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Studies on the morphological and rheological properties of granular cold water soluble corn and potato starches
Food Hydrocolloids 17 (2003) 63±72
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Studies on the morphological and rheological properties of granular cold water soluble corn and potato starches Jaspreet Singh, Narpinder Singh* Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 143 005, Punjab, India
Abstract Granulated cold water soluble (GCWS) starches were prepared from corn starch and starches separated from four potato cultivars using alcoholic-alkaline method. The morphological, thermal and rheological properties of GCWS corn and potato starches were studied. The amylose content of the GCWS starches from corn and potato starches were signi®cantly lower as compared to their counterparts native starches. GCWS potato starches showed higher cold water solubility than GCWS corn starch. Cold water solubility of GCWS starches prepared from different potato cultivars also differed signi®cantly. Native potato starch granules were larger in size (15±45 mm), smooth, oval and irregular or cuboidal shaped while native corn starch granules were smaller in size (5±18 mm), less smooth, rounded and angular shaped. Both corn and potato starches were distorted and indented during conversion to GCWS starches, however, this effect was more pronounced in potato starches than corn starch. The extent of distortion in GCWS starches differed signi®cantly in starches prepared from different potato cultivars. The potato cultivars having starch with large sized granules showed more granular distortion as compared in those having small sized granules during GCWS starch production. Native corn starch showed higher transition temperatures and lower enthalpy of gelatinization (DHgel) than native potato starches. GCWS corn and potato starches did not show any gelatinization endotherm during heating between 20 and 100 8C. The rheological properties such as G 0 , G 00 , h 0 and Tan d of GCWS corn and potato starches also showed signi®cant variation, when subjected to frequency sweep testing. GCWS potato starches showed higher G 0 , G 00 , h 0 and lower Tan d than GCWS corn starch. The G 0 , G 00 , h 0 of the GCWS starches from both corn and potato increased and Tan d decreased with the increase in temperature. The turbidity of GCWS and native corn and potato starches increased during storage at 4 8C, however, the increase was less pronounced in GCWS starches. q 2002 Published by Elsevier Science Ltd. Keywords: Granulated cold water soluble; Corn starch; Potato starch; Morphological; Rheological
1. Introduction Native starch granules are birefringent and show the characteristic cross shaped shadow under polarized light. Starch granule gelatinize in water when the temperature is raised to 60±70 8C. The granules normally swell to form a paste or solution above 70 8C. A loss of the birefringence occurs during gelatinization and the starch granule may disintegrate into molecules and fragments. Most of pre-gelatinized starches are made by applying starch slurry to a hot drum and grinding the thin sheet to a ®ne powder. Rehydration of drum-cooked starch at room temperature gives a paste of reduced consistency with a dull, grainy appearance and gels of reduced strength (Rajagopalan & Seib, 1992a). Many methods have been developed to prepare granulated cold water soluble (GCWS) starches, such as heating of starches in aqueous alcohol, high temperature and pressure condi* Corresponding author. Tel.: 191-183-258802; fax: 191-183-258820. E-mail address: [email protected] (N. Singh). 0268-005X/03/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0268-005 X(02)00 036-X
tions and alcoholic-alkaline treatment (Chen & Jane, 1994a; Eastman & Moore, 1984; Rajagopalan & Seib, 1992a,b). The GCWS starches prepared by these methods exhibited different cold water solubility. GCWS starches gives greater viscosity, smoother texture and have more processing tolerance than traditional pregelatinized starches (Light, 1990). Jane and Seib (1991) proposed an alcoholic-alkaline method for preparing GCWS starches which involved the treatment of starch with a mixture of ethanol and alkali to swell starch granules and neutralization with hydrochloric acid (HCl), washing with alcohol and drying. Chen and Jane (1994a) reported solubility ranged between 11.7 and 93.3% for waxy and high amylose maize starches prepared by different alcoholic-alkaline treatments and concluded that the effectiveness of the method depends mainly on the starch variety, concentration of ethanol and sodium hydroxide (NaOH) and reaction temperature. Many workers have used X-ray diffraction, differential scanning calorimetry (DSC), pasting curves and scanning electron microscopy (SEM) to examine GCWS wheat, corn, tapioca, hydroxypropylated and
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cross-linked wheat starches (Chen & Jane, 1994a,b; Jane, Craig, Seib, & Hoseney, 1986; Rajagopalan & Seib, 1992b). Jane et al. (1986) observed the distorted shape of starch granules compared to native granules after treatment with aqueous alcohol. Chen and Jane (1994a) observed that the granules of GCWS maize starches prepared by alcoholicalkaline treatment had indented appearance and were larger than those of the original native granules. Light microscopy revealed that GCWS starches prepared from wheat, corn, tapioca and hydroxy propylated cross linked starch by propan-1,2-diol treatment had fractured internally but contained an intact shell at the surface (Rajagopalan & Seib, 1992b). Chen and Jane (1994b) studied the pasting behaviour of waxy and normal GCWS maize starches using viscoamylograph and concluded that these starches had viscosity higher than those of their native counterparts. Potato starch is used for its characteristics, which differ signi®cantly from those of starch from other plant sources (Madsen & Christensen, 1996). Identi®cation of native starch sources is required for desired functionality and unique properties (Duxbury, 1989). Barichello and Yada (1991) suggested that paste characteristics and other physicochemical properties of starches vary with genotype and cultural practices. Hopkins and Gormley (2000) reported the rheological properties of pastes and gels made from starch isolated from different Irish potato cultivars. Kim, Wiesenborn, Orr, and Grant (1995) and Wiesenborn, Orr, Casper, and Tacke (1994) reported the paste behaviour from various potato genotypes and correlated the physicochemical characteristics with functional properties. The objectives of the present work were to compare morphological, thermal and rheological properties of GCWS starch prepared from corn and potato starches. 2. Material and methods
starches. Starches (100 g, dwb) were dispersed in 1100 g of ethanol solution (40% w/w) in a 3000 ml beaker and placed on rotary shaker. NaOH solution (2.5 M, 200 g) was weighed and added in the starch±ethanol slurry during shaking at the rate of 5 g/min. The reaction temperature was maintained at 25 8C. The mixture was then allowed to rest for 10 min. Additional 900 g of ethanol solution (40% w/w) was added and the slurry was shaked for 10 min and left at room temperature (25 8C) until the starch settled down. The clear supernatant was separated and another 500 g ethanol solution (40% w/w) was added and neutralized with HCl (2 M in absolute ethanol). The neutralized starch solution was washed with ethanol solutions of 60, 95% and ®nally with 100% concentration. The dehydrated starch was then dried at 50 8C in an air oven. The dried starch was grounded and passed through no: 100 (BSS) sieve and stored in air tight containers at room temperature. 2.3. Amylose content Amylose content of the native and GCWS corn and potato starches was determined by the method given by Williams, Kuzina, and Hlynka (1970). 2.4. Cold water solubility (%) The method of Eastman and Moore (1984) was slightly modi®ed to determine cold water solubility of GCWS corn and potato starches. A 100 ml (1%) suspension of GCWS starch was shaked thoroughly for 30 min on a rotary shaker. The starch suspension was transferred to a 250 ml centrifuge bottle and centrifuged at 1200 £ g for 10 min. A 25 ml aliquot of the supernatant was taken in a preweighed aluminum moisture dish and dried in an air oven at 110 8C for 4 h. The cold water solubility (as swollen, non-sedimented granules as soluble material) was calculated as CWS% grams of solid in supernatant £ 4/grams of sample £ 100.
