effect of protein on the rheological properties of rice flour

EFFECT OF PROTEIN ON THE RHEOLOGICAL PROPERTIESOF RICE FLOUR

JIANPING SUN, CAIYUN HOU1 and SHAOYING ZHANG

College of Food Science and Nutritional EngineeringChina Agricultural University

Beijing, China

Accepted for Publication April 24, 2008

ABSTRACT

The rheological properties of rice flour during pasting, cooling andreheating were measured. The effect of protein on the rheological character-istics during temperature sweeping was studied. The results indicated thatprotein decreased the temperature of the start of the pasting, at the same timemaking the pasting of the rice flour difficult through competition with starchgranules. A high protein content in rice is helpful to increase heat-resistantcapacity and keep the hardness and stickiness of the gel when the temperatureis changed. It was also deduced that rice with a high protein content needsmore water to cook and the hardness and stickiness of the cooked rice weremore stable than rice with a lower protein content when subjected to a changein temperature.

PRACTICAL APPLICATIONS

Protein is one of the main components in rice which influences thenutrition value and the eating quality of rice. The relationship between ricequality and protein content is ambiguous, which causes the evaluation of ricequality to be difficult. The influence of protein on the gelatinization propertiesof rice can be studied objectively using a dynamic rheometer and the influenceon the eating and cooking quality of rice can be deduced. The result is helpfulto evaluating the quality of rice correctly and gives some references to buildingan evaluation standard for the quality of rice.

1 Corresponding author. C. HOU, PO BOX 112, No. 17, Qinghua Dong Lu, Haidian, Beijing 100083,China. TEL/FAX: +86-010-62737613; EMAIL: [email protected]

Journal of Food Processing and Preservation 32 (2008) 987–1001.© 2008 The Author(s)Journal compilation © 2008 Wiley Periodicals, Inc.

987

mailto:[email protected]

INTRODUCTION

Rice is a staple food for more than half of the world’s population. Chinais one of the countries with a broad planting area and high consumption of rice;both japonica and indica are widely planted and consumed. A small amount ofthe rice crop is used to make ingredients for processed foods and feed, but thebulk is consumed as cooked rice (Zhou et al. 2002). Eating quality is consid-ered the most important trait to evaluate the quality of cooked rice. The eatingquality of rice has long been ascribed to starch content, which accounts for upto 90% of the dry matter in a milled rice grain. However, rice with a similarcontent of amylose sometimes shows very different cooking and eating qualitywhich may be attributed to the heredity variability.

Protein is one of the main components in rice, its content in commonChinese rice varieties ranges from 5 g/100 g to 11 g/100 g (Simpson et al.1965; Resurreccion et al. 1979). The protein content in indica rice is usually10–20% higher than that in japonica rice. Although protein influences thenutrition value and the eating quality of rice, it is a little known fact that couldexplain some of the properties of cooked rice that starch cannot explain.

Few studies have been done about the correlation between the proteinproperties and eating quality of rice, most of them have focused their attentionon the correlation between the protein content and the sensory scores ofcooked rice. However, problems that arise because the sensory scores ofcooked rice are easily influenced by the panelists’ subjective preferences;therefore, the opinions of the relationship between eating quality of rice andprotein content are not yet consistent among different studies or countries.Japan is one of the main japonica rice growing countries, the lower amyloseand protein contents are a necessary characteristic for rice with good eatingquality for the Japanese market. The suitable range of protein content isconsidered to be 6 g/100 g–7 g/100 g (Ohtsubo et al. 1993; Isono et al. 1994;Matsue and Ogata 1998). India is one of the largest rice growing countries ofthe world, where the major rice grown is indica. The protein content in Indiancommon rice varieties, including scented ones, ranges from 7.1 g/100 g to8.9 g/100 g. Basmati rice, which is famous for its good quality, compares witha protein content of 0.5 g/100 g–2.0 g/100 g, the highest among the commonrice varieties with respect to protein content (Bhattacharjee et al. 2002).

