Notes
Evaluation of Staling of Cooked Rice by Kinetic Analysis of Texture Changes
2018 Volume 24 Issue 3 Pages 427-434
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2018 Volume 24 Issue 3 Pages 427-434
Five polished rice varieties with different amylose contents were cooked, sealed, and stored at 4°C for 168 h. The slope of first-order plots of hardness and stickiness changed during storage; the change process was divided into two periods: the first and latter halves of the period, each with different rate constants. The rate constant of hardness increase in the first half period was greater for rice with a higher amylose content; conversely, in the latter half period, it was greater for rice with a lower amylose content. The rate constant for stickiness decrease in the latter half was greater than that in the first half, and the higher the amylose content, the larger the rate constant in both periods. By simulating a change in the stickiness-hardness ratio using kinetic parameters with texture changes, the time taken for the quality of cooked rice to deteriorate during storage was predicted.
Rice, a major Japanese crop, is a staple food in Japan. The quality of cooked rice is influenced by starch characteristics such as amylose content and the amylose and amylopectin fine structure (Juliano et al., 1981; Li et al., 2016). In general, cooked rice with a high amylose content is firm and not sticky, whereas rice with a low amylose content is soft and sticky. During low-temperature storage after cooking, cooked rice becomes firmer and less sticky, and undergoes undesirable changes in quality such as staling (Otahara et al., 1995). Such changes in the texture of cooked rice are attributed to the retrogradation of starch. Retrogradation of gelatinized starch involves the rapid re-ordering of the amylose chains over a short time and a slower reorganization of the amylopectin after longer periods (Miles et al., 1985; Biliaderis and Zawistowski, 1990; Lai et al., 2000). Regarding the staling of cooked rice during storage, the changes in textural properties such as hardness and stickiness and the retrogradation of starch in cooked rice have been assessed (Lima and Singh, 1993; Perdon et al., 1999; Takami et al., 1998, 2004; Yu et al., 2009). In these reports, the assessment of changes over time was conducted every 24 h. Despite the fact that cooked rice is often consumed within 24 h of storage, only a few studies have reported on the changes over time within 24 h after cooking (Otahara et al., 1995; Ikawa et al., 2002).
In the previous reports, the measured values of textural properties such as hardness and stickiness of cooked rice are compared among varieties and before and after storage. However, the measured values of these properties just after cooking vary among samples, as does the maximum amount of texture changes during storage. Accordingly, a comparison of the factors related to staling among samples is complicated. Therefore, kinetic analysis is required to clarify the characteristics of changes in textural properties during storage of cooked rice.
To analyze the changes in physical properties that accompany the storage of starchy foods, kinetic analysis of viscoelastic changes using a creep test was performed (Katsuta et al., 1992a, 1992b; Amano et al., 1995). Regarding bread staling, a study applying the Avrami equation for changes in the modulus of elasticity in bread crumbs was performed by Conford et al. (1964), and the equation has been subsequently used as the Conford model for analyzing bread staling (Yamauchi et al., 1994, 1999).
For the kinetic analysis of cooked rice, reports used the Avrami equation to assess the retrogradation of starch in cooked rice (Yao et al., 2002; Yoshida et al., 2010). However, these reports are analyses of starch retrogradation. A change in textural properties, which directly indicates a deterioration in the quality of cooked rice, is important; however, kinetic analysis of changes in textural properties has not yet been conducted.
This study aimed to evaluate the staling of five varieties of cooked rice by kinetic analysis of the changes in textural properties during storage, including the changes during the early stage of storage. In addition, using the rate constant obtained by kinetic analysis, we simulated the deterioration in quality based on the changes in the textural properties of cooked rice.
Materials Five rice varieties, namely Hoshiyutaka (from Saga Prefecture), Nipponbare (from Shiga Prefecture), Koshihikari (from Saitama Prefecture), Milky Queen (from Toyama Prefecture), and Himenomochi (from Chiba Prefecture), that were harvested in Japan in 2014 were used in this study. Hoshiyutaka is generally classified as a high-amylose-containing rice variety in Japan, Milky Queen as a low-amylose-containing rice, and Himenomochi as a glutinous rice. Koshihikari is the most popular variety in Japan and is rated as having a better taste than Nipponbare. The apparent amylose content of each cultivar was reported (Toyoshima et al., 1999; Umekuni et al., 2003) as follows: 26.4%–28.5% for Hoshiyutaka, 17.1%–21.4% for Nipponbare, 14.8%–19.2% for Koshihikari, 8.5%–9.3% for Milky Queen, and 0% for Himenomochi. Approximately 90% of all polished rice samples was purchased from retail stores and stored in plastic bags (nylon-polyethylene, 480 × 340 mm) in a room at 4°C until the experiments were conducted.
