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Influence of molecular structural characteristics on pasting and thermal properties of acid-methanol-treated rice starches
Food Hydrocolloids 26 (2012) 441e447
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Influence of molecular structural characteristics on pasting and thermal properties of acid-methanol-treated rice starches Jheng-Hua Lin a, Ciao-Ling Pan b, Harinder Singh b, Yung-Ho Chang b, * a b
Department of Hospitality Management, MingDao University, 369 Wen-Hwa Road, Peetow 52345, Taiwan Department of Food and Nutrition, Providence University, 200 Chung-Chi Road, Shalu 43301, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2010 Accepted 24 November 2010
Rice starches from TKW1, TNG67 and TCS17 varieties, differing widely in amylose contents (0.1, 18.3 and 29.2%) were treated at 45 C for 1 h in methanol containing various amounts of HCl. The recovery, pasting properties, thermal behaviors, molecular size and chain length distribution of starch were observed. Starches exhibited widely different pasting and thermal behavior upon acid-methanol treated (AMT). Degradation of starches upon AMT affected the leaching extent and chain length of amylose. No obvious changes were found on chain length and content of chain fractions of amylopectin. The pasting viscosity of rice starch decreased with increasing concentration of HCl, and the pasting profiles depended on the variety of rice. The pasting profile of AMT-TNG67 starch showed a two-step increasing pattern during heating, while TKW1 and TCS17 starches showed smoothly increasing pasting curves. The relationship between pasting patterns of AMT-TNG67 starches with amylose leaching and two stages of swelling behavior of starch granules was investigated. Results indicated that the pasting of starch granules depend on the amount, as well as the chain length, of amylose in granules. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Rice starch Pasting properties Amylose leaching Chain length Acidemethanol treatment
1. Introduction Starch is composed of two polysaccharides amylose and amylopectin, the ratio of these two depends upon botanical origin. Starch is the main component of rice grains. Gelatinization, morphology characteristics of starches from rice vary widely depending on environmental and genetic factors. Starch in its native form suffers from high retrogradation and low shear stress resistance. Therefore, starch is usually modified physically, chemically and enzymatically to reduce shear breakdown and retrogradation. Acid hydrolysis has been used to modify starch granule structure and produce “soluble starch” for many years. Industrially, acidmodified starch is prepared with dilute HCl or H2SO4 at 25e55 C for various reaction durations. Although the viscosity or fluidity of acid-hydrolyzed starch varies with the conditions used during modification, the yield of modified starch consistently decreases with increase in acid concentration. Acid-alcohol treatment is hydrolysis of starch in the presence of alcohol, which has high recovery of starch and uses less amount of acid than that of starch hydrolyzed by acid in water (Ma & Robyt, 1987). Gelatinization of
* Corresponding author. Tel.: þ886 4 2632 8001x15302; fax: þ886 4 2653 0027. E-mail address: [email protected] (Y.-H. Chang). 0268-005X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2010.11.016
starch is disruption of molecular order revealed in irreversible changes in granular properties such as swelling, pasting crystallite melting, loss of birefringence, uncoiling and dissociation of the double helices, solubility (Singh, Singh, Kaur, Sodhi, & Gill, 2003). Wang, Truong, and Wang (2003) observed little changes in amylopectin chain length distributions, gelatinization and retrogradation characteristics of corn starch after treatment in different concentrations of HCl in aqueous medium. Jayakody and Hoover (2002) reported faster hydrolysis of waxy maize starch than normal maize with 2.2 N aqueous HCl. Vandeputte, Vermeylen, Geeroms, and Delcour (2003a) observed dependence of the rice starch gelatinization and swelling behavior on absolute, free and lipid-complexed amylose content and amylopectin chain length distributions. In our previous paper (Chang, Lin, & Pan, 2010), we observed that hydrochloric acid caused maximum degradation in molecular weights of rice starches in methanol than H2SO4 and HNO3. We also found out that degradation extent in starches increased with higher acid concentrations and amylose content of starches influenced the degradation in significant way. However, the effect of different concentrations of hydrochloric acid on pasting and thermal characteristics of rice starches was not reported previously. The present study was aimed at investigation on relationship of chain lengths of amylose and amylopectin with the functional properties of acid-methanol-treated starch.
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447
2. Materials and methods 2.1. Materials Polished rice kernels of TKW1, a waxy rice, and TNG67, a japonica rice, were obtained from Agricultural Research Institute of Taiwan (COA), Wufong, Taichung, Taiwan. Polished rice kernels of TCS17, an indica rice, were obtained from Taichung District Agricultural Research and Extension Station (COA), Datsun, Changhua, Taiwan. All reagents used were of analytical grade. 2.2. Methods 2.2.1. Isolation of starch The starch was isolated by an alkaline-steeping method (Yang, Lai, & Lii, 1984) with some modification. Polished rice kernels (2 kg) was steeped overnight in 5 L of 0.1% NaOH solution, then the supernatant was decanted and the kernels were washed with fresh 0.1% NaOH solution. After washed, the kernels were milled with 10 L of 0.1% NaOH solution by using of a Stone Wet-Mill (CL-010, Ladyship, Taoyuan, Taiwan). The slurry was diluted to 35 L and poured into a glass container (30 cm in diameter and 60 cm in height). After standing for 10 min, the slurry separated into three layers. The top and bottom layers were yellow in color, which were impurity layers. The middle layer was starch layer, which was siphoned out. The impurity layers were collected to repeat the diluting with 0.1% NaOH solution, standing, and siphoning procedures until the starch layer was clear. The collected starch layers were centrifuged (10,000 g) by using of a continuous phase centrifuge (T1A, Sharples, Warminster, PA, USA). The precipitate was suspended in distilled water, neutralized with 0.1% HCl, then the slurry was repeatedly washed and centrifuged until the absence of salt, detecting by 1% AgNO3, in the supernatant. The precipitate was washed with 95% ethanol, air-oven dried at 40 C, then ground and passed through a 100-mesh sieve. The moisture content of the isolated TKW1, TNG67, and TCS17 rice starch was 8.9%, 10.7%, and 9.1%, respectively. Amylose content of the isolated native starch, as determined from its iodine affinity value (BeMiller, 1964), was 0.1%, 18.3% and 29.2% for TKW1, TNG67, and TCS17 starch, respectively. 2.2.2. Acid-methanol treatment Acid-methanol-treated starches were prepared following the procedures of Chang et al. (2010). The recovery of rice starch granules after acid-methanol treatment ranged from 95.4 to 98.2%. Acid-methanol-treated starches are indicated with the abbreviation AMT and the subscripts with the abbreviation indicate the concentration of acid used in the treatment. 2.2.3. Average molecular weight and chain length distribution The molecular weight distribution and chain length distribution of starch after debranched by isoamylase were determined by highperformance size-exclusion chromatography (HPSEC) following the procedures of Chang, Lin, and Chang (2006). 2.2.4. Blue value, lmax and amylose leaching Blue value and lmax of starches was determined using the method as described in Chang et al. (2006). The solution prepared as above was scanned for different wavelengths. Amylose leaching at 80 C of undefatted and defatted starches was determined with the method reported in Jayakody and Hoover (2002).
