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Retrogradation properties of high amylose rice flour and rice starch by physical modification
LWT - Food Science and Technology 43 (2010) 492–497
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Retrogradation properties of high amylose rice flour and rice starch by physical modification Yue Wu a, Zhengxing Chen a, *, Xiaoxuan Li a, Zhenjiong Wang a a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2009 Received in revised form 9 September 2009 Accepted 18 September 2009
A new physical modification applied to prevent the retrogradation of rice flour and rice starch was investigated. This study examined the retrogradation properties of treated rice flour or rice starch paste by three stirring or heating–stirring, or without. The results proved that the retrogradations of rice flour and rice starch were both not substantially affected by three stirring modifications. However, three heating–stirring treatments had a marked effect on retarding the retrogradation of rice flour, but did not affect that of isolated rice starch. In the differential scanning calorimetry (DSC) analysis, modified rice flour by three heating–stirring exhibited the lowest retrogradation enthalpy (3.04 J/g dry matter) as compared to the control RF (5.93 J/g dry matter) and by three stirring (5.08 J/g dry matter). Meantime, it had almost the least recrystallization of retrogradation by X-ray diffraction (XRD). It was also found through scanning electron microscopy (SEM) that the granule structure of this modified rice flour had a more honeycomb-like structure and the lowest crystallinity as compared to the others. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Physical modification Retrogradation Differential scanning calorimetry (DSC) X-ray diffraction (XRD) Scanning electron microscopy (SEM)
1. Introduction Rice is one of the major food crops in the world, and more than 50% of the world’s population depend on rice as their primary caloric source (FAO, 2001). Rice products are staple foods, especially in Oriental countries, and most of these products are made of rice flour. Sharing the market of instant foods, fresh-cooked rice products such as cooked rice grains, rice noodles, rice cakes, and rice pasta have profound commercial potential. In recent years, rice flour has been increasingly used as novel foods like tortillas, beverages, processed meat, puddings, salad dressing, and glutenfree bread because of its unique functional properties such as being hypoallergenic, colorless and bland (Kadan & Ziegler, 1989; Kadan, Robinson, Thibodeux, & Pepperman, 2001; McCue, 1997). However, rice flour products are known to become hard, and they decline in texture and taste over time. This phenomenon is generally called ‘‘retrogradation’’ thus increasing the level of enzyme-resistant starch through recrystallization (Englyst, Kingman, & Cummings, 1992). Retrogradation is a process in which the molecules of gelatinized starch reassociate to form crystallites upon cooling, which imply fully reversible recrystallization in the case of amylopectin and partially irreversible recrystallization in the case
* Corresponding author. Tel.: þ86 510 85917025. E-mail address: [email protected] (Z. Chen). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.09.017
of amylose (Bjo¨ck, 1996). Therefore, the technology of preparing rice flour products with extended shelf life or using rice flour effectively requires efficient retarding of starch retrogradation. Traditionally, starches are chemically modified to prevent starch retrogradation. However, often questionable chemicals used will be banned in the health-conscious period. In addition, when adding one or more enzymes before cooking or after cooking the starch products, it could be to prevent retrogradation by enzymatic action (Martin & Hoseney,1991; Yao, Zhang, & Ding, 2003). It is pity that these products finally lose their firmness and suitable chewiness, and their taste is impaired. Since the decomposition reaction proceeds from the surface successively to the inside, their surface becomes overly soft. Although the loss in quality during the storage of starch-based products also can be counteracted by additives such as known trehalose, hydrocolloids (Funami, Kataoka, Omoto, Goto, Asai, & Nishinari, 2005; Lee, Baek, Cha, Park, & Lim, 2002), emulsifiers or surfactants (Ghiasi, Varriano-Marston, & Hoseney, 1982; Harbitz, 1983; Krog, 1981; Rao, Nussinovitch, & Chinachoti, 1992), and other materials, these materials are expensive or their effect is not ideal. The physical modification of starch is mainly applied to change its granular structure and convert native starch into cold watersoluble starch or small-crystallite starch. This method accomplished without any chemicals or even biological agents meets these requirements. It is thus desirable to develop a new type of physical modification to prevent the retrogradation of starch in rice flour. It should be also noticed that rice flour and rice starch are
Y. Wu et al. / LWT - Food Science and Technology 43 (2010) 492–497
different systems. Therefore, this study aims to investigate performance of the retrogradation as compared to rice flour (RF) and rice starch (RS) by applied a physical modification. The effects of this modification were systemically investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). 2. Materials and methods 2.1. Rice flour Freshly harvested high-amylose content (Juliano, 1982) rice was purchased in a local market. Rice was milled to flour using a mechanical grinder. Flour sample was screened through a 100-mu sieve, packed in airtight plastic bags, and stored at room temperature for further use. The rice flour contained starch (75 g/100 g), protein (8.1 g/100 g), fat (1.1 g/100 g), moisture (14.2 g/100 g) (AOAC, 2000), and amylose (28.6 g/100 g) (Gunaratne & Hoover, 2000). 2.2. Extraction of rice starch Starch was extracted using the modified method described by Lin and Chang (2006) involving steeping rice flour in NaOH (0.1 g/100 g) solution. The isolated starch was dried in an oven at 38 C for about 24 h to produce a moisture content close to that of rice flour (about 14 g/100 g). The dried starch was ground into powder and was passed through a 100-mu sieve. The isolated starch had protein content (0.5 g/100 g) (AOAC, 2000). 2.3. Physical modification of rice flour or starch The methods of physical modification of rice flour or starch are presented in Table 1. The methods only differ when heating in a boiling water bath (100 C) was applied along with stirring. The stirring machine used RW 20 digital head stirrer (IKA, German) at 500 rpm. 2.4. Gelatinization and retrogradation The samples (1:2 dry matter-to-water ratio) with treated or not were gelatinized by steam heating for 20 min in a closed thermostat water bath (100 C). Subsequently, the gelatinized samples were stored at 4 C for 7 days to allow them to retrograde. Afterward, these samples were lyophilized. The freeze-dried samples were ground, and then the powder samples were passed through a 100-mu sieve before DSC, XRD, and SEM testing.
