2018 Volume 24 Issue 4 Pages 739-746
Application of high-speed shear treatment (HST) to gelatinized high-amylose rice created rice gels with unique viscoelastic properties. This study examined the quantitative relationship between HST conditions and texture, assessing the dynamic viscoelastic properties of high-amylose rice gels based on a second-order polynomial model. Twenty-four HST high-amylose rice gel samples were prepared by varying the three conditions of shearing speed and time, and temperature during HST. Dynamic viscoelasticity of the rice gel samples was measured to evaluate their physical properties. Results showed that dynamic viscoelastic moduli decreased concomitantly with increasing shearing time and speed, whereas they increased concomitantly with increasing temperature. Analyses of the models revealed that shearing speed was the most dominant factor affecting rice gel preparation in HST, while shearing time and temperature strongly affected the texture. These results are expected to be applicable to the design of HST equipment for rice gel production.
Consumption of rice, a staple food in Southeast Asia, has been decreasing in Japan because of the transition to a western-style diet, even though rice remains Japan's primary agricultural product. Rice consumption per capita in Japan has dropped to 54.5 kg/year, which is less than half of that in 1962 (118.3 kg/year), representing an 80,000 ton per year decrease on averagei). Several attempts have been made to boost rice consumption. Rice noodles, a well-known food in Southeast Asian countries, have not been popular in Japan because eating rice noodles is not a dietary habit of the Japanese. Thus, several studies have examined substitution of all or part of wheat flour with rice in bread making, by milling rice grain into flour (Iwashita et al., 2011; Nakamura et al., 2009; Sabanis and Tzia, 2009). However, making rice flour bread with a similar volume to wheat flour bread is difficult because the rice protein does not develop a similar viscoelastic structure to gluten, in which the network retains gases inside the dough during mixing and proofing (Kawamura-Konishi et al., 2013; Marco and Rosell, 2008).
Through attempts to obtain rice bread with sufficient volume by processing and modifying rice, Shibata et al. (2012) found that rice gel made by high-speed shear treatment (HST) of gelatinized high-amylose rice has distinct gel properties. The HST high-amylose rice gels showed a higher storage modulus and lower tanδ (ratio of loss to storage modulus) than gels from intermediate or low-amylose varieties. Reportedly, the values of tanδ of HST high-amylose rice gels changed only slightly, indicating that the texture did not change markedly with storage. Moreover, it was reported that the addition of HST high-amylose rice gel to wheat bread dough modified the bread quality by significantly increasing the specific volume of bread (Shibata et al., 2015). In addition, Kokawa et al. (2017) demonstrated that cooling treatment of gelatinized rice before HST decreased the values of storage and loss moduli of the rice gels, which contained fewer and larger air bubbles. Furthermore, bread made from rice gel processed with cooling and HST showed higher specific volume than bread made from HST rice gel without cooling.
Numerous other studies of the modification of rice or rice starch by physical treatments have been reported. The gelatinization properties of rice starch differed between dry heat processing with an oven and moist heat processing with an autoclave (Takahashi et al., 2005). Extrusion cooking modifies the functional and digestive properties of rice flour (Bryant et al., 2001). Applying high-pressure treatment to rice starch changes its crystal structure (Hibi et al., 1993; Stute et al., 1996). Wu et al. (2010) reported that heating and stirring strongly affect rice flour properties, but not the rice starch itself. Katsuno et al. (2010) reported that shear and heat milling machine (SHMM) treatments of milled rice produce amorphous rice starch. However, no technique similar to HST has been investigated so far.
Therefore, HST is highly anticipated as a new technique for producing low allergen food material made from rice; however, knowledge is limited in relation to HST conditions of the rice gels and their physical properties because earlier studies (Kokawa et al., 2017; Shibata et al., 2015; Shibata et al., 2012) examined rice gel prepared under constant conditions. For wider application of rice gel by HST in the food industry, relevant data are necessary. Consequently, our objective is to specify the effects of manufacturing conditions on the dynamic viscoelastic properties of high-amylose rice gel by HST. We also clarified the contribution of HST manufacturing conditions to the physical properties of rice gel, which will facilitate the effective control of those properties and support food industry processes.