2.1. Materials
2.5. Scanning electron microscopy
The potatoes of four cultivars, viz. Kufri Chandermukhi, Kufri Badshah, Kufri Jyoti and Kufri Sindhuri were procured from Sangha Potato Farms, Jalandhar, India from 2000 harvest. Uniform sized potatoes were selected from each cultivar before starch isolation. Corn starch was supplied by Sukhjit Starch Ltd, Phagwara (India). Analytical grade sodium hydroxide and hydrochloric acid was procured from CDH, Bombay (India) and ethanol was obtained from Hayman Ltd, Essex (UK).
Scanning electron micrographs of native and GCWS starches were obtained with a scanning electron microscope (Jeol JSM-6100, Jeol Ltd, Tokyo, Japan). Starch samples were sprinkled on double stick tape ®xed on an alminium stub, and the starch was coated with gold±palladium (60:40).
2.1.1. Potato starch isolation Potato starch was isolated as described earlier (Singh & Singh, 2001). 2.2. Preparation of granular cold water soluble starches The method of Chen and Jane (1994a) with slight modi®cation was used to prepare GCWS corn and potato
2.6. Differential scanning calorimetry Thermal properties of native and GCWS corn and potato starches were analysed using DSC-821 e (Mettler Toledo, Switzerland) equipped with a thermal analysis data station. Starch (3.5 mg, dwb) was weighed into a 40 ml capacity aluminium pan (Mettler, ME-27331) and distilled water was added with the help of Hamilton microsyringe to achieve a starch±water suspension containing 70% water. Samples were hermetically sealed and allowed to stand for 1 h at room temperature before heating in DSC. The DSC
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Table 1 Amylose content and cold water solubility of native and GCWS corn and potato starches (values with similar superscripts in column do not differ signi®cantly (p , 0.05); GCWS, granulated cold water soluble) Starch source
Amylose content (%) Native
Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
cd
28.1 29.0 d 31.6 e 26.6 bc 22.1 a
Cold water solubility (%) GCWS 25.2 d 23.3 b 27.8 e 24.2 c 20.1 a
analyser was calibrated using indium and an empty aluminum pan was used as reference. Sample pans were heated at a rate of 10 8C/min from 20 to 100 8C for potato starches and 20 to 120 8C for corn starch. Onset temperature (To); peak temperature (Tp); conclusion temperature (Tc) and enthalpy of gelatinization (DHgel) were calculated. 2.7. Rheological properties A small amplitude oscillatory rheological measurement was made for GCWS corn and potato starches, with a dynamic rheometer (Carri-Med CSL 2-100, TA Instruments Ltd, Surrey, England) equipped with 1.598 steel cone geometry (4 cm dia). The strain was set at 1.5%. The GCWS starch samples were subjected to frequency sweep testing with a range of 0.1±20 Hz at different temperatures. The dynamic rheological properties such as storage modulus (G 0 ), loss modulus (G 00 ), loss factor Tan d and dynamic viscosity (h 0 ) were determined for GCWS corn and potato starches. GCWS starch slurries of 20% (w/w) concentration were loaded on the ram of rheometer and covered with a thin layer of low-density silicone oil (to minimize evaporation losses). 2.8. Turbidity Turbidity of native and GCWS corn and potato starches was measured as described by Perera and Hoover (1999). An aqueous suspension (2%) of native starch from corn and potato starches was heated in a boiling water bath for 1 h with constant stirring. The suspension was cooled for 1 h at 30 8C. The GCWS starches were dispersed in water and mixed thoroughly. The samples were stored for 72 h at 4 8C in a refrigerator and turbidity was determined every 24 h by measuring absorbance at 640 nm against water blank with a Shimadzu UV-1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). 2.9. Statistical analysis The data reported in all the tables are average of triplicate observations. The data were subjected to statistical analysis using Minitab Statistical Software (Minitab Inc., USA).
85.2 b 93.0 d 93.8 d 89.4 c 63.0 a
3. Results and discussion 3.1. Amylose content The amylose content of native potato starches ranged between 25.5 and 31.6% while corn starch had amylose content of 22.1%. Among the potato starches, Kufri Jyoti potato starch had the highest amylose content whereas Kufri Sindhuri potato starch had the lowest. Kim et al. (1995) and Wiesenborn et al. (1994) also reported similar ranges of amylose content for starches from American potato cultivars. Corn starch showed lower amylose content as compared to all potato starches. GCWS corn and potato starches had lower amylose content than their native starches (Table 1). The amylose content of GCWS potato starches ranged between 23.31 and 27.8% while GCWS corn starch had amylose content of 20.14%. The amylose has been reported to leach out during preparation of GCWS starches by alcoholic-alkaline treatment (Chen & Jane, 1994a; Jane et al., 1986). Jane et al. (1986) suggested that the amylose intertwine with amylopectin to prevent dispersion of starch granules during heating in aqueous solution. Among the GCWS potato starches, Kufri Jyoti potato starch had the highest amylose content while Kufri Sindhuri potato starch showed the lowest. The difference in the amylose content of GCWS corn and potato starches might be due to the difference in the amylose content of their native starches and granule morphology which in turn depends on their biological origin (Svegmark & Hermansson, 1993). The extent of dropdown in amylose content is higher in potato starches than corn starch during preparation of GCWS starches. This may be attributed to the fragile nature of the potato starch granules (Singh, Singh, & Saxena, 2002). 3.2. Cold water solubility (%) The cold water solubility of starch prepared by alcoholicalkaline treatment represents mainly swollen, non-sedimented granules as soluble material and depends on the starch source and granular structure. Cold water solubility of GCWS corn and potato starches differed signi®cantly. The cold water solubility of GCWS corn starch was 63% and ranged between 85.2 and 93.8% for GCWS potato starches
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Fig. 1. (a) SEM of native corn and potato starches (A) Kufri Chandermukhi, (B) Kufri Badshah, (C) Kufri Jyoti, (D) Kufri Sindhuri, and (E) corn ( £ 400, Bar 10 mm); (b) SEM of GCWS corn and potato starches (A) Kufri Chandermukhi, (B) Kufri Badshah, (C) Kufri Jyoti, (D) Kufri Sindhuri, and (E) corn ( £ 400, Bar 10 mm); (c) SEM showing granular indentation and fragmentation in GCWS starches (A) GCWS Kufri Badshah, and (B) GCWS Kufri Jyoti ( £ 1200, Bar 10 mm).