Rice cooking In brief, 150 g of rice was washed with 300 g of reverse osmosis (RO) water by stirring 10 times with a plastic spoon and the water was discarded after every washing; this was repeated five times. Thereafter, RO water was added for cooking. The amount of water added for cooking was 1.5 times the weight of Hoshiyutaka, Nipponbare, Koshihikari, and Milky Queen rice varieties and 1.2 times the weight of Himenomochi rice variety. Except for Himenomochi, all other rice samples were cooked after soaking for 60 min in water; however, because Himenomochi is glutinous, it was cooked immediately after washing. The rice samples were cooked using an electric rice cooker (SR-03F, Panasonic Co., Osaka, Japan) connected to a voltage regulator (bolt slider-V-130-5, Yamabishi Denki Co., Ltd., Tokyo, Japan) at 100 V until the cooking water reached 98°C, which was measured using a thermocouple (K-type, Anritsu Meter Co. Ltd., Kanagawa, Japan) and a data recorder (Thermologger AM-8000K, Anritsu Meter Co. Ltd., Kanagawa, Japan), and the samples were then cooked at 60 V for 12 min. The cooking time at 100 V for Hoshiyutaka, Nipponbare, Koshihikari, and Milky Queen was 13 min and that for Himenomochi was 12 min. The cooked rice samples were held in the rice cooker for an additional 15 min. Finally, the pot of the rice cooker was covered with paper towel, which was moistened with RO water, covered with a wrap film (made from polyvinylidene chloride), and left to cool for 1 h at 20°C in a room.
Storage of cooked rice After cooling, the cooked rice on the surface of the pot was removed using a plastic spoon, and 130 g of cooked rice from the central part of the pot was taken as a sample for storage. Each of the 6-g samples was placed in a glass Petri dish (Φ, 30 mm; height, 14 mm), covered with a lid, and sealed with parafilm. The cooked rice samples were then stored in a room at 4 ± 0.5°C for 0 (no storage), 6, 12, 16, 20, 24, 48, 72, 96, and 168 h. Each sealed glass Petri dish containing cooked rice was taken out from the room after each of the different storage times and placed for 1 h in a room at 20°C before assessing its texture.
Texture assessment of cooked rice The hardness and stickiness of cooked rice were measured using a texturometer (GTX-2, Zenken Co., Ltd., Tokyo, Japan). Three grains of cooked rice were placed on the sample stage; the height of the peak when compressing the grains with the probe and that of the negative peak when pulling up the probe away from the compressed grains were measured as hardness and stickiness, respectively. Ten measurements were averaged to give one measured value, which was repeated 4–6 times. The measurement conditions were as follows: probe, Lucite resin with Φ of 18 mm; compression speed, 2 mm s−1; and clearance, 0.2 mm.
Changes in texture of cooked rice during storage The texture properties of cooked rice stored at 4°C until 168 h were evaluated at intervals (Fig. 1). Before storage (0-h storage), Hoshiyutaka had the highest value for hardness, followed by Nipponbare and Koshihikari, and both Milky Queen and Himenomochi had the lowest hardness values. Regarding stickiness at 0-h storage, Himenomochi and Milky Queen had the highest values, followed by Koshihikari and Nipponbare, and Hoshiyutaka had the lowest value for stickiness. Among the five varieties, rice with a higher amylose content had a higher hardness value and a lower stickiness value, which is in agreement with a previous report (Toshima et al., 1999).
Changes in texture of cooked rice during storage at 4°C
mean ± standard deviation (n = 4–6)
Ten measurements were averaged to give one mean value.
