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Effects of repeated freezing and thawing treatments on retrogradation of starch in rice

T. Maeda1, N. Matsuura2, T. Utsunomiya2, T. Sitsuji2 and N. Morita2

1Department of Life and Health Sciences, Hyogo University of Teacher Education, 942-1, Shimokume, Yashiro, Hyogo 673-1494, Japan.
2
Lab. of Food Chemistry, Graduate School of Life and Environmental Sciences, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan.

Introduction

Recently, various kinds of frozen foods have been sold and eaten since the convenience stores and family restaurants have been in vogue in Japan. Furthermore, as the frozen foods are quite useful and necessary according to the coming of an aging society, occidentalisation of life style, etc, the frozen system has been used for not only side dishes but also staple diets, such as rice, bread and noodle. Effects of freezing or thawing treatments on the qualities of cooked rice have been studied from the view points of retrogradation of starch (Matsunaga and Kainuma, 1981; Kim et al, 1997; Jacobson and Bemiller, 1998). As the result, the freezing storage could suppress the retrogradation of cooked foods, as compared with storages under the cold or room temperatures (Matsunaga and Kainuma, 1981), and also the speed for freezing or temperature for thawing was correlated with the retrogradation level of starch (Jacobson and Bemiller, 1998). However, the storage conditions such as storage length or temperature after purchasing frozen foods from shopping store would be considerably changed by consumers, and the stability of frozen foods could not be controlled. The boiled rice has been empirically known to change the colour and texture by severe condition of repeated freezing and thawing. In this study, to improve the qualities of frozen foods under the unstable or stern conditions, especially using boiled rice, effects of various additives on the damage caused by repeated freezing and thawing were investigated.

Materials and methods

Rice and chemicals

Ingredients and their composition for the present experiments were similar to those used for commercial products in a frozen rice factory as shown in Table 1: that is, rice of Koshihikari harvested in 2004 and vinegar including trehalose and sugar were used. Various additives as improvers were added to these ingredients at the amounts of 0.1-0.3 % on a rice weight basis as shown in Table 2.

Preparation of boiled rice and its conditions of freezing and thawing

The boiled rice was prepared with additives using an electric rice cooker (Matsushita Co., Ltd., Osaka, Japan), and addition of ingredients and handling procedures were shown in Figure 1. The first freezing was conducted for the boiled rice samples at -20C for 24-48 hr, and then the frozen rice samples were completely thawed at room temperature (I) and refrigerator (II) for 4 hr, followed by the second freezing at -20C for 20-24 hr. Finally, all samples were completely thawed at room temperature for 4hr, and the samples with processes I and II were named RRFT-B and -C, respectively. In addition, the sample treated by only process I without the second freezing was named RRFT-A. Two kinds of control samples were prepared as follows: the first was thawed by microwave oven after the first freezing (RTMO); the second was treated by the same three processes described above without additives (CON-A, -B and -C).

Change of weight of rice during boiling

The rice samples were weighed before and after boiling, and the changing ratio of weight during boiling was calculated from its increasing amount.

Rheological properties of boiled rice

Rheological properties of rice samples as described above were determined by fractural and textural tests (Morita et al, 2002; Maeda et al, 2004). To determine the properties of rice samples packed into a plastic vessel (25 mm i.d. x 25 mm), thus prepared samples were stood for 10 min at 30C, and then the textural properties were measured with two times’ compression using a mono-axial compression-elongation type Fudoh rheometer (Model RT-2002D・D, Rheotec Co., Ltd., Tokyo, Japan) as reported previously (Morita et al, 2002). A disc plunger (10 mm i.d. x 2 mm, Product No. 3) was used to measure compression stress and cohesiveness of the aggregate of boiled rice grains at 30C. The penetration speed of plunger was 30 cm/min, and the baseline and stroke were controlled at 40 mm and 30 mm, respectively. On the other hand, to determine the rheological properties of one rice grain, the wedge-shaped type of plunger (20 mm i.d. x 2 mm, Product No. 13) was used to measure the fracture stress with the compression deformation controlled at 1 mm. The plunger speed was 6 cm/min and also tested by the same rheometer as described above. The data were processed using a computer program, Rheosoft TR-06 (Rheotech Co., Ltd., Tokyo, Japan).

