Classification of rice cultivars based on cluster analysis of hydration ...

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time, pasting temperature, peak viscosity, trough viscosity, final viscosity, breakdown, and setback. Other important parameters which represent processability are hydration properties: water absorption index, water solubility, and swelling power (Crosbie,1991; Li & Yeh, 2001). The hydration properties <strong>of</strong> starches can be correlated with their pasting behaviors which are important for the determination <strong>of</strong> starch quality. In many cases, a single parameter from different measures rarely reflects whole characteristics <strong>of</strong> a starch. In addition, it is difficult to classify the <strong>rice</strong> starches intuitively <strong>based</strong> on the parameters because <strong>of</strong> the complexity <strong>of</strong> them. To overcome these issues, statistical categorization such as clustering can be used as an alternative approach to obtain a common pattern from complex data sets <strong>of</strong> hydration and pasting parameters. Cluster analysis is a statistical method to convert various characteristics <strong>of</strong> objects to quantitative measures (so called similarity distance) and correspondingly to cluster them at relatively close distances into a category (Endo, Okada, Nagao, & D’applonia, 1990). We hypothesize that the cluster analysis can be used to categorize the <strong>rice</strong> <strong>cultivars</strong> having similar hydration and pasting parameters into single group. In this approach, starches from different <strong>cultivars</strong> but with similar processability properties can be assumed as a single commodity for easy identification and selection by starch industry for specific application purposes. In this study, <strong>rice</strong> starches were extracted from 12 different <strong>cultivars</strong> and their processability parameters were measured; they are categorized into several groups by cluster analysis <strong>of</strong> the processability parameters. In addition, the texture properties <strong>of</strong> these <strong>rice</strong> starches were also determined to validate the categorization. 2. Materials and methods 2.1. Materials Twelve <strong>rice</strong> <strong>cultivars</strong> are listed in Table 1 and were grown at the National Institute <strong>of</strong> Crop Science, Rural Development Administration, Korea in 2009. The grains <strong>of</strong> each <strong>rice</strong> cultivar was polished to separate husk and bran and subsequently stored for further starch preparation. 2.2. Rice starch preparation Rice starch from each cultivar was isolated by an alkaline steeping method (Yamamoto, Sawada, & Onogaki, 1973). 2.3. Amylose content measurement Amylose content <strong>of</strong> the <strong>rice</strong> starches was determined by using the iodine test (Williams, Kuzina, & Hlynka, 1970). Table 1 Amylose contents <strong>of</strong> <strong>rice</strong> starches extracted from different <strong>cultivars</strong>. Symbols Amylose content (g/100 g) Name <strong>of</strong> the cultivar A High amylose 33.84 0.47 a Goami B 29.16 0.71 b Thailand C Medium amylose 22.05 1.33 c Seolgaeng D 21.73 0.51 dc Hiami E 21.44 0.2 d Ilmi F 21.35 0.32 d Hanareum G Low amylose 19.44 0.17 e Hopyeong H 17.97 0.37 f Deuraechan I 17.92 0.18 f Hanmauem J 17.62 0.23 f Dongjin 1 K Waxy 1.70 0.12 g Boseokchal L 1.32 0.56 g Shinseonchal Means with the different letters in the same column are significantly different at the 5% level. I. Lee et al. / LWT - Food Science and Technology 48 (2012) 164e168 165 2.4. Measurement <strong>of</strong> water absorption index, water solubility, and swelling power Water absorption index (WAI) was measured according to the method <strong>of</strong> Medcalf and Gilles (1965). Water solubility (WS) and swelling power (SP) were determined following the method <strong>of</strong> Schoch (1964). For the analysis <strong>of</strong> hydration properties, starch suspension