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Effect of pH, corn starch and phosphates on the pasting properties of rice flour
Journal of Food Engineering 46 (2000) 133±138
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Eect of pH, corn starch and phosphates on the pasting properties of rice ¯our Hai-Hong Wang a,1, Da-Wen Sun a,*, Qingxiao Zeng b, Yinquan Lu b b
a Department of Agricultural and Food Engineering, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland Department of Food Engineering, College of Food and Bioengineering, South China University of Technology, Guangzhou 510641, People's Republic of China
Received 23 November 1999; accepted 15 April 2000
Abstract Pasting properties of rice ¯our were studied with the Brabender visco-amylograph using a non-waxy Chinese rice ¯our with intermediate amylose content, medium gel consistency and high birefringence-endpoint temperature (BEPT). The eect of pH, corn starch and phosphates including sodium tripolyphosphate (STP) and sodium phosphate dibasic (SPD) on the pasting characteristics of rice ¯our was investigated. The corresponding rice ¯our noodles were evaluated by sensory analysis for hardness and slipperiness. Cooking loss and swelling index of rice ¯our noodles were also determined. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Amylograph; Brabender; Corn starch; Noodle quality; Pasting properties; Phosphates; Rice ¯our
1. Introduction Rice ¯our and starch are important ingredients in both traditional and novel foods. In Western countries, rice is widely used to manufacture products such as puddings, infant foods, pued grains and breakfast cereals. In order to control better production processes, it is necessary to understand the properties of rice ¯our and starch. Researchers have previously investigated the properties of rice ¯our and starch to reveal their eects on product quality. Resmini and Pagani (1983) observed that the starch in rice noodles lost its granular structure and showed some ®brillar structures. Mestres, Colonna and Buleon (1988) studied the structural organisation of starch within rice ¯our noodles. Yeh and Yeh (1993) evaluated the characteristics of hydroxypropylated and cross-linked rice starch for their further applications in food processing. The eect of particle size on the viscoamylographic behaviour of rice ¯our and vermicelli quality was reported by Hemavathy and Bhat (1994) and the in¯uence of rice variety, rice ¯our concentration and enzyme levels on composite bread quality was determined by Noomhorm, Bandola and Kongseree
(1994). However, rice ¯our is often subjected to dierent pH levels, or is combined with other crop starches or additives during processing in order to obtain desirable product quality. Unfortunately, little information about this is available. In the current study, experiments were undertaken to determine the pasting properties of rice ¯our with respect to the eect of pH and the eect of the presence of corn starch and phosphates. The quality of rice noodles under these conditions was evaluated as well. 2. Materials and methods 2.1. Materials The rice ¯our was obtained from a popular Chinese winter rice variety, ÔDongmiÕ. The rice was cleaned and soaked in water at room temperature overnight. Water was then drained and the rice was ground in a grinder (Chongqing Equipment, China), passing an 80-mesh sieve and dried naturally afterwards. The ¯our was stored in an air-tight glass jar before using. 2.2. Physicochemical analysis
* 1
Corresponding author. Tel.: +353-1-7067493; fax: +353-1-4752119. E-mail address: [email protected] (D.-W. Sun). Tel.: +353-1-7067493; fax: +353-1-4752119
Contents of moisture and protein (AOAC, 1980), crude starch (Wang, 1987) and amylose (Juliano et al.,
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 7 7 - 7
134
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
1981) were determined. Birefringence-endpoint temperature (BEPT) was determined by using a polarising microscope with a hot stage (Perez, Pascual & Juliano, 1979). Gel consistency was evaluated by the method described by Cagampang, Perez and Juliano (1973) and the length of the gel was measured after the test tubes had been laid horizontally for 60 min. 2.3. Pasting properties The pasting behaviour was studied with a Brabender visco-amylograph (C.W. Brabender Instruments) using rice ¯our slurry with a concentration of 6% on a dry basis (d.b.). The levels of pH were adjusted with 1 N HCl or 0.9 N NaOH. Corn starch (provided by the Institute of Light Chemical Engineering at South China University of Technology), sodium tripolyphosphate (STP) (Shanghai Reagent, China) and sodium phosphate dibasic (SPD) (Shanghai Reagent, China) were added when appropriate. The mixture was made up to 460 g with distilled water and heated from the room temperature to 95°C at a rate of 1.5°C/min. The slurry was allowed to remain at that temperature for 60 min and then cooled to 50°C at the same rate and held for 50 min. 2.4. Noodle preparation Noodles were prepared using a noodle steamer (Taishan Rice Noodle Equipment, Guangdong, China). The steamer has four drawer type plates and was put horizontally on a wok with enough boiling water. Dry ¯our or starch was mixed with water in the ratio of 1:7 (w/v). The pH of the mixture was adjusted and phosphates (STP or SPD) were added when appropriate. Sixty-®ve milliliters of the slurry obtained was taken and transferred to a drawer plate that was removed from the steamer and smeared with a ®lm of edible oil. The plate was then inserted back into the steamer. After heating for 8 min, the plate was removed again and the thin sheet (about 1 mm in depth) of gel formed was taken out carefully and spread to cool at room temperature. Noodles were obtained by cutting the gel sheet into strips 1 mm in width and 20 mm in length after cooling for 30 min.