35 C for 1.5 min, heated to 95 C at a rate of 6 C/min, maintained at 95 C for 5 min, then cooled to 35 C at a rate of 6 C/min, and maintained at 35 C for 5 min. Paddle speed was set at 960 rpm for the first 10 s and then 160 rpm for the rest of the analysis. 2.2.6. Light microscopy Starch suspension was heated by RVA, and paste samples were collected at 75 C and 95 C. Paste samples were stained with 0.2% I2/KI solution and observed under light microscope (BX-41, Olympus, Tokyo, Japan). 2.2.7. Thermal properties Thermal properties of starch during heating were determined by using a differential scanning calorimeter (DSC 2910, TA Instruments, Surrey, England). Starch sample (about 2.5 mg, db) was weighed in the sample pan, mixed with distilled water (about 7.5 mg), and sealed. The sample pans were heated at a rate of 10 C/ min from 25 to 140 C. Gelatinization temperatures and gelatinization enthalpy change were quantified. 3. Results and discussion 3.1. Average molecular weight and chain length distribution Weight-average degree of polymerization (DPw) of AMT starches decreased with increasing concentration of acid (Fig. 1). Degradation extent of TNG67 was observed to be higher than two other rice starches. This may be due to preferential hydrolysis of long chains of starch by acid in methanol. Amylose disrupts the crystalline structure of amylopectin and hence presence of longer chains may have resulted into more imperfect crystallites. All the three starches showed a little degradation extent with 25 mN of HCl used. However, TKW1 and TCS17 starches showed almost similar degradation with 25 and 50 mN of HCl used in methanol (Chang et al., 2010). Acid degradation is dependant upon factors like amylose-lipid complexes, weakness of crystallinity, branching extent of amylopectin and presence and packing of double helices in crystallites (Jayakody & Hoover, 2002; Wang et al., 2003). The HPSEC profile of debranched TNG67 and TCS17 starches showed trimodal profile. The first fraction (f1) of HPSEC profile corresponds to amylose, and the second (f2) and third (f3) fraction to the long and short chains of amylopectin, respectively. While TKW1 starch showed bimodal chain length distribution profile with fractions corresponding to long chain (f2) and short chain (f3) of 1e+7 TKW1 TNG67 TCS17
Mw (log DPw)
442
1e+6
1e+5
1e+4 0
2.2.5. Pasting properties Pasting properties of starch was determined by using of a Rapid Visco-Analyzer (RVA 3Dþ, Newport Scientific, Warriewood, Australia). Each starch suspension (7%, w/w, 28 g total weight) was equilibrated at
25
50
75
100
Acid concentration (mN) Fig. 1. Plot of the log value of weight-average degree of polymerization (DPw) against the concentration of acid used in the treatment.
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much different. However, undefatted AMT-TNG67 and AMT-TCS17 starches showed higher lmax than their native counterparts. The lmax of starch indicates degree of polymerization (DP) and average chain length of amylose and amylopectin (Banks, Greenwood, & Khan,1971; Fales, 1980). Fujita, Yamamoto, Sugimoto, Morita, and Yamamori (1998) reported lmax from 594 to 604 nm for non-waxy and from 524 to 534 nm for waxy wheat starches. Waxy rice starch TKW1 showed lmax of 522 nm and upon defatting it decreased to 512 nm. Defatted AMT-TKW1 starches showed lower lmax than undefatted AMT-TKW1 starches, while the defatted AMT-TNG67 and AMT-TCS17 starches showed higher lmax than undefatted one. This difference in behavior of starches can be attributed to the amylose content and branching architecture. Defatted TNG67 and TCS17 starches showed lmax above 600 nm which indicates higher amylose content or longer chains in amylopectin. lmax showed positive correlations (r ¼ 0.718, p < 0.001; r ¼ 0.933, p < 0.001) with leaching extent of amylose and blue value of starches. Chain lengths of amylose and short chains of amylopectin also affected lmax (r ¼ 0.808, p < 0.001; r ¼ 0.786, p < 0.001). Chang et al. (2006) observed lmax of 527 and 615 nm respectively for waxy and normal corn starches. Sokhey and Chinnaswamy (1993) observed lmax of 520 nm for amylopectin, branched or short chain fractions and 630 nm for amylose, linear or longbranched chain fractions. Branching architecture was thus observed to show major changes only in TNG67 starches which was also confirmed from HPSEC results (Table 1). However we also inferred that high molecular weights chains of both amylose and amylopectin got degraded in the starches mentioned. Native TNG67 and TCS17 starches showed amylose leaching to the extent of 1.9 and 2.3%, respectively. Amylose leached to greater extent upon AMT in both starches. Amylose leaching ranged from 9.2 to 12.6% and from 7.7 to 13.2% for AMT-TNG67 and AMT-TCS17 starches, respectively. Leaching extent was lower in TCS17 starch although its amylose content was much higher than TNG67. This finding on leaching is consistent with our observations on change in lmax of these starches. Amylose leaching showed significant positive correlations with blue value (r ¼ 0.708, p < 0.001) and short chains of amylopectin (r ¼ 0.832, p < 0.001). Amylose leaching depends upon many factors like rigidity of starch granules, amylose chain length, amylose-lipid complex formation, molecular weight and temperature. Extent of solubilization of starches was found to be in the order of TKW1 > TNG67 > TCS17 starch with
amylopectin. Table 1 summarizes the weight percentage and average chain length (CLw) for each fraction of the HPSEC profiles for starches studied. The percentage and CLw of AMT-TKW1 were similar to those of the native counterpart. After acid-methanol treatment, the CLw of f1 of TNG67 and TCS17 starches decreased with increasing acid concentration. No obvious changes on the content and CLw of f3 of TNG67 and TCS17 starches after treatment were found. The branch chain length distribution of amylopectin was not much affected as degradation of high molecular weight amylopectin produced the lower molecular weight fractions which remain with f2 and f3 in HPSEC elution. Similar results of HPSEC profile of corn starch amylopectin chains after degradation with HCl have been reported by Wang et al. (2003). The results indicated that amylose of starch was preferentially degraded during the condition used in acid-methanol treatment of this study. 3.2. Blue value, lmax and amylose leaching Blue value for defatted and undefatted TCS17 starch was found to be the highest among the three rice starches studied (Table 2). The difference between blue values of defatted and undefatted native rice starches was found to be the highest in TNG67 followed by TKW1. This difference relates to the amylose-lipid complex content of starches. All the starches showed lower blue value on AMT. The decrease in blue value was dependent upon concentration of acid used. The decrease in blue value of defatted starches upon AMT was higher in TCS17 starch, which may be due to preferential degradation of long chains of starch, essentially amylose, with acid. It was also observed that blue value of undefatted AMT-TKW1 and AMTTNG67 starches was higher than their native counterparts, which is due to the solubilization of lipids in methanol during AMT. On the other hand, the decrease of blue values for AMT75- and AMT100TCS17 starches was due to the significantly shortening of the chain length (Table 1) after treated at high concentration of acid. Blue value showed significant positive correlations with chain lengths of amylose and short chains of amylopectin (r ¼ 0.757, p < 0.001; r ¼ 0.649, p < 0.001). However the weight proportions of short and long chains of amylopectin affected blue value negatively (r ¼ 0.954, p < 0.001; r ¼ 0.997, p < 0.001). The maximum starcheiodine absorbance (lmax) was observed to be the highest for TCS17 followed by TNG67 starch. Generally defatted TKW1 and TNG67 starches showed lower lmax upon AMT, but not
Table 1 Weight percentage and weight-average chain length (CLw) of HPSEC fractions of isoamylase-debranched native and acid-methanol-treated rice starches. Starch
Distribution (%)
CLw
f1
f2
TKW1 Native AMT25a AMT50 AMT75 AMT100
e e e e e
23.7 23.5 23.2 23.5 23.1
0.7 0.4 0.4 0.3 0.2
76.3 76.5 76.8 76.5 76.9
TNG67 Native AMT25 AMT50 AMT75 AMT100
17.8 17.6 17.3 16.6 16.6
0.1 0.2 0.5 0.4 0.3
19.0 18.3 19.3 19.1 20.3
0.4 0.5 0.2 0.3 0.6
63.2 64.1 63.4 64.3 63.1
TCS17 Native AMT25 AMT50 AMT75 AMT100
27.9 29.0 27.7 28.2 27.6
0.4 0.1 0.2 0.7 0.3
18.4 17.4 18.3 18.3 18.4
0.1 0.1 0.2 0.3 0.1
53.7 53.6 53.9 53.5 54.0
a
f3
f1
f2
0.7 0.4 0.4 0.3 0.2
e e e e e
62.9 62.4 61.9 62.6 62.3
0.8 0.5 0.3 0.2 0.3
18.0 18.2 18.1 18.2 18.1
0.0 0.2 0.1 0.1 0.0
0.6 0.6 0.6 0.1 0.6
3241 2344 987 813 770
73 85 32 22 22
