Functional and Pharmaceutical Properties of Physically and Chemically Modified Rice Bean (Vigna umbellata) Starches

Abstract

This study explored the functional and pharmaceutical properties of native and modified starches derived from rice bean (Vigna umbellata) using physical (pregelatinization) and chemical (phosphorylation, carboxymethylation) modifications. Native starch (NRBS) exhibited a 27.5% amylose content. Modifications significantly influenced physicochemical characteristics. Swelling power increased from 12.25 g/g in NRBS to 16.34 g/g (pregelatinized, PGRBS) and 18.91 g/g (carboxymethylated, CMRBS), while solubility reached 53.12% in CMRBS. X-ray diffraction study estimated degrees of crystallinity of 26.5%, 19.4%, 22.8%, and 14.5% for NRBS, PGRBS, phosphate crosslinked (CLRBS), and CMRBS, respectively. Oil absorption capacity was highest in CMRBS (1.67 g/g), while its free swelling capacity reached 6.12 g/g at 37 °C. In vitro digestibility showed resistant starch (RS) contents of 11.31%, 5.49%, 17.38%, and 21.65% for NRBS, PGRBS, CLRBS, and CMRBS, respectively. Flowability and compressibility analysis demonstrated that CLRBS had the best flow (Carr’s Index: 12.16%, Hausner ratio: 1.14), while CMRBS exhibited superior tablet hardness across compression forces. These findings highlight rice bean starch, particularly in its modified forms, as a sustainable and multifunctional excipient and ingredient for food and pharmaceutical applications.

1. Introduction

Rice bean (Vigna umbellata (Thunb.) Ohwi & H. Ohashi) is an annual leguminous crop native to the hilly and tropical regions of South and Southeast Asia. Traditionally cultivated as an intercrop alongside maize and other staple crops, rice bean is gaining increasing attention due to its high nutritional value and agricultural adaptability [1,2,3]. Despite its nutritional richness, rice bean remains an underutilized legume, often overshadowed by more commercially prominent pulses such as soybean, mung bean, and cowpea [4,5]. Less explored Vigna species, including moth bean (Vigna aconitifolia), adzuki bean (Vigna angularis), and bambara groundnut (Vigna subterranea) have recently attracted increasing interest in starch research [6,7,8]. This growing attention highlights their potential for the development and application of native and modified starches in both the food and pharmaceutical industries. Rice bean seeds are a valuable source of macronutrients, particularly starch, which constitutes approximately 52–57% of the seed’s dry weight [9]. The starch isolated from rice beans is characterized by its relatively high amylose content, reported to be up to 60% depending on genotype, environmental conditions, and extraction methods [10]. This is notably higher than many conventional starch sources such as rice (15–35%), corn (17–25%), potato (17–24%), and cassava (19–22%). High-amylose starch is of considerable interest due to its superior film-forming ability, slower digestibility, and potential to produce resistant starch beneficial for gut health and glycemic control [11]. The functional properties of rice bean starch, such as its water and oil absorption capacities, swelling power, solubility, emulsion capacity, and freeze–thaw stability, further enhance its attractiveness for industrial applications [12]. These properties enable rice bean starch to serve as a thickener, gelling agent, and fat replacer in low-calorie foods, and biodegradable material for packaging films. Moreover, the presence of resistant starch makes it suitable for developing diabetic-friendly and gluten-free products [13].

Despite these promising attributes, rice bean starch has not yet been widely explored or utilized in the food or pharmaceutical industries. Compared to traditional starch sources, there is limited research on the physicochemical modification of rice bean starch to optimize its functional properties for targeted industrial applications [14,15,16]. Modification techniques, including physical (pregelatinization) and chemical methods (such as phosphorylation and carboxymethylation), have been effectively used in other starches to improve properties like swelling, solubility, oil absorption, flowability, and digestibility [17]. These modifications are crucial for expanding the applicability of starch as a pharmaceutical excipient or a functional food ingredient. In pharmaceutical formulation, starch plays essential roles as a binder, disintegrant, filler, and controlled-release agent. Modified starches with enhanced flowability, compactibility, and swelling capacity are particularly valuable in tablet formulation, where they facilitate efficient processing and effective drug release [18]. Additionally, the development of starches with increased resistant starch content has gained attention due to their potential for use in colon-targeted drug delivery systems [19]. Considering the rising global demand for sustainable, biodegradable, and health-promoting materials, the exploration of starch from underutilized legumes like rice bean presents a significant opportunity. Rice bean starch could serve as an eco-friendly and cost-effective alternative to conventional starch, particularly in rural and marginal farming systems [20].

