Effect of Sodium Chloride on Glassy and Crystalline ... - naldc - USDA

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DOI 10.1002/star.200700669

Meera Kweona Louise Sladeb Harry Levineb

Effect of Sodium Chloride on Glassy and Crystalline Melting Transitions of Wheat Starch Treated with High Hydrostatic Pressure: Prediction of Solute-induced Barostability from Nonmonotonic Solute-induced Thermostability

a

USDA, ARS, Soft Wheat Quality Lab., Wooster, OH, USA b Food Polymer Science Consultancy, Morris Plains, NJ, USA

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Wheat starch was high hydrostatic pressure (HHP)-treated in aqueous solutions with various sodium chloride (NaCl) concentrations (0 to near-saturation), in order to explore the effects of salt on glassy and crystalline transitions of starch during the treatment, using differential scanning calorimetry (DSC). For wheat starch at atmospheric pressure, glass and crystalline melting transitions of amylopectin (reported as gelatinization peak temperature) increased up to 2 M NaCl, and then decreased with further increase in NaCl concentration, but the gelatinization peak temperature was higher in all NaCl solutions than in water alone. In contrast, the melting transition for the amylose-lipid complex (reported as peak temperature) increased continuously with increasing NaCl concentration. When 50% (w/w) starch slurries were HHP-treated in water and various NaCl concentrations (0.1, 2 and 5 M) for 15 min at 257C, the presence of salt significantly protected glass and crystalline transitions of starch during the HHP treatment. Although the baroprotective effect was maximal near the lyotropic 2 M NaCl concentration [1-5], all NaCl concentrations were more baroprotective than was water alone for HHP-treated wheat starch. As reported previously [6] for corn starches in a lyotropic concentration of NaCl (ionic solvent) or a non-equilibrium concentration of sucrose (glass-forming solvent) [7], solute-induced thermostabilization of the wheat starch gelatinization transition predicted solute-induced barostabilization.

1 Introduction It is well known that starch gelatinization is affected by various electrolytes, and the effects depend on the concentration and type of anions and cations [8]. Using a 40% wheat starch suspension in water, 6.25%, and 21.57% sucrose, or 0.11 M and 0.22 M NaCl, Chinachoti et al. [9] reported that these relatively low concentrations of solutes increased the gelatinization temperature of the starch as observed by DSC. 13C NMR spectra acquired during heating (25, 55, 65, and 957C) of the starchsucrose-D2O and starch-NaCl-D2O mixtures showed contrasting changes in intensity and line width during starch gelatinization [9]. Jane [10] has reported that various neutral salts affected the onset temperature and heat of transition during corn starch gelatinization, and the effect differed by type and concentration of salt. However, Correspondence: Meera Kweon, USDA, ARS, Soft Wheat Quality Lab., 1680 Madison Ave., Wooster, OH 44691, USA. Phone: 11-330-263-3984, Fax: 11-330-263-3651, e-mail: [email protected].

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the temperature and heat of transition were affected by anions or cations in a complex mode. In particular, a cation series of chloride salts showed a nonmonotonic increase and subsequent decrease in the temperature and heat of transition with increasing salt concentration [10]. Chiotelli et al. [11] have shown similar nonmonotonic results for the effect of NaCl on wheat and potato starches. Villwock and BeMiller [12] have reported that different salts affected the reaction efficiency of hydroxypropylation of normal corn starch, and the efficiencies differed by type of anion or cation, due to different extents of lyotropic effects, resulting in varying inhibition of starch swelling and gelatinization. Ahmad and Williams [13], using both anion and cation series, also have shown that the presence of salts during heating of sago starch affected the peak temperature and heat of gelatinization, swelling properties, storage modulus, gel strength, and gelation rate constant, depending on the type and concentration of salt. As an alternative to gelatinization by heat, the structure and functional properties of starches can be modified www.starch-journal.com

Research Paper

Keywords: Wheat starch; NaCl; High hydrostatic pressure; Thermostabilization; Barostabilization

