Particle size and concentration of ground rice on tribological properties and in vitro starch digestibility of rice porridge

ABSTRACT

Rice porridge is a food for general consumption; however, it may vary in food texture and starch digestibility depending on particle sizes and concentrations. This study investigated the tribological properties, steady shear flow, and in vitro starch digestibility of rice porridge with different particle sizes of ground rice (small size, <0.2 mm; large size, 0.2–0.45 mm; based on sieving) and concentrations (5, 7.5, or 10% w/v). The results showed that a high concentration of large-sized ground rice promoted lubrication properties, with the maximum friction coefficient reducing from ~ 0.7 in the 5% w/v concentration to ~ 0.4 in the 10% w/v concentration. In addition, the reduction in the friction started at a sliding speed of ~ 2 mm/s for the high concentration ground rice compared to ~ 20 mm/s for the low concentration rice. The results of high viscosity, small hysteresis loops, and swollen rice particles under morphological observation showed greater porridge stability for the high concentration, thus promoting its lubrication properties. These physical characteristics of the porridge could also play a role in starch digestibility during in vitro gastric digestion. Therefore, the management of ground rice to obtain a large size and a high concentration in rice porridge could benefit food preparation for the elderly and people having difficulty swallowing.

KEYWORDS:

• Ground rice
• Tribology
• Friction coefficient
• Starch digestibility
• Microstructure

Introduction

Rice porridge is a starch-based food commonly consumed by elderly people. There are varying grain sizes in ground rice, which might have different properties of rheology and lubrication. Nowadays, the rheological and lubrication properties of foods are of great interest in the food preparation for patients with dysphagia and for older adults who may have difficulty swallowing.^[1–4]

Rheology is associated with the viscosity and consistency of foods, with one expected goal being to use such knowledge to slowing down the flow of liquids to prevent choking during food consumption. A shear rate of 50 s⁻¹ occurs during swallowing and is used as a factor in classifying the level or consistency of the liquid based on the National Dysphagia Diet (NDD) categories to provide food that is safe for consumption by people with dysphagia.^[1,2] Tribological studies has been used to investigate the lubrication properties of foods that had been tailor-made for the elderly population with various oral insufficiencies and for people with swallowing disorder to ensure oral comfort when eating.^[4–6]

Tribology is the science of the friction, wear, and lubrication of surfaces in relative motion,^[7] which has been increasingly recognized with respect to oral processing and has been used to explain some complex mouthfeel attributes such as smoothness, slipperiness, and creaminess.^[1,8,9] Lubrication behavior, which has been observed for food colloids, is classically presented using a Stribeck curve with three regions of lubrication: boundary, mixed, and hydrodynamic.^[8]

For starch-based foods, textural characterization and their starch digestibility depend on the characteristics of the starch granules, particle size, and the gelatinization of starch.^[10–12] Characteristics of the gelatinized starch granules, such as swollen starch granules, combine with their concentration to play a role in the lubrication and rheological properties of starch suspensions.^[12–14] A good lubrication property of a suspension is displayed through a reduction in the friction coefficient in the boundary and mixed regions because the particles became entrained in the contact zone, which decreases the friction.^[12,14] It has been reported that a high concentration of granule ghosts promoted the lubrication properties of a starch suspension.^[14] However, dense packing of gelatinized rice particles with increasing concentration could increase the friction due to an increase in the gel modulus.^[13] In addition, the shape and surface of particles had an impact on friction and the lubrication properties of the starch suspension; for example, irregularly shaped particles and stickiness of molecules resisted surface movement and therefore increased the friction.^[13]

Besides particle characteristics, leached-out starch polymers from granules, and stability of the food system also influence the lubrication properties. The stickiness of leached-out starch polymers, such as amylopectin, may cause increased friction in gelatinized rice starch,^[13] while an increase in viscosity due to amylose leaching out into aqueous phase could produce negative or positive effects on lubrication properties.^[12–14]

The in vitro digestibility of rice starch has been investigated using different particle sizes of rice grains by increasing the grinding time and the degree of homogenization to obtain a slurry state of the cooked rice, which resulted in increased the starch digestibility in the early stage of digestion due to the absence of physical barriers, such the endosperm cell wall, and the high degree of homogenization.^[15,16]

Even though rheological properties, tribological properties, and starch digestibility of rice starch suspensions have been reported, research has been limited on their properties based on particle size and the concentration of the ground rice used to make rice porridge. Therefore, the aim of this study was to investigate the lubrication properties, steady shear flow, and in vitro starch digestibility of rice porridge with varied concentrations (5–10% w/v) and sizes of ground rice (based on sieving, <0.2 mm as a small size and 0.2–0.45 mm as a large size). The morphology of the gelatinized rice particles, particle size distribution, and steady shear flow were studied to understand the food matrix characteristics of the porridge. The results from this study should be helpful in the development of suitable rice-based foods, such as rice porridge for the elderly or for people having difficulty swallowing, to ensure its consumption is both pleasant and safe.