The objectives of this study were to extract starch from rice bean seeds and to evaluate the effects of physical (pregelatinization) and chemical (phosphorylation and carboxymethylation) modifications on physicochemical and functional properties. The modified starches were further assessed for their potential use as excipients for pharmaceuticals and nutraceuticals. This study aims to expand the current understanding of rice bean starch and unlock its potential as a versatile, sustainable ingredient in both the food and pharmaceutical industries.

2. Materials and Methods

2.1. Chemicals and Reagents

Seeds of V. umbellata were collected from Chiang Dao district, Chiang Mai province, Thailand. Monochloroacetic acid (MCA; CAS No. 79-11-8) was purchased from Merck (Hohenbrunn, Germany). Sodium trimetaphosphate (STMP; CAS No. 7785-84-4) was obtained from Aldrich (Wyoming, IL, USA). All other chemicals used were of analytical reagent (AR) grade or equivalent.

2.2. Proximate Analysis of Rice Bean Seed Powder

Moisture, ash, protein, and fat contents of rice bean powder were determined according to AOAC methods 925.19, 942.05, 992.23, and 920.39, respectively.

2.3. Starch Extraction

Starch was extracted from rice beans following the method of Sandhu et al. [21], with slight modifications. Rice bean seeds (500 g) were soaked in 1.5 L of 0.05 M sodium metabisulfite solution for 15 h at room temperature, then blended to form a slurry. The slurry was diluted with water, filtered through a 75 µm mesh, and allowed to settle for 12 h. The supernatant was discarded, and the sediment was washed three times with distilled water. The starch suspension was adjusted to pH 7.0 using 1 M HCl, centrifuged at 3000 rpm for 5 min, and the supernatant removed. The starch was further washed repeatedly with distilled water (500 mL each), centrifuged, and finally collected by vacuum filtration. The extracted starch was dried at 50 °C for 12 h and ground to pass through a 60-mesh (0.250 mm) sieve prior to use.

2.4. Starch Modification

2.4.1. Pregelatinization

Rice bean starch (50 g) was dispersed in distilled water (500 mL) and heated with continuous stirring for 15 min until a paste-like consistency was achieved. After cooling, methanol (200 mL) was gradually added with continuous stirring. Excess methanol was removed, and the starch was dried in a hot-air oven at 50 °C for 24 h. The dried pregelatinized starch was ground and passed through a 60-mesh sieve, then stored in sealed plastic bags under cool, dry conditions until use.

2.4.2. Phosphorylation

Phosphorylation of rice bean starch was carried out according to Jubril et al. [22]. Rice bean starch (50 g) was dispersed in a 300 mL aqueous solution containing 15 g of monosodium phosphate. The mixture was stirred for approximately 10 min, with the pH adjusted to 6. The soaked starch was recovered by vacuum filtration and subsequently dry heating in a hot air oven at 135 °C for 3 h to induce phosphorylation. The dried starch was washed with 80% v/v methanol and further dried at 50 °C for 6 h. The product was passed through an 80-mesh sieve. The final starch was stored in a tightly sealed plastic bag for further use.

2.4.3. Carboxymethylation

Carboxymethylation of rice bean starch was conducted following Adeyanju et al. [23], with slight modifications. Monochloroacetic acid (25 g) was dissolved in 100 mL of 2-propanol and diluted to 500 mL with additional 2-propanol. Rice bean starch (50 g) was dispersed in this solution, followed by gradual addition of 50 mL of 30% (w/v) sodium hydroxide under continuous stirring. The reaction was maintained at 70 ± 1 °C for 60 min, after which the pH was adjusted to 6.0 using 50% glacial acetic acid. The product was filtered, washed repeatedly with 80% methanol until no chloride remained (confirmed by a silver nitrate test), then given a final wash with absolute methanol. The starch was dried at 50 °C for 6 h, ground through a 60-mesh sieve, and stored in sealed plastic bags under cool, dry conditions until further use.

2.5. Physicochemical Property Evaluation

2.5.1. Amylose Content

Amylose content (AC) of native starch was determined using an Amylose/Amylopectin assay kit (K-AMYL, Megazyme, Ireland), following the method described by Gibson et al. [24]. The method selectively removes amylopectin through complexation with concanavalin A, and amylose content was measured spectrophotometrically after enzymatic hydrolysis to glucose.

2.5.2. Scanning Electron Microscopic (SEM)

SEM analysis to visualize granule morphology and surface characteristics was conducted using a JEOL JSM-5410LV microscope (JEOL, Tokyo, Japan) equipped with a tungsten filament (K-type). Samples were mounted on copper stubs with carbon tape and gold-coated before imaging. Images were acquired at an acceleration voltage of 15 kV under low vacuum conditions (0.7–0.8 torr) at 2000× magnification.