64.6 64.6 66.8 66.4 68.2
0.4 0.7 0.5 0.4 0.5
18.0 18.0 17.9 17.9 17.9
0.1 0.0 0.0 0.1 0.1
0.5 0.1 0.2 0.9 0.3
2252 1813 1674 1400 1285
35 8 42 65 39
65.4 63.2 65.4 66.2 65.6
0.3 0.1 0.1 0.5 0.4
18.3 18.2 18.3 18.3 18.3
0.0 0.1 0.0 0.0 0.0
AMT stands for acidemethanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
f3
444
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Table 2 Blue value, lmax and amylose leaching of native and acidemethanol-treated rice starches. Starch
lmax (nm)
Blue value Undefatted
Defatted
Undefatted
Amylose leaching (%, db)a Defatted
Based on starch
Based on amylose
0.0 0.3 0.8 0.5 0.6
e e e e e
e e e e e
0.9 0.5 0.6 0.5 0.6
1.9 9.2 10.3 11.5 12.6
10.3 52.3 59.0 68.4 76.1
0.8 1.0 0.6 0.3 0.3
2.3 7.7 10.5 11.7 13.2
8.0 26.7 37.8 42.7 48.2
TKW1 Native AMT25b AMT50 AMT75 AMT100
0.078 0.105 0.100 0.101 0.091
0.003 0.003 0.002 0.002 0.002
0.148 0.129 0.125 0.124 0.123
0.001 0.011 0.002 0.001 0.000
522.2 520.2 519.2 519.7 519.3
0.3 0.3 0.3 0.3 0.6
512.0 509.2 508.8 509.0 508.8
TNG67 Native AMT25 AMT50 AMT75 AMT100
0.269 0.426 0.421 0.419 0.413
0.012 0.003 0.005 0.009 0.004
0.477 0.460 0.452 0.449 0.434
0.002 0.002 0.002 0.003 0.006
581.7 602.5 602.0 602.0 602.3
0.3 0.0 0.0 0.0 0.3
609.0 611.0 609.2 603.5 603.7
TCS17 Native AMT25 AMT50 AMT75 AMT100
0.656 0.659 0.653 0.638 0.633
0.008 0.007 0.005 0.002 0.009
0.713 0.662 0.680 0.668 0.666
0.021 0.002 0.003 0.003 0.003
607.0 610.2 608.8 609.0 608.3
0.0 0.8 0.3 0.5 0.8
615.3 615.3 614.8 610.8 610.8
a b
Standard deviation < 3.0%. AMT stands for acid-methanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
decreasing molecular weight on AMT (Chang et al., 2010). TNG67 showed higher degradation extent and smaller CLw of amylose (f1) than TCS17 starches on AMT (Table 1). The weight proportion of long and short chains of amylopectin affected the amylose leaching negatively (r ¼ 0.746, p < 0.001; r ¼ 0.724, p < 0.001). Positive correlation between weight proportion of amylose and amylose leaching was observed (r ¼ 0.562, p < 0.05). Amylose leaching can be attributed to the degradation extent of starch, weight proportion and chain length of amylose, as well as, amylose-amylose and amylose-amylopectin interactions. 3.3. Pasting property Pasting viscosity profiles of native and AMT rice starches as determined by RVA are shown in Fig. 2. Among the three rice starches, waxy starch TKW1 showed higher peak viscosity (PV) and breakdown (BD) than other two starches. PV largely depends upon amylose, friction between swollen granules and amylopectin content. Vandeputte, Vermeylen, Geeroms, and Delcour (2003b) showed that amylose played no role in swelling of rice starches in the temperature range of 55e85 C. These authors, however, showed lower PV for waxy rice starches and attributed this to low rigidity of waxy rice starch granules. Han and Hamaker (2001) did not observe any significant correlation of amylopectin architecture with PV of RVA result. Mua and Jackson (1997) observed higher PV for amylopectin fraction isolated from corn starch than amylose fractions. Higher BD in starches can be attributed to higher crystallinity and lower amylose content (Singh, Inouchi, & Nishinari, 2006). Vandeputte et al. (2003b) observed that BD of normal rice starches was stabilized by amylose content. Han and Hamaker (2001) reported that long (DP > 100) and short amylopectin chains (average DP ¼ w17) were negatively and positively correlated with BD respectively. They attributed this to two possible reasons: (1) involvement of the long amylopectin chains in more than one cluster and hence causing fewer tendencies to be dispersed because of entanglement with other amylopectin molecules, and (2) increase in the gyration radius of amylopectin molecules because of higher proportion of long chains which affects increase in viscosity. AMT starches showed decrease in PV, BD, FV (final viscosity) and SB (setback) for all the three rice varieties. This decrease in pasting parameters was observed to become severe as higher concentration
of acid was used. AMT-TKW1 starches showed lower pasting parameters than counterpart TNG67 and TCS17 starches. Among the AMT starches, AMT100-TKW1 showed the lowest PV, FV, BD and SB. PV of AMT rice starches ranged from 33 to 635 cP, 268 to 1072 cP, and 562 to 1819 cP for TKW1, TNG67 and TCS17, respectively. The decrease in PV of starches on AMT may be attributed to hydrolysis of amorphous regions and production of low molecular weight dextrins. The greater decrease in pasting parameters of waxy rice starch on AMT over other two starches suggests loose packing of double helices of amylopectin in amorphous region which allows easy penetration of acid. But in normal rice starch, both amylose and amylopectin might have been affected although the reaction duration was short, ie. 1 h. Wang et al. (2003) reported that high molecular weight fractions of both amylose and amylopectin got affected in corn starches with treatment in HCl in water. SB is indirect measurement of retrogradation of starches. SB of AMT rice starches varied between 45e243 cP, 128e854 cP and 344e1287 cP for TKW1, TNG67 and TCS17, respectively. SB showed significant positive correlations with chain lengths of amylose and weight proportion of amylose chains (r ¼ 0.697, p < 0.001; r ¼ 0.585, p < 0.05). SB was affected negatively with weight proportions of short and long chains of amylopectin (r ¼ 0.519, p < 0.05; r ¼ 0.542, p < 0.05). Singh, Sodhi, and Singh (2009) and Thirathumthavorn and Charoenrein (2005) observed lower SB for sorghum starch after treatment with 2.2 N HCl and attributed this to Newtonian behavior of gel and insufficient time for starch molecules to align themselves. FV was observed to be 3985, 2486 and 1761 cP for native TCS17, TNG67 and TKW1 starch, respectively. FV depends upon leaching of amylose and granule architecture. Peak temperature for native TKW1 and TCS17 starches were observed to be 66.7 and 79.1 C, respectively. While peak temperature of AMT rice starches from TKW1 and TCS17 ranged between 66.7e67.9 C and 68.8e72.1 C, respectively. Vandeputte et al. (2003b) did not observe any correlation between amylopectin chain length distribution and pasting parameters except peak temperature. We also observed a positive correlation of peak temperature with long chains of amylopectin and amylose chains (r ¼ 0.518, p < 0.05; r ¼ 0.639, p < 0.05). Pasting properties of starch have been reported to be influenced by amylose, amyloseelipid complex, branching architecture of amylopectin and the ratio of amylose/amylopectin (Tester & Morrison, 1990).
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445
this study the granule size of native or AMT-TNG67 starch ranged from 2.10 to 8.75 mm, which is a very narrow range. Therefore, distribution of granular size should not have significant effect on the stepwise pasting profile. The stepwise pasting curve can be related to two stages of swelling which might have been affected by amylose leaching behavior. The difference of pasting behaviors of AMT-TNG67 and AMT-TCS17 starches may be due to differences in chain length and content of amylose (f1) fraction present in these starches (Table 1). The stepwise pasting profile of TNG67 starch could also be attributed to variety of factors such as amylose content, amylose chain length, heterogeneity of crystallites amyloseeamylopectin interactions, melting of amyloseelipid helices and amylose leaching. 3.4. Morphology To further probe the two-step pasting behavior of AMT-TNG67 starches light microscopy was used to observe the native and AMTTNG67 starch granules after heated in RVA to 75 C (critical point temperature) and 95 C (Fig. 3). At 75 C, native starch had a higher degree of swelling than AMT starches. With increase in acid concentration, the content of broken granules increased and formed more small granules or fractions. This reflected acid-methanol treatment caused some granules to easily swell and rupture. This observation is consistent with the increase in extent of amylose leaching and BD upon AMT. The content of swelled granules or broken starch granules heated to 95 C was lesser than those at 75 C. This may be due to increase solubilization of starch granules upon heating. The role of proportion of continuous phase, rigidity and shear resistance of starch granules in determining pasting behavior of AMT-TNG67 starches versus their native counterparts was evident from the micrographs. 3.5. Gelatinization thermal properties
Fig. 2. Pasting profiles of native (__) and AMT rice starches: AMT25 (C), AMT50 (:), AMT75 (-) and AMT100 (;). AMT stands for acid-methanol-treated starch and the subscripts stand for the concentration of acid used in the treatment.