(Table 1). The lower cold water solubility of GCWS corn starch could be attributed to the more rigid structure of the corn starch granules. Potato starch granules are fragile in nature (Singh et al., 2002) and the alcoholic-alkaline treatment may have affected potato starch to the greater extent as compared to corn starch during preparation of GCWS starches. The potato starch granules swelled much more during the treatment, therefore the resulting GCWS potato
starches had greater cold water solubilities. Kufri Badshah and Kufri Jyoti potato GCWS starches showed the highest cold water solubility among GCWS potato starches. SEM results showed that their native starches contained large oval and irregular or cubiodal shaped granules in fairly large number that may have swelled to greatest extent. The differences among cold water solubilities of GCWS potato starches may be attributed to the differences in granular
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Fig. 1. (continued)
structure and amylose content. Amylopectin branch chain length may also affect the cold water solubility of different starches (Chen & Jane, 1994a). 3.3. Scanning electron microscopy The granular structure of native corn and potato starches showed signi®cant variation in size and shape when viewed by SEM (Fig. 1a). The granule size ranged between 15 and
20 mm for small and 20±45 mm for large granules in potato starch. The size of corn starch granules was ranged between 5 and 7 mm for small and 15±18 mm for large granules. The native potato starch granules were observed to be smooth, oval and irregular or cuboidal shaped. Native corn starch granules appeared to be less smooth, rounded and angular shaped. Kufri Jyoti potato starch showed larger oval and few small granules while the Kufri Badshah potato starch showed large irregular or cuboidal shaped granules in fairly
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J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72 Table 2 Differential scanning calorimetry thermal properties of native corn and potato starches (To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; R, gelatinization range (Tc 2 To); DHgel, enthalpy of gelatinization (dwb, based on starch weight). Values with similar superscripts in column do not differ signi®cantly (p , 0.05)) Cultivar Potato (Kufri Chandermuki) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
To (8C) bc
60.27 59.72 a 59.86 ab 60.70 c 69.82 d
Tp (8C) a
63.39 63.45 a 63.26 a 64.58 b 74.87 c
Tc (8C) a
67.28 68.35 c 67.66 ab 70.34 d 78.1 d
DHgel (J/g) 12.55 b 13.85 d 13.68 d 13.38 c 10.92 a
during alcoholic-alkaline treatment of maize starch. All the GCWS starch granules showed internal fragmentation, which clearly indicates the swelling, and shrinkage in that region of the granule (Fig. 1c). 3.4. Differential scanning calorimetry
Fig. 1. (continued)
large number. The variation in the size and shape in starch granules may be due to the biological origin (Svegmark & Hermansson, 1993). The morphology of starch granules depends on the biochemistry of the chloroplast or amyloplast, as well as physiology of the plant (Badenhuizen, 1969). The size and shape of the GCWS corn and potato starches differed from their native starches (Fig. 1b). The effect of alcholic-alkaline treatment was observed to be brought more changes in potato starch than corn starch. The potato starch granules swelled more than corn starch granules. The alcoholic-alkaline treatment caused indentation of potato starch granules. The indentation of potato starches separated from different cultivars differed signi®cantly. Kufri Badshah and Kufri Jyoti potato GCWS starches showed higher indentation in granules while it was lower for Kufri Chandermukhi and Kufri Sindhuri potato GCWS starches. These differences may be attributed to the variation in number of large and small granules in starches. SEM pictures of GCWS starches clearly reveals that small starch granules were less affected by alcoholicalkaline treatment than large starch granules. Chen and Jane (1994a) also observed more swelling of large granules
Native corn and potato starches showed endothermic peaks between 60 and 80 8C. The transition temperatures (To; Tp; and Tc), and enthalpies of gelatinization (DHgel) of starches from native corn and potato starches differ signi®cantly (Table 2). Corn starch showed higher transition temperatures and lower DHgel than potato starches. These differences in potato and corn starch may be attributed to the differences in the granular structure, amylose content and gelatinization temperature of starches (Singh et al., 2002; Singh & Singh, 2001). Among the native potato starches, Kufri Badshah potato starch showed highest DHgel value (13.85 J/g) and Kufri Chandermukhi potato starch showed the lowest DHgel value (12.55 J/g). Kufri Sindhuri potato starch had highest To (60.70 8C) followed by Kufri Chandermukhi potato starch (60.27 8C) while it was lowest for Kufri Badshah potato starch (59.72 8C). Kim et al. (1995) also reported similar ranges of transition temperatures and enthalpies of gelatinization for starches from 42 American potato cultivars. Double helical and crystalline structures are disrupted in starches during gelatinization. This order±disorder phase transition showed melting of crystals which was illustrated by DSC endotherms in the range of 50±70 8C for various native starches (Jacobs, Eerlingen, Clauwaert, & Delcour, 1995). A higher enthalpy for Kufri Badshah potato starch may be attributed to the presence of higher percentage of irregular or cuboidal shaped and large granules while Kufri Chandermukhi potato starch, which contained small and oval granules, may be responsible for its low DHgel. The starch from potato cultivars having smaller starch granules showed lower DHgel and vice versa. The GCWS corn and potato starches did not show any gelatinization endotherm when heated between 20 and 100 8C. The absence of gelatinization endotherm after alcoholic-alkaline treatment during preparation of GCWS maize starches has been reported earlier (Chen & Jane, 1994a). This con®rmed the changed nature of the
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Fig. 2. (a) Storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 40 8C; (b) storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 50 8C; and (c) storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 60 8C.
GCWS corn and potato starch granules during alcoholicalkaline treatment. 3.5. Rheological properties The rheological parameters like G 0 , G 00 , Tan d, and h 0 showed signi®cant variation among GCWS corn and potato starches when subjected to frequency testing ranged from 0.1 to 20 Hz at different temperatures (Fig. 2a±c). The G 0 , G 00 , increased while h 0 decreased with the increase in frequency. GCWS corn starch showed lower G 0 , G 00 and h 0 values than GCWS potato starches (Table 3). This may be attributed to more rigid nature of corn starch granules (Singh et al., 2002). Among the GCWS potato starches, Kufri Badshah potato starch showed the highest G 0 , G 00 and h 0 while these were lowest for Kufri Sindhuri potato starch. These differences may be due to the differences in granular size and shape of the native starches. Kufri Badshah potato native starch contained large irregular and cuboidal granules and very few or negligible small granules. Kufri Jyoti potato native starch also contains large oval granules in fairly large number. The large starch granules are likely to be more indented during alcoholic-alkaline treatment (Chen & Jane, 1994a). The native Kufri Badshah and Kufri Jyoti potato native starches had lowest To and higher DHgel values while starches from other potato culti-
vars showed higher transition temperatures and lower DHgel values (Table 2). The native starches with lower transition temperature might be more susceptible to alcoholic-alkaline treatment as compare to starches with high transition temperature. High transition temperatures in native starches have been reported to result from a high degree of crystallinity, which provided structural stability and made the granule more resistant to gelatinization (Barichello, Yada, Cof®n, & Stanley, 1990). This may possibly affect the swelling of starch granules during alcoholic-alkaline treatment that resulted in the variation in rheological properties of GCWS starches. The native starches from Kufri Chandermukhi and Kufri Sindhuri potato cultivars contained a large number of small granules which may be responsible for low G 0 and G 00 and h 0 . Tan d of GCWS corn and potato starches decreased with increase in frequency. GCWS corn starch exhibited highest Tan d value of 0.2751. Kufri Badshah and Kufri Jyoti GCWS potato starches showed lowest Tan d values of 0.0766 and 0.081, respectively, which was highest (0.091) for Kufri Sindhuri GCWS starch (Table 3). Higher G 0 , G 00 , h 0 and lower Tan d values for Kufri Badshah and Kufri Jyoti potato GCWS starches suggested that these starches formed a more rigid gel structure as compare to gels from other potato GCWS starches. The G 0 , G 00 , h 0 of the GCWS corn and potato starches increased with the increase in temperature while the Tan d decreased during
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Fig. 2. (continued)
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Table 3 Rheological properties of GCWS corn and potato starches at different temperatures (at angular frequency of 125.7 rad/s) Starch
Temperature (8C)
G 0 (Pa)
G 00 (Pa)
Tan d
h 0 (Pa s)
Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
40 40 40 40 40 50 50 50 50 50 60 60 60 60 60
27 510 40 102 35 430 24 890 1950 79 880 15 1900 90 150 68 390 5386 366 900 425 100 387 500 348 500 32 590
7867 11 188 10 097 7342 6025 11 486 20 340 12 520 10 668 1573 30 840 32 562 31 387 31 713 8966
0.286 0.279 0.285 0.295 0.321 0.1438 0.1339 0.1389 0.156 0.2922 0.082 0.0766 0.081 0.091 0.2751
58.6 94 81 50.8 5.2 91 153 110 82 10.62 177 237 218 148 71
frequency sweep testing (Fig. 2a±c). The GCWS starches swelled more in water with increase in temperature as compared to their counterpart native starches. The starch granules also swelled to greater extent with increase in reaction temperature during preparation of GCWS starches with alcoholic-alkaline treatment (Chen & Jane, 1994a). Lancaster and Conway (1968) also reported an exponential relationship between temperature and the swelling rate of
starch granules in NaOH solution. Rajagopalan and Seib (1992b) suggested that the indented GCWS starch granules have an empty cavity inside the helices which left after the drying of alcohol. This empty cavity may provide ambient space to form bonding between starch and water molecules. This bonding may be facilitated with increase in temperature, which ultimately led to an increase in G 0 , G 00 , h 0 and decrease in Tan d. The reduction in water activity in GCWS starch±water system may also be responsible for the increase in G 0 and G 00 . The gelatinization temperature has been reported to increase in aqueous alcohol and starch± water systems which may occurred due to changes in water structure, reduction in water activity and speci®c solute± starch interactions in starch granule (Rajagopalan & Seib, 1992a). The rheological properties of the native starches have been reported to depend on granular structure, amylose to amylopectin ratio, presence of phosphate esters (Singh & Singh, 2001; Wiesenborn et al., 1994). These factors may possibly have affected the rheological properties during preparation of GCWS starches by alcoholic-alkaline treatment. 3.6. Turbidity
Fig. 3. (a) Effect of storage conditions on the turbidity of native corn and potato starches; and (b) Effect of storage conditions on the turbidity of GCWS corn and potato starches.