2.5. Sensory analysis Sensory evaluation was carried out by a randomly selected informal panel of eight members, approximately 1 h after steaming. A seven-point scale was used for the rating with 3 and )3 for very high and very low intensity, respectively. The panelists were trained in the use of the rating method and sensory properties of noodles. 2.6. Cooking loss and swelling index Cooking loss and swelling index of rice ¯our noodles were determined using the methods described by Mestres et al. (1988) and Lii and Chang (1981) with some modi®cations. Twenty-®ve grams of noodles were cut into 5 cm lengths, added to a 300 ml beaker with 200 ml boiling distilled water, and cooked for 40 min with slight stirring using a glass rod. Boiling distilled water was added every 10 min to compensate for evaporation loss. The mixture was ®ltered through 60-mesh screen. The ®lter residue was drained for 5 min and its dry matter content was then determined using a 130°C oven. The ®ltrate was centrifuged at 4000 rpm for 20 min. Dry matter content of the centrifugation sediment was also determined with a 130°C oven. Cooking loss and swelling index were calculated with the following equations: TCL% 25 DM ÿ W2 100= 25 DM;
1
SDL% W3 100= 25 DM;
2
SBL% TCL% ÿ SL%;
3
SI% W1 ÿ W2 100=W2 ;
4
where TCL is total cooking loss, DM the dry matter content (%), 25 the weight of noodle sample (g), SDL the solid loss, SBL the soluble loss, SI the swelling index, W1 the weight of ®lter residue after draining (g), W2 the constant weight of ®lter residue after drying in the 130°C oven (g), and W3 the constant weight of centrifugation sediment after 130°C drying (g).
Table 1 Physicochemical properties of rice ¯our and corn starch Source
Moisture (%)
Protein (% d.b.)
Crude starch (% d.b.)
Amylose (% d.b.)
BEPT (°C)
Gel consistency (mm)
Rice Corn
14.0 13.4
8.6 0.5
86.4 96.5
23.6 33.2
78.5 75.5
46 128
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
3. Results and discussion 3.1. Physicochemical properties Some physicochemical properties of rice ¯our and corn starch are shown in Table 1. The rice ¯our was non-waxy with intermediate amylose content, medium gel consistency and high BEPT (Wang, Sun, Zeng & Lu, 1999). 3.2. Pasting behaviour 3.2.1. Eect of pH Fig. 1 shows the Brabender viscograms of 6% rice ¯our at pH 6.20 (normal) and pH 4.10. It can be seen from Fig. 1 that, at pH 6.20, 6% rice ¯our exhibited a restricted swelling pattern (type C), according to the classi®cation of Schoch and Maywald (1968). When temperature was maintained at 95°C, the Brabender curve increased rather than decreased, indicating rice starch granules were resistant to fragmentation during cooking. The paste viscosity increased rapidly while cooling from 95°C to 50°C, which is an obvious re¯ection of retrogradation. During 50°C hold, rice ¯our showed a decrease in viscosity. The viscosity pattern in this study, especially the change in viscosity during 95°C cooking, is quite dierent from that reported by Kumar and Ali (1991), which could be classi®ed as type A (high swelling) (Schoch & Maywald, 1968). One reason lies in dierent amylose content of the samples. Kumar and Ali (1991) used high-amylose cultivars. Their starch granules are rigid and hard and thus do not rupture immediately when they are subjected to heating and shearing. Therefore, a high peak of viscosity displayed as a result of the swelling of starch granules. However, under
Fig. 1. Eect of pH on the pasting behaviour of rice ¯our (6% d.b.).
