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Atmospheric pressure cold plasma (ACP) treatment of wheat flour
N.N. Misra, Seeratpreet Kaur, Brijesh K. Tiwari, Amritpal Kaur, Narpinder Singh, P.J.Cullen
PII: S0268-005X(14)00295-1
DOI: 10.1016/j.foodhyd.2014.08.019
Reference: FOOHYD 2703
To appear in: Food Hydrocolloids
Received Date: 21 June 2014
Revised Date: 26 August 2014
Accepted Date: 29 August 2014
Please cite this article as: Misra, N.N., Kaur, S., Tiwari, B.K, Kaur, A., Singh, N., Cullen, P.J.,Atmospheric pressure cold plasma (ACP) treatment of wheat flour, Food Hydrocolloids (2014), doi:10.1016/j.foodhyd.2014.08.019.
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Atmospheric pressure cold plasma (ACP) treatment of wheat flour 1
N.N. Misra1, Seeratpreet Kaur
2, Brijesh K Tiwari
3*, Amritpal Kaur
2, Narpinder Singh
2 and P.J. Cullen
1,4 2
1BioPlasma Research Group, School of Food Science and Environmental Health, Dublin Institute of 3
Technology, Dublin 1, Ireland 4
2Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 05, India 5
3Department of Food Biosciences, Teagasc Food Research Centre, Dublin, Ireland. 6
4School of Chemical Engineering, University of New South Wales, Sydney, Australia. 7
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Address for corresponding author: [email protected]; Tel: +353 1 805 9785; Fax: +353 1 805 17
9550 18
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Highlights 20
• Atmospheric pressure cold plasma (ACP) treatment effects on flour quality were evaluated. 21
• ACP effects on rheological properties were evaluated using Mixograph and Dynamic 22
rheometery. 23
• Changes caused by ACP in secondary structures of dough proteins were assessed. 24
• Optimum conditions to obtain desirable effects of ACP of dough vary with wheat type. 25
26
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Abstract 28
Atmospheric pressure cold plasma (ACP) is an emerging advanced oxidation process which has 29
recently drawn considerable interest from food scientists. The objective of this study was to 30
investigate the effect of ACP treatments on the rheological properties of hard and soft wheat flour. 31
Optical emission characterisation of the dielectric barrier discharge ACP source and ozone 32
measurements revealed the generation of reactive oxygen and excited nitrogen species. The 33
rheological properties of flours were studied using mixogram and oscillation rheometry. Mixographs 34
revealed an improvement in the dough strength and optimum mixing time for both strong and weak 35
wheat flours. The elastic and viscous moduli of strong wheat flour progressively increased with 36
applied voltage and treatment time. A significant variation in the tan δ was not found. Changes in 37
the secondary structure of proteins were evaluated using FTIR spectroscopy and revealed a decrease 38
in β-sheets and increase in α-helix and β-turns, for both strong and weak wheat flour. The results 39
indicate that ACP can be exploited as a means to modulate functionality of wheat flour. 40
Keywords: 41
Cold Plasma, Wheat flour, Mixing, FTIR, Amide 42
43
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1. Introduction 44
Wheat flour is used in making an array of bakery products, with various product classes demanding 45
different functionality including bread, pasta, noodles, cakes, biscuits and pastries. The baking 46
industry is influenced by wheat flour properties, which is an ensemble of physicochemical and 47
rheological parameters, giving a first-hand indication about flour performance during production (Li 48
Vigni, Baschieri, Marchetti, & Cocchi, 2013). Gluten proteins are the principal structure forming 49
elements of most baked products, contributing to the elasticity, cohesiveness and viscosity 50
characteristics of the dough. Thus, gluten proteins substantially control the quality of wheat flour 51
based products (MacRitchie, 1984). Dough formation involves various steps: hydration, formation of 52
gluten network and disruption as well as evolution in response to mixing energy (Migliori & Correra, 53
2013). Apart from the gluten protein, starch and starch-protein interactions also dictate the quality 54
of the products and the respective production processes. Lipids are also important ingredient of 55
baking industry and their oxidation affects the quality of the bakery products. Oxidised lipids cross-56
link with proteins and modify the texture of the products (Patrignani, Conforti, & Lupano, 2014). 57
In order to improve the functionality of wheat flours several oxidising treatments have been 58
evaluated with a number currently being employed at commercial level. Examples of such 59