The turbidity value of gelatinized starch suspension from the native corn and potato starches measured as absorbance at 640 nm differed signi®cantly (Fig. 3a). The GCWS starches showed higher turbidity values than their native starches (Fig. 3b). This may be due to the altered nature of starch granules and leaching out of amylose during alcoholic-alkaline treatment. Native corn starch showed higher turbidity values than native potato starches which may be attributed to the difference in granular rigidity of these starches. Among the GCWS potato starches, Kufri Chandermukhi and Kufri Sindhuri potato starch suspension showed higher turbidity values. Starches from the potato cultivars having larger size granules showed lower turbidity values while those having smaller size granules showed
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higher turbidity values (Singh & Singh, 2001). Factors such as granule swelling, granule remnants, leached amylose and amylopectin, amylose and amylopectin chain lengths, intra or intermolecular bonding, lipids, cross-linking and substitution have been reported to be responsible for turbidity development in native starches during storage (Jacobson, Obanni, & BeMiller, 1997). The turbidity of the native and GCWS starches increased with increase in storage period. The increase in turbidity values was very little in GCWS potato starches and almost negligible for corn starch. The slow increase in turbidity values of GCWS starches during storage may be attributed to the low amylose content, altered nature of the granules, low granular remnants and less aggregation of amylose and amylopectin molecules. 4. Conclusion Alcoholic-alkaline treatment brought substantial changes in physico-chemical, morphological, thermal and rheological properties in corn and potato starches during preparation of GCWS starches. This treatment caused indentation and distortion in granular structure in both corn and potato starches, however, potato starches were found to be more susceptible to these changes. The properties of GCWS starches were found to depend on the native granule morphology. The larger the starting granule population, the greater the amylose lost and subsequent swelling of the treated granules. GCWS potato starches showed higher G 0 , G 00 , h 0 and lower Tan d than GCWS corn starch. The rheological properties of GCWS potato starches were found to vary with source of native starch. G 0 , G 00 , h 0 of GCWS starches increased and Tan d decreased with the increase in temperature. GCWS corn and potato starches showed lower turbidity values than their native counterpart starches during storage at 4 8C. Acknowledgements We thank Dr S.K. Saxena, Director, Food Research and Analysis Center, New Delhi for providing us the facility for rheological studies. References Badenhuizen, N. P. (1969). The biogenesis of starch granules in higher plants, New York: Appleton Crofts. Barichello, V., & Yada, R. Y. (1991). Starch properties of various potato (Solanum tuberosum L.) cultivars susceptible and resistant to low temperature sweetening. Journal of the Science of Food and Agriculture, 56, 385±397.
Barichello, V., Yada, R. Y., Cof®n, R. H., & Stanley, D. W. (1990). Low temperature sweetening in susceptible and resistant potatoes: starch structure and composition. Journal of Food Science, 54, 1054±1059. Chen, J., & Jane, J. (1994a). Preparation of granular cold-water soluble starches by alcoholic-alkaline treatment. Cereal Chemistry, 71, 618± 622. Chen, J., & Jane, J. (1994b). Properties of granular cold-water soluble starches prepared by alcoholic-alkaline treatment. Cereal Chemistry, 71, 623±626. Duxbury, D. D. (1989). Modi®ed starch functionalitiesÐno chemicals or enzymes. Food Processing, 50, 35±37. Eastman, J. E., & Moore, C. O. (1984). Cold water soluble granular starch for gelled food compositions. US Patent, 4,465,702. Hopkins, S., & Gormley, R. (2000). Rheological properties of pastes and gels made from starch separated from different potato cultivars. Lebensmittel-Wissenchaft und-Technologie, 33, 388±396. Jacobs, H., Eerlingen, R. C., Clauwaert, W., & Delcour, J. A. (1995). In¯uence of annealing on the pasting properties of starches from varying botanical sources. Cereal Chemistry, 72, 480±487. Jacobson, M. R., Obanni, M., & BeMiller, J. N. (1997). Retrogradation of starches from different botanical sources. Cereal Chemistry, 74, 571± 578. Jane, J., Craig, S. A. S., Seib, P. A., & Hoseney, R. C. (1986). Characterization of granular cold water soluble starch. Starch, 38, 258±263. Jane, J., & Seib, P. A. (1991). Preparation of granular cold-water swelling/ soluble starches by alcoholic-alkaline treatments. US Patent, 5057,157. Kim, S. Y., Wiesenborn, D. P., Orr, P. H., & Grant, L. A. (1995). Screening potato starch for novel properties using differential scanning calorimetry. Journal of Food Science, 60, 1060±1065. Lancaster, E. B., & Conway, H. F. (1968). Alkali sorption and swelling of starch. Cereal Science Today, 13, 248±253. Light, J. M. (1990). Modi®ed food starches: Why, where, and how. Cereal Foods World, 35, 1081±1084. Madsen, M. H., & Christensen, D. H. (1996). Changes in viscosity properties of potato starch during growth. Starch, 48, 245±249. Perera, C., & Hoover, R. (1999). In¯uence of hydroxypropylation on retrogradation properties of native, defatted and heat-moisture treated potato starches. Food Chemistry, 64, 361±375. Rajagopalan, S., & Seib, P. A. (1992a). Granular cold-water soluble starches prepared at atmospheric pressure. Journal of Cereal Science, 16, 13±28. Rajagopalan, S., & Seib, P. A. (1992b). Properties of granular cold-water soluble starches prepared at atmospheric pressure. Journal of Cereal Science, 16, 29±40. Singh, J., & Singh, N. (2001). Studies on the morphological, thermal and rheological properties of starch separated from some Indian potato cultivars. Food Chemistry, 75, 67±77. Singh, J., Singh, N., & Saxena, S. K. (2002). Effect of fatty acids on the rheological properties of corn and potato starch. Journal of Food Engineering, 52, 9±16. Svegmark, K., & Hermansson, A. M. (1993). Microstructure and rheological properties of composites of potato starch granules and amylose: A comparison of observed and predicted structure. Food Structure, 12, 181±193. Wiesenborn, D. P., Orr, P. H., Casper, H. H., & Tacke, B. K. (1994). Potato starch paste behaviour as related to some physical/chemical properties. Journal of Food Science, 59, 644±648. Williams, P. C., Kuzina, F. D., & Hlynka, I. (1970). A rapid colorimetric procedure for estimating the amylose content of starches and ¯ours. Cereal Chemistry, 47, 411±420.