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standard. For retrogradation studies, these stored samples (about 2 mg) were accurately weighed into aluminum DSC pans, and de-ionized water was added by micropipette to achieve a water– sample ratio of 2:1. The sample pans were sealed and equilibrated at room temperature for 24 h before analysis. The samples were heated at a rate of 10 C/min in a temperature range of 10–85 C using an empty pan as reference. The onset temperature To, peak temperature Tp, and conclusion temperature Tc were respectively determined from the heating DSC curves. Retrogradation enthalpy (DHr) was evaluated based on the area of the main endothermic peak. Analyses were performed in triplicate. 2.6. X-ray diffraction (XRD) Recrystallization of retrograded samples was determined using a Bruker D8 Advance speed X-ray diffractometer (Bruker AXS, Rheinfelden, Germany) equipped with a copper tube operating at 40 kV and 200 mA, producing CuKa radiation of 0.154 nm wavelength. Diffractograms were obtained by scanning from 4 to 40 (2q) at a rate of 4 /min, a step size of 0.02 , a divergence slit width (DS) of 1, a receiving slit width (RS) of 0.02 mm, and a scatter slit width (SS) of 1. Each sample was measured triplicate. MDI Jade 5.0 software was used to analyze the diffractograms. Crystallinity was quantified by integrating the area under the fitting crystalline peaks (14, 17, 20 and 23 ). The relative crystallinity (XRC) was expressed as XRC ¼ (Is/Ic) 100, where Is is the integrated area of crystalline peaks in the treated samples and Ic is the integrated area of crystalline peaks in the control (Primo-Martin, van Nieuwenhuijzen, Hamer, & van Vliet, 2007). The integrated areas were obtained after smoothing and fitting the original peak using the ˜ o´n, 2004). PeakFit v4.12 software (Ribotta, Cuffini, Leo´n, & An 2.7. Scanning electron microscopy (SEM) The structural properties of the samples were studied using a scanning electron microscope (Quanta-200, FEI Company, Netherlands). Dried, finely ground samples were mounted on an aluminum stub using a double-sided stick tape and were coated with a thin film of gold (10 nm), then examined at an accelerating voltage of 10 kV. 2.8. Statistical analysis The mean, standard deviations and significant differences of the data collected were calculated and reported using SAS version 8 (SAS Institute Inc., 2000). Whenever differences were reported as significant, a confidence level of 95% was considered. The data reported in all tables were the average of triplicate observations.
2.5. Differential scanning calorimetry (DSC) 3. Results and discussion The retrogradation properties of the samples were determined from the DSC curves using Pyris 1-DSC (Perkin-Elmer Corp., Norwalk, CT, USA). The calorimeter was calibrated with an indium Table 1 The methods of physical modification of rice flour or starch. The control
Method A
Method B
5 g flour or starch was mixed with 10 ml de-ionized water.
5 g flour or starch mixed with 6 ml de-ionized water was stirred for 20 s, and then 2 ml of water was added. The stirring/adding water cycles were repeated once. Finally, the sample was stirred for 20 s again.
The method is same as Method A, only when the samples were stirred with heating in a boiling water bath (100 C).
3.1. Thermal analysis After storage at 4 C for seven days, gelatinized starch molecules reassociate to an ordered structure, and this is referred to as retrogradation. The enthalpy values of the retrograded starch reflect the melting of the crystallites formed by the association between adjacent double helices during gel storage (Hoover & Senanayake, 1996). This endotherm peak was due to the melting of retrograded amylopectin (Fearn & Russell, 1982; Karim, Norziah, & Seow, 2000) rather than amylose. Fig. 1 shows retrogradation thermograms of treated rice flour or rice starch and their control. There was a clear decreasing enthalpy of treated RF by Method B as compared with the control and treated RF by Method A (Fig. 1a). But thermograms of treated RS or not
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Y. Wu et al. / LWT - Food Science and Technology 43 (2010) 492–497
Fig. 1. Retrogradation thermograms of rice flour (a) and rice starch (b) gel. The curves were the control, modified using Method A and Method B from top to below, respectively.
seem to be almost same (Fig. 1b). Meantime, Table 2 lists the retrogradation transition temperature (To, Tp, Tc) and the enthalpy of retrogradation (DHr) of samples after seven days storage. The retrogradation peak temperature all of these samples was around 48 C, which is the typical melting temperature of retrograded starch (Sievert & Pomeranz, 1990). However, the DHr of the control of RF was a little higher than that of isolated starch. This may be due to the slight difference in the actual starch concentration in the flour and the isolated starch suspensions. It was also clearly shown that modified RF using Method B has the lowest DHr (3.04 J/g dry flour) as compared to the control (5.93 J/g dry flour) and RF using Method A (5.08 J/g dry flour). In contrast, as compared to that of the control of RS (5.3 J/g dry starch), there was almost no difference between the treated RS samples by Method A (5.13 J/g dry starch) and Method B (5.1 J/g dry starch). It could therefore be considered that Method B could prevent the retrogradation of RF but not RS. The different result also indicates that rice protein appears to be
a crucial factor influencing the retrogradation properties of modified RF using Method B. It may be that this treatment induced some characteristic changes in rice protein such as improving emulsifying ability. Eventually, these changes in rice protein facilitated protein–starch interactions or higher water-binding capacity to prevent starch molecular recrystallization during the storage time. Numerous reports have shown that flour protein is an important factor in the rate of bread staling. Some researchers had confirmed that protein retarded bread staling. Gluten plays an important role on bread staling by forming an extensible protein network that keeps the crumb structure soft by slowing the movement of water from crumb to crust (Roach & Hoseney, 1995). Martin and Hoseney (1991) also reported that interactions exist between swollen starch granules and the gluten network, through hydrogen bonding, preventing the staling of bread. Rice proteins are found to be mostly within endosperm (storage proteins) cells, which are situated in protein bodies between the
Table 2 Retrogradation temperatures and enthalpy of gelatinized rice flour and rice starch with physical modification or without after 7 days’ storage at 4 C. To ( C)
Samples RF Modified Modified RS Modified Modified
Tp ( C) a
of RF using Method A of RF using Method B of RS using Method A of RS using Method B
39.0 0.79 40.4 1.62a 41.5 1.82b 39.8 2.26a 39.8 1.25a 40.2 1.87a
a
47.6 0.29 49.6 1.69b 49.2 1.32b 48.9 2.15b 48.7 0.95b 49.1 2.17b
Tc ( C)
DHr (J/g dry starch)
DHr (J/g dry flour)
57.2 0.22a 58.7 1.99b 57.4 1.57a 58.8 1.37b 58.5 0.72b 59.1 1.84b
– – – 5.13 0.59a 5.14 0.69a 5.34 0.30a
5.93 0.67a 5.08 0.25b 3.04 0.63c – – –
To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; DHr, enthalpy of retrogradation. Values are means SD (n ¼ 3). Values followed by the same letter in the same column are not significantly different (P < 0.05).
