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Texture determinants of cooked, parboiled rice. II: Physicochemical properties and leaching behaviour of rice
Journal of Cereal Science 21 (1995) 261–269
Texture determinants of cooked, parboiled rice. II: Physicochemical properties and leaching behaviour of rice M. H. Ong and J. M. V. Blanshard University of Nottingham, Department of Applied Biochemistry and Food Science, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. Received 23 June 1994
ABSTRACT This paper reports the pasting, gelatinisation and leaching behaviour of 11 cultivars of rice, the starch structural properties of which were determined in the preceding paper. The results show that the contents of leached amylose in the cooking water, as determined by both size exclusion–high performance liquid chromatography (SE–HPLC) and iodine colorimetry, were correlated positively with the texture of cooked rices, which possessed total amylose contents in the range 18·4–29·5%. The amount of leached amylose depended on the total amylose content of the rice. A similar correlation between the conventional ‘setback’ value, measured using the Viscoamylograph, and the texture of cooked rice may be a result of the leached starch content. The gelatinisation temperatures of rice starches determined by differential scanning calorimetry (DSC) were not correlated with the texture of cooked rice, but were significantly related to the crystallinity of the rice starch. The longest chain population (92–98 DPn), which had been detected previously in the hard rice samples, was not found in their corresponding leached starches. This observation may well support the suggestion in the preceding paper that the longest amylopectin chains could interact with other components in rice, the resultant complexes being retained in the cooked grain and inhibiting softening.
‘setback’ values but negatively with stickiness2,3. The gelatinisation temperature determined from the alkaline spreading values (an index of gelatinisation of rice4) has also been used for many years to categorise cooked rice properties2. Recently, increasing cross breeding activities have generated rices that exhibit different eating properties, even though they may possess similar amylose contents5, pasting and/or gelatinisation characteristics6. Although DSC is a recommended method for determining the temperatures and enthalpies of the gelatinisation of starch7, the results depend on whether rice flour, rice starch or whole grains are measured8–10. Other constituents of milled rice are protein (about 7% depending on the cultivar and environment) and minor components such as lipid, minerals, vitamins and the non-starch polysaccharides, especially
INTRODUCTION Since starch granules contribute about 90% of the dry weight of milled rice grains, their physicochemical properties (e.g. pasting and gelatinisation characteristics) have been used for decades to predict the eating properties of rice1–3. The hardness of non-waxy cooked rice has been shown to be correlated positively with Amylograph ‘breakdown’, hot and cold paste viscosities and
: DSC=differential scanning calorimetry; SE–HPLC=size exclusion–high performance liquid chromatography; DP=degree of polymerisation; Mn, MW=number and weight average molecular weights, respectively; Ve=elution volume. Corresponding author: J. M. V. Blanshard. 0733–5210/95/030261+09 $08.00/0
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those from the endosperm cell wall components11–14. Although present in smaller amounts, protein and lipid15–19 have both received attention because of the generation of possible intra and/ or inter-molecular interactions during the cooking of whole rice grains. Studies on the properties of materials in the cooking water derived from the leaching of the gelatinised granules from the rice grain during cooking take into account the molecular interactions of the constituents in the intact kernels. Priestley20 showed a reduction in the stickiness of parboiled rice with increased starch extractability. Further tests on the apparent extractabilities of starches from 48 cultivars of rice showed that the amounts of extractable amylopectin (14–72%) were correlated with the amylose contents (r=−0·82∗∗) of rice: they may be related to the stickiness of cooked rices21. Starch–iodine blue values and solids in the cooking water have been used by some laboratories for determining rice grain quality3. A further instance of the significance of the leached starch has been demonstrated in the manufacturing of canned rice, in which pre-leaching of starch on hydrating instant parboiled rice ensures the retention of grain integrity after canning22. Waterunextractable amylose (which is the total minus the extractable amylose determined from rice flour23), instead of the total amylose content of rice, has been found to correlate significantly with the pasting behaviour and textural attributes of rice24,25. The unextractable amylose-equivalent was later reported to be an index of the amylopectin structure, which is correlated with the texture of cooked rice26. Recently, Murugesan and co-workers27 characterised the hot-water-extractable components of four commercial starch granules (maize, wheat, potato and sweet potato) but not rice starch. As no report has been found in the literature detailing the structures of the leached starch from cooked rice or hot-water-extractable rice starch, this study was undertaken to investigate the structures of such leached/extractable starch components (i.e. amylose and amylopectin) from the cooked raw rice grain by size exclusion–high performance liquid chromatography (SE–HPLC). In addition, this paper completes the report on the texture determinants of cooked parboiled rice by comparing the pasting, gelatinisation and extractability characteristics of the eleven rice cultivars whose textural and structural properties were discussed in the preceding paper28.
30 800
95
95
50°C Cold paste viscosity
700 600 Brabender units
262
Peak viscosity
500 400 300 Hot paste viscosity
200 100 0
Pasting temperature 10
20
30
40 50 60 Time (min)
70
80
90 100
Figure 1 A typical Viscoamylograph trace of a 10% (w/v) rice flour slurry.
EXPERIMENTAL
Eating quality of cooked rice, extraction of starch from rice grain and statistical correlation analysis. As reported in the preceding paper. Pasting characteristics The pasting characteristics of rices were determined with a 10% (w/v) rice flour slurry using a Brabender Viscoamylograph type E, which was operated according to the AACC 22-10 method (Final Approval 5-5-60). The rice flour slurry was heated from 30°C to 95°C, held for 20 min at 95°C and cooked to 50°C. The heating and cooking took place at 1·5°C/min. The parameters, ‘breakdown’ (peak viscosity minus hot paste viscosity), ‘setback’ (cold paste viscosity minus peak viscosity) and ‘consistency’ (cold paste viscosity minus hot paste viscosity) were derived from the Viscoamylographs (Fig. 1). Gelatinisation temperatures and enthalpies of rice starches Rice starches were prepared and defatted by the methods described previously28. A Perkin Elmer DSC-2 equipped with a thermal analysis data system was used in the study. A rice starch:water mixture of ratio 1:2 (w/w), which gave approximately a water content of 70% (w/w). The sample (approximately 10–14 mg of a well-mixed suspension) was transferred to a pre-weighed, aluminium pan, which was hermetically sealed and
Physicochemical and leaching properties of rice
reweighed. At least 2 h after sealing the pan, it was heated from 300–380 K at 10 K/min. An empty pan was used as reference. Two measurements were performed for each sample. Both gelatinisation temperatures and enthalpies were corrected by the use of an indium standard.