2.5.3. X-Ray Diffraction

XRD analysis of starch samples was performed using a Siemens D-500 X-ray diffractometer (Siemens AG, Munich, Germany). Diffractograms were recorded over a 2θ range from 5° to 40° at a scan rate of 2.5°/min with a step size of 0.02°. The degree of crystallinity of each sample was then estimated, as described by Dome [25]. The calculation was based on the determination of the area under the peaks of the crystalline and the amorphous regions of starch. The percentage ratio between the combined area of the crystalline to the total area under the XRD curve represents the degree of crystallinity.

2.5.4. FT-IR

FT-IR spectra were recorded using a Nicolet 510 FT-IR spectrophotometer (Nicolet Instrument Corp., Madison, WI, USA) with the KBr disc method. Each spectrum was collected in % transmittance mode over 64 scans at a resolution of 4 cm⁻¹. Baseline correction and peak analysis were performed using Omnic software version 6.2. The region at 1200–800 cm⁻¹ was deconvoluted, the absorbance values at 1047 and 1022 cm⁻¹ were determined. The ratio of 1047/1022 cm⁻¹ was calculated to determine the amount of short-range ordering for each sample.

2.5.5. Moisture Content

Moisture content was determined using an Ohaus MB25 moisture analyzer (Ohaus Corp., Parsippany, NJ, USA) with a halogen heating unit. Approximately 2 g of starch was weighed, heated at 105 °C until constant weight, and moisture content was calculated based on the percentage weight loss from the initial sample.

2.5.6. Water Solubility (WS) and Swelling Power (SP) at 70 °C

Starch samples (0.1 g) were weighed into pre-weighed centrifuge tubes, mixed with 10 mL of distilled water, and vortexed for 1 min. The mixtures were heated at 70 °C with intermittent stirring for 10 min, then cooled and centrifuged at 3000× g for 15 min. The clear supernatant was transferred to a pre-weighed crucible and dried at 120 °C to constant weight (W1) to determine WS. The sediment was weighed (W2) to calculate SP. Each sample was analyzed in five replicates. The calculations of WS and SP were based on the following equations, as described by Zhang [26], with slight modifications.

$$\mathrm{WS}={\displaystyle \frac{\mathrm{W}1}{0.1} \times 100\%}$$

(1)

$$\mathrm{SP}={\displaystyle \frac{\mathrm{W}2}{(0.1\times (100\%-\mathrm{W}\mathrm{S})}}$$

(2)

2.5.7. Free-Swelling Capacity at 37 °C

Free-swelling capacity was determined using the teabag method described by Heß et al. [27], with minor modifications. A 1.0 g starch sample was placed in a dry, pre-weighed teabag, which was then sealed and immersed in a water bath at 37 °C for 15 min. The teabag was removed and suspended to drain excess water. The swollen weight was recorded, and FSC (q) was calculated based on the weight gain as follows:

$$q_{FSC} = {\displaystyle \frac{m_t - m_{tb} - m_w}{m_s}}$$

(3)

where m_t is the total weight of the teabag and the swelling content; m_tb is the weight of the empty, dry teabag; m_w is the weight of the water absorbed by the empty teabag; and m_s is the weight of the dry sample.

2.5.8. Oil Absorption Capacity (OAC)

OAC was determined using a modified method of Bhosale and Singhal [28]. Starch (0.1 g) was weighed into a microcentrifuge tube, followed by 1 g of mineral oil. After vortexing for 1 min, the mixture was left to stand at room temperature for 30 min and then centrifuged at 4000× g for 20 min. The supernatant oil was discarded, and the weight of the oil-absorbed starch was recorded. OAC was expressed as grams of oil absorbed per gram of starch.

2.5.9. Starch Digestibility

The digestibility of native and modified rice bean starches was assessed using a Megazyme Resistant Starch Assay Kit (AOAC Method 2002.02), in comparison with high amylose corn starch (HAC, Hi-Maize 260, Ingredion GmbH, Hamburg, Germany). For each sample, three screw-capped tubes containing 100 mg starch and 4.0 mL of enzyme solution (pancreatic α-amylase, 10 mg/mL; amyloglucosidase, 3 U/mL; pH 6.0) were incubated in a shaking water bath at 37 °C. Aliquots were collected at 20 min, 2 h, and 16 h to determine rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS), respectively. Reactions were terminated with 4 mL ethanol and centrifuged at 4000× g for 10 min. The supernatant was diluted in 100 mM sodium acetate buffer and incubated with amyloglucosidase (10 µL, 300 U/mL) at 50 °C for 20 min. The residue was dissolved in 2 M KOH (2 mL) on ice, neutralized with 1.2 M sodium acetate buffer (8 mL), and hydrolyzed with amyloglucosidase (0.1 mL, 3300 U/mL) at 50 °C for 30 min. Glucose content in both fractions was determined using the GOPOD reagent, with absorbance measured at 510 nm. Starch content was calculated as glucose × 0.9. Total starch was the sum of resistant and digested starch, and RDS, SDS, and RS were expressed as percentages of total starch.