Material and methods

Materials

Broken-milled rice (Jasmine variety) was obtained from a local supermarket (Thailand). Alpha-amylase from porcine pancreas (Type VI-B, ≥5 units/mg solid), pepsin (from porcine gastric mucosa in lyophilized powder, 3,200–4,500 units/mg protein), pancreatin (from porcine pancreas, activity 4×USP/g), porcine bile extract and invertase (from baker’s yeast (S. cerevisiae), grade VII, ≥ 300 units/mg solid) were purchased from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Amyloglucosidase (3,260 U/mL) and D-glucose assay kit (glucose oxidase/peroxide, GOPOD format) were purchased from Megazyme International Ireland Co. Ltd. (Wicklow, Bray, Ireland). All chemical analysis materials for electrolyzes preparation including simulated salivary fluid (SSF), simulated gastric fluid (SGF), and simulated intestinal fluid (SIF) used were of analytical reagent grade.

Preparation of ground rice

A sample of the broken-milled rice was ground in a powder grinder (Spring Green Evolution Co. Ltd., Bangkok, Thailand). The ground rice was separated into two different sizes using two screen sieves (40 mesh sieve and 80 mesh sieve) to obtain small (<0.2 mm) and large (0.2–0.45 mm) sizes, respectively. The small and large sizes of ground rice were mixed 1:1 and defined as a mixed size which was included in this study.

Preparation of rice porridge

The ground rice was dispersed in reverse osmosis water to obtain 5, 7.5, and 10% (w/v) concentrations of ground rice, followed by heating on a hotplate stirrer at 80 ± 5°C for 5 min with constant shearing using a magnetic stirrer. After heating, the cooked rice samples were equilibrated at room temperature (25–30°C). All gelatinized rice suspensions were freshly prepared on the day of measurement.

Microscopy

A light microscope (Olympus BX51, Tokyo, Japan) was used to observe the microstructural characteristics of the gelatinized ground rice in the porridge samples with the different sieve sizes and concentrations. The samples were diluted with water before staining with 10 µL iodine solution, and then placed on a glass slide for observation of the starch. The images were recorded at 20X magnification.

Particle size determination

Particle size analysis was carried out using a laser particle size analyzer (Mastersizer 3000, Malvern Instruments Ltd., Worcestershire, England), following the method of Liu et al. (2016).^[13] The refractive indices of the rice samples (both raw and gelatinized rice particles) and water were set at 1.472 and 1.33, respectively. An absorption index of 0.01 was used for the rice particles and the particle size was reported as volume weighted mean (D [4,3]). The volume of the native and swollen granules was estimated from the measured size (D [4,3]) assuming the particles to be completely spherical.^[12] The gelatinized rice particles were added to circulating distilled water until an obscuration value of > 10% was recorded. All the measurements were performed in triplicate.

Steady-shear measurement

The steady shear flow of the porridge was measured at 37°C (representing oral processing temperature) using a rheometer (Anton Paar MCR 302, Graz, Austria), fitted with a 50 mm of a 1° cone-and-plate geometry system (CP50–1) with a gap of 0.210 mm. Prior to the test, the sample was equilibrated between the cone and plate at the measurement gap at 37°C for 60 s to achieve a steady state. The shear rate was increased in logarithmic steps from 0.1 to 1,000 s⁻¹ and then decreased from 1,000 to 0.1 s⁻¹. All the measurements were performed in duplicate.

Tribological measurement

A tribology test of the porridge was performed using a rheometer (Anton Paar MCR 302, Graz, Austria), with a ball-on-three-pins test configuration to estimate the lubricating properties of the samples on the surfaces to mimic the oral cavity. Polydimethylsiloxane (PDMS) pins were used in this study. The samples were measured at 37°C and investigated in three runs of each measurement with increasing sliding speed in logarithmic steps from 10⁻¹ to 10³ mm/s, based on Pondicherry et al. (2018).^[17] The load applied between the upper surfaces and samples was set to 1 N. The data of the second and third run were reported. The friction coefficient (μ) was calculated based on the friction force divided by the load as a function of sliding speed.