Starch components in cooking water by the SE–HPLC method The raw rice was cooked for 15 min in excess water (i.e. 875 g rice/500 ml water for 15 min). The starch in the supernatant of the freshly cooked rice (1 ml) was debranched according to the method described by Ong and co-workers29 except that the sample was mixed in hot water and no heating was required before the addition of acetate buffer and isoamylase. The molecular weight of the amylose and the chain lengths of the amylopectin of the starch in the cooking water were determined by the gel permeation chromatography system with multi-angle laser light scattering and refractive index detectors as described previously28. The unresolved amylopectin peak was divided into two fractions, namely long and short amylopectin chains. The chain length distribution was calculated by the conventional division analysis reported by Ong and co-workers29. Leached amylose content in cooking water The leached amylose content of the cooking water was estimated by the Williams’s method30 as described previously28, expect that the cooking water (10 ml) was centrifuged at 625 g before analysis. The results recorded were the mean of three values. Table I
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Total extractable solids content in cooking water The total extractable solids content of the cooking water was determined by drying 5 g of the cooking water overnight in a conventional oven at 110°C. The % extractable solids content was calculated from the weight difference before and after drying. The tests were carried out in duplicate. Turbidity of cooking water The turbidity of the cooking water was determined by measuring the transmittance at a wavelength of 650 nm in a spectrophotometer. Distilled water was used in the reference cell which gave a transmittance of 100. RESULTS AND DISCUSSION Pasting characteristics of rice The Viscoamylograph in Figure 1 illustrates the peak, hot and cold paste viscosities and the pasting temperature of a typical rice sample. All the rice samples showed a similar profile, but had individually distinct viscosities and pasting temperatures, which are shown in Table I. ‘Setback’ was the only viscosity parameter found to correlate significantly with the texture of the rices examined (r=0·7995, P<0·01∗∗). Gelatinisation properties of rice determined from the rice starch The temperatures at which starch gelatinises (given as Tonset, Tpeak and Tconclusion in Figure 2) and the enthalpies of gelatinisation for the eleven rice
Pasting characteristics of eleven cultivars of rice with different eating properties
Cultivar) (texture)
Pasting temperature (°C)
Peak viscosity (BU)
Setback (BU)
Breakdown (BU)
Consistency (BU)
Hot paste viscosity (BU)
Cold paste viscosity (BU)
A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
72·0 70·0 70·0 69·0 63·0 62·5 67·5 59·0 69·0 69·5 69·0
330 530 450 485 525 505 435 435 460 430 510
235 50 100 150 105 45 40 60 115 115 160
160 255 210 210 225 215 215 210 220 220 205
395 305 310 360 330 260 255 270 335 335 365
170 275 240 275 300 290 220 225 240 210 305
565 580 550 635 630 550 475 495 575 545 670
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M. H. Ong and J. M. V. Blanshard Tpeak
(based on Tpeak), were found to be weakly correlated with the texture (hardness) of cooked rice (r= 0·524, P<0·05∗). Therefore, in contrast to the alkaline spreading values, which were found to be related to the texture of rice2,3, this study shows that the gelatinisation temperatures of rice starches determined by DSC were not correlated (r=0·269, P=0·467) with the texture of cooked rice. On the other hand, the gelatinisation temperature was correlated positively with the crystallinity of starch (r=0·8876, P<0·01∗∗) as determined previously28. This finding is in agreement with the literature reports, and the underlying cause for this correlation is not known31–33, but crystal perfection34 and an increased glass transition temperature (Tg)35 have been suggested. The type of material used for DSC analysis, i.e. whether rice starch, flour or whole rice grain, can give different results for the gelatinisation temperatures even after taking into account possible cultivar differences. The gelatinisation temperatures and enthalpies obtained for the rice starches in this study were found to fall within a range similar to those reported in the literature8,9. The Tpeak and Tconclusion values (but not the Tonset) and the enthalpy values were much lower (about 10 K and 3 J/g lower) than the values determined for whole rice grain10,17. Normand and Marshall10 have demonstrated that the values of Tpeak for rice flour were closer to those of rice starch, but lower than those for the whole rice grain. This difference is not surprising as there will be a significant time lag in thermal conductance from the periphery to the centre of the grain.
Tconclusion
Rice A Tonset Rice B Rice C Rice D Rice E
Endothermic heat flow
Rice F
Rice G Rice H
Rice I
Rice J Rice K
310
320
330
340
350
360
370
Temperature (K)
Figure 2 DSC thermograms of starches showing the gelatinisation endotherms of eleven different cultivars of rice.
samples were determined for the defatted starches using DSC; the results are given in Table II and Figure 2. The enthalpies, but not the temperatures
Table II
Gelatinisation temperatures and enthalpies of the starches derived from eleven cultivars of rices Gelatinisation temperature (°C)∗
Cultivar (texture)
Tonset
Tpeak
Tconclusion
Enthalpy (cal/g d.w.b.)