135
shearing at a temperature as high as 95°C, the greatly swollen granules disintegrate due to crowding and mutual pressure (Sandhya Rani & Bhattacharya, 1995), resulting in dramatic decrease in viscosity following the viscosity peak. The other reason may be due to the lower rice ¯our concentration of 6% used in the current study, as compared to over 10% used by Kumar and Ali (1991), since lower paste concentration led to less possibility of crowding and mutual shear of starch granules. In this situation, starch granules could swell freely and gradually with increasing temperature and prolonged time of cooking, and hence resulted in continuous increase in viscosity during heating and 95°C hold. This explanation could be con®rmed by the results of Abbas, Scheerens, Tinsley and Berry (1986). They found that tepary starch gave a mixed viscosity pattern resembling type C at lower concentrations (4% and 6%) and type B at higher concentrations (7%, 8% and 10%). Fig. 1 also shows that the rice ¯our paste displayed a lower viscosity pro®le at pH 4.10 than at pH 6.20, and its viscosity pattern was proximately type B (Schoch & Maywald, 1968), which could be characterised by a low viscosity peak and little thinning during cooking. The data obtained from Brabender visco-amylograph are given in Table 2. Paste viscosity slightly decreased during the holding period of 95°C at pH 4.10, illustrating that starch granules became fragile and broke down relatively rapidly after acidi®cation. Acidic rice ¯our paste exhibited less tendency of retrogradation. This may be ascribed to the short-chain starch molecules appeared due to acid hydrolysis, which were too active to form a highly ordered crystalline structure. It can also be seen from Fig. 1 that the viscosity was more stable at pH 4.10 than at pH 6.20 during 50°C hold. The gelatinisation temperature of rice ¯our given in Table 2 was higher than that reported by Noomhorm et al. (1994), which may be due to the dierence in rice varieties. The gelatinisation temperature read from the amylograph (Table 2) was higher than BEPT as well due to the dierent measuring techniques used. 3.2.2. Mixing with corn starch In the processing of rice products such as noodles and rice desserts, ¯ours or starches from other plants are sometimes added for lower cost or better quality. The pasting properties of rice ¯our mixed with corn starch were compared with those of rice ¯our and corn starch in the present study and the results are shown in Fig. 2. The corn starch used contains 13.4% moisture, 0.46% protein and 33.18% amylose with the BEPT of 75.5°C as presented in Table 1. Fig. 2 shows that the corn starch exhibited the lowest viscosity at 6% (d.b.) concentration throughout the amylogram as a result of low resistance of corn starch granules to disintegration. The corn starch displayed a type B viscosity pattern (Schoch & Maywald, 1968). A low viscosity peak was observed
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Table 2 Pasting properties of rice ¯our Sample
pH 6.20 pH 4.10 RFd /CSe 3/1 RF/CS 1/3 CS 0.2% STP 0.2% SPD
GTa (°C)
87.5 86.5 83.0 84.0 83.0 86.5 85.0
Viscosity (BU) Peak (P)
95°C/0 min
95°C/60 min (H)
50°C/0 min (C)
50°C/50 min
260 260 220 240 220 240 220
260 260 210 230 210 220 220
340 240 220 170 140 310 200
980 560 740 470 370 950 610
870 550 680 440 360 810 590
SBb
BDr c
720 300 520 230 150 710 390
)0.125 0.063 0 0.304 0.348 )0.109 0.049
a
Gelatinization temperature. Setback C ) P, a measure of starch retrogradation. c Relative breakdown (P ) H)/(C ) H). d Rice ¯our. e Corn starch. b
during heating, followed by a slight decrease during holding at 95°C and an increase on cooling. The key data from Fig. 2 were listed in Table 2. Corn starch had lower gelatinisation temperature than rice ¯our, which is consistent with its less resistance of granules to swelling and deformation. Pasting behaviour of the rice ¯our mixed with corn starch was dependent on mixing ratios (Fig. 2). The gelatinisation temperatures of the mixtures at weight ratios of 3:1 and 1:3 were obviously lower than rice ¯our, but almost the same as corn starch (Table 2). This could be considered as the result of the precedent swelling of corn starch granules upon cooking. When the viscosity of corn starch started to decrease, the rice starch granules kept on swelling, thus providing a viscosity compensation. Consequently, the breakdown was reduced with the increasing proportion of rice ¯our in
the mixtures. However, both mixtures exhibited type B viscosity pattern, which was similar to corn starch. As the result of less retrogradation tendency of corn starch (Table 2), some of the rice starch ghosts and corn starch ghosts were entrapped in the network of solubilized corn starch and therefore fewer amylose molecules were available for recrystallisation. This accounted for smaller amounts of setback with increase in the ratio of corn starch (Table 2).
Fig. 2. Pasting properties of rice ¯our, corn starch and their mixtures (6% d.b.).
Fig. 3. Eect of 0.2% STP and 0.2% SPD on the pasting behaviour of rice ¯our (6% d.b.).