It can be found from the above opinions that the relationship between ricequality and protein content is ambiguous. The influence of protein on ricequality should be studied using a more objective approach. Besides the sensoryscores and relevant cooking properties, both viscosity characteristics and gela-tinization quality are important to reflect the eating and cooking quality of ricemore exactly (Ju and Mittal 1995). Martin investigated the effects of protein onviscosity by comparing the viscosity curve of different varieties using Rapid

988 J. SUN, C. HOU and S. ZHANG

Visco Analyzer (RVA), then concluded that protein influences the viscositycurves through the binding of water and a network linked by disulphide bonds.Some relevant studies using RVA have also been done (Lim et al. 1999; Martinand Fitzgerald 2002; Xie et al. 2006).

The dynamic rheometer is a good tool for recording the gelatinizationproperties of rice flour. Through consistent sweeping, the rheological proper-ties of the rice paste or slurry during the pasting and gelatinization steps can berecorded automatically. Because the cooking and eating quality of rice isreflected by these rheological properties, the influence of protein on the eatingand cooking quality of the rice can be studied. However, generally the dynamicrheometer has only been used to study the characteristics of starch, whereasthe research on rice flour and protein is few (Li and Yeh 2001; Sodhi and Singh2002; Singh et al. 2003; Singh et al. 2006; Gunathilake and Abeyrathne 2007;Lu et al. 2008).

The objectives of this work were to study the rheological characteristicsof rice flour with or without protein in the presence of variant temperatures, aswell as the influence of the protein to the eating and cooking quality of the rice.The results may be significant for building a standard to evaluate the rice withhigh quality.

MATERIALS AND METHODS

Materials

Two commercially available milled rice varieties (A and B), both prod-ucts of China, were purchased from a local market in Guangxi province andHeilongjiang province, to be the testing samples of indica rice and japonicarice, respectively. The protein content and compositions of A and B are varianton purpose, their relevant characteristics were predetermined, and the resultsare shown in Table 1.

Milling

The original rice samples were milled and passed through a sieve with amesh screen aperture of 150 nm, then numbered as A-1 and B-1 and packedafter drying.

Removing Protein in Rice

Protein was removed from the rice by dissolving it in alkali lye. Rice flour(10 g) was incubated with caustic lye of soda, 0.05 M, 50 mL, and shaken for24 h at 40C (oscillator with water bath, model HZS-H, Dongming Iatrical

989PROTEIN INFLUENCE ON THE RHEOLOGICAL PROPERTIES OF RICE

Instrument CO., Ltd., Ha’erbin, China), then centrifuged (15 min, 10,000 g) todiscard the clarified solution and upper yellow protein. The residue waswashed 5–10 times with distilled water until the pH of the washed waterreached 7.0.

The washed residue and upper yellow protein were dried to a 6% mois-ture content with live steam at 40C separately, then milled and passed througha sieve with a mesh size of 150 nm. After that, the treated rice flours andprotein powder was dried to a constant weight at 60C and packed. The ricesamples without protein were numbered A-2 and B-2, respectively. Anotherpair of samples numbered A-3 and B-3 was prepared by adding part of thedried protein powder back to the treated rice flour from which it was removed.The protein content of all the samples was determined by the Kjeldahl method(American Association for Clinical Chemistry). The protein content of samplemixture was approximately 3 g/100 g.

Preparation of Rice Paste for Rheological Measurements

Rice flour (1.00 g) and distilled water (10 mL) were placed in a glassbeaker and hand mixed with a glass rod. The glass beaker was put into boiledwater with constant mixing to gelate the rice slurry, and then cooled to roomtemperature. After resupplying the lost water, 5 mL of distilled water and4.00 g rice flour was poured into the beaker, mixed with a glass rod for 20 minto a rice paste and kept for 30 min at room temperature (20 � 1C) for hydra-tion balance purposes.

