The effect of sodium chloride on the glass transition of potato and cassava starches at low moisture contents

The effect of sodium chloride on the glass transition of potato and cassava starches at low moisture contents

Food Hydrocolloids 23 (2009) 1483–1487 Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhy...

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Food Hydrocolloids 23 (2009) 1483–1487

Contents lists available at ScienceDirect

Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd

The effect of sodium chloride on the glass transition of potato and cassava starches at low moisture contents Asgar Farahnaky a, *, Imad A. Farhat b, John R. Mitchell c, Sandra E. Hill c a

Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran Firmenich S.A., Rue de la Berge`re 7, Meyrin, CH1217 Geneva, Switzerland c Food Sciences Division, School of Biosciences, Sutton Bonington Campus, University of Nottingham, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 July 2008 Accepted 11 August 2008

The effect of NaCl on the glass transition of cassava and potato starches at low water levels (<20% dwb) was investigated. Sodium chloride (up to 6% of the starch dry weight) was mixed thoroughly with cassava and potato starches using a twin-screw extruder. The samples were equilibrated over saturated salt solutions (LiCl, CH3COOK, MgCl2, NaBr, CuCl2 and NaCl) to give a range of moisture contents. The addition of sodium chloride caused a considerable reduction in the DSC measured glass transition temperature for both starches. For example, the Tg of cassava starch without and with 6% NaCl equilibrated at relative humidity of 11% was 166 and 136  C, respectively. Similar reductions were found for potato starch. Although the starch sorption isotherms are affected by the addition of salt when Tg is plotted against water content as opposed to relative humidity a Tg reduction on salt addition is still observed. The possible reasons for the plasticization of starch by salt are discussed. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Sodium chloride Potato starch Cassava starch Glass transition temperature Sorption isotherm

1. Introduction The major ingredient for breakfast cereals and many snack foods is starch. These products will generally contain salt and/or sugars. Different sources of starch result in different physical properties of the final products, such as texture and degree of expansion on puffing. Along with the starch type, sugars and salt can have profound effects on the physical characteristics of the final products. The effect of some ingredients on starch based products has been investigated by a number of researchers, for example, Carvalho and Mitchell (2001), Fan, Mitchell, and Blanshard (1996a, 1996b) and Moore, Sanei, Van Hecke, and Bouvier (1990) have studied the effect of sugars on the physical and structural properties of extrudates and indicated that these components can decrease the glass transition of starches and often reduce expansion. Many researchers have reported the impact of salt on dough properties. Kojima et al. (1995) investigated the addition of salt to the flour and reported that salt markedly affected the physical characteristics of the dough. Combinations of organic acids plus 1.5% NaCl increased mixing time and dough stability. The original mixing profile of flour could be restored by decreasing the level of NaCl from 1.5 to 1.0%, or by adding 40–80 ppm cysteine–HCl

* Corresponding author. Tel.: þ98 711 613 8229; fax: þ98 711 628 9017. E-mail address: [email protected] (A. Farahnaky). 0268-005X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2008.08.007

(Tanaka, Furukaw, Matsumoto, & Chemistry, 1967). Studies on elastic and viscous moduli of dough by Larsson (2002) showed that NaCl at concentrations up to 2% (0.5, 1 and 2%) increased the elastic modulus of wheat flour dough as measured in a rotational rheometer. Salovaara (1982) in his study concluded that salt increases the machinability of dough. In most cases the action of salt on dough rheology is attributed to the gluten fraction and the fact that salt toughens the gluten and gives a less sticky (more machinable) dough. Salt slows down the rate of fermentation, and its addition is sometimes delayed until the dough has been partly fermented. Salt also affects the brown colour of breads and in the absence of salt the bread will be whiter (Kent & Evers, 1994). Although the impact of salt on dough rheology and bread has been the subject of many research projects the role of salt in snack and breakfast cereals has not been much studied. The inclusion of salt in breakfast cereals has some important technological roles, e.g. structure formation and flavour and colour generation. Salt plays a key role in the expansion of low moisture extruded starch based products. For example Chinnaswamy and Hanna (1998) reported that during the extrusion-cooking at 140  C the expansion ratio of starch increased from 13 to 16.9 as the sodium chloride concentration was increased from 0 to 1 g per 100 g of starch (dry wt. basis) and then decreased. Jomduang and Mohamed (1994) indicated that salt helped improve puffed product quality of a traditional Thai glutinous rice-based puffed snack. The effect of salt on colour formation and generation of low moisture breakfast cereals has been also reported. It is believed that the presence of salt can

