2017 Volume 23 Issue 2 Pages 221-228
In this study, the storage characteristics of high-amylose rice gel and emulsified rice gel, two intermediate food materials, were investigated. High-amylose rice was gelatinized and sheared at a high speed to create rice gel. Cooking oil was added to the rice gel and the mixture was mixed at a high speed to create emulsified rice gel. The ratio of rice to liquid, which is water in the rice gel and water and oil in the emulsified rice gel, was set to 1:1.5 or 1:4. The emulsified rice gel and rice gel were stored under refrigerated (4°C) and freezing (−20°C) conditions. Temporal changes in the physical properties were compared by measuring the dynamic viscoelasticity after 1, 7, and 14 days of storage. The results show that the temporal changes in E* and tanδ for the emulsified rice gel were smaller than those for the rice gel. The results suggest that the emulsification process can effectively suppress changes in the physical properties of rice gel over time.
In recent years, rice consumption in Japan has decreased owing to the diversification of lifestyles and a transition to a Western diet. The consumption of rice, which was 118.3 kg per capita in 1962, had decreased to 56.9 kg by 2013.i
Since the amount of rice consumed as cooked rice has decreased, the use of rice flour to produce foods such as bread (Takahashi et al., 2009; Takahashi et al., 2011), confectioneries (Kusunose, 2009), and noodles (Iwamori and Murayama, 2009; Kita et al., 2006) has been studied actively. However, when using rice flour for these purposes, the rice must be milled to sufficiently small particles to replace wheat flour and other starches. Consequently, it is difficult to reduce manufacturing costs (Kainuma and Tanaka, 2009), and the increasing use of rice flour has slowed in recent years.ii
Regarding this technical challenge associated with the use of rice flour, Shibata et al. and Matsuyama et al. proposed the use of a novel gel-like intermediate food material obtained from high-amylose rice (referred to as rice gel) (Shibata et al., 2012; Matsuyama et al., 2014). Rice gel is a novel material developed by Sugiyama et al., and is obtained by shearing cooked rice at a high speed (Sugiyama et al., 2015).
Rice gel has the following advantageous properties: 1) it is easier to produce than rice flour, 2) it has unique viscoelastic properties, 3) it does not release water easily, and 4) its physical properties can be easily controlled. Currently, the properties of rice gel are being studied in detail, and applications to new products are being investigated.
On the other hand, in the process of considering the applications of rice gel to new products, rice gel was found to exhibit strong emulsifying properties and was confirmed to be suitable for use as a modifier, thickener, and emulsifier (patent pending). This “emulsified rice gel” is considered to be useful as an intermediate food material. For industrial applications, however, it is necessary to obtain an in-depth understanding of its properties and characteristics. In this study, we examined the changes in the physical properties of emulsified rice gel over time, which will be important during manufacturing and distribution.
There have been several studies on the effect of adding oil to rice in the context of texture profile (Maruyama et al., 1980; Hibi, 1993) and amylogram viscosity (Ojima, 1986; Ojima et al., 1986). However, no studies using dynamic viscoelasticity measurement have been conducted on this issue. Dynamic viscoelasticity measurement can be effectively applied to the study of changes in rice gel and emulsified rice gel during storage, since measurement of the elasticity as well as the viscosity of a gel can be easily and accurately obtained in a similar state to when it is actually used. In this paper, we report the results of our study, which confirmed that emulsification is effective in suppressing the changes in the physical properties of rice gel.
Materials A high-amylose rice cultivar, “Momiroman” (harvested in Saitama Prefecture in 2008), was used. Its amylose content (28.8%) was measured in accordance with Juliano (1971). Canola oil (Nisshin canola oil; Nisshin Oillio Group, Ltd., Tokyo, Japan) was used as the cooking oil.