2.6. Pharmaceutical Functionality and Evaluation as Potential Excipients

2.6.1. Density and Flow

Bulk and tapped densities of starch samples were determined following the USP standard method [29]. Carr’s Index (CI) and Hausner Ratio (HR) were calculated using the following equations:

$$\mathrm{Carr}’\mathrm{s}\mathrm{index}={\displaystyle \frac{(tapped density-bulk density)}{tapped density} \times 100\%}$$

(4)

$$\mathrm{Hausner} \mathrm{ratio}={\displaystyle \frac{Tapped density}{Bulk density}}$$

(5)

Powder flow properties were evaluated using the fixed-funnel method to determine the angle of repose (AR). A 100 g starch sample was poured through a 15 cm diameter glass funnel positioned 15 cm above a flat surface. Upon forming a conical pile, the height (h) and radius (r) were measured, and the angle of repose was calculated as follows:

$$\mathrm{Angle}\mathrm{of} \mathrm{repose} (\mathrm{AR}, \mathsf{\theta }) = {\tan }^{-1} {\displaystyle \frac{h}{r}}$$

(6)

2.6.2. Compactibility

Starch samples (250 mg) were compressed into tablets using a 4.6 mm flat-face punch and die set on a hydraulic press (Carver Inc., Wabash, IN, USA) at compression forces of 0.5, 1.0, 1.5, and 2.0 tons. Tablet hardness was measured in triplicate using a hardness tester (Erweka, Langen, Germany). A pressure–hardness profile (PHP) was constructed by plotting tablet hardness against the applied compression force.

2.7. Statistical Analysis

All experiments were performed in triplicate unless stated otherwise. Data are presented as mean values. Statistical analysis was conducted using one-way ANOVA in SPSS (v19.0), with Tukey’s HSD test for post hoc comparisons. Student’s t-test was used where appropriate. A p-value of < 0.05 was considered statistically significant.

3. Results and Discussion

3.1. Proximate Analysis and Amylose Content

Proximate analysis showed that the rice bean sample contained 66.1% carbohydrate, 19.3% protein, 4.0% ash, and 0.1% fat. These values were in line with those reported by Bepary et al. [15] on rice bean varieties in India, which averaged 63.4, 18.4, 3.5, and 1.5%, respectively. A study by Kaur [14] also found similar values, which calculated to 61.0, 23.8, 5.0, and 1.6%, respectively, while a study by Shweta et al. [30] reported the values of 57.22, 20.8, 3.1, and 1.59%, respectively. The starch extraction yield was 51.2 ± 3.8%. The amylose content (AC) of native rice bean starch, determined using the lectin concanavalin A method, was 27.5%. The value was significantly lower than the values reported in many previous studies, especially those using iodine method for amylose determination [9,10]. Physical modification (pregelatinization) and chemical modifications (carboxymethylation and phosphorylation) showed minimal effects on the AC and the values were not significantly changed.

3.2. SEM

The SEM images of starch and modified starches granules are presented in Figure 1. Native rice bean starch (NRBS) granules (Figure 1A) appeared oval to polygonal in shape, similar to those of other legume starches, with mostly smooth surfaces, indicating that they are intact and undegraded. There was noticeable variation in granule size, ranging from 5 to 25 µm in diameter. The smooth surfaces indicated that no chemical or physical modification took place. A few granules showed indentations or slight fissures, which were common due to processing or sample preparation. The granules appeared closely packed with slight overlap. Pregelatinized rice bean starch (PGRBS) granules (Figure 1B) appeared deformed or fragmented. The shape was irregular, with loss of well-defined edges, a mark of pregelatinization. The surfaces were rough, wrinkled, or cracked, indicating structural damage and loss of native crystalline order. The noticeable plate-like structures or amorphous masses were likely gelatinized starch residues. For crosslinked rice bean starch (CLRBS) (Figure 1C), most granules retained their basic polygonal or oval shapes, but slight deformation or edge roughening were also observed. There were visible fine cracks, pores, and a slightly rough surface or erosion, possibly due to chemical reaction. In contrast, carboxymethylated rice bean starch (CMRBS) (Figure 1D) largely lost the original shape. The granules were no longer uniformly smooth but exhibited noticeable surface roughness and irregularities. Many granules appeared pitted or eroded, with fissures and cracks indicative of structural disruption from carboxymethylation. Some granules appeared swollen or collapsed, suggesting partial gelatinization or weakened cohesion.

