Effect of heat treatment on the protein fraction of rice

 Abstract: The effects of different treatment temperatures and times on the protein fractions of rice were investigated by using protein fraction systematic separation analysis. The results showed that the heat treatment temperature and time had different degrees of influence on the protein fractions in the samples. The clear protein content decreased gradually with the increase of temperature and time. Below 100 ℃, there was little difference between the globulin and the control, while the alkaline soluble protein increased. When the temperature was increased to 200 ℃, both of them decreased significantly.

 

Protein is an important component of rice, and its content is generally about 8.0% of the dry weight of rice. Protein is an important component of rice, and its content is generally about 8.0% of the dry weight of the rice. Proteins are usually divided into two main groups according to their chemical composition: simple and complex proteins. Simple proteins consist of a variety of amino acids, whereas complex proteins consist of simple proteins with a variety of non-protein components (cofactors). Simple proteins are categorized according to their solubility in different solvents: albumen (soluble in water), globular proteins (soluble in dilute salt), alcohol soluble proteins (soluble in 60% to 80% ethanol), and alkaline soluble proteins (soluble in dilute alkali). Quantitative analysis of protein fractions in wheat, followed by studies on the relationship between protein fractions and some qualities, has been reported both domestically and internationally [1-2]. However, there are few studies on the content of protein components in rice grain, especially after heat treatment of rice, the change of protein components in the body and the relationship with the nutritional quality. This study adopts a systematic separation and quantification method of protein components to study the interrelationships between the protein components of rice and the temperature and heating time, and to make a preliminary discussion on reducing and avoiding nutrient losses during the processing process.

 

1 Materials and equipment

Rice: Variety Yong Japonica 18, collected from Zhuangshi Town, Ningbo City, harvested in the same year. The samples were hulled in the laboratory, pulverized and sieved through a 40-mesh sieve.

Main instruments and equipment: LD4-2 centrifuge (Beijing Medical Centrifuge Factory), micro nitrogen fixing instrument, constant temperature box, constant temperature water bath, Soxhlet extractor, muffle furnace, etc..

 

2 Heat treatment of rice

Two factors, temperature and heating time, were set. The temperature factor was set at four levels, with the room temperature treatment as the control and other heat treatment temperatures of 100℃, 150℃ and 200℃; the heating time factor was set at four levels, i.e., at each temperature, there were three treatments with different constant temperature times, i.e., 10, 20, and 30 min. There were 10 treatments in total, and each treatment was repeated 3 times. As shown in Table 1. The heating method is electric thermostat.

 

3 Isolation and Determination of Protein Fractions

The detailed separation procedure is described in Determination of Crop Seed Protein Fractions [3]. The separated protein fractions were transferred into Kjeldahl bottles and the nitrogen content was determined by Kjeldahl method. The nitrogen content of the samples was multiplied by the protein conversion factor of 5.92 to obtain the protein fractions. The nitrogen content of the sample was multiplied by the protein conversion factor of 5.92 to obtain the content of each protein fraction. The moisture content of the test samples was also determined and the results were converted to dry basis for comparison. The reagents used were analytically pure.

 

4 Results and discussion

4 . 1 Effect of heat treatment temperature on protein fractions of rice

Rice samples were separated and analyzed for protein fractions to obtain Table 2.

 

Table 1 Heat treatment tests on rice

As can be seen from Table 2, the protein components in the Yong-Japonica 18 rice samples were higher in albumen, followed by alkaline soluble protein and globular protein, while alcohol soluble protein was not detected.

Most food proteins have functional properties only within a narrow temperature range. From a nutritional point of view, the changes caused by mild heat treatment are usually beneficial, but heat treatment beyond a certain limit can also lead to amino acid destruction, such as desulfurization, deamidation, isomerization, and other chemical modifications of amino acids, and in some cases further formation of toxic compounds [4]. Therefore, the study of the content of protein components can help to understand the changes in the nutritional composition of rice. The contents of the protein components were heated at 100℃, 150℃ and 200℃ for 10, 20 and 30 min, respectively, and compared with the original samples (i.e., the control), as shown in Figures 1-4. In the test, except for the alcohol soluble protein, which was not detected, the other protein fractions had different degrees of changes with temperature. The clear protein decreased gradually with the increase of temperature (Fig. 1), and the clear protein content decreased by 29.5%~100% compared with the control when the temperature was increased from 100℃ to 200℃. From 100°C to 200°C, the clear protein content decreased by 29.5% to 100% compared with the control. Globulin did not differ significantly from the control at 100°C (Figure 2), but increased at 150°C, by 9.3% to 53.3% over the control. 3% ~ 53 . At 150℃, there was an increase of 9.3%~53.5% compared with the control, but when the temperature was increased to 200℃, it decreased to undetectable. Alkaline soluble protein increased by 38.8%~85.5% at 100℃ compared with the control. 8% ~ 85.9%. 9% at 100℃, and decreased significantly when the temperature was further increased. The residual protein at 100℃ did not differ much from the control, but increased when the temperature was raised. The total protein did not change much throughout the heat treatment and was basically maintained within a variation of ± 3.1%. The total protein did not change much throughout the heat treatment and was basically maintained within a variation of ± 3.1%.

 

4 . 2 Effect of heat treatment time on protein fractions of rice

Heat treatment time is also an important factor (Figures 1~4). From the figure, it can be seen that at a certain heating temperature, if the heating time is extended, for example, the time is extended from 10 min to 30 min at 100 ℃, the whiteness gradually decreased, and the content of whiteness decreased from 1.76% of the control to 0.0% of the control. 76% to 0.64% (Figure 1). 64% (Figure 1); while the alkaline soluble protein increased (Figure 3); globulin and residual protein did not change much (Figure 2, Figure 4). At 150℃, the clear protein continued to decrease with the heating time, while the globular protein increased gradually, and the residual protein also increased. When heated to 200 ℃ for 10 min, the clear protein and globulin still had some residual protein. Continue to extend the time, both disappear.

 

5 Summary

In summary, during heat treatment, the various protein components in rice were changed to different degrees by temperature and time. The clear protein content gradually decreased with the increase of temperature and time. Below 100 ℃, the difference between globulin and control is not significant, while the alkaline soluble protein increases. When the temperature was increased to 200 ℃, both of them decreased significantly. Among the protein fractions, clear protein is particularly sensitive to temperature and time, and heat treatment can rapidly reduce its content, and at 200 ℃ temperature. It is rapidly reduced by heat treatment, and at a temperature of 200°C. Data [5] show that clear egg white and globular egg white are rich in lysine. Therefore, for rice with lysine as the first limiting essential amino acid, in order to minimize the destruction of amino acids in the process, we should try to avoid higher temperatures or longer processing, or in the processing, add a certain amount of lysine to supplement its loss, to ensure that the nutritional value of the stability of the rice. The content of alkaline soluble protein increases at 100℃, but decreases above 150℃. In addition, whether there is a possibility of mutual conversion between protein fractions due to heat treatment needs to be further investigated in the future.

 

References:

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[2] WANG Hui, MA Chuanxi, XU Feng, et al.  Effect of heat treatment on protein fractions of wheat kernels ［J］. Journal of Wheat Crops, 2001, 21(4): 64 - 66 .

[3] Hua Tianmao, ed. Agricultural products analysis method [M]. Northwest Agricultural University, 1994 . 21 - 23 .

[4] O . R . Fenema, Wang Zhang, et al .  Food Chemistry [M]. Beijing: China Light Industry Publishing House, 1991.

[5] Laszity R .  The chemistry of cereal proteins [M].  cpcp press, Inc., 1980 .