Effects of Ripening Temperature on Starch Structure and

Mar 17, 2015 - Rice flour from rice grown in the coolest season, consequently, with a higher amylose content, has a lower peak viscosity and higher se...
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Effects of Ripening Temperature on Starch Structure and Gelatinization, Pasting, and Cooking Properties in Rice (Oryza sativa) Areum Chun,*,† Ho-Jin Lee,§ Bruce R. Hamaker,# and Srinivas Janaswamy# †

Rice Research Division, National Institute of Crop Science, Rural Development Administration, Suwon 441-707, Korea Department of Plant Science, Seoul National University, Seoul 151-921, Korea # Department of Food Science, Purdue University, West Lafayette, Indiana 47907, United States §

ABSTRACT: The effect of ripening temperature on rice (Oryza sativa) grain quality was evaluated by assessing starch structure and gelatinization, pasting, and cooking properties. As the ripening temperature increased, the amylose content and number of short amylopectin chains decreased, whereas intermediate amylopectin chains increased, resulting in higher gelatinization temperatures and enthalpy in the starch. These results suggested that an increase in cooking temperature and time would be required for rice grown at higher temperatures. A high ripening temperature increased the peak, trough, and final viscosities and decreased the setback due to the reduction in amylose and the increase in long amylopectin chains. With regard to starch crystallinity and amylopectin molecular structure, the highest branches and compactness were observed at 28/20 °C. Rice that was grown at temperatures above 28/20 °C showed a deterioration of cooking quality and a tendency toward decreased palatability in sensory tests. KEYWORDS: rice, Oryza sativa, amylose, amylopectin, starch, temperature, gelatinization, pasting, cooking

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and amylopectin structure,6 respectively. However, little is known about the effects of ripening temperature on the fine structure and molecular characteristics of amylopectin and on starch crystallinity. The growing conditions can change the pasting and thermal properties of starch gelatinization. Rice flour from rice grown in the coolest season, consequently, with a higher amylose content, has a lower peak viscosity and higher setback value than that from other growing seasons.7 These pasting properties are commonly used to predict the texture of cooked rice. The onset and complete gelatinization temperatures and the enthalpy of endosperm starch grown at 30 °C are higher than those of rice grown at 25 °C.8 Only fragmentary literature is available on the effect of ripening temperature on the gelatinization properties of rice starch, and there is little information on their relationship with the texture, cooking, and eating quality of cooked rice. The present study describes the effects of ripening temperature on starch composition, fine structure, and molecular characteristics, on thermal and pasting properties during gelatinization, and the final impact on eating quality and cooking properties in rice.

INTRODUCTION Rice, Oryza sativa L., a food staple for over half of the world’s population, is mainly consumed in Asia. As living conditions continue to improve in this region of the world, the rice market has begun to focus on improving rice quality. Rice quality may also be affected by climate change. However, the existing literatures on rice quality changes is inadequate to make provisions for variable climate conditions, especially with regard to the effect of high temperature on grain filling and quality. Starch is the major component in rice, comprising approximately 90% of the total weight. Consequently, starch plays an important role in determining the quality of cooked rice and the rice flour end-product. Starch is generally separated into amylose and amylopectin: amylose is a linear chain of (1→ 4)-linked α-D-glucopyranosyl units,1 and amylopectin is a branched polysaccharide composed of thousands of short (1→ 4)-α-D-glucan chains linked to each other by α-(1→6) linkages. 2 Initially, the apparent amylose content was considered the most important factor affecting the eating quality of rice. However, after the introduction of gel permeation chromatography separating methods to starch research and the recognition of the affinity of iodine for the long chain in amylopectin, the focus moved from amylose to amylopectin.3 The chain length distribution of amylopectin was subsequently revealed to have a significant impact on starch properties,4 and amylopectin fine structure plays a critical role in determining rice quality. In terms of the impact of climate change on rice starch, a high temperature during the grain-filling stage decreases the levels of amylose and long-chain-enriched amylopectin.5 The subsequent reduced expression of granule-bound starch synthase I, as well as the activity of the starch-branching enzyme IIb at a high ripening temperature, was considered to be primarily responsible for the changes in amylose content5 © XXXX American Chemical Society

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MATERIALS AND METHODS

Rice Samples. Two rice cultivars (O. sativa L. ssp. japonica cv. Ilpum and Chucheong) were grown at the National Institute of Crop Science, Rural Development Administration in Suwon, South Korea. Pots with three seedlings were transferred 1 day after flowering to phytotrons, which were set at four different temperature conditions. Received: October 11, 2014 Revised: January 14, 2015 Accepted: January 15, 2015

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DOI: 10.1021/jf504870p J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

Table 1. Characteristics of Grain Shape, Grain Weight, and Head Rice Percentage According to Ripening Temperaturea grain size characteristics of milled rice cultivar

a

ripening temperature (°C)

length (mm)

width (mm)

thickness (mm)

length/width

1000-grain weight (g)

head rice (%)

