Phase transition and swelling behaviour of different starch granules over a wide range of water content

LWT - Food Science and Technology xxx (2014) 1e8

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Phase transition and swelling behaviour of different starch granules over a wide range of water content Shujun Wang a, *, Caili Li a, 1, Jinglin Yu b, 1, Les Copeland c, Shuo Wang a a Key Laboratory of Food Nutrition and Safety, Ministry of Education of China, College of Food Engineering and Biotechnology, Tianjin University of Science & Technology, Tianjin 300457, China b Research Centre of Modern Analytical Technique, Tianjin University of Science & Technology, Tianjin 300457, China c Faculty of Agriculture and Environment, The University of Sydney, NSW 2006, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2014 Received in revised form 30 May 2014 Accepted 10 June 2014 Available online xxx

The relationship between swelling behaviour and thermal transitions of starch was investigated by differential scanning calorimetry (DSC) in combination with swelling power of starch. Sodium dodecyl sulphate (SDS)-treated wheat, waxy maize and potato starches showed increasing swelling power with increasing water/starch ratio. In contrast, swelling power of untreated wheat and high-amylose maize starches increased initially with increasing water/starch ratio, and then remained essentially unchanged above a certain ratio. The main endotherm G of native and SDS-treated wheat starch broadened progressively with increasing water/starch ratio up to 10:1. SDS-treated wheat, waxy maize and potato starches showed a typical endotherm over the whole range of water/starch ratios from 0.33:1 to 25:1, but the maximum enthalpy change occurred at different water/starch ratios. Our results indicate that thermal transition behaviour of starch granules is a very complex process, which involves swelling and leaching of starch polymer molecules rather than the dissociation of double helices or melting of crystallites. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Wheat starch Potato starch Maize starch Swelling Thermal transition

1. Introduction When heated in excess water, starch granules undergo an irreversible phase transition, referred to as gelatinization, in which the highly ordered structure is disrupted. Starch gelatinization has been broadly defined as the “collapse (disruption) of molecular orders (breaking of hydrogen bonds) within the starch granule manifested in irreversible changes in properties such as water uptake, granular swelling, crystallite melting, unwinding of double helices, loss of birefringence, starch solubilisation and viscosity development”(Atwell, 1988; BeMiller, 2011; Biliaderis, 2009). This definition implies that at least three distinct changes occur during gelatinization: granule swelling, disruption of ordered structures (crystalline and molecular) and solubilisation of starch molecules (Wang & Copeland, 2012a). The extent to which these changes occur is a major determinant of the functional properties of starch, including its susceptibility to enzymatic digestion, and depends on the type of starch and the moisture and heating conditions during

* Corresponding author. Tel.: þ86 60601430; fax: þ86 22 60601332. E-mail address: [email protected] (S. Wang). 1 Equal contribution.

hydrothermal processing (Biliaderis, 2009; Goldstein, Nantanga, & Seetharaman, 2010). Starch gelatinization has been studied extensively using a variety of techniques, of which DSC is accepted widely as most suitable for quantitative and qualitative analyses following pioneering work of Stevens and Elton (1971) and Donovan (1979). Other techniques including wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), 13C nuclear magnetic resonance (NMR), FTIR, and microscopy (light microscopy, electron microscopy and transmission microscopy) have also been used directly or in combination with DSC, to examine multiple aspects of starch gelatinization simultaneously (Biliaderis, 2009; Ratnayake & Jackson, 2009; Wang & Copeland, 2013a, 2013b; Derycke et al., 2005; Jenkins & Donald, 1998; Le Bail et al., 1999; Vermeylen et al., 2006a; Vermeylen et al., 2006b). Donovan's pioneering work considered the effect of water content on thermal transitions of starch granules (Donovan, 1979). He observed a single constant endotherm (called endotherm G) with potato starch at a water/starch ratio of 1:1.5 or greater, which was assumed to represent the complete gelatinization of starch (Donovan, 1979). However, other studies have obtained different DSC profiles, indicating that Donovan's observations and conclusions for potato starch, which is known to have granules with a

http://dx.doi.org/10.1016/j.lwt.2014.06.028 0023-6438/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

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Starch solubility

C

14 12

10

10

8

8

6

6

B

Starch solubility (%)

Swelling power

12

4

4

2

A

2

0 0 --

30:1

25:1

20:1

15:1

10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

Water/starch ratio (%) Fig. 1. Effect of water/starch ratio (40 mg of starch was used) on swelling power and solubility of wheat starch granules. The zones marked A, B and C correspond to the stages of swelling discussed in the text, namely A: limited swelling without leaching; B: incomplete swelling with some leaching; C: complete swelling with continuous leaching.