3.2.3. In¯uence of phosphates Pasting characteristics of rice ¯our with 0.2% STP or 0.2% SPD are shown in Fig. 3, and the key data are presented in Table 2. Rice ¯our pastes exhibited a type C viscosity pattern (Schoch & Maywald, 1968) in the presence of STP, but a type B pattern (Schoch &
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
137
Maywald, 1968) with SPD added. The viscosity was suppressed by both phosphates throughout processing. Gelatinisation temperatures were slightly lower in the presence of phosphates as compared to that of rice ¯our pastes (Table 2). The viscosity of rice ¯our with STP displayed a tendency to increase while holding at 95°C, similar to rice ¯our, but the viscosity was decreased by the addition of SPD (Fig. 3). As far as the cooling period is concerned, the setback was slightly changed in the presence of STP, but signi®cantly suppressed when SPD was added (Table 2). The repulsion between phosphate ester groups prevents starch molecules from hydrogen bonding to each other, accounting for the reduced retrogradation (Craig, Maningat, Seib & Hoseney, 1989). As is evident in Fig. 3, the viscosity of rice ¯our paste became notably stable during the holding period at 50°C when SPD was added, but decreased more rapidly in the presence of STP, as compared with rice ¯our. 3.3. Noodle qualities 3.3.1. Sensory evaluation Rice ¯our noodles were evaluated by sensory analysis for hardness and slipperiness. The total scores of eight panelists are presented in Table 3. Rice ¯our noodles made from the sole rice ¯our slurry were given the highest score for hardness. Acidi®cation, mixing with corn starch and addition of SPD all suppressed hardness. The hardness score for noodles with STP was similar to that for normal rice ¯our noodles. An almost inverse trend was found in slipperiness scores. Normal rice ¯our noodles were least slippery. Slipperiness scores were a little higher at a relatively lower pH or in the presence of corn starch. Phosphates were the most effective to improve slipperiness, especially SPD. However, from an overall point of view, adding STP might be a better choice to improve noodle quality, since the noodles obtained had moderate hardness and high slipperiness. According to the discussion with the panelists, noodles from acidi®ed rice ¯our slurries tasted little sour and the transparency of noodles was improved by the addition of corn starch. The latter could
Fig. 4. Eect of pH on the cooking loss and swelling index of rice ¯our noodles.
Table 3 Sensory evaluation of rice ¯our noodles
4. Conclusions
a b
Sample
Hardness
Slipperiness
pH 6.20 pH 4.10 RFa /CSb 3/1 RF/CS 1/3 CS 0.2% SPD 0.2% STP
6 )11 )8 )15 )16 )5 5
)1 2 )1 1 3 10 6
Rice ¯our. Corn starch.
be attributed to the higher light transmittance of corn starch than rice ¯our (data not shown here). 3.3.2. Cooking loss and swelling index Fig. 4 presented the cooking loss and swelling index of rice ¯our noodles at dierent pH values. A notable in¯uence of pH was found on cooking loss and swelling index. Cooking loss was the lowest at pH 6.20 and increased beyond this level. This could be related to the more pronounced breakdown of starch granules after addition of acid (Fig. 1), and similar explanation may be sought under alkaline conditions. The soluble loss contributed more than the solid loss to the total cooking loss. This conforms to the result of Mestres et al. (1988). The change of swelling index with pH is similar to that of cooking loss and the smallest swelling index was observed at normal pH.
Rice ¯our paste exhibited a type C viscosity pattern at the concentration of 6%. The viscosity pro®le was lower at lower pH and in the presence of corn starch, STP and SPD. Retrogradation of rice starch was notably suppressed by decreasing pH value, and adding corn starch or SPD. With the addition of corn starch, gelatinisation temperature of rice ¯our decreased. Sensory evaluation showed that normal rice noodles scored the highest for hardness but the lowest for slipperiness. Hardness became lower at lower pH or in the presence of corn
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starch. SPD improved slipperiness more than STP. The lowest cooking loss and swelling index were observed at pH 6.20. References Abbas, I. R., Scheerens, J. C., Tinsley, A. M., & Berry, J. W. (1986). Tepary bean starch. Part II. Rheological properties and suitability for use in foods. Starch/St arke, 38, 351±354. AOAC. (1980). Ocial methods of analysis (13th ed.). Washington, DC: Association of Ocial Analytical Chemists. Cagampang, G. B., Perez, C. M., & Juliano, B. O. (1973). A gel consistency test for eating quality of rice. Journal of the Science of Food and Agriculture, 24, 1589±1594. Craig, S. A. S., Maningat, C. C., Seib, P. A., & Hoseney, R. C. (1989). Starch paste clarity. Cereal Chemistry, 66, 173. Hemavathy, J., & Bhat, K. K. (1994). Eect of particle size on viscoamylographic behaviour of rice ¯our and vermicelli quality. Journal of Texture Studies, 25, 469±476. Juliano, B. O., Perez, C. M., Blakeney, A. B., Castillo, T., Kongseree, N., Laignelet, B., Lapis, E. T., Murty, V. V. S., Paule, C. M., & Webb, B. D. (1981). International cooperative testing on the amylose content of milled rice. Starch/St arke, 33, 157±162. Kumar, K. R., & Ali, S. Z. (1991). Properties of rice starch from paddy stored in cold and at room temperature. Starch/St arke, 43, 165± 168.