An interesting observation was recorded for pasting curves of AMT-TNG67 starches. Pasting curve of native TNG67 showed smoothly increasing viscosity from pasting temperature to peak temperature, but AMT-TNG67 starches showed two-step increase in viscosity (Fig. 2). The first step increased from pasting temperature to temperature of critical point (w75 C), and the second step from the critical temperature to peak temperature. Therefore, two slopes could be obtained from the increasing viscosity profiles of AMT-TNG67 starches. The slopes of the first increasing profiles were comparable for the starches treated with different acid concentrations, however, the slopes of the second increasing profiles decreased with increasing concentration of acid. Native or AMT-TKW1 and TCS17 starches had smooth increase in viscosity; they didn’t show stepwise increase in viscosity. The phenomenon of two-step increase in viscosity has also been observed for canna and sweet potato starches (Thitipraphunkul, Uttapap, Piyachomkwan, & Takeda, 2003). A slight shoulder pasting profile reflected the heterogeneous nature of the canna starch sample. Broad size distribution of canna starch granules (10e100 mm) was responsible for the un-coincident of starch gelatinization. The same is true for sweet potato starch. However, in
The gelatinization properties of native and acid-methanoltreated starches as measured on DSC are presented in Table 3. AMT starches showed lower gelatinization onset (To) and peak (Tp) temperatures than counterpart native starches. Waxy starch TKW1 showed higher To, Tp, gelatinization conclusion temperature (Tc), temperature range (R) and gelatinization enthalpy change (DHgel) than other two starches. TKW1, TNG67 and TCS17 native starches showed Tc, of 78.1, 76.5 and 74.8 C, respectively. Sasaki et al. (2009) showed Tc, between 76.7 and 88.3 C for three waxy rice starches. AMT rice starches showed Tc in the range of 79.4e80.2, 75.3e76.6 and 75.8e77.5 C for TKW1, TNG67 and TCS17, respectively. While, the AMT rice starches recorded R ranging from 19.6 to 20.6, 16.7 to 17.9, and 17.2 to 18.9 C for TKW1, TNG67 and TCS17, respectively. AMT starches showed higher R values than their native counterparts which indicate increasing crystallite heterogeneity. AMT-TKW1 and AMT-TNG67 starches showed lower DHgel than counterpart native starches in contrast to AMT-TCS17 starches. Gelatinization temperatures and DHgel showed significant negative correlation with amylose leaching, lmax, blue value, and weight proportion and chain length of amylose and long chain of amylopectin. Noda et al. (1998) attributed the low To, Tp and Tc to short amylopectin chains. Short chains of amylopectin facilitate gelatinization as observed by Vandeputte et al. (2003a) for rice starches. These authors observed that amylose plays no role in swelling of rice starches in the temperature range between 55 and 85 C. Biliaderis, Page, Maurice, and Juliano (1986) observed a decrease in gelatinization enthalpies (DHgel) of waxy and non-waxy rice starches with amylose content. AMT resulted into shortening of amylose and amylopectin chains. DHgel indicates loss of molecular order rather than loss of crystallinity (Cooke & Gidley, 1992).
446
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447
Fig. 3. Micrographs of native and AMT-TNG67 rice starches after heated to 75 C and 95 C in RVA. AMT stands for acid-methanol-treated starch and the subscripts stand for the concentration of acid used in the treatment.
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447 Table 3 Gelatinization thermal propertiesa of native and acid-methanol-treated rice starches. Starch
To ( C)
Tp ( C)
Tc ( C)
R ( C)
DH (J/g)
TKW1 Native AMT25b AMT50 AMT75 AMT100
60.1 59.8 59.7 59.6 59.7
68.1 67.7 67.5 67.6 68.1
78.1 79.4 79.6 79.6 80.2
18.0 19.6 20.0 20.0 20.6
14.6 14.6 14.5 14.4 13.9
TNG67 Native AMT25 AMT50 AMT75 AMT100
60.3 58.4 58.9 58.6 58.7
66.5 64.5 65.2 64.9 64.9
76.5 76.3 76.6 75.3 75.9
16.2 17.9 17.7 16.7 17.3
12.9 12.6 12.3 12.0 12.0
TCS17 Native AMT25 AMT50 AMT75 AMT100
59.8 58.6 58.7 58.6 58.6
65.0 64.3 64.3 64.4 64.3
74.8 75.9 76.5 75.8 77.5
15.1 17.3 17.7 17.2 18.9
11.1 11.2 11.4 11.5 11.3
a To, Tp, Tc and R stand for the onset, peak, conclusion and temperature range of gelatinization, respectively, and DH stands for enthalpy change of gelatinization. Standard deviations are &0.5 C and &0.3 J/g for parameters of temperature and enthalpy change, respectively. b AMT stands for acid-methanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
Gelatinization parameters depend upon variety of factors such as side chain architecture of amylopectin, extent of co-crystallization of amylose with amylopectin chains, amount of absolute and lipidcomplexed amylose, granule shape, architecture and sizes and phosphate groups (Morrison, Tester, Snape, Law, & Gidley, 1993; Singh et al., 2003; Tester, 1997). 4. Conclusions This study showed the importance of amylose leaching, weight percentage and chain lengths of amylose and amylopectin in determining the pasting and thermal behavior of native and AMT starches. The decrease of pasting viscosities of AMT starches resulted from the degradation of starch molecules. The two-step pasting pattern of AMT-TNG67 starch was attributed to the insufficient amount of amylose in starch granules, as well as the shortening in chain length of amylose after AMT. TCS17 starch, with high amylose content, showed a smooth pasting curve and lower pasting temperature after AMT as compared to the counterpart native starch. The amount, as well as the chain length, of amylose in starch granules can result into two stages of swelling. We propose that AMT can be used to study fine structure of starch chains and interactions between them. This study can be used to predict the behavior of starches from various botanical sources upon different modifications like oxidation and acid hydrolysis. Acknowledgment We thank the National Science Council, Taiwan, for financial support (NSC 95-2313-B-126-007-MY3).
447
References Banks, W., Greenwood, C. T., & Khan, K. M. (1971). The interaction of linear, amylose oligomers with iodine. Carbohydrate Research, 17, 25e33. BeMiller, J. N. (1964). Iodimetric determination of amylose. In R. L. Whistler, R. J. Smith, J. N. BeMiller, & M. L. Wolfrom (Eds.), Methods in carbohydrate chemistry, Vol. IV (pp. 165e168). New York: Academic Press. Biliaderis, C. G., Page, C. M., Maurice, T. J., & Juliano, B. O. (1986). Thermal characterisation of rice starches: a polymeric approach to phase transitions of granular starch. Journal of Agricultural and Food Chemistry, 34, 6e14. Chang, Y. H., Lin, J. H., & Chang, S. Y. (2006). Physicochemical properties of waxy and normal corn starches treated in different anhydrous alcohols with hydrochloric acid. Food Hydrocolloids, 20, 332e339. Chang, Y. H., Lin, J. H., & Pan, C. L. (2010). Type and concentration of acid on solubility and molecular size of acid-methanol-treated rice starches differing in amylose content. Carbohydrate Polymers, 79, 762e768. Cooke, D., & Gidley, M. J. (1992). Loss of crystalline and molecular order during starch gelatinization: origin of the enthalpic transition. Carbohydrate Research, 227, 103e112. Fales, F. W. (1980). The linear relationship between iodine staining and average chain length of the unbranched amyloglucans. Biopolymers, 19, 1535e1542. Fujita, S., Yamamoto, H., Sugimoto, Y., Morita, N., & Yamamori, M. (1998). Thermal and crystalline properties of waxy wheat (Triticum aestivum L.) starch. Journal of Cereal Science, 27, 1e5. Han, X.-Z., & Hamaker, B. R. (2001). Amylopectin fine structure and rice starch paste breakdown. Journal of Cereal Science, 34, 279e284. Jayakody, L., & Hoover, R. (2002). The effect of lintnerization on cereal starch granules. Food Research International, 35, 665e680. Ma, P. O., & Robyt, J. F. (1987). Preparation and characterization of soluble starches having different molecular sizes and composition, by acid hydrolysis in different alcohols. Carbohydrate Research, 166, 283e297. Morrison, W. R., Tester, R. F., Snape, C. E., Law, R., & Gidley, M. J. (1993). Swelling and gelatinisation of cereal starches. IV. Some effects of lipid-complexed amylose and free amylose in waxy and normal barley starches. Cereal Chemistry, 70, 385e391. Mua, J. P., & Jackson, D. S. (1997). Fine structure of corn amylose and amylopectin fractions with various molecular weights. Journal of Agricultural and Food Chemistry, 45, 3840e3847. Noda, T., Takahata, Y., Sato, T., Suda, I., Morishita, T., Ishiguro, K., et al. (1998). Relationships between chain length distribution of amylopectin and gelatinization properties within the same botanical origin for sweet potato and buckwheat. Carbohydrate Polymers, 37, 153e158. Sasaki, T., Kohyama, K., Suzuki, Y., Okamoto, K., Noel, T. R., & Ring, S. G. (2009). Physicochemical characteristics of waxy rice starch influencing the in vitro digestibility of a starch gel. Food Chemistry, 116, 137e142. Singh, N., Inouchi, N., & Nishinari, K. (2006). Structural, thermal and viscoelastic characteristics of starches separated from normal, sugary and waxy maize. Food Hydrocolloids, 20, 923e935. Singh, N., Singh, J., Kaur, L., Sodhi, N. S., & Gill, B. S. (2003). Morphological, thermal and rheological properties of starches from different botanical sources. Food Chemistry, 81, 219e231. Singh, H., Sodhi, N. S., & Singh, N. (2009). Structure and physicochemical properties of acid thinned sorghum starch. International Journal of Food Properties,12, 