In vitro starch digestion

The samples were digested following the standard methods, developed by the COST Action INFOGEST.^[18] Samples (5 g) of the porridge were weighed and dispersed in 5 mL of SSF electrolyte stock solution (pH 7) containing salivary α–amylase. The reaction mixture was incubated in a shaking water bath (Memmert GmbH + Co. KG, Schwabach, Germany) at 37°C for 2 min to simulate the oral phase. After that, 10 mL of SGF electrolyte stock solution (pH 3) containing pepsin was added to the digesta and incubated for 30 min at 37°C. After 30 min incubation in the gastric phase, 20 mL of SIF electrolyte stock solution (pH 7) containing pancreatin and porcine bile extract were added. The digestion in the intestinal phase was allowed to proceed for another 120 min.

During the gastrointestinal digestion, 0.5 mL of the digesta were withdrawn at 2, 17, 32, 37, 42, 47, 62, 92, 122, and 152 min of digestion time and each digesta sample was mixed with 1 mL EtOH to inactivate the enzymes and precipitate dextrin for 30 min. The supernatant was hydrolyzed using amyloglucosidase and invertase to yield glucose^[19] that was determined using glucose determination reagent (GOPOD).^[20] The absorbance at 510 nm was measured using a microplate reader (TECAN, Tecan Trading AG, Switzerland). The cumulated released glucose content at each time increment was reported as the weight (g) of released glucose per 100 g of ground rice. The area under the digestion curve (AUC) was determined graphically.

Statistical analysis

Each treatment was conducted in duplicate with new samples. The data are presented as mean ± standard deviation (SD) and were analyzed by IBM SPSS Statistics Software for Windows (IBM Corp., Armonk, NY). The significant effect of samples on the analysis parameters was determined using one-way ANOVA. The Duncan multiple range test was used to continue the comparison test if there was a significant difference at the 95% confidence level.

Results and discussion

Morphology of gelatinized ground rice porridge

The appearance of the gelatinized ground rice in the porridge, at 10% w/v concentration is shown in Figure 1(a – b). The morphology of the swollen rice particles in the porridge was characterized using light microscopy; the obtained images are shown in Figure 1(c–f). The images showed that morphological changes in the gelatinized ground rice were induced by the different sizes of ground rice and their concentration. After cooking, the small-sized ground rice particles (Figure 1c) seemed to have more swollen granules than the large-sized rice ones (Figure 1d). In addition, there was agglomeration of the swollen rice particles at high concentration (10% w/v) in the porridge made with the large-sized ground rice particles that had the dark blue color of Lugol’s iodine stain, as shown in Figure 1f, while the porridge which had the small-sized ground rice particles had no color (Figure 1e). This could have been due to water absorption of the ground rice being limited by the large-sized particles, thus reducing the swelling capacity. This effect has been reported in other studies, where the swelling power of starch granules was related to the particle size.^[10,11] The results showed that the swelling power, water solubility, and water absorption index of rice flours were significantly reduced with an increase in particle size, because the larger particles were less easily gelatinized and possibly due to the higher rigidity and lower surface area of the rice flours.^[10,11]

Figure 1. Appearance of gelatinized ground rice in porridge (a – b) and light micrographs (c – f) for difference particle sizes and concentrations of ground rice in porridge (20X magnification). (a) 10% w/v concentration of small-sized particles, (b) 10% w/v concentration of large-sized particles, (c) 5% w/v concentration of small-sized particles, (d) 5% w/v concentration of large-sized particles, (e) 10% w/v concentration of small-sized particles, and (f) 10% w/v concentration of large-sized particles.

Particle size distribution of gelatinized ground rice porridge

The curve of particle size distribution of native ground rice (ungelatinized) and gelatinized ground rice with different sizes and concentrations are shown in Figure 2. The measured particle sizes (D[4,3]) of the small-sized and large-sized native ground rice were 111 ± 4 µm and 378 ± 14 µm, respectively. The mixed size of ground rice (comprised of equal parts of the large- and small- sized components) had an averaged particle size of 255 ± 5 µm, as shown in Table 1. After cooking, the change in the particle size distribution was controlled by the concentration of ground rice in the porridge. At a high concentration (10% w/v) of ground rice, there was a lower average particle size than at the low concentration (5% w/v) for all sizes of ground rice (p < 0.05). The presence of a high concentration of ground rice could limit the expansion of swollen rice particles during the gelatinization process and consequently produce a lower size for the gelatinized ground rice in the porridge. Therefore, a smaller size of gelatinized ground rice could promote stability of the rice particles in the porridge and thereby they displayed a dark blue color with Lugol’s iodine stain of the particles under microscope observation.