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by high pressure, even at room temperature [14]. The mechanism for pressure-induced gelatinization has been described to be similar to heat-induced gelatinization: first-stage swelling of amorphous regions of amylopectin, followed by melting of crystalline regions of amylopectin [15]. The gel strength of pressureinduced wheat, corn, and pea starch gels is significantly different from that of gels obtained by thermal gelatinization [16, 17]. Cereal starches and tuber starches have been reported to exhibit different responses to HHP treatment [16, 18, 19]. In most of the numerous papers describing HHP treatments of starches, the experiments have been conducted with starch suspensions in water. The effects of other diluents, such as sugar-water or saltwater, on glassy and crystalline transitions of starch during the HHP treatment have not been reported. In order to explore the effects of lyotropic concentrations (high salt concentrations that are relevant for the Hofmeister series, far above the concentration region where electrostatics dominates [1-5]) of salt on glassy and crystalline transitions of starch during HHP treatments, wheat starch was HHP-treated in various NaCl concentrations, and the treated samples were analyzed with DSC.

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2.2.2 High Hydrostatic Pressure (HHP) treatment for 15 min at 257C A Multivessel Apparatus Unipress 111 (Warsaw, Poland) was used for HHP treatments, as described previously [6]. Starch and either water or aqueous NaCl solution were mixed in a weight ratio of 1:1. The starch mixture was transferred to a 1 mL pressure vial (NUNC cryotube vial, Denmark), and the vial was closed tightly with a cap. Then the vial was placed into the pressure vessel filled with the pressure medium (silicone oil M40.165.10), and the pressure was raised to the target within 1 min. Although the temperature of the pressure vessel increased due to adiabatic heating during pressurization (maximally 107C at 600 MPa), the pressure vessel returned to the experimental target temperature within 1 min after reaching the target pressure. At the end of the HHP treatment, the sample was depressurized, removed from the pressure vessel, and used for DSC analysis.

2.2.3 Differential Scanning Calorimetry About 40 mg of a starch sample was transferred to a stainless steel DSC sample pan and sealed. Each sample was heated in the DSC instrument (DSC-7, Perkin-Elmer, Norwalk, CT, USA) from 30 to 1307C with a 107C/min heating rate, and an empty pan was used as a reference. Temperature and enthalpy calibrations were performed as described previously [21].

2 Materials and Methods 2.1 Materials

3 Results and Discussion

Wheat starch (Aytex P) was kindly supplied by ADM (Decatur, IL, USA). All other chemicals were reagent grade.

3.1 Effect of NaCl concentration on apparent rvp and pH values

Sodium chloride solutions were prepared to cover a wide range of concentrations from 0 to 5 M (near the saturation concentration of 5.145 M at 257C [20]), for use in DSC and HHP experiments. Their apparent rvp values (equilibrium and non-equilibrium) were measured with a relative humidity instrument (AquaLab CX2, Decagon, Pullman, WA, USA). Their apparent pH values were measured with a pH meter (360i, Corning Inc., Corning, NY, USA), calibrated with standard pH 4 (0.05M potassium biphthalate) and 7 (0.05M potassium phosphate monobasic - sodium hydroxide) buffers.

The physical behavior of the NaCl solutions used for DSC and HHP experiments was characterized by measuring their apparent rvp and pH values. Apparent rvp values can be used to define equilibrium (rvp 0.95–1) and non-equilibrium (rvp ,0.95) ranges of concentration [22]. As the concentration of NaCl increased, the apparent rvp values of the solutions decreased monotonically from 0.996 to 0.777 (Fig. 1A), with a change in slope showing a greater depression of rvp by salt near 2 M (rvp 0.929), in the non-equilibrium concentration range. Chiotelli et al. [11] have reported that “water activity” values of wheat and potato starch dispersions were decreased similarly down to 0.83 with increasing NaCl concentration above 4 M. In contrast to the effect of salt on apparent rvp, the apparent pH values of the NaCl solutions were almost unchanged (6.2 to 6.36) up to 1 M, and then increased monotonically to apparent pH 8.5 at 5 M


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2.2.1 Relative vapor pressure (rvp) and pH of NaCl solutions

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Fig. 1. Apparent rvp (A) and apparent pH (B) for NaCl solutions at various concentrations (237C 6 0.3).