www.elsevier.com/locate/foodhyd
Studies on the morphological and rheological properties of granular cold water soluble corn and potato starches Jaspreet Singh, Narpinder Singh* Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 143 005, Punjab, India
Abstract Granulated cold water soluble (GCWS) starches were prepared from corn starch and starches separated from four potato cultivars using alcoholic-alkaline method. The morphological, thermal and rheological properties of GCWS corn and potato starches were studied. The amylose content of the GCWS starches from corn and potato starches were signi®cantly lower as compared to their counterparts native starches. GCWS potato starches showed higher cold water solubility than GCWS corn starch. Cold water solubility of GCWS starches prepared from different potato cultivars also differed signi®cantly. Native potato starch granules were larger in size (15±45 mm), smooth, oval and irregular or cuboidal shaped while native corn starch granules were smaller in size (5±18 mm), less smooth, rounded and angular shaped. Both corn and potato starches were distorted and indented during conversion to GCWS starches, however, this effect was more pronounced in potato starches than corn starch. The extent of distortion in GCWS starches differed signi®cantly in starches prepared from different potato cultivars. The potato cultivars having starch with large sized granules showed more granular distortion as compared in those having small sized granules during GCWS starch production. Native corn starch showed higher transition temperatures and lower enthalpy of gelatinization (DHgel) than native potato starches. GCWS corn and potato starches did not show any gelatinization endotherm during heating between 20 and 100 8C. The rheological properties such as G 0 , G 00 , h 0 and Tan d of GCWS corn and potato starches also showed signi®cant variation, when subjected to frequency sweep testing. GCWS potato starches showed higher G 0 , G 00 , h 0 and lower Tan d than GCWS corn starch. The G 0 , G 00 , h 0 of the GCWS starches from both corn and potato increased and Tan d decreased with the increase in temperature. The turbidity of GCWS and native corn and potato starches increased during storage at 4 8C, however, the increase was less pronounced in GCWS starches. q 2002 Published by Elsevier Science Ltd. Keywords: Granulated cold water soluble; Corn starch; Potato starch; Morphological; Rheological
1. Introduction Native starch granules are birefringent and show the characteristic cross shaped shadow under polarized light. Starch granule gelatinize in water when the temperature is raised to 60±70 8C. The granules normally swell to form a paste or solution above 70 8C. A loss of the birefringence occurs during gelatinization and the starch granule may disintegrate into molecules and fragments. Most of pre-gelatinized starches are made by applying starch slurry to a hot drum and grinding the thin sheet to a ®ne powder. Rehydration of drum-cooked starch at room temperature gives a paste of reduced consistency with a dull, grainy appearance and gels of reduced strength (Rajagopalan & Seib, 1992a). Many methods have been developed to prepare granulated cold water soluble (GCWS) starches, such as heating of starches in aqueous alcohol, high temperature and pressure condi* Corresponding author. Tel.: 191-183-258802; fax: 191-183-258820. E-mail address: [email protected] (N. Singh). 0268-005X/03/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0268-005 X(02)00 036-X
tions and alcoholic-alkaline treatment (Chen & Jane, 1994a; Eastman & Moore, 1984; Rajagopalan & Seib, 1992a,b). The GCWS starches prepared by these methods exhibited different cold water solubility. GCWS starches gives greater viscosity, smoother texture and have more processing tolerance than traditional pregelatinized starches (Light, 1990). Jane and Seib (1991) proposed an alcoholic-alkaline method for preparing GCWS starches which involved the treatment of starch with a mixture of ethanol and alkali to swell starch granules and neutralization with hydrochloric acid (HCl), washing with alcohol and drying. Chen and Jane (1994a) reported solubility ranged between 11.7 and 93.3% for waxy and high amylose maize starches prepared by different alcoholic-alkaline treatments and concluded that the effectiveness of the method depends mainly on the starch variety, concentration of ethanol and sodium hydroxide (NaOH) and reaction temperature. Many workers have used X-ray diffraction, differential scanning calorimetry (DSC), pasting curves and scanning electron microscopy (SEM) to examine GCWS wheat, corn, tapioca, hydroxypropylated and
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J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
cross-linked wheat starches (Chen & Jane, 1994a,b; Jane, Craig, Seib, & Hoseney, 1986; Rajagopalan & Seib, 1992b). Jane et al. (1986) observed the distorted shape of starch granules compared to native granules after treatment with aqueous alcohol. Chen and Jane (1994a) observed that the granules of GCWS maize starches prepared by alcoholicalkaline treatment had indented appearance and were larger than those of the original native granules. Light microscopy revealed that GCWS starches prepared from wheat, corn, tapioca and hydroxy propylated cross linked starch by propan-1,2-diol treatment had fractured internally but contained an intact shell at the surface (Rajagopalan & Seib, 1992b). Chen and Jane (1994b) studied the pasting behaviour of waxy and normal GCWS maize starches using viscoamylograph and concluded that these starches had viscosity higher than those of their native counterparts. Potato starch is used for its characteristics, which differ signi®cantly from those of starch from other plant sources (Madsen & Christensen, 1996). Identi®cation of native starch sources is required for desired functionality and unique properties (Duxbury, 1989). Barichello and Yada (1991) suggested that paste characteristics and other physicochemical properties of starches vary with genotype and cultural practices. Hopkins and Gormley (2000) reported the rheological properties of pastes and gels made from starch isolated from different Irish potato cultivars. Kim, Wiesenborn, Orr, and Grant (1995) and Wiesenborn, Orr, Casper, and Tacke (1994) reported the paste behaviour from various potato genotypes and correlated the physicochemical characteristics with functional properties. The objectives of the present work were to compare morphological, thermal and rheological properties of GCWS starch prepared from corn and potato starches. 2. Material and methods
starches. Starches (100 g, dwb) were dispersed in 1100 g of ethanol solution (40% w/w) in a 3000 ml beaker and placed on rotary shaker. NaOH solution (2.5 M, 200 g) was weighed and added in the starch±ethanol slurry during shaking at the rate of 5 g/min. The reaction temperature was maintained at 25 8C. The mixture was then allowed to rest for 10 min. Additional 900 g of ethanol solution (40% w/w) was added and the slurry was shaked for 10 min and left at room temperature (25 8C) until the starch settled down. The clear supernatant was separated and another 500 g ethanol solution (40% w/w) was added and neutralized with HCl (2 M in absolute ethanol). The neutralized starch solution was washed with ethanol solutions of 60, 95% and ®nally with 100% concentration. The dehydrated starch was then dried at 50 8C in an air oven. The dried starch was grounded and passed through no: 100 (BSS) sieve and stored in air tight containers at room temperature. 2.3. Amylose content Amylose content of the native and GCWS corn and potato starches was determined by the method given by Williams, Kuzina, and Hlynka (1970). 2.4. Cold water solubility (%) The method of Eastman and Moore (1984) was slightly modi®ed to determine cold water solubility of GCWS corn and potato starches. A 100 ml (1%) suspension of GCWS starch was shaked thoroughly for 30 min on a rotary shaker. The starch suspension was transferred to a 250 ml centrifuge bottle and centrifuged at 1200 £ g for 10 min. A 25 ml aliquot of the supernatant was taken in a preweighed aluminum moisture dish and dried in an air oven at 110 8C for 4 h. The cold water solubility (as swollen, non-sedimented granules as soluble material) was calculated as CWS% grams of solid in supernatant £ 4/grams of sample £ 100.
2.1. Materials
2.5. Scanning electron microscopy
The potatoes of four cultivars, viz. Kufri Chandermukhi, Kufri Badshah, Kufri Jyoti and Kufri Sindhuri were procured from Sangha Potato Farms, Jalandhar, India from 2000 harvest. Uniform sized potatoes were selected from each cultivar before starch isolation. Corn starch was supplied by Sukhjit Starch Ltd, Phagwara (India). Analytical grade sodium hydroxide and hydrochloric acid was procured from CDH, Bombay (India) and ethanol was obtained from Hayman Ltd, Essex (UK).
