In this study, we developed a method for adjusting the viscoelasticity of rice batter by adding a starch batter that was kneaded. The rice bread batter was mixed with extruder-kneaded starch batter to evaluate baking properties and measure dynamic viscoelasticity. The impacts of extruder kneading on starch structure were investigated by measuring chain-length distribution and molecular weight distribution. The starch components forming the rice batter were viewed under an optical microscope, whereas the structure of the bread batter was observed under a scanning electron microscope. Adding extruder-conditioned starch batter to rice batter was found to increase the storage modulus G′ in the low-temperature region and the expansion ratio during fermentation. Kneading additive starch batter in an extruder readily reduces the molecular weight of starch, thereby decreasing the G′ of bread batter.

Rice consumption in Japan is decreasing every year. This has resulted in a significant surplus of rice, which creates the problem of food waste. In recent years, the price of wheat has surged, which has drawn attention to rice flour. In this context, research on processed rice foods has been vigorous. Bread (Hera et al., 2013, 2014; Kang et al., 2015; Murakami et al., 2016a, 2016b; Nakano et al., 2018; Sivaramakrishnan et al., 2004; Yano et al, 2016, 2020) and pasta (Sozer, 2009; Hsi-Lai, 2002; Nishioka et al., 2019) are subjects of research. When making processed foods with rice flour, it is important to adjust the viscoelasticity of the rice batter to suit the production process of each food. In this study, we propose adjusting the viscoelasticity of rice batter by adding starch batter kneaded using a twin-screw extruder.

Rice gel (Nakano et al, 2018; Shibata et al, 2018) and amorphous rice flour (Katsuno et al., 2010; Murakami et al., 2016b, 2018; Kanke et al., 2021) have been proposed as materials to modify the viscoelasticity of rice batter. Rice puree was also suggested as an additive to adjust wheat batter (Nitta et al., 2019). In this study, a novel method that involves a twin-screw extruder and an additive starch batter, was used to prepare a new viscosity modifier. The effect of kneading history on the viscoelasticity of the added starch batter was investigated. The foamability of the batter was also evaluated by the expansion ratio and specific volume of the batter after baking.

Materials A rice flour and a nonglutinous rice starch were commercially available, MURASAKIJYUNNAMAKO (Hinomonto-Kokufun Co., Ltd.) and FINESNOW (Joetsu Starch Co., Ltd.) respectively. Nonglutinous rice starch was labeled as “FS flour.” The particle size of the rice flour and FS flour were 84 um and 7 μm, respectively, as measured by laser particle size analyzer (Mastersizer 2000; Malvern Instruments, Ltd., Worcestershire, UK).

Preparation of additive starch batter The additive starch batter was prepared by adding water to FS flour to make a 25 wt% starch suspension in water and processing the mixture in a twin-screw extruder (Process 11; Thermo Fisher Scientific, Co., Ltd., Tokyo, Japan). We prepared two additive starch batters with different kneading histories. The first sample, named “ad-Gelatinization,” which underwent only gelatinization, was processed at a screw rotation speed of 50 rpm in full-flight screw element with a length of 440 mm and a temperature of 90 °C. The second sample, named “ad-Kneading,” was obtained by passing the “ad-Gelatinization” sample through the twin-screw extruder at a screw rotation speed of 150 rpm and a temperature of 30 °C in full-flight screw element of 120 mm, followed by a kneading section of 120 mm.

Bread batter compositions The compositions of bread batters used to examine the effects of additive starch batters are listed in Table 1. The control batter, named “Control,” consisted of 160 g of rice flour and 160 g of water. For the bread batter named FS, a 5 g portion of rice flour of Control was replaced by FS flour. For the bread batters named Gelatinization and Kneading, 20 g of the Control batter was replaced by ad-Gelatinization and ad-Kneading, respectively. The weight of the starch material, including rice starch and additive starch, was kept equal to the weight of water in the batter.

Bread baking experiment Bread batters for baking experiments were obtained by compounding the ingredients listed in Table 1 with an additional 20 g of sugar, 3 g of dry yeast, and 2 g of salt in a mixer (Kitchen Aid KSW150WH; FMI, Tokyo, Japan). The Control and FS bread batters were mixed for 12 min. For bread batters requiring gelatinization and kneading, initial mixing was performed for 10 min without ad-Gelatinized or ad-Kneading starch batter additives. After 10 min of mixing and adding the additive starch batter, the bread batter containing the additive starch batter was additionally mixed for 2 min. In this study, the total mixing time was 12 min for each bread batter.