3.3. X-Ray Diffraction

The X-ray diffraction (XRD) patterns of four rice bean starch samples, including native (NRBS), pregelatinized (PGRBS), phosphate-crosslinked (CLRBS), and carboxymethyl (CMRBS) (Figure 2), exhibited clear and progressive changes in crystallinity resulting from physical and chemical modifications. NRBS showed strong, visible peaks at approximately 15.2°, 17.0°, 18.0°, and 23.1° Bragg’s angle (2θ). While these peaks were characteristic of an A-type crystalline polymorph, the overlapping peaks in the 17–18° range, along with a notable shoulder near 20°, suggested a possible C-type crystalline pattern, which was typical of legume starches and reflected a hybrid structure containing both A- and B-type crystallites [9]. The absence of a characteristic B-type low-angle peak at ~5.6° 2θ, which corresponded to a large d-spacing from water channels in the crystalline lattice, could be due to low moisture in the sample [31]. C-type starches offered a unique balance of crystallinity and functional flexibility, which made them responsive to various modification techniques. PGRBS exhibited broadened and less intense peaks, indicating partial gelatinization and disruption of the granular crystalline order. Though some residual crystallinity remains, the reduced peak intensities at 17° and 23° suggested breakdown of the lamellar structure, consistent with previous reports on heat-moisture treated starches [32]. This partial amorphization was responsible for improved solubility and swelling capacity, which can be advantageous in food processing and pharmaceutical applications. CLRBS, chemically modified by phosphorylation using sodium trimetaphosphate, showed further peak broadening and intensity loss, consistent with the formation of crosslinks between hydroxyl groups on starch chains. These phosphate crosslinks restricted chain mobility and hindered the formation of double helices necessary for crystallinity [33]. This resulted in a more disordered semi-crystalline structure. The functional benefits of this modification included increased thermal stability and resistance to enzymatic degradation—traits desirable in controlled-release pharmaceutical formulations. CMRBS, an etherified starch prepared through carboxymethylation in propanol, displayed an almost amorphous pattern, with the fading of distinct peaks and the appearance of broad humps. The carboxymethyl groups interfered with hydrogen bonding and molecular alignment, effectively eliminating crystalline order [20]. The degree of crystallinity (%) of NRBS, PGRBS, CLRBS, and CMRBS was determined to be 26.5, 19.4, 22.8, and 14.5%, respectively.

3.4. FT-IR

The FT-IR spectrum of native rice bean starch (Figure 3A) reveals characteristic absorption bands indicative of starch, including a 3284 cm⁻¹ broad O–H peak, 2924 cm⁻¹ C–H stretching vibrations from –CH and –CH₂ groups, 1637 cm⁻¹ and 1535 cm⁻¹ peaks due to adsorbed water and possible H-O-H bending vibrations. The peaks at 1317 cm⁻¹ and 1242 cm⁻¹ represented C–H bending and C–O–H bending vibrations, respectively, within the starch matrix, while the 1149 cm⁻¹ and 1076 cm⁻¹ bands corresponded to strong C–O–C and C–O stretching vibrations, respectively, which confirmed the presence of glycosidic linkages typical of amylose and amylopectin. Pregelatinized starch (Figure 3B) exhibited an almost identical IR profile to that of native starch, suggesting that the core starch skeleton remained intact. The slightly different pattern observed in the 1000–700 cm⁻¹ region could be attributed to granule swelling and partial breakdown of ordered regions. The IR spectrum of phosphorylated starch (Figure 3C) showed slightly broader and shifted peaks at 3522, 3439, and 3287 cm⁻¹, which suggested increased hydrogen bonding or structural rearrangement due to phosphate group incorporation. The peak at 1243 cm⁻¹ was characteristic of the newly formed P=O bond [34]. As for CMRBS (Figure 3D), the sharp peak at 1593 cm⁻¹ was due to the asymmetric stretching of carboxylate anion (COO⁻), which confirmed the presence of carboxymethyl groups in the structure, while the peak at 1409 cm⁻¹ was likely the symmetric COO⁻ stretching, which further supported carboxymethyl substitution on the starch. The ratio of peaks 1047/1022 cm⁻¹ for NRBS, PGRBS, CLRBS, and CMRBS were 0.82, 0.75, 0.88, and 0.79, respectively. The value represented the amount of short-range ordering for each sample. A higher ratio value marked a greater degree of short-range order, which indicated more crystalline structure and double helix formation in the starch molecule [26].