Ilpum

22/14 25/17 28/20 31/23

4.81c 5.12a 4.90b 4.79c

3.01a 3.04a 2.97ab 2.90b

2.09b 2.20a 2.22a 2.03b

1.60b 1.69a 1.65a 1.65a

21.7a 21.5a 20.3b 20.1c

62.5b 82.0a 83.9a 71.0b

Chucheong

22/14 25/17 28/20 31/23

4.84a 4.78a 4.70b 4.81a

2.80b 2.92a 2.81b 2.84b

2.09ab 2.12ab 2.06b 2.14a

1.73a 1.64c 1.68bc 1.69ab

19.8a 19.8a 19.5b 18.9c

77.7c 80.1c 88.7a 84.8b

Values with the same letter in a column are not significantly different according to a Duncan’s multiple-range test (p < 0.05). added to an aliquot (1 mL) of gelatinized polyglucan. Hydrolysis was achieved by the addition of 0.5 μL of isoamylase from Pseudomonas amyloderamosa (Sigma I2758, 1,250,000 units) and incubation at 37 °C with shaking at 150 rpm for 24 h. The homogenate was centrifuged at 15000 rpm for 5 min, and then a 1 mL aliquot was filtered through a 0.2 μm syringe filter (PTFE). The HPAEC system (ICS-3000; Dionex Corp., Sunnyvale, CA, USA) components were an ICS 3000 dual pump, an EO1 eluent organizer, a gold electrode with an AgCl reference electrode, and an ED electrochemical detector. Samples were analyzed with a CarboPac PA1 column (2 × 250 mm) with a CarboPac PA1 guard column (2 × 50 mm). Samples (50 μL) were eluted by a 60 min linear gradient from 100 to 475 mM sodium acetate in 150 mM sodium hydroxide at a flow rate of 0.250 mL/min. Peak area ratios (%) were calculated using Dionex Chromeleon (version 6.80). Gelatinization and Pasting Properties. The gelatinization properties of rice starch were analyzed using a differential scanning calorimeter (DSC) (TA Q1000; TA Instruments, New Castle, DE, USA) following the method of Song et al.15 Rice starch (20 mg) was weighed into aluminum sample pans (TA Instruments) and mixed with 40 μL of distilled water. The pans were sealed and allowed to reach equilibrium for 1 h. Thermal curves were obtained from 30 to 150 °C with a heating rate of 10 °C/min. All DSC analyses were conducted in triplicate. The pasting properties of rice flour were determined in triplicate with a Rapid Visco Analyzer (RVA; Newport Scientific model 3D, Warriewood, NSW, Australia) following AACC method 61-02. X-ray Diffraction. The crystallinity of the rice starch was analyzed according to the method of Han et al.16 Amorphous starch was used to generate a background pattern, as used by Cairns et al.17 X-ray diffraction patterns were obtained using a Philips PW3710 diffractometer with a step width of 0.010° in the 2θ range of 3−38°. Approximately 0.5−1 g of rice flour was firmly packed into an aluminum sample holder. X-ray data were collected at a rate of 3 s/ step, and there were 3501 steps. Cu Kα (λ = 1.5418 Å) radiation was used from a generator operating at 40 kV and 25 mA. Data were analyzed with Philips PC-APD (version 3.6) software. Cooking Properties. Cooking properties were evaluated according to Chun’s method in triplicate.18 First, 8 g of rice grains in a stainless steel mesh container was washed to eliminate surface residue. The container was fixed in a 300 mL tall beaker with 160 mL of distilled water using a glass rod, and these grains were cooked in an electric rice cooker (CR-3020 V; Cuckoo Electronics Co. Ltd., Yangsan, Republic of Korea). After 20 min for equilibrium, the water absorption ratio (w/w) and expansion volume of the cooked rice were measured. The eluate was filtered and dried at 105 °C, and the soluble solid was weighed from it. The pH and iodine index were also measured from filtrate of eluate with 2% iodine solution and using absorbance at 600 nm, respectively. The shape of cooked rice kernels (length, width, and thickness) was measured 40 times using a Mitutoyo Digimatic Caliper. Sensory Evaluation. The sensory properties of cooked rice were evaluated by six trained panels in triplicate.19 A modified preference