centrifuged at 13, 000  g for 10 min. When a supernatant was obtained, it was transferred carefully to a glass evaporating dish, evaporated to dryness on a steam bath, and dried further for 24 h at 80  C. SDS-treated wheat starch, potato starch and waxy maize starch formed a soft gel, making it difficult to separate a clear supernatant even after centrifugation. For these starches, the starch solubility was considered to be zero. The sedimented, swollen granules and dried soluble fraction were weighed to determine swelling power (g H2O absorbed/g dry starch) and solubility (% of dry starch) using the following formulae:

Solubility; S ¼ ½weight of solubles=  ½dry weight of original starch Swelling power; SP ¼ ½weight of swollen granules=  ½dry weight of original starch

40 35

To understand the swelling behaviour of starches during the DSC heating, the water:starch ratios used in the DSC measurements were largely consistent with those used in the swelling power test, which can be seen in Figs. 1e5. For direct comparison of the effect of water/starch ratio on swelling power and DSC thermal transition, the water/starch ratio used in Figures of DSC thermal transition parameters was adjusted correspondingly.

30

Swelling power (g/g)

2.2. Sample preparation

1.0

0.8

Swelling power Starch solubility

25

0.6

20 0.4

15 10

Starch solubility (%)

Wheat, potato, and waxy maize starches were obtained from commercial sources, with amylose content of 27%, 25% and 3%, respectively. High-amylose maize starch (85% amylose) was obtained from the National Starch and Chemical Company (Shanghai, China). The starches were used without further purification. To investigate the influence of surface proteins and lipids on starch swelling and thermal transition, wheat starch was treated with 2% sodium dodecyl sulphate (SDS) solution at room temperature for 24 h to remove surface proteins and lipids (Debet & Gidley, 2006). All other chemicals were analytic grade from SigmaeAldrich Chemical Corporation (Shanghai, China).

16

14

0.67:1

2.1. Materials

18

16

0.5:1

2. Materials and methods

20 18

0.33:1

typical swelling property, do not apply generally. Increasing experimental evidence is showing that the endotherm G at a water/ starch ratio above 1.5 shifts to higher temperature with increasing water/starch ratio and does not represent the full gelatinization of e, & Thoen, 2002; Liu, Yu, Xie, starch granules (Cruz-Orea, Pitsi, Jame & Chen, 2006; Randzio, Flis-Kabulska, & Grolier, 2002; Tananuwong & Reid, 2004; Wang & Copeland, 2012a, 2012b). In addition to water content, the surface proteins and lipids were shown to influence the swelling behaviour and thermal properties of starch granules (Debet & Gidley, 2006; Wang et al., 2014). Sodium dodecyl sulphate (SDS) solution has been proven to remove proteins and lipids from surface of starch granules sufficiently (Debet & Gidley, 2006). In a previous study (Wang & Copeland, 2012a), we investigated the effect of water content on the thermal transitions of pea starch granules over a wide range of water/starch ratios. The endothermic transition of pea starch granules was proposed to reflect the swelling behaviour of starch during DSC heating, and not complete gelatinization. The present study is aimed at determining how generally this relationship between DSC thermal transitions and swelling behaviour, as observed for pea starch, applies, thereby increasing our understanding of what the DSC endothermic transitions for starch actually represent. To this end, the swelling power and DSC profiles of wheat starch, waxy maize starch, potato starch and high-amylose maize starch were investigated over a wide range of water/starch ratios (from 0.33:1 to 25:1 or 30:1). To our knowledge, this is the first study to investigate the thermal transitions of different starches over such a wide range of water/starch ratios.

Swelling power (g/g)

2

0.2

2.3. Swelling power and starch solubility

0

0.0 --

30:1

25:1

20:1

15:1

10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.33:1

0.5:1

Swelling power and solubility of starch were determined in triplicate according to the method described elsewhere (Wang & Copeland, 2012c) as follows. Exactly 40 mg (wet basis) of starch were weighed into a 2 ml screw cap plastic test tube and water was added. After the lid was screwed on tightly, the starch-water mixtures were heated in a water bath at 92.5  C for 30 min with regular shaking. The samples were cooled at 20  C for 3 min and

5

Water/starch ratio (w/w) Fig. 2. Effect of water/starch ratio on swelling power and starch solubility of SDStreated wheat starch.