Scanning electron micrographs of native and GCWS starches were obtained with a scanning electron microscope (Jeol JSM-6100, Jeol Ltd, Tokyo, Japan). Starch samples were sprinkled on double stick tape ®xed on an alminium stub, and the starch was coated with gold±palladium (60:40).
2.1.1. Potato starch isolation Potato starch was isolated as described earlier (Singh & Singh, 2001). 2.2. Preparation of granular cold water soluble starches The method of Chen and Jane (1994a) with slight modi®cation was used to prepare GCWS corn and potato
2.6. Differential scanning calorimetry Thermal properties of native and GCWS corn and potato starches were analysed using DSC-821 e (Mettler Toledo, Switzerland) equipped with a thermal analysis data station. Starch (3.5 mg, dwb) was weighed into a 40 ml capacity aluminium pan (Mettler, ME-27331) and distilled water was added with the help of Hamilton microsyringe to achieve a starch±water suspension containing 70% water. Samples were hermetically sealed and allowed to stand for 1 h at room temperature before heating in DSC. The DSC
J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
65
Table 1 Amylose content and cold water solubility of native and GCWS corn and potato starches (values with similar superscripts in column do not differ signi®cantly (p , 0.05); GCWS, granulated cold water soluble) Starch source
Amylose content (%) Native
Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
cd
28.1 29.0 d 31.6 e 26.6 bc 22.1 a
Cold water solubility (%) GCWS 25.2 d 23.3 b 27.8 e 24.2 c 20.1 a
analyser was calibrated using indium and an empty aluminum pan was used as reference. Sample pans were heated at a rate of 10 8C/min from 20 to 100 8C for potato starches and 20 to 120 8C for corn starch. Onset temperature (To); peak temperature (Tp); conclusion temperature (Tc) and enthalpy of gelatinization (DHgel) were calculated. 2.7. Rheological properties A small amplitude oscillatory rheological measurement was made for GCWS corn and potato starches, with a dynamic rheometer (Carri-Med CSL 2-100, TA Instruments Ltd, Surrey, England) equipped with 1.598 steel cone geometry (4 cm dia). The strain was set at 1.5%. The GCWS starch samples were subjected to frequency sweep testing with a range of 0.1±20 Hz at different temperatures. The dynamic rheological properties such as storage modulus (G 0 ), loss modulus (G 00 ), loss factor Tan d and dynamic viscosity (h 0 ) were determined for GCWS corn and potato starches. GCWS starch slurries of 20% (w/w) concentration were loaded on the ram of rheometer and covered with a thin layer of low-density silicone oil (to minimize evaporation losses). 2.8. Turbidity Turbidity of native and GCWS corn and potato starches was measured as described by Perera and Hoover (1999). An aqueous suspension (2%) of native starch from corn and potato starches was heated in a boiling water bath for 1 h with constant stirring. The suspension was cooled for 1 h at 30 8C. The GCWS starches were dispersed in water and mixed thoroughly. The samples were stored for 72 h at 4 8C in a refrigerator and turbidity was determined every 24 h by measuring absorbance at 640 nm against water blank with a Shimadzu UV-1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). 2.9. Statistical analysis The data reported in all the tables are average of triplicate observations. The data were subjected to statistical analysis using Minitab Statistical Software (Minitab Inc., USA).
85.2 b 93.0 d 93.8 d 89.4 c 63.0 a
3. Results and discussion 3.1. Amylose content The amylose content of native potato starches ranged between 25.5 and 31.6% while corn starch had amylose content of 22.1%. Among the potato starches, Kufri Jyoti potato starch had the highest amylose content whereas Kufri Sindhuri potato starch had the lowest. Kim et al. (1995) and Wiesenborn et al. (1994) also reported similar ranges of amylose content for starches from American potato cultivars. Corn starch showed lower amylose content as compared to all potato starches. GCWS corn and potato starches had lower amylose content than their native starches (Table 1). The amylose content of GCWS potato starches ranged between 23.31 and 27.8% while GCWS corn starch had amylose content of 20.14%. The amylose has been reported to leach out during preparation of GCWS starches by alcoholic-alkaline treatment (Chen & Jane, 1994a; Jane et al., 1986). Jane et al. (1986) suggested that the amylose intertwine with amylopectin to prevent dispersion of starch granules during heating in aqueous solution. Among the GCWS potato starches, Kufri Jyoti potato starch had the highest amylose content while Kufri Sindhuri potato starch showed the lowest. The difference in the amylose content of GCWS corn and potato starches might be due to the difference in the amylose content of their native starches and granule morphology which in turn depends on their biological origin (Svegmark & Hermansson, 1993). The extent of dropdown in amylose content is higher in potato starches than corn starch during preparation of GCWS starches. This may be attributed to the fragile nature of the potato starch granules (Singh, Singh, & Saxena, 2002). 3.2. Cold water solubility (%) The cold water solubility of starch prepared by alcoholicalkaline treatment represents mainly swollen, non-sedimented granules as soluble material and depends on the starch source and granular structure. Cold water solubility of GCWS corn and potato starches differed signi®cantly. The cold water solubility of GCWS corn starch was 63% and ranged between 85.2 and 93.8% for GCWS potato starches
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Fig. 1. (a) SEM of native corn and potato starches (A) Kufri Chandermukhi, (B) Kufri Badshah, (C) Kufri Jyoti, (D) Kufri Sindhuri, and (E) corn ( £ 400, Bar 10 mm); (b) SEM of GCWS corn and potato starches (A) Kufri Chandermukhi, (B) Kufri Badshah, (C) Kufri Jyoti, (D) Kufri Sindhuri, and (E) corn ( £ 400, Bar 10 mm); (c) SEM showing granular indentation and fragmentation in GCWS starches (A) GCWS Kufri Badshah, and (B) GCWS Kufri Jyoti ( £ 1200, Bar 10 mm).
