreatin (2 mg/mL) and 58 U/mL amyloglucosidase in the same sodium acetate buffer solution (5 mL) was added to the digesta, and the mixture was incubated at 37 °C and at 85 rpm. A liquots (0.2 mL) were obtained at various intervals up to 180 min during the small intestine phase, and digestion was inhibited by the addition of 0.8 mL of 95 % ethanol. The digestibility of starch (%) was calculated based on the quantity of glucose released in the supernatant, which was translated to the mass of digested starch using a factor of 0.9. A first-order equation model, as presented by Goñi, Garcia-Alonso, and Saura-Calixto (1997), was employed to characterize the kinetics of starch hydrolysis. The hydrolysis index (HI), representing the starch hydrolysis of a sample, was determined by dividing the area under the starch hydrolysis curve during the simulated small intestine phase (AUCsample) by the starch hydrolysis area of a reference sample (AUCwhite bread). The estimated glycemic index (eGI) was computed using the formula provided by Goñi et al. (1997): eGI = 39.71 + 0.549HI.

2.4. Data analysis

All experiments were conducted in triplicate, and results are expressed as mean ± standard deviation. Statistical analysis was performed using IBM SPSS Statistics 20, and differences among means were evaluated by one-way ANOVA followed by Duncan's multiple range test at a significance level of p < 0.05.

3. Results and discussion

3.1. Antioxidant properties

The content of total phenolic compounds and antioxidant activity of VD20 white rice were 2.43 mgGAE/g and 10.23 %, respectively. Brown rice exhibited significantly higher values than white rice. Rice bran is rich in numerous health-enhancing antioxidant components, including phenolic acid, flavonoids, anthocyanins, proanthocyanidins, tocopherol, vitamin E, γ-oryzanol, and phytic acids, particularly. As most of these beneficial phytochemicals are contained in the bran layer of the rice kernel, they might be significantly eliminated during the polishing process (Singh et al., 2017). After germination, the content of antioxidants increased dramatically, especially with the assisted of “Cẩm” leaves extract. The value of antioxidant content in the germinated sample was double that of brown rice. Enzymes activated during germination can degrade the cell walls of the plant, releasing phenolic compounds and subsequently leading to an increase in total phenolic compounds (Chen, Zhu, & Qin, 2022). However, it can also be seen clearly in Table 1 that the content of germinated rice with “Cẩm” leaves extract has the content of antioxidant compounds remarkably higher than the germinated rice flour without extract. The “Cẩm” leave extract contained high antioxidant compounds (Thuy, Tien, Van Tai, & Minh, 2022), which stuck on the grain during the soaking and germination process, as shown in Fig. 1. Therefore, the germinated rice + PES sample had a significantly high content of antioxidants. Moreover, recent reports presented that there is a strong positive correlation between total phenolic content and antioxidant activity. Higher total phenolic content generally indicates greater antioxidant capacity (Chinma et al., 2015; Qadir & Wani, 2023). In this study also presented as the same trend.

Table 1.

Antioxidant properties of different rice samples.

Sample TPC (mgGAE/100 g) DPPH (%)
White 2.43 ± 0.12a 10.23 ± 0.22a
Brown 12.34 ± 0.33b 30.45 ± 0.34b
Germinated rice 26.64 ± 0.23c 75.65 ± 0.24c
Germinated rice + PES 53.45 ± 0.44d 90.34 ± 0.12d
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Note: Values are expressed as mean ± SD, n = 3. Means followed by different letters in a column are significantly different (P < 0.05).

Fig. 1.

Fig. 1

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Visual appearance of VD20 rice flour.