Figure 2. Particle size distribution of (a) ground rice (before cooking) and (b – d) gelatinized ground rice (after cooking) with different concentrations; (b) small-sized, (c) mixed-sized, and (d) large-sized particles in ground rice in porridge.

Table 1. Averaged particle size (D [4,3]), yield stress, and viscosity at shear rate of 50 s⁻¹.

Sample type	Rice particles in porridge	Averaged particle size (µm, D [4,3])	Yield stress (mPa)	Viscosity (mPa.s) at 50 s^−1
Raw	Small sized	111 ± 4i	-	-
	Mixed sized	255 ± 5f	-	-
	Large sized	378 ± 14c	-	-
5%	Small sized	201 ± 1g	6,515 ± 176e	345 ± 1g
	Mixed sized	355 ± 3d	43,521 ± 3,271b	1,740 ± 28e
	Large sized	506 ± 5a	54,904 ± 3,230a	1,880 ± 99e
7.5%	Small sized	201 ± 3g	7,176 ± 562e	1,120 ± 99f
	Mixed sized	292 ± 3e	47,609 ± 926b	2,395 ± 106d
	Large sized	404 ± 1b	55,196 + 4,961a	2,610 ± 141d
10%	Small sized	179 ± 6h	20,490 ± 1920d	3,380 ± 156c
	Mixed sized	242 ± 16f	31,887 ± 136c	5,220 ± 42b
	Large sized	366 ± 2cd	42,279 ± 1835b	5,995 ± 247a

Values shown as mean±SD values.

Different lowercase letters in same column indicate significant differences (p < 0.05) based on ANOVA.

Values for yield stress were predicted using a Herschel-Bulkley model at shear rates of 0.2–20 s⁻¹, Coefficient of determination > 0.9.

Steady shear property of porridge

Figure 3a–c show the flow curves of the porridge containing different sizes and concentrations of ground rice. The apparent viscosity of the porridge decreased with an increase in shear rate, indicating the porridge with 5–10% w/v concentration of ground rice exhibited shear-thinning behavior. The change in the viscosity of the porridge depended on the size and concentration of the ground rice, with a significant increase with an increase in the particle size and concentration of the ground rice. At shear rate of 50 s⁻¹ (Table 1), the porridge which contained the large-sized ground rice had a higher viscosity compared to the porridge which contained the small-sized ground rice (p < 0.05), increasing by ~six times in 5% concentration of ground rice (1,880 ± 99 mPa.s compared to 345 ± 1 mPa.s, respectively) and ~two times in the 10% concentration of ground rice (5,995 ± 247 mPa.s compared to 3,380 ± 156 mPa.s, respectively). Keawkaika et al.^[21] explained that the viscosity of rice porridge was correlated with the cooked rice volume fraction due to water adsorption. In addition, rice porridge with a high concentration promoted viscosity due to deformation of the cooked rice when it was packed into a compacted system where available space was limited.

Figure 3. Viscosity (a – c) and shear stress (d – f) versus shear rate of the porridge with different particle sizes and concentrations of ground rice over a shear rate range of 0.1–1,000 s⁻¹ at 37°C.

The shear stress curves of the porridge were fitted with the Herschel-Bulkley model and the obtained yield stress values are shown in Table 1. The yield stress of a material is related to the strength of the coherent network structure and corresponds to the minimum shear stress that must be applied to the material to initiate flow.^[22] The current study showed that the presence of the large-sized ground rice in the porridge resulted in high yield stress for all concentrations. However, an increase of ground rice concentration in the porridge up to 10% w/v resulted in a slight decrease in the yield stress in the samples containing mixed and large-sized ground rice. This could have been due to a reduction in the particle sizes in the porridge with high concentration (Figure 2 and Table 1); however, this effect was not evident in the small-sized ground rice. An increase in the small-sized ground rice in the porridge up to 10% w/v increased the yield stress from ~ 7,000 mPa to ~ 20,000 mPa. Therefore, this study revealed that the large-sized ground rice increased the yield stress in rice porridge for all concentrations, while small-sized ground rice promoted yield stress at a high concentration (10% w/v) of ground rice.