Appearance of boiled rice

Appearance of boiled rice was determined from the colour values using a colour reader CR-13 (Minolta, Co., Ltd., Tokyo, Japan). Fifty grams of rice samples and the same amount of distilled water were mixed by MX-X60 (Matsushita, Co., Ltd., Osaka, Japan) for 1 min and the whiteness was determined.

Results and discussions

Effects of additives on the changing weight of rice during boiling

Normally, the weight of rice increased by absorbing water during boiling, the water holding ability is related to the firmness of boiled rice. Therefore, the increasing weight means that the rice can sufficiently hold water without its evaporation during boiling. KG, TG, PG, SE-S1670, TO+SE-S1670, OL+SE-S1670, PFAE-MM750, PFAE-MSW7S, OL, XG, SE-P1670, OL+SE-P1670, TO+PFAE-MSW7S, TO+SA-GMB, TO+TG, TO+XG and GE improved the water holding ability, as compared with those of control samples (RTMO or CON) as shown in Table 3.

Effects of additives on appearance of rice

Colour appearance of CON-A was the same as that of RTMO, and those of CON samples changed distinctly depending on the storage conditions. Especially, the value is quite high for CON-B including thawing at room temperature, while low for CON-C thawed in refrigerator. Generally, the high value of whiteness is considered to highly retrograde starch in boiled rice. Therefore, CON-C might be exceedingly staled by stern conditions, and the low value was suggested to produce complicated damages except for only the retrogradation. Making comparison with RTMO, all samples with additives showed high values, but they did not change regardless of the freezing or thawing conditions. Particularly, additions of TO or KG to RRFT-C made relatively similar values to those of RTMO.

Figure 1. Flow-diagram of boiling, freezing and thawing rice samples.

Effects of additives on the rheological properties of rice

The firmness of a rice grain sample was greater for RRFT-B rather than RRFT-A, regardless of additives. PFAE-MSW7S, TO+PFAE-MM750, TO+SA-GMB and GE did not increase the value for RRFT-C and the firmness was almost similar to that of RTMO. Especially, the improving tendency was clear for RRFT-C added with PFAE. Normally, the progressed retrogradation of rice starch would increase the fragile values, but RRFT-C samples including above additives could keep the same fragile values as that of RMTO. Therefore, PFAE, TO and GE could make the rice with soft and elastic properties despite the repeated freezing or thawing treatments. As to aggregate of boiled rice grains packed into the case, the firmness increased in the order of RRFT-A, -B and -C. However, PFAE-MSW7S, TO+PFAE-MM750, TO+ PFAE-GP120, TO+SA-GMB and GE decreased the values more than that with RTMO, and any treatments could not affect the firmness of RRFT. Cohesiveness and elasticity of RRFT were larger than those of RTMO and didn’t depend on the used additives. Therefore, all additives were considered to suppress the damage or breakdown of RRFT starch exposed by stern treatments of frozen foods.

Table 1. Compositions of ingredients for the boiling rice.

Ingredients

Amount

Rice

960 g

Water

2208 g*

Vinegar including trehalose and sugar

222 ml

Additive

0.1-0.3 %*

*Amounts of water and additives were 2.3 times and 0.1-0.3 % on the rice weight basis, respectively.

Table 2. Summary of additives used for the boiling rice.

Additive

Abbreviation

No additive

CON

Oligotose

OL

Trehalose

TO

Glycine

GL

Gelatin

GE

Potassium D-Gluconate

PG

Calcium D-Gluconate

CG

Sodium D-Gluconate

SG

Sodium L-Lactate

SL

Soyafibe-S-DN (Galacturonan, Rhamnogalacturonan, Arabinan, Galactan)

SF-DN

Soyafibe-S-LNP (Galacturonan, Rhamnogalacturonan, Arabinan, Galactan)

SF-LNP

Fiberon (D-Galactose-D-mannoglycan)

FS

Kerukogel (Gellan gum)

KG

Glyride, Tamarind gum (Galactoxyloglucan)

TG

Xanthan gum

XG

Healtygum, Psyllium seed gum

HG

Sucrose fatty acid ester (Stearic acid)

SE-S1170

Sucrose fatty acid ester (Stearic acid)

SE-S1670

Sucrose fatty acid ester (Palmitic acid)