Each batter weighing 300 g was poured into a loaf pan before being fermented in an electric fermentation machine (SK-15; Taisho Electric Co., Ltd., Kusatsu, Japan) for 30 min at 40 °C. The batter was then baked for 30 min at 180 °C in a gas oven (OZ100BOEC; Ozaki Co., Ltd., Tokyo, Japan).

The baked bread was evaluated by measuring the expansion ratio and specific volume. The expansion ratio was defined by the following equation (Yano et al., 2016):

The height was calculated as the average of three points, including the center of the sample. An expansion ratio of 100 % indicates no size change from the original batter before fermentation. The specific volume was defined by the following equation:

The volume of bread after baking was determined by the product of the bottom area and the height. The weight of the bread after baking was measured immediately after removal from the oven.

Dynamic viscoelasticity measurements Rice batters of corresponding compositions of Table 1 without adding sugar, dry yeast, and salt for dynamic viscoelastic measurements were compounded by using the mixer (Kitchen Aid KSW150WH; FMI, Tokyo, Japan). The mixing scheme for each batter was the same as described in the bread-baking experiment section, except that sugar, dry yeast, and salt were not added. The storage modulus G′ was measured using a coaxial cylinder with an inner diameter of 24 mm and an outer diameter of 26 mm equipped with a rheometer (Physica MCR 301; Anton Paar Co., Ltd., Graz, Austria). A parallel plate fixture with a diameter of 25 mm was also used as a rheometer instead of the coaxial cylinder to ensure accuracy, depending on the experiments. We measured with an angular velocity of 10 rad/s and a constant strain in the linear viscoelastic region determined after a strain sweep experiment. The storage modulus G′ in the fermentation temperature range was evaluated by constant temperature measurements at 25 °C, 30 °C, and 35 °C. The storage modulus G′ in the baking process was evaluated by temperature-increasing measurements from 35 °C to 90 °C at a rate of 17 °C/min to imitate the temperature-increasing speed in the experimental baking process.

Analysis of starch structure The chain-length distribution of amylopectin in the FS flour and additive starch batter was measured by capillary electrophoresis, according to the methods of previous studies (O'Shea et al., 1996; Fujita et al., 2001).

The molecular size distribution of total starch was analyzed by gel filtration chromatography. Up to 1.6 mL of distilled water was added to 20 mg FS flour or 80 mg additive starch batter and stirred. In addition, 0.4 mL of 5 M NaOH was added and suspended. Samples were incubated for 30 min to gelatinize them. Next, 2 mL of distilled water and 2 mL of eluent were added, stirred, and filtered through a 5-μm filter. The samples were applied to a four-column system, consisting of two Toyopearl HW75S, one HW65S, and one HW55S columns (20 mm diameter, 300 mm long) pre-equilibrated with 0.05 M NaOH/0.2 % NaCl. A refractive index detector (RI-8020; Tosoh Corporation) was used to analyze the progress. Samples were eluted with the same solution at a flow rate of 1 mL/min.

Observation of batter under a polarizing microscope A polarized optical microscope (BX50, Olympus, Tokyo, Japan) was used to observe mesoscopic views of the rice flour, FS flour, and additive starch batter in cross-polarizers with a susceptible tint plate at 530 nm wavelength. The amount of water was added arbitrarily to each sample to obtain a clear view.

Observation of batter under a scanning electron microscope (SEM) Before fermentation, each rice batter sample was freeze-dried using a freeze dryer (DC401/800; Yamato Scientific Co., Ltd., Tokyo, Japan). Dried samples were deposited in gold and observed under a SEM (VE-9800; KEYENSE, Osaka, Japan).

Effect of adding starch batter on baking properties of rice batter Fig. 1 shows a cross-sectional photograph of each rice bread. Upper surfaces of the Control and FS breads were sloped with one peak. Upper surfaces of Gelatinization and Kneading bread were in a flat shape. These different expansion patterns are explained by the rate of bubbling in the baking process after fermentation, as shown in Fig. 2.

Fig. 1.

Cross-sectional photographs of rice bread samples.

Fig. 2.

Expansion ratios for each sample.