A hysteresis loop was observed in the porridge results (Figure 3d-f). Several studies explained that the hysteresis loop can be interpreted as structural breakdown by the shear field to alter a structure or form a new structure.^[23–25] Generally, a larger hysteresis loop area suggests a greater extent of destruction in the gel structure.^[24] The current study showed that the porridge with low (5% w/v) and medium (7.5% w/v) concentrations of ground rice had a wide hysteresis loop, not including the 7.5% w/v of small-sized ground rice (Figure 3d). The small hysteresis loop of the porridge with the 7.5% w/v concentration of small-sized ground rice could imply stability of the food matrix, which contained gelatinized rice particles and the leaching out of starch polymers in the aqueous phase. Therefore, an increase in ground rice to the 10% w/v concentration in the porridge could promote shear stability of the porridge, resulting in a smaller hysteresis loop in all sizes of ground rice.

Tribological properties of porridge

Figure 4 shows the friction curves of the porridge samples containing different sizes of ground rice at 5, 7.5, and 10% concentration. The lubrication properties of foods have been investigated using a tribology study, presenting the friction occurring on the surfaces. The food could form a lubricant film between the surfaces, resulting in a friction increase in the boundary regime of the Stribeck curve. After that, the friction decreases in the mixed regime where there is complete separation of the surfaces.^[8] As shown in Figure 4, the friction curves of the porridge behaved differently for the different sizes and concentrations of the ground rice. The porridge samples with low (5% w/v) and medium (7.5% w/v) concentrations of ground rice showed prolonged boundary regimes followed by a decrease in the maximum friction coefficient in the mixed regime at sliding speeds of ~10-30 mm/s. However, in the porridge with high concentration (10% w/v), the boundary regime started rapidly and the maximum friction coefficient (μ) decreased at sliding speeds of <5 mm/s. In particular, the μ values sharply decreased in the porridge which contained large-sized ground rice (Figure 4c), having the μ values of 0.4. Therefore, the presence of large-sized ground rice with 10% w/v concentration in the porridge resulted in an efficient decrease in the friction, thus promoting the lubrication properties of the porridge, compared to the porridge with lower concentrations of the small-sized and mixed-sizes of ground rice. This effect was probably caused by the characteristics of the gelatinized rice particles, viscosity, and the shear stability of the porridge, which are discussed in next paragraph.

Figure 4. Stribeck curves of porridge with different particle sizes and concentrations of ground rice; (a) small-sized, (b) mixed-sized, and (c) large-sized particles.

The characteristics of particles in foods, namely food emulsion and starch dispersion, have been reported to influence the lubrication properties. This study showed that the presence of large-sized ground rice particles at 5% concentration (Figure 1d) and 10% concentration (Figure 1f) in the porridge produced a dark blue color of rice particles, stained using Lugol’s iodine solution, as shown in the light micrographs. This indicated that the large-sized ground rice could promote a reduction in friction and thus enhance the lubrication properties. This was mentioned by Ji et al. (2022),^[12] where the presence of starch granules and the large particle size of the starch led to lower friction due motion of the granules, providing lubrication via particle sliding between two surfaces in a tribology test. However, the particles could indirectly increase surface roughness and asperity contacts, producing an extended boundary regime for the friction curves.^[13,14] As shown in the current study, the small-sized ground rice in the porridge, which resulted in the full gelatinization of the rice particles (Figure 1c and Figure 1e), could adhere to surfaces and thus resist movement, resulting in increased friction.

The presence of smaller averaged-sized particle in the porridge containing a high concentration of ground rice (Figure 2b-d and Table 1) could influence the lubrication properties. Swollen rice particles in the porridge that limited water absorption for a high concentration, could lead to particle stability under shear and therefore promote lubrication properties, resulting in decreased friction (Figure 4). In addition, the presence of smaller particle sizes or a lower concentration of ground rice in the porridge, or both, could produce low stability in the porridge, resulting in a sharp drop in shear stress for shear rates of 10–100 s⁻¹ (Figure 3a–c) and a display of wide hysteresis loops (Figure 3d–f), and thereby increase the friction.