TABLE 1.RELEVANT PHYSICAL–CHEMISTRY PROPERTIES OF TESTING MATERIALS*

Rice cultivar A B

Amylose content (g/100 g) 24.12 � 0.34 17.22 � 0.28Alkali spreading value 2.3 � 0.5 6.7 � 0.4Gelatinization temperature High (�75C) Low (�69C)Gel consistency (mm) 72 � 1 73 � 1Taste score† 73 � 1 76 � 1

Sample number A-1 A-2 A-3 B-1 B-2 B-3

Protein content (g/100 g) 7.51 0.34 3.07 6.65 0.37 2.98Moisture content (g/100 g) Before heating 63.1 63.4 63.1 63.5 63.3 63.2

After heating 62.7 63.2 62.8 63.1 63.0 62.8WHC (%) Before heating 36% 39% 37% 48% 52% 49%

After heating 82% 93% 85% 80% 92% 85%

* The average of results of triplicate repeat measurements.† Taste score of rice was measured with sensory evaluation method, which is the Chinese National

Standard method.


Rheological Measurements

The rheological measurements of the different rice paste were carried outusing a dynamic rheometer (AR2000ex, TA Instruments Ltd., Crawley, UK). Aparallel plate geometry (40 mm diameter) was used and the gap between twoplates was 1,000 mm. The sample temperature was internally controlled by apeltier system (from -20 to 200C with an accuracy of �0.1°C) attached to awater circulation unit. A platinum resistance thermometer positioned at thecenter of the plate ensured proper temperature control and measurement. Foreach test, a measured volume of thoroughly mixed sample was placed betweenthe rheometer plates for 5 min for stress relaxation and temperature equilibra-tion before the actual measurements.

The treated rice paste was loaded onto the ram of the rheometer andcovered with a thin layer of low-density silicon oil to minimize the water lossduring the measurements. The moisture tests of rice paste were performedbefore and after heating to confirm that water was conserved. A temperaturesweep was conducted with three steps continuously at a heating rate of 5C/min:(1) from 20C to 95C; (2) from 95C to 20C; and (3) from 20C to 95C again. Withthe three temperature change steps, the rheological properties of the rice flourduring the pasting, gel cooling and the heat destroying period were measured.The strain and angular frequency were set at 2% and 6.283 rad/s, respectively,for all determinations. The dynamic rheological properties, such as storagemodulus (G′), loss modulus (G″) and loss factor (tan d) were directly obtainedfrom the manufacturer supplied computer software (Rheology Advantage DataAnalysis Program, TA Instruments) determined for each rice flour sample. Thedeviations did not exceed 5% between duplicate runs. The average of theduplicate runs was reported as the measured value.

Water-Holding Capacity (WHC) Tests

The WHC of the paste was evaluated using a centrifuge method (Vega-Warner et al. 1999). A 10 mm ¥ 3 mm paste section was placed into a micro-centrifuge tube with polypropylene mesh with a hole size of 10 mm andcentrifuged at 3000 rpm for 30 min at 4C. The WHC was calculated as theweight of water released divided by the paste weight multiplied by 100. WHCtests of rice paste were performed before and after heating from each of thethree replicates, and the influence of protein on the WHC was studied.

RESULTS AND DISCUSSION

Protein Content of Samples

Table 1 shows the protein content of the six samples. It can be observedfrom Table 1 that the protein content of A-2 and B-2 is less than 0.5 g/100 g.


It means that more than 90% of the protein was removed after treatment andthe extracting rates of each sample are close. The protein content of A-3 andB-3 is 3.07 g/100 g and 2.98 g/100 g. The effect of protein on the rheologicalcharacteristics can be obtained from the comparison of the rheological curveswith the original and treated samples.