Fig. 2 shows the expansion ratios of each rice batter sample during the fermentation and baking processes. After fermentation, the baking process resulted in a rapid increase in the foam rate of the Control and FS, which swelled into a slope. Batters with rapidly expanding bubbles during baking become sloped at the top. However, Gelatinization and Kneading, which had flat surfaces, did not gain any expansion ratio in the baking process. It is expected that in these batters of Gelatinization and Kneading, bubbles formed and expanded during fermentation, and then these bubbles that were retained during baking resulted in little shape change to the top flat surface. However, bubbles expanded primarily during baking in the Control and FS batters. Position difference of solidification rate with bubble expansion causes the mountainlike shape. Fig. 3 shows the specific volume of each rice bread. Similar trends were seen between the expansion rate and the measured volumes after baking.

Fig. 3.

Specific volumes of baked bread samples.

Effect of adding starch batter on rheological properties of rice batter Fig. 4 shows the temperature dependency of the storage modulus G′ for each rice batter. At low temperatures, the G′ values of the FS were lower than those of the Control. In contrast, the G′ values for Gelatinization and Kneading were higher than those of the Control at low temperatures. These results demonstrate that adding starch batter made by a twin-screw extruder to rice batter increases the elastic modulus of the batter.

Fig. 4.

Storage modulus G′ as a function of temperature for rice batter samples.

The results shown in Fig. 4 agree with the bread-baking properties shown in Fig. 2. The G′ value shown in Fig. 4 is low for the Control and FS in the fermentation temperature range (25 °C–35 °C) compared with Gelatinization and Kneading. Fig. 5 shows the relationship between G′ at 25 °C and expansion ratio after fermentation for each rice batter. Fig. 5 summarizes the relationship between the G′ of each batter at 25 °C in Fig. 4 and the expansion ratio of each sample after fermentation in Fig. 2. Batter of higher G′ at 25 °C tends to have a higher expansion ratio during fermentation. Control and FS had lower expansion ratios after fermentation because they could not retain the bubbles generated by the yeast due to low G′ at 25 °C. In contrast, Gelatinization and Kneading batters show higher expansion ratios after fermentation because the higher G′ at 25 °C kept the bubbles in the batters.

Fig. 5.

Relationship between the expansion ratio after fermentation of each sample in Fig. 2 and the storage modulus G′ at 25°C in Fig. 4.

During baking, the batter solidifies at a gelatinization temperature of 35 °C–70 °C to become bread. At the gelatinization temperature, bubbles expanded and solidified in Control and FS batters as G′ increased rapidly from low values. Therefore, a higher expansion ratio was achieved for bread after baking than for batter after fermentation before baking. However, bubbles in the Gelatinization and Kneading batters were prevented from expanding by the high G′ value of the batter in the baking process, resulting in a lower expansion ratio in the bread after baking than in the batter after fermentation.

Furthermore, it was found that the viscoelasticity of the batter can be controlled by the kneading condition of an additive starch batter. Compared to Gelatinization, Kneading showed lower G′ values at lower temperatures, as shown in Fig. 4. This is because of the decrease in molecular weight of amylopectin caused by the kneading shear in the twin-screw extruder. In general, the G′ value decreases as the molecular weight decreases. This is examined in the next section devoted to starch structure analyses.

Analyses of starch structure Fig. 6 shows the results of amylopectin chain-length distribution measurements for FS flour, ad-Gelatinization, and ad-Kneading. The chain-length distribution of the three samples was the same. This shows that starch gelatinization by a twin-screw extruder and the kneading of starch batter does not affect the amylopectin chain-length distribution.

Fig. 6.

Chain-length distribution of amylopectin extracted from FS flour, Gelatinization, and Kneading analyzed by capillary electrophoresis.

Fig. 7 shows the results of gel filtration chromatography for FS flour, ad-Gelatinization, and ad-Kneading. Three peaks, I, II, and III, are shown in Fig. 7. It is reported that amylopectin contributes to peak I and amylose to peak III, and both contributions from amylopectin and amylose are included in peak II (Nitta et al., 2019). A significant retention time corresponds to a lower molecular weight. Fig. 7 shows that the large molecules corresponding to peak I slightly decreased by kneading FS flour batter to obtain ad-Gelatinization, and considerably decreased after kneading ad-Gelatinization to achieve ad-Kneading. After kneading, molecules from peak I were transferred to the retention time range around peak II. These results indicate that twin-screw extruder kneading lowers the molecular weight of amylopectin, preserving the branched structure of starch, as shown by the analytical results of Fig. 6. The lower molecular weight of Kneading agrees with the results of Fig. 4, showing that batter Kneading has a lower G′ than Gelatinization.