The 24 h temperature regimens for day/night during the grain-filling period were 22/14, 25/17, 28/20, and 31/23 °C, changed at 1 h intervals. This 8 °C gap between day and night during the grain-filling period was based on the climate data (30 year average) of 1971−2000 from the Korea Meteorological Administration. The relative humidity in the room was maintained at 70 ± 5%. Rice samples were harvested at an accumulated temperature of 1000 °C from heading, the panicle exsertion. Harvested grain samples were dried to achieve 15% moisture content. The rough rice was dehulled and milled using a rice sheller (Kiya ST-50; Yanmar Co., Ltd., Osaka, Japan) and miller (Pearlest; Kett Electric Laboratory, Tokyo, Japan). Milled rice was kept in sealed bags under refrigeration (4 °C) until analysis. Ripening and Physicochemical Characteristics. Grain size characteristics (e.g., length, width, and thickness) were measured from 10 head rice kernels using a Mitutoyo Digimatic Caliper (CD-15CP; Mitutoyo Corp., Kawasaki, Japan). A 1000-grain weight from each sample was determined randomly in triplicate and weighed separately. The head rice of the milled rice was assessed by using a standard consisting of a kernel or a piece of kernel with its length equal to or greater than three-fourths of the average length of the unbroken kernel and kernels with half or less opaque like the color of chalk and brittle. The amylose contents were tested by two methods, iodine and concanavalin A, to confirm the results. The iodine test was performed according to the method of Juliano,9 and the concanavalin A method was performed by using an amylose/amylopectin assay kit (Megazyme International Ltd. Co., Wicklow, Ireland).10 Protein content was determined by using the Kjeldahl method with the conversion factor for nitrogen to protein of 5.95.11 Hardness was determined with 10 head rice kernels using a TA-XT2 texture analyzer (Stable Micro Systems Ltd., Godalming, Surrey, UK) with a 2 mm probe following the method of Song et al.12 Amylopectin Fine Structure. The molecular structure of amylopectin were determined by using HPSEC-MALLS-RI (highperformance size-exclusion chromatography on a system equipped with multiangle laser-light scattering and refractive index detectors) following Patindol’s method.13 The weight-average molecular weight (Mw), z-average radius of gyration (Rz), and polydispersity (ratio of weight-average and number-average molar masses: Mw/Mn) were determined using HPSEC-MALLS-RI (Wyatt Technology, Santa Barbara, CA, USA), and the density (ρ = Mw/Rz3) and specific volume for gyration (SVg = 2.522 Rz3/Mw) were calculated from these values.13 Amylopectin chain length distribution was determined by highperformance anion-exchange chromatography (HPAEC) with a pulsed-amperometric detector as performed by Nishi et al. with a slight modification.14 The milled rice was ground with a mortar and pestle, and then 5−7 mg of the resulting powder was suspended in 5 mL of methanol and allowed to dissolve for 1 day. The homogenate was centrifuged at 2500g for 5 min. The pelleted polyglucan was washed twice with 5 mL of 90% (v/v) methanol, suspended in 5 mL of distilled water, and then boiled for 3 h. Then, 50 μL of 600 mM sodium acetate buffer (pH 4.4) and 10 μL of 2% (w/v) NaN3 was B

DOI: 10.1021/jf504870p J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry Table 2. Physicochemical Properties of Milled Rice According to Ripening Temperaturea amylose (%) cultivar

a

ripening temperature (°C)

iodine

concanavalin A

protein (%)

hardness (g)

Ilpum

22/14 25/17 28/20 31/23

17.3a 16.4b 15.0c 14.8c

19.1b 20.6a 16.6c 14.6d

6.66b 6.86b 6.90b 7.86a

4433b 5470b 4504b 8249a

Chucheong

22/14 25/17 28/20 31/23

17.4a 16.5b 15.7c 14.7d

22.8a 19.8b 18.9c 15.2d

6.30c 6.79b 7.54a 7.55a

4585b 4131b 4946ab 5705a

Values with the same letter in a column are not significantly different according to a Duncan’s multiple-range test (p < 0.05).

test was used involved scoring the cooked rice quality on a 7-point hedonic scales from −3 to +3. The sensory evaluation test provided information on appearance, aroma, taste, hardness, stickiness, and overall quality of cooked rice.20 Statistical Analysis. Data were processed with Statistical Analysis System software (version 9.1.; SAS Institute Inc., Cary, NC, USA). The data were statistically analyzed using Duncan’s multiple-range test at a probability level of 0.05. Homogeneity of variance was calculated with Levene’s test. General linear model analysis and correlation analysis with Pearson’s correlation coefficient were also performed. Quantification analysis was conducted to evaluate the relative importance of various factors for the textural properties of the cooked rice by using the SAS PROC REG program, with PCORR2 in the model.21 The squared partial correlation coefficients of PCORR2 measure the marginal contribution of one explanatory variable when all others are already included in multiple linear regression models.