Description of the Rheological Properties of the Rice Flour During theTemperature Sweep

Figure 1 shows the rheological behavior of the original rice flour during theperiod of rising temperature from 20C to 95C. The rheological trendlines of A(indica rice) and B (japonica rice) were similar to those with rice starch. Boththe G′ and G″ of all the rice cultivars changed slowly in the earlier heatingperiod, then increased with an increase in temperature with lift-off (T0). Thisindicates that the starch granules swell to form a dispersed phase, and the riceflour transforms into a “sol,” G′ and G″ increased to peaks (G′p, G″p) at a certaintemperature (Tp). This indicates a sol-to-gel transition, mainly attributed to thesolubilization of amylose to form a three-dimensional gel network, reinforcedby the strong interaction among the swollen starch particles (Eliasson 1986; Hsuet al. 2000; Martin and Fitzgerald 2002). With a further increase in temperature,G′ and G″ in all rice cultivars decreased rapidly, indicating that the gel structureis destroyed during prolonged heating (Tsai et al. 1997). This destruction iscaused by the melting of the crystalline region remaining in the swollen starchgranule, which deforms and loosens the particles (Eliasson 1986; Keetels et al.1996a,b) The relevant modulus and temperatures at the turning points of therheological curves are summarized in Table 2.

The rheological curves of the original rice samples (Fig. 1: A-1 and B-1)show that G′ of B is always higher than that of A during the temperaturesweeping. It was found from Table 2 that T0 and Tp of B are 10–15C lower thanthat of A, which indicates that indica rice with high amylose content requiresa higher temperature to be swollen and gelated than japonica rice with a moremoderate amylose content, which relates to the effects of amylose on thethree-dimensional gel network (Ring 1985).

Figure 2 shows the rheological behaviors of rice flour during the fallingtemperature sweep from 95C to 20C. During the falling temperature, both G′and G″ increase with a decrease in temperature, which attributes to the retro-gradation of amylose and initiates the development of hydrogen bonds. Theinteraction of starch granules slows down with a decrease in temperature, andthen the gel system tends to steady with more rigidity and intensity. Therigidity of the gel can be measured with tan d at falling 20C (Table 3).

Figure 3 shows the rheological behaviors of rice flour during the secondrising temperature sweep from 20C to 95C, where the heat-resistant capacity


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FIG. 1. STORAGE MODULUS (G′), LOSS MODULUS (G″) AND LOSS FACTOR (TAN d) OFRICE FLOUR DURING THE RISING TEMPERATURE PERIOD FROM 20C TO 95C

(�) A-1, original indica rice; (�) A-2, indica rice with protein removed; (¥) A-3, indica rice withprotein replaced; (�) B-1, original japonica rice; (D) B-2, japonica rice with protein removed; and

(+) B-3, japonica rice with protein replaced.


of the rice gel was investigated. During the reheating period, the G′ and G″decrease with an increase in temperature, which indicates the gel system wasdestroyed again. The heat-resistant capacity of the gel was used to reflect thedestructive degree of the rice gel during the reheating sweep, which wascalculated by the deviation of G′ (DG′) at falling 80C and reheating 80C(Table 3).

The factors governing the rheological behaviors of rice include swellingof the starch granules to form a disproportionate amount of water and solubi-lization of amylose to form the viscous phase. Without proteins, the pastingand gelatinization of rice flour are reflected only by the swelling of the starch.For each cultivar, the difference in the rheological properties of the originaland treated rice flour indicates the contribution of the protein.