Y. Wu et al. / LWT - Food Science and Technology 43 (2010) 492–497
3.2. Recrystallization The final recrystallization was investigated by XRD to further prove the effect of physical modification on the retrogradation. The XRD patterns and corresponding crystallinity observed from the gelatinized RF or RS systems with treatment or not are shown in Figs. 2 and 3. The retrograded starch from gel gave several peaks around 2q ¼ 17 and 20 and others (Kim, Kim, Shin, & Kwangju, 1997), which was typical pattern of retrograded starch (B-type) and was clearly distinct from the pattern of the raw starch. This is accompanied by gradual increases in rigidity and phase separation between the polymer and the solvent (syneresis). B-type crystallinity is characterized by a well-defined peak at 17 (2q). The formation of this peak was the result of the crystallization of the amorphous starch melt, which is mainly of the amylopectin fraction that was increased during storage (Thire´, Sima´o, & Andradeb, 2003;
“B” type
Intensity (Counts)
250
“V” type
300 250
Intensity (Counts)
starch granules and are tightly associated with rice starch (Adoration, Li, Okita, & Juliano, 1993; Lasztity, 1996). Therefore, it may be assumed that this unique structure of rice protein and starch might undergo the effect of anti-retrogradation through three heating– stirring modifications. The grains/granules began to swell and then burst with heating to release protein. At this time, rice protein had better emulsifying ability as several stirring to delay the retrogradation. That is known as the ability of the emulsifiers to retard firming and retrogradation (Krog, 1981; Ghiasi et al., 1982; Harbitz, 1983; Rao et al., 1992) by forming a complex with starch. Much work has been carried out on the interaction of starch with cereal proteins. Rice grains are rich in a storage protein called rice glutelin or oryzenin (over 80–90 g/100 g of total proteins) which interacts with starch by binding to amylose and/or amylopectin (Chrastil, 1990). Dahle (1971) studied the wheat starch– protein interactions in a solution by measuring the absorbances of amylose–iodine and amylopectin–iodine complexes in the presence of wheat proteins. An interaction of gelatinized wheat starch and wheat protein occurred, which interfered with the blue color associated with the starch–iodine complex. In addition, protein is supposed to influence the formation of resistant starch type 3 that is also retrograded starch (Haralampu, 2000). The presence of bovine serum albumin decreased the yield of resistant starch type 3 (Escarpa, Gonza´lez, Morales, & Saura-Calixto, 1997). However, a strong similarity exists between the retrogradation behavior of rice starch in cooked rice grains and that in purified form (Yao, Zhang, & Ding, 2002). These contrary results may be due to difference of gelatinization condition or protein characteristics to cause association difficult or easy of protein and starch.
200
a
150
b 100
c
50 0 5
a
c
50 0 25
25
30
35
40
Fig. 3. X-ray diffraction patterns of gelatinized rice starch without physical modification (a), with modification using Method A (b), and using Method B (c) after 7 days’ storage at 4 C.
Osella, Sa´nchez, Carrara, de la Torre, & Pilar, 2005). This peak at 20 found in corresponds to the presence of crystalline V-type amylose–lipid complexes formed during processing (Zobel & Kulp, 1996). Two major broad peaks were identified in the all retrogradated RF and RS samples at 2q angles of 17 and 20 (Figs. 2 and 3). As shown in Fig. 2, the intensity of a peak close to 17 of the modified RF samples using Method B seemed to be very small in a representative X-ray diffractogram. However, the XRD diffraction patterns of treated RS by Method A, Method B, or the control seemed to be the same as shown in Fig. 3. Because XRD peak of retrograded starch was normally the dispersion broad peak (Kim et al., 1997; Primo-Martin et al., 2007), so PeakFit software was used to smooth and fit these peaks in order to calculate crystallinity. The result of fitting was that four peaks at 14, 17, 20 and 23 were found in each original pattern (process not shown). Therefore, crystallinity was quantified based on integrating the area under the fitting crystalline peaks (14, 17, 20 and 23 ). The relative crystallinity modified RF using Method B (Table 3) was significantly decreased to 51.47% as compared with the control (100%) and modified RF using Method A (93.87%). This implies that Method B could retard the recrystallization or retrogradation behavior of gelatinized RF. At the same time, the relative crystallinity value was no evidently discrepancy between modified RS using Method A (90.44%), using Method B (100.21%) and the control (100%) (Table 3). This indicated that this physical modification had not effect on the retrogradation of RS. These observations agreed with the foregoing results on the retrogradation endotherm value in DSC.