3.2. Hydration properties

Swelling power (SP) describes the capacity of starch or starchy food to absorb water at specified temperatures, influencing their pasting behaviors and technological characteristics, fundamentally reflecting the interactions between starch hydroxyl groups and water molecules (Van Ngo et al., 2024). Fig. 2 illustrates that the SP value of white and brown rice flour from the VD20 variety differed, with values of 8.97 % and 7.21 %, respectively. A recent study indicated that brown rice consistently exhibited lower swelling power than white rice across various cooking temperatures (Van Ngo et al., 2024). The elevated protein content in the outer layers of brown rice inhibits the expansion of the starchy endosperm. Furthermore, flour proteins and components like carbohydrates and phosphates enhance the water absorption capacity of flours in viscous foods by absorbing water and improving the consistency of the products (Qadir & Wani, 2023). Polishing resulted in a considerable increase in the water absorption capacity of rice flours. Substantial lipids in brown rice flours may have hindered the hydration of starch chains and limited the swelling capacity of the sample due to their non-polar constituents (Devisetti, Yadahally, & Bhattacharya, 2014). Germination process also altered the solubility of sample, however, the value was change insignificantly. Bhavadharani and Gurumoorthi (2025) reported that germination can enhance protein solubility, facilitating enzymatic activity and the formation of new tissue. But the alteration in solubility during germination was minor.

Fig. 2.

Fig. 2

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Hydration properties of different rice samples Note: Different letters above the bars indicate statistically significant difference at P < 0.05 (one-way ANOVA, n = 3).

The variations in SP for native and germinated samples may result from the enhanced hydrophilic components and structural alterations of flour particles occurring during germination (Bhinder, Kaur, Singh, Yadav, & Singh, 2020). The elevated specific surface area may result from disrupting the flour's crystalline and short-range ordered structures, thereby enhancing the permeability of water molecules into the interior of wheat particles and increasing their capacity for interaction with flour macromolecules. The germination process enhances the amylase and protease activities in glutinous brown rice, leading to the degradation of starch and protein macromolecules and an increase in hydrophilic groups (Islam & Becerra, 2012). Consequently, the swelling extent of germinated brown rice flours progressively increased. Brown rice's solubility (SB) was higher than white rice's. After germination, the solubility value of the sample increased. Germination breaks down complex carbohydrates and proteins, thereby increasing their solubility (Singh et al., 2017). Ma, Bykova, and Igamberdiev (2017) reported that when the germination process begin, the more soluble carbohydrates and water-soluble proteins was found, which also could explained the change in value of SB in different samples.

3.3. Rheological properties

The rheological properties of flour paste are crucial for assessing process design, unit operations, and the quality of end products. Fig. 3 depicts the dynamic rheological characteristics of rice flour from white, brown, and germinated rice flour. The rheograms indicate that the storage modulus (G′) exceeded the loss modulus (G″), with both moduli increasing with oscillation frequency, and no crossover between the two moduli was detected across the entire test frequency range. This behavior can be rheologically categorized as a conventional weak gel structure (Clark, 1991). The result also presented that the highest value of G' and G“ was found on germinated rice with PES sample. The storage modulus (G') signifies a material's elastic or “solid-like“ characteristics, indicating the energy retained during deformation. An elevated storage modulus signifies a more rigid, stretchy material. While loss modulus (G") represents the viscous or “liquid-like” behavior, reflecting the energy released as heat during deformation.

Fig. 3.

Fig. 3

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Rheological properties of different rice samples.

The rheological behavior of the rice flour pastes revealed that samples germinated with PES exhibited significantly higher viscoelastic moduli (G′ and G″) compared to all other treatments, suggesting the development of a more structured and elastic gel network. Germination could lead to an increase in the storage and loss modulus. The germination of rice flour enhances both the storage modulus (G') and the loss modulus (G") through alterations in starch structure and heightened enzyme activity, resulting in a more robust and elastic gel-like structure (Chun & Yoo, 2004; Ding et al., 2023). These findings suggest that the enhanced rheological properties in the PES-germinated sample were more likely resulted from polyphenol–starch interactions established during germination. This interaction may involve hydrogen bonding between polyphenolic hydroxyl groups and starch chains, or the formation of V-type amylose inclusion complexes, both of which can contribute to gel reinforcement and increased elasticity (Wu et al., 2024). These results are consistent with prior observations linking phenolic incorporation to improved starch retrogradation and viscoelastic strength in functional food matrices. A recent study indicated that polyphenols can establish hydrogen bonds with the hydroxyl groups of starch molecules, potentially resulting in a more rigid network within the starch structure (Wu et al., 2024), which might be also linked with the solubility data. The interactions enhance the elasticity of the starch gel, thereby improving its capacity to store and release energy during deformation. Increased levels of olyphenols can enhance the viscosity of the starch solution, resulting in a higher loss modulus (G") (Kaur, Mehta, & Kumar, 2025).