A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
69·9±0·5 73·0±0·6 73·2±0·3 73·8±0·2 62·1±0·3 68·6±0·1 73·7±0·2 62·2±0·2 61·2±0·5 60·7±0·4 59·8±0·4
75·3±0·5 78·2±0·1 77·6±0·1 78·0±0·1 67·0±0·5 72·4±0·2 77·1±0·1 66·7±0·2 65·8±0·4 63·8±0·4 64·2±0·2
83·4±0·9 84·6±0·0 84·0±0·1 84·3±0·5 75·3±0·5 80·3±0·1 83·1±0·3 75·8±0·6 72·7±0·9 71·2±0·9 68·0±0·9
3·0±0·4 3·2±0·0 3·2±0·1 3·6±0·1 2·5±0·1 3·2±0·1 3·4±0·1 2·5±0·1 4·3±0·1 3·3±0·2 3·1±0·0
∗ Means±SD of two duplicate determinations
Physicochemical and leaching properties of rice
Table III
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Composition and size distribution determined by the SE–HPLC method of the leached starch components from cooked milled rice grain % Proportion of leached starch Amylose
Cultivar (texture) A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
18·7 8·0 24·0 28·1 8·1 9·4 10·4 11·3 21·3 28·7 30·5
Amylopectin
Molecular weight of leached amylose×105
Chain length (DPn) of leached amylopectin
Long
Short
Mw
Mn
Mw/Mn
Long
Short
23·2 21·7 21·0 19·9 23·9 23·9 22·4 23·3 22·5 18·2 18·2
58·1 70·3 55·0 52·0 68·0 67·0 67·2 65·4 56·2 49·0 51·3
5·42 4·20 3·42 5·57 6·69 6·74 6·19 7·22 6·20 5·68 5·80
2·66 1·70 1·08 2·72 2·90 3·69 2·61 3·09 2·73 2·49 2·81
2·04 2·47 3·17 2·05 2·31 1·83 2·37 2·34 2·27 2·28 2·06
45 42 42 42 44 43 44 44 46 46 44
16 16 16 16 15 15 16 16 16 15 16
Volts
Volts
(a)
20
25
30 Elution volume (ml)
35
(b)
20
25
30 Elution volume (ml)
35
Volts
Figure 4 SE–HPLC chromatogram of the leached starch from the defatted rice grain; the arrow highlights the longest chains from the amylopectin.
20
25
30 Elution volume (ml)
35
Figure 3 SE–HPLC chromatograms showing the amylose and amylopectin peaks of the leached starch after debranching with isoamylase for (a) a typical hard cooking rice and (b) a typical soft cooking rice.
Molecular properties of leached starch The SE–HPLC chromatograms in Figure 3 show that the amylose and amylopectin fractions of the isoamylase debranched starch from the hard and soft cooking rice samples. Amylose and amylopectin in varying proportions were both leached
out into the cooking water. When the structures of these leached starches were compared with the profiles of the extracted native starches (preceding paper28, Fig. 4),it was clear that the distinctive profiles that were observed for the native starches were still retained, but that the multiple components were less distinct and the long chain region component (i.e. Ve=28·5 ml, DP≈90), which was detected in the native starch of the hard cooking rices (for variations A, C, D, I, J and K28), was not detected in the extractable/leached starch in any of the 11 samples. Figure 4, however, shows the detection of the longest chain amylopectin at Ve 28·5 ml (DP 92– 98) in the SE–HPLC profile of the leached starch from rice cultivar A, which had been defatted. This profile is very similar to that for native starch (see Fig. 4 of preceding paper28). The detection of
Volts
a b
20
25
30 Elution volume (ml)
35
Figure 5 SE–HPLC chromatograms comparing the leached amylose content from the (a) defatted and (b) undefatted starch suspension.
the longest chain amylopectin in the defatted but not the undefatted rice grains confirms our previous conclusion that this long amylopectin chain of hard rices is involved in molecular interactions with other components in the native state, or that starch components interact with others during the cooking of rice grains. The complexation of amylose and lipid was also shown when a 1% (w/v) starch suspension was heated at 90°C for 6 min and the leached components were compared for the defatted and nondefatted starches. Figure 5 shows the higher relative proportion of leached amylose from the defatted (27·0%) than from the undefatted (18·6%) starches extracted from rice cultivar variety A. The total solids contents and the turbidities of the cooking waters were also examined, and the results are included in Table IV. The extracted total solids in the cooking waters varied from 5·5 Table IV Cultivars (texture) A B C D E F G H I J K
Hardness score of cooked rice
M. H. Ong and J. M. V. Blanshard
Hardness score of cooked rice
266
10 9 8 7 6 5 4 3 2 1 10 9 8 7 6 5 4 3 2 1 0
(a) 2
y = 1·9689 + 0·11759x r = 0·566
0
4 8 12 16 20 24 28 Leached amylose content (% from SE–HPLC)
32
(b) 2
y = 1·2067 + 0·83777x r = 0·630
0·5 1·0 1·5 2·0 2·5 3·0 3·5 4·0 4·5 5·0 5·5 6·0 Leached amylose content (g/100 g rice)
Figure 6 Relationship between the leached amylose contents and the texture (hardness) of cooked rices, (a) from the SE–HPLC test, and (b) from the iodine colorimetry test.
to 12·2% of the weight of the rice grains. The turbidity of the cooking waters differed substantially only for sample B (Table IV). There was no significant correlation between texture and the extractable solids content (r=0·2819, P=0·201) nor with the turbidity (r=−0·3814, P=0·124) of the cooking water.
Leached amylose contents, total extractable solids and turbidities of cooking water of eleven cultivars of rice grains Leached amylose content (g/100 g)
Total extractable solids content (%)∗
Turbidity (0–turbid) (100–clear)
4·34±0·03 0·93±0·02 3·54±0·09 4·06±0·02 2·21±0·06 1·78±0·05 3·32±0·09 3·28±0·04 4·69±0·09 4·93±0·07 4·79±0·09
12·2±0·04 6·7±0·04 5·5±0·04 6·7±0·04 9·7±0·04 7·6±0·00 9·7±0·00 9·7±0·00 11·4±0·01 9·7±0·00 9·3±0·01
15·2 35·5 6·8 11·7 9·1 8·6 9·8 10·3 8·1 8·5 9·2
∗ Proportion (%) of the total weight of solids in cooking water over the original weight of rice grains. The values given are means±SD of duplicate determinations.