the grain-filling rate and decreases the grain-filling duration, decreasing the grain weight.22,25 The head rice yield, considering not only the shape but also the appearance of the grain tissue, was decreased in both cultivars when the grain-filling temperature was higher or lower than 28/20 °C. There is more non-head rice (i.e., chalky, immature, and dead rice) when the grain-filling temperature is higher or lower than the optimum temperature.25 Low temperature causes spikelet sterility, a delay of grain filling, and reductions in yield.6,26,27 The head rice percentage were lowest at 22/14 °C, peaked at 28/20 °C, and re-decreased at 31/23 °C in both IP and CC cultivars. Non-head rice is one of the primary issues in the rice market due to its undesirability to consumers. The palatability of cooked rice is decreased by an increased chalky rice proportion in sensory evaluation tests, even at just a 5% blend of non-head to head rice.28 A high ripening temperature reduces cereal yield by increasing imperfect or empty grains.29 Here, head rice percentage decreased at 31/23 °C ripening temperature in both IP and CC cultivars. IP showed a steeper decrease in head rice than CC at 31/23 °C. The optimum temperature for these cultivars for producing head rice during grain filling has previously been reported: IP and CC show the highest yield and grain filling near 28/20 °C, and IP has a slightly lower optimum temperature (0.6 °C lower) than CC.30 Starch Compositions, Structures, and Crystallinities. With regard to starch composition, amylose content and short amylopectin chains decreased and intermediate amylopectin chains increased with an increased grain-filling temperature (Table 2; Figure 1). The amylose contents of IP and CC grown at the high ripening temperature were clearly decreased in both analyses. As previously reported, the high-temperature-ripened grain showed decreased levels of amylose,7,8,31 which might be due to the repressed expression of granule-bound starch synthase I (GBSSI) at high temperatures, through which the amylose in endosperm of rice grain is synthesized.5,32,33 In the amylopectin chain length profile, the degree of polymerization (DP) for the peak chain length distribution of amylopectin was equal (at 11) in both cultivars. The proportion of short chains clearly reduced as the ripening temperature increased, ranging from DP 5 to 12 for IP and from DP 5 to 13 for CC. On the other hand, the relative amounts of amylopectin intermediate chains, DP 20−30 for IP and DP 18−33 for CC, increased when the ripening temperature increased. However, the longer chains above DP 30 or 33 were unaffected by the maturation temperature in both cultivars. A similar tendency was reported in wheat,34 in which amylopectin branch chains were classified into three groups as the maturation temperature

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RESULTS AND DISCUSSION Grain Characteristics. The weight, shape, appearance, and hardness of the rice grains changed according to the ripening temperature (Table 1). The 1000-grain weight ranged from 20.1 to 21.7 g for Ilpum (IP) and from 18.9 to 19.8 g for Chucheong (CC). Even though the grains of CC weighed less than those of IP in each ripening temperature state, the higher the ripening temperature of the rice, the lower the 1000-grain weight of the rice kernels, with significant differences for both cultivars. These results are consistent with previous studies stating that a high temperature at the grain-filling stage decreases grain weight.5,22 The decreased grain weight in response to a higher ripening temperature is caused by various factors. The decrease in weight seemed to be related to the sink size or kernel shape. The grain size of milled rice depended on cultivar and temperature. Grains grown at a high ripening temperature tend to be shorter and thinner.23 Likewise, the grain shape (i.e., grain length, width, and thickness) of IP was stable until 28/20 °C, but showed a significant reduction in shape at the highest ripening temperature (31/23 °C). On the other hand, the grain size of CC did not noticeably change with ripening temperature, meaning that ripening temperature affected the grain size of IP more than that of CC. Testing the interaction effects (cultivar × ripening temperature) using general linear model analysis revealed a significant interaction effect for length and thickness (p < 0.001) and grain width (p < 0.1). This result also means that there is an interaction effect of both cultivar and ripening temperature on grain size properties. The grain weight is also positively associated with the grain-filling duration and negatively with the grain-filling rate in rice cultivars during ripening.24 Thus, a high ripening temperature slightly increases C

DOI: 10.1021/jf504870p J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

and 1.20−1.24 for IP and CC than the polydispersity at a lower ripening temperature (22/14, 25/17 °C; 1.83−1.96 and 1.66− 1.99 for IP and CC, respectively). These lower polydispersity values meant that amylopectin molecules of rice ripened at a high temperature had a more homogeneous mass distribution than those of rice ripened at a low temperature. On the other hand, amylopectin molecules had the smallest specific volume for gyration (SVg) value when ripened at 28/20 °C, 0.16 and 0.19 for IP and CC, respectively. The specific volume for gyration, the theoretical gyration volume per unit of molar mass, is significantly reduced by branching.35 The Rz, the theoretical probability of finding a molecule at a given distance from the center, showed a similar tendency to SVg. These results revealed that the amylopectin molecule was more highly branched at 28/20 °C than at other ripening temperatures. Similarly, the density (ρ) had the largest values at 28/20 °C, 15.4 and 13.0 g/mol/nm3 for IP and CC, respectively. These results suggest that the appropriate ripening temperature may be around 28/20 °C for amylopectin branches and compactness compared with SVg, Rz, and ρ at lower or higher ripening temperatures. X-ray powder diffraction pattern analysis of the crystallinity of rice flour also showed that the highest compactness and largest crystalline region were at the optimum temperature (28/20 °C; Table 4). X-ray diffraction provides data on the

Figure 1. HPAEC determination of changes in the amylopectin chain length distribution according to ripening temperature: (IP) degree of polymerization of the branched amylopectin of Ilpum; (CC) degree of polymerization of the branched amylopectin of Chucheong.