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

S. Wang et al. / LWT - Food Science and Technology xxx (2014) 1e8

-2

Endothermic heat flow

-4

2.4. Differential scanning calorimetry

a

3:1

Native wheat starch

3:1.5 3:2

-6

3:3

-8

3:4 3:5

-10

3:6 3:9

-12

3:12

Endo Exo

3:15 3:22.5 3:30

-14 -16

2:30

-18

1:25

-20 50

60

70

80

90

100

110

120

o

Temperature ( C) 80

Start temperature Peak temperature End temperature

Transition temperature ( o C)

75

b

70

65

60

Thermal transition measurements of starch were made over a wide range of water content using a Modulated Differential Scanning Calorimeter MDSC 2920 instrument (TA Instruments Inc., Delaware, USA) equipped with a thermal analysis data station and data recording software. Exactly 3 mg of starch were weighed into 40 mL aluminium pans. Different amounts of distilled water were added to the starch in the DSC pans with a microsyringe. For samples with low water content (water/starch ratios of 1:3 and 1:2), a pin was used to mix gently starch with water. When larger volumes of water were added, care was taken to ensure that the starch granules were completely immersed in the water by gentle shaking. The pans were sealed and reweighed to determine the amount of water added. The sealed pans were allowed to stand overnight at room temperature before DSC analysis. An empty pan was used as a reference. The pans were heated from 30 to 120  C at a scanning rate of 10  C/min. The instrument was calibrated using indium as a standard. The analysis was done in duplicate, and the precision of water addition based on the weight of the sealed pans before heating was estimated to be ±0.5%. The Universal Analysis 2000 software was used to analyse the main endotherm of the DSC traces for start (Ts), peak (Tp) and end (Te) temperatures and enthalpy change (DH). The start and conclusion temperatures were defined, respectively, as the point at which the DSC trace first starts and finally ceases to deviate from a flat baseline. The peak temperature was defined as the point of maximum endothermic heat flow relative to the baseline. The DH was defined as the area under the line drawn from the start temperature to the end temperature. 2.5. Statistical analysis All analyses were replicated at least twice and mean values and standard deviation values are reported. Analysis of variance (oneway ANOVA) by Duncan's test (p < 0.05) were conducted using the SPSS 10.0 Statistical Software Program (SPSS Inc. Chicago, IL, USA).

55

50 10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.5:1

0.33:1

Water/starch ratio (w/w)

3. Results 3.1. Effect of water content on swelling power and solubility of wheat starch

14

c 12

Enthalpy change J/g

3

10 8 6 4 2 0

15:1

7.5:1

10:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.5:1

0.33:1

Water/starch ratio (w/w) Fig. 3. a: Effect of water/starch ratio (3 mg of starch was used) on DSC thermograms of wheat starch (the numbers under each line represent the actual water/starch ratio (w/

Swelling power of native wheat starch increased from 1.5 g/g at a water/starch ratio of 0.33:1 to 16.7 g/g at a water/starch ratio of 15:1, above which swelling power decreased slightly (Fig. 1). No supernatant was obtained on centrifugation when the water/starch ratio was below 10:1, indicating that all of the water added was absorbed into the swollen granules and remained inside the starch gel. At a water/starch ratio of 10:1, a supernatant was obtained after centrifugation, even though the swelling power of starch had not reached a maximum (Fig. 1). The amount of wheat starch solubilized (i.e., leached out of the swollen granules into the supernatant) at a water/starch ratio of 10:1 was 1.2%. The solubility of wheat starch increased to a value of 13.1% at a water/starch ratio of 30:1 (Fig. 1). In comparison, SDS-treated wheat starch displayed a quite different swelling power curve (Fig. 2). Swelling power increased continuously with increasing water content up to a value of 34.6 g/g

w); b. Effect of water/starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC transition temperatures of wheat starch; c. Effect of water/ starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC enthalpy changes of wheat starch.