Fig. 7.

Gel filtration chromatography of whole starch from FS flour, ad-Gelatinization, and ad-Kneading.

Polarized optical microscope image Fig. 8 shows the compositional structure of each batter sample under polarized light microscopy. The rice flour contains crystalline rice starch granules and lumps of combined crystalline rice starch granules. In existing systems, crystalline rice starch granules and lumps are insoluble starch components in water at room temperature. FS flour contains only crystalline rice starch granules. The absence of particles in microscopic images of ad-Gelatinization and ad-Kneading shows that starch is uniformly resolved in water during the twin-screw extruder process. The Control and FS batters consisted of the insoluble starch components of the surrounding water. In contrast, batters Gelatinization and Kneading consisted of insoluble starch components in a viscous water solution of starch introduced by ad-Gelatinization or ad-Kneading.

Fig. 8.

Microscopic images of rice flour, FS flour, and ad-Gelatinization, ad-Kneading of cross-polarizers with sensitive tint plates.

SEM observations Fig. 9 shows the results of SEM observations of each rice batter sample before fermentation. For Control and FS samples, individual starch grains can be seen. Gelatinization and Kneading show a stranded, linear structure of starch granules. We consider that the strand structure was composed of gelatinized starch. The addition of starch batter caused linked starch granules. This strand structure in the wheat batter is composed of gluten. This study shows that gelatinized starch forms a gluten-like strand structure in gelatinized starch-added rice batter.

Fig. 9.

SEM observations of rice batter samples before fermentation.

Effect of additive starch batter on rice batter The formation of the strand structure shown in Gelatinization and Kneading in Fig. 9 is a remarkable result of an additive starch batter. The Control and FS consisted of crystalline rice starch granules and lumps of crystalline rice starch granules in low-viscosity water surrounding them, as is expected from rice flour and FS flour shown in Fig. 8. As shown in Fig. 8, ad-Gelatinization and ad-Kneading consisted of gelatinized starch without starch lumps and granules. When added to the control batter, the strand structure of Gelatinization and Kneading was induced, as shown in Fig. 9. This structure increases the G′ in the fermentation temperature range of around 30 °C, suppressing bubble escape during fermentation and facilitating the formation of larger bubbles. Gelatinization and Kneading of the high G′ of Fig. 4 in the fermentation temperature range with the strand structure of Fig. 9 resulted in a high expansion ratio after fermentation, as shown in Fig. 2.

Effect of kneading history on additive starch batter Fig. 4 shows that the kneading history of additive starch batter significantly affected the results. When FS flour was added to the control batter without processing in the extruder, the G′ value decreased in the low-temperature range. In contrast, adding ad-Gelatinization or ad-Kneading increased the G′ value at this temperature range. The difference was due to starch morphology in the additive starch batter. As shown in Fig. 8, starch granules from the FS flour disappeared in ad-Gelatinization and ad-Kneading after the extrusion process. At a high temperature of 90 °C, the shear in the screw and kneaders of the extruder destroyed the starch granule structure, transforming it into gelatinized starch. Adding starch granules to the batter induced a thinning effect, whereas adding gelatinized starch induced a thickening effect.

The kneading condition of additive gelatinized starch also influenced the results. The degree of increase after adding ad-Gelatinization and ad-Kneading was different when comparing the results of Gelatinization and Kneading in Fig. 4. As a result of the decrease in molecular weight during the extruding process, Kneading results in a lower G′ than Gelatinization. This was confirmed by comparing the results of the gel filtration chromatography ad-Gelatinization and ad-Kneading, as shown in Fig. 7.

The kneading history of starch batter in a twin-screw extruder was found to affect the baking properties of rice bread. This study demonstrated four effects of adding starch batter with a kneading history to the rice batter, as follows:

These results indicate that the rheological properties of rice bread batter are adjusted by the kneading condition of an additive starch batter.

Acknowledgements We would like to thank our lab members Ms. Yu Kanke and Mr. Naoya Kano for their valuable input and discussions that propelled our research. This work was supported by JSPS KAKENHI Grant Number JP19H02904.

Conflict of interest There are no conflicts of interest to declare.

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