30
35
XRC %a,b
Rice flour
The control Modification using Method A Modification using Method B
100 93.87 4.66 51.47 5.23
Rice starch
The control Modification using Method A Modification using Method B
100 90.44 3.78 100.21 6.43
100
20
20
2-Theta (°)
b
15
15
Sample
150
10
10
Table 3 Relative crystallinity (XRC) in retrograded samples as determined using X-ray diffraction.
200
5
495
40
2-Theta (°) Fig. 2. X-ray diffraction patterns of gelatinized rice flour without physical modification (a), with modification using Method A (b), and using Method B (c) after 7 days’ storage at 4 C.
a XRC, the relative crystallinity, is calculated using: XRC ¼ (Is/Ic) 100, where Is is the integrated area of fitting crystalline peaks (14, 17, 20 and 23 ) in the treated samples and Ic is the integrated area of fitting crystalline peaks (14, 17, 20 and 23 ) in the control. Relative crystallinity of the control (rice flour or rice starch) is a hundred, respectively. b Values are means SD (n ¼ 3).
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3.3. Morphological properties The scanning electron micrographs at magnification of 1200 of the retrograded RF and RS granules with treatment or not are illustrated in Fig. 4. The modified RF granular structure (Fig. 4c) using Method B as compared to the control (Fig. 4a) or the modified one using Method A (Fig. 4b) exhibited significant variations in shape when viewed by scanning electron microscopy (SEM). These granular (Fig. 4c) appeared to have a honeycomb-like structure formed due to the loss of a lot of water during freeze-drying treatment. It might state that the RF through Method B processing exhibited a high water holding capacity and less syneresis during storage. Since the retrogradation is often associated with water
separation from the gel (syneresis) due to starch chains associating to form thick structure (Hoover & Manuel, 1995; Whistler & Daniel, 1985; Yeh & Yeh, 1993). Thus the amount of syneresis is a useful indicator for the tendency of starch to retrograde (Karim et al., 2000). The extent of syneresis increases due to an increase in amylopectin retrogradation in the starch-rich phase (Yuan & Thompson, 1998). From the comparison of syneresis with Fig. 4c, it was readily visible that the control (Fig. 4d), modified RS using Method A (Fig. 4e), and Method B (Fig. 4f) all had very high solid and compact structure. It also may mean that this structure has high crystallinity that had a high extent of retrogradation (Gonza´lez-Soto, Mora-Escobedo, Herna´ndez-Sa´nchez, Sa´nchez-Rivera, & Bello-Pe´rez, 2007).
Fig. 4. Scanning electron micrograph of gelatinized rice flour and rice starch samples with physical modification using Method A or Method B or without after 7 days’ storage at 4 C: without modified RF (a), modified RF using Method A (b), modified RF using Method B (c), without modified RS (d), modified RS using Method A (e), and modified RS using Method B (f).
Y. Wu et al. / LWT - Food Science and Technology 43 (2010) 492–497
4. Conclusions It could be speculated through combining these results with those obtained from DSC, XRD, and SEM stated above that the physical modification of three heating–stirring could prevent the retrogradation of RF but not RS. Furthermore, the presence of proteins in rice flour was considered responsible for this due to alterations in some protein characteristics following this modification. These observations were supported by the contrasting results of rice flour and rice starch. Althrough research into the retrogradation of rice starch is extensive, the proteins of rice are a less-known modified factor that may prevent the retrogradation of rice starch. In future, it would pay more attention in this aspect of investigation. Acknowledgments This research work was supported by the earmarked fund for Modern Agro-industry Technology Research System, PCSIRT 0627, and 111 project B07029. References Adoration, P. R., Li, X. L., Okita, T. W., & Juliano, B. O. (1993). Characterization of poorly digested protein of cooked rice protein bodies. Cereal Chemistry, 70, 101–105. AOAC. (2000). In W. Horwitz (Ed.), Official methods of analysis of AOAC International (17th ed.). Maryland, Gaithersburg, USA: AOAC International. Bjo¨ck, I. (1996). Starch: nutritional aspects. In A.-C. Eliasson (Ed.), Carbohydrates in food (pp. 505–553). New York: Marcel Dekker. Chrastil, J. (1990). Protein–starch interactions in rice grains. Influence of storage of oryzenin and starch. Journal of Agricultural and Food Chemistry, 38, 1804–1809. Dahle, L. K. (1971). Wheat protein–starch interaction. I. Some starch binding effects of wheat-flour proteins. Cereal Chemistry, 48, 706–714. Englyst, H. N., Kingman, S. M., & Cummings, J. H. (1992). Classification and measurement of nutritionally important starch fractions. European Journal of Clinical Nutrition, 46, 33–50. Escarpa, A., Gonza´lez, M. C., Morales, M. D., & Saura-Calixto, F. (1997). An approach to the influence of nutrients and other food constituents on resistant starch formation. Food Chemistry, 60, 527–532. FAO. (2001). FAO statement on biotechnology. Rome: Food and Agricultural Organization of the United Nations. www.fao.org. Fearn, T., & Russell, P. L. (1982). A kinetic study of bread staling by differential scanning calorimetry: the effect of specific volume. Journal of the Science of Food and Agriculture, 33, 537–541. Funami, T., Kataoka, Y., Omoto, T., Goto, Y., Asai, I., & Nishinari, K. (2005). Effects of non-ionic polysaccharides on the gelatinization and retrogradation behavior of wheat starch. Food Hydrocolloids, 19, 1–13. Ghiasi, K., Varriano-Marston, E., & Hoseney, R. C. (1982). Gelatinization of wheat starch. II. Starch–surfactant interaction. Cereal Chemistry, 59, 86–88. Gonza´lez-Soto, R. A., Mora-Escobedo, R., Herna´ndez-Sa´nchez, H., Sa´nchezRivera, M., & Bello-Pe´rez, L. A. (2007). The influence of time and storage temperature on resistant starch formation from autoclaved debranched banana starch. Food Research International, 40, 304–310. Gunaratne, A., & Hoover, R. (2000). Effect of heat-moisture treatment of the structure and physicochemical properties of tuber and root starches. Carbohydrate Polymers, 49, 425–437. Haralampu, S. G. (2000). Resistant starchda review of the physical properties and biological impact of RS3. Carbohydrate Polymers, 41, 285–292.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Retrogradation properties of high amylose rice flour and rice starch by physical modification Yue Wu a, Zhengxing Chen a, *, Xiaoxuan Li a, Zhenjiong Wang a a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2009 Received in revised form 9 September 2009 Accepted 18 September 2009