Preparation and sampling of rice gels and emulsified rice gels As shown in Fig. 1, the amount of water used in the rice gels was 1) 1.5 times or 2) four times the amount of polished rice, and the amount of liquid (oil and water) used in the emulsified rice gel was 3) 1.5 times or 4) four times the amount of polished rice. The amount of cooking oil added to the emulsified gel was 20% of the rice weight, and this amount was subtracted from the total amount of liquid stated above to give the amount of water. The weight of each experimental batch was 1 kg.
Composition of each sample
After the polished rice was washed and left to absorb water for 2 h, gelatinized rice was prepared with a rice cooker (NP-NC10; Zojirushi Corporation, Osaka, Japan) set to “rice porridge mode”. After gelatinization, the gelatinized rice was immediately transferred to a food processor (Blixer 5 Plus; Robot Coupe, Jackson, MS, USA) and sheared for 4 min at 3000 rpm. The cooking oil was divided in two portions, and added at the beginning and in the middle of the shearing step. After shearing, the samples were filled into the stainless steel sanitary pipes (inner diameter of 18.4 mm and height of 20 mm) and sandwiched at both ends with stainless steel plates. The molded gels were sealed in a zippered plastic bag and stored under refrigerated (4°C) or freezing conditions (−20°C), respectively, for periods ranging from 1 to 14 days after gel preparation, and dynamic viscoelasticity measurements were obtained after storage.
Dynamic viscoelasticity measurement The rice gel and emulsified gel samples were allowed to stand for 1 h at room temperature (20°C), and after reaching room temperature, the measurements were carried out. After the rice gel and emulsified gel were removed from the stainless steel sanitary pipes, both ends of the sample were glued with Aron Alpha (Toagosei Co., Ltd., Tokyo, Japan) as an adhesive, and measured with a dynamic viscoelastic tester in vertical oscillation mode (Rheolograph-Sol Special; Toyo Seiki Seisaku-Sho, Ltd., Tokyo, Japan). A sinusoidal strain of 0.100 mm amplitude was applied in the axial direction using a 20-mm-diameter plunger. The frequency during measurement was 1 Hz. Four samples were measured for each batch of rice gel or emulsified gel.
Statistical analysis The results of the dynamic viscoelasticity measurement were expressed as the mean ± standard deviation. To evaluate differences in mean values, Welch's t-test was employed using the statistical analysis program MEPHASiii.
Comparison of E* at D+1 Figure 2 shows the complex modulus (E*) of the emulsified gel and rice gel that were stored for one day (hereinafter referred to D+1). E* was significantly lower for the rice gels with a high water ratio than for those with a low water ratio, regardless of the addition of oil and the storage temperature (p < 0.01). This result is consistent with the report of Matsuyama et al. (2014), in which the dynamic storage, loss moduli, and complex modulus of rice gel with a low water ratio were higher than those of rice gel with a high water ratio. Comparing the emulsified gel and rice gel stored at 4°C, the gels created with both the low (rice-to-liquid ratio of 1:1.5) and high (rice-to-liquid ratio of 1:4) liquid ratios showed significantly higher E* values for emulsified rice gel than for rice gel (p < 0.01).
Composition of E* at D+1
A) ×1.5 amount of water added to rice
B) ×4 amounts of water added to rice
The first reason for this observation is the difference in the water ratio between the rice gel and the emulsified rice gel. As shown in Fig. 1, the water ratio of the rice gel was 60% in the low-water-ratio samples and 80% in the high-water-ratio samples. On the other hand, the water ratio of the emulsified rice gel was 52% in the low-water-ratio samples and 76% in the high-water-ratio samples, since an amount of water equivalent to 20% of the weight of the rice was replaced with oil. Thus, this difference in the water ratio may have been the cause of the observed difference in E*.