3.4. Pasting properties

The pasting properties of rice flour exhibited significant variation across different rice kinds, as illustrated in Fig. 4 and Table 2. The maximum peak viscosity was observed in the white rice sample. Peak viscosity reflects the capacity of the starch granule to associate with water through hydrogen bonding (Balet, Guelpa, Fox, & Manley, 2019; Renzetti, van den Hoek, & van der Sman, 2021). Augmented water binding diminishes the free water among swollen granules, leading to heightened frictional contact, quantified as increased viscosity. Brown rice consistently exhibits lower peak viscosity compared to white rice. The augmentation of starch components following the removal of the rice bran layer may lead to elevated peak viscosity in rice (Van Ngo et al., 2024), which is also positively connected with swelling power. However, the peak viscosity decreased upon germination, and the breakdown value exhibited a similar trend, but these change was not siginificant different. The decrease in peak viscosity and disintegration of rice flours post-germination may be ascribed to starch degradation induced by α-amylase activity. Alpha-amylase activity is swiftly activated and augmented during the germination phase, leading to the degradation of starch (Muralikrishna & Nirmala, 2005). Starch constitutes the primary component of rice flour, and a reduction in starch concentration during germination contributes to diminished pasting viscosities (Wu, Na, Alhassane, Zhengyu, & Xu, 2013). The elevated setback and ultimate viscosity were observed in germinated flour, particularly in germinated rice within the solution of “Cẩm” leaves. The disparity in pasting characteristics between germinated and non-germinated rice flours may be ascribed to variations in starch, protein, and amylose concentration (Chinma et al., 2015). Notably, it also was found that the peak viscosity of germinated rice + PES is reported to be the lowest, but the final viscosity is the highest. Germination activates amylolytic enzymes that partially hydrolyze rice starch, resulting in shorter chains and reduced swelling capacity of the granules, which significantly decreased the peak viscosity during pasting (Helland et al., 2002). Moreover, germinated grains, when immersed in a polyphenol-rich extract, exhibit binding of phenolic compounds to starch through hydrogen bonding and other non-covalent interactions. The complexation of polyphenols with starch limits granule swelling, thereby minimizing peak viscosity, and may also suppress residual amylase activity from germination, which prevents excessive starch degradation during heating (Shen et al., 2015). The soaking process may leach simple sugars and potentially bind or precipitate lipids and proteins from the flour, thereby eliminating components that typically dilute paste thickness or disrupt starch re-association (Wu et al., 2024). Lipids and proteins present in unmalted cereal flours are recognized for their role in suppressing final viscosity; their removal results in increased final viscosity. Upon cooling, the starch chains, now containing shorter linear fragments due to germination, readily re-crystallize and retrograde (Liu et al., 2024). This process is facilitated by the retained amylose content and the lack of inhibitory solutes. The robust re-association of starch, with amylose reforming into a gel network during cooling, accounts for the notably high final viscosity observed. In addition, germination effectively pre-digests starch, leading to a reduction in peak viscosity. The observed reduction in peak viscosity in germinated samples can be attributed to elevated α-amylase activity, which partially hydrolyzes starch granules and disrupts their swelling capacity during heating (Helland et al., 2002). However, final viscosity was notably higher in the PES-treated germinated rice compared to both white rice and germinated rice without PES, suggesting enhanced retrogradation during cooling. Polyphenols present in the PES extract likely contribute to this behavior by forming non-covalent interactions (e.g., hydrogen bonding, hydrophobic interactions) with starch chains or short amylose fragments released during germination, reinforcing gel network formation (Ngo et al., 2022; Wu et al., 2024). While polyphenols may exhibit mild α-amylase inhibitory effects, their impact during germination appears limited; however, during cooling, they may facilitate retrogradation and enhance gel strength (Chou et al., 2020). These findings support a two-step mechanism whereby enzymatic degradation lowers peak viscosity, followed by polyphenol-mediated stabilization of the starch network during cooling (Renzetti et al., 2021; Wu et al., 2024).

