Qin Ke-xin, Liu Lin-lin, Liu Tian-yi, Cheng Wen-hong, and Shi Yan-guo

College of Food Engineering, Harbin University of Commerce, Harbin 150076, China

Correlation Between Physicochemical Properties and Eating Qualities of Rice

Qin Ke-xin, Liu Lin-lin, Liu Tian-yi, Cheng Wen-hong, and Shi Yan-guo*

College of Food Engineering, Harbin University of Commerce, Harbin 150076, China

Correlation among physicochemical properties, which include amylose content, alkali spreading values, gel consistency, water absorption, expansion rate, solid content of rice-water, protein content and fat content, and eating qualities of six kinds of rice samples planted in Heilongjiang Province were studied. Correlation analysis showed that amylose content, water absorption and expansion rate were negatively correlated with eating qualities, yet gel consistency, alkali spreading values, solid content of ricewater and fat content were positively correlated with eating qualities. Among them, eating quality had an obvious correlation with amylose content and gel consistency, but no significant correlation with protein content. The regression equation, which described the relationship of the eating quality scores and physicochemical indexes, was Y=94.439–12.711X1–23.721X2–0.701X3+0.570X4+186.938X5(X1, X2, X3, X4and X5represented amylose content, water absorption, expansion rate gel, consistency and fat content). The single factor analysis of variance showed that six kinds of rice existed significant differences in quality category.

rice, physicochemical property, eating quality, correlation

Introduction

Appearance quality, milling quality, cooking and eating quality and nutrition quality are primary indexes to evaluate rice quality (Hu et al., 2010). Eating qualities can reflect the consumer acceptance more directly than others, so eating qualities are viewed as the most important aspect to evaluate rice quality. Sensory evaluation, composition determination and physical properties determination are principal methods to evaluate eating quality of rice. Generally, People judge the quality of rice by sensory evaluation (Zhang et al., 2010), because of its intuition. However, due to personal hobbies, habits and different environments, there are some differences in sensory evaluation. It is a new means for rice eating quality evaluation to determine the taste value by modern analytical instrument. In Japan, taste testers have been widely applied to determine the sensory quality of rice (Sun et al., 2008). Taste testers detect the light quantity of the reflected light in 540 nm and 970 nm and transmission light in 540 nm and 640 nm. Collecting visible spectrum and near infrared spectrum, through the correlation among visible spectrum, near infrared spectrum and sensory taste value, by PLS, acquires the best mathematical model, which makes the sensory experiment numerical and evaluates the rice taste level objectively. Rice taste testers have a lot of advantages, such as the accuracy of the testingresults, easy operation and the more testing samples once (above 20 at a time). Besides, the whiteness and gloss of rice, physical property, such as hardness and viscoelasticity, are measured by photoelectric sensor and texture analyzer, respectively, then by means of sensory evaluation and computer technology to predict taste values of rice, and the mathematical model is built (Wang et al., 2011). Nowadays, electronic nose, a novel instrument of fast detection, has been developed, which is a smell scanner to evaluate the smell entirely instead of human sense organ (Zheng et al., 2009; Jonsson et al., 1997; Olsson et al., 2002)..

Testing some compositions and properties of rice are less affected by external and human factors, so it attracts more and more people's attention to study the correlation between some physicochemical properties and sensory qualities to evaluate rice quality by measuring physicochemical properties. Early in 1997, correlation among eating quality (total points), appearance quality, cooking quality and nutrition quality was investigated systematically (Chen et al., 1997). Afterwards, the characteristics between cooking and eating qualities of 22 kinds of rice (Jin et al., 2001), and the relevance between eating quality (total points) and amylose content, alkali spreading values and gel consistency of 390 kinds of rice (Cheng et al., 2006), have been studied. Recently, the correlation between physicochemical properties and eating qualities of eight kinds of rice has been researched (Huang, 2012). However, every result was not the same. This paper chose six kinds of representative rice, planted in Heilongjiang Province, to detect each physicochemical index values of cooking quality and nutrition quality, eating index values by sensory evaluation combined with the taste values by rice taste tester as the standard to reflect eating quality of rice. The correlation between physicochemical properties and eating qualities were studied by the method of correlation coefficient and regression analyses, and the varieties difference of these six kinds of rice were analyzed by the single factor analysis of variance.

Materials and Methods

Materials

Experiment materials were six kinds of japonica rice which produced from Jamusi City, Fangzheng County, Suihua City, Mudanjiang City, Tailai County and Wuchang City in Heilongjiang Province, China. They were purchased separately at the local farmers' market. All the chemical reagents were of analytical grade.

Equipments

Rice taste tester (STA1B), Shredder (FZ102), Visible spectrophotometer (722E), Kjeltec auto analyzer (KDN-F), Electro-thermostatic blast oven (DHG-9203A).