713e725. Sokhey, A. S., & Chinnaswamy, R. Q. (1993). Chemical and molecular properties of irradiated starch extrudates. Cereal Chemistry, 70, 260e268. Tester, R. F. (1997). Starch: the polysaccharide fractions. In P. J. Frazier, P. Richmond, & A. M. Donald (Eds.), Starch, structure and functionality (pp. 163e171). London, UK: Royal Society of Chemistry. Tester, R. T., & Morrison, W. R. (1990). Swelling and gelatinization of cereal starches. I. Effect of amylopectin, amylose, and lipids. Cereal Chemistry, 67, 551e557. Thirathumthavorn, D., & Charoenrein, S. (2005). Thermal and pasting properties of acid-treated rice starches. Starch, 57, 217e222. Thitipraphunkul, K., Uttapap, D., Piyachomkwan, K., & Takeda, Y. (2003). A comparative study of edible canna (Canna edulis) starch from different cultivars. Part I. Chemical composition and physicochemical properties. Carbohydrate Polymers, 53, 317e324. Vandeputte, G. E., Vermeylen, R., Geeroms, J., & Delcour, J. A. (2003a). Rice starches. I. Structural aspects provide insight into crystallinity characteristics and gelatinisation behaviour of granular starch. Journal of Cereal Science, 38, 61e68. Vandeputte, G. E., Vermeylen, R., Geeroms, J., & Delcour, J. A. (2003b). Rice starches. II. Structural aspects provide insight into swelling and pasting properties. Journal of Cereal Science, 38, 53e59. Wang, Y. J., Truong, V. N., & Wang, L. (2003). Structures and rheological properties of corn starch as affected by acid hydrolysis. Carbohydrate Polymers, 52, 327e333. Yang, C. C., Lai, H. M., & Lii, C. Y. (1984). The modified alkaline steeping method for isolation of rice starch. Food Science (Taiwan), 11, 158e162.
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Influence of molecular structural characteristics on pasting and thermal properties of acid-methanol-treated rice starches Jheng-Hua Lin a, Ciao-Ling Pan b, Harinder Singh b, Yung-Ho Chang b, * a b
Department of Hospitality Management, MingDao University, 369 Wen-Hwa Road, Peetow 52345, Taiwan Department of Food and Nutrition, Providence University, 200 Chung-Chi Road, Shalu 43301, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2010 Accepted 24 November 2010
Rice starches from TKW1, TNG67 and TCS17 varieties, differing widely in amylose contents (0.1, 18.3 and 29.2%) were treated at 45 C for 1 h in methanol containing various amounts of HCl. The recovery, pasting properties, thermal behaviors, molecular size and chain length distribution of starch were observed. Starches exhibited widely different pasting and thermal behavior upon acid-methanol treated (AMT). Degradation of starches upon AMT affected the leaching extent and chain length of amylose. No obvious changes were found on chain length and content of chain fractions of amylopectin. The pasting viscosity of rice starch decreased with increasing concentration of HCl, and the pasting profiles depended on the variety of rice. The pasting profile of AMT-TNG67 starch showed a two-step increasing pattern during heating, while TKW1 and TCS17 starches showed smoothly increasing pasting curves. The relationship between pasting patterns of AMT-TNG67 starches with amylose leaching and two stages of swelling behavior of starch granules was investigated. Results indicated that the pasting of starch granules depend on the amount, as well as the chain length, of amylose in granules. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Rice starch Pasting properties Amylose leaching Chain length Acidemethanol treatment
1. Introduction Starch is composed of two polysaccharides amylose and amylopectin, the ratio of these two depends upon botanical origin. Starch is the main component of rice grains. Gelatinization, morphology characteristics of starches from rice vary widely depending on environmental and genetic factors. Starch in its native form suffers from high retrogradation and low shear stress resistance. Therefore, starch is usually modified physically, chemically and enzymatically to reduce shear breakdown and retrogradation. Acid hydrolysis has been used to modify starch granule structure and produce “soluble starch” for many years. Industrially, acidmodified starch is prepared with dilute HCl or H2SO4 at 25e55 C for various reaction durations. Although the viscosity or fluidity of acid-hydrolyzed starch varies with the conditions used during modification, the yield of modified starch consistently decreases with increase in acid concentration. Acid-alcohol treatment is hydrolysis of starch in the presence of alcohol, which has high recovery of starch and uses less amount of acid than that of starch hydrolyzed by acid in water (Ma & Robyt, 1987). Gelatinization of
* Corresponding author. Tel.: þ886 4 2632 8001x15302; fax: þ886 4 2653 0027. E-mail address: [email protected] (Y.-H. Chang). 0268-005X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2010.11.016
starch is disruption of molecular order revealed in irreversible changes in granular properties such as swelling, pasting crystallite melting, loss of birefringence, uncoiling and dissociation of the double helices, solubility (Singh, Singh, Kaur, Sodhi, & Gill, 2003). Wang, Truong, and Wang (2003) observed little changes in amylopectin chain length distributions, gelatinization and retrogradation characteristics of corn starch after treatment in different concentrations of HCl in aqueous medium. Jayakody and Hoover (2002) reported faster hydrolysis of waxy maize starch than normal maize with 2.2 N aqueous HCl. Vandeputte, Vermeylen, Geeroms, and Delcour (2003a) observed dependence of the rice starch gelatinization and swelling behavior on absolute, free and lipid-complexed amylose content and amylopectin chain length distributions. In our previous paper (Chang, Lin, & Pan, 2010), we observed that hydrochloric acid caused maximum degradation in molecular weights of rice starches in methanol than H2SO4 and HNO3. We also found out that degradation extent in starches increased with higher acid concentrations and amylose content of starches influenced the degradation in significant way. However, the effect of different concentrations of hydrochloric acid on pasting and thermal characteristics of rice starches was not reported previously. The present study was aimed at investigation on relationship of chain lengths of amylose and amylopectin with the functional properties of acid-methanol-treated starch.
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447
2. Materials and methods 2.1. Materials Polished rice kernels of TKW1, a waxy rice, and TNG67, a japonica rice, were obtained from Agricultural Research Institute of Taiwan (COA), Wufong, Taichung, Taiwan. Polished rice kernels of TCS17, an indica rice, were obtained from Taichung District Agricultural Research and Extension Station (COA), Datsun, Changhua, Taiwan. All reagents used were of analytical grade. 2.2. Methods 2.2.1. Isolation of starch The starch was isolated by an alkaline-steeping method (Yang, Lai, & Lii, 1984) with some modification. Polished rice kernels (2 kg) was steeped overnight in 5 L of 0.1% NaOH solution, then the supernatant was decanted and the kernels were washed with fresh 0.1% NaOH solution. After washed, the kernels were milled with 10 L of 0.1% NaOH solution by using of a Stone Wet-Mill (CL-010, Ladyship, Taoyuan, Taiwan). The slurry was diluted to 35 L and poured into a glass container (30 cm in diameter and 60 cm in height). After standing for 10 min, the slurry separated into three layers. The top and bottom layers were yellow in color, which were impurity layers. The middle layer was starch layer, which was siphoned out. The impurity layers were collected to repeat the diluting with 0.1% NaOH solution, standing, and siphoning procedures until the starch layer was clear. The collected starch layers were centrifuged (10,000 g) by using of a continuous phase centrifuge (T1A, Sharples, Warminster, PA, USA). The precipitate was suspended in distilled water, neutralized with 0.1% HCl, then the slurry was repeatedly washed and centrifuged until the absence of salt, detecting by 1% AgNO3, in the supernatant. The precipitate was washed with 95% ethanol, air-oven dried at 40 C, then ground and passed through a 100-mesh sieve. The moisture content of the isolated TKW1, TNG67, and TCS17 rice starch was 8.9%, 10.7%, and 9.1%, respectively. Amylose content of the isolated native starch, as determined from its iodine affinity value (BeMiller, 1964), was 0.1%, 18.3% and 29.2% for TKW1, TNG67, and TCS17 starch, respectively. 2.2.2. Acid-methanol treatment Acid-methanol-treated starches were prepared following the procedures of Chang et al. (2010). The recovery of rice starch granules after acid-methanol treatment ranged from 95.4 to 98.2%. Acid-methanol-treated starches are indicated with the abbreviation AMT and the subscripts with the abbreviation indicate the concentration of acid used in the treatment. 2.2.3. Average molecular weight and chain length distribution The molecular weight distribution and chain length distribution of starch after debranched by isoamylase were determined by highperformance size-exclusion chromatography (HPSEC) following the procedures of Chang, Lin, and Chang (2006). 2.2.4. Blue value, lmax and amylose leaching Blue value and lmax of starches was determined using the method as described in Chang et al. (2006). The solution prepared as above was scanned for different wavelengths. Amylose leaching at 80 C of undefatted and defatted starches was determined with the method reported in Jayakody and Hoover (2002).