Table 4. X-ray Diffraction Pattern Analysis of the Crystallinity of Rice Flour According to Ripening Temperature

increased: amylopectin branch chains with DP 6−12 decreased, the proportion of DP 13−34 increased, and those of DP ≥ 35 showed no significant difference. The high-temperature-ripened rice grain was also reported to contain intermediate and longchain-enriched amylopectin, which might be due to the repressed expression of branching enzymes, especially the branching enzyme IIb.5,6 HPSEC-MALLS-RI analysis of the molecular structure of amylopectin revealed a different tendency in response to ripening temperature (Table 3). Amylopectin showed the highest density (ρ) at the optimal temperature for grain filling, suggesting that amylopectin had the highest compactness. The Mw of amylopectin ranged from 1.64 × 108 to 2.20 × 108 g/mol and from 1.52 × 108 to 2.02 × 108 g/mol for IP and CC, respectively, similar to Patindol’s results for rice amylopectin.13 The polydispersity (Mw/Mn), an indication of the narrowness of the distribution,13 of amylopectin grown at higher ripening temperatures (28/20, 31/23 °C) had lower values of 1.15−1.42

ripening temperature (°C)

crystallinity (%)

Ilpum

cultivar

22/14 25/17 28/20 31/23

19.7 21.2 21.6 21.5

Chucheong

22/14 25/17 28/20 31/23

20.5 21.2 25.2 24.3

relative amounts of crystallites and amorphous phases for elucidating the crystal structures of the crystallites of the starch granules. By integrating the X-ray scattering intensity separately over the peak and the background, a number is obtained and interpreted as the “X-ray crystallinity”.36 The crystallinities for

Table 3. HPSEC-MALLS-RI Determination of the Molecular Characteristics of Amylopectin According to Ripening Temperature cultivar

ripening temperature (°C)

Mwa (108 g/mol)

Ilpum

22/14 25/17 28/20 31/23

1.96 2.20 1.64 1.90

(0.07) (0.07) (0.05) (0.13)

Chucheong

22/14 25/17 28/20 31/23

1.52 1.86 1.58 2.02

(0.05) (0.10) (0.03) (0.11)

Rzb (nm)

f

Mw/Mnc

ρd (g/mol/nm3)

SVge (nm3/g)

278 330 220 239

(15) (32) (9) (11)

1.83 1.96 1.42 1.15

(0.01) (0.05) (0.03) (0.04)

9.1 6.1 15.4 13.9

0.28 0.41 0.16 0.18

289 269 230 275

(3) (9) (8) (9)

1.66 1.99 1.20 1.24

(0.04) (0.05) (0.09) (0.05)

6.3 9.5 13.0 9.7

0.40 0.26 0.19 0.26

a

Weight-average molecular weight. bz-average radius of gyration. cPolydispersity. dDensity (ρ) = Mw/Rz3. eSpecific volume for gyration (SVg) = 2.522 Rz3/Mw. fStandard deviation. D

DOI: 10.1021/jf504870p J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry Table 5. Gelatinization Properties of Rice Starch According to Ripening Temperaturea ripening temperature (°C)

To (°C)

Tp (°C)

ΔH (J/g)

Tc (°C)

ΔT (°C)

Ilpum

22/14 25/17 28/20 31/23

68.5b 70.1a 70.7a 70.8a

74.8d 77.0c 79.7b 81.3a

4.18c 5.58b 6.11b 7.32a

86.6d 89.8c 92.1b 94.2a

18.1d 19.8c 21.4b 23.6a

Chucheong

22/14 25/17 28/20 31/23

67.7c 68.3c 70.6b 71.5a

74.8d 76.7c 79.5b 81.6a

4.69b 6.56c 6.16b 8.26a

88.0d 90.3c 92.3b 94.9a

20.2b 21.7ab 21.7ab 23.4a

cultivar

To, onset temperature; Tp, peak temperature; Tc, completion temperature; ΔH, gelatinization enthalpy; ΔT = Tc − To. Values with the same letter in a column are not different significantly according to a Duncan’s multiple-range test (p < 0.05). a

The rice starch matured at a high temperature had consistently higher gelatinization temperatures (e.g., To, Tp, and Tc) and enthalpies, and the gelatinization temperatures increased with an increased ripening temperature. The gelatinization starting (onset) temperature, To, increased by 2.3 °C, from 68.5 to 70.8 °C, for IP and by 3.8 °C, from 67.7 to 71.5 °C, for CC. The peak temperature (Tp), completion temperature (Tc), and gelatinization enthalpy (ΔH) also significantly increased with increases in the ripening temperature in both cultivars, reflecting differences in the amylopectin crystalline order. These findings are consistent with those of a previous study, which suggested that the DSC characteristics of rice correlated with the environmental temperature.8 Our results confirmed that a higher ripening temperature increased the gelatinization temperatures of rice starch. Previous reports found that a longer chain length of an individual double helix of amylopectin increases the stability of the amylopectin double helix in gelatinization conditions.42 In addition, the gelatinization enthalpy mainly reflects a loss of the double-helical order of amylopectin.38 The formation of the double-helical structure that leads to crystallization requires a chain length of at least 10 for pure malto-oligosaccharides.42 Therefore, more long amylopectin chains (DP > 10) forming double helices due to a higher ripening temperature would lead to more stable gelatinization. The temperature range of gelatinization (ΔT) of starch also tended to widen in response to ripening temperature increases. This increased range would mean that the starch of rice grown in a high ripening temperature condition would require more energy in terms of cooking time and temperature than that grown in a low ripening temperature condition. This trend was more obvious for IP starch than for CC starch. In the IP cultivar, the ΔT of the starch showed a significant tendency to increase from 18.1 to 23.6 °C under elevated ripening temperature conditions. On the other hand, the ΔT of CC starch also increased, but the increase was not statistically significant. The ΔT is a function of both the proportion of amylopectin short chains and their DP.1 The ΔT of starch was reported to positively correlate with the amount of DP 6−9 in amylopectin due to an increase in the relative crystallinity in comparison with other rice varieties.43 However, here, the ΔT increased with an increase in the crystallinity at a high ripening temperature, despite a reduction in the amount of DP 6−9 in amylopectin (Tables 4 and 5; Figure 1). The increase in the ΔT was mainly due to an increase in the Tc. Moreover, a negative correlation between the ΔT and short chains of amylopectin was identified (r = −0.841; p < 0.01). It seems that the ΔT is

