2025 Volume 72 Issue 4 Article ID: 7204104
Rice bread, a gluten-free alternative to wheat bread, often suffers from poor texture due to inadequate viscoelasticity during baking. This study aimed to propose a baking method for pure rice bread by investigating the effects of hydroxypropylated potato starches (HPPS) with varying degrees of substitution on the baking performance of rice batter. First, the particle size distribution and thermal properties of HPPS were analyzed to characterize their fundamental attributes. Rice bread was then prepared using each type of HPPS, and their foaming properties were assessed. Additionally, dynamic viscoelastic measurements were performed during heating to assess rheological changes during baking. Results showed that a higher degree of substitution in HPPS reduced in the gelatinization onset temperature. Moreover, HPPS addition improved the cross-sectional structure of the rice bread. Notably, highly substituted HPPS suppressed the formation of large internal voids caused by bubble coalescence. These findings suggest that HPPS with a high degree of substitution enhances the overall quality of rice bread.
DSC, differential scanning calorimetry; HPPS, hydroxypropylated potato starch; RVA, rapid visco analyzer; SHMM, Shear and Heat Milling Machine
In recent years, increasing health consciousness among consumers has driven demand for gluten-free foods. Consequently, there is growing interest in improving the quality of rice-based bread made without wheat. One of the major challenges in using rice flour as a substitute for wheat flour is its poor foaming ability compared to wheat bread [1, 2]. Wheat bread is typically prepared by mixing wheat flour and water, which, through kneading, forms gluten. This gluten network is highly elastic and plays a crucial role in retaining air bubbles during baking. It facilitates the formation of variably sized bubbles, resulting in soft, fluffy bread.
In contrast, rice flour does not form gluten like protein network when mixed and kneaded with water, making it difficult to achieve the same level of expansion and texture as wheat bread. To address this issue, previous studies have attempted the addition of various thickening agents-such as hydroxypropyl methylcellulose, xanthan gum, guar gum, and pectin-to enhance the viscoelastic properties of rice batter and stabilize air bubbles during baking [3, 4, 5].
In our previous research, we proposed an alternative approach that avoids conventional thickening agents by incorporating amorphous rice flour. The addition of highly viscoelastic amorphous rice flour improved bubble stability and overall foaming performance in rice bread. This led to the development of a novel milling system, the Shear and Heat Milling Machine (SHMM), which enables the rapid production of amorphous rice flour [6]. By optimizing SHMM milling conditions, we recently succeeded in producing well-foamed rice batter without any added thickening agents [7]. Bread produced with this technique contains fine, uniform air bubbles, resulting in enhanced texture and quality.
Notably, both the storage modulus of the batter during fermentation and its increase at the gelatinization temperature during baking significantly influence baking performance [7, 8, 9]. Moreover, modifying the rice cultivar has been found to change the gelatinization temperature, thereby allowing control over bubble size in the final bread.
In this study, we propose a new approach to controlling the gelatinization temperature of rice batter through the use of modified starch. Modified starches are functional ingredients whose physical and chemical properties have been altered to enhance food quality. Specifically, we focus on hydroxypropylated potato starch (HPPS), in which some hydroxyl groups in the starch molecules are replaced by hydroxypropyl groups, increasing hydrophilicity. This modification not only suppresses retrogradation but also lowers the gelatinization temperature [10]. We aim to leverage this property to regulate the gelatinization behavior of rice batter and improve bread formability [11].
This study aimed to clarify how differences in the degree of substitution in HPPS affect the quality of rice bread. We analyzed the particle size distribution and thermal properties of HPPS with varying degrees of substitution and evaluated the foaming performance of rice bread prepared with each type. Finally, we assessed the viscoelastic behavior of HPPS-added batters to better understand their processability during baking.
Materials
Three types of HPPS with varying degrees of substitution were provided by Nippon Starch Chemical Co., Ltd. (Osaka, Japan). The samples were designated as HPPS-L (low), HPPS-M (medium), and HPPS-H (high), corresponding to degrees of substitution of 1.93, 3.75, and 5.49, respectively. Native potato starch (PS; Nippon Starch Chemical Co., Ltd.) was used as the control. The rice flour used for bread-making was Mizuhochikara, supplied by Kumamoto Flour Milling Co., Ltd. (Kumamoto, Japan).