Physicochemical and leaching properties of rice
Leached amylose
back’ value and the leached amylose content with the texture of cooked rice may well indicate that the former is, in fact, a measure of the leached amylose content. The weight (Mw) and number (Mn) average molecular weights of the leached amylose, measured by laser light scattering are given in Table III. It appears that the molecular weights of the leached amyloses for all the rice samples were lower than those of the native starches28. A narrower range of polydispersity (Mw/Mn), from 1·83–3·17, was obtained for the leached amyloses than for the previous range of 3·4–8·4 for the corresponding native starches28. The absence of the longer chain amylopectin fraction and the presence of only the lower molecular weight amylose in the leaached starch in cooking water strongly suggests that, on cooking, the starch components, especially the higher molecular weights amylose and longer chain amylopectin, interact with other constituents in the rice. This observation is supported by the detection of amylose-lipid complexes in rices from X-ray diffraction and DSC studies19,36,37. Priestley36 proposed that the less-sticky behaviour of parboiled rice was due to the presence of these insoluble amylose complexes and not to retrogradation of starch as commonly assumed (M. H. Ong and J. M. V. Blanshard, unpublished results). These complexes have been thought to be responsible for the reduction in leaching of the solids and
Table III shows the relative proportions of leached amylose, long and short chain amylopectin (the amylopectin was divided into only two fractions). SE–HPLC method (Table III) gives the relative proportions of amylose and amylopectin, whilst iodine colorimetry (Table IV) records the absolute amylose content in the cooking water. From the iodine colorimetry, the cooking water for the 11 rice samples contained 0·93 to 4·93 g of amylose/100 g of the original rice grains. The relative leached amylose content from the SE– HPLC method (Table III) was correlated positively with the amylose content of the rices (r=0·7525, P<0·01∗∗) and with the textures of the cooked rices (r=0·7828, P<0·01∗∗). A slightly lower but significant correlation was also obtained between the absolute leached amylose content determined by iodine colorimetry (r=0·6527, P<0·05∗) (Table IV) and the textures of the cooked rices. Figure 6 shows the linear correlation between the leached amylose content determined from the SE–HPLC (r2=0·566) and iodine colorimetry (r2=0·630) tests with the hardness of cooked rice. This study shows that hardness after cooking of the rice samples was correlated with the values of ‘setback’ derived using the Viscoamylograph. ‘Setback’ was also reported to be an indicator of the texture of cooked rice2,3. Since the parameter measures changes in paste viscosities after heating (Fig. 1), the positive correlation between the ‘set-
Table V
267
Correlation analysis of the texture and selected properties of rice Texture of cooked rice
Correlation parameters Amylose content Amylopectin chain length DP 92–98 DP 22–25 DP 18 DP 10–11 Pasting temperature Peak viscosity ‘Setback’ Gelatinisation temperature Enthalpy Crystallinity Molecular weight of amylose Leached amylose content (SE–HPLC) Leached amylose content (I2)
r 0·7220
P 0·004∗
0·7535 −0·5493 −0·5668 −0·5545 0·6213 −0·4294 0·7995 0·269 0·524 −0·2747 0·2850 0·6527
0·040∗ 0·040∗ 0·030∗ 0·040∗ 0·016∗ 0·082 0·001∗∗ 0·467 0·040∗ 0·194 0·185 0·011∗
0·7828
0·001∗∗
∗ and ∗∗ significant at P<0·05 and 0·01 levels of probability respectively.
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M. H. Ong and J. M. V. Blanshard
solubilisation of the kernel, which make parboiled rice less prone to disintegration. Recently, the presence of types I and II amylose-lipid complexes has been found to depend on the amylose contents and the parboiling processes19.
Leached amylopectin Table III also shows the relative proportions and chain lengths of amylopectins of the leached starches from the eleven rice samples. The relative proportion of the shorter chains (DPn 15–16) were found to vary to a greater extent (i.e. 49·0–70·3%) than those of the longer (DPn 42–46) amylopectin chains (18·2–23·9%). It is also apparent from Table III that the soft rices (cultivars B, E, F, G and H) have higher proportions (65·4–70·3%) of short amylopectin chains than the hard rices (49·0– 58·1%). This observation seems to support the previous finding28 that the soft and hard rices have different short chain populations.
Texture determinant of cooked rice Table V summarises some correlations between texture and selected properties of rice determined in both this and the previous study28. The difference in amylose contents can be measured indirectly from the physicochemical tests, such as from the changes in viscosities (‘setback’) or the leaching behaviour of rices. The presence of distinctive SE–HPLC profiles reported in the first paper supports the view that amylopectin affects the final textural properties of rice grains significantly. The intermolecular interactions of the longer chain amylopectin and amylose in the rice grain were evident indirectly from the different structures of starch determined from leaching tests. We conclude, therefore, that the content of amylose and the structure of the amylopectin of starch profoundly affect the final textural properties of cooked rice.
Acknowledgement The authors thank Dr K. Jumel for assistance with the SE–HPLC system and the multi-angle laser light scattering and refractive index detectors.
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19. Biliaderis, C.G., Tonogai, J.R., Perez, C.M. and Juliano, B.O. Thermophysical properties of milled rice starch as influenced by variety and parboiling method. Cereal Chemistry 70 (1993) 512–516. 20. Priestley, R.J. Studies on parboiled rice – Part 3: Characteristics of parboiled rice on recooking. Food Chemistry 2 (1977) 43–50. 21. Priestley, R.J. Solubilization of starch fractions in rice varieties during cooking. In ‘Developments in Food Carbohydrate’ (Birch, G.G. and Shallenberger, R.S., eds) Applied Science Publication, UK (1977) pp. 141–151. 22. Gerdes, D.L. and Burns, E.E. Techniques for canning instant parboiled rice. Journal of Food Science 47 (1982) 1734–1735. 23. Shanthy, A.P., Sowbhagya, C.M. and Bhattacharya, K.R. Simplified determination of water-insoluble amylose content of rice. Starch/Sta¨rke 32 (1980) 409–411. 24. Juliano, B.O., Cartano, A.V. and Vidal, A.J. Note on a limitation of the starch-iodine blue test for milled rice amylose. Cereal Chemistry 45 (1968) 63–65. 25. Battacharya, K.R., Sowbhagya, C.M. and Swamy, Y.M.I. Importance of insoluble amylose as a determinant of rice quality. Journal of Science Food Agriculture 29 (1978) 359–364. 26. Reddy, K.R., Ali, S.Z. and Bhattacharya, K.R. The fine structure of rice starch amylopectin and its relation to the texture of cooked rice. Carbohydrate Polymers 22 (1993) 267–275. 27. Muregesan, G., Shibanuma, K. and Hizukuri, S. Characterisation of hot-water-soluble components of starches. Carbohydrate Research 242 (1993) 203–215. 28. Ong, M.H. and Blanshard, J.M.V. Texture determinants of cooked, parboiled rice. I: Rice starch amylose and the fine structure of amylopectin. Journal of Cereal Science 21 (1995) 251–260.