Effect of Protein in the Pasting Period

The rheological properties of the original and treated rice during therising temperature sweep are compared in Fig. 1. It indicates the peak heights(G′p and G″p) of rice without protein (A-2 and B-2) are lower than that of theoriginal rice (A-1 and B-1) and the treated rice with additional protein (A-3and B-3), although the temperatures with peak height (Tp) are nearly the same(Table 2). Primal loss factors (tan d) of rice without protein (A-2 and B-2) arelower than that of the original rice (A-1 and B-1) and treated rice withadditional protein (A-3 and B-3), as their rising velocity (slope of the curves)increased. The tan d of rice without protein (A-2 and B-2) is higher than thatof the original samples (A-1 and B-1) and the treated rice with additionalprotein (A-3 and B-3) after the primal short acceleration and changes moresensitively than rice samples with protein (A-1, B-1, A-3 and B-3). This

TABLE 2.RELEVANT MODULUS AND TEMPERATURES AT TURNING POINTS OF

RHEOLOGICAL CURVES

Samplenumber

T0*(C)

Tp(G′)†(C)

G′p‡(pa)

Tp(G″)†(C)

G″p‡(pa)

A-1 67.4–70.6 83.9 13,000 81.3 1,579A-2 73.2–74.9 83.9 1,009 83.4 305.9A-3 72.2–73.7 83.9 3,622 83.4 654.9

B-1 55.7–57.3 72.2 19,870 67.4 2,377B-2 59.9–61.5 71.6 8,826 67.9 1,255B-3 56.2–57.3 71.6 13,240 67.4 1,724

* The temperature when G′ and G″ began to increase evidently.† The temperature when G′ or G″ reached the peak values.‡ The G′ and G″ at Tp, respectively.


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FIG. 2. STORAGE MODULUS (G′), LOSS MODULUS (G″) AND LOSS FACTOR (TAN d) OFRICE FLOUR DURING THE FALLING TEMPERATURE PERIOD FROM 95C TO 20C




indicates that the rigidity and intensity of the rice gel decreased after removingthe protein and increased after supplying protein, both of which were acutelyinfluenced by the temperature. The WHC of each rice gel after heating wascompared in Table 1, and it can be seen that the WHC of the rice gel withoutthe protein (A-2 and B-2) is approximately 10% higher than that of originalrice gel. The WHC of the treated rice with additional protein (A-3 and B-3)increased after replacing the protein. It means that the starch binds with thewater more compactly than with the protein. Therefore, the rice protein com-petes with the starch to bind with water and decrease the swelling and col-lapsing of the starch granules and the leaching amylase. Protein is also boundwith amylose through disulphide bonds to form a network, and then thesurface hardness and intensity of the gel is increased. It is shown in Table 2that T0 of both A-2 and B-2 are 3C–5C higher than that of A-1 and B-1, whichindicates that the protein advances the pasting course of the rice flour. Thiscould be attributed to the fact that the glass transition of protein is slightlybelow that of starch.

According to the rheological properties during the rising temperaturesweep, the effect of protein on the eating and cooking quality of rice can beunderstood. Among the similar varieties, rice with the higher protein contentusually needs more water to swell. When the same amount of water wasabsorbed, the higher protein content could promote cooked rice to be morehard and rigid.

Effect of Protein in the Cooling Gelatinizing Period

The rheological properties of original and treated rice during the fallingtemperature sweep were compared in Fig. 2. It shows that the G′ and G″ of rice

TABLE 3.MEASUREMENT OF THE RIGIDITY AND HEAT-RESISTANT

CAPACITY OF RICE SAMPLES

Sample number Tan d0* G′1† G′2‡ DG′§

A-1 0.0723 6,101 10,220 4,119A-2 0.0674 487.1 1,485 997.9A-3 0.0698 1,724 3,804 2,080

B-1 0.0960 8,070 10,120 2,050B-2 0.1055 883.4 1,568 684.6B-3 0.1006 2,670 3,903 1,233

* Tan d at falling 20C, measure the rigidity and intensity of coolinggel.

† G′ at falling 80C.‡ G′ at reheating 80C.§ The difference of G′1 and G′2, measure the heat-resistant capacity

of gel.