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

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70

at a water/starch ratio 30:1, at which point it had not reached a plateau, unlike untreated wheat starch as shown in Fig. 1. A theoretical value for swelling power of starch can be calculated from the weight of starch plus added water divided by the dry weight of the starch (Wang & Copeland, 2012a). At a water/starch ratio of 30:1, the actual swelling power of SDS-treated starch was slightly less than the theoretical value. No supernatant was obtained on centrifugation of the swollen granules after heating and cooling, indicating that no starch was leached out of the SDS-treated starch granules. Potato starch and waxy maize starch presented similar swelling power curves as that of SDS-treated wheat starch (not shown). The swelling power of both potato and waxy maize starches starch increased with increasing water/starch ratio without reaching a plateau, and no supernatant was obtained after centrifugation of the swollen granules, indicative of the strong binding of water by the swollen granules. The measured swelling power was close to the calculated theoretical value (22.9 g/g and 23.3 g/g, respectively, for potato starch at a water/starch ratio of 20:1). The swelling trend of high-amylose maize starch resembled that of untreated wheat starch (data not shown). Swelling power increased to a maximum of 4.4 g/g at a water/starch ratio of 3:1, above which swelling power decreased slightly to a plateau value of 4.0 g/g above a water/starch ratio of 10:1. Below a water/starch ratio of 3:1, no starch was leached out of the swollen granules after centrifugation, but starch solubility increased to a maximum value of 5.8% at a water/starch ratio of 10:1.

65

3.2. Thermal transitions of starch at different water/starch ratios

Endothermic heat flow

-2 -4

3:1 3:1.5 3:2

-6

3:3 3:4

a

potato starch

M1

G

3:5

-8

3:6 3:9

-10

3:12 3:15

-12

3:22.5 3:30 2:30

-14

1:25 -16 50

60

70

80

90

100

110

120

o

Temperatrue ( C) 85

Transition temperature ( o C)

80

Start temperature Peak temperature End temperature

b

75

60 55 50 25:1

15:1

10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.5:1

0.33:1

Water/starch ratio (w/w) 25

c Enthalp change (J/g)

20

15

10

5

0

25:1

15:1

10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.5:1

0.33:1

Water/starch ratio (w/w) Fig. 4. a: Effect of water/starch ratio on DSC thermograms of potato starch (the numbers under each line represent the actual water/starch ratios); b: Effect of water/ starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC transition temperatures of potato starch; c: Effect of water/starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC enthalpy changes of potato starch.

DSC thermograms of native wheat starch are shown in Fig. 3a, with the effect of different water/starch ratios on transition temperatures and enthalpy change shown in Fig. 3b and c, respectively. A well-defined endothermic transition (endotherm G) was observed between 50 and 85  C for water/starch ratios below 15:1. At greater water/starch ratios, the initial endothermic trend seemed to flatten out at the maximum heat flow (Fig. 3a). A second shallow endotherm occurring at about 90e100  C was also observed, which appeared to overlap with a subsequent exothermic transition (Fig. 3a). With increasing water/starch ratios, endotherm G shifted to a higher temperature, as shown by a gradual increase in the peak (Tp) and end (Te) temperatures. In contrast, the start temperature (Ts) of endotherm G remained essentially unchanged across the entire range of water/starch ratios (Fig. 4c). Tp and Te of endotherm G increased from 61.8 to 65.1  C, and from 66.8 to 73.3  C, respectively, whereas the enthalpy change of endotherm G increased gradually from 1.3 to 13.4 J/g with increasing water/starch ratio up to 10:1. When the water/starch ratio was above 10:1, the endotherm G flattened out from the maximum heat flow, making identification of Tc of endotherm G extremely difficult. DSC thermograms of SDS-treated wheat starch were similar to those shown for native wheat starch. Tp and Te increased from 62.4 to 65.8  C and from 65.8 to 71.1  C, respectively, as the water/starch ratio increased up to 15:1 (results not shown). The enthalpy change increased from 1.4 (water/starch ratio of 0.33:1) to 12.5 J/g as the water/starch ratio increased up to 10:1, and then decreased to 10.2 J/g at a water/starch ratio of 15:1. A weak endothermic transition was still observed at a water/starch ratio of 25:1, although Tc was not identified clearly. Potato starch showed a typical endothermic transition over the wide range of water/starch ratios from 0.33:1 to 25:1 (Fig. 4a). At water/starch ratios of 0.33:1 and 0.5:1, the endothermic transition was small, and at water/starch ratios of 0.67:1 and 1:1, a shoulder on the high-temperature side of endotherm G developed, which

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

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a

-2

Waxy maize starch

Endothermic heat flow

3:1 -4

3:2 3:3

-6

3:4

-8

3:5 3:6 3:9

-10

3:12

-12

3:15 3:22.5

M1

G

3:30 -14

2:30 1:25

-16 -18

50

60

70

80

90

100

110

120

o

Temperature ( C) 95

Start temperature Peak temperature End temperature

b

85

o

Transition temperature ( C)

90

80 75 70 65 60 55 50 25:1

15:1

10:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.33:1

Water/starch ratio (w/w)