SE-P1670

Alginic acid

AA-HFD

Sodium alginate

SA-DMF

Sodium alginate

SA-GMB

Polyglycerol fatty acid ester (Stearic acid ester)

PFAE-MSW7S

Polyglycerol fatty acid ester (Myristic acid ester)

PFAE-MM750

Polyglycerol fatty acid ester (Behenic acid ester)

PFAE-GP120

Trehalose+Polyglycerol fatty acid ester (Steraric acid ester)

TO+PFAE-MSW7S

Trehalose+Polyglycerol fatty acid ester (Myristic acid ester)

TO+PFAE-MM750

Trehalose+Polyglycerol fatty acid ester (Behenic acid ester)

TO+PFAE-GP120

Trehalose+Potassium D-Gluconate

TO+PG

Trehalose+Sodium alginate

TO+SA-GMB

Trehalose+Glyride, Tamarind gum (Galactoxyloglucan)

TO+TG

Trehalose+Xanthan gum

TO+XG

Trehalose+Kerukogel (Gellan gum)

TO+KG

Trehalose+Sucrose fatty acid ester (Palmitic acid)

TO+SE-P1670

Trehalose+Sucrose fatty acid ester (Stearic acid)

TO+SE-S1670

Oligotose+Sucrose fatty acid ester (Stearic acid)

OL+SE-S1670

Oligotose+Sucrose fatty acid ester (Palmitic acid)

OL+SE-P1670

Kerukogel (Gellan gum)+Sucrose fatty acid ester (Stearic acid)

KG+SE-S1670

Table 3. Changing ratio of rice weight during boiling.

Sample

(%)

Sample

(%)

CON

217.97*

AA-HFD

215.5

OL

219.6

SA-DMF

214.4

TO

216.9

SA-GMB

215.6

GL

218.1

PFAE-MSW7S

219.6

GE

225.2

PFAE-MM750

219.4

PG

220.4

PFAE-GP120

218.1

CG

217.3

TO+PFAE-MSW7S

221.9

SG

217.6

TO+PFAE-MM750

217.8

SL

216.7

TO+PFAE-GP120

217.5

SF-DN

215.6

TO+SA-GMB

220.9

SF-LNP

216.7

TO+TG

219.8

FS

217.1

TO+XG

219.6

KG

219.0

TO+KG

217.5

TG

219.2

TO+SE-S1170

217.5

XG

221.7

TO+SE-S1670

223.9

HG

217.9

OL+SE-S1670

220.4

SE-S1170

216.3

OL+SE-P1670

221.7

SE-S1670

223.7

KG+SE-S1670

217.8

SE-P1670

221.2

   

Conclusions

Characteristics of boiled rice treated with repeated freezing and thawing in the presence of various additives were compared with those of rice samples thawed by microwave oven and in absence of additives. Some additives increased the water holding ability of rice during boiling, as compared with the control samples without additives. Addition of all additives could not change the whiteness of RRFT during storage. Regardless of freezing and thawing treatments, the firmness of one rice grain or aggregate of boiled rice grains of RRFT was the same as that of RTMO by additions of PFAE-MSW7S, TO+PFAE-MM750, TO+PFAE-GP120, TO+SA-GMB and GE. Therefore, PFAE could improve the tolerance of rice to be repeated freezing and thawing, resulting in improving softness, elasticity and a high water holding ability, as compared with samples thawed by microwave oven or without additives.

Acknowledgements

The authors wish to thank Fuji Oil Co., Ltd. (Osaka, Japan), Mitsubishi-Kagaku Foods Co. (Tokyo, Japan), Dainippon Pharmaceutical Co., Ltd. (Osaka, Japan) and Sakamoto Yakuhin Kogyo Co., Ltd. (Hyogo, Japan) for providing various additives.

References

Jacobson, M., R., and Bemiller. J. N. (1998). Cereal Chemistry 75: 22-29.

Matsunaga, A., and Kainuma, K. (1981). Journal of Home Economics of Japan 32: 653-659 (in Japanese).

Kim, J., O., Kim, W. S., and Shin, M. S. (1997). Starch 49: 71-75.

Maeda, T., Kim, J. H., and Morita, N. (2004). Cereal Chemistry 81: 660-665.

Morita, N., Maeda, T., Miyazaki, M., Yamamori, M., Miura, H., and Ohtsuka, I. (2002). Food Science and Technology Research 8: 119-124.

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