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FIG. 3. STORAGE MODULUS (G′), LOSS MODULUS (G″) AND LOSS FACTOR (TAN d)OF RICE FLOUR DURING THE REHEATING PERIOD FROM 20C TO 95C




without protein (A-2 and B-2) are lower than that of the original rice (A-1 andB-1) and the treated rice with additional protein (A-3 and B-3) in the wholefalling temperature sweep. Their rising velocity increased more slowly thanthat of rice with protein. When the rice gel was cooled, amylose began toretrograde and more hydrogen bonds were developed. The existence of proteininhibits those transformations, which has been proved in the thermal transitiontests of rice flours and rice starch, therefore more energy is required in thecooling of the gel with protein (Ding and Wang 2004). The tan d of sampleswithout protein (A-2 and B-2) decreased rapidly in primal course with fallingtemperature (Fig. 2). This indicates that the starch retrogrades more easilywithout protein and the intensity and hardness of the gel increased, whereasthe tan d of the original samples (A-1 and B-1) and treated samples withadditional protein (A-3 and B-3) changed slightly. From Table 3, it is evidentthat tan d0 is similar between the original and treated samples, indicating thatthe intensity and hardness of the original and treated gels are similar afterabsolute cooling.

According to the cooling gel, the effect of protein on the eating quality ofcooling cooked rice can be understood. During the cooling temperature sweep,the protein prevents the retrogradation of starch; therefore, it is more apt tokeep the certain hardness and stickiness of the cooked rice with a high proteincontent than rice with a low protein content with the falling temperaturesweep.

Effect of Protein on the Heat-Resistant Capacity of Gelatinization

The rheological properties of original and treated rice during the reheat-ing temperature sweep are compared in Fig. 3. It is shown that G′ and G″ fromthe samples without protein (A-2 and B-2) are lower than that of the originalrice (A-1 and B-1) and treated rice with additional protein (A-3 and B-3), andtheir falling velocities (slope of the curves) are also lower. The tan d of thetreated rice (A-2 and B-2) increases rapidly after approximately 70C indicat-ing that the rigidity and intensity of the rice gel without protein increasedagain, whereas the rigidity and intensity of the rice gel with protein changedunsensitively. It means that protein limited the recollapse of the starch throughconnecting with starch to a network.

The thermal-induced phase transition temperature of rice flour is 80Capproximately, then the deviation of G′ (DG′) at falling 80C and reheating 80Cmeasured the change of energy stored in the rice gel during the reheat tem-perature sweep, i.e., the heat-resistant capacity of rice gel. The change of G′ at80C (Table 3) shows DG′ of samples without protein (A-2 and B-2) are muchlower than that of the original samples (A-1 and B-1), whereas DG′ increasedafter partially replacing the protein. It means that the gel system that contains


protein has better heat-resistant capacity than the samples without protein. It isconcluded that protein keeps the heat-resistant capacity of the rice gel systemwhen the temperature is changed.

CONCLUSIONS

After removing the protein, the rheological properties of the rice flourobviously change. For each cultivar, the difference in the rheological proper-ties indicates what the contribution of protein is. During the pasting of the rice,protein decreased the beginning temperature of the rice pasting slightly,increased the transform of the modulus and surface hardness and intensity ofthe gel through competing with the starch granules to bind water and amylose.During the cooling of the gel, protein inhibits the retrogradation of amyloseand the development of hydrogen bonds to keep the steady hardness andstickiness of the gel. When the gel was reheated, the protein helps to increasethe heat-resistant capacity of the rice gel structure.

Through the study of the rheological properties of rice employingchanges in temperature, the eating and cooking quality of rice was deduced.For the similar cultivars, the rice with a higher protein content needs morewater to cook to ensure the full swelling of the starch granules. As well, moreprotein is helpful to increase the heat-resistant capacity and to keep a certainhardness and stickiness of the cooked rice whenever cooling and reheating.Therefore, the presence of protein does not always decrease the eating qualityof rice, but instead the relevant cooking conditions need to be developed foreach cultivar.

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effect of protein on the rheological properties of rice flour

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