High-shear treatment system Figure 1 shows a schematic of the HST treatment system flow, which applied high shear to the gelatinized rice. The system comprises an 8 L bowl with a φ245 mm stainless-steel cutting blade, a boiler, and a vacuum pump. The bowl was covered with a jacket in which tap water or steam from the boiler circulated to control the inner bowl temperature. The sheared sample temperature was controlled by regulating the bowl temperature. In this study, the temperature of the sample was regarded as equal to that of the bowl by setting the initial sample temperature equal to the bowl temperature. During shearing treatment, the bowl was perfectly sealed to prevent water from evaporating.
Schematic views of the high-shear treatment system.
Experimental design Full factorial design was applied to three manufacturing conditions of the rice gel samples: bowl temperature (25, 80 °C), which was regarded as sample temperature, shearing speed (500, 1500, 3000 rpm) and shearing time (1, 3, 5, 10 min). In all, 24 experiments were conducted to assess all of those conditions.
Sample preparation Polished high-amylose rice, Momiroman cultivar, harvested in 2008 in Saitama Prefecture, Japan was used. The amylose content was 28.8 %, which was measured using the method described by Juliano et al. (1971). After 300 g of the rice was washed with filtered water, it was immersed in 1500 g of filtered water for 2 h, and gelatinized using a rice cooker (NP-NC10; Zojirushi Corp., Osaka, Japan). The gelatinization of the polished rice was performed by increasing the temperature at a rate of 5 °C /min and holding the temperature at 100 °C for 40 min. During the gelatinization treatment, approximately 50 g of water evaporated. Filtered water of the same amount as lost during gelatinization was added to the gelatinized rice before the shearing process to control the amount of water in HST rice gel samples. Prior to the shearing treatment, the gelatinized rice temperature was adjusted to the bowl temperature of 25 °C or 80 °C. Next, HST was applied to the gelatinized rice using the HST system under the designed experimental conditions at a constant bowl temperature. After the treatment, the HST gelatinized rice was formed into a disk of φ20×2 mm for dynamic viscoelastic measurement. The formed samples gelated at a room temperature of approximately 25 °C for 3 h.
Dynamic viscoelastic measurement Dynamic viscoelastic properties of the rice gel samples, storage modulus G′ and loss modulus G″, were measured using a stress-controlled rheometer (AR-G2; TA Instrument, Newcastle, DE, USA) with a 20-mm-diameter parallel plate. The gap separating the bottom and the parallel plate was set to 2 mm. A sinusoidal shear stress of 1 N was applied to the HST rice gel samples. The sample temperature was set at 25 °C by controlling the bottom plate temperature using a Peltier device. All tests were confirmed to be performed in the linear region, where the deformation of the samples was proportional to the applied stress, by preliminary stress sweep tests.
Water content Water contents of the HST rice gel samples were measured using an oven drying method. After the dynamic viscoelastic measurements, the samples (each sample weighed approximately 2 g) were placed in aluminum cans and dried at 130 °C for 24 h. Later, the dried samples in the cans were cooled in a desiccator. Finally, water contents on a wet basis of the samples were calculated from changes in their weights before and after drying.
Statistical analysis The experimental data were subjected to multiple regression analysis to quantify the effects of the manufacturing conditions on the viscoelastic properties of the rice gel samples. A second-order polynomial model (Myers et al., 2009) was fitted by setting the manufacturing conditions as explanatory variables and the dynamic viscoelastic moduli as response variables. The model formula is expressed as Eq. 1 as shown below.
 |
Where, Y is the response (dynamic viscoelastic moduli), β0 is the intercept coefficient, βi is the linear coefficient, βij is the interaction coefficient, and βii is the quadratic coefficient. Subscripts i, s, and t denote the bowl temperature, shearing speed, and shearing time, respectively. A quadratic term of bowl temperature was omitted from the equation because it had only two levels. Before the analysis, natural logarithmic conversion was applied to the values of Y to avoid statistical biases resulting from the difference of error in each experimental point. All statistical analyses were conducted using software (JMP Pro 12.2.0; SAS Institute Inc., Cary, NC, USA).