A new physical modification applied to prevent the retrogradation of rice flour and rice starch was investigated. This study examined the retrogradation properties of treated rice flour or rice starch paste by three stirring or heating–stirring, or without. The results proved that the retrogradations of rice flour and rice starch were both not substantially affected by three stirring modifications. However, three heating–stirring treatments had a marked effect on retarding the retrogradation of rice flour, but did not affect that of isolated rice starch. In the differential scanning calorimetry (DSC) analysis, modified rice flour by three heating–stirring exhibited the lowest retrogradation enthalpy (3.04 J/g dry matter) as compared to the control RF (5.93 J/g dry matter) and by three stirring (5.08 J/g dry matter). Meantime, it had almost the least recrystallization of retrogradation by X-ray diffraction (XRD). It was also found through scanning electron microscopy (SEM) that the granule structure of this modified rice flour had a more honeycomb-like structure and the lowest crystallinity as compared to the others. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Physical modification Retrogradation Differential scanning calorimetry (DSC) X-ray diffraction (XRD) Scanning electron microscopy (SEM)
1. Introduction Rice is one of the major food crops in the world, and more than 50% of the world’s population depend on rice as their primary caloric source (FAO, 2001). Rice products are staple foods, especially in Oriental countries, and most of these products are made of rice flour. Sharing the market of instant foods, fresh-cooked rice products such as cooked rice grains, rice noodles, rice cakes, and rice pasta have profound commercial potential. In recent years, rice flour has been increasingly used as novel foods like tortillas, beverages, processed meat, puddings, salad dressing, and glutenfree bread because of its unique functional properties such as being hypoallergenic, colorless and bland (Kadan & Ziegler, 1989; Kadan, Robinson, Thibodeux, & Pepperman, 2001; McCue, 1997). However, rice flour products are known to become hard, and they decline in texture and taste over time. This phenomenon is generally called ‘‘retrogradation’’ thus increasing the level of enzyme-resistant starch through recrystallization (Englyst, Kingman, & Cummings, 1992). Retrogradation is a process in which the molecules of gelatinized starch reassociate to form crystallites upon cooling, which imply fully reversible recrystallization in the case of amylopectin and partially irreversible recrystallization in the case
* Corresponding author. Tel.: þ86 510 85917025. E-mail address: [email protected] (Z. Chen). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.09.017
of amylose (Bjo¨ck, 1996). Therefore, the technology of preparing rice flour products with extended shelf life or using rice flour effectively requires efficient retarding of starch retrogradation. Traditionally, starches are chemically modified to prevent starch retrogradation. However, often questionable chemicals used will be banned in the health-conscious period. In addition, when adding one or more enzymes before cooking or after cooking the starch products, it could be to prevent retrogradation by enzymatic action (Martin & Hoseney,1991; Yao, Zhang, & Ding, 2003). It is pity that these products finally lose their firmness and suitable chewiness, and their taste is impaired. Since the decomposition reaction proceeds from the surface successively to the inside, their surface becomes overly soft. Although the loss in quality during the storage of starch-based products also can be counteracted by additives such as known trehalose, hydrocolloids (Funami, Kataoka, Omoto, Goto, Asai, & Nishinari, 2005; Lee, Baek, Cha, Park, & Lim, 2002), emulsifiers or surfactants (Ghiasi, Varriano-Marston, & Hoseney, 1982; Harbitz, 1983; Krog, 1981; Rao, Nussinovitch, & Chinachoti, 1992), and other materials, these materials are expensive or their effect is not ideal. The physical modification of starch is mainly applied to change its granular structure and convert native starch into cold watersoluble starch or small-crystallite starch. This method accomplished without any chemicals or even biological agents meets these requirements. It is thus desirable to develop a new type of physical modification to prevent the retrogradation of starch in rice flour. It should be also noticed that rice flour and rice starch are
Y. Wu et al. / LWT - Food Science and Technology 43 (2010) 492–497
different systems. Therefore, this study aims to investigate performance of the retrogradation as compared to rice flour (RF) and rice starch (RS) by applied a physical modification. The effects of this modification were systemically investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). 2. Materials and methods 2.1. Rice flour Freshly harvested high-amylose content (Juliano, 1982) rice was purchased in a local market. Rice was milled to flour using a mechanical grinder. Flour sample was screened through a 100-mu sieve, packed in airtight plastic bags, and stored at room temperature for further use. The rice flour contained starch (75 g/100 g), protein (8.1 g/100 g), fat (1.1 g/100 g), moisture (14.2 g/100 g) (AOAC, 2000), and amylose (28.6 g/100 g) (Gunaratne & Hoover, 2000). 2.2. Extraction of rice starch Starch was extracted using the modified method described by Lin and Chang (2006) involving steeping rice flour in NaOH (0.1 g/100 g) solution. The isolated starch was dried in an oven at 38 C for about 24 h to produce a moisture content close to that of rice flour (about 14 g/100 g). The dried starch was ground into powder and was passed through a 100-mu sieve. The isolated starch had protein content (0.5 g/100 g) (AOAC, 2000). 2.3. Physical modification of rice flour or starch The methods of physical modification of rice flour or starch are presented in Table 1. The methods only differ when heating in a boiling water bath (100 C) was applied along with stirring. The stirring machine used RW 20 digital head stirrer (IKA, German) at 500 rpm. 2.4. Gelatinization and retrogradation The samples (1:2 dry matter-to-water ratio) with treated or not were gelatinized by steam heating for 20 min in a closed thermostat water bath (100 C). Subsequently, the gelatinized samples were stored at 4 C for 7 days to allow them to retrograde. Afterward, these samples were lyophilized. The freeze-dried samples were ground, and then the powder samples were passed through a 100-mu sieve before DSC, XRD, and SEM testing.