treatments include the use of chemical methods viz. chlorination (Sinha, Yamamoto, & Ng, 1997), 60
use of KIO3, azodicarbonamide, KBrO3, ascorbic acid (Junqueira, et al., 2007), benzoyl peroxide (Saiz, 61
Manrique, & Fritz, 2000) and recently ozone treatments, and enzymatic approaches such as use of 62
lipoxygenase, pentosanases, proteolytic enzymes, redox enzymes, and reducing agents in 63
combination (Lamsal & Faubion, 2009). Among these, ozone is of relevance for the present study as 64
it is a strong oxidising agent and yet it leaves no residues. Due to its oxidising properties, ozone has 65
been reported to act as an alternative to potassium bromate, chlorine, and benzoyl peroxide for 66
treatment of wheat flour (Sandhu, Manthey, & Simsek, 2012). The use of ozone in grain processing 67
has been recently reviewed (Tiwari, et al., 2010). It should be noted that ozone enjoys a GRAS 68
(Generally Recognized as Safe) status in the United States as a food additive, following U.S. Food and 69
Drug Administration (FDA) approval in 1997 (Cullen, Tiwari, O'Donnell, & Muthukumarappan, 2009; 70
Joshi, Mahendran, Alagusundaram, Norton, & Tiwari, 2013). 71
In order to achieve industrial adoption of ozone treatments for flour modification, it is important to 72
develop efficient ozone generation processes. Ozone production often involves use of corona 73
discharges in oxygen rich gas. Dielectric barrier discharges (DBD) are one of the most efficient 74
methods to produce ozone (Alonso, García, Calleja, Ribas, & Cardesin, 2005; Amjad, Salam, Facta, & 75
Ishaque, 2012). A DBD comprises of two electrodes with a large potential difference across them, 76
with a dielectric material in-between to prevent the transition into arc regime. The discharge from a 77
dielectric barrier set-up results in a cold plasma, which is an assemblage of reactive ions, electrons 78
and neutral species with emission of photons. Cold plasmas at atmospheric pressure are often 79
characterised by a net gas temperature close to ambience and a thermodynamic disequilibrium 80
between the temperature of neutral and electron species (Misra, Keener, Bourke, Mosnier, & Cullen, 81
2014). Atmospheric cold plasma (ACP) has recently drawn considerable attention of food scientists 82
and engineers for decontamination purpose (Misra, Tiwari, Raghavarao, & Cullen, 2011). It is well-83
known that ACP also modifies the structure of materials at the micro- to nano-metre ranges (Attri, 84
Arora, & Choi, 2013; Pankaj, Bueno-Ferrer, Misra, Milosavljević, et al., 2014; Pankaj, Bueno-Ferrer, 85
Misra, O'Neill, et al., 2014). 86
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The authors have recently demonstrated the applicability of DBDs to generate ACPs inside sealed 87
packages filled with air, at large gaps of order of centimetres, by applying sufficiently high voltages 88
(Misra, Patil, et al., 2014; Misra, Ziuzina, Cullen, & Keener, 2013; Pankaj, Misra, & Cullen, 2013). The 89
ACP generated in air is not only a source of ROS (including ozone and singlet oxygen), but also 90
excited molecular nitrogen (Misra, Pankaj, et al., 2014). Further, this set-up requires low power input 91
and is amenable to scale-up for continuous online processing. 92
Previous studies have demonstrated that ozone modifies the functional properties of wheat flour 93
(Chittrakorn, 2008). Considering this, we explore the possible effects of ACP generated in air (which 94
has many active species, including ozone) on wheat flours. The objective of this work was (1) to carry 95
out ACP treatments of soft and hard wheat flours, (2) optically characterise the plasma source, and 96
(3) evaluate the effects on selected technological and chemical properties of wheat flour. 97
2. Materials and Methods 98
2.1. Cold Plasma Treatment of Flour 99
Samples of soft and hard wheat flours were obtained from Teagasc Food Research Centre, Ashtown. 100
The samples were divided into 250 g unit portions for further experiments. The ACP source 101
comprises of a DBD powered from a step-up transformer (Phenix Technologies, Inc., USA), the input 102
to which is regulated using a variac. The DBD unit includes two aluminium electrodes of circular 103
geometry (outer diameter = 158 mm), resting over two polypropylene (PP) dielectric layers (2 mm 104
thick). Further details regarding the plasma source set-up can be found elsewhere (Misra, Patil, et 105
al., 2014). The wheat flour samples (250±10 g) were placed in commercial 270 µm thick polyethylene 106
terephthalate trays (150 mm × 150 mm × 35 mm), sealed with a high barrier 50 µm films. The 107