Lii, C. -Y., & Chang, S. -M. (1981). Charaterization of red bean (Phaseolus radiatus var Aurea) starch and its noodle quality. Journal of Food Science, 46, 78±81. Mestres, C., Colonna, P., & Buleon, A. (1988). Characteristics of starch networks within rice ¯our noodles and mungbean starch vermicelli. Journal of Food Science, 53, 1809±1812. Noomhorm, A., Bandola, D. C., & Kongseree, N. (1994). Eect of rice variety, rice ¯our concentration and enzyme levels on composite bread quality. Journal of the Science of Food and Agriculture, 64, 433±440. Perez, C. M., Pascual, C. G., & Juliano, B. O. (1979). Eating quality indicators for waxy rices. Food Chemistry, 4, 179±184. Resmini, P., & Pagani, M. A. (1983). Ultrastructure studies of pasta ± A review. Food Microstructure, 2, 1±12. Sandhya Rani, M. R., & Bhattacharya, K. R. (1995). Microscopy of rice starch granules during cooking. Starch/St arke, 47, 334± 337. Schoch, T. J., & Maywald, E. C. (1968). Preparation and properties of various legume starches. Cereal Chemistry, 45, 564±573. Wang, H. -H., Sun, D. -W., Zeng, Q. -X., & Lu, Y. -Q. (1999). Flow behaviour and rheological models of rice ¯our pastes. Journal of Food Process Engineering, 22, 191±200. Wang, W. J. (1987). Standardisation of the method for determining crude starch in cereals and root crops. National Sciences Journal of Hunan Normal University, 10, 47±53. Yeh, A. -I., & Yeh, S. -L. (1993). Some characteristics of hydroxypropylated and cross-linked rice starch. Cereal Chemistry, 70, 596±601.
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Eect of pH, corn starch and phosphates on the pasting properties of rice ¯our Hai-Hong Wang a,1, Da-Wen Sun a,*, Qingxiao Zeng b, Yinquan Lu b b
a Department of Agricultural and Food Engineering, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland Department of Food Engineering, College of Food and Bioengineering, South China University of Technology, Guangzhou 510641, People's Republic of China
Received 23 November 1999; accepted 15 April 2000
Abstract Pasting properties of rice ¯our were studied with the Brabender visco-amylograph using a non-waxy Chinese rice ¯our with intermediate amylose content, medium gel consistency and high birefringence-endpoint temperature (BEPT). The eect of pH, corn starch and phosphates including sodium tripolyphosphate (STP) and sodium phosphate dibasic (SPD) on the pasting characteristics of rice ¯our was investigated. The corresponding rice ¯our noodles were evaluated by sensory analysis for hardness and slipperiness. Cooking loss and swelling index of rice ¯our noodles were also determined. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Amylograph; Brabender; Corn starch; Noodle quality; Pasting properties; Phosphates; Rice ¯our
1. Introduction Rice ¯our and starch are important ingredients in both traditional and novel foods. In Western countries, rice is widely used to manufacture products such as puddings, infant foods, pued grains and breakfast cereals. In order to control better production processes, it is necessary to understand the properties of rice ¯our and starch. Researchers have previously investigated the properties of rice ¯our and starch to reveal their eects on product quality. Resmini and Pagani (1983) observed that the starch in rice noodles lost its granular structure and showed some ®brillar structures. Mestres, Colonna and Buleon (1988) studied the structural organisation of starch within rice ¯our noodles. Yeh and Yeh (1993) evaluated the characteristics of hydroxypropylated and cross-linked rice starch for their further applications in food processing. The eect of particle size on the viscoamylographic behaviour of rice ¯our and vermicelli quality was reported by Hemavathy and Bhat (1994) and the in¯uence of rice variety, rice ¯our concentration and enzyme levels on composite bread quality was determined by Noomhorm, Bandola and Kongseree
(1994). However, rice ¯our is often subjected to dierent pH levels, or is combined with other crop starches or additives during processing in order to obtain desirable product quality. Unfortunately, little information about this is available. In the current study, experiments were undertaken to determine the pasting properties of rice ¯our with respect to the eect of pH and the eect of the presence of corn starch and phosphates. The quality of rice noodles under these conditions was evaluated as well. 2. Materials and methods 2.1. Materials The rice ¯our was obtained from a popular Chinese winter rice variety, ÔDongmiÕ. The rice was cleaned and soaked in water at room temperature overnight. Water was then drained and the rice was ground in a grinder (Chongqing Equipment, China), passing an 80-mesh sieve and dried naturally afterwards. The ¯our was stored in an air-tight glass jar before using. 2.2. Physicochemical analysis
* 1
Corresponding author. Tel.: +353-1-7067493; fax: +353-1-4752119. E-mail address: [email protected] (D.-W. Sun). Tel.: +353-1-7067493; fax: +353-1-4752119
Contents of moisture and protein (AOAC, 1980), crude starch (Wang, 1987) and amylose (Juliano et al.,
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 7 7 - 7
134
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