Fig. 4.

Fig. 4

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Pasting properties of different rice samples.

Table 2.

Pasting parameters and thermal properties of different rice samples.

White Brown Germinated rice Germinated rice + PES
Peak vicosity (Pa) 3022.5 ± 14.8b 2201.5 ± 44.5a 2121 ± 117.4a 2091.5 ± 98.3a
Breakdown (Pa) 1442.5 ± 126.6b 849.5 ± 132.2a 750 ± 49.5a 766 ± 96.2a
Final vicosity(Pa) 2746.5 ± 81.3a 2678.5 ± 91.2a 3183 ± 104.7b 3246 ± 121.6b
Setback (Pa) 276 ± 96.2a 477 ± 135.8b 1062 ± 12.7c 1154.5 ± 23.3c
Onset temperature
(To, oC)		 63.34 ± 0.34b 65.34 ± 0.12d 62.44 ± 0.34a 64.54 ± 0.33c
Peak temperature
(Tp, oC)		 71.23 ± 0.23a 73.87 ± 0.23c 72.43 ± 0.23b 72.34 ± 0.34b
Conclusion temperature
(Tc, oC)		 79.34 ± 0.23b 80.54 ± 0.21c 77.45 ± 0.23a 79.22 ± 0.23b
Enthaphyl (ΔH, J/g) 4.57 ± 0.34b 5.23 ± 0.34c 4.02 ± 0.23a 4.67 ± 0.23b
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Note: Values are expressed as mean ± SD, n = 3. Means followed by different letters in a row are significantly different (P < 0.05).

3.5. Thermal properties

The germination and polishing processes resulted in a notable decrease in thermal properties (onset, peak, and conclusion temperatures, excluding enthalpy value) of rice flours (Table 2). The highest value of onset, peak, and conclusion temperature was found on brown rice flour sample, which was remarkable significant different. The non-starches components such as protein, fat, and fiber, which inhibit gelatinization, imparted thermal characteristics to the starch (Qadir & Wani, 2023). The decrease in thermal characteristics of germinated rice flours may be attributed to enzymatic starch modification during germination, which leads to a reduction in gelatinization temperatures. Germination markedly alters starch structure due to the activation of hydrolytic enzymes such as α- and β-amylases and debranching enzymes, which partially degrade amylose and amylopectin into shorter chains and simple sugars. As a result, germinated starch typically exhibits lower gelatinization temperatures and enthalpy, reflecting reduced crystalline order (Helland et al., 2002). Additionally, the accumulation of low-molecular-weight sugars during germination may contribute to a slight decrease in gelatinization enthalpy (Baek, Yoo, & Lim, 2004). These findings are consistent with Li, Jeong, Lee, and Chung (2020), who reported a reduction in the gelatinization parameters of brown rice following germination. Interestingly, when rice samples were soaked in Peristrophe bivalvis extract (PES) prior to germination, the thermal properties were higher than those observed in white rice. This may be due to the presence of polyphenols, which are known to interact non-covalently with starch and influence its gelatinization behavior. In many cases, polyphenolic compounds increase gelatinization temperature by stabilizing the starch structure or competing with starch for available water. Polyphenols can hydrogen-bond with starch hydroxyl groups and may also form inclusion complexes with amylose helices, thereby restricting granule swelling and delaying thermal transitions (Wu et al., 2024). Therefore, the combined effects of germination and PES enrichment on starch thermal properties are governed by distinct yet overlapping mechanisms. While germination promotes starch depolymerization and loss of crystallinity—leading to lower gelatinization parameters—polyphenols may counteract these effects by stabilizing starch chains through hydrogen bonding or complex formation. The extent and direction of these changes are influenced by processing conditions, particularly germination duration, extract concentration, and polyphenol composition. Collectively, starch depolymerization, amylose–polyphenol complexation, and changes in crystallinity are key factors dictating whether gelatinization is enhanced or hindered. Furthermore, rice flour possesses the most resilient starch structure, comprising an amylose-lipid complex or an amylose-protein complex, which results in an elevation of the gelatinization temperature due to the amylose-polyphenol interaction (Qadir & Wani, 2023). Moreover, it also could be seen that the DSC endotherm profiles (Table 2) showed slight increases in gelatinization enthalpy and shifts in thermal transition points in the PES-treated samples, which may indirectly reflect the presence of thermally stable starch–polyphenol interactions. Additionally, increased setback and final viscosity observed in RVA profiles, along with elevated G′ and G″ moduli in rheological tests, support the hypothesis that PES polyphenols interact with starch molecules, contributing to retrogradation and gel reinforcement.