atmospheric air condition at the time of treatment was 45±1% relative humidity (RH) and 20±2°C, 108
measured using a humidity-temperature probe connected to a data logger (Testo 176T2, Testo Ltd., 109
UK). Treatments were done in duplicates. Two discrete voltages of 60 and 70 kV were applied across 110
the electrodes for 5 and 10 min. 111
2.2. Ozone Concentration 112
The ozone concentration inside the packages was measured after treatment using short-term 113
chemical ozone detection tubes (Product No. 18M, Gastec Corp., Kanagawa, Japan) with ±10% 114
accuracy. Gas sampling was conducted using a handheld pump (Gastec, Japan) and a needle with 115
septum placed at the point of sampling. The tubes were pre-calibrated for specific volume of the gas 116
and in this study, we sampled 10 mL volume. 117
2.3. Optical Emission Spectroscopy 118
The light emitted by the plasma discharge carries information regarding the chemical species 119
present in the plasma. To capture this information optical emission spectroscopy of the discharge in 120
an empty package, covering the ultraviolet-visible region (200-850 nm) was employed. Initial 121
attempts to acquire emission spectrum revealed that the fine flour particles interfere with the 122
optical path, which ruled out this approach for packaged filled with samples. Emission spectrum of 123
the discharge was acquired with an Ocean-Optics spectrometer HR2000+ spectrometer at a 124
resolution of 0.8 nm by coupling the light using an optical fibre (UV-Vis, 600 μm core). The spectra 125
were averaged over 10 samples accumulated for 50 s each to maximize the signal to noise ratio 126
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within the wavelength region. All spectra were corrected for noise levels by subtracting the noise 127
from background scans. Spectroscopic data on emission lines was based on National Institute of 128
Standards and Technology (2012) atomic spectra database and Herron and Green (2001). 129
2.4. Mixographic studies 130
Mixographs were recorded electronically using a 10 g bowl (National Mfg. Co. Lincoln, NE, USA) with 131
the spring fixed at the 10th
position in the scale. For construction of mixograms, the moisture and 132
protein contents of the samples were determined according to the method of the American 133
Association of Cereal Chemists (2000). Midline curve parameters were used for analysis as midline 134
parameters are reported to be very repeatable compared to envelop curve parameters (Martinant, 135
et al., 1998). Parameters evaluated were peak time, left peak value, right peak value, peak width, 136
right peak width, slope and integral, obtained during the first 8 min of the mixing process. 137
2.5. Dynamic Rheometry 138
Oscillation measurements were performed using a Haake Rheostress 6000 controlled stress 139
rheometer (Thermo Fischer, Karlsruhe, Germany) equipped with a Phoenix II P1-C25P refrigeration 140
circulation bath. A plate and plate geometry was used with a diameter of 30 mm and a plate gap of 2 141
mm. A thin layer of silicon oil was applied to the exposed surface of the sample between the plates 142
to prevent drying during testing. The sample was rested for 15 min to allow relaxation of stresses 143
generated during sample loading. Stress sweep tests (1 Hz at 25 °C) were made to determine the 144
linear viscoelastic region (LVR) of all samples; a stress value of 1 Pa was chosen for all the frequency 145
tests. Frequency sweep tests (mechanical spectra) from 0.1 to 10 Hz were performed at 25 °C. The 146
elastic modulus (G’) and viscous modulus (G’’), phase angle (tan δ) were derived at 25 °C. Three or 147
more replications were conducted for each dynamic test. 148
2.6. FTIR Spectroscopy 149
The secondary structure of the gluten protein for the dough samples was determined using FTIR 150
spectroscopy. Spectra were recorded on a Vertex 70 FTIR spectrometer (Bruker, Germany), 151
equipped with an ATR (Attenuated Total Reflection) cell and a deuterated L-alanine doped triglycine 152
sulphate (DLaTGS) detector. Spectra of an empty cell were taken as a background. Measurements 153
were performed over wavenumbers ranging from 800 to 2000 cm-1
(fingerprint region) with 4 cm-1
154
resolution using OPUS software. All spectra were the average of 200 scans. The spectrum for water 155
vapour was measured and subtracted from the sample spectra, following which they were corrected 156
for baseline shifts. Contribution of H2O to absorption in the amide I region was also subtracted by 157
using H2O-D2O (Bock & Damodaran, 2013). The spectrum for water vapour was measured and 158
subtracted from the sample spectra, following which they were corrected for baseline shifts. The 159