was heated at 80 C. Three hydration parameters were calculated using the following equation: Water Absorption Index ðWAIÞ ¼ Water Solubility ðWS; %Þ ¼ wet sediment weight dry sample weight dry supernatant weight dry sample weight 100 wet sediment weight Swelling Power ðSPÞ ¼ WS ð%Þ dry sample weight 1 100 2.5. Measurement <strong>of</strong> pasting properties Pasting properties <strong>of</strong> starch samples were measured using a rheometer equipped with a starch pasting cell (AR 1500ex, TA instrument, New Castle, DE, USA) which can be operated like a rapid visco-analyzer (RVA). Three grams (dry weight) <strong>of</strong> starch sample was added to 25 g <strong>of</strong> distilled water in a aluminum vessel (37 mm internal diameter and 65 mm height) installed on the rheometer. The starch suspension was stirred at 25 C for 1 min by a plastic paddle. During the measurement, a time-temperature schedule was applied; the starch suspension was heated to 95 C (starting from 25 C, at a rising temperature rate <strong>of</strong> 12 C/min), held at 95 C for 2.5 min, cooled down to 50 C at a rate <strong>of</strong> 12 C/min, and held at that temperature for 2 min. The pasting parameters, peak time, pasting temperature, and peak (P), trough (T), final (F), breakdown (PeT), and setback (FeT) viscosity parameters were determined in situ for the starch suspension. 2.6. Texture pr<strong>of</strong>ile analysis <strong>of</strong> starch gels For the texture pr<strong>of</strong>ile analysis, 2.4 g <strong>of</strong> each starch sample (dry weight) was added to 27.6 mL <strong>of</strong> distilled water in a 100 mL screw cap bottle (40 mm opening) at 25 C under stirring conditions (700 rpm for 30 min). The bottles were heated by immersion in a water bath at 85 C for 30 min and then cooled to 25 C. Subsequently, the bottles were sealed with screw cap and stored at 4 C for 24 h. Each bottle including the starch gel prepared was placed on a texture analyzer (TMS-Pro, Food Technology Co., Sterling, VA, USA) equipped with a 25 N load cell. The starch gel was compressed 5 mm at pretest a speed <strong>of</strong> 50 mm/min, test speed <strong>of</strong> 20 mm/min, and post-test speed <strong>of</strong> 20 mm/min using a 20 mm diameter cylindrical probe with a flat end. Texture parameters, hardness (maximum peak during first compression), cohesiveness (the area under the second peak/the area under the first peak), and gumminess (hardness cohesiveness) were determined for individual samples. 2.7. Duncan’s multiple range tests The hydration and the pasting parameters were statistically analyzed on the basis <strong>of</strong> the Duncan’s multiple range tests using SAS s<strong>of</strong>tware (v. 9.1, SAS Institute Inc., Cary, NC, USA).
166 2.8. Cluster analysis for <strong>rice</strong> <strong>cultivars</strong> classification Hierarchical clustering method was applied to the processability data for classification <strong>of</strong> <strong>rice</strong> <strong>cultivars</strong>. The processability data sets, the hydration and the pasting parameters <strong>of</strong> the starch suspensions, were used to classify 12 <strong>rice</strong> <strong>cultivars</strong> into several groups. Hydration parameters were normalized whereas other parameters were the values measured instrumentally. Hierarchical clustering method is <strong>based</strong> on the average linkage clustering a method <strong>of</strong> calculating mean (Euclidean) distance between elements <strong>of</strong> each cluster. The average linkage clustering was carried out using SPSS s<strong>of</strong>tware (v. 12.0, SPSS Inc., Chicago, IL, USA). 