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increase the rate of caramelization in these products on toasting at high temperatures. Two mechanisms have been reported for such an effect. Firstly, the increase in the molecular mobility that occurs in starch based products in the presence of salt and secondly the action of salt as a catalyst for caramelization which is due to the hydrolysis of starch to glucose, by a large number of ions (Chinnaswamy & Hanna, 1998; Kunlan et al., 2001). Singh, Kaur, Singh, and Skhon (1999) investigated the effect of addition of Na2HPO4 and NaH2PO4 to rice grits and claimed that NaH2PO4 decreased die pressure, SME of extruder and expansion of extrudates and increased water absorption, solubility indices and aqueous dispersion viscosity of extrudates. Na2HPO4 addition also showed a similar effect on the die pressure, SME of extruder and expansion of extrudates whereas the reverse was observed for water absorption and solubility indices and aqueous dispersion viscosities of extrudates. Due to concerns over health related issues of high intake of salt by the consumers, recently the reduction of salt in different food products has become the focus of many research projects, conferences and workshops. For example Farahnaky and Hill (2007) modelled the effect of salt, water and temperature on dough rheology using surface response methodology and concluded that salt, water and temperature all had negative effects on dough consistency and the effect of salt was low compared to the temperature and water level. Salt reduction in processed foods is a major challenge for the industry that is under increasing pressures from the consumers, health professionals and regulatory bodies. Therefore the reduction of salt in the breakfast cereals is of great importance, however, its reduction not only affects taste but causes some important technological problems that need to be addressed. To assist in achieving these reductions it is important to understand the role of salt on the phase transitions of starch the main biopolymer present. The purpose of this study is to investigate the glass rubber transition behaviour of amorphous starch– salt systems. The glass transition temperature (Tg) was measured using a differential scanning calorimeter.

2. Materials and methods 2.1. Materials Potato and cassava starches and all other chemicals were bought from Sigma Co. (USA) and were of analytical grade unless otherwise mentioned.

2.2. Preparation and extrusion of starch–salt mixtures Using a twin-screw extruder cassava and potato starches were extruded with different levels of NaCl. To have a homogenous mix, prior to the extrusion process starch and salt powders were mixed thoroughly in a laboratory scale mixer for 5 min. Extrusion was performed using a Clextral BC-21 (Clextral Ltd., Firminy, France) corotating, inter-meshing twin-screw with a useful length of 400 mm, a barrel length to diameter ratio of 16:1 and screw speed 200 rpm. The extruder was equipped with a pre-calibrated K-Tron Type T20 twin-screw volumetric feeder and a DKM-Clextral Type TD/2 water pump, which were used to control the solid feed and water inputs, respectively. The water flow rate was adjusted to give a moisture content of approximately 35% (wet basis) for the extruded products (ribbons) coming out of the slit die. The identification and extrusion conditions for potato starch are summarized in Table 1. The extrusion conditions for cassava starch were very similar to potato starch.