H2O-D2O reference spectra were used to digitally subtract contribution of H2O to absorption in the 160
amide I region (1600-1700 cm-1
) to obtain information on gluten secondary structure in dough. D2O 161
did not show absorption in the 3000-3800 cm-1
as well as in the amide I regions (Figure 1).The 162
quantitative estimation of gluten secondary structure in dough was determined from second-163
derivative spectra in the amide I region. Intermolecular β–sheets + antiparallel β-sheets (1612-1632 164
cm-1
), α-helix (1650-1660 cm-1
) and β-turn+ β-sheets (1665-1670 cm-1
) were calculated from the area 165
of the peaks obtained at different wave numbers (Wang, et al., 2014). 166
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[Figure 1 about here] 167
2.7. Statistical Analysis 168
The significance of differences among measured parameters of control and ACP treated flours was 169
statistically analysed using Tukeys multiple comparison test at a significance level of p=0.05. 170
Correlation matrix analysis for the mixograph parameters was carried out. Statistical analysis was 171
carried out using SPSS statistical package (SPSS ver 19, SPSS Inc., Chicago, IL). 172
3. Results and Discussion 173
3.1. Ozone Concentration 174
The electrical discharge in the inter-electrode region of the DBD generates energetic electrons, 175
which dissociate oxygen molecules by direct impact. Singlet oxygen (O•) resulting from this process 176
combines with oxygen molecules (O2) to form ozone (O3). The ozone concentrations for both wheat 177
flours after 5 and 10 min treatment at 60 kV (RMS) was 100 ± 20 ppm and 200 ± 40 ppm, 178
respectively (see Table 1). Treatment of flour at 70 kV (RMS) for 5 and 10 min, respectively, caused 179
an increase in ozone generation to 160 ± 40 and 240 ± 20 ppm. These concentrations were much 180
lower compared to those observed in empty packages, which was typically in the order of thousands 181
of ppm (Misra, Patil, et al., 2014). The possible reasons for this could be reduced amount of available 182
free gas volume following loading of flour into each package. Furthermore, the flour formed a bed of 183
up to 1.3-1.5 cm thickness in the package, which limits the electric field, thereby restricting the 184
collision processes leading to ozone formation. With regards to treatment of fresh produce, previous 185
study revealed that the ozone levels can vary significantly depending on the type of produce, which 186
modify the humidity levels in the package. This complicates the process optimisation for each type 187
of produce. However, considering that the flour samples do not vary significantly in their moisture 188
levels, the type of flour will not affect the concentration. 189
3.2. Optical Emission Spectroscopy 190
[Figure 2 about here] 191
The optical emission spectrum of the discharge in a window of 200-850 nm is presented in Figure 2. 192
The spectra revealed the presence of second positive system (SPS) vibrational bands of N2 indicating 193
low electron excitation energies. The C-B (0-0) transition of the SPS of N2 at 337 nm corresponds to 194
electron excitation energies of about 11 eV, much lower than the 19 eV required for the lowest N2+ 195
molecular ion band of the N2+ First Negative System (FNS) - which should be observed at 391 nm. 196
The singlet oxygen transition line at 777.4 nm was also observed, however, at relatively low 197
intensity, which was most likely an outcome of quenching of O(3P) and O(
5P) in the air plasma 198
(Walsh, Liu, Iza, Rong, & Kong, 2010). The small peak around 300 nm was identified as OH• 199
transition. The emission spectrum, together with the observed ozone concentrations indicated that 200
the cold plasma setup was a source of reactive nitrogen species (RNS) and reactive oxygen species 201
(ROS). 202
3.3. Mixographic Properties 203
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Dough rheological evaluation techniques are well accepted as a means of predicting pasta-cooking 204
and bread-making qualities. A mixograph provides practical information about the performance of 205
the wheat flour during product preparation. The results of mixographic studies for untreated and 206
ACP treated weak and strong wheat flours are summarised in Table 2. The high-resolution 207
mixographs (see supplementary material) indicated that the mixing action in the pin mixer is the 208
superposition of a large number of stretching and folding of the dough around the pins (Mann, et al., 209
2008). Different curve height measurements gave information about dough consistency while the 210
width of the curves and slopes expressed mixing tolerance. The correlation matrix for the mixograph 211