Differences in the viscosity of water and oil are the second reason. Since the viscosity of canola oil (70 – 85 mPa · s) is higher than that of water, the loss modulus of the emulsified rice gel to which canola oil was added was higher than that of the rice gel to which the same amount of water was added (Huang et al., 2015). As a result, the complex elastic modulus E*, which is the square root of the sum of the squares of the storage and loss moduli, increased. As shown in Fig. 2, the two reasons described above are supported by the fact that E* for the emulsified rice gel (1.5×) at 4°C showed an approximately 260% increase with the same condition of rice gel, while the other gel (4×) at 4°C showed a limited increase of up to 170%.
In the high-water-ratio samples (4×), the difference in the water ratio between the emulsified and rice gels was 4%, while in the low-water-ratio samples (1.5×), the difference was 8%, as shown in Fig. 1. For the low-water-ratio samples (1.5×) stored under the freezing (−20°C) condition, the average E* value of the emulsified rice gel was slightly higher than that of the rice gel; however, the difference was not significant (Fig. 2A (right)). On the other hand, a significant difference at 4°C was observed between them in Fig. 2A (left). There may be several reasons for the smaller difference between the emulsified and rice gels stored at −20°C than those stored at 4°C (Fig. 2A). One reason may have been that the change between the gel immediately after preparation and the D+1 sample was smaller at −20°C than at 4°C because of suppression of retrogradation (Takahashi R., 1996).
Furthermore, for the high-water-ratio (4×) samples stored at −20°C, E* for the rice gel was significantly higher than that for the emulsified rice gel, shown in Fig. 2B (right).
Both gels were strongly influenced by freezing and become spongy. More specifically, crystallization of water in the gel by freezing is thought to bring about some changes in the gel matrix structure, such as the association and cohesion of starch chains due to an increase of ice volume, resulting in a corresponding change in the physical properties of the gels. It is also considered that the reason for the lower E* for the emulsified rice gel than for the rice gel is that more water remained within, attributable to the complex formation of amylose and fatty acids (Doguchi et al., 2015).
According to these results, the industrial use of high-water-ratio samples stored at −20°C is expected to be difficult; thus, these samples were omitted from further experiments.
Comparison of temporal changes in E* Since the objective of this experiment is a comparison of the relative value difference from initial E*, a relative value was adopted in the Y-axis. Figures 3 and 4 show the relative changes in E* for the samples with low-water-ratio (1.5×, Fig. 3) and high-water-ratio (4×, Fig. 4), respectively, which were stored from D+1 to D+14 at the prescribed temperature. The relative values of E* were normalized by setting the D+1 value equal to 100.
Changes in relative value of E* with storage time
A) 1.5 × amount of water added to rice, stored at 4°C
B) 1.5 × amount of water added to rice, stored at −20°C
Changes in relative value of E* with storage time 4 × amount of water added to rice, stored at 4°C
In the case of the low-water-ratio samples stored at 4°C, E* for the rice gel increased significantly among the D+1, D+7, D+7, and D+14 samples (p < 0.05) in Fig. 3A. On the other hand, E* for the emulsified gel increased significantly between the D+1 and D+7 samples (p < 0.05), but did not show a significant difference between the D+7 and D+14 samples (p > 0.05).
In the case of the low-water-ratio samples stored at −20°C, E* for the rice gel did not show a significant difference between the D+1 and D+7 samples (p > 0.05), but values between D+7 and D+14 samples decreased significantly (p < 0.05), as shown in Fig. 3B. On the other hand, E* for the emulsified gel did not show a significant difference among the D+1, D+7, and D+14 samples (p > 0.05).
Furthermore, in the case of the high-water-ratio samples, E* for both the rice gel and emulsified gel stored at 4°C increased significantly during storage, and the relative values of E* for the emulsified gel showed smaller changes than those for the rice gel, as shown in Fig. 4. Therefore, the emulsified rice gel is suggested to show less change in E* than the rice gel under refrigeration.
These results indicate that the changes in E* during storage up to D+14 for the emulsified gel were always smaller than those for the rice gel. The viscoelastic data suggest that the addition of oil can effectively suppress changes in the physical properties, as previously reported (Maruyama et al., 1978; Hibi, 1993).