2.3. Preparation of starch–salt mixtures with different moisture contents The extruded samples (ribbons) were dried in a freeze drier and ground using a disc mill with 0.5 mm sieve (Cyclotec model 1093, Foss Tecator Amersfoort, the Netherlands) and then the powders (particle size range 125 to 225 mm) of extruded potato and cassava starches with different levels of NaCl (0, 1.5, 3, 4.5 and 6% of starch dry weight basis) were equilibrated over different super-saturated salt solutions (LiCl, CH3COOK, MgCl2, NaBr, CuCl2 and NaCl for relative humidities of 11, 23, 33, 58, 65 and 75, respectively) at 25  C to give a range of moisture contents. The equilibrated powders were separately stored at 25  C in sealed glass bottles for further experiments. The moisture contents of the equilibrated samples were measured gravimetrically by heating the samples at 80  C until dried to constant weight. 2.4. Glass transition determination using differential scanning calorimetry A DSC-7 (Perkin–Elmer, Beaconsfield, UK), calibrated with indium and cyclohexane, was used to analyse the samples (10– 15 mg) of known water contents. High pressure stainless steel pans containing samples were scanned at a heating rate of 10 K min1, from 20 to 200  C depending on their moisture contents to measure their glass transition temperatures. An empty stainless steel pan was used as reference. Pyris software version 3.5 (Perkin– Elmer Corporation, USA) was used to analyse the DSC traces. To determine the glass transition temperature points were first chosen on the DSC curves before and after the transition. The software then extrapolates tangents from these points and the inflection point of the curve. The onset and endset temperatures are the intersection of these tangents. 2.5. Sorption isotherm curves using dynamic vapour sorption DVS (dynamic vapour sorption) is a relatively novel technique to the food industry. The technique enables the study of hydration and dehydration of a material by monitoring changes in mass when the sample is subjected to different relative humidities. It is a ‘dynamic equilibration method,’ i.e. relies on the continuous circulation of RH controlled gas over the sample, and hence is relatively rapid as compared to static equilibration methods. The required humidity is achieved by mixing dry and water-vapour-saturated nitrogen gas flows, with the help of flow controllers, before they enter the chambers (Teoh, Schmidt, Day, & Faller, 2001). Using this method of obtaining humidified gas can achieve relative humidity values ranging from 0 to 95% RH. This gas then exits the chambers above the pans. 3. Results and discussion The physically mixed powders of starch and NaCl were extruded to obtain a homogenous mixture of starch and salt. To mix the two components (starch and salt) at a molecular level the conditions of the extrusion (water content and temperature) were chosen in a way to result in molten extruded materials. The added water was adjusted to control the moisture content of the extruded starch– salt melts to be about 35% (wb). From the temperature profile of the four zones of the extruder (Table 1) it can be concluded that the starch has been fully gelatinised after the extrusion as confirmed by X-ray diffraction spectrum (data not shown). Extrusion process parameters in Table 1 show that the inclusion of salt in starch did not affect the specific mechanical energy (SME) of the system. This means that the energy consumed for the extrusion of the powder

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Table 1 Extrusion conditions for the preparation of potato starch–salt mixtures Salt level (%)

T1

T2

T3

T4

Water (kg/h)

Feed rate (kg/h)

Torque (Nm)

Speed (rpm)

Throughput (kg/h)

SME (Wh/kg)

0 1.5 3 4.5 6

19 18 19 19 19

74 74 74 74 74

121 120 121 124 122

69 67 69 68 66

1.64 1.64 1.64 1.64 1.64

6.04 6.04 6.04 6.04 6.04

14.3 14.7 14.3 14.3 14.7

199 200 200 200 200

7.68 7.68 7.68 7.68 7.68

77.6 80.2 78.0 78.0 80.2

T1, T2, T3 and T4 are the temperatures of each zone from the raw material to the die end.

Heat Flow Endo Up (mW)

was not affected by the change of salt content at least when the moisture level was about 35%. DSC traces of cassava starches with different levels of salt (0–6%) equilibrated over saturated solution of LiCl (relative humidity of 11%) are given in Fig. 1. From bottom to top as the salt content increases the glass transition regions of the samples shift to the left, i.e. lower temperatures. This graph shows a change of about 25  C for the inclusion of 6% salt in the cassava starch powder. This indicates that the presence of salt decreases the glass transition temperature and the higher the salt content the lower the Tg of the starch–salt mixture for the samples equilibrated at relative humidity of 11%. The glass transition data obtained from the analysis of the DSC traces of cassava and potato starches with different levels of sodium chloride equilibrated over super-saturated salt solutions to give relative humidities of 11, 23, 33, 58, 65 and 75%, respectively, at 25  C are presented in Figs. 1 and 2. Both figures show that for all relative humidities when starch–salt mixtures were equilibrated over salt solutions, the presence of salt caused a reduction in the glass transition. For example, Tg of cassava starch without and with 6% NaCl equilibrated at relative humidity of 11% was 166 and 136  C, respectively. Tg of potato starch without and with 6% salt was 167 and 137  C, respectively (Fig. 3). For the cassava starches without and with 6% salt equilibrated at relative humidity of 75%, Tg were 59 an 39  C, respectively. Tg of potato starch without and with 6% salt equilibrated at relative humidity of 75% were 63 and 39  C, respectively. It is important to see whether in the presence of NaCl the total amount of water increases and the changes can be attributed to the presence of more water (acting as a plasticizer) at a known relative humidity (i.e. change in sorption isotherm) or whether this is only due to the plasticizing effect of NaCl molecules. Therefore the sorption isotherm of cassava and potato powders with different salt levels (0–6%) was studied using a DVS (for more details see Section 2) at 25  C. As seen in Figs. 4 and 5 the presence of salt affects the