35 C for 1.5 min, heated to 95 C at a rate of 6 C/min, maintained at 95 C for 5 min, then cooled to 35 C at a rate of 6 C/min, and maintained at 35 C for 5 min. Paddle speed was set at 960 rpm for the first 10 s and then 160 rpm for the rest of the analysis. 2.2.6. Light microscopy Starch suspension was heated by RVA, and paste samples were collected at 75 C and 95 C. Paste samples were stained with 0.2% I2/KI solution and observed under light microscope (BX-41, Olympus, Tokyo, Japan). 2.2.7. Thermal properties Thermal properties of starch during heating were determined by using a differential scanning calorimeter (DSC 2910, TA Instruments, Surrey, England). Starch sample (about 2.5 mg, db) was weighed in the sample pan, mixed with distilled water (about 7.5 mg), and sealed. The sample pans were heated at a rate of 10 C/ min from 25 to 140 C. Gelatinization temperatures and gelatinization enthalpy change were quantified. 3. Results and discussion 3.1. Average molecular weight and chain length distribution Weight-average degree of polymerization (DPw) of AMT starches decreased with increasing concentration of acid (Fig. 1). Degradation extent of TNG67 was observed to be higher than two other rice starches. This may be due to preferential hydrolysis of long chains of starch by acid in methanol. Amylose disrupts the crystalline structure of amylopectin and hence presence of longer chains may have resulted into more imperfect crystallites. All the three starches showed a little degradation extent with 25 mN of HCl used. However, TKW1 and TCS17 starches showed almost similar degradation with 25 and 50 mN of HCl used in methanol (Chang et al., 2010). Acid degradation is dependant upon factors like amylose-lipid complexes, weakness of crystallinity, branching extent of amylopectin and presence and packing of double helices in crystallites (Jayakody & Hoover, 2002; Wang et al., 2003). The HPSEC profile of debranched TNG67 and TCS17 starches showed trimodal profile. The first fraction (f1) of HPSEC profile corresponds to amylose, and the second (f2) and third (f3) fraction to the long and short chains of amylopectin, respectively. While TKW1 starch showed bimodal chain length distribution profile with fractions corresponding to long chain (f2) and short chain (f3) of 1e+7 TKW1 TNG67 TCS17
Mw (log DPw)
442
1e+6
1e+5
1e+4 0
2.2.5. Pasting properties Pasting properties of starch was determined by using of a Rapid Visco-Analyzer (RVA 3Dþ, Newport Scientific, Warriewood, Australia). Each starch suspension (7%, w/w, 28 g total weight) was equilibrated at
25
50
75
100
Acid concentration (mN) Fig. 1. Plot of the log value of weight-average degree of polymerization (DPw) against the concentration of acid used in the treatment.
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447
443
much different. However, undefatted AMT-TNG67 and AMT-TCS17 starches showed higher lmax than their native counterparts. The lmax of starch indicates degree of polymerization (DP) and average chain length of amylose and amylopectin (Banks, Greenwood, & Khan,1971; Fales, 1980). Fujita, Yamamoto, Sugimoto, Morita, and Yamamori (1998) reported lmax from 594 to 604 nm for non-waxy and from 524 to 534 nm for waxy wheat starches. Waxy rice starch TKW1 showed lmax of 522 nm and upon defatting it decreased to 512 nm. Defatted AMT-TKW1 starches showed lower lmax than undefatted AMT-TKW1 starches, while the defatted AMT-TNG67 and AMT-TCS17 starches showed higher lmax than undefatted one. This difference in behavior of starches can be attributed to the amylose content and branching architecture. Defatted TNG67 and TCS17 starches showed lmax above 600 nm which indicates higher amylose content or longer chains in amylopectin. lmax showed positive correlations (r ¼ 0.718, p < 0.001; r ¼ 0.933, p < 0.001) with leaching extent of amylose and blue value of starches. Chain lengths of amylose and short chains of amylopectin also affected lmax (r ¼ 0.808, p < 0.001; r ¼ 0.786, p < 0.001). Chang et al. (2006) observed lmax of 527 and 615 nm respectively for waxy and normal corn starches. Sokhey and Chinnaswamy (1993) observed lmax of 520 nm for amylopectin, branched or short chain fractions and 630 nm for amylose, linear or longbranched chain fractions. Branching architecture was thus observed to show major changes only in TNG67 starches which was also confirmed from HPSEC results (Table 1). However we also inferred that high molecular weights chains of both amylose and amylopectin got degraded in the starches mentioned. Native TNG67 and TCS17 starches showed amylose leaching to the extent of 1.9 and 2.3%, respectively. Amylose leached to greater extent upon AMT in both starches. Amylose leaching ranged from 9.2 to 12.6% and from 7.7 to 13.2% for AMT-TNG67 and AMT-TCS17 starches, respectively. Leaching extent was lower in TCS17 starch although its amylose content was much higher than TNG67. This finding on leaching is consistent with our observations on change in lmax of these starches. Amylose leaching showed significant positive correlations with blue value (r ¼ 0.708, p < 0.001) and short chains of amylopectin (r ¼ 0.832, p < 0.001). Amylose leaching depends upon many factors like rigidity of starch granules, amylose chain length, amylose-lipid complex formation, molecular weight and temperature. Extent of solubilization of starches was found to be in the order of TKW1 > TNG67 > TCS17 starch with
amylopectin. Table 1 summarizes the weight percentage and average chain length (CLw) for each fraction of the HPSEC profiles for starches studied. The percentage and CLw of AMT-TKW1 were similar to those of the native counterpart. After acid-methanol treatment, the CLw of f1 of TNG67 and TCS17 starches decreased with increasing acid concentration. No obvious changes on the content and CLw of f3 of TNG67 and TCS17 starches after treatment were found. The branch chain length distribution of amylopectin was not much affected as degradation of high molecular weight amylopectin produced the lower molecular weight fractions which remain with f2 and f3 in HPSEC elution. Similar results of HPSEC profile of corn starch amylopectin chains after degradation with HCl have been reported by Wang et al. (2003). The results indicated that amylose of starch was preferentially degraded during the condition used in acid-methanol treatment of this study. 3.2. Blue value, lmax and amylose leaching Blue value for defatted and undefatted TCS17 starch was found to be the highest among the three rice starches studied (Table 2). The difference between blue values of defatted and undefatted native rice starches was found to be the highest in TNG67 followed by TKW1. This difference relates to the amylose-lipid complex content of starches. All the starches showed lower blue value on AMT. The decrease in blue value was dependent upon concentration of acid used. The decrease in blue value of defatted starches upon AMT was higher in TCS17 starch, which may be due to preferential degradation of long chains of starch, essentially amylose, with acid. It was also observed that blue value of undefatted AMT-TKW1 and AMTTNG67 starches was higher than their native counterparts, which is due to the solubilization of lipids in methanol during AMT. On the other hand, the decrease of blue values for AMT75- and AMT100TCS17 starches was due to the significantly shortening of the chain length (Table 1) after treated at high concentration of acid. Blue value showed significant positive correlations with chain lengths of amylose and short chains of amylopectin (r ¼ 0.757, p < 0.001; r ¼ 0.649, p < 0.001). However the weight proportions of short and long chains of amylopectin affected blue value negatively (r ¼ 0.954, p < 0.001; r ¼ 0.997, p < 0.001). The maximum starcheiodine absorbance (lmax) was observed to be the highest for TCS17 followed by TNG67 starch. Generally defatted TKW1 and TNG67 starches showed lower lmax upon AMT, but not