The surface separation under sliding speeds in a tribology test, presenting low friction, could be produced by particle stability and also high viscosity in the foods. The mixed lubrication regime appeared rapidly at low speeds (~2 mm/s), with a friction coefficient of 0.4, as shown in Figure 4c. The current study showed that the presence of large-sized ground rice with a high concentration in the porridge produced stability of the rice particles (Figure 1f), high viscosity (~6,000 mPa.s at 50 s⁻¹, as shown in Table 1), and small hysteresis loops (Figure 3). However, the high yield stress of the porridge did not promote lubrication properties. Therefore, the gelatinized rice particles of the large-sized ground rice, which were dispersed in the high concentration porridge, could be stable under shear.

In vitro starch digestion of porridge

An increased particle size and concentration of ground rice in the porridge significantly decreased the cumulated release of glucose in the gastric phase, obtained from the digestive enzyme from the oral phase. For all sizes of ground rice, the presence of 7.5% or 10% concentrations in the porridge produced significantly (p < 0.05) lower AUC values compared to the 5% concentration, as shown in Figure 5a. In addition, the highest concentration had a low AUC in the intestinal phase (Figure 5b), but there were no significant differences in the AUC values for the different rice sizes. Therefore, the characteristic of gelatinized ground rice, presenting low swollen granules and flow behavior of the porridge with high viscosity could delay starch digestibility, which are discussed in next paragraph.

Figure 5. In vitro starch digestion of porridge with different particle sizes and concentrations, measured as area under curve (AUC) of cumulative released glucose.

(a) during gastric phase (G-phase) and (b) during intestinal phase (I-phase). Values with different superscript letters indicate significant differences (p < 0.05).

¹AUC was calculated based on the cumulated released glucose (g) per 100 g of ground rice at each time increment.

The presence of large-sized ground rice in the porridge could limit water absorption during cooking and thus delay enzyme accessibility. This study showed that the characteristics of large-sized ground rice in the porridge, produced low swelling of granules (Figure 1d) and a larger particle size (Table 2) that significantly reduced starch digestion in the gastric phase. However, the presence of the large-sized ground rice in the porridge did not reduce starch digestibility in the intestinal digestion. This could have been due to an increase in the enzyme concentration in the digestive system. Other studies showed that the presence of rice grain and rice flour, which are more stable due to their increasing particle size, could act as a physical barrier to starch digestion.^[10,15,16] Farooq et al. (2018)^[10] explained that the digestion rate of fine particles was higher compared to the medium or coarse fractions, due to the lower particle rigidity and higher surface area of the smaller sizes.

The flow behavior of the porridge with high viscosity could also inhibit enzyme activity. This study showed that the porridge with high viscosity, caused by the increasing rice concentration, could delay starch digestion. As shown in the presence of small-sized ground rice in the porridge, the increased viscosity (at 50 s⁻¹) of the 5% and 7.5% concentrations of ground rice (from 345 mPa.s to 1,120 mPa.s, Table 1) produced a significant reduction in starch digestibility (Figure 5). Other studies explained that the viscosity of the substrates caused a mass transfer limitation of enzyme activity.^[19,26,27] However, the increased viscosity of the porridge, for example, from 7.5% to 10% concentration, did not reduce starch digestibility during intestinal digestion. As shown in Figure 5b, there were no significant differences in the AUC values of the porridge. Therefore, an increase in the concentration of ground rice and viscosity in the porridge resulted in a reduction in starch digestibility. However, there was no significant reduction in starch digestibility in the intestinal phase of the porridge with a high concentration (7.5% compared to 10% concentration).

Conclusion

The viscosity, lubrication properties, and starch digestibility of rice porridge could be modulated by the particle size of the ground rice and its concentration. This study revealed that large-sized ground rice (0.2–0.45 mm based on sieving) resulted in low swollen granules, and thus delayed starch digestion in the gastric phase. In addition, the presence of ground rice with a high concentration (10% w/v) in the porridge resulted in high viscosity and high stability in the porridge and low increased particle sizes of the gelatinized ground rice. These characteristics could have played a role in determining the lubrication properties and in vitro starch digestion. A combination of large-sized ground rice at a 10% w/v concentration also reduced the friction coefficient and starch digestibility in the gastric phase. Therefore, the desirable lubrication properties and starch digestibility of porridge containing large-sized ground rice at a high concentration could contribute to designing a rice porridge product for the elderly and people having difficulty swallowing.

Acknowledgments

This research was financially supported by the Kasetsart University Research and Development Institute (KURDI) under Grant [FF(KU) 37.66], Bangkok, Thailand. Assoc. Prof. Parichat Hongsprabhas contributed helpful discussion and suggestions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Kasetsart University Research and Development Institute (KURDI) [FF(KU) 37.66].