3.5. Moisture Content (MC)

NRBS yielded a MC of 10.47%, a typical value for native starch. A slight decrease observed in PGRBS (9.89%) was not significant (

Table 1. Physicochemical properties of native and modified rice bean starches.

Sample	MC (%)	Swelling Power at 80 °C (g/g)	Solubility at 80 °C (%)	Free Swelling Capacity at 37 °C (g/g)	Oil Absorbance Capacity (g/g)
NRBS	10.47 ± 0.62 b	12.25 ± 0.50 c	19.48 ± 0.85 c	1.99 ± 0.04 c	1.08 ± 0.04 c
PGRBS	9.89 ± 0.83 b	16.34 ± 0.78 b	21.63 ± 1.24 b	2.11 ± 0.09 b	1.02 ± 0.03 d
CLRBS	8.45 ± 0.73 c	4.11 ± 0.23 d	2.28 ± 0.12 d	1.63 ± 0.08 d	1.22 ± 0.08 b
CMRBS	12.19 ± 1.14 a	18.91 ± 1.25 a	53.12 ± 8.29 a	6.12 ± 0.23 a	1.67 ± 0.11 a

Values under the same heading with identical superscript are not significantly different (p < 0.05).

), while the 8.45% MC of CLRBS was likely a result of phosphate crosslinking that reduced the number of free –OH group to bind water. An increase in MC of CMCBS was the effect of the hydrophilic –COOH groups which also existed in the form of –COONa.

3.6. Swelling Power and Solubility

The swelling power (SP) of PGRBS was 16.34 g/g, which was 30% higher than that of NRBS (12.25 g/g). The two chemically modified starches yielded opposite SP results. CLRBS showed an extremely low SP (4.11 g/g), while CMRBS exhibited the highest SP (18.91 g/g). The solubility of starch samples followed a similar trend. CMRBS exhibited the highest solubility (53.12%), followed by PGRBS (21.63%), NRBS (19.48%), and CLRBS (2.28%). Pregelatinization disrupted the starch crystalline structure, resulting in an increase in the water uptake, which enhanced the swelling and solubility of PGRBS at high temperature. The formation of covalent bonds by phosphate crosslinking restricted granule expansion in CLRBS, thus limiting the penetration of water molecules, which led to low swelling and solubility. A similar result was observed in Madua starch [34]. The introduction of the hydrophilic carboxymethyl group enhanced the dispersion of CMRBS in water and promoted water uptake into the starch chain. The SP of CMRBS would have been higher if not for the increased solubility that converted the gel into solution as the temperature reached the target setting. The swelling power and solubility of starch are critical functional properties that determine its behavior in food processing, as well as playing an important role in the appearance of food products, particularly in terms of clarity, opacity, and color of starch-thickened systems [35]. In pharmaceuticals, these are crucial parameters to be determined for certain excipients, including tablet disintegrant, binder, and gelling agent [17,19,36].

3.7. Free-Swelling Capacity

The free-swelling capacity (FSC) of native and modified starches was determined at 37 °C to evaluate their functionality as tablet disintegrants, excipients incorporated into pharmaceutical formulations to enhance tablet disintegration and facilitate drug release upon oral administration. The test results collected after 15 min revealed the FSC values in the order of CMRBS > PGS > NRBS > CLRBS (

Table 2. Amounts (%DW of total starch) of rapidly (RDS) and slowly (SDS) digestible starches, and resistant starch (RS) in native, pregelatinized, phosphate-crosslinked, and carboxymethyl rice bean starches, in comparison with high amylose corn starch (HAC), a known resistant starch.

Sample	RDS	SDS	RS
NRBS	23.54 ± 3.62 ab	63.70 ± 5.29 a	11.31 ± 0.67 d
PGRBS	29.42 ± 4.74 a	62.87 ± 7.11 a	5.49 ± 1.07
CLRBS	15.83 ± 2.89 c	64.59 ± 4.89 a	17.38 ± 2.17 c
CMRBS	22.01 ± 1.59 b	55.31 ± 4.37 b	21.65 ± 1.59 b
HAC	5.04 ± 0.88 d	39.77 ± 2.73 c	53.19 ± 6.77 a

All values were reported on dry basis of starch. Data are mean ± SD. Means within the same column with different letters indicate significant difference (p < 0.05) by Duncan’s multiple range test.

). Like previous studies, carboxymethylated starch exhibited the best FSC value and could be further developed into a tablet disintegrant [17,19].