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29. Ong, M.H., Jumel, K., Tokarczuk, P.F., Blanshard, J.M.V. and Harding, S.E. Simultaneous determination of the molecular weight distributions of amyloses and the fine structures of amylopectins of native starches. Carbohydrate Research 260 (1994) 99–117. 30. Williams, P.C., Kuzina, F.D. and Hylnka, I. A rapid calorimetric procedure for estimating the amylose content of starches and flours. Cereal Chemistry 47 (1970) 411–420. 31. Zobel, H.F. Starch granule structure. In ‘Starch Chemistry and Technology’ (R.L. Whistler, J.N. BeMiller and E.F. Paschall, eds) Academic Press, USA (1984) pp. 285–309. 32. Blanshard, J.M.V. Starch granule structure and function: a physicochemical approach. In ‘Starch: Properties and Potential’, (Galliard, T., ed.), John Wiley and Sons, UK (1987) pp. 16–54. 33. Shi, Y-C and Seib, P.A. The structure of four waxy starches related to gelatinization and retrogradation. Carbohydrate Research 227 (1992) 131–145. 34. Tester, R.F. and Morrison, W.R. Swelling and gelatinization of cereal starches. II. Waxy rice starch. Cereal Chemistry 67 (1990) 558–563. 35. Slade, L. and Levine, H. Non-equilibrium melting of native granular starch: Part I. temperature location of the glass transition associated with gelatinization of Atype cereal starches. Carbohydrate Polymers 8 (1988) 183– 208. 36. Priestley, R.J. Studies on parboiled rice: Part 1 – comparison of the characteristics of raw and parboiled rice. Food Chemistry 1 (1976) 5–14. 37. Bhattacharya, K.R. Parboiling of rice. In ‘Rice Chemistry and Technology’ (B.O. Juliano, ed.) AACC (1985) pp. 289–348.
Texture determinants of cooked, parboiled rice. II: Physicochemical properties and leaching behaviour of rice M. H. Ong and J. M. V. Blanshard University of Nottingham, Department of Applied Biochemistry and Food Science, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. Received 23 June 1994
ABSTRACT This paper reports the pasting, gelatinisation and leaching behaviour of 11 cultivars of rice, the starch structural properties of which were determined in the preceding paper. The results show that the contents of leached amylose in the cooking water, as determined by both size exclusion–high performance liquid chromatography (SE–HPLC) and iodine colorimetry, were correlated positively with the texture of cooked rices, which possessed total amylose contents in the range 18·4–29·5%. The amount of leached amylose depended on the total amylose content of the rice. A similar correlation between the conventional ‘setback’ value, measured using the Viscoamylograph, and the texture of cooked rice may be a result of the leached starch content. The gelatinisation temperatures of rice starches determined by differential scanning calorimetry (DSC) were not correlated with the texture of cooked rice, but were significantly related to the crystallinity of the rice starch. The longest chain population (92–98 DPn), which had been detected previously in the hard rice samples, was not found in their corresponding leached starches. This observation may well support the suggestion in the preceding paper that the longest amylopectin chains could interact with other components in rice, the resultant complexes being retained in the cooked grain and inhibiting softening.
‘setback’ values but negatively with stickiness2,3. The gelatinisation temperature determined from the alkaline spreading values (an index of gelatinisation of rice4) has also been used for many years to categorise cooked rice properties2. Recently, increasing cross breeding activities have generated rices that exhibit different eating properties, even though they may possess similar amylose contents5, pasting and/or gelatinisation characteristics6. Although DSC is a recommended method for determining the temperatures and enthalpies of the gelatinisation of starch7, the results depend on whether rice flour, rice starch or whole grains are measured8–10. Other constituents of milled rice are protein (about 7% depending on the cultivar and environment) and minor components such as lipid, minerals, vitamins and the non-starch polysaccharides, especially
INTRODUCTION Since starch granules contribute about 90% of the dry weight of milled rice grains, their physicochemical properties (e.g. pasting and gelatinisation characteristics) have been used for decades to predict the eating properties of rice1–3. The hardness of non-waxy cooked rice has been shown to be correlated positively with Amylograph ‘breakdown’, hot and cold paste viscosities and
: DSC=differential scanning calorimetry; SE–HPLC=size exclusion–high performance liquid chromatography; DP=degree of polymerisation; Mn, MW=number and weight average molecular weights, respectively; Ve=elution volume. Corresponding author: J. M. V. Blanshard. 0733–5210/95/030261+09 $08.00/0
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those from the endosperm cell wall components11–14. Although present in smaller amounts, protein and lipid15–19 have both received attention because of the generation of possible intra and/ or inter-molecular interactions during the cooking of whole rice grains. Studies on the properties of materials in the cooking water derived from the leaching of the gelatinised granules from the rice grain during cooking take into account the molecular interactions of the constituents in the intact kernels. Priestley20 showed a reduction in the stickiness of parboiled rice with increased starch extractability. Further tests on the apparent extractabilities of starches from 48 cultivars of rice showed that the amounts of extractable amylopectin (14–72%) were correlated with the amylose contents (r=−0·82∗∗) of rice: they may be related to the stickiness of cooked rices21. Starch–iodine blue values and solids in the cooking water have been used by some laboratories for determining rice grain quality3. A further instance of the significance of the leached starch has been demonstrated in the manufacturing of canned rice, in which pre-leaching of starch on hydrating instant parboiled rice ensures the retention of grain integrity after canning22. Waterunextractable amylose (which is the total minus the extractable amylose determined from rice flour23), instead of the total amylose content of rice, has been found to correlate significantly with the pasting behaviour and textural attributes of rice24,25. The unextractable amylose-equivalent was later reported to be an index of the amylopectin structure, which is correlated with the texture of cooked rice26. Recently, Murugesan and co-workers27 characterised the hot-water-extractable components of four commercial starch granules (maize, wheat, potato and sweet potato) but not rice starch. As no report has been found in the literature detailing the structures of the leached starch from cooked rice or hot-water-extractable rice starch, this study was undertaken to investigate the structures of such leached/extractable starch components (i.e. amylose and amylopectin) from the cooked raw rice grain by size exclusion–high performance liquid chromatography (SE–HPLC). In addition, this paper completes the report on the texture determinants of cooked parboiled rice by comparing the pasting, gelatinisation and extractability characteristics of the eleven rice cultivars whose textural and structural properties were discussed in the preceding paper28.