1981) were determined. Birefringence-endpoint temperature (BEPT) was determined by using a polarising microscope with a hot stage (Perez, Pascual & Juliano, 1979). Gel consistency was evaluated by the method described by Cagampang, Perez and Juliano (1973) and the length of the gel was measured after the test tubes had been laid horizontally for 60 min. 2.3. Pasting properties The pasting behaviour was studied with a Brabender visco-amylograph (C.W. Brabender Instruments) using rice ¯our slurry with a concentration of 6% on a dry basis (d.b.). The levels of pH were adjusted with 1 N HCl or 0.9 N NaOH. Corn starch (provided by the Institute of Light Chemical Engineering at South China University of Technology), sodium tripolyphosphate (STP) (Shanghai Reagent, China) and sodium phosphate dibasic (SPD) (Shanghai Reagent, China) were added when appropriate. The mixture was made up to 460 g with distilled water and heated from the room temperature to 95°C at a rate of 1.5°C/min. The slurry was allowed to remain at that temperature for 60 min and then cooled to 50°C at the same rate and held for 50 min. 2.4. Noodle preparation Noodles were prepared using a noodle steamer (Taishan Rice Noodle Equipment, Guangdong, China). The steamer has four drawer type plates and was put horizontally on a wok with enough boiling water. Dry ¯our or starch was mixed with water in the ratio of 1:7 (w/v). The pH of the mixture was adjusted and phosphates (STP or SPD) were added when appropriate. Sixty-®ve milliliters of the slurry obtained was taken and transferred to a drawer plate that was removed from the steamer and smeared with a ®lm of edible oil. The plate was then inserted back into the steamer. After heating for 8 min, the plate was removed again and the thin sheet (about 1 mm in depth) of gel formed was taken out carefully and spread to cool at room temperature. Noodles were obtained by cutting the gel sheet into strips 1 mm in width and 20 mm in length after cooling for 30 min.
2.5. Sensory analysis Sensory evaluation was carried out by a randomly selected informal panel of eight members, approximately 1 h after steaming. A seven-point scale was used for the rating with 3 and )3 for very high and very low intensity, respectively. The panelists were trained in the use of the rating method and sensory properties of noodles. 2.6. Cooking loss and swelling index Cooking loss and swelling index of rice ¯our noodles were determined using the methods described by Mestres et al. (1988) and Lii and Chang (1981) with some modi®cations. Twenty-®ve grams of noodles were cut into 5 cm lengths, added to a 300 ml beaker with 200 ml boiling distilled water, and cooked for 40 min with slight stirring using a glass rod. Boiling distilled water was added every 10 min to compensate for evaporation loss. The mixture was ®ltered through 60-mesh screen. The ®lter residue was drained for 5 min and its dry matter content was then determined using a 130°C oven. The ®ltrate was centrifuged at 4000 rpm for 20 min. Dry matter content of the centrifugation sediment was also determined with a 130°C oven. Cooking loss and swelling index were calculated with the following equations: TCL% 25 DM ÿ W2 100= 25 DM;
1
SDL% W3 100= 25 DM;
2
SBL% TCL% ÿ SL%;
3
SI% W1 ÿ W2 100=W2 ;
4
where TCL is total cooking loss, DM the dry matter content (%), 25 the weight of noodle sample (g), SDL the solid loss, SBL the soluble loss, SI the swelling index, W1 the weight of ®lter residue after draining (g), W2 the constant weight of ®lter residue after drying in the 130°C oven (g), and W3 the constant weight of centrifugation sediment after 130°C drying (g).
Table 1 Physicochemical properties of rice ¯our and corn starch Source
Moisture (%)
Protein (% d.b.)
Crude starch (% d.b.)
Amylose (% d.b.)
BEPT (°C)
Gel consistency (mm)
Rice Corn
14.0 13.4
8.6 0.5
86.4 96.5
23.6 33.2
78.5 75.5
46 128
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
3. Results and discussion 3.1. Physicochemical properties Some physicochemical properties of rice ¯our and corn starch are shown in Table 1. The rice ¯our was non-waxy with intermediate amylose content, medium gel consistency and high BEPT (Wang, Sun, Zeng & Lu, 1999). 3.2. Pasting behaviour 3.2.1. Eect of pH Fig. 1 shows the Brabender viscograms of 6% rice ¯our at pH 6.20 (normal) and pH 4.10. It can be seen from Fig. 1 that, at pH 6.20, 6% rice ¯our exhibited a restricted swelling pattern (type C), according to the classi®cation of Schoch and Maywald (1968). When temperature was maintained at 95°C, the Brabender curve increased rather than decreased, indicating rice starch granules were resistant to fragmentation during cooking. The paste viscosity increased rapidly while cooling from 95°C to 50°C, which is an obvious re¯ection of retrogradation. During 50°C hold, rice ¯our showed a decrease in viscosity. The viscosity pattern in this study, especially the change in viscosity during 95°C cooking, is quite dierent from that reported by Kumar and Ali (1991), which could be classi®ed as type A (high swelling) (Schoch & Maywald, 1968). One reason lies in dierent amylose content of the samples. Kumar and Ali (1991) used high-amylose cultivars. Their starch granules are rigid and hard and thus do not rupture immediately when they are subjected to heating and shearing. Therefore, a high peak of viscosity displayed as a result of the swelling of starch granules. However, under
Fig. 1. Eect of pH on the pasting behaviour of rice ¯our (6% d.b.).