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standard. For retrogradation studies, these stored samples (about 2 mg) were accurately weighed into aluminum DSC pans, and de-ionized water was added by micropipette to achieve a water– sample ratio of 2:1. The sample pans were sealed and equilibrated at room temperature for 24 h before analysis. The samples were heated at a rate of 10 C/min in a temperature range of 10–85 C using an empty pan as reference. The onset temperature To, peak temperature Tp, and conclusion temperature Tc were respectively determined from the heating DSC curves. Retrogradation enthalpy (DHr) was evaluated based on the area of the main endothermic peak. Analyses were performed in triplicate. 2.6. X-ray diffraction (XRD) Recrystallization of retrograded samples was determined using a Bruker D8 Advance speed X-ray diffractometer (Bruker AXS, Rheinfelden, Germany) equipped with a copper tube operating at 40 kV and 200 mA, producing CuKa radiation of 0.154 nm wavelength. Diffractograms were obtained by scanning from 4 to 40 (2q) at a rate of 4 /min, a step size of 0.02 , a divergence slit width (DS) of 1, a receiving slit width (RS) of 0.02 mm, and a scatter slit width (SS) of 1. Each sample was measured triplicate. MDI Jade 5.0 software was used to analyze the diffractograms. Crystallinity was quantified by integrating the area under the fitting crystalline peaks (14, 17, 20 and 23 ). The relative crystallinity (XRC) was expressed as XRC ¼ (Is/Ic) 100, where Is is the integrated area of crystalline peaks in the treated samples and Ic is the integrated area of crystalline peaks in the control (Primo-Martin, van Nieuwenhuijzen, Hamer, & van Vliet, 2007). The integrated areas were obtained after smoothing and fitting the original peak using the ˜ o´n, 2004). PeakFit v4.12 software (Ribotta, Cuffini, Leo´n, & An 2.7. Scanning electron microscopy (SEM) The structural properties of the samples were studied using a scanning electron microscope (Quanta-200, FEI Company, Netherlands). Dried, finely ground samples were mounted on an aluminum stub using a double-sided stick tape and were coated with a thin film of gold (10 nm), then examined at an accelerating voltage of 10 kV. 2.8. Statistical analysis The mean, standard deviations and significant differences of the data collected were calculated and reported using SAS version 8 (SAS Institute Inc., 2000). Whenever differences were reported as significant, a confidence level of 95% was considered. The data reported in all tables were the average of triplicate observations.
2.5. Differential scanning calorimetry (DSC) 3. Results and discussion The retrogradation properties of the samples were determined from the DSC curves using Pyris 1-DSC (Perkin-Elmer Corp., Norwalk, CT, USA). The calorimeter was calibrated with an indium Table 1 The methods of physical modification of rice flour or starch. The control
Method A
Method B
5 g flour or starch was mixed with 10 ml de-ionized water.
5 g flour or starch mixed with 6 ml de-ionized water was stirred for 20 s, and then 2 ml of water was added. The stirring/adding water cycles were repeated once. Finally, the sample was stirred for 20 s again.
The method is same as Method A, only when the samples were stirred with heating in a boiling water bath (100 C).
3.1. Thermal analysis After storage at 4 C for seven days, gelatinized starch molecules reassociate to an ordered structure, and this is referred to as retrogradation. The enthalpy values of the retrograded starch reflect the melting of the crystallites formed by the association between adjacent double helices during gel storage (Hoover & Senanayake, 1996). This endotherm peak was due to the melting of retrograded amylopectin (Fearn & Russell, 1982; Karim, Norziah, & Seow, 2000) rather than amylose. Fig. 1 shows retrogradation thermograms of treated rice flour or rice starch and their control. There was a clear decreasing enthalpy of treated RF by Method B as compared with the control and treated RF by Method A (Fig. 1a). But thermograms of treated RS or not
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Fig. 1. Retrogradation thermograms of rice flour (a) and rice starch (b) gel. The curves were the control, modified using Method A and Method B from top to below, respectively.
seem to be almost same (Fig. 1b). Meantime, Table 2 lists the retrogradation transition temperature (To, Tp, Tc) and the enthalpy of retrogradation (DHr) of samples after seven days storage. The retrogradation peak temperature all of these samples was around 48 C, which is the typical melting temperature of retrograded starch (Sievert & Pomeranz, 1990). However, the DHr of the control of RF was a little higher than that of isolated starch. This may be due to the slight difference in the actual starch concentration in the flour and the isolated starch suspensions. It was also clearly shown that modified RF using Method B has the lowest DHr (3.04 J/g dry flour) as compared to the control (5.93 J/g dry flour) and RF using Method A (5.08 J/g dry flour). In contrast, as compared to that of the control of RS (5.3 J/g dry starch), there was almost no difference between the treated RS samples by Method A (5.13 J/g dry starch) and Method B (5.1 J/g dry starch). It could therefore be considered that Method B could prevent the retrogradation of RF but not RS. The different result also indicates that rice protein appears to be
a crucial factor influencing the retrogradation properties of modified RF using Method B. It may be that this treatment induced some characteristic changes in rice protein such as improving emulsifying ability. Eventually, these changes in rice protein facilitated protein–starch interactions or higher water-binding capacity to prevent starch molecular recrystallization during the storage time. Numerous reports have shown that flour protein is an important factor in the rate of bread staling. Some researchers had confirmed that protein retarded bread staling. Gluten plays an important role on bread staling by forming an extensible protein network that keeps the crumb structure soft by slowing the movement of water from crumb to crust (Roach & Hoseney, 1995). Martin and Hoseney (1991) also reported that interactions exist between swollen starch granules and the gluten network, through hydrogen bonding, preventing the staling of bread. Rice proteins are found to be mostly within endosperm (storage proteins) cells, which are situated in protein bodies between the
Table 2 Retrogradation temperatures and enthalpy of gelatinized rice flour and rice starch with physical modification or without after 7 days’ storage at 4 C. To ( C)
Samples RF Modified Modified RS Modified Modified
Tp ( C) a
of RF using Method A of RF using Method B of RS using Method A of RS using Method B
39.0 0.79 40.4 1.62a 41.5 1.82b 39.8 2.26a 39.8 1.25a 40.2 1.87a
a
47.6 0.29 49.6 1.69b 49.2 1.32b 48.9 2.15b 48.7 0.95b 49.1 2.17b
Tc ( C)
DHr (J/g dry starch)
DHr (J/g dry flour)
57.2 0.22a 58.7 1.99b 57.4 1.57a 58.8 1.37b 58.5 0.72b 59.1 1.84b
– – – 5.13 0.59a 5.14 0.69a 5.34 0.30a
5.93 0.67a 5.08 0.25b 3.04 0.63c – – –
To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; DHr, enthalpy of retrogradation. Values are means SD (n ¼ 3). Values followed by the same letter in the same column are not significantly different (P < 0.05).