The present study has sought to increase our understanding of the starch gelatinization processes that DSC thermograms represent and thereby advance our knowledge of the mechanism of starch gelatinization. This is the first study to investigate swelling power and solubility of water/starch systems over a wide range of water/starch ratios for wheat, waxy maize, potato and highamylose starches at fixed temperature (92.5  C). DSC thermal transitions of the starches over the same wide range of water/starch ratios were also recorded. The swelling power and starch solubility of starch granules were measured at the same water/starch ratios as were thermal transitions, with the aim of relating the swelling behaviour of granules to thermal transitions.

c

16

Enthalpy change (J/g)

was termed endotherm M1. Similar to SDS-treated wheat starch, potato starch also showed a typical endotherm G transition at a water/starch ratio of 25:1. At water/starch ratios greater than 1.33:1, the system showed a characteristic endothermic transition with well-defined Ts, Tp and Te values. Tp increased from 65.1 to 68.0  C as the water/starch ratio increased up to 10:1. Tc increased gradually from 66.2 to 71.0  C as the water/starch ratio was increased from 0.33:1 to 1:1, followed by a sharp increase to 77.8  C at a water/starch ratio of 1.33:1. At higher water/starch ratio, Tc did not vary greatly until the water/starch ratio was 10:1, above which Tc decreased slightly. The enthalpy change increased from 1.0 to 17.0 J/g as the water/starch ratio increased up to 1.33:1. Above this ratio, the enthalpy remained essentially unchanged till the water/ starch ratio of 10:1, above which there was a slight decrease (Fig. 4b and c). Typical endothermic transitions occurring between 50 and 80  C were observed for waxy maize starch at all of the water/starch ratios measured, although at the lowest water/starch ratio endotherm G was small (Fig. 5a). Ts was not greatly affected by the water/ starch ratio. With water/starch ratio increasing, the peak of endothermic transition shifted slowly to higher temperature. Tp increased from 70 to 73.4  C as the water/starch ratio increased from 0.67:1 to 25:1. Te increased from 72.6 to 83.1  C as the water/ starch ratio increased from 0.67:1 to 2:1 and did not change significantly at higher ratios. The enthalpy change increased sharply from 1.0 to 16.3 J/g at a water/starch ratio of 2:1 and then did not change greatly at higher water/starch ratios (Fig. 5b and c). Similar to potato starch, waxy maize starch also showed a biphasic transition at certain water/starch ratios and a typical endothermic transition at a water/starch ratio of 25:1. No distinct endothermic transitions were observed over the entire range of water/starch ratios for high-amylose starch. A broad endothermic trend was observed, which was appeared to be followed by an exothermic transition occurring at 100e120  C (data not shown). 4. Discussion

20 18

5

14 12 10 8 6 4

4.1. Effect of water content on swelling power and starch solubility

2 0 --

25:1

10:1

15:1

7.5:1

5:1

4:1

3:1

2:1

1.67:1

1.33:1

1:1

0.67:1

0.33:1

Water/starch ratio (w/w) Fig. 5. a: Effect of water/starch ratio on DSC thermograms of waxy maize starch (the numbers under each line represent the actual water/starch ratios); b: Effect of water/ starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC transition temperatures of waxy maize starch; c: Effect of water/starch ratio (the ratio was modified for comparison with that for swelling power test) on DSC enthalpy changes of waxy maize starch.

Native wheat starch granules absorbed all the added water on heating to 92.5  C, and no starch polymer molecules were leached from the swollen granules after centrifugation when the water/ starch ratio was less than 10:1. Hence, under these conditions, swelling of native wheat starch was incomplete. Starch polymer molecules started to leach out of the swollen granules at a water/ starch ratio of 10:1, even though granule swelling had not reached a maximum value. At a water/starch ratio of 15:1, starch granules were swollen to their maximum extent under the conditions and starch polymer molecules were leached in increasing amounts from the swollen granules. Starch polymer molecules continued to leach out of the granules as the water/starch ratio increased,