Starch characterization
Particle size distribution
Particle size distributions of the starch samples and rice flour were measured using a laser diffraction particle size analyzer (Mastersizer 2000; Malvern Panalytical Ltd., Malvern, UK). Measurements were conducted with a measuring time of 5 s and an accumulation count of 5. The mode diameter was determined from the distribution data.
Rapid visco analyzer measurement
To investigate the effects of increasing hydroxypropyl substitution on starch pasting behavior, rapid visco analyzer (RVA) measurements were performed following the method described by Toyoshima et al., which conforms to standard RVA analysis protocols [12]. Each sample contained 2 g of starch suspended in 18 g of deionized water. A rotational rheometer (Physica MCR 302; Anton Paar GmbH, Graz, Austria) equipped with a paddle stirrer (ST24/2D-2V-2V) was used instead of a conventional RVA instrument. The temperature and stirring conditions followed the standard protocol: heating from 50 to 93 °C at a rate of 10.75 °C/min, holding at 93 °C for 7 min, cooling to 50 °C at the same rate, and holding for an additional 3 min. The stirring speed was maintained at 160 rpm. Pasting parameters including peak viscosity, trough viscosity, final viscosity, and setback were extracted from the resulting profiles. RVA measurements were performed in triplicate, and each curve in the figure represents a typical result of the three.
Differential scanning calorimetry measurement
To examine the gelatinization behavior of starches with different levels of hydroxypropyl substitution, differential scanning calorimetry (DSC) measurements were performed for HPPS-L, HPPS-M, and HPPS-H. For each measurement, a high-sensitivity differential scanning calorimeter (Microcalvet Ultima; Setaram, KEP Technologies SA, Caluire-et-Cuire, France) was used for 200 mg of starch and 600 mg of distilled water sealed in a stainless steel sample pan (Hastelloy C276). Distilled water was used as the reference. The samples were heated from 25 to 110 °C at a rate of 1 °C/min.
Baking test
To evaluate baking performance, rice batter was prepared by mixing 400 g of rice flour, 315 g of water, 40 g of sugar, 6 g of salt, and 4 g of dry yeast. Batter name ‘Control’ is for the pure rice bread without adding PS nor HPPS type starch. For starch-added samples, 10 wt% of the rice flour was replaced with PS or one of the HPPS types. Batter name ‘Add PS’, ‘Add HPPS-L’, ‘Add HPPS-M’ and ‘Add HPPS-H’ are, respectively, for the rice batters: added PS, added HPPS-L, added HPPS-M, and added HPPS-H. Mixing was performed using a stand mixer (Kitchen Aid KSM5WH; FMI Co., Ltd., Tokyo, Japan) at low speed for 5 min, followed by a 20-min rest at 21.4 ± 1.1 °C. The batter was then mixed again at high speed for 5 min, and 700 g was transferred into a loaf pan.
Fermentation was performed at 40 °C for 40 min in a proofing chamber (SK-15; Taisho Electric Co., Ltd., Kusatsu, Japan), followed by baking in a gas oven (OZ100BOEC; Ozaki Co., Ltd., Tokyo, Japan) at 200 °C for 40 min. After baking, the bread was cooled at room temperature for 24 h before photographing the exterior and cross-section.
The expansion ratio and specific volume were calculated using the volumes of the batter before baking and the bread after baking. The batter volume was measured with a graduated cylinder prior to pan transfer. The volume of the baked bread was measured by the rapeseed displacement method, using plastic beads to fill the container holding the cooled bread (24 h after baking), with the displaced volume used to calculate the final bread volume.
Void size was evaluated from cross-sectional images using image analysis software (Mac-View Version 4; Mountech Co., Ltd., Tokyo, Japan). Baking test was performed in triplicate, and the results were expressed as averages having standard deviations.
Dynamic viscoelasticity measurements
To observe rheological changes in the rice batter during heating, dynamic viscoelasticity measurements were conducted using a rotational rheometer (Physica MCR 302; Anton Paar GmbH). The batter was prepared using the base formulation, excluding sugar, salt, and dry yeast to eliminate gas generation during heating, which could interfere with rheological measurements. A 25-mm parallel plate geometry with a 1-mm gap was used. The test conditions were set to an amplitude of 0.02 rad and an angular frequency of 1 Hz. The temperature was increased from 32 to 98 °C at a rate of 1.65 °C/min, based on actual temperature profiles recorded during baking.