NaCl, with an exaggerated increment in apparent pH with increase in NaCl concentration from 2 M to 2.5 M (Fig. 1B).

3.2 Effect of NaCl concentration on glassy and crystalline melting transitions of wheat starch at atmospheric pressure In preparation for the exploration of HHP treatment of wheat starch in NaCl solutions, the effects of NaCl concentration on glassy and crystalline transitions of wheat starch were examined at atmospheric pressure. The peak temperature of the amylopectin gelatinization transition increased nonmonotonically up to 2 M aqueous NaCl, and then decreased from 2 M to 5 M (Tab. 1, Fig. 2). The observed increase in amylopectin peak temperature, without an increase in peak symmetry, is diagnostic of the effect of low concentrations of salt to increase the glass transition temperature of the amorphous regions of amylopectin [23, 24]. However, the nonmonotonic response to high concentrations of salt is diagnostic of the effect of salt on the melting temperature of the crystalline regions of amylopectin [1, 23, 24]. In contrast to the peak temperature for gelatinization of amylopectin, the peak temperature for melting of amylose-lipid complex increased with increasing NaCl concentration up to 4.0 M (Tab. 1, Fig. 2). For both amylopectin and amylose-lipid complex, there was no exaggerated increment in peak temperature with increase in NaCl concentration from 2 M to 2.5 M, as


Fig. 2. Effect of NaCl concentration on thermal transitions of wheat starch at atmospheric pressure.

observed for the effect of NaCl on apparent pH (Fig. 1). Moreover, the nonmonotonic effect of NaCl on the peak transition temperatures was not predicted by the monotonic effects of NaCl on apparent rvp or apparent pH values (Fig. 1). The measured physical behavior of the NaCl solutions did not account for the effect of NaCl on the thermal behavior of wheat starch at atmospheric pressure. Although the effect of NaCl on the peak transition temperature of amylopectin was nonmonotonic with a maximum near 2 M NaCl, all NaCl concentrations were thermostabilizing, compared to water alone. The DSC thermograms for wheat starch in a complete series of NaCl concentrations are shown in Fig. 3. Lii and Lee [25] have reported similar results for seven A-, B-, and C-type, corn, potato, canna, lotus tuber, and rice starches, for which the gelatinization temperatures increased and then decreased, as the NaCl concentration increased. Similar results have been reported by Chiotelli et al. [11] for potato and wheat starches; Chungcharoen and Lund [26] for rice starch; Jane [10] for corn starch; and Ahmad and Williams [13] and Ghani et al. [27] for sago starch. www.starch-journal.com

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Tab. 1. Peak temperatures and heats of transition of amylopectin and amylose-lipid complex (alc) for the wheat starch samples in various NaCl concentrations at atmospheric pressure. Average and standard deviation for duplicate DSC scans. NaCl concentration [M]

Tpeak [7C]

DQpeak [J/g]

Talc [7C]

DQalc [J/g]

0.00 0.01 0.04 0.10 0.50 1.00 2.00 2.50 3.00 4.00 5.00

62.4 6 0.4 63.4 6 0.1 64.0 6 0.1 65.6 6 0.1 68.2 6 0.0 70.7 6 0.5 72.1 6 0.0 71.8 6 0.1 71.4 6 0.1 69.4 6 0.1 64.0 6 0.1

5.27 6 0.40 5.47 6 0.32 5.65 6 0.06 5.40 6 0.18 5.48 6 0.11 5.63 6 0.32 5.81 6 0.16 5.85 6 0.13 5.42 6 0.23 5.54 6 0.04 5.18 6 0.07