(Table 1). The lower cold water solubility of GCWS corn starch could be attributed to the more rigid structure of the corn starch granules. Potato starch granules are fragile in nature (Singh et al., 2002) and the alcoholic-alkaline treatment may have affected potato starch to the greater extent as compared to corn starch during preparation of GCWS starches. The potato starch granules swelled much more during the treatment, therefore the resulting GCWS potato
starches had greater cold water solubilities. Kufri Badshah and Kufri Jyoti potato GCWS starches showed the highest cold water solubility among GCWS potato starches. SEM results showed that their native starches contained large oval and irregular or cubiodal shaped granules in fairly large number that may have swelled to greatest extent. The differences among cold water solubilities of GCWS potato starches may be attributed to the differences in granular
J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
67
Fig. 1. (continued)
structure and amylose content. Amylopectin branch chain length may also affect the cold water solubility of different starches (Chen & Jane, 1994a). 3.3. Scanning electron microscopy The granular structure of native corn and potato starches showed signi®cant variation in size and shape when viewed by SEM (Fig. 1a). The granule size ranged between 15 and
20 mm for small and 20±45 mm for large granules in potato starch. The size of corn starch granules was ranged between 5 and 7 mm for small and 15±18 mm for large granules. The native potato starch granules were observed to be smooth, oval and irregular or cuboidal shaped. Native corn starch granules appeared to be less smooth, rounded and angular shaped. Kufri Jyoti potato starch showed larger oval and few small granules while the Kufri Badshah potato starch showed large irregular or cuboidal shaped granules in fairly
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J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72 Table 2 Differential scanning calorimetry thermal properties of native corn and potato starches (To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; R, gelatinization range (Tc 2 To); DHgel, enthalpy of gelatinization (dwb, based on starch weight). Values with similar superscripts in column do not differ signi®cantly (p , 0.05)) Cultivar Potato (Kufri Chandermuki) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
To (8C) bc
60.27 59.72 a 59.86 ab 60.70 c 69.82 d
Tp (8C) a
63.39 63.45 a 63.26 a 64.58 b 74.87 c
Tc (8C) a
67.28 68.35 c 67.66 ab 70.34 d 78.1 d
DHgel (J/g) 12.55 b 13.85 d 13.68 d 13.38 c 10.92 a
during alcoholic-alkaline treatment of maize starch. All the GCWS starch granules showed internal fragmentation, which clearly indicates the swelling, and shrinkage in that region of the granule (Fig. 1c). 3.4. Differential scanning calorimetry
Fig. 1. (continued)
large number. The variation in the size and shape in starch granules may be due to the biological origin (Svegmark & Hermansson, 1993). The morphology of starch granules depends on the biochemistry of the chloroplast or amyloplast, as well as physiology of the plant (Badenhuizen, 1969). The size and shape of the GCWS corn and potato starches differed from their native starches (Fig. 1b). The effect of alcholic-alkaline treatment was observed to be brought more changes in potato starch than corn starch. The potato starch granules swelled more than corn starch granules. The alcoholic-alkaline treatment caused indentation of potato starch granules. The indentation of potato starches separated from different cultivars differed signi®cantly. Kufri Badshah and Kufri Jyoti potato GCWS starches showed higher indentation in granules while it was lower for Kufri Chandermukhi and Kufri Sindhuri potato GCWS starches. These differences may be attributed to the variation in number of large and small granules in starches. SEM pictures of GCWS starches clearly reveals that small starch granules were less affected by alcoholicalkaline treatment than large starch granules. Chen and Jane (1994a) also observed more swelling of large granules
Native corn and potato starches showed endothermic peaks between 60 and 80 8C. The transition temperatures (To; Tp; and Tc), and enthalpies of gelatinization (DHgel) of starches from native corn and potato starches differ signi®cantly (Table 2). Corn starch showed higher transition temperatures and lower DHgel than potato starches. These differences in potato and corn starch may be attributed to the differences in the granular structure, amylose content and gelatinization temperature of starches (Singh et al., 2002; Singh & Singh, 2001). Among the native potato starches, Kufri Badshah potato starch showed highest DHgel value (13.85 J/g) and Kufri Chandermukhi potato starch showed the lowest DHgel value (12.55 J/g). Kufri Sindhuri potato starch had highest To (60.70 8C) followed by Kufri Chandermukhi potato starch (60.27 8C) while it was lowest for Kufri Badshah potato starch (59.72 8C). Kim et al. (1995) also reported similar ranges of transition temperatures and enthalpies of gelatinization for starches from 42 American potato cultivars. Double helical and crystalline structures are disrupted in starches during gelatinization. This order±disorder phase transition showed melting of crystals which was illustrated by DSC endotherms in the range of 50±70 8C for various native starches (Jacobs, Eerlingen, Clauwaert, & Delcour, 1995). A higher enthalpy for Kufri Badshah potato starch may be attributed to the presence of higher percentage of irregular or cuboidal shaped and large granules while Kufri Chandermukhi potato starch, which contained small and oval granules, may be responsible for its low DHgel. The starch from potato cultivars having smaller starch granules showed lower DHgel and vice versa. The GCWS corn and potato starches did not show any gelatinization endotherm when heated between 20 and 100 8C. The absence of gelatinization endotherm after alcoholic-alkaline treatment during preparation of GCWS maize starches has been reported earlier (Chen & Jane, 1994a). This con®rmed the changed nature of the
J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
69
Fig. 2. (a) Storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 40 8C; (b) storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 50 8C; and (c) storage modulus (G 0 ), loss modulus (G 00 ), dynamic viscosity (h 0 ) and loss factor (Tan d) of GCWS corn and potato starches at 60 8C.
GCWS corn and potato starch granules during alcoholicalkaline treatment. 3.5. Rheological properties The rheological parameters like G 0 , G 00 , Tan d, and h 0 showed signi®cant variation among GCWS corn and potato starches when subjected to frequency testing ranged from 0.1 to 20 Hz at different temperatures (Fig. 2a±c). The G 0 , G 00 , increased while h 0 decreased with the increase in frequency. GCWS corn starch showed lower G 0 , G 00 and h 0 values than GCWS potato starches (Table 3). This may be attributed to more rigid nature of corn starch granules (Singh et al., 2002). Among the GCWS potato starches, Kufri Badshah potato starch showed the highest G 0 , G 00 and h 0 while these were lowest for Kufri Sindhuri potato starch. These differences may be due to the differences in granular size and shape of the native starches. Kufri Badshah potato native starch contained large irregular and cuboidal granules and very few or negligible small granules. Kufri Jyoti potato native starch also contains large oval granules in fairly large number. The large starch granules are likely to be more indented during alcoholic-alkaline treatment (Chen & Jane, 1994a). The native Kufri Badshah and Kufri Jyoti potato native starches had lowest To and higher DHgel values while starches from other potato culti-
vars showed higher transition temperatures and lower DHgel values (Table 2). The native starches with lower transition temperature might be more susceptible to alcoholic-alkaline treatment as compare to starches with high transition temperature. High transition temperatures in native starches have been reported to result from a high degree of crystallinity, which provided structural stability and made the granule more resistant to gelatinization (Barichello, Yada, Cof®n, & Stanley, 1990). This may possibly affect the swelling of starch granules during alcoholic-alkaline treatment that resulted in the variation in rheological properties of GCWS starches. The native starches from Kufri Chandermukhi and Kufri Sindhuri potato cultivars contained a large number of small granules which may be responsible for low G 0 and G 00 and h 0 . Tan d of GCWS corn and potato starches decreased with increase in frequency. GCWS corn starch exhibited highest Tan d value of 0.2751. Kufri Badshah and Kufri Jyoti GCWS potato starches showed lowest Tan d values of 0.0766 and 0.081, respectively, which was highest (0.091) for Kufri Sindhuri GCWS starch (Table 3). Higher G 0 , G 00 , h 0 and lower Tan d values for Kufri Badshah and Kufri Jyoti potato GCWS starches suggested that these starches formed a more rigid gel structure as compare to gels from other potato GCWS starches. The G 0 , G 00 , h 0 of the GCWS corn and potato starches increased with the increase in temperature while the Tan d decreased during
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Fig. 2. (continued)
J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
71
Table 3 Rheological properties of GCWS corn and potato starches at different temperatures (at angular frequency of 125.7 rad/s) Starch
Temperature (8C)
G 0 (Pa)
G 00 (Pa)
Tan d
h 0 (Pa s)
Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn Potato (Kufri Chandermukhi) Potato (Kufri Badshah) Potato (Kufri Jyoti) Potato (Kufri Sindhuri) Corn
40 40 40 40 40 50 50 50 50 50 60 60 60 60 60
27 510 40 102 35 430 24 890 1950 79 880 15 1900 90 150 68 390 5386 366 900 425 100 387 500 348 500 32 590
7867 11 188 10 097 7342 6025 11 486 20 340 12 520 10 668 1573 30 840 32 562 31 387 31 713 8966
0.286 0.279 0.285 0.295 0.321 0.1438 0.1339 0.1389 0.156 0.2922 0.082 0.0766 0.081 0.091 0.2751
58.6 94 81 50.8 5.2 91 153 110 82 10.62 177 237 218 148 71
frequency sweep testing (Fig. 2a±c). The GCWS starches swelled more in water with increase in temperature as compared to their counterpart native starches. The starch granules also swelled to greater extent with increase in reaction temperature during preparation of GCWS starches with alcoholic-alkaline treatment (Chen & Jane, 1994a). Lancaster and Conway (1968) also reported an exponential relationship between temperature and the swelling rate of
starch granules in NaOH solution. Rajagopalan and Seib (1992b) suggested that the indented GCWS starch granules have an empty cavity inside the helices which left after the drying of alcohol. This empty cavity may provide ambient space to form bonding between starch and water molecules. This bonding may be facilitated with increase in temperature, which ultimately led to an increase in G 0 , G 00 , h 0 and decrease in Tan d. The reduction in water activity in GCWS starch±water system may also be responsible for the increase in G 0 and G 00 . The gelatinization temperature has been reported to increase in aqueous alcohol and starch± water systems which may occurred due to changes in water structure, reduction in water activity and speci®c solute± starch interactions in starch granule (Rajagopalan & Seib, 1992a). The rheological properties of the native starches have been reported to depend on granular structure, amylose to amylopectin ratio, presence of phosphate esters (Singh & Singh, 2001; Wiesenborn et al., 1994). These factors may possibly have affected the rheological properties during preparation of GCWS starches by alcoholic-alkaline treatment. 3.6. Turbidity
Fig. 3. (a) Effect of storage conditions on the turbidity of native corn and potato starches; and (b) Effect of storage conditions on the turbidity of GCWS corn and potato starches.