IP and CC ranged from 19.7 to 21.6% and from 20.5 to 25.2%, respectively. In all ripening conditions, CC had a higher degree of crystallinity than IP. The crystallinity, related to the double helices of amylopectin in X-ray diffraction analysis,37 was greater at high ripening temperatures (28/20, 31/23 °C) than at a low ripening temperature (21/14, 25/17 °C) in both cultivars. The largest crystalline regions were at 28/20 °C, with values of 21.6 and 25.2% for IP and CC, respectively. The crystallinity of rice flour was slightly decreased, by 0.1% for IP and by 0.9% for CC, from 28/20 to 31/23 °C. In this study, amylopectin intermediate chains, which construct double helices, increased under elevated temperature conditions (Figure 1). In addition, the samples had a higher proportion of crystalline regions when ripened at high temperatures of 28/20 and 31/23 °C in both cultivars. The increase in crystallinity with an increased ripening temperature was likely due to the increased amount of amylopectin intermediate chains. However, this increase could not explain the reduced crystallinity at 31/23 °C. Instead of the amylopectin chain length distribution from a previous study,38 the crystallinity of rice flour was highly correlated with the head rice percentage, with a correlation coefficient of 0.790 (p < 0.05). As stated above, the head rice percentage of both cultivars was highest at 28/20 °C. Therefore, one likely reason for the decreased crystallinity was the increased chalky rice at high ripening temperatures. The amylopectin chain length distribution and DSC properties were determined with only head rice to reduce the error due to the differences between head rice and chalky rice,39 but the crystallinity was measured using rice flour, which included chalky rice. Chalky rice has less starch accumulation in the tissue. In addition, crystallinity is related to the density of the packing arrangement,40 as also shown here by the decreased head rice percentage and reduced packing density (based on SVg, Rz, and ρ values) at 31/23 °C in Table 3. These results revealed that the crystallinity of rice flour was related to the head rice percentage, as well as the amylopectin chain length distribution, and confirmed the decreases in head rice yield, the compactness of amylopectin molecular structure, and the crystallinity of rice flour at a high grain-filling temperature above the optimal temperature. Gelatinization and Pasting Properties. The gelatinization properties of rice are closely related to the eating quality.41 In this study, the gelatinization properties of starch markedly changed as the ripening temperature changed (Tables 5), supporting our findings that rice starch grown at a high temperature would require a higher cooking temperature and longer cooking time. E

DOI: 10.1021/jf504870p J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry Table 6. Pasting Properties of Rice Flour According to Ripening Temperaturea ripening temperature (°C)

peak viscosity (RVU)

trough viscosity (RVU)

final viscosity (RVU)

breakdown (RVU)

setback (RVU)

Ilpum

22/14 25/17 28/20 31/23

147.8c 152.0b 180.6a 181.6a

102.3d 109.6c 131.0b 135.0a

198.6d 202.3c 217.5a 210.0b

45.4ab 42.4b 49.6a 46.6ab

50.9a 50.3a 36.9b 28.4c

Chucheong

22/14 25/17 28/20 31/23

130.4d 138.4c 170.3b 183.3a

84.6d 88.5c 116.3b 133.3a

174.2c 177.3c 203.4b 212.3a

45.8c 49.9b 54.0a 49.9b

43.8a 38.8b 33.1c 29.1d

cultivar

a

Values with the same letter in a column are not significantly different according to a Duncan’s multiple-range test (p < 0.05).