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3.2. Recrystallization The final recrystallization was investigated by XRD to further prove the effect of physical modification on the retrogradation. The XRD patterns and corresponding crystallinity observed from the gelatinized RF or RS systems with treatment or not are shown in Figs. 2 and 3. The retrograded starch from gel gave several peaks around 2q ¼ 17 and 20 and others (Kim, Kim, Shin, & Kwangju, 1997), which was typical pattern of retrograded starch (B-type) and was clearly distinct from the pattern of the raw starch. This is accompanied by gradual increases in rigidity and phase separation between the polymer and the solvent (syneresis). B-type crystallinity is characterized by a well-defined peak at 17 (2q). The formation of this peak was the result of the crystallization of the amorphous starch melt, which is mainly of the amylopectin fraction that was increased during storage (Thire´, Sima´o, & Andradeb, 2003;
“B” type
Intensity (Counts)
250
“V” type
300 250
Intensity (Counts)
starch granules and are tightly associated with rice starch (Adoration, Li, Okita, & Juliano, 1993; Lasztity, 1996). Therefore, it may be assumed that this unique structure of rice protein and starch might undergo the effect of anti-retrogradation through three heating– stirring modifications. The grains/granules began to swell and then burst with heating to release protein. At this time, rice protein had better emulsifying ability as several stirring to delay the retrogradation. That is known as the ability of the emulsifiers to retard firming and retrogradation (Krog, 1981; Ghiasi et al., 1982; Harbitz, 1983; Rao et al., 1992) by forming a complex with starch. Much work has been carried out on the interaction of starch with cereal proteins. Rice grains are rich in a storage protein called rice glutelin or oryzenin (over 80–90 g/100 g of total proteins) which interacts with starch by binding to amylose and/or amylopectin (Chrastil, 1990). Dahle (1971) studied the wheat starch– protein interactions in a solution by measuring the absorbances of amylose–iodine and amylopectin–iodine complexes in the presence of wheat proteins. An interaction of gelatinized wheat starch and wheat protein occurred, which interfered with the blue color associated with the starch–iodine complex. In addition, protein is supposed to influence the formation of resistant starch type 3 that is also retrograded starch (Haralampu, 2000). The presence of bovine serum albumin decreased the yield of resistant starch type 3 (Escarpa, Gonza´lez, Morales, & Saura-Calixto, 1997). However, a strong similarity exists between the retrogradation behavior of rice starch in cooked rice grains and that in purified form (Yao, Zhang, & Ding, 2002). These contrary results may be due to difference of gelatinization condition or protein characteristics to cause association difficult or easy of protein and starch.
200
a
150
b 100
c
50 0 5
a
c
50 0 25
25
30
35
40
Fig. 3. X-ray diffraction patterns of gelatinized rice starch without physical modification (a), with modification using Method A (b), and using Method B (c) after 7 days’ storage at 4 C.
Osella, Sa´nchez, Carrara, de la Torre, & Pilar, 2005). This peak at 20 found in corresponds to the presence of crystalline V-type amylose–lipid complexes formed during processing (Zobel & Kulp, 1996). Two major broad peaks were identified in the all retrogradated RF and RS samples at 2q angles of 17 and 20 (Figs. 2 and 3). As shown in Fig. 2, the intensity of a peak close to 17 of the modified RF samples using Method B seemed to be very small in a representative X-ray diffractogram. However, the XRD diffraction patterns of treated RS by Method A, Method B, or the control seemed to be the same as shown in Fig. 3. Because XRD peak of retrograded starch was normally the dispersion broad peak (Kim et al., 1997; Primo-Martin et al., 2007), so PeakFit software was used to smooth and fit these peaks in order to calculate crystallinity. The result of fitting was that four peaks at 14, 17, 20 and 23 were found in each original pattern (process not shown). Therefore, crystallinity was quantified based on integrating the area under the fitting crystalline peaks (14, 17, 20 and 23 ). The relative crystallinity modified RF using Method B (Table 3) was significantly decreased to 51.47% as compared with the control (100%) and modified RF using Method A (93.87%). This implies that Method B could retard the recrystallization or retrogradation behavior of gelatinized RF. At the same time, the relative crystallinity value was no evidently discrepancy between modified RS using Method A (90.44%), using Method B (100.21%) and the control (100%) (Table 3). This indicated that this physical modification had not effect on the retrogradation of RS. These observations agreed with the foregoing results on the retrogradation endotherm value in DSC.
30
35
XRC %a,b
Rice flour
The control Modification using Method A Modification using Method B
100 93.87 4.66 51.47 5.23
Rice starch
The control Modification using Method A Modification using Method B
100 90.44 3.78 100.21 6.43
100
20
20
2-Theta (°)
b
15
15
Sample
150
10
10
Table 3 Relative crystallinity (XRC) in retrograded samples as determined using X-ray diffraction.