3.8. Oil Absorption Capacity (OAC)

OAC of starch significantly influences oil retention, textural attributes, creaminess, and the stability of fat-containing products. This functional property plays a crucial role in improving the sensory qualities of formulations, particularly mouthfeel and flavor retention [37]. Furthermore, OAC serves as a valuable indicator of the potential functionality of starch as an emulsifying agent in pharmaceutical manufacturing. The OAC of NRBS (1.08 g/g) was in the lower range compared to other native starches. The value increased significantly in CMRBS (1.67 g/g) and CLRBS (1.22 g/g), while PGRBS (1.02 g/g) showed a slightly lower OAC than that of NRBS. Carboxymethylation increased OAC due to the presence of more porous structures and hydrophobic sites created by modification, facilitating oil retention. For CLRBS, phosphate group incorporation may induce steric effects within the starch chains, creating additional space and fine capillary structures that accommodated greater oil uptake. Starch granules entrapped the oil molecules within the helical structure, forming starch–lipid complexes [34]. The disruption of the crystalline structure of starch upon pregelatinization caused the granules to become less ordered and more susceptible to water uptake, resulting in an increased solubility and water-holding capacity while lowering the ability to absorb oil.

3.9. In Vitro Digestibility

The digestibility profile of native (NRBS), pregelatinized (PGRBS), crosslinked (CLRBS), and carboxymethylated (CMRBS) rice bean starches were evaluated by measuring their contents of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) (Table 2). The RDS content of PGRBS (29.42%) increased by 6% from that of native starch (23.54%), indicating a greater proportion of starch rapidly hydrolyzed into glucose within 20 min of digestion. This was likely due to the pregelatinization process, which disrupted the crystalline structure of starch granules, enhancing enzyme accessibility and increasing digestibility. In contrast, CLRBS showed a significantly lower RDS content (15.83%). Phosphate crosslinking introduced covalent bonds between starch molecules, reinforcing the granular structure and making it more resistant to enzymatic hydrolysis. The RDS content of CMRBS (22.01%) was not significantly different from that of NRBS, despite being chemically modified starch. The standard resistant starch contained only 5% RDS. The SDS contents were relatively high in NRBS (63.70%) and remained unchanged in both PGRBS (62.87%) and CLRBS (64.59%). The value of CMRBS was slightly lower (55.31%), while the standard HAC yield approximately 40% SDS. The RS contents varied significantly among the starch sample. NRBS contained 11.31% RS, which was considerably higher than that of most commercial starches. However, pregelatinization markedly reduced the RS content to 5.49%, a change that correlated with a reduction in the short-range molecular order of the starch. This was evidenced by a decrease in the FT-IR 1047/1022 cm⁻¹ peak ratio from 0.82 in NRBS to 0.75 in PGRBS, alongside a drop in XRD degree of crystallinity from 26.5% to 19.4%. Interestingly, CMRBS displayed a substantially higher RS content (21.65%) than NRBS, despite having a lower 1047/1022 ratio (0.79) and crystallinity (14.5%). This suggests that RS formation is influenced not only by crystalline or ordered structures but also by other factors such as molecular accessibility and enzyme–starch interactions. The introduction of carboxymethyl groups into the starch structure hinders enzymatic digestion, thereby enhancing RS levels. Similarly, crosslinked rice bean starch (CLRBS) showed an elevated RS content of 17.38%, attributable to the formation of crosslinks that restrict enzymatic penetration and stabilize the starch matrix. A comparable increase in enzymatic resistance was previously reported in Mandua starch modified with STPP/STMP [34]. High-amylose corn starch (HAC), known for its naturally high amylose content, demonstrated the highest RS level at 53.19%, serving as a benchmark for RS-rich starches. These findings indicate that chemical modifications such as carboxymethylation and phosphate crosslinking can effectively alter the digestibility profile of rice bean starches by increasing their resistant starch content. Resistant starch serves as a source of dietary fiber and offers various health-promoting and therapeutic benefits [11]. It contributes to improved glycemic control, enhanced gut health, and increased satiety, while also enhancing product texture, extending shelf life, and enriching fiber content without adversely affecting sensory attributes [38]. Additionally, RS has attracted considerable interest in pharmaceutical applications, particularly in colon-targeted drug delivery systems, sustained-release formulations, and as a biodegradable filler-binder in tablets, due to its resistance to enzymatic digestion in the upper gastrointestinal tract [21].

3.10. Pharmaceutical Functionality

3.10.1. Density and Powder Flow

The bulk and tapped density values of native and modified rice bean starches (

Table 3. Density and flow properties of native and modified rice bean starches.