▴, Hoshiyutaka; ■, Nipponbare; □, Koshihikari; ●, Milky Queen; ○, Himenomochi
Changes in the values for hardness and stickiness of cooked rice tended to become less pronounced with increasing storage time. Hoshiyutaka, which had the highest value for hardness at 0 h, showed little increase in its hardness value during storage from 48 to 168 h. Further, the change in the hardness value from 0 to 168 h was the lowest. In contrast, the hardness value of Himenomochi was low until 24-h storage, but was the highest at 168 h, and the change in its hardness value from 0 to 168 h was the highest among the five rice varieties. Changes in the stickiness value could not be detected after 16-h storage for Hoshiyutaka. Nipponbare and Koshihikari at 72 h and Milky Queen and Himenomochi at 96 h showed a change in stickiness value of almost 0. A previous study (Takami et al., 1998) reported that the hardness values of Milky Queen and Himenomochi after 24-h storage were lower and the stickiness values after 24-h storage were higher than those of Koshihikari. This corresponds with our results, in which Milky Queen and Himenomochi at the same storage time showed lower hardness values and higher stickiness values than those of Koshihikari. Furthermore, Milky Queen showed a stickiness value that was close to that of Himenomochi.
Regarding texture changes as shown above, values at 0-h storage and those of the change from 0 to 168 h varied among the five rice varieties. Thus, to investigate the rates of change in hardness and stickiness during storage, kinetic analysis was performed.
Rate of change in texture properties of cooked rice during storage As changes in the values of hardness and stickiness of cooked rice tended to become less pronounced with increasing storage time, we assumed that the first-order rate law could be used to analyze the changes in hardness and stickiness of cooked rice over time during storage.
The hardness value was replaced by a dimensionless value. The ratio of hardness increase xh (−) was obtained using the following equation:
 |
where h is the hardness value at time t; h0 is the initial hardness value (N), which is the value at 0-h storage in each cooked rice sample; and he is the equilibrium hardness value (N), which is the value at 168-h storage in each cooked rice sample used in this study. The first-order rate law was used to assess the change in hardness of cooked rice sample during storage as follows:
 |
where kh is the rate constant of hardness increase (h−1). [Eq. 3] is derived from [Eq. 2].
 |
The stickiness value was replaced by a dimensionless value. The ratio of stickiness decrease xs (–) was obtained using the following equation:
 |
where s is the stickiness value at time t; s0 is the initial stickiness value (N), which is the value at 0-h storage in each cooked rice sample; and se is the equilibrium stickiness value (N), which is used as the value at 168 h in each cooked rice sample. The first-order rate law was used to assess the change in stickiness of cooked rice during storage as follows:
 |
where ks is the rate constant of stickiness decrease (h−1). The following equation is derived from [Eq. 5].
 |
The ratio of hardness increase (xh) and that of stickiness decrease (xs) were calculated based on experimental values in Fig. 1, and accordingly, the first-order plots shown in Figs. 2 and 3 were obtained.
First-order plots of hardness increase in cooked rice of five rice varieties
xh, (h − h0)/(he − h0); h0, the initial hardness value (the value at 0 h of storage); he, the equilibrium hardness value (the value at 168 h of storage); h, hardness value at time t.
First-order plots of stickiness decrease in cooked rice of five rice varieties
xs, (s0 − s)/(s0 − se); s0, the initial stickiness value (the value at 0 h of storage); se, the equilibrium stickiness value (the value at 168 h of storage); s, stickiness value at time t.
Figure 2 shows the first-order plots of the hardness increase in cooked rice. In Hoshiyutaka, the slope of the line changed at 48-h storage, and the slope of the latter half period was less steep than that of the first half period. Nipponbare and Koshihikari had a slight change in the slope of the line at 48- and 24-h storage, respectively, but the distinction between the first and latter half periods was unclear. In Milky Queen and Himenomochi, the slope of the line changed at 20- and 24-h storage, respectively, and the slope in the latter half period was steeper than that in the first half period. Figure 3 shows the first-order plot of the stickiness decrease in cooked rice, and the slope of the line of each plot of four of the five varieties (excluding Hoshiyutaka) changed at some point during storage. The time at which the slope changed depended on the rice variety and ranged from 12 to 16 h. In Hoshiyutaka, stickiness was not detected after 16 h, and the plot was linear until 12 h.
The above results indicated that the slope of the first-order plots both with respect to the hardness increase and stickiness decrease changed halfway, and the process of texture change could be divided into the first half period and the latter half period by different rate constants. Therefore, the changes in the texture properties of cooked rice were separately analyzed for two periods.