Table 1 Weight percentage and weight-average chain length (CLw) of HPSEC fractions of isoamylase-debranched native and acid-methanol-treated rice starches. Starch
Distribution (%)
CLw
f1
f2
TKW1 Native AMT25a AMT50 AMT75 AMT100
e e e e e
23.7 23.5 23.2 23.5 23.1
0.7 0.4 0.4 0.3 0.2
76.3 76.5 76.8 76.5 76.9
TNG67 Native AMT25 AMT50 AMT75 AMT100
17.8 17.6 17.3 16.6 16.6
0.1 0.2 0.5 0.4 0.3
19.0 18.3 19.3 19.1 20.3
0.4 0.5 0.2 0.3 0.6
63.2 64.1 63.4 64.3 63.1
TCS17 Native AMT25 AMT50 AMT75 AMT100
27.9 29.0 27.7 28.2 27.6
0.4 0.1 0.2 0.7 0.3
18.4 17.4 18.3 18.3 18.4
0.1 0.1 0.2 0.3 0.1
53.7 53.6 53.9 53.5 54.0
a
f3
f1
f2
0.7 0.4 0.4 0.3 0.2
e e e e e
62.9 62.4 61.9 62.6 62.3
0.8 0.5 0.3 0.2 0.3
18.0 18.2 18.1 18.2 18.1
0.0 0.2 0.1 0.1 0.0
0.6 0.6 0.6 0.1 0.6
3241 2344 987 813 770
73 85 32 22 22
64.6 64.6 66.8 66.4 68.2
0.4 0.7 0.5 0.4 0.5
18.0 18.0 17.9 17.9 17.9
0.1 0.0 0.0 0.1 0.1
0.5 0.1 0.2 0.9 0.3
2252 1813 1674 1400 1285
35 8 42 65 39
65.4 63.2 65.4 66.2 65.6
0.3 0.1 0.1 0.5 0.4
18.3 18.2 18.3 18.3 18.3
0.0 0.1 0.0 0.0 0.0
AMT stands for acidemethanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
f3
444
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447
Table 2 Blue value, lmax and amylose leaching of native and acidemethanol-treated rice starches. Starch
lmax (nm)
Blue value Undefatted
Defatted
Undefatted
Amylose leaching (%, db)a Defatted
Based on starch
Based on amylose
0.0 0.3 0.8 0.5 0.6
e e e e e
e e e e e
0.9 0.5 0.6 0.5 0.6
1.9 9.2 10.3 11.5 12.6
10.3 52.3 59.0 68.4 76.1
0.8 1.0 0.6 0.3 0.3
2.3 7.7 10.5 11.7 13.2
8.0 26.7 37.8 42.7 48.2
TKW1 Native AMT25b AMT50 AMT75 AMT100
0.078 0.105 0.100 0.101 0.091
0.003 0.003 0.002 0.002 0.002
0.148 0.129 0.125 0.124 0.123
0.001 0.011 0.002 0.001 0.000
522.2 520.2 519.2 519.7 519.3
0.3 0.3 0.3 0.3 0.6
512.0 509.2 508.8 509.0 508.8
TNG67 Native AMT25 AMT50 AMT75 AMT100
0.269 0.426 0.421 0.419 0.413
0.012 0.003 0.005 0.009 0.004
0.477 0.460 0.452 0.449 0.434
0.002 0.002 0.002 0.003 0.006
581.7 602.5 602.0 602.0 602.3
0.3 0.0 0.0 0.0 0.3
609.0 611.0 609.2 603.5 603.7
TCS17 Native AMT25 AMT50 AMT75 AMT100
0.656 0.659 0.653 0.638 0.633
0.008 0.007 0.005 0.002 0.009
0.713 0.662 0.680 0.668 0.666
0.021 0.002 0.003 0.003 0.003
607.0 610.2 608.8 609.0 608.3
0.0 0.8 0.3 0.5 0.8
615.3 615.3 614.8 610.8 610.8
a b
Standard deviation < 3.0%. AMT stands for acid-methanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
decreasing molecular weight on AMT (Chang et al., 2010). TNG67 showed higher degradation extent and smaller CLw of amylose (f1) than TCS17 starches on AMT (Table 1). The weight proportion of long and short chains of amylopectin affected the amylose leaching negatively (r ¼ 0.746, p < 0.001; r ¼ 0.724, p < 0.001). Positive correlation between weight proportion of amylose and amylose leaching was observed (r ¼ 0.562, p < 0.05). Amylose leaching can be attributed to the degradation extent of starch, weight proportion and chain length of amylose, as well as, amylose-amylose and amylose-amylopectin interactions. 3.3. Pasting property Pasting viscosity profiles of native and AMT rice starches as determined by RVA are shown in Fig. 2. Among the three rice starches, waxy starch TKW1 showed higher peak viscosity (PV) and breakdown (BD) than other two starches. PV largely depends upon amylose, friction between swollen granules and amylopectin content. Vandeputte, Vermeylen, Geeroms, and Delcour (2003b) showed that amylose played no role in swelling of rice starches in the temperature range of 55e85 C. These authors, however, showed lower PV for waxy rice starches and attributed this to low rigidity of waxy rice starch granules. Han and Hamaker (2001) did not observe any significant correlation of amylopectin architecture with PV of RVA result. Mua and Jackson (1997) observed higher PV for amylopectin fraction isolated from corn starch than amylose fractions. Higher BD in starches can be attributed to higher crystallinity and lower amylose content (Singh, Inouchi, & Nishinari, 2006). Vandeputte et al. (2003b) observed that BD of normal rice starches was stabilized by amylose content. Han and Hamaker (2001) reported that long (DP > 100) and short amylopectin chains (average DP ¼ w17) were negatively and positively correlated with BD respectively. They attributed this to two possible reasons: (1) involvement of the long amylopectin chains in more than one cluster and hence causing fewer tendencies to be dispersed because of entanglement with other amylopectin molecules, and (2) increase in the gyration radius of amylopectin molecules because of higher proportion of long chains which affects increase in viscosity. AMT starches showed decrease in PV, BD, FV (final viscosity) and SB (setback) for all the three rice varieties. This decrease in pasting parameters was observed to become severe as higher concentration
of acid was used. AMT-TKW1 starches showed lower pasting parameters than counterpart TNG67 and TCS17 starches. Among the AMT starches, AMT100-TKW1 showed the lowest PV, FV, BD and SB. PV of AMT rice starches ranged from 33 to 635 cP, 268 to 1072 cP, and 562 to 1819 cP for TKW1, TNG67 and TCS17, respectively. The decrease in PV of starches on AMT may be attributed to hydrolysis of amorphous regions and production of low molecular weight dextrins. The greater decrease in pasting parameters of waxy rice starch on AMT over other two starches suggests loose packing of double helices of amylopectin in amorphous region which allows easy penetration of acid. But in normal rice starch, both amylose and amylopectin might have been affected although the reaction duration was short, ie. 1 h. Wang et al. (2003) reported that high molecular weight fractions of both amylose and amylopectin got affected in corn starches with treatment in HCl in water. SB is indirect measurement of retrogradation of starches. SB of AMT rice starches varied between 45e243 cP, 128e854 cP and 344e1287 cP for TKW1, TNG67 and TCS17, respectively. SB showed significant positive correlations with chain lengths of amylose and weight proportion of amylose chains (r ¼ 0.697, p < 0.001; r ¼ 0.585, p < 0.05). SB was affected negatively with weight proportions of short and long chains of amylopectin (r ¼ 0.519, p < 0.05; r ¼ 0.542, p < 0.05). Singh, Sodhi, and Singh (2009) and Thirathumthavorn and Charoenrein (2005) observed lower SB for sorghum starch after treatment with 2.2 N HCl and attributed this to Newtonian behavior of gel and insufficient time for starch molecules to align themselves. FV was observed to be 3985, 2486 and 1761 cP for native TCS17, TNG67 and TKW1 starch, respectively. FV depends upon leaching of amylose and granule architecture. Peak temperature for native TKW1 and TCS17 starches were observed to be 66.7 and 79.1 C, respectively. While peak temperature of AMT rice starches from TKW1 and TCS17 ranged between 66.7e67.9 C and 68.8e72.1 C, respectively. Vandeputte et al. (2003b) did not observe any correlation between amylopectin chain length distribution and pasting parameters except peak temperature. We also observed a positive correlation of peak temperature with long chains of amylopectin and amylose chains (r ¼ 0.518, p < 0.05; r ¼ 0.639, p < 0.05). Pasting properties of starch have been reported to be influenced by amylose, amyloseelipid complex, branching architecture of amylopectin and the ratio of amylose/amylopectin (Tester & Morrison, 1990).