Nagura et al. (1981) examined the histochemical changes of cooked rice in the presence of oil and with refrigeration. They reported that the oil formed small droplets, which permeated from the surface to the inside. Delayed changes of the tissue structure of cooked rice during refrigeration were also observed. Because rice gel does not maintain a rice grain shape, it is possible that the influence of oil permeation appears more strongly.
From these results, the addition of oil to rice gel can effectively maintain E*, particularly with respect to industrial use during the D+1 to D+14 period. The tendency of the storage elastic modulus E′ and loss modulus E″ in each test category showed almost the same tendency as the complex elastic modulus E*.
Comparison of tanδ at D+1 Figure 5 shows the loss tangent (tanδ) of the emulsified gel and rice gel after storage for one day. The tanδ values for gels with a high-water-ratio were always higher than those with a low-water-ratio, regardless of the addition of oil and storage temperature. This result indicates that a low-water-ratio and high-solid-concentration result in a solid characteristic. This is consistent with the results of Matsuyama et al. (2014).
Comparison of tanδ at D+1
A) 1.5 × amount of water added to rice
B) 4 × amount of water added to rice
In a comparison of the emulsified gel and rice gel stored at 4°C, both gels had a significantly lower tanδ value for the emulsified rice gel than for the rice gel (p < 0.01). As stated previously, this may have been due to differences in the water ratio between the rice gel and emulsified rice gel or differences in the viscosity of the water and oil added to the emulsified rice gel, whose viscosity (loss moduli) is higher than that of water, which is reflected in the dynamic viscoelasticity of the gel.
For the samples stored at −20°C, a significant difference in tanδ was not observed between the rice gel and the emulsified gel (p > 0.05). As mentioned in Comparison of temporal changes in E*, the reason may have been that the change between the gel immediately after preparation and the D+1 sample was smaller at −20°C than at 4°C, possibly because of suppressed retrogradation.
Comparison of temporal changes in tanδ Figures 6 and 7 show the relative changes in tanδ value over time. The relative values of tanδ were normalized by setting the D+1 value equal to 100. In the case of the low-water-ratio samples stored at 4°C (Fig. 6A), the rice gel and emulsified gel tended to exhibit similar changes. Tanδ significantly decreased from D+1 to D+7 in both cases (p < 0.05), with minimal change (p > 0.05) between D+7 and D+14. The overall decrease in tanδ from D+1 to D+14 for the emulsified gel was smaller than that for the rice gel. In the case of the low-water-ratio samples stored at −20°C (Fig. 6B), both the rice gel and the emulsified gel showed no significant differences between D+1 and D+7 or between D+7 and D+14 (p > 0.05).
Changes in relative value of tanδ with storage time
A) 1.5 × amount of water added to rice, stored at 4°C
B) 1.5 × amount of water added to rice, stored at −20°C
Changes in relative value of tanδ with storage time 4 × amount of water added to rice, stored at 4°C
Furthermore, in the case of the high-water-ratio samples stored at 4°C (Fig. 7), a significant difference was observed between D+1 and D+7 as well as between D+7 and D+14 (p < 0.05). In addition, the relative decrease in tanδ from D+1 to D+14 for the emulsified gel was smaller than that for the rice gel, similar to that observed for the low-water-ratio samples. The results indicate that gel emulsification suppresses both changes in E* (hardness) during the storage period as well as tanδ (gel texture) in comparison with the rice gel.
The results summarized below reveal the effect of oil addition on the rice gel.
Thus, the results of these experiments suggest that changes in the relative values of E* and tanδ can be decreased by emulsification with oil to form a rice gel. This indicates that the effect of suppressing the changes in the physical properties of rice gel over time is similar to that of gelatinous rice, which was previously investigated in terms of the degree of gelatinization and the texture profile (Maruyama et al., 1980; Hibi, 1993). These results provide us with a means of modifying the physical properties of the rice gel as well as an effective means of processing and distribution.