Characteristics of hardness changes in five cooked rice varieties Table 1 shows the kinetic parameters of the hardness increase in cooked rice. The rate constant kh1 was calculated from the slope of the first half period, and kh2 was calculated from the slope of the latter half period in Fig. 2.
| Hoshiyutaka | Nipponbare | Koshihikari | Milky Queen | Himenomochi | |
|---|---|---|---|---|---|
| kh1 (×10−2 h−1) | 3.72 | 2.91 | 1.84 | 1.39 | 0.89 |
| kh2 (×10−2 h−1) | 1.30 | 2.43 | 2.33 | 2.75 | 3.32 |
| Period of the first half (h) | 0–48 | 0–48 | 0–24 | 0–20 | 0–24 |
| Period of the latter half (h) | 48–96 | 48–96 | 24–96 | 20–96 | 24–96 |
| kh1 / kh2 | 2.9 | 1.2 | 0.8 | 0.5 | 0.3 |
| h0 (N) | 37.8 | 32.9 | 30.3 | 28.6 | 28.4 |
| he (N) | 49.5 | 52.0 | 50.6 | 50 | 54.6 |
| he − h0 (N) | 11.7 | 19.1 | 20.3 | 21.4 | 26.1 |
kh1 and kh2 were calculated from the slope of the first half period and the latter half period in Fig. 2, respectively. h0, the initial hardness value (the value at 0 h of storage); he, the equilibrium hardness value (the value at 168 h of storage); he − h0, the maximum change amount of hardness.
The value of kh1 was in the order of Hoshiyutaka > Nipponbare > Koshihikari > Milky Queen > Himenomochi, which showed that among the five rice varieties, rice with a higher amylose content had a higher rate constant kh1. The value of kh2 was the lowest for Hoshiyutaka, followed by Nipponbare and Koshihikari, both of which shared a near identical value, and the value of Himenomochi was the highest. Contrary to the result of kh1, among the five rice varieties, rice with a lower amylose content tended to have a higher rate constant kh2.
From the value of each rate constant and the ratio of the rate constants (kh1/kh2), it was revealed that the change in hardness of cooked rice for each variety could be classified into the following three types: type 1, hardening early with a larger rate constant in the first half period; type 2, hardening at a rate that is similar in the first half period and the latter half period; and type 3, late start hardening with a higher rate constant in the latter half period. Hoshiyutaka showed the features of type 1, Nipponbare and Koshihikari showed the features of type 2, and Milky Queen and Himenomochi showed the features of type 3.
The results in which the slope of the first-order plots of hardness changed halfway suggested that the process of hardness change during storage was not simple. Changes in textural properties during low-temperature storage of cooked rice are related to starch retrogradation. Retrogradation of gelatinized starch involves the rapid re-ordering of the amylose chains over a short time and a slower reorganization of the amylopectin after longer periods (Miles et al., 1985; Biliaderis and Zawistowski, 1990; Lai et al., 2000). Our results indicated that rice with a higher amylose content had a higher rate of change during the initial stage of storage; whereas rice with a higher amylopectin content had a higher rate of change during the latter stage of storage. The results suggest that the influence of amylose is greater during the change in hardness in the first half period, and the influence of amylopectin is greater during the change in hardness in the latter half period.
In the above, we calculated the rate constant with respect to the dimensionless hardness change. The actual hardness value of cooked rice during storage is determined by the following three kinetic parameters: the initial hardness value of cooked rice (h0), the maximum change amount of hardness (he − h0), and the rate constant of hardness increase (kh1, kh2). Regarding Himenomochi, because the initial value (h0) was low and the rate constant of the first half period (kh1) was the lowest among the five varieties, the measured hardness value up to 24 h was the lowest. However, because the maximum change (he − h0) and the rate constant in the latter half period (kh2) were the highest, the measured hardness value of Himenomochi became larger than that of other varieties in the latter half period.
Characteristics of stickiness changes in five cooked rice varieties Table 2 shows the kinetic parameters of the stickiness decrease in cooked rice. The rate constant ks1 was calculated from the slope of the first half period, and ks2 was calculated from the slope of the latter half period (Fig. 3). The values of ks2 of each variety were higher than the corresponding ks1 values. In addition, the higher the amylose content, the higher the rate constant in both ks1 and ks2. In the case of Himenomochi and Milky Queen, because the initial stickiness values (s0) were high and the rate constants (both ks1 and ks2) were low, the stickiness values of each storage time were high. Among the five varieties, it was inferred that both the initial stickiness value (s0) and the rate constant of stickiness decrease (ks) of cooked rice were greatly influenced by the amylopectin content.