Determination of physicochemical indexes

Amylose content was determined according to standard ISO 6647-1: 2007 (rice-determination of amylose content). Alkali spreading values were determined according to the measurement method of gelatinization temperature of standard NY/T 83-1988 (measurement method of rice quality), China. Gel consistency was determined according to Appendix A of standard GB/ T 17891 (high quality paddy), China. Determination of water absorption, expansion rate and solid content of rice-water was according to the measurement method of cooking quality in Ruan Shaolan's report (Ruan and Mao, 2004). Kjeltec auto analyzer was used to determine protein content according to operation instruction. Fat content was determined by soxhlet extraction of GB/T 14772-2008 (determination of fat in foods), China.

Determination of eating quality

Eating index values of rice were got by sensory evaluation. Preparation of rice, requirement of evaluation and the method of sensory evaluation were according to standard GB/T 15682-2008 (inspection of grain and oils-method for sensory evaluation of paddy or rice cooking and eating quality), China. Taste values weredetermined by rice taste tester (STA1B).

Statistical analysis

SPSS, the data processing software was used to analyze experimental data.

Results and Discussion

Experiment values

Table 1 shows measurement values of each physicochemical index. It can be seen that the samples with high amylase content showed high water absorption and expansion rate, but low alkali spreading values, gel consistency and solid content. Fig. 1 shows the scores of each eating quality index, total points by sensory evaluation and taste values by rice taste testers. The index of sensory evaluation included smell, exterior structure, palatability, taste and cold rice texture, and the taste value was the total points of eating quality by taste testers. As we can see in Fig. 1, the order from high to low of total points was Rice 3, Rice 6, Rice 5, Rice 1, Rice 4 and Rice 2, but the smell of Rice 6 and Rice 5 was better than that of Rice 3. Taste value determined by rice taste testers was significant positive correlation with the total points of sensory evaluation (P＜0.01) and the correlation coefficient being 0.995, so taste value could be used to evaluate the eating quality of rice. Determining taste value of rice by rice taste testers not only made the determination of rice eating quality more simple and convenient, but also saved the raw material cost and test time. The method has many advantages, such as using less samples, testing more samples at one time and high efficiency.

However, there were some uncertain relationships between some physicochemical indexes and the scores of the eating quality. So these values were analyzed by SPSS to confirm the relationship among them.

Table 1 Summary sheet of measurement results of physicochemical properties

Fig. 1 Sensory evaluation scores of eating quality of six kinds of rice samples

Correlation between each cooking quality index of rice

Table 2 shows the correlation coefficient among cooking quality indexes of six kinds of rice. It can be seen that the samples with higher amylose content showed the lower alkali spreading value, gel consistency and solid content of rice-water, but the bigger water absorption, expansion rate, and vice versa. There being close correlation among cooking qualities which included amylose content, alkali spreading value, gel consistency, water absorption, expansion rate and solid content of rice-water. It was consistent with the result of Ruan and Mao (2004).

Table 2 Correlation coefficient among cooking quality indexes

Amylose content is a constituent part of rice starch, with its structure-anhydroglucose unit connected by α-1, 4 glycosidic bond, and intramolecular hydrogen bonding curled up into a spiral chain, forming a hollow screw structure, namely the double helix. Amylose content had close relationship with the gelatinization temperature of starch. With high amylose content, gelatinization temperature, and low alkali spreading values, the rice was dry and soft, and had poor viscosity, but was not easy broken in the process of cooking.

On the contrary, with low amylose content, gelatinization temperature, and high alkali spreading values, the rice had strong viscosity and was easy broken. Amylase content also affected the water absorption and expansion rate in the process of cooking. Generally, lower amylose content meant tasty. It is related to the pecular structure, anti swelling property, good tensile strength and gel properties of amylose closely.

Correlation between eating quality and physicochemical index and regression analysis

Table 3 shows the correlation coefficient between eating quality index and physicochemical index of six kinds of rice. From experimental result, sensory total points of eating quality were significantly negative correlation with amylose content (p＜0.01), water absorption and expansion rate (p＜0.05), but significantly positive correlation with gel consistency (p＜0.01), alkali spreading values, solid content of rice-water and fat content (p＜0.05). It was consistent with the result of Cheng et al. (2006). Effect result of water absorption and expansion rate on eating qualities was consistent with the result of Xiong et al. (2002) roughly, but it was opposite with the result of Meng et al. (2008). But there was no significant correlation between eating quality and protein content. Thus, the samples which had a higher amylose content, water absorption and expansion rate showed a lower alkalispreading value, gel consistency and solid content of rice-water, a worse eating quality and vice versa. The trend of taste value determined by rice taste testers was consistent with the result of sensory evaluation and the correlation with each physicochemical index was much the same.

Main indexes influencing rice smell were amylose content, alkali spreading values, gel consistency and water absorption. Amylose content, alkali spreading values, water absorption and expansion rate mainly affected exterior structure, and the indexes of gel consistency, solid content of rice-water and fat content were secondary. Amylose content mainly affected palatability, and secondly, alkali spreading values, water absorption and expansion rate and solid content of rice-water also influenced palatability. Main indexes impacting on taste were amylose content, alkali spreading values and water absorption, and secondly were gel consistency, expansion rate, solid content of rice-water and fat content. Main indexes affecting cold rice texture were amylose content, alkali spreading values and gel consistency, followed by the water absorption and expansion rate and fat content.