30 800
95
95
50°C Cold paste viscosity
700 600 Brabender units
262
Peak viscosity
500 400 300 Hot paste viscosity
200 100 0
Pasting temperature 10
20
30
40 50 60 Time (min)
70
80
90 100
Figure 1 A typical Viscoamylograph trace of a 10% (w/v) rice flour slurry.
EXPERIMENTAL
Eating quality of cooked rice, extraction of starch from rice grain and statistical correlation analysis. As reported in the preceding paper. Pasting characteristics The pasting characteristics of rices were determined with a 10% (w/v) rice flour slurry using a Brabender Viscoamylograph type E, which was operated according to the AACC 22-10 method (Final Approval 5-5-60). The rice flour slurry was heated from 30°C to 95°C, held for 20 min at 95°C and cooked to 50°C. The heating and cooking took place at 1·5°C/min. The parameters, ‘breakdown’ (peak viscosity minus hot paste viscosity), ‘setback’ (cold paste viscosity minus peak viscosity) and ‘consistency’ (cold paste viscosity minus hot paste viscosity) were derived from the Viscoamylographs (Fig. 1). Gelatinisation temperatures and enthalpies of rice starches Rice starches were prepared and defatted by the methods described previously28. A Perkin Elmer DSC-2 equipped with a thermal analysis data system was used in the study. A rice starch:water mixture of ratio 1:2 (w/w), which gave approximately a water content of 70% (w/w). The sample (approximately 10–14 mg of a well-mixed suspension) was transferred to a pre-weighed, aluminium pan, which was hermetically sealed and
Physicochemical and leaching properties of rice
reweighed. At least 2 h after sealing the pan, it was heated from 300–380 K at 10 K/min. An empty pan was used as reference. Two measurements were performed for each sample. Both gelatinisation temperatures and enthalpies were corrected by the use of an indium standard.
Starch components in cooking water by the SE–HPLC method The raw rice was cooked for 15 min in excess water (i.e. 875 g rice/500 ml water for 15 min). The starch in the supernatant of the freshly cooked rice (1 ml) was debranched according to the method described by Ong and co-workers29 except that the sample was mixed in hot water and no heating was required before the addition of acetate buffer and isoamylase. The molecular weight of the amylose and the chain lengths of the amylopectin of the starch in the cooking water were determined by the gel permeation chromatography system with multi-angle laser light scattering and refractive index detectors as described previously28. The unresolved amylopectin peak was divided into two fractions, namely long and short amylopectin chains. The chain length distribution was calculated by the conventional division analysis reported by Ong and co-workers29. Leached amylose content in cooking water The leached amylose content of the cooking water was estimated by the Williams’s method30 as described previously28, expect that the cooking water (10 ml) was centrifuged at 625 g before analysis. The results recorded were the mean of three values. Table I
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Total extractable solids content in cooking water The total extractable solids content of the cooking water was determined by drying 5 g of the cooking water overnight in a conventional oven at 110°C. The % extractable solids content was calculated from the weight difference before and after drying. The tests were carried out in duplicate. Turbidity of cooking water The turbidity of the cooking water was determined by measuring the transmittance at a wavelength of 650 nm in a spectrophotometer. Distilled water was used in the reference cell which gave a transmittance of 100. RESULTS AND DISCUSSION Pasting characteristics of rice The Viscoamylograph in Figure 1 illustrates the peak, hot and cold paste viscosities and the pasting temperature of a typical rice sample. All the rice samples showed a similar profile, but had individually distinct viscosities and pasting temperatures, which are shown in Table I. ‘Setback’ was the only viscosity parameter found to correlate significantly with the texture of the rices examined (r=0·7995, P<0·01∗∗). Gelatinisation properties of rice determined from the rice starch The temperatures at which starch gelatinises (given as Tonset, Tpeak and Tconclusion in Figure 2) and the enthalpies of gelatinisation for the eleven rice
Pasting characteristics of eleven cultivars of rice with different eating properties
Cultivar) (texture)
Pasting temperature (°C)
Peak viscosity (BU)
Setback (BU)
Breakdown (BU)
Consistency (BU)
Hot paste viscosity (BU)
Cold paste viscosity (BU)
A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
72·0 70·0 70·0 69·0 63·0 62·5 67·5 59·0 69·0 69·5 69·0
330 530 450 485 525 505 435 435 460 430 510
235 50 100 150 105 45 40 60 115 115 160
160 255 210 210 225 215 215 210 220 220 205
395 305 310 360 330 260 255 270 335 335 365
170 275 240 275 300 290 220 225 240 210 305
565 580 550 635 630 550 475 495 575 545 670
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(based on Tpeak), were found to be weakly correlated with the texture (hardness) of cooked rice (r= 0·524, P<0·05∗). Therefore, in contrast to the alkaline spreading values, which were found to be related to the texture of rice2,3, this study shows that the gelatinisation temperatures of rice starches determined by DSC were not correlated (r=0·269, P=0·467) with the texture of cooked rice. On the other hand, the gelatinisation temperature was correlated positively with the crystallinity of starch (r=0·8876, P<0·01∗∗) as determined previously28. This finding is in agreement with the literature reports, and the underlying cause for this correlation is not known31–33, but crystal perfection34 and an increased glass transition temperature (Tg)35 have been suggested. The type of material used for DSC analysis, i.e. whether rice starch, flour or whole rice grain, can give different results for the gelatinisation temperatures even after taking into account possible cultivar differences. The gelatinisation temperatures and enthalpies obtained for the rice starches in this study were found to fall within a range similar to those reported in the literature8,9. The Tpeak and Tconclusion values (but not the Tonset) and the enthalpy values were much lower (about 10 K and 3 J/g lower) than the values determined for whole rice grain10,17. Normand and Marshall10 have demonstrated that the values of Tpeak for rice flour were closer to those of rice starch, but lower than those for the whole rice grain. This difference is not surprising as there will be a significant time lag in thermal conductance from the periphery to the centre of the grain.