110.0 6 0.4 110.9 6 0.4 110.8 6 0.1 111.5 6 0.1 111.2 6 0.1 115.0 6 0.5 118.5 6 0.1 119.6 6 0.1 120.2 6 0.4 122.4 6 0.4 121.7 6 0.6

0.69 6 0.04 0.68 6 0.02 0.70 6 0.03 0.69 6 0.10 0.68 6 0.01 0.76 6 0.01 0.79 6 0.01 0.77 6 0.03 0.82 6 0.01 0.79 6 0.01 0.77 6 0.03

Tab. 2. Peak temperatures and heats of transition of amylopectin and amylose-lipid complex (alc) for the HHP-treated wheat starch samples in various NaCl concentrations. Average and standard deviation for triplicate DSC scans. HHP pressure [MPa]

NaCl concentration [M]

Tpeak [7C]

DQpeak [J/g]

Talc [7C]

DQalc [J/g]

Atmospheric

0.0 0.1 2.0 5.0

62.4 6 0.4 65.6 6 0.1 72.1 6 0.0 64.0 6 0.1

5.27 6 0.40 5.40 6 0.18 5.81 6 0.16 5.18 6 0.07

110.0 6 0.4 111.5 6 0.1 118.5 6 0.1 121.7 6 0.6

0.69 6 0.04 0.69 6 0.10 0.79 6 0.01 0.77 6 0.03

300

0.0 0.1 2.0 5.0

62.6 6 0.7 65.6 6 0.2 72.7 6 0.4 64.1 6 0.3

4.84 6 0.11 5.36 6 0.24 5.52 6 0.02 5.06 6 0.16

110.0 6 0.1 112.2 6 0.5 118.6 6 0.3 122.2 6 0.5

0.67 6 0.01 0.71 6 0.02 0.79 6 0.00 0.83 6 0.02

450

0.0 0.1 2.0 5.0

66.5 6 0.1 68.1 6 0.5 72.4 6 0.1 63.8 6 0.3

3.68 6 0.14 3.73 6 0.04 5.65 6 0.08 4.55 6 0.19

112.7 6 0.4 112.0 6 0.5 118.9 6 0.1 120.8 6 0.5

0.80 6 0.05 0.74 6 0.02 0.80 6 0.00 0.86 6 0.02

600

0.0 0.1 2.0 5.0

80.8 6 0.2 80.1 6 0.8 81.4 6 0.3 76.4 6 0.1

1.23 6 0.03 1.26 6 0.03 2.91 6 0.14 2.58 6 0.04

112.7 6 0.2 112.4 6 0.4 119.0 6 0.4 121.8 6 0.1

0.78 6 0.03 0.79 6 0.03 0.84 6 0.01 0.86 6 0.02

3.3 Effect of selected NaCl concentrations on glassy and crystalline melting transitions of wheat starch with HHP treatment When wheat starch was HHP-treated in water at various pressures from 300 to 600 MPa for 15 min at 257C, the heat of the amylopectin gelatinization transition near 60oC decreased progressively with increase in pressure, showing a small change due to treatment at 300 MPa and larger changes due to treatment at 450 and 600 MPa (Tab. 2, Fig. 4). This reflects the partial gelatinization of the starch granules by HHP. The extent of gelatinization at

600 MPa was sufficient to result in a slight retrogradation observed near 50oC. Treatment at 450 MPa in water resulted in a small extent of annealing (crystal perfection), and annealing was exaggerated by treatment at 600 MPa (increase in peak temperature of amylopectin crystal melting to 80oC). In contrast, Stute et al. [16] observed partial gelatinization and retrogradation resulting from HHP treatment of wheat starch in water, but they did not observe annealing due to HHP treatment. The more dilute wheat starch suspension (less than 30%, w/w) used by Stute et al. [16] allowed complete gelatinization above the amylopectin glass transition temperature and subsequent



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Fig. 3. DSC thermograms for wheat starch suspended in solutions (1:1, w/w) of various NaCl concentrations at atmospheric pressure.