100

6% 4.5%

=1 mW

3% 1.5% 0%

120

140

160

180

Temperature (C) Fig. 1. Differential scanning calorimetry traces of cassava starch samples with different levels of salt (0–6%) equilibrated over saturated salt solution of LiCl (aw ¼ 0.11). The arrows show the position of the midpoint as the glass transition temperature of each sample (for more details see text).

sorption isotherm of both starches particularly at high relative humidities. This means if two samples one starch and the other a starch–salt mixture are equilibrated at a known relative humidity, the final moisture content of the two samples can differ. The isotherms of starch–salt mixtures indicate that the inclusion of salt will make the starch more hygroscopic (i.e. absorb more water) in comparison with starch by itself. This phenomenon was more dramatic for the samples equilibrated at relative humidities higher than 70%. The isotherms of the starch–salt mixtures showed that the equilibration of the samples at a known relative humidity may result in different moisture contents. Therefore it was necessary to determine the moisture content of the samples and plot their glass transition temperatures versus moisture levels for both starches with different salt levels. In Figs. 6 and 7, the glass transition of cassava and potato starches with different NaCl levels against moisture content is presented. For both starches the presence of salt reduced Tg of the samples at constant water content, i.e. salt acted as a plasticizer and the greater the NaCl level the lower the Tg. The reduction of Tg at low moisture levels was more dramatic than at high moisture levels for both starches. For cassava and potato starches a 6% NaCl caused about 45 and 38  C decline (respectively) in the Tg of starch–salt mixtures compared to the controls without NaCl. However, a 6% change in the moisture level of the cassava and potato starches without NaCl caused a 73 and 72  C drop in the Tg of the samples, respectively. These values indicated that the effect of water on the Tg of starchy systems is greater than NaCl when an equal amount of salt or moisture is changed. A possible mechanism that should be considered is that the presence of salt results in water partition. The presence of the salt resulting in greater water content in the starch phase. To test this hypothesis we calculated the effect on the Tg of starch assuming all the salt ‘‘phase’’ was completely anhydrous and all the added water was in the starch phase. When this done there was still a significant reduction in the Tg based on the starch water content, e.g. a 6% NaCl caused 32  C reduction in the Tg of potato starch. Although salt crystals are anhydrous, X-ray diffraction patterns of the samples showed no evidence for crystalline salt in any of these samples, so the extreme assumption of no water in the salt phase seems unlikely. Similar reports are well documented for other small molecules (e.g. small sugars, water and glycerol) to decrease Tg of the mixtures when mixed with large polymers (Carvalho & Mitchell, 2001; Chinnaswamy & Hanna, 1998; Moore et al., 1990). The reason for choosing cassava and potato starches was to study the effect of NaCl on two polymers being different in terms of the charge on the molecules. Potato starch is a charged molecule (the amylopectin is phosphorylated) while cassava starch is not. The results showed that in principle the plasticizing effect of NaCl in both cassava and potato starches was seen and therefore the mechanism would be similar. Baroni, Sereno, and Hubnger (2003) studied the thermal transitions of osmotically dehydrated tomato at low moisture levels by modulated temperature DSC and reported that compared to the control the presence of sucrose and NaCl in the dried tomatoes had a negative effects on the glass transition. For example for completely dried tomato samples, the inclusion of

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Glass transition temperature (C)

180 0% 1.50% 3% 4.50% 6%

160 140 120 100 80 60 40 20 0

LiCl

CH3COOK

MgCl2

NaBr

CuCl2

NaCl

Super saturated salt solutions Fig. 2. Glass transition temperature of cassava starches (determined using the DSC) with different levels of NaCl (0, 1.5, 3, 4.5, and 6%) equilibrated over different super-saturated salt solutions at 25  C.