200
5
495
40
2-Theta (°) Fig. 2. X-ray diffraction patterns of gelatinized rice flour without physical modification (a), with modification using Method A (b), and using Method B (c) after 7 days’ storage at 4 C.
a XRC, the relative crystallinity, is calculated using: XRC ¼ (Is/Ic) 100, where Is is the integrated area of fitting crystalline peaks (14, 17, 20 and 23 ) in the treated samples and Ic is the integrated area of fitting crystalline peaks (14, 17, 20 and 23 ) in the control. Relative crystallinity of the control (rice flour or rice starch) is a hundred, respectively. b Values are means SD (n ¼ 3).
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3.3. Morphological properties The scanning electron micrographs at magnification of 1200 of the retrograded RF and RS granules with treatment or not are illustrated in Fig. 4. The modified RF granular structure (Fig. 4c) using Method B as compared to the control (Fig. 4a) or the modified one using Method A (Fig. 4b) exhibited significant variations in shape when viewed by scanning electron microscopy (SEM). These granular (Fig. 4c) appeared to have a honeycomb-like structure formed due to the loss of a lot of water during freeze-drying treatment. It might state that the RF through Method B processing exhibited a high water holding capacity and less syneresis during storage. Since the retrogradation is often associated with water
separation from the gel (syneresis) due to starch chains associating to form thick structure (Hoover & Manuel, 1995; Whistler & Daniel, 1985; Yeh & Yeh, 1993). Thus the amount of syneresis is a useful indicator for the tendency of starch to retrograde (Karim et al., 2000). The extent of syneresis increases due to an increase in amylopectin retrogradation in the starch-rich phase (Yuan & Thompson, 1998). From the comparison of syneresis with Fig. 4c, it was readily visible that the control (Fig. 4d), modified RS using Method A (Fig. 4e), and Method B (Fig. 4f) all had very high solid and compact structure. It also may mean that this structure has high crystallinity that had a high extent of retrogradation (Gonza´lez-Soto, Mora-Escobedo, Herna´ndez-Sa´nchez, Sa´nchez-Rivera, & Bello-Pe´rez, 2007).
Fig. 4. Scanning electron micrograph of gelatinized rice flour and rice starch samples with physical modification using Method A or Method B or without after 7 days’ storage at 4 C: without modified RF (a), modified RF using Method A (b), modified RF using Method B (c), without modified RS (d), modified RS using Method A (e), and modified RS using Method B (f).
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4. Conclusions It could be speculated through combining these results with those obtained from DSC, XRD, and SEM stated above that the physical modification of three heating–stirring could prevent the retrogradation of RF but not RS. Furthermore, the presence of proteins in rice flour was considered responsible for this due to alterations in some protein characteristics following this modification. These observations were supported by the contrasting results of rice flour and rice starch. Althrough research into the retrogradation of rice starch is extensive, the proteins of rice are a less-known modified factor that may prevent the retrogradation of rice starch. In future, it would pay more attention in this aspect of investigation. Acknowledgments This research work was supported by the earmarked fund for Modern Agro-industry Technology Research System, PCSIRT 0627, and 111 project B07029. References Adoration, P. R., Li, X. L., Okita, T. W., & Juliano, B. O. (1993). Characterization of poorly digested protein of cooked rice protein bodies. Cereal Chemistry, 70, 101–105. AOAC. (2000). In W. Horwitz (Ed.), Official methods of analysis of AOAC International (17th ed.). Maryland, Gaithersburg, USA: AOAC International. Bjo¨ck, I. (1996). Starch: nutritional aspects. In A.-C. Eliasson (Ed.), Carbohydrates in food (pp. 505–553). New York: Marcel Dekker. Chrastil, J. (1990). Protein–starch interactions in rice grains. Influence of storage of oryzenin and starch. Journal of Agricultural and Food Chemistry, 38, 1804–1809. Dahle, L. K. (1971). Wheat protein–starch interaction. I. Some starch binding effects of wheat-flour proteins. Cereal Chemistry, 48, 706–714. Englyst, H. N., Kingman, S. M., & Cummings, J. H. (1992). Classification and measurement of nutritionally important starch fractions. European Journal of Clinical Nutrition, 46, 33–50. Escarpa, A., Gonza´lez, M. C., Morales, M. D., & Saura-Calixto, F. (1997). An approach to the influence of nutrients and other food constituents on resistant starch formation. Food Chemistry, 60, 527–532. FAO. (2001). FAO statement on biotechnology. Rome: Food and Agricultural Organization of the United Nations. www.fao.org. Fearn, T., & Russell, P. L. (1982). A kinetic study of bread staling by differential scanning calorimetry: the effect of specific volume. Journal of the Science of Food and Agriculture, 33, 537–541. Funami, T., Kataoka, Y., Omoto, T., Goto, Y., Asai, I., & Nishinari, K. (2005). Effects of non-ionic polysaccharides on the gelatinization and retrogradation behavior of wheat starch. Food Hydrocolloids, 19, 1–13. Ghiasi, K., Varriano-Marston, E., & Hoseney, R. C. (1982). Gelatinization of wheat starch. II. Starch–surfactant interaction. Cereal Chemistry, 59, 86–88. Gonza´lez-Soto, R. A., Mora-Escobedo, R., Herna´ndez-Sa´nchez, H., Sa´nchezRivera, M., & Bello-Pe´rez, L. A. (2007). The influence of time and storage temperature on resistant starch formation from autoclaved debranched banana starch. Food Research International, 40, 304–310. Gunaratne, A., & Hoover, R. (2000). Effect of heat-moisture treatment of the structure and physicochemical properties of tuber and root starches. Carbohydrate Polymers, 49, 425–437. Haralampu, S. G. (2000). Resistant starchda review of the physical properties and biological impact of RS3. Carbohydrate Polymers, 41, 285–292.
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