To prevent drying during the measurement, the exposed batter surface was coated with silicone oil (KF-96-50cs, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan). The batter was held at the initial temperature for 5 min before measurement to ensure thermal equilibration. Dynamic viscoelasticity measurements were performed in triplicate, and each curve in the figure represents a typical result of the three.
Statistical analysis
Measurements were performed in triplicate except for the DSC measurements. Data having standard deviations are averages of the three measurements. To evaluate the statistical significance of differences in void size, expansion ratio, specific volume and RVA parameters for Table S1 (see J. Appl. Glycosci. Web site), Tukey’s test was conducted manually for three replicates at a significance level of p = 0.05.
Material properties based on starch modification
Particle size distribution
Figure 1 presents the particle size distributions of PS, HPPS-L, HPPS-M, HPPS-H, and the base rice flour. The horizontal axis represents particle diameter, and the vertical axis indicates frequency by volume percentage. PS, HPPS-L, HPPS-M, and HPPS-H showed similar particle size distributions ranging from 10 to 100 μm, all with mode diameters of 46 μm. In contrast, the rice flour showed a broader distribution, ranging from 5 to 210 μm, with a mode diameter of 23 μm. The median diameters were approximately 21 μm for Mizuhochikara, 38 μm for PS, and approximately 40 μm for all HPPS samples.
These results indicate no significant differences in average particle size among the starch samples. Previous studies have shown that rice flour particle size can influence batter viscosity and ultimately affect baking performance [7, 13]. However, in this study, the differences in particle size distribution among the added starches were minimal and are unlikely to meaningfully impact baking performance.
RVA results
Figure 2 shows the RVA profiles of aqueous suspensions for the starch samples PS, HPPS-L, HPPS-M, and HPPS-H. These curves reflect the gelatinization behavior of starch granules, indicated by increasing viscosity as the granules swell upon heating. The RVA parameters obtained from the measurements was summarized in Table S1 (see J. Appl. Glycosci. Web site).
Figure 2 and Table S1 (see J. Appl. Glycosci. Web site) show that the pasting onset temperature was significantly lower for HPPS-H and HPPS-M compared to PS, indicating that higher degrees of substitution effectively promoted earlier gelatinization. This is attributed to the increased hydrophilicity imparted by hydroxypropyl groups, which reduces intramolecular hydrogen bonding and allows starch granules to absorb water more readily and swell at lower temperatures.
Peak viscosity values were similar among PS, HPPS-L, and HPPS-M; however, HPPS-H exhibited a markedly lower peak viscosity. This is likely due to earlier disintegration of starch granules during heating in HPPS-H [11]. While swelling increases viscosity, granule collapse causes a decline. In the case of HPPS-H, its high hydrophilicity promoted rapid swelling and early collapse, thus reducing the peak viscosity.
Trough viscosities were comparable across all starch samples. In contrast, the final viscosity was significantly lower in HPPS-H and HPPS-M compared to PS, suggesting that hydroxypropylation effectively suppressed retrogradation. After the peak viscosity, the rapid changes in viscosity due to starch granule disintegration resulted in the curve containing noise.
DSC results
Figure 3 presents the results of the DSC measurements for PS, HPPS-L, HPPS-M, and HPPS-H. All samples exhibited endothermic peaks between 45 and 70 °C, corresponding to the gelatinization of starch. The gelatinization endothermal peak temperature decreased as the degree of hydroxypropyl substitution increased. This aligns with the RVA results discussed previously. The lower peak temperatures observed in the DSC measurements are consistent with the pasting onset temperature in the RVA analysis.
Baking properties of rice bread depending on the modification of the added starch
Appearance and foaming properties of the rice bread
The bread sample without added starch is referred to as the control, while the other bread samples are designated according to the added starch: PS, HPPS-L, HPPS-M, or HPPS-H. Figure 4 shows the exterior appearance, cross-sectional images, and foaming characteristics (void size, expansion ratio, and specific volume) of the rice bread samples.
Data are shown as averages having standard deviations for triplicate measurements.
Among these parameters, the expansion ratio directly reflects the batter’s ability to expand during baking, while the specific volume is closely related to bubble density and serves as an indicator of the bread’s lightness and texture. From the appearance images, all samples showed dome-shaped expansion at the top of the loaf. The cross-sectional images revealed that the control and PS-added samples developed a large central void, whereas the HPPS-added samples showed progressively smaller voids with increasing degrees of substitution.