Table 7. Pearson Correlation Coefficients between Parameters Describing the Starch Structure and Pasting Properties of Rice Flour According to Ripening Temperature

a

factor

peak viscosity

trough viscosity

final viscosity

breakdown

setback

ripening temperature amylose DP 5−12 DP 13−24 DP 25−36 DP 37−60 protein

0.912**a −0.912** −0.898** 0.700b 0.863** − 0.833*

0.883** −0.909** −0.838** 0.748* 0.805* − 0.803*

0.716* −0.803* −0.713* 0.793* − − −

−c − − − − 0.802* −

−0.888** 0.731* 0.861** − −0.943** − −0.805*

* and **, significant at p < 0.05 and p < 0.01, respectively. bSignificant at p < 0.10. cNo correlation.

amylopectin fine structure has been reported to be related to the breakdown of swollen granules and viscosity, with a negative correlation with long chains and a positive correlation with short chains.52 We confirmed the negative correlation between amylose content and peak (r = −0.912, p < 0.01) and trough (r = −0.909, p < 0.01) viscosities (Table 7). Thus, rice flour made from rice ripened at higher temperatures swelled more easily and resisted shear more readily than that of rice grown at low temperatures. On the other hand, the breakdown had a slightly different tendency with maximal values at 28/20 °C in both cultivars, showing that the capacity of starch granules to rupture after cooking was higher near the optimum temperature for grain weight, especially in the CC cultivar. Breakdown has also been reported to be positively correlated with the stickiness of cooked rice.53 Setback also has a positive correlation with cohesiveness of mass in cooked rice, a sensory textural attribute.54 Here, the lower amylose content at higher ripening temperatures is related to decreased setback (r = 0.731, p < 0.05). Those values in the breakdown and setback, together with the lower amylose content mentioned above, showed that cooked rice grown at high ripening temperatures may have lower hardness and cohesiveness and higher stickiness. However, in this study, these results were not confirmed by analysis of the textural properties of the cooked rice using a texture analyzer (data not shown). The higher protein content in rice grown at higher ripening temperatures may explain the texture differences, because protein content correlates with cooked rice texture. Protein is also involved in providing structural support to the rice kernel during cooking, thereby restricting starch granule swelling. Thus, treatment with protease, an enzyme that cleaves protein, significantly decreases cooked rice firmness.55 In addition, protein influences the stickiness of cooked rice, with increased stickiness after protein disruption.56 We concluded that these contradictory roles of protein in the texture of cooked rice offset the predicted effects

related to the packing density, which is related to crystallinity, and not directly with the amount of short chains of amylopectin. Amylography using the RVA is a commonly used pasting property test that predicts the texture of cooked rice. With regard to the pasting properties of rice flour, the viscosity increased with an increased ripening temperature in both cultivars (Table 6). Peak and trough viscosities were clearly increased and setback was decreased with increasing ripening temperature. The CC cultivar had a higher increase in peak viscosity, with 52.9 RVA units (RVUs) compared with the 33.8 RVUs of IP. The trough viscosity of CC also increased to 48.7 RVUs compared with the 32.7 RVUs of IP. The breakdown displayed maximum values of 49.6 RVUs for IP and 54.0 RVUs for CC at 28/20 °C, with no statistically significant difference of IP. Setback was markedly decreased with an increase in the ripening temperature. However, the decreased amount of the CC cultivar was lower than that of the IP (14.7 vs 22.5 RVUs, respectively), with a different trend from above. These results concur with a report stating that a lower peak viscosity and higher setback are seen in rice with a higher amylose content (from the coldest growing season) than in rice with a lower amylose content.7 Pasting properties are affected by amylose content and by the branch chain length distribution of amylopectin.44 The amylose content is generally thought to be a critical determinant of starch pasting properties because amylose suppresses starch swelling. The amylose helix has an internal hydrophobic tube,45 providing a space for hydrophobic complexing agents such as lipids.46 Amylose−lipid complexes restrict granule swelling,47 which consequently affects pasting.48 Hence, the apparent amylose content was negatively correlated with flour swelling volume49 and stickiness50 and was positively correlated with the hardness50 of cooked rice. However, the correlation between amylose content and pasting properties is variable,50 because amylopectin structure also affects pasting properties.51 The F

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Journal of Agricultural and Food Chemistry Table 8. Comparison of Cooking Properties of Rice According to Ripening Temperaturea ripening temperature (°C)

water absorption ratio

expansion volume (cm3)

pH

iodine index

soluble solid (mg)

Ilpum

22/14 25/17 28/20 31/23

2.67b 2.69b 2.69b 2.84a

34.85a 34.65a 33.95a 35.37a

6.81a 6.79a 6.72b 6.66c

0.14a 0.13ab 0.11bc 0.10c

5.16a 5.07a 5.00a 5.16a

Chucheong

22/14 25/17 28/20 31/23

2.67c 2.71c 3.00b 3.15a

34.77a 35.76a 34.95a 37.33a

6.79a 6.75b 6.73b 6.76ab

0.16a 0.16a 0.12b 0.12b

6.41a 6.08a 4.83b 5.58ab

cultivar

a

Values with the same letter in a column of the same cultivar are not significantly different according to a Duncan’s multiple-range test (p < 0.05).