Glass transition Temperature (C)

180 160

0%

140

1.50%

120

3%

100

4.50% 6%

80 60 40 20 0 LiCl

CH3COOK

MgCl2

NaBr

CuCl2

NaCl

Super saturated salt solutons Fig. 3. Glass transition temperature of potato starches (determined using the DSC) with different levels of NaCl (0, 1.5, 3, 4.5, and 6%) equilibrated over different super-saturated salt solutions at 25  C.

To explore the plasticizing effect of salt on starches, one may focus on the charge effect of the dissociated atoms of NaCl molecules and the possible role they may play to reduce the hydrogen bonds by the occupation of the number of available sites.

40

Moisture content (g water/g dry matter)

4.6% NaCl caused a 15  C depression of the Tg. They also stated that NaCl changed the sorption characteristic of the dried tomato significantly particularly at relative humilities greater than 50%. The result was related to the impregnation of the solid matrix with a solute of lower molecular weight. NaCl also has been reported to decrease the glass transition of bovine serum albumin in dilute solutions (Inoue & Ishikawa, 2000). Dielectric relaxation studies of frozen wheat dough with and without NaCl have also indicated that salt can depress the glass transition and the onset of melting temperature of ice in frozen dough (Laaksonen & Roos, 2001) and the effect seen was thought to be due to the higher conductivity of the frozen material with salt. There is very little information on the mechanism of glass transition depression by NaCl. At low moistures’ levels as studied in this research (<20% dwb) one can assume that the NaCl molecules will not dissociate to their ions and remain as ion pairs. If this is the case NaCl molecules would act like other small molecules as a plasticizer. It has been well documented that in polysaccharide systems the glass transition of the system is dictated by the average molecular size and the presence of small molecules can decrease the average molecular weight resulting in the depression of the Tg. Sodium chloride can perform like other small molecules, e.g. sucrose and glucose.

6% 4.5% 30 3% 1.5% 0%

20

10

0

0

10

20

30

40

50

60

70

80

90

Relative humidity (%) Fig. 4. Sorption isotherms of potato starches with different levels of NaCl (0, 1.5, 3, 4.5, and 6%) determined using the DVS at 25  C.

A. Farahnaky et al. / Food Hydrocolloids 23 (2009) 1483–1487

Moisture content (g water/g dry matter)

40 6% 4.5% 30

3% 1.5% 0%

20

10

0 0

10

20

30

40

50

60

70

80

90

Glass transition temperature (C)

Fig. 5. Sorption isotherms of potato starches with different levels of NaCl (0, 1.5, 3, 4.5, and 6%) determined using the DVS at 25  C.

180

90

60

30

5

8

11

14

17

20

Moisture content (%, dry basis) Fig. 6. Glass transition temperature of potato starch with different levels of NaCl (0–6%) versus moisture content.

Glass transition temperature (C)

Although on the X-ray diffraction spectrums of the starch samples with different salt levels (data not presented) there was no evidence of crystallized NaCl (which could be due to the low levels of salt in the samples), the state of NaCl (crystallized and/or amorphous forms) and its effect on the water distribution in the system can be of great importance. Hsieh, Peng, and Huff (1990) reported that the addition of salt in corm meals extruded using a twin-screw extruder caused a decrease in the specific mechanical energy, die pressure, bulk density and lightness of the extruded products. Salt also increased

180 0% 1.50% 3% 4.50% 6%

150 120 90 60 30

5

8

11

14

17

Due to the importance of the role of NaCl in starch based systems the effect of NaCl on the glass transition of two starch systems (cassava and potato) was investigated. The results obtained in this research confirmed that the inclusion of NaCl in the starch systems reduces the glass transition temperature. References

0% 1.50% 3% 4.50% 6%

120

the yellowness and the gas cell diameter of the extruded corm meals. The impact of NaCl on the Tg of the starch based systems can help explaining the visible changes seen during the processing of breakfast cereals and baked products at low moisture levels in the presence of NaCl, e.g. structure formation (more expansion), flavour and colour generation. Finally, the work invites questions as to the effect of salt to the vitrification patterns of non-starchy polysaccharides and to the corresponding behaviour using mechanical measurements where it has been demonstrated that in certain formulations results deviate from those obtained by DSC (Kasapis, 2008). 4. Conclusions

Relative Humidity (%)

150

1487

20

Moisture content (%, dry basis) Fig. 7. Glass transition temperature of cassava starch with different levels of NaCl (0–6%) versus moisture content.

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