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Fig. 5. DSC thermograms for wheat starch after HHP treatment in 0.1 M NaCl (1:1, w/w) for 15 min at 257C.

induced by 0.1 M NaCl predicted the small NaCl-induced extent of barostabilization. Baroprotection of wheat starch by 2 M NaCl was greatly enhanced compared to that by 0.1 M NaCl (Tab. 2, Fig. 5), as shown by the greater residual amylopectin structure after treatment at 450 MPa and greater annealed amylopectin structure after treatment at 600 MPa (Tab. 2, Fig. 6). Baroprotection by 5 M NaCl (Tab. 2, Fig. 7) showed a similar trend compared to that by 0.1 M NaCl: greater residual structure after treatment at 450 MPa and greater annealed structure after treatment at 600 MPa, but maximal baroprotection was enabled by treatment in 2 M NaCl.

In contrast, treatment of wheat starch at 300 MPa in a low NaCl concentration (0.1 M) did not result in gelatinization (Tab. 2, Fig. 5). Treatment at 450 MPa in 0.1 M aqueous NaCl resulted in a significant extent of gelatinization, but less than that due to treatment in water. The extents of gelatinization and retrogradation resulting from treatment at 600 MPa in 0.1 M NaCl were similar to the results for treatment in water, but the extent of annealing was slightly greater due to treatment in 0.1 M NaCl compared to treatment in water. The results indicated that the small extent of thermostabilization

Numerous publications have reported on the HHP treatment of starches, but there are few reports on the baroprotective effect of NaCl on the glassy and crystalline thermal transitions and structural features of amylopectin. Unlike the role of concentrated sucrose solutions to monotonically raise the glass transition temperature of amylopectin (gelatinization temperature), compared to water alone, the nonmonotonic trend of the effect of NaCl concentration on the peak transition temperature of amylopectin (Fig. 3) demonstrated that NaCl raises the amylopectin glass transition temperature both directly, at lower salt concentrations up to 0.5 M (Fig. 2), and indirectly, at higher salt concentrations from 1 - 2 M (Fig. 2). In this lyotropic concentration range, NaCl modulates the stability of helical structure and helix association in amylopectin crystallites, and the amylopectin glass transition temperature is indirectly modulated by the variation in chain length distribution of the accessible branches in the amorphous regions of amylopectin. This thermostabilizing effect of NaCl on wheat starch at atmospheric pres-



Fig. 4. DSC thermograms for wheat starch after HHP treatment in water (1:1, w/w) for 15 min at 257C.

extensive retrogradation, whereas the higher concentration (50%, w/w) used in our experiments allowed partial gelatinization and extensive annealing.

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Starch/Stärke 60 (2008) 127–133 amylopectin structure than did treatment in water alone, the barostabilization at 25oC was maximal near 2 M NaCl. This study has demonstrated that the concentration-dependent pattern of the solute-induced barostabilization of wheat starch by NaCl at room temperature was predicted by the concentration-dependent pattern of the soluteinduced thermostabilization at atmospheric pressure, even though the extent of thermostabilization depended nonmonotonically on the concentration of NaCl.

References

Fig. 6. DSC thermograms for wheat starch after HHP treatment in 2 M NaCl (1:1, w/w) for 15 min at 257C.

Fig. 7. DSC thermograms for wheat starch after HHP treatment in 5 M NaCl (1:1, w/w) for 15 min at 257C. sure also accounts for the baroprotective effect of NaCl during HHP treatment of wheat starch up to 600 MPa.

4 Conclusions Although all concentrations of NaCl from 0.01 to 5 M caused an increase in the gelatinization peak temperature for wheat starch, compared to water alone, thermostabilization at atmospheric pressure was maximal near 2 M NaCl. HHP treatment of the starch at 25oC had a significant progressive impact on glassy and crystalline transitions of starch with increasing pressure, resulting in a progressive extent of gelatinization and annealing. Although HHP treatment in all of the tested concentrations of NaCl from 0.1 to 5 M resulted in a greater retention of


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