135
shearing at a temperature as high as 95°C, the greatly swollen granules disintegrate due to crowding and mutual pressure (Sandhya Rani & Bhattacharya, 1995), resulting in dramatic decrease in viscosity following the viscosity peak. The other reason may be due to the lower rice ¯our concentration of 6% used in the current study, as compared to over 10% used by Kumar and Ali (1991), since lower paste concentration led to less possibility of crowding and mutual shear of starch granules. In this situation, starch granules could swell freely and gradually with increasing temperature and prolonged time of cooking, and hence resulted in continuous increase in viscosity during heating and 95°C hold. This explanation could be con®rmed by the results of Abbas, Scheerens, Tinsley and Berry (1986). They found that tepary starch gave a mixed viscosity pattern resembling type C at lower concentrations (4% and 6%) and type B at higher concentrations (7%, 8% and 10%). Fig. 1 also shows that the rice ¯our paste displayed a lower viscosity pro®le at pH 4.10 than at pH 6.20, and its viscosity pattern was proximately type B (Schoch & Maywald, 1968), which could be characterised by a low viscosity peak and little thinning during cooking. The data obtained from Brabender visco-amylograph are given in Table 2. Paste viscosity slightly decreased during the holding period of 95°C at pH 4.10, illustrating that starch granules became fragile and broke down relatively rapidly after acidi®cation. Acidic rice ¯our paste exhibited less tendency of retrogradation. This may be ascribed to the short-chain starch molecules appeared due to acid hydrolysis, which were too active to form a highly ordered crystalline structure. It can also be seen from Fig. 1 that the viscosity was more stable at pH 4.10 than at pH 6.20 during 50°C hold. The gelatinisation temperature of rice ¯our given in Table 2 was higher than that reported by Noomhorm et al. (1994), which may be due to the dierence in rice varieties. The gelatinisation temperature read from the amylograph (Table 2) was higher than BEPT as well due to the dierent measuring techniques used. 3.2.2. Mixing with corn starch In the processing of rice products such as noodles and rice desserts, ¯ours or starches from other plants are sometimes added for lower cost or better quality. The pasting properties of rice ¯our mixed with corn starch were compared with those of rice ¯our and corn starch in the present study and the results are shown in Fig. 2. The corn starch used contains 13.4% moisture, 0.46% protein and 33.18% amylose with the BEPT of 75.5°C as presented in Table 1. Fig. 2 shows that the corn starch exhibited the lowest viscosity at 6% (d.b.) concentration throughout the amylogram as a result of low resistance of corn starch granules to disintegration. The corn starch displayed a type B viscosity pattern (Schoch & Maywald, 1968). A low viscosity peak was observed
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Table 2 Pasting properties of rice ¯our Sample
pH 6.20 pH 4.10 RFd /CSe 3/1 RF/CS 1/3 CS 0.2% STP 0.2% SPD
GTa (°C)
87.5 86.5 83.0 84.0 83.0 86.5 85.0
Viscosity (BU) Peak (P)
95°C/0 min
95°C/60 min (H)
50°C/0 min (C)
50°C/50 min
260 260 220 240 220 240 220
260 260 210 230 210 220 220
340 240 220 170 140 310 200
980 560 740 470 370 950 610
870 550 680 440 360 810 590
SBb
BDr c
720 300 520 230 150 710 390
)0.125 0.063 0 0.304 0.348 )0.109 0.049
a
Gelatinization temperature. Setback C ) P, a measure of starch retrogradation. c Relative breakdown (P ) H)/(C ) H). d Rice ¯our. e Corn starch. b
during heating, followed by a slight decrease during holding at 95°C and an increase on cooling. The key data from Fig. 2 were listed in Table 2. Corn starch had lower gelatinisation temperature than rice ¯our, which is consistent with its less resistance of granules to swelling and deformation. Pasting behaviour of the rice ¯our mixed with corn starch was dependent on mixing ratios (Fig. 2). The gelatinisation temperatures of the mixtures at weight ratios of 3:1 and 1:3 were obviously lower than rice ¯our, but almost the same as corn starch (Table 2). This could be considered as the result of the precedent swelling of corn starch granules upon cooking. When the viscosity of corn starch started to decrease, the rice starch granules kept on swelling, thus providing a viscosity compensation. Consequently, the breakdown was reduced with the increasing proportion of rice ¯our in
the mixtures. However, both mixtures exhibited type B viscosity pattern, which was similar to corn starch. As the result of less retrogradation tendency of corn starch (Table 2), some of the rice starch ghosts and corn starch ghosts were entrapped in the network of solubilized corn starch and therefore fewer amylose molecules were available for recrystallisation. This accounted for smaller amounts of setback with increase in the ratio of corn starch (Table 2).