Figures 2 and 3 show changes in storage modulus G′ and loss modulus GM″, respectively, of the rice gel samples in relation to temperature, shearing speed and time. The values are classified into three graphs in each figure by shearing speed at the bowl temperature. In addition, we were unable to prepare the rice gel sample at 25 °C and 500 rpm because the gelatinized rice at 25 °C was so hard that the torque at the shearing speed was not sufficiently strong to move the cutting blade within the gelatinized rice. The values of G′ and G″ of the rice gel samples decreased concomitantly with increasing shearing speed and time. High shear by increasing the shearing speed or time converted the gelatinized rice grain into a paste, leading to lower G′ and G″ of the samples with shearing time. Additionally, the samples produced at 80 °C had higher G′ than those produced at 25 °C. Kokawa et al. (2017) obtained a similar result for HST rice gels in axial dynamic viscoelasticity of E’ and E″. They cooled gelatinized rice to less than 15 °C before shearing, which decreased the E’ and E″ of rice gel samples considerably.
Changes of storage modulus G′ of the rice gel in relation to temperature, shearing speed, and shearing time. Each temperature represents the temperature of the samples during HST. Error bars denote standard deviations of each data point.
Changes of loss modulus G″ of the rice gel in relation to temperature, shearing speed, and shearing time. Each temperature represents the temperature of the samples during HST. Error bars denote standard deviations of each data point.
Figure 4 presents changes in tanδ (= G″/G′) of the rice gel samples in relation to temperature, shearing speed and time. Overall, the value tended to increase concomitantly with increasing shearing time and speed; however, at shearing speed of 500 rpm and 80 °C, the value seemed to be almost constant with shearing time. Moreover, tanδ was lower at a high temperature than at a low temperature. The higher the shearing speed, the greater the degree of increase in tanδ with shearing time.
Changes of tanδ of the rice gel in relation to temperature, shearing speed, and shearing time. Each temperature represents the temperature of the samples during HST. Error bars denote standard deviations of each data point.
Figure 5 shows changes in the water content of the rice gel samples with temperature, shearing time and speed. The water contents of the samples hardly changed at 25 °C with shearing time, whereas they changed slightly at 80 °C. The decrease of water content at 80 °C was attributed to the separation of water from the rice gel due to the heat during the shearing. Changes from the initial values at 1 min of shearing time at 80 °C were 1 % at most. In addition, the difference between the samples at 25 °C and 80 °C with the same shearing time was less than 2 %. Although the changes in water contents might affect the dynamic viscoelasticity of the samples, they were so small that they could not explain all the changes of the dynamic viscoelasticity of the rice gel samples.
Changes of water content of the rice gel with temperature, shearing speed and shearing time. Each temperature represents the temperature of the samples during HST. Error bars denote standard deviations of each data point.
Figure 6 presents the response surface of the fitted second-order models of dynamic viscoelastic moduli versus HST conditions. The figures consist of log (G′), log (G″), and log (tanδ) as longitudinal axes, and shearing time and speed as horizontal axes at the high temperature condition of 80 °C. The adjusted coefficients of determination R2adj of the models for predicting the values of G′, G″, and tanδ were 0.98, 0.91, and 0.97, respectively. The high values of R2adj indicated that the second-order models were sufficient to correlate the dynamic viscoelastic moduli of the HST rice gel samples with the HST conditions.
Response surface of logarithmically converted dynamic viscoelastic moduli of rice gel samples versus HST conditions at 80 °C.
Table 1 presents the results of statistical analyses, which include standardized regression coefficients and adjusted coefficients of determination, R2adj, of the second-order predictive models for viscoelastic moduli of the HST rice gel samples. Several standardized coefficients were significant by t-test, which means that they were regarded as non-zero values statistically. Effects of the first order of shearing speed and time, and temperature were almost significant, except for temperature in G″. In addition, most of the second-order effects were significant. Furthermore, some interactions were observed among experimental conditions.
| Responses of respective models | |||
|---|---|---|---|
| Regression coefficient | log (G′) | log (G″) | log (tanδ) |
| βs | −0.760** | −0.386** | 0.377** |
| βt | −0.408** | −0.196** | 0.213** |
| βi | 0.248** | 0.074 | −0.176** |
| βst | −0.015 | 0.079* | 0.094* |
| βis | 0.114** | 0.075* | −0.039 |
| βit | −0.007 | 0.04 | 0.046 |
| βss | 0.232** | 0.164* | −0.072** |
| βtt | 0.151** | 0.081 | −0.070* |
| R2adj | 0.98 | 0.91 | 0.97 |
For each viscoelastic modulus, shearing speed gave the largest coefficient, which means that it had the greatest effect on the viscoelastic properties of HST rice gel. Second and third factors were the shearing time and temperature, respectively. To our knowledge, no report in the relevant literature has described effects of shearing speed on starch or starch products in the case of shearing applied after starch gelatinization. This is the first manuscript to report the effect of shearing speed on gelatinized rice or rice starch quantitatively.