3. Results and discussion 3.1. Amylose content <strong>of</strong> <strong>rice</strong> starches Amylose content <strong>of</strong> <strong>rice</strong> starches are listed in Table 1. Rice <strong>cultivars</strong> are generally classified depending on the amylose content <strong>of</strong> their starches into waxy, low amylose, medium amylose, and high amylose varieties which contain 0e2, 9e20, 20e25, and over 25 g/100 g amylose, respectively (Mitchell, 2009). In this study, the <strong>rice</strong> starches <strong>of</strong> <strong>cultivars</strong> A and B, <strong>cultivars</strong> C, D, E and F, <strong>cultivars</strong> G, H, I and J, and <strong>cultivars</strong> K and L were categorized into groups <strong>of</strong> high amylose, medium amylose, low amylose, and waxy <strong>rice</strong> starches, respectively. 3.2. Hydration properties <strong>of</strong> <strong>rice</strong> starches Hydration properties including WAI, WS, and SP <strong>of</strong> <strong>rice</strong> starches at 80 C are showed in Table 2. The WAI values <strong>of</strong> the <strong>rice</strong> starches from the <strong>cultivars</strong> B, E, and H were similar but their amylose levels showed a significant difference as shown in Table 1. Especially, the WAI <strong>of</strong> the starch in cultivar B was relatively high in spite <strong>of</strong> containing high amylose content. These results are not in agreement with previous findings that the WAI <strong>of</strong> the starches with high amylose content tend to be relatively low since amylose is hardly converted into amorphous state compared to amylopectin (Lee, Han, Lee, Kim, & Chung, 1989). The WAI <strong>of</strong> the starches <strong>of</strong> waxy <strong>rice</strong> <strong>cultivars</strong> (high amylopectin) were relatively low. The low WAI values <strong>of</strong> the waxy starches were not due to their low waterbinding capacity, but because amount <strong>of</strong> soluble fraction in the waxy starches was relatively larger than that present in the nonglutinous starches. The waxy starches are easily solubilized by water so that their wet sediment weights fall <strong>of</strong>f after the centrifugation for the WAI analysis. Herein, the amylose content does not Table 2 Pasting and hydration properties <strong>of</strong> <strong>rice</strong> starches. WAI WS (%) SP PV (Pa s) T (Pa s) FV (Pa s) BD (Pa s) SB (Pa s) Rice A 7.00 e 9.19 d 7.70 c 2.19 g 1.45 cd 3.83 b 0.75 g 2.38 a B 12.05 a 17.29 c 13.26 b 3.79 c 1.61 b 4.09 a 2.18 dc 2.47 a C 8.80 cd 4.32 d 9.20 c 3.23 e 1.31 e 2.44 ef 1.92 e 1.13 d D 9.31 cbd 4.60 d 9.76 cb 3.24 e 1.30 e 2.46 ef 1.95 e 1.17 d E 12.31 a 7.42 d 13.29 b 4.25 b 1.52 cb 3.17 c 2.74 a 1.66 b F 10.64 b 5.38 d 11.24 cb 4.31 ba 1.82 a 3.20 c 2.49 b 1.37 c G 8.61 d 4.72 d 9.04 c 3.39 de 1.37 ed 2.48 e 2.03 de 1.12 d H 12.27 a 6.92 d 13.18 b 4.13 b 1.35 ed 2.72 d 2.78 a 1.37 c I 10.14 cb 6.25 d 10.83 cb 3.39 de 1.12 f 2.38 ef 2.27 c 1.26 dc J 7.90 ed 4.37 d 8.26 c 2.51 f 1.49 c 3.09 c 1.03 f 1.61 b K 8.16 ed 53.92 b 17.63 a 4.55 a 1.72 a 2.56 de 2.83 a 0.84 e L 4.75 f 79.41 a 22.76 a 3.61 dc 1.52 cb 2.27 f 2.09 dce 0.75 e WAI: water absorption index, WS: water solubility, SP: swelling power, PV: peak viscosity, T: though viscosity, FV: final viscosity, BD: breakdown, SB: setback. Means with the different letters in the same column are significantly different at the 5% level. I. Lee et al. / LWT - Food Science and Technology 48 (2012) 164e168 represent properly the processability properties including WAI, which exhibited difference among the <strong>rice</strong> starches even with similar amylose content. Cultivars L and K, the high amylopectin content but low amylose varieties, showed high WS compared to others. On the contrary, water solubility <strong>of</strong> the <strong>rice</strong> starches <strong>of</strong> <strong>cultivars</strong> A to J was less than 10%. Upon heating insoluble starch swells by penetrated water and weakened infrastructure hydrogen bonds, and correspondingly some fragments <strong>of</strong> the starch could be solubilized upon heating. Thus, WS depends on amount <strong>of</strong> soluble starch, which will vary with cultivar and processing conditions including heat treatment. Especially, hydrogen bonds can be easily weakened in the starch granules with high amylopectin content. High swelling powers were observed with the <strong>rice</strong> starches <strong>of</strong> <strong>cultivars</strong> L and K (Table 2). On the contrary, the SP <strong>of</strong> starch <strong>of</strong> cultivar A was the lowest. The SP <strong>of</strong> a starch depends on the ratio and molecular weights <strong>of</strong> amylose and amylopectin, and also intra- and inter-molecular interactions. In general, amylose acts as an inhibitor <strong>of</strong> swelling but amylopectin is likely to swell due to the weakness <strong>of</strong> the intra- and inter-molecular coherence in starch (Morrison, Tester, Snape, Law, & Gidley, 1993; Tester & Morrison, 1990). 