The central voids are likely caused by bubble coalescence during baking. Heat is transferred primarily through the interface between the batter and the loaf pan, resulting in a temperature gradient between the outer and central regions. The center remains cooler for longer, resulting in a delayed viscosity increase compared to the periphery. This prolonged softness in the center facilitates bubble coalescence.
In contrast, HPPS-added samples exhibited higher viscosity even at lower temperatures, likely preventing excessive coalescence and thus suppressing the formation of large voids. Therefore, modifying gelatinization properties of starches through hydroxypropyl substitution is effective in controlling the viscoelastic behavior of rice batter and reducing unwanted void formation during baking.
Considering the expansion ratio and specific volume shown in Fig. 4, all bread samples exhibited favorable values for both parameters, with no significant differences. According to the results of Tukey’s test, no significant differences were observed in expansion ratio and specific volume. Therefore, based on the comparison of void sizes, which did show significant differences, HPPS-H demonstrated the best formability among the samples, as it showed no voids in the cross-sectional image.
Dynamic viscoelasticity measurement
Figure 5 presents the complex modulus |G*| of the simulated rice batter samples prepared as the Control, Add PS, Add HPPS-L, Add HPPS-M, and Add HPPS-H. As shown in Fig. 5, all samples exhibited similar |G*| values at both the beginning and end of the measurement. However, differences were observed in the onset temperature of the |G*| increase. The HPPS-added samples displayed an increase in |G*| within the temperature range of 45-60 °C. This increase corresponds to starch gelatinization, which enhances viscoelasticity, and aligns with the gelatinization onset temperatures observed in the DSC measurements (Fig. 3).
Focusing on the onset temperature it was found that the onset temperature decreased as the degree of substitution increased. This is likely due to the increased hydrophilicity of the starch. The complex modulus |G*| rises as starch granules swell. The highly substituted HPPS-H, being the most hydrophilic, swelled more readily even at lower temperatures and exhibited higher |G*| values in the 45-60 °C range.
We also show the storage modulus (G′) (Fig. S1; see J. Appl. Glycosci. Web site), loss modulus (G″) (Fig. S2; see J. Appl. Glycosci. Web site) and the loss tangent tan δ (Fig. S3; see J. Appl. Glycosci. Web site) in the Supplementary data section. The onset temperatures of the |G*| increase of Fig. 5 are accompanied by the tan δ peaks of Fig. S3 (see J. Appl. Glycosci. Web site).
We expect that rice batters with added HPPS suppressed bubble coalescence more effectively than those without additives or with PS, particularly in this temperature range. During 45-60 °C, bubbles grow rapidly, and the viscoelasticity of the batter plays a key role in controlling expansion [7, 8, 9]. Above 40 °C, yeast becomes inactive, and further expansion is driven mainly by steam. If the batter viscosity is low at this stage, bubbles easily coalesce, leading to the rise and rupture of large bubbles on the surface. The extent of bubble rupture varies significantly depending on the batter’s viscoelastic properties. Since bubble coalescence reduces overall expansion, viscoelasticity in the 45-60 °C range is critical. The instability observed in the rheological curves from low temperatures up to around 50 °C is likely due to the heterogeneous microstructure of the slurry-like batter. At lower temperatures, the batter remains structurally heterogeneous; however, as the temperature increases, starch gelatinization progresses, resulting in a more homogeneous matrix that reduces measurement noise in the rheological curves.
The correlation between the lower temperature onset of |G*| increase shown in Fig. 5 and the more effective suppression of void formation seen in Fig. 4 suggests that differences in |G*| behavior within this temperature range significantly influenced bread quality. In this study, batter with HPPS-H effectively suppressed void formation and demonstrated the most favorable baking performance.
This study investigated the effects of varying degrees of hydroxypropyl substitution in HPPS added to rice bread batter. RVA and DSC analyses revealed that higher substitution levels corresponded to lower gelatinization temperatures. The addition of HPPS reduced internal void formation in the bread, with HPPS-H exhibiting the most favorable baking performance. Notably, the increase in |G*| within the 45-60 °C temperature range contributed to suppressing void formation. Therefore, adding highly substituted HPPS effectively improves the quality of rice bread.
The authors declare no conflict of interest.
Potato starch and hydroxypropylated potato starch with different degrees of substitution (HPPS-L, HPPS-M, and HPPS-H) were kindly provided by Nippon Starch Chemical Co., Ltd.