Fig. 2. Pasting properties of rice ¯our, corn starch and their mixtures (6% d.b.).
Fig. 3. Eect of 0.2% STP and 0.2% SPD on the pasting behaviour of rice ¯our (6% d.b.).
3.2.3. In¯uence of phosphates Pasting characteristics of rice ¯our with 0.2% STP or 0.2% SPD are shown in Fig. 3, and the key data are presented in Table 2. Rice ¯our pastes exhibited a type C viscosity pattern (Schoch & Maywald, 1968) in the presence of STP, but a type B pattern (Schoch &
H.-H. Wang et al. / Journal of Food Engineering 46 (2000) 133±138
137
Maywald, 1968) with SPD added. The viscosity was suppressed by both phosphates throughout processing. Gelatinisation temperatures were slightly lower in the presence of phosphates as compared to that of rice ¯our pastes (Table 2). The viscosity of rice ¯our with STP displayed a tendency to increase while holding at 95°C, similar to rice ¯our, but the viscosity was decreased by the addition of SPD (Fig. 3). As far as the cooling period is concerned, the setback was slightly changed in the presence of STP, but signi®cantly suppressed when SPD was added (Table 2). The repulsion between phosphate ester groups prevents starch molecules from hydrogen bonding to each other, accounting for the reduced retrogradation (Craig, Maningat, Seib & Hoseney, 1989). As is evident in Fig. 3, the viscosity of rice ¯our paste became notably stable during the holding period at 50°C when SPD was added, but decreased more rapidly in the presence of STP, as compared with rice ¯our. 3.3. Noodle qualities 3.3.1. Sensory evaluation Rice ¯our noodles were evaluated by sensory analysis for hardness and slipperiness. The total scores of eight panelists are presented in Table 3. Rice ¯our noodles made from the sole rice ¯our slurry were given the highest score for hardness. Acidi®cation, mixing with corn starch and addition of SPD all suppressed hardness. The hardness score for noodles with STP was similar to that for normal rice ¯our noodles. An almost inverse trend was found in slipperiness scores. Normal rice ¯our noodles were least slippery. Slipperiness scores were a little higher at a relatively lower pH or in the presence of corn starch. Phosphates were the most effective to improve slipperiness, especially SPD. However, from an overall point of view, adding STP might be a better choice to improve noodle quality, since the noodles obtained had moderate hardness and high slipperiness. According to the discussion with the panelists, noodles from acidi®ed rice ¯our slurries tasted little sour and the transparency of noodles was improved by the addition of corn starch. The latter could
Fig. 4. Eect of pH on the cooking loss and swelling index of rice ¯our noodles.
Table 3 Sensory evaluation of rice ¯our noodles
4. Conclusions
a b
Sample
Hardness
Slipperiness
pH 6.20 pH 4.10 RFa /CSb 3/1 RF/CS 1/3 CS 0.2% SPD 0.2% STP
6 )11 )8 )15 )16 )5 5
)1 2 )1 1 3 10 6
Rice ¯our. Corn starch.
be attributed to the higher light transmittance of corn starch than rice ¯our (data not shown here). 3.3.2. Cooking loss and swelling index Fig. 4 presented the cooking loss and swelling index of rice ¯our noodles at dierent pH values. A notable in¯uence of pH was found on cooking loss and swelling index. Cooking loss was the lowest at pH 6.20 and increased beyond this level. This could be related to the more pronounced breakdown of starch granules after addition of acid (Fig. 1), and similar explanation may be sought under alkaline conditions. The soluble loss contributed more than the solid loss to the total cooking loss. This conforms to the result of Mestres et al. (1988). The change of swelling index with pH is similar to that of cooking loss and the smallest swelling index was observed at normal pH.
Rice ¯our paste exhibited a type C viscosity pattern at the concentration of 6%. The viscosity pro®le was lower at lower pH and in the presence of corn starch, STP and SPD. Retrogradation of rice starch was notably suppressed by decreasing pH value, and adding corn starch or SPD. With the addition of corn starch, gelatinisation temperature of rice ¯our decreased. Sensory evaluation showed that normal rice noodles scored the highest for hardness but the lowest for slipperiness. Hardness became lower at lower pH or in the presence of corn
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