Table 9. Sensory Evaluation of the Eating Quality of Cooked Rice According to Ripening Temperaturea ripening temperature (°C)

appearance

flavor

taste

stickiness

hardness

palatability

Ilpum

22/14 25/17 28/20 31/23

0.46a 0.42a 0.04ab −0.17b

0.08a −0.04a 0.13a −0.25a

0.08ab 0.29a 0.17a −0.33b

0.25a 0.83a 0.33a 0.38a

0.46a 0.29a 0.29a 0.00a

0.25a 0.17a 0.08a −0.13a

Chucheong

22/14 25/17 28/20 31/23

0.54ab 0.88a 0.00b 0.42ab

−0.13a 0.08a −0.08a −0.29a

0.21a 0.38a 0.13a 0.17a

0.21a 0.50a 0.50a 0.29a

0.25a 0.67a 0.25a 0.38a

0.17a 0.54a 0.21a 0.13a

cultivar

a

Values with the same letter in a column of the same cultivar are not significantly different according to a Duncan’s multiple range test (p < 0.05). Evaluation score: from −3 (bad) to +3 (good).

Table 10. Relative Importance of Starch Properties and Protein Content for the Palatability of Cooked Rice through Quantification Analysis amylopectin palatability (%) a

DP 6−12

DP 13−24

DP 25−36

DP 37−60

amylose

protein

9.2a

6.9

29.4

1.4

27.6

25.6

Squared partial correlation coefficients (%) through quantification analysis.

The starch−iodine blue value of the residual liquid after cooking has been negatively correlated with eating quality.60 Here, the iodine index was significantly lower at a high ripening temperature than at a low temperature. This low iodine index may be due to the low amylose content of the samples. However, the amylose content did not seem to be affected by the leaching of starch exudates. Despite the low amylose content, the amount of leaching of starch exudates was decreased during the cooking of rice grown at a high ripening temperature, although high levels of amylose reduced the amount of starch exudates leaching into the solution, because amylose suppresses starch swelling.49 Enriched long amylopectin may be related, because lower levels of soluble solid were seen in both cultivars at 28/20 °C, the optimum temperature for amylopectin density. Finally, we performed a sensory evaluation of the palatability of the cooked rice. Palatability showed a nonsignificant tendency to decrease in response to an increase in the ripening temperature (Table 9). The relative importance or contribution of starch components and protein content for the palatability of cooked rice was analyzed using the quantification method type I61 (Table 10). The amylopectin chains of DP 25−36 showed the greatest effect on palatability at 29.4% among the rice samples grown with various ripening temperatures. In addition, the amylopectin chains of DP 25−36 had a high effect on the gelatinization properties of rice starch, as discussed above.

of a low amylose content on the pasting properties discussed above at high ripening temperatures. Furthermore, the setback value reflects the degree of retrogradation of starch paste and shows that starch grown at high ripening temperatures had less retrogradation. However, the short amylopectin chain of DP 6−9 has the opposite effect on retrogradation, showing a negative correlation.57 Thus, the retrogradation did not appear to be affected by an increased ripening temperature, shown by the percentage retrogradation, calculated by the ΔHretrograded starch/ΔHgelatinization in the rice starch tested (data not shown).44 Cooking Properties and Palatability. The cooking properties of the rice were predicted to be best at the optimum temperature according to water absorbability and a low iodine index and soluble solid value (Table 8). The water absorption ratio was significantly higher at 31/23 °C in both cultivars. Water absorbability is highly related to the gelatinization and eating quality of rice.58 This high water absorption following a high ripening temperature is related to the starch granular structure and low amylose content. The rigidity of the starch granular structure is in proportion to its amylose content and in inverse proportion to the degree of granular swelling.59 In other words, a high temperature during grain filling caused a loose starch granular structure due to low amylose content, increasing water absorbability. G

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Journal of Agricultural and Food Chemistry

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However, the amylose and protein contents also had high relative importances of 27.6 and 25.6%, respectively. Therefore, we could conclude that palatability was mostly related to the following chemical properties: amylopectin, amylose, and protein content. Of amylopectin chains, the DP 25−36 chains had the greatest relative importance on eating quality, because of their effect on starch gelatinization.

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AUTHOR INFORMATION

Corresponding Author

*(A.C.) Phone: +82-31-695-0603. Fax: +82-31-695-4085. Email: [email protected]. Funding

This work was carried out with the support of “Cooperative Research Program for Agriculture Science & Technology Development (Project title: Identification of useful grain quality characteristics in rice mutants using reverse and forward genetics, Project No. PJ00923907)”, Rural Development Administration, Republic of Korea. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate Y.-J. Wang for MALLS analysis. ABBREVIATIONS USED IP, Ilpum; CC, Chucheong; HPSEC-MALLS-RI, high-performance size exclusion chromatography with multiangle laser-light scattering and refractive index detectors; HPAEC, highperformance anion-exchange chromatography; DSC, differential scanning calorimeter; GBSSI, granule-bound starch synthase I; RVA, Rapid Visco Analyzer; DP, degree of polymerization; Mw, weight-average molecular weight; Rz, zaverage radius of gyration; Mn, number-average molar mass; ρ, density; SVg, specific volume for gyration; To, onset temperature; Tp, peak temperature; Tc, completion temperature; ΔH, gelatinization enthalpy

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I

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