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

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S. Wang et al. / LWT - Food Science and Technology xxx (2014) 1e8

whereas swelling power decreased slightly. Thus, swelling behaviour of wheat starch can be divided into three different stages which occur progressively as the water/starch ratio increases: limited swelling at low water/starch ratio (stage A), incomplete swelling with starch leaching at high water/starch ratio (stage B), and complete swelling with continuous starch leaching in excess water (stage C) (Fig. 1). The swelling profile of wheat starch was comparable with that of pea starch granules (Wang & Copeland, 2012a). In comparison, swelling power of SDS-treated wheat starch did not reach a limiting value as water/starch ratios were increased up to 30:1. The measured swelling power of SDS-treated starch was comparable to the theoretical values, consistent with starch polymer molecules not being leached during the swelling power test. No differences in swelling power and starch solubility were observed between native wheat and SDS-treated starches at water/ starch ratios below 7.5:1, as the swelling of starch granules was incomplete in both cases. However, as the water/starch ratio increased, SDS-treated starch held more water, resulting in higher swelling power and lower starch solubility. The increased swelling capability of SDS-treated starch was assumed to be associated with the removal of surface proteins and lipids, which are considered to inhibit swelling of most cereal starches (Chan, Bhat, & Karim, 2010; Debet & Gidley, 2006; Tester & Morrison, 1990). The swelling power profiles of potato and waxy maize starches, which were similar to that of SDS-treated wheat starch, did not reach a limiting value at a water/starch ratio of 30:1. No leaching of starch polymer molecules occurred over the whole range of water/ starch ratios, and the measured swelling power of these starches was close to the theoretical value. Potato starch and waxy starch are known to have very high swelling capacity. The high swelling power of potato starch has been proposed to be related to repulsion between phosphate groups weakening the bonding within the crystalline domain, thereby increasing potential for swelling (Galliard & Bowler, 1987), whereas for waxy starch high swelling power has been attributed to the high content of amylopectin, which is assumed to absorb and hold more water within its threedimensional branched structure than the linear amylose (Wang, Sharp, & Copeland, 2011). In contrast to the other starches, high-amylose maize starch had much lower swelling power, as has been reported for high-amylose starches generally (Case et al., 1998; Liu, Ramsden, & Corke, 1997; Shi, Capitani, Trzasko, & Jeffcoat, 1998). The lower swelling power of high-amylose starch is considered to be associated with the lower content of amylopectin and the presence of amyloseelipid complexes (Tester & Morrison, 1990). The gelatinization temperature of high-amylose starches is generally higher than for other starches (Carciofi et al., 2012; Li, Jiang, Campbell, Blanco, & Jane, 2008; Naguleswaran, Vasanthan, Hoover, & Bressler, 2013; Shi et al., 1998; Wei et al., 2011). High-amylose starches begin to swell below 100  C, but temperatures greater than 130  C are required to fully disperse these starches due to the greater involvement of amylose molecules in the crystalline regions than in other starches (Shi et al., 1998). Hence, it is possible that at the temperature used in the present experiments (92.5  C), the peak swelling power value, which was observed at a water/starch ratio of 3:1, did not represent granules that had swollen completely. 4.2. Effect of water content on thermal transition behaviour of starches The thermograms of native wheat starch showed that endotherm G shifted to a higher temperature and that Tp, Tc and DH increased gradually with increasing water/starch ratio up to 10:1. At water/starch ratios greater than 2:1, Tp and Tc have been reported to