As related researches, several reports have described that HST affected the physical or physicochemical properties of starch. Wang et al. (2015) applied HST to a cornstarch suspension with a homogenizer equipped with an 18-cm-diameter rotor, using various shearing speeds. They reported that the crystalline type did not change and the relative crystallinity decreased slightly and concomitantly with increasing shearing speed. Slight damage to the starch granule structure was observed. They concluded that native starch displayed resistance to mechanical force. Kaur et al. (2013) prepared kidney bean starch suspension samples and applied shearing with different shearing times at a constant speed of 15,000 rpm. They found that some samples showed increased apparent amylose contents, change of thermal properties, and decreased viscoelastic properties, which resulted from the breakage and fragmentation of raw starch granules during high-speed shearing, as indicated in another study (Herceg et al., 2010). Some results agreed with our own, such as the decrease of viscoelastic properties, likely due to the destruction of starch granules.
However, the main difference between these earlier studies and the present study is the timing of HST application to starch or rice. We applied HST after gelatinization, whereas the earlier studies implemented HST before gelatinization. Consequently, it is unlikely that the change in viscoelastic property of rice gel was attributable to the change in swelling ability through HST-induced destruction of starch granules, because G′ and G″ of the samples with a high temperature were higher than those of the samples with a lower temperature. A high temperature could facilitate the destruction of starch granules, which would result in decreases of G′ and G″ of the rice gel.
One possible explanation for the increase of viscoelastic moduli at a higher temperature is that during HST the protein in the gelatinized rice aggregated, resulting in the hardening of the HST rice gel. Isaka et al. (2018) reported that retort treatment caused hardening of HST high-amylose rice gel samples, although the relative crystallinity of starch in the samples decreased after the treatment. Moreover, Mujoo et al. (1998) revealed that the molecular weight of protein in rice flake samples increased after the flaking and roasting process as a result of increased disulphide bonding. They also inferred that the conversion of mechanical energy to bonding energy was the main factor for the aggregation, since protein aggregation occurred more in the flaking than the roasting process. Moreover, the aggregation occurred more at high-temperature than low-temperature flaking. From these results, protein aggregation during HST processing could give the characteristic rheological properties to HST rice gel.
In the food industry, HST has potential for wide application because it is simple and requires only a mixer. Moreover, this methodology is easily applicable to non-rice cereals. In developing gluten-free products for celiac disease, the demand for alternative food material using rice is growing globally. Actually, HST shows great potential in the development of gluten-free food using rice, highlighting the need for additional chemical investigation at a molecular level to elucidate microscopic phenomena during HST.
This study was conducted to investigate the effects of HST conditions on the dynamic viscoelastic properties of high-amylose rice gel and to quantify the relationship between HST conditions and dynamic viscoelastic moduli of HST rice gel samples based on a second-order polynomial model. We prepared 24 high-amylose rice gel samples with differences in three conditions: shearing speed, shearing time and temperature during HST. Dynamic viscoelastic moduli G′, G″, and tanδ of the rice gel samples were measured to evaluate their physical properties. G′ and G″ decreased concomitantly with increasing shearing time and speed, although they increased concomitantly with increasing temperature. Relationships between rice gel dynamic viscoelastic properties and rice gel manufacturing conditions were modeled using a second-order polynomial model with R2 greater than 0.91, indicating that shearing speed was the most effective condition for the preparation of rice gel in HST. In fact, HST is a simple process and is potentially applicable not only to rice but also to other cereals. More studies investigating chemical changes during HST should be conducted to facilitate large-scale production of rice gel using HST.
Acknowledgment In this research work, we used the HST treatment system at the Food Research Institute, National Agricultural and Food Research Organization, and this work was financially supported by a Grant-in-Aid for JSPS Fellows, Grant Number 13J07094.
high-speed shear treatment