3.3. Pasting properties <strong>of</strong> <strong>rice</strong> starches Pasting properties <strong>of</strong> the <strong>rice</strong> starches are listed in Table 2.TheP values, maximum viscosity during sol/gel transition upon heating, <strong>of</strong> the starches with high amylose content were relatively low since they do not swell easily or get thicker. On the other hand, waxy starches have higher P values because they can easily swell and become pastes. The highest P value was observed with cultivar K while the lowest value was with cultivar A. The P values <strong>of</strong> the <strong>rice</strong> starches <strong>of</strong> the <strong>cultivars</strong> D and E were significantly different but their amylose contents were similar. On the contrary, the P values <strong>of</strong> starches <strong>of</strong> the <strong>cultivars</strong> C and J were similar though their amylose contents were varied considerably. In conclusion, amylose content is not correlated closely with the P value which is one <strong>of</strong> important parameters that need to be considered for the starch processability. It is advantageous to characterize starches <strong>of</strong> different <strong>cultivars</strong> <strong>based</strong> on the processing parameters including P value, instead <strong>of</strong> amylose content. T values, which represent the lowest viscosity upon heating, <strong>of</strong> the <strong>rice</strong> starches were presumably observed at the end <strong>of</strong> heating at 95 C. The highest T value observed with cultivar F while cultivar I exhibited the lowest among the <strong>cultivars</strong> studied. F values represent the viscosity <strong>of</strong> starch paste after cooling to 50 C. The F values <strong>of</strong> <strong>cultivars</strong> A and B were the highest. The high amylose <strong>rice</strong> starches showed relatively high F values since starch pastes formed after heating likely to become harder dramatically upon cooling. On the contrary, amylopectin-rich starch (cultivar L) developed into a clear paste with low viscosity. The F values <strong>of</strong> the <strong>rice</strong> starches <strong>of</strong> the <strong>cultivars</strong> I and J were differed significantly but their amylose contents are similar. However, the F values <strong>of</strong> starches <strong>of</strong> the <strong>cultivars</strong> D and G exhibited close similarity but their amylose contents are varied considerably. Breakdown values reflect the heat stability at 95 C. After initial gelatinization, continuous exposure <strong>of</strong> starch paste to high temperature lowers its viscosity. In general, hardness <strong>of</strong> starch paste is high at low temperature, vice versa, as temperature and texture are mutually dependent and in reversible manner. Hence a high breakdown value may be considered as a relatively low heat stability point because low value indicates thermal stability (Karim, Norziah, & Seow, 2000). Breakdown values <strong>of</strong> the <strong>rice</strong> starches <strong>of</strong> the <strong>cultivars</strong> D and E were differed significantly but their amylose contents are similar. On the contrary, the breakdown values <strong>of</strong> the <strong>rice</strong> starches in the <strong>cultivars</strong> E and H present similar breakdown values but their amylose contents are varied considerably.
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