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this study the granule size of native or AMT-TNG67 starch ranged from 2.10 to 8.75 mm, which is a very narrow range. Therefore, distribution of granular size should not have significant effect on the stepwise pasting profile. The stepwise pasting curve can be related to two stages of swelling which might have been affected by amylose leaching behavior. The difference of pasting behaviors of AMT-TNG67 and AMT-TCS17 starches may be due to differences in chain length and content of amylose (f1) fraction present in these starches (Table 1). The stepwise pasting profile of TNG67 starch could also be attributed to variety of factors such as amylose content, amylose chain length, heterogeneity of crystallites amyloseeamylopectin interactions, melting of amyloseelipid helices and amylose leaching. 3.4. Morphology To further probe the two-step pasting behavior of AMT-TNG67 starches light microscopy was used to observe the native and AMTTNG67 starch granules after heated in RVA to 75 C (critical point temperature) and 95 C (Fig. 3). At 75 C, native starch had a higher degree of swelling than AMT starches. With increase in acid concentration, the content of broken granules increased and formed more small granules or fractions. This reflected acid-methanol treatment caused some granules to easily swell and rupture. This observation is consistent with the increase in extent of amylose leaching and BD upon AMT. The content of swelled granules or broken starch granules heated to 95 C was lesser than those at 75 C. This may be due to increase solubilization of starch granules upon heating. The role of proportion of continuous phase, rigidity and shear resistance of starch granules in determining pasting behavior of AMT-TNG67 starches versus their native counterparts was evident from the micrographs. 3.5. Gelatinization thermal properties
Fig. 2. Pasting profiles of native (__) and AMT rice starches: AMT25 (C), AMT50 (:), AMT75 (-) and AMT100 (;). AMT stands for acid-methanol-treated starch and the subscripts stand for the concentration of acid used in the treatment.
An interesting observation was recorded for pasting curves of AMT-TNG67 starches. Pasting curve of native TNG67 showed smoothly increasing viscosity from pasting temperature to peak temperature, but AMT-TNG67 starches showed two-step increase in viscosity (Fig. 2). The first step increased from pasting temperature to temperature of critical point (w75 C), and the second step from the critical temperature to peak temperature. Therefore, two slopes could be obtained from the increasing viscosity profiles of AMT-TNG67 starches. The slopes of the first increasing profiles were comparable for the starches treated with different acid concentrations, however, the slopes of the second increasing profiles decreased with increasing concentration of acid. Native or AMT-TKW1 and TCS17 starches had smooth increase in viscosity; they didn’t show stepwise increase in viscosity. The phenomenon of two-step increase in viscosity has also been observed for canna and sweet potato starches (Thitipraphunkul, Uttapap, Piyachomkwan, & Takeda, 2003). A slight shoulder pasting profile reflected the heterogeneous nature of the canna starch sample. Broad size distribution of canna starch granules (10e100 mm) was responsible for the un-coincident of starch gelatinization. The same is true for sweet potato starch. However, in
The gelatinization properties of native and acid-methanoltreated starches as measured on DSC are presented in Table 3. AMT starches showed lower gelatinization onset (To) and peak (Tp) temperatures than counterpart native starches. Waxy starch TKW1 showed higher To, Tp, gelatinization conclusion temperature (Tc), temperature range (R) and gelatinization enthalpy change (DHgel) than other two starches. TKW1, TNG67 and TCS17 native starches showed Tc, of 78.1, 76.5 and 74.8 C, respectively. Sasaki et al. (2009) showed Tc, between 76.7 and 88.3 C for three waxy rice starches. AMT rice starches showed Tc in the range of 79.4e80.2, 75.3e76.6 and 75.8e77.5 C for TKW1, TNG67 and TCS17, respectively. While, the AMT rice starches recorded R ranging from 19.6 to 20.6, 16.7 to 17.9, and 17.2 to 18.9 C for TKW1, TNG67 and TCS17, respectively. AMT starches showed higher R values than their native counterparts which indicate increasing crystallite heterogeneity. AMT-TKW1 and AMT-TNG67 starches showed lower DHgel than counterpart native starches in contrast to AMT-TCS17 starches. Gelatinization temperatures and DHgel showed significant negative correlation with amylose leaching, lmax, blue value, and weight proportion and chain length of amylose and long chain of amylopectin. Noda et al. (1998) attributed the low To, Tp and Tc to short amylopectin chains. Short chains of amylopectin facilitate gelatinization as observed by Vandeputte et al. (2003a) for rice starches. These authors observed that amylose plays no role in swelling of rice starches in the temperature range between 55 and 85 C. Biliaderis, Page, Maurice, and Juliano (1986) observed a decrease in gelatinization enthalpies (DHgel) of waxy and non-waxy rice starches with amylose content. AMT resulted into shortening of amylose and amylopectin chains. DHgel indicates loss of molecular order rather than loss of crystallinity (Cooke & Gidley, 1992).
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Fig. 3. Micrographs of native and AMT-TNG67 rice starches after heated to 75 C and 95 C in RVA. AMT stands for acid-methanol-treated starch and the subscripts stand for the concentration of acid used in the treatment.
J.-H. Lin et al. / Food Hydrocolloids 26 (2012) 441e447 Table 3 Gelatinization thermal propertiesa of native and acid-methanol-treated rice starches. Starch
To ( C)
Tp ( C)
Tc ( C)
R ( C)
DH (J/g)
TKW1 Native AMT25b AMT50 AMT75 AMT100
60.1 59.8 59.7 59.6 59.7
68.1 67.7 67.5 67.6 68.1
78.1 79.4 79.6 79.6 80.2
18.0 19.6 20.0 20.0 20.6
14.6 14.6 14.5 14.4 13.9
TNG67 Native AMT25 AMT50 AMT75 AMT100
60.3 58.4 58.9 58.6 58.7
66.5 64.5 65.2 64.9 64.9
76.5 76.3 76.6 75.3 75.9
16.2 17.9 17.7 16.7 17.3
12.9 12.6 12.3 12.0 12.0
TCS17 Native AMT25 AMT50 AMT75 AMT100
59.8 58.6 58.7 58.6 58.6
65.0 64.3 64.3 64.4 64.3
74.8 75.9 76.5 75.8 77.5
15.1 17.3 17.7 17.2 18.9
11.1 11.2 11.4 11.5 11.3
a To, Tp, Tc and R stand for the onset, peak, conclusion and temperature range of gelatinization, respectively, and DH stands for enthalpy change of gelatinization. Standard deviations are &0.5 C and &0.3 J/g for parameters of temperature and enthalpy change, respectively. b AMT stands for acid-methanol-treated starch and the subscript stands for the concentration of acid used in the treatment.
Gelatinization parameters depend upon variety of factors such as side chain architecture of amylopectin, extent of co-crystallization of amylose with amylopectin chains, amount of absolute and lipidcomplexed amylose, granule shape, architecture and sizes and phosphate groups (Morrison, Tester, Snape, Law, & Gidley, 1993; Singh et al., 2003; Tester, 1997). 4. Conclusions This study showed the importance of amylose leaching, weight percentage and chain lengths of amylose and amylopectin in determining the pasting and thermal behavior of native and AMT starches. The decrease of pasting viscosities of AMT starches resulted from the degradation of starch molecules. The two-step pasting pattern of AMT-TNG67 starch was attributed to the insufficient amount of amylose in starch granules, as well as the shortening in chain length of amylose after AMT. TCS17 starch, with high amylose content, showed a smooth pasting curve and lower pasting temperature after AMT as compared to the counterpart native starch. The amount, as well as the chain length, of amylose in starch granules can result into two stages of swelling. We propose that AMT can be used to study fine structure of starch chains and interactions between them. This study can be used to predict the behavior of starches from various botanical sources upon different modifications like oxidation and acid hydrolysis. Acknowledgment We thank the National Science Council, Taiwan, for financial support (NSC 95-2313-B-126-007-MY3).
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