3.6. In vitro starch digestibility

The structure of starch has two molecular types: branching amylopectin and linear, slightly branched amylose. The fundamental unit of both molecules is an α-d-glucopyranose residue, which establishes α-1,4-glucosidic bonds in the linear structure of amylose and incorporates additional α-1,6-glycosidic branches in amylopectin (Wang, Ou, Al-Maqtari, He, & Othman, 2024). The starch content in rice grains varies between 75 % and 80 % (Liu et al., 2024). In this study, the expected glycemic index of white and brown VD20 rice were 70.34 ± 0.23 and 67.24 ± 0.17, respectively. It was comparable with Hommali rice in the study of Van Ngo et al. (2024). White rice always has the higher digestion rate than that of brown rice in the same variety (Sun et al., 2010). High content of protein and fat molecules surrounding starch particles in brown rice flour impede the enzymatic conformation required for hydrolysis, potentially resulting in the lowest rates of starch digestion (Qadir & Wani, 2023). However, germination process could reduce the estimated glycemic index (eGI) of rice. The germinated rice and germinated rice + PES had the eGI value of 64.23 ± 0.56 and 58.23 ± 0.34, respectively. The reduction in eGI observed in PES-treated germinated rice may result from two complementary mechanisms as inhibition of digestive enzymes by polyphenols and starch–polyphenol interactions that reduce enzymatic accessibility (Ngo et al., 2022). The PES extract demonstrated moderate inhibitory activity against α-amylase, suggesting a partial contribution to digestion delay. These findings indicate that physical interactions between starch and polyphenols such as hydrogen bonding or inclusion complex formation, may play a more substantial role by creating a compact matrix that limits enzyme penetration. This structural barrier likely reduces the rate of starch hydrolysis and contributes more significantly to the observed lowering of GI, consistent with prior studies (Zhang, Feng, Wang, & He, 2024). Recent findings have shown that germination promotes enzymatic activities such as amylase, protease, and lipase, which modify the starch–protein–lipid matrix and increase the accessibility of polyphenols to interact with starch molecules (Guzmán-Ortiz et al., 2019). Varrious research reported that there is an increase in phenolic compounds during rice germination, there is a lack of information concerning the understanding of metabolic dynamic during this process for reducing the digestion. However, it also possible for interacting between phenolic compounds and starch during germination process, which forming new complex structure. Moroever, the polyphenol could modulate the digestion behaviors in different mechanisms (Ngo et al., 2022). One primary mechanism is the formation of polyphenol–starch complexes through hydrogen bonding and hydrophobic interactions, which inhibit enzyme access to the starch chains (Zhang et al., 2024). These interactions may not always form crystalline regions but can lead to reduced gelatinization and lower enzymatic hydrolysis rates. Once germination is a very complex process that involves several procedures, phenolic compounds can interact with amylose and amylopectin in different ways, in order to form a short-range order structure of crystalline regions in resistant starch (Van Ngo et al., 2024; Van Ngo & Luangsakul, 2025). This happens when amylose interacts by covalent bonds with small molecules and forms inclusion complexes in the shape of left-handed helices named V-type amylose (Shen et al., 2015). These V-type inclusion complexes are thermally stable and less susceptible to α-amylase, thereby contributing to the increase in slowly digestible and resistant starch fractions (Romero Hernández, Gutiérrez, & Bello-Pérez, 2022). In addition, phenolic compounds may influence the pasting behavior of starch by stabilizing its granular structure or altering water absorption. This could lead to lower peak viscosity during heating and higher final viscosity during cooling, as observed in germinated rice treated with polyphenol extracts. The higher final viscosity suggests stronger retrogradation and re-association of starch chains, possibly mediated by polyphenol cross-linking (Wu et al., 2024). It is also important to consider that polyphenols can act as enzyme inhibitors, particularly against α-amylase and α-glucosidase, by binding to their active sites or altering their tertiary structure. This dual action—modifying the starch structure and inhibiting hydrolytic enzymes—may provide a synergistic effect in lowering starch digestibility (Ćorković, Gašo-Sokač, Pichler, Šimunović, & Kopjar, 2022). In other words, part of polyphenols could be embedded into the starch structure. The formation of these interactions depends on the chemical structure of starch, the concentration and type of phenolic compound, and food processing. High antioxidant in the germinated rice samples led to reduction the digestion rate of starch, especially the sample was enriched with PES. Thus, integrating germination with polyphenol fortification from sources such as PES provides a promising strategy to design rice-based functional foods with reduced glycemic response. This approach not only improves nutritional value but may also contribute to managing postprandial hyperglycemia, making it relevant for dietary interventions in populations at risk of type 2 diabetes.