Tconclusion
Rice A Tonset Rice B Rice C Rice D Rice E
Endothermic heat flow
Rice F
Rice G Rice H
Rice I
Rice J Rice K
310
320
330
340
350
360
370
Temperature (K)
Figure 2 DSC thermograms of starches showing the gelatinisation endotherms of eleven different cultivars of rice.
samples were determined for the defatted starches using DSC; the results are given in Table II and Figure 2. The enthalpies, but not the temperatures
Table II
Gelatinisation temperatures and enthalpies of the starches derived from eleven cultivars of rices Gelatinisation temperature (°C)∗
Cultivar (texture)
Tonset
Tpeak
Tconclusion
Enthalpy (cal/g d.w.b.)
A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
69·9±0·5 73·0±0·6 73·2±0·3 73·8±0·2 62·1±0·3 68·6±0·1 73·7±0·2 62·2±0·2 61·2±0·5 60·7±0·4 59·8±0·4
75·3±0·5 78·2±0·1 77·6±0·1 78·0±0·1 67·0±0·5 72·4±0·2 77·1±0·1 66·7±0·2 65·8±0·4 63·8±0·4 64·2±0·2
83·4±0·9 84·6±0·0 84·0±0·1 84·3±0·5 75·3±0·5 80·3±0·1 83·1±0·3 75·8±0·6 72·7±0·9 71·2±0·9 68·0±0·9
3·0±0·4 3·2±0·0 3·2±0·1 3·6±0·1 2·5±0·1 3·2±0·1 3·4±0·1 2·5±0·1 4·3±0·1 3·3±0·2 3·1±0·0
∗ Means±SD of two duplicate determinations
Physicochemical and leaching properties of rice
Table III
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Composition and size distribution determined by the SE–HPLC method of the leached starch components from cooked milled rice grain % Proportion of leached starch Amylose
Cultivar (texture) A (Hard) B (Soft) C (Hard) D (Hard) E (Soft) F (Soft) G (Soft) H (Soft) I (Hard) J (Hard) K (Hard)
18·7 8·0 24·0 28·1 8·1 9·4 10·4 11·3 21·3 28·7 30·5
Amylopectin
Molecular weight of leached amylose×105
Chain length (DPn) of leached amylopectin
Long
Short
Mw
Mn
Mw/Mn
Long
Short
23·2 21·7 21·0 19·9 23·9 23·9 22·4 23·3 22·5 18·2 18·2
58·1 70·3 55·0 52·0 68·0 67·0 67·2 65·4 56·2 49·0 51·3
5·42 4·20 3·42 5·57 6·69 6·74 6·19 7·22 6·20 5·68 5·80
2·66 1·70 1·08 2·72 2·90 3·69 2·61 3·09 2·73 2·49 2·81
2·04 2·47 3·17 2·05 2·31 1·83 2·37 2·34 2·27 2·28 2·06
45 42 42 42 44 43 44 44 46 46 44
16 16 16 16 15 15 16 16 16 15 16
Volts
Volts
(a)
20
25
30 Elution volume (ml)
35
(b)
20
25
30 Elution volume (ml)
35
Volts
Figure 4 SE–HPLC chromatogram of the leached starch from the defatted rice grain; the arrow highlights the longest chains from the amylopectin.
20
25
30 Elution volume (ml)
35
Figure 3 SE–HPLC chromatograms showing the amylose and amylopectin peaks of the leached starch after debranching with isoamylase for (a) a typical hard cooking rice and (b) a typical soft cooking rice.
Molecular properties of leached starch The SE–HPLC chromatograms in Figure 3 show that the amylose and amylopectin fractions of the isoamylase debranched starch from the hard and soft cooking rice samples. Amylose and amylopectin in varying proportions were both leached
out into the cooking water. When the structures of these leached starches were compared with the profiles of the extracted native starches (preceding paper28, Fig. 4),it was clear that the distinctive profiles that were observed for the native starches were still retained, but that the multiple components were less distinct and the long chain region component (i.e. Ve=28·5 ml, DP≈90), which was detected in the native starch of the hard cooking rices (for variations A, C, D, I, J and K28), was not detected in the extractable/leached starch in any of the 11 samples. Figure 4, however, shows the detection of the longest chain amylopectin at Ve 28·5 ml (DP 92– 98) in the SE–HPLC profile of the leached starch from rice cultivar A, which had been defatted. This profile is very similar to that for native starch (see Fig. 4 of preceding paper28). The detection of
Volts
a b
20
25
30 Elution volume (ml)
35
Figure 5 SE–HPLC chromatograms comparing the leached amylose content from the (a) defatted and (b) undefatted starch suspension.
the longest chain amylopectin in the defatted but not the undefatted rice grains confirms our previous conclusion that this long amylopectin chain of hard rices is involved in molecular interactions with other components in the native state, or that starch components interact with others during the cooking of rice grains. The complexation of amylose and lipid was also shown when a 1% (w/v) starch suspension was heated at 90°C for 6 min and the leached components were compared for the defatted and nondefatted starches. Figure 5 shows the higher relative proportion of leached amylose from the defatted (27·0%) than from the undefatted (18·6%) starches extracted from rice cultivar variety A. The total solids contents and the turbidities of the cooking waters were also examined, and the results are included in Table IV. The extracted total solids in the cooking waters varied from 5·5 Table IV Cultivars (texture) A B C D E F G H I J K
Hardness score of cooked rice
M. H. Ong and J. M. V. Blanshard
Hardness score of cooked rice
266
10 9 8 7 6 5 4 3 2 1 10 9 8 7 6 5 4 3 2 1 0
(a) 2
y = 1·9689 + 0·11759x r = 0·566
0
4 8 12 16 20 24 28 Leached amylose content (% from SE–HPLC)
32
(b) 2
y = 1·2067 + 0·83777x r = 0·630
0·5 1·0 1·5 2·0 2·5 3·0 3·5 4·0 4·5 5·0 5·5 6·0 Leached amylose content (g/100 g rice)
Figure 6 Relationship between the leached amylose contents and the texture (hardness) of cooked rices, (a) from the SE–HPLC test, and (b) from the iodine colorimetry test.
to 12·2% of the weight of the rice grains. The turbidity of the cooking waters differed substantially only for sample B (Table IV). There was no significant correlation between texture and the extractable solids content (r=0·2819, P=0·201) nor with the turbidity (r=−0·3814, P=0·124) of the cooking water.