| Hoshiyutaka | Nipponbare | Koshihikari | Milky Queen | Himenomochi | |
|---|---|---|---|---|---|
| ks1 (×10−2 h−1) | 9.30 | 3.91 | 2.97 | 1.86 | 1.41 |
| ks2 (×10−2 h−1) | — | 5.68 | 4.04 | 3.80 | 3.70 |
| Period of the first half (h) | 0–12 | 0–16 | 0–16 | 0–16 | 0–12 |
| Period of the latter half (h) | — | 16–72 | 16–72 | 16–72 | 12–72 |
| ks1/ks2 | — | 0.7 | 0.7 | 0.5 | 0.4 |
| s0 (N) | 0.8 | 6.0 | 6.2 | 7.6 | 7.2 |
| se (N) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| s0 − se (N) | 0.8 | 6.0 | 6.2 | 7.6 | 7.2 |
ks1 and ks2 were calculated from the slope of the first half period and the latter half period in Fig. 3, respectively. s0, the initial stickiness (the value at 0 h of storage); se, the equilibrium stickiness (the value at 168 h of storage); s0 − se, the maximum change amount of stickiness.
Simulation of the change in quality of cooked rice based on the stickiness-hardness ratio obtained from calculated values of hardness and stickiness Changes in hardness and stickiness during storage were calculated from the values mentioned in Tables 1 and 2, respectively. From the calculated values of hardness and stickiness, the value of the stickiness-hardness ratio (stickiness value/hardness value) at certain times was calculated, and the change in quality of cooked rice was simulated. The value of the stickiness-hardness ratio has been used to evaluate the quality of cooked rice, and rice with a large ratio is highly valued in Japan (Okabe, 1979).
To calculate the change in hardness of cooked rice during storage, the ratio of hardness increase at time t1 in the first half period is expressed by the following equation, which is derived from [Eq. 3]:
 |
where kh1 is the rate constant of hardness increase in the first half period. Therefore, the hardness of cooked rice at time t1 was calculated from [Eq. 7] and [Eq. 1].
The ratio of hardness increase at time t2 in the latter half period is expressed by the following equation:
 |
where kh2 is the rate constant of hardness increase in the latter half period and A is an intercept, with the y axis obtained by extending the straight line of the latter half period as shown in Fig. 2. The following equation is derived from [Eq. 8].
 |
Therefore, the hardness of the cooked rice at time t2 was calculated using [Eq. 9] and [Eq. 1].
The calculated value of stickiness of cooked rice during storage was obtained in the same manner as described above.
Figure 4 shows the calculated values of hardness and stickiness at certain times and calculated values of the stickiness-hardness ratio obtained from both properties. When the ratio is <0.1, the quality of cooked rice is inferior (Okabe, 1979). Therefore, in this study, a stickiness-hardness ratio of ≤0.09 was used as an indicator of deterioration in the quality of cooked rice by staling. The time to reach a value 0.09 was obtained from the results shown in Fig. 4 for each rice variety. The time when the ratio reached 0.09 was 0 h for Hoshiyutaka, 13 h for Nipponbare, 22 h for Koshihikari, and 30 h for Milky Queen and Himenomochi. The stickiness-hardness ratio for Hoshiyutaka was ≤0.09 even at 0 h. This was not because of the staling of cooked rice but rather because of the characteristics of Hoshiyutaka, which has a high hardness value and a low stickiness value owing to its high amylose content. Simulations for the stickiness-hardness ratios of the other four rice varieties quantitatively showed that the change in quality of Koshihikari during storage was slower than that of Nipponbare, and the change in quality of Milky Queen was as slow as that of Himenomochi.
Change in the stickiness–hardness ratio obtained from calculated values of hardness and stickiness of cooked rice
, Hoshiyutaka; 
, Nipponbare; 
, Koshihikari; 
, Milky Queen; 
, Himenomochi
In our study, we showed the characteristics of changes in textural properties of cooked rice during storage by kinetic analysis. In addition, we demonstrated the possibility of simulating changes in quality using the stickiness-hardness ratio calculated from rate constants and other parameters obtained by kinetic analysis. The evaluation of staling of cooked rice using kinetic analysis, as shown in this study, is a useful method that can be used to discriminate among rice varieties and analyze the influence of cooking conditions and other factors on the staling of cooked rice.