increase with increasing water content (Cruz-Orea et al., 2002; Fukuoka, Ohta, & Watanable, 2002; Liu et al., 2006; Randzio et al., 2002; Tananuwong & Reid, 2004; Wang & Copeland, 2012a; Wong & Lelievre, 1982). Similarly, the DH of endotherm G has also been reported to increase with increasing water/starch ratio for wheat starch (Randzio et al., 2002; Tananuwong & Reid, 2004), rice starch (Biliaderis, Page, Maurice, & Juliano, 1986), legume starches (Wang & Copeland, 2012a) and maize starch (Liu et al., 2006). At higher water/starch ratios, endotherm G flattened out at the point of maximum heat flow and did not have a clearly identified end point. Endotherm G has been proposed to represent water absorption and swelling by starch granules, with the end point corresponding to the completion of these processes under the particular experimental conditions (Wang & Copeland, 2012a). We propose that the change from a well-defined sharp endotherm G to a broadening thermogram without clearly defined features represents the transition from granules absorbing water and swelling to swelling of granules accompanied by leaching of starch polymer molecules, considered to be predominantly amylose. Accordingly, at a water/starch ratio of 2:1, swelling of wheat starch granules is incomplete and endotherm G reflects the partial swelling behaviour of starch granules rather than full gelatinization. This interpretation is substantiated by previous findings that not all crystalline and lamellar structures are disrupted at the end of this so-called gelatinization endotherm (Jenkins & Donald, 1998; Vermeylen et al., 2006a). The thermograms of SDS-treated wheat starch were generally similar to those of native wheat starch, with similar Ts, Tp and DH changes over the wide range of water/starch ratios, although SDStreated starch had lower Te values. However, unlike native wheat starch, the endotherm was still clearly defined at water/starch ratios of 15:1 and 25:1, when the starch granules had not reached maximum swelling and there was no leaching of starch polymer molecules. This finding supports the proposal that endotherm G represents the completion of water absorption and granule swelling. SDS-treated starch displayed a more rapid and greater swelling tendency as seen from decreased pasting temperature and increased peak viscosity in RVA profiles (data not shown), and from the swelling power data presented in this paper. The faster completion of granule swelling was consistent with the lower Te compared to native wheat starch at the same water/starch ratios. DSC enthalpy change of starch reflects the loss of ordered structure (double helices/crystallites) of granules (Cooke & Gidley, 1992), and the similarity in DH for native and SDS-treated wheat starches is consistent with SDS treatment not significantly affecting the crystalline structure of starch granules (data not shown). For SDStreated starch, the greater swelling tendency at a given water/ starch ratio implies comparatively more disruption of ordered structures, which would tend to increase enthalpy change. On the other hand, decreased Te indicates the end-point of granule swelling reached under the water-limited conditions, occurred at a lower temperature, which may cause less disruption to ordered structure in the granules (for example, some of the more stable helical regions are not affected at the lower temperature). The balance between these two scenarios could result in the little or no net change in enthalpy, as was observed. The thermograms of potato and waxy maize starches had several features in common. Firstly, both starches presented a weak shoulder on the high-temperature side of endotherm G at low water/starch ratios, as has been observed with other starches (Donovan, 1979), although not in this and some previous studies (Biliaderis et al., 1986; Fredriksson, Silvrio, Andersson, Eliasson, & Aman, 1998; Patel & Seetharaman, 2010; Wang & Copeland, 2012a). Secondly, both starches reached the maximum enthalpy change at low water/starch ratios (1.33:1 for potato starch and 2:1

Please cite this article in press as: Wang, S., et al., Phase transition and swelling behaviour of different starch granules over a wide range of water content, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.06.028

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for waxy maize starch), when both starches had swollen to only a limited extent. Similar results were also reported for potato starch (Cruz-Orea et al., 2002; Donovan, 1979) and waxy maize starch (Liu et al., 2006), although potato and waxy maize starch had higher maximum swelling power and greater maximum enthalpy changes than other starches. Swelling power of potato and waxy maize starch increased with increasing water/starch ratio up to 30:1, but the enthalpy change did not follow this trend, and hence did not correlate with the swelling of granules. The reason for this observation is unclear and needs to be investigated further. Finally, similar to SDS-treated wheat starch, potato and waxy maize starches both showed a clearly defined endotherm even at a water/ starch ratio of 25:1, when the all the water added was absorbed and granule swelling occurred without leaching of starch polymer molecules. Again, this is consistent with our hypothesis that endotherm G reflects the water absorption and swelling of starch granules. The thermogram of high-amylose maize starch displayed an endothermic trend over the whole range of water/starch ratios rather than a typical endotherm, indicating that gelatinization of high-amylose maize starch was limited at both low and high water/ starch ratios. As high-amylose starch requires high temperature for complete gelatinization, the lack of a well-defined endotherm may reflect limited swelling of granules at low water/starch ratios, and swelling of granules with partial leaching of starch molecules at high water/starch ratios. 4.3. Nature of overlapping endothermic and exothermic transitions in DSC thermograms Overlapping endothermic and exothermic transitions were observed for all the starch-water systems, although the effect was weak at some water/starch ratios. Similar observations have been reported in other studies and have been attributed to the dissociation, melting and subsequent recrystallization of amyloseelipid complexes (Derycke et al., 2005; Le Bail et al., 1999). While this could apply to cereal starches, it is less likely to apply to tuber, legume starches, and especially waxy starches, all of which contain very small amounts of lipid. In a previous study, these overlapping transitions were attributed to a phase transition of water from vapour to liquid (Wang & Copeland, 2012a; 2012b). We now propose that this occurs by the following mechanism. Firstly, water is absorbed into the granules causing them to swell and giving rise to endotherm G. With increasing temperature, the liquid water molecules redistribute in the swollen granules, concomitant with ongoing melting of residual crystallites, before the water molecules vaporise and escape from the starch gel. The condensation of water droplets on the inside of the pan lids results in the DSC exothermic transition. 5. Conclusions In this study, the effect of water/starch ratio on swelling behaviour and DSC thermal transition of starch granules was investigated. Swelling behaviour of starch granules as a function of water/starch ratio at 92.5  C can be divided into three categories: limited to complete swelling with partial leaching of starch polymer molecules (as observed for wheat and pea starches), swelling without leaching of starch polymer molecules (waxy maize and potato starches), and limited swelling with some leaching of starch polymer molecules (high-amylose starch). The endothermic transition over a wide range of water/starch ratio reflects different aspects of the swelling behaviour of starch granules during DSC heating. For wheat and pea starches, the endotherm G reflects the change from limited swelling to maximum swelling with partial