In this experiment, there was no linear relation between protein contents and eating qualities. However, Lin et al. (2011) showed that there was a significant negative correlation between protein contents and eating qualities, but Shi et al. (2010) thought that protein content had no effect on eating quality. Recent studies have showed that the effect of protein contents on eating qualities was twofold (Li et al., 2009). The primarily protein in rice are glutelin and prolamine. Glutelin is rich with essential amino acid such as lysine, tryptophan and methionine, and high nutritional value, and it is helpful in digestive absorption. Prolamine is less with lysine, it can't be degraded by trypsin and difficult to be broken during cooking, and thus it can affect the surface texture of rice and debase the eating qualities (Xie et al., 2008; Matsue et al., 1995). Thus, if the sample has a higher protein content, the superior amino acids will be higher, the nutritive value will be richer, and easier for the body to absorb and the positive influence on eating qualities. The sample with lower protein content negatively influenced on eating qualities . However, there was a high prolamine content to the sample with very high protein content, and it negatively influenced on eating qualities. Jin et al. (2007) studied the correlation between protein contents and eating qualities of rice, and the result showed that there was a dynamic change relationship between protein contents and eating qualities, at first, the taste value increased as protein contents decreased, and then, when protein contents decreased to a critical value, the taste value decreased as protein contents decreased.

Table 3 Correlation coefficient (r) between eating quality index and physicochemical index

Data were processed by linear regression analysis to reflect the effect of physicochemical properties on eating qualities. We got the regression formula:

Y=94.439–12.711X1–23.721X2–0.701X3+0.570X4+ 186.938X5

Y referred to taste value, X1referred to amylose content, X2referred to water absorption, X3referred to expansion rate, X4referred to gel consistency and X5referred to fat content.

Regression formula had some limitations because factors were not quite comprehensive. We should consider the appearance qualities, such as grain shape, chalky rice rate, chalkiness, light transmittance, milling quality and moisture content of rice in actually work. Xia et al. (2008) studied rice planted in Anhui Province, China, and the result of regression analysis showed that milled rice ratio, grain length, length width ratio, transmittance, gel consistency and protein content could evaluate scores of eating quality better. Mo et al. (2014) found the eating quality had close relationship with starch properties and gelatinization characteristics of rice. So, we should consider more comprehensive factors to build the regression equation between physicochemical indexes and eating qualities. Otherwise, the rice varieties were studied less in the experiment. If we increase rice varieties, we can get a much more accurate result.

Analysis of varieties difference of six kinds of rice

Table 4 shows the result of the single factor analysis of variance. We can see the varieties of these six rice samples were significant different (p＜0.01). In terms of variable coefficient, the variable coefficient of fat content was the maximum in these indexes, followed by the alkali spreading values, solid content of ricewater, taste value, gel consistency and amylose content, and the variable coefficient of water absorption and expansion rate were the minimum. Variable coefficient of fat content was more than five times as water absorption, and the variable coefficient of alkali spreading values and solid content of rice-water was more than twice as water absorption. Those showed that the variation ranges of fat content, alkali spreading values and solid content of rice-water were relatively large.

Table 4 Six kinds of different rice varieties

Conclusions

Correlation analysis showed that there were close correlations among cooking qualities which included amylose content, alkali spreading values, gel consistency, water absorption, expansion rate and solid content of rice-water. Eating quality was significantlynegative correlation with amylose content (p＜0.01), but significantly positive correlation with gel consistency (p＜0.01).

Eating quality was significantly negative correlation with water absorption and expansion rate (p＜0.05), and yet significantly positive correlation with alkali spreading values, solid content of rice-water and fat content (p＜0.05).

The regression equation between eating quality scores and physicochemical indexes was Y=94.439–12.711X1–23.721X2–0.701X3+0.570X4+186.938X5. It showed that some physicochemical indexes of rice could evaluate the eating quality indirectly. The single factor analysis of variance showed that varieties of these six kinds of rice samples were remarkably different (p＜0.01), especially fat content, alkali spreading values, solid content of rice-water, gel consistency and protein content.

Some limitations still exist, in order to obtain the exact relationship between physicochemical properties and eating quality of rice in Heilongjiang Province accurately, more rice varieties will be chosen in the future researches, and we will consider various indexes comprehensively which can reflect the rice qualities. Especially, the composition of amylose and amylopectin and their starch spectrum characteristics will be researched comprehensively and in depth.

There are many methods to evaluate the rice quality. Sensory score is the most direct method that reflects the rice quality, but is greatly influenced by subjective factors. There are still many effective and advanced methods, such as texture analyzer, photoelectric sensor, near infrared spectroscopy, electronic nose, which should be widely applied by researchers, to evaluate rice quality.

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S511

A

1006-8104(2014)-03-0060-08

Received 23 March 2014

Supported by Program for Technological Innovation Research Team in University of Heilongjiang Province (2010td04)

Qin Ke-xin (1988-), female, Master, engaged in the research of cereal, oil and plant protein engineering. E-mail: 460040223@qq.com

* Corresponding author. Shi Yan-guo, professor, supervisor of Ph.D student, engaged in the research of soybean processing technology. E-mail: 13603681425@163.com