Sample *	Density (g/cm3)		%CI	HR	AR
	Bulk	Tapped
NRBS	0.64 ± 0.01 c	0.86 ± 0.00 b	25.52 ± 0.75 b	1.34 ± 0.01 b	37.75 ± 3.58 a
PGRBS	0.60 ± 0.01 d	0.70 ± 0.01 d	14.47 ± 1.06 c	1.17 ± 0.02 c	33.39 ± 3.08 b
CLRBS	0.66 ± 0.01 b	0.75 ± 0.00 c	12.16 ± 0.70 d	1.14 ± 0.01 d	32.79 ± 2.29 b
CMRBS	0.69 ± 0.01 a	0.94 ± 0.01 a	27.14± 1.03 a	1.37 ± 0.02 a	36.08 ± 4.49 a

* NRBS, native starch; PGRBS, pre-gelatinized starch; CLRBS, phosphate-crosslinked starch; CMRBS, carboxymethyl starch. Values under the same heading with identical superscript are not significantly different (p < 0.05).

) showed clear variations depending on the type of modification applied. The bulk density ranged from 0.60 g/cm³ to 0.69 g/cm³, while the tapped density ranged from 0.70 g/cm³ to 0.94 g/cm³. Among these, CMRBS exhibited the highest bulk (0.69 g/cm³) and tapped density (0.94 g/cm³). This could be attributed to the introduction of carboxymethyl groups, which may have reduced particle porosity, increasing packing ability. In contrast, pregelatinized rice bean starch (PGRBS) displayed the lowest bulk (0.60 g/cm³) and tapped density (0.70 g/cm³), possibly due to its more porous and loose structure after pregelatinization. CLRBS and NRBS showed intermediate density values, with CLRBS values slightly higher than those of NRBS, suggesting that crosslinking improved compactness without significantly altering particle morphology.

Flow properties were assessed using three parameters: Carr’s Compressibility Index (CI), Hausner Ratio (HR), and Angle of Repose (AR) (Table 3). CI values ranged from 12.16% to 27.14%. CMRBS exhibited the highest CI (27.14%), indicating poor flowability, which could be due to its higher cohesiveness. CLRBS showed the lowest CI (12.16%), suggesting excellent flow characteristics, likely due to improved particle uniformity after crosslinking. HR values ranged from 1.14 to 1.37. CLRBS again showed the best flow with the lowest HR (1.14), while CMRBS exhibited the highest HR (1.37), confirming its poor flow behavior. AR values ranged from 32.79° to 37.75°. NRBS had the highest AR (37.75°), indicating relatively poor flow, despite having moderate CI and HR values. CLRBS presented the lowest AR (32.79°), which corresponds to its good flow as indicated by CI and HR. Interestingly, CMRBS with poor CI and HR still exhibited a lower AR (36.08°) than NRBS, suggesting that AR alone might not fully reflect flowability compared to CI and HR measurements.

3.10.2. Powder Compactibility

The pressure–hardness profile (PHP, Figure 4) illustrates the compressibility of different starch powders, measured as tablet hardness (N) in response to increasing compression force (T). Among the samples, CMRBS exhibited the highest hardness at all compression levels, with a steep linear slope indicating excellent compactibility and tablet integrity—making it effective as a binder, while the data on FSC also suggested its functionality as a disintegrant. PGRBS also performed strongly, achieving significant hardness at lower pressures due to its enhanced plastic deformation properties, supporting previous findings that pregelatinization improves compressibility. CLRBS showed moderate hardness, with a gradual slope characteristic of elastic deformation, making it more suitable for disintegrant or controlled-release applications rather than as a primary binder. NRBS demonstrated the lowest performance, with a shallow slope and poor compressibility, requiring higher compaction forces and often necessitating additional formulation aids such as binders or granulation.

4. Conclusions

This study demonstrated that rice bean (Vigna umbellata) starch, when modified through physical and chemical methods, exhibits significantly enhanced functional and pharmaceutical properties. Pregelatinization increased swelling power to 16.34 g/g and solubility to 21.63%, while phosphorylation improved flowability with the lowest Carr’s Index (12.16%) and Hausner ratio (1.14). Carboxymethylated starch showed the highest swelling power (18.91 g/g), solubility (53.12%), oil absorption capacity (1.67 g/g), and free-swelling capacity (6.12 g/g). Notably, it also exhibited the highest resistant starch content at 21.65%, nearly double that of native starch (11.31%), indicating strong potential for dietary and controlled-release applications. Tablet compactibility was also greatest in carboxymethylated starch, supporting its role as a multifunctional excipient. These findings suggest that rice bean starch, particularly in its modified forms, holds great promise as a sustainable, underutilized source for developing functional food ingredients and pharmaceutical excipients, contributing to the advancement of health-oriented and environmentally friendly product formulations.