The turbidity value of gelatinized starch suspension from the native corn and potato starches measured as absorbance at 640 nm differed signi®cantly (Fig. 3a). The GCWS starches showed higher turbidity values than their native starches (Fig. 3b). This may be due to the altered nature of starch granules and leaching out of amylose during alcoholic-alkaline treatment. Native corn starch showed higher turbidity values than native potato starches which may be attributed to the difference in granular rigidity of these starches. Among the GCWS potato starches, Kufri Chandermukhi and Kufri Sindhuri potato starch suspension showed higher turbidity values. Starches from the potato cultivars having larger size granules showed lower turbidity values while those having smaller size granules showed
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J. Singh, N. Singh / Food Hydrocolloids 17 (2003) 63±72
higher turbidity values (Singh & Singh, 2001). Factors such as granule swelling, granule remnants, leached amylose and amylopectin, amylose and amylopectin chain lengths, intra or intermolecular bonding, lipids, cross-linking and substitution have been reported to be responsible for turbidity development in native starches during storage (Jacobson, Obanni, & BeMiller, 1997). The turbidity of the native and GCWS starches increased with increase in storage period. The increase in turbidity values was very little in GCWS potato starches and almost negligible for corn starch. The slow increase in turbidity values of GCWS starches during storage may be attributed to the low amylose content, altered nature of the granules, low granular remnants and less aggregation of amylose and amylopectin molecules. 4. Conclusion Alcoholic-alkaline treatment brought substantial changes in physico-chemical, morphological, thermal and rheological properties in corn and potato starches during preparation of GCWS starches. This treatment caused indentation and distortion in granular structure in both corn and potato starches, however, potato starches were found to be more susceptible to these changes. The properties of GCWS starches were found to depend on the native granule morphology. The larger the starting granule population, the greater the amylose lost and subsequent swelling of the treated granules. GCWS potato starches showed higher G 0 , G 00 , h 0 and lower Tan d than GCWS corn starch. The rheological properties of GCWS potato starches were found to vary with source of native starch. G 0 , G 00 , h 0 of GCWS starches increased and Tan d decreased with the increase in temperature. GCWS corn and potato starches showed lower turbidity values than their native counterpart starches during storage at 4 8C. Acknowledgements We thank Dr S.K. Saxena, Director, Food Research and Analysis Center, New Delhi for providing us the facility for rheological studies. References Badenhuizen, N. P. (1969). The biogenesis of starch granules in higher plants, New York: Appleton Crofts. Barichello, V., & Yada, R. Y. (1991). Starch properties of various potato (Solanum tuberosum L.) cultivars susceptible and resistant to low temperature sweetening. Journal of the Science of Food and Agriculture, 56, 385±397.
Barichello, V., Yada, R. Y., Cof®n, R. H., & Stanley, D. W. (1990). Low temperature sweetening in susceptible and resistant potatoes: starch structure and composition. Journal of Food Science, 54, 1054±1059. Chen, J., & Jane, J. (1994a). Preparation of granular cold-water soluble starches by alcoholic-alkaline treatment. Cereal Chemistry, 71, 618± 622. Chen, J., & Jane, J. (1994b). Properties of granular cold-water soluble starches prepared by alcoholic-alkaline treatment. Cereal Chemistry, 71, 623±626. Duxbury, D. D. (1989). Modi®ed starch functionalitiesÐno chemicals or enzymes. Food Processing, 50, 35±37. Eastman, J. E., & Moore, C. O. (1984). Cold water soluble granular starch for gelled food compositions. US Patent, 4,465,702. Hopkins, S., & Gormley, R. (2000). Rheological properties of pastes and gels made from starch separated from different potato cultivars. Lebensmittel-Wissenchaft und-Technologie, 33, 388±396. Jacobs, H., Eerlingen, R. C., Clauwaert, W., & Delcour, J. A. (1995). In¯uence of annealing on the pasting properties of starches from varying botanical sources. Cereal Chemistry, 72, 480±487. Jacobson, M. R., Obanni, M., & BeMiller, J. N. (1997). Retrogradation of starches from different botanical sources. Cereal Chemistry, 74, 571± 578. Jane, J., Craig, S. A. S., Seib, P. A., & Hoseney, R. C. (1986). Characterization of granular cold water soluble starch. Starch, 38, 258±263. Jane, J., & Seib, P. A. (1991). Preparation of granular cold-water swelling/ soluble starches by alcoholic-alkaline treatments. US Patent, 5057,157. Kim, S. Y., Wiesenborn, D. P., Orr, P. H., & Grant, L. A. (1995). Screening potato starch for novel properties using differential scanning calorimetry. Journal of Food Science, 60, 1060±1065. Lancaster, E. B., & Conway, H. F. (1968). Alkali sorption and swelling of starch. Cereal Science Today, 13, 248±253. Light, J. M. (1990). Modi®ed food starches: Why, where, and how. Cereal Foods World, 35, 1081±1084. Madsen, M. H., & Christensen, D. H. (1996). Changes in viscosity properties of potato starch during growth. Starch, 48, 245±249. Perera, C., & Hoover, R. (1999). In¯uence of hydroxypropylation on retrogradation properties of native, defatted and heat-moisture treated potato starches. Food Chemistry, 64, 361±375. Rajagopalan, S., & Seib, P. A. (1992a). Granular cold-water soluble starches prepared at atmospheric pressure. Journal of Cereal Science, 16, 13±28. Rajagopalan, S., & Seib, P. A. (1992b). Properties of granular cold-water soluble starches prepared at atmospheric pressure. Journal of Cereal Science, 16, 29±40. Singh, J., & Singh, N. (2001). Studies on the morphological, thermal and rheological properties of starch separated from some Indian potato cultivars. Food Chemistry, 75, 67±77. Singh, J., Singh, N., & Saxena, S. K. (2002). Effect of fatty acids on the rheological properties of corn and potato starch. Journal of Food Engineering, 52, 9±16. Svegmark, K., & Hermansson, A. M. (1993). Microstructure and rheological properties of composites of potato starch granules and amylose: A comparison of observed and predicted structure. Food Structure, 12, 181±193. Wiesenborn, D. P., Orr, P. H., Casper, H. H., & Tacke, B. K. (1994). Potato starch paste behaviour as related to some physical/chemical properties. Journal of Food Science, 59, 644±648. Williams, P. C., Kuzina, F. D., & Hlynka, I. (1970). A rapid colorimetric procedure for estimating the amylose content of starches and ¯ours. Cereal Chemistry, 47, 411±420.