The diagram illustrates how germination and Peristrophe bivalvis extract (PES) enrichment improve the functionality of VD20 rice flour (Fig. 5). Germination promotes enzymatic starch degradation, while PES provides polyphenols that enhance antioxidant capacity and interact with starch molecules. These polyphenol–starch interactions reduce enzyme accessibility and digestibility, contributing to a lower glycemic response. The combined process yields a functional flour with improved antioxidant activity and reduced glycemic potential.

Fig. 5.

Fig. 5

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Mechanistic map for this study.

4. Conclusion

This study provides new insight into the combined effect of germination and polyphenol enrichment using PES on the functional and nutritional quality of VD20 rice flour. The synergistic application of these treatments resulted in improved antioxidant activity, reduced starch digestibility, enhanced swelling capacity, and increased rheological stability, suggesting potential for use in health-oriented food formulations. While germination contributed to starch breakdown and lowered gelatinization temperatures, PES enrichment appeared to partially counterbalance this effect by forming polyphenol–starch interactions that supported retrogradation and enhanced viscoelastic properties. Importantly, the combination of both processes significantly reduced the estimated glycemic index, highlighting its relevance in the development of low-GI food ingredients. This approach demonstrates a cost-effective, natural strategy for value-adding to local rice varieties through mild bioprocessing and plant-based functionalization. However, the study is limited by the lack of direct structural evidence for starch–polyphenol interactions, as FTIR and XRD analyses were not accessible. Future work should focus on characterizing these molecular interactions to better understand their roles in starch functionality and to support the formulation of targeted functional food systems.

CRediT authorship contribution statement

Le Thi Kim Loan: Writing – review & editing, Writing – original draft, Visualization, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Pham Do Trang Minh: Writing – review & editing, Visualization, Supervision, Project administration, Investigation. Pham Thi Minh Hoang: Writing – review & editing. Bach Long Giang: Writing – review & editing. Chaiyut Mansamut: Writing – review & editing, Methodology.

Declaration of competing 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.

Acknowledgment

The authors would like to thank the Department of Science and Technology of Dong Thap province (Vietnam) for providing funding and Tien Giang University for providing facilities to cary out this study.

Contributor Information

Le Thi Kim Loan, Email: lethikimloan@agu.edu.vn.

Pham Do Trang Minh, Email: phamdotrangminh@tgu.edu.vn.

Data availability

Data will be made available on request.

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