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leaching of starch polymer molecules. For potato and waxy starches with very high swelling power, the endotherm G only reflects swelling of starch granules. However, the swelling behaviour of high-amylose starch during DSC heating is not as easily characterized. Based on the results of the present study and previous work (Wang & Copeland, 2012a), we propose that endotherm G in DSC profiles of commonly used starch/water systems represents the energy change mainly associated with water absorption and granule swelling. As most DSC measurements are made at low water/starch ratios (2:1 or 3:1), the extent to which endotherm G represents starch gelatinization will depend on how gelatinization is defined. If gelatinization includes solubilisation of starch polymers, endotherm G obtained at water/starch ratios below which starch polymers begin to leach out of granules will not represent complete gelatinization. If gelatinization is defined as the complete disruption of granular structure, a clearly defined endotherm G would indicate this has occurred even though swelling of the granules is incomplete and no starch polymer molecules have leached out. As the endotherm obtained at a water/starch ratio of 2:1 does not represent the complete gelatinization of most starch granules, the enthalpy change under these conditions does not totally relate to the quantity or quality of starch crystalline structure. The relationships between the loss of ordered structure and swelling behaviour of starch granules during thermal transition need to be studied further to better understand the extent to which the energy absorbed is for the unwinding of amylopectin helices, or for the swelling of starch granules. Acknowledgements SW gratefully acknowledges the financial support from the National Natural Science Foundation of China (31371720) and the Natural Science Foundation of Tianjin (13JCYBJC38100). SW also greatly appreciates the financial support of Haihe River Scholar Program (000050401) from Tianjin University of Science & Technology. References * Atwell, W. A., Hood, L. F., Lineback, D. R., Varriano-Marston, E., & Zobel, H. F. (1998). The terminology and methodology associated with basic starch phenomena. Cereal Foods World, 33, 306e311 (This reference dealt with the general definition of starch gelatinization, which is very important for readers to compare our results and conclusions with general definition). * BeMiller, J. (2011). Pasting, paste, and gel properties of starchhydrocolloid combinations. Carbohydrate Polymers, 86(2), 386e423 (This reference dealt with the general definition of starch gelatinization, which is very important for readers to compare our results and conclusions with general definition). Biliaderis, C. G. (2009). Structural transitions and related physical properties of starch. In J. BeMiller, & R. Whistler (Eds.), Starch chemistry and technology (pp. 293e372). USA.: Academic Press. Biliaderis, C. G., Page, C. M., Maurice, T. J., & Juliano, B. O. (1986). Thermal characterization of rice starches: a polymeric approach to phase transitions of granular starch. Journal of Agricultural and Food Chemistry, 34(1), 6e14. Carciofi, M., Blennow, A., Jensen, S. L., Shaik, S. S., Henriksen, A., Buleon, A., et al. (2012). Concerted suppression of all starch branching enzyme genes in barley produces amylose-only starch granules. BMC Plant Biology, 12, 223e238. Case, S. E., Capitani, T., Whaley, J. K., Shi, Y.-C., Trzasko, P., Jeffcoat, R., et al. (1998). Physical properties and gelation behavior of a low-amylopectin maize starch and other high-amylose maize starch. Journal of Cereal Science, 27(3), 301e314. Chan, H.-T., Bhat, R., & Karim, A. A. (2010). Effects of sodium dodecyl sulphate and sonication treatment on physicochemical properties of starch. Food Chemistry, 120(3), 703e709. Cooke, D., & Gidley, M. J. (1992). Loss of crystalline and molecular order during starch gelatinization: origin of the enthalpic transition. Carbohydrate Research, 227(6), 103e112. e, P., & Thoen, J. (2002). Phase transitions in the starchCruz-Orea, A., Pitsi, G., Jame water system studied by adiabatic scanning calorimetry. Journal of Agricultural and Food Chemistry, 50(6), 1335e1344. Debet, M. R., & Gidley, M. J. (2006). Three classes of starch granule swelling: influence of surface proteins and lipids. Carbohydrate Polymers, 64(3), 452e465.

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