Leached amylose contents, total extractable solids and turbidities of cooking water of eleven cultivars of rice grains Leached amylose content (g/100 g)
Total extractable solids content (%)∗
Turbidity (0–turbid) (100–clear)
4·34±0·03 0·93±0·02 3·54±0·09 4·06±0·02 2·21±0·06 1·78±0·05 3·32±0·09 3·28±0·04 4·69±0·09 4·93±0·07 4·79±0·09
12·2±0·04 6·7±0·04 5·5±0·04 6·7±0·04 9·7±0·04 7·6±0·00 9·7±0·00 9·7±0·00 11·4±0·01 9·7±0·00 9·3±0·01
15·2 35·5 6·8 11·7 9·1 8·6 9·8 10·3 8·1 8·5 9·2
∗ Proportion (%) of the total weight of solids in cooking water over the original weight of rice grains. The values given are means±SD of duplicate determinations.
Physicochemical and leaching properties of rice
Leached amylose
back’ value and the leached amylose content with the texture of cooked rice may well indicate that the former is, in fact, a measure of the leached amylose content. The weight (Mw) and number (Mn) average molecular weights of the leached amylose, measured by laser light scattering are given in Table III. It appears that the molecular weights of the leached amyloses for all the rice samples were lower than those of the native starches28. A narrower range of polydispersity (Mw/Mn), from 1·83–3·17, was obtained for the leached amyloses than for the previous range of 3·4–8·4 for the corresponding native starches28. The absence of the longer chain amylopectin fraction and the presence of only the lower molecular weight amylose in the leaached starch in cooking water strongly suggests that, on cooking, the starch components, especially the higher molecular weights amylose and longer chain amylopectin, interact with other constituents in the rice. This observation is supported by the detection of amylose-lipid complexes in rices from X-ray diffraction and DSC studies19,36,37. Priestley36 proposed that the less-sticky behaviour of parboiled rice was due to the presence of these insoluble amylose complexes and not to retrogradation of starch as commonly assumed (M. H. Ong and J. M. V. Blanshard, unpublished results). These complexes have been thought to be responsible for the reduction in leaching of the solids and
Table III shows the relative proportions of leached amylose, long and short chain amylopectin (the amylopectin was divided into only two fractions). SE–HPLC method (Table III) gives the relative proportions of amylose and amylopectin, whilst iodine colorimetry (Table IV) records the absolute amylose content in the cooking water. From the iodine colorimetry, the cooking water for the 11 rice samples contained 0·93 to 4·93 g of amylose/100 g of the original rice grains. The relative leached amylose content from the SE– HPLC method (Table III) was correlated positively with the amylose content of the rices (r=0·7525, P<0·01∗∗) and with the textures of the cooked rices (r=0·7828, P<0·01∗∗). A slightly lower but significant correlation was also obtained between the absolute leached amylose content determined by iodine colorimetry (r=0·6527, P<0·05∗) (Table IV) and the textures of the cooked rices. Figure 6 shows the linear correlation between the leached amylose content determined from the SE–HPLC (r2=0·566) and iodine colorimetry (r2=0·630) tests with the hardness of cooked rice. This study shows that hardness after cooking of the rice samples was correlated with the values of ‘setback’ derived using the Viscoamylograph. ‘Setback’ was also reported to be an indicator of the texture of cooked rice2,3. Since the parameter measures changes in paste viscosities after heating (Fig. 1), the positive correlation between the ‘set-
Table V
267
Correlation analysis of the texture and selected properties of rice Texture of cooked rice
Correlation parameters Amylose content Amylopectin chain length DP 92–98 DP 22–25 DP 18 DP 10–11 Pasting temperature Peak viscosity ‘Setback’ Gelatinisation temperature Enthalpy Crystallinity Molecular weight of amylose Leached amylose content (SE–HPLC) Leached amylose content (I2)
r 0·7220
P 0·004∗
0·7535 −0·5493 −0·5668 −0·5545 0·6213 −0·4294 0·7995 0·269 0·524 −0·2747 0·2850 0·6527
0·040∗ 0·040∗ 0·030∗ 0·040∗ 0·016∗ 0·082 0·001∗∗ 0·467 0·040∗ 0·194 0·185 0·011∗
0·7828
0·001∗∗
∗ and ∗∗ significant at P<0·05 and 0·01 levels of probability respectively.
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solubilisation of the kernel, which make parboiled rice less prone to disintegration. Recently, the presence of types I and II amylose-lipid complexes has been found to depend on the amylose contents and the parboiling processes19.
Leached amylopectin Table III also shows the relative proportions and chain lengths of amylopectins of the leached starches from the eleven rice samples. The relative proportion of the shorter chains (DPn 15–16) were found to vary to a greater extent (i.e. 49·0–70·3%) than those of the longer (DPn 42–46) amylopectin chains (18·2–23·9%). It is also apparent from Table III that the soft rices (cultivars B, E, F, G and H) have higher proportions (65·4–70·3%) of short amylopectin chains than the hard rices (49·0– 58·1%). This observation seems to support the previous finding28 that the soft and hard rices have different short chain populations.
Texture determinant of cooked rice Table V summarises some correlations between texture and selected properties of rice determined in both this and the previous study28. The difference in amylose contents can be measured indirectly from the physicochemical tests, such as from the changes in viscosities (‘setback’) or the leaching behaviour of rices. The presence of distinctive SE–HPLC profiles reported in the first paper supports the view that amylopectin affects the final textural properties of rice grains significantly. The intermolecular interactions of the longer chain amylopectin and amylose in the rice grain were evident indirectly from the different structures of starch determined from leaching tests. We conclude, therefore, that the content of amylose and the structure of the amylopectin of starch profoundly affect the final textural properties of cooked rice.
Acknowledgement The authors thank Dr K. Jumel for assistance with the SE–HPLC system and the multi-angle laser light scattering and refractive index detectors.
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