properties is presented in Table 3 which indicates the relation between individual parameters. It 212
may be noted from Table 3 that the peak time (PT) correlates well with the right peak value (RPV), 213
integral and, dough consistency (8V) and mixing tolerance (8W) at the end of mixing cycle. 214
[Table 2 about here] 215
[Table 3 about here] 216
The dough mixing curves of the weak flour reached the peak in a shorter time compared to the 217
strong wheat flours, which is a characteristic of the respective flours. Following ACP treatments, an 218
increase in the peak time (PT) and peak integral of both strong and weak flour was found, which was 219
positively correlated with the treatment time and voltage. The peak time is a measure of the dough 220
development time and the peak integral indicates the strength of the dough. A high correlation was 221
observed between PT and peak integral for both strong and weak wheat flour (Table 3). The 222
statistical analysis revealed that only treatment times beyond 5 min and applied voltage above 60 kV 223
had a significant effect (p=0.05) on the peak time and peak integral. Chittrakorn (2008) observed 224
that the ozone treatment of soft wheat flour increased time to peak during the mixograph test. 225
While it is known that the viscoelasticity rendered by the development stage in dough mixing 226
involves mainly large molecular size glutenins, the addition of oxidizing or reducing agents can be 227
used to modify the dough properties. It is also known that ozone promotes the oxidation of 228
sulfhydryl groups and the subsequent formation of disulphide bonds between cysteine moieties. 229
In earlier studies an increase in unextractable polymeric proteins following ozone treatment of soft 230
wheat flour has been associated with an increase in disulphide bonding between protein subunits 231
(Chittrakorn, 2008). Notably, disulphide bonds actively dictate the dough properties and exposure to 232
oxidants can increase the dough strength by the oxidation of sulfhydryl groups to disulfide bonds 233
(Yeh & Shiau, 1999). Unextractable polymeric proteins have been positively correlated with dough 234
strength and thus increase the dough development time (Singh, Singh, & MacRitchie, 2011). 235
Therefore, high dose and short exposure to ozone improved dough strength, by promoting 236
disulphide bond formation. 237
The breakdown behaviour of the dough is reflected in the tailing region of the curve, where mixing 238
continues beyond the PT (optimum dough development time) and is referred to as ‘mixing 239
tolerance’. The slope of the tailing region indicates the rate of breakdown, and shows the stability of 240
the dough and its sensitivity to mechanical treatment. Treatment for 10 min alone was found to 241
have a significant effect (p=0.05) on the mixing tolerance at the end of the mixing cycle. Both 242
treatment time and applied voltage were found to be insignificant (p=0.05) for inducing changes to 243
the optimum mixing tolerance (PW) of both hard and soft wheat flours. 244
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Earlier ozonation studies of soft to medium wheat flours were reported to cause an increase in the 245
resistance to extension. It was also hypothesized that excess formation of disulphide bonds during 246
longer treatment times, with the accompanying loss of sulfhydryl groups, may hinder disulphide 247
bond interchange and thus lead to a more rapid breakdown of dough (Wieser, 2003). Oxidants 248
facilitate the formation of disulfide bonds between glutenin subunits that result in improved dough 249
strength by making dough more elastic and thus enhancing the gas retention capacity (Bloksma, 250
1972). The effects of high concentrations of ozone were also observed in the present study. 251
Therefore, the observed improvement in dough strength (intergral of mixograph) and optimum 252
mixing time (PT) was most likely due to formation of disulphide linkages. Disulphide bonds among 253
flour proteins form strong cross-links within and between polypeptide chains, which stabilizes other 254
energetic bonding such as hydrogen and hydrophobic interactions (Sivam, Sun-Waterhouse, Quek, & 255
Perera, 2010). In an earlier study Mendez, Maier, Mason, and Woloshuk (2003) exposed wheat 256
grains to ozone and reported that the treatments did not significantly change the bread-making 257
properties of hard wheat, including tolerance of the dough to over-mixing, absorption of water and 258
mixing time to bake. This was most likely due to the resistance offered by the intact structure of the 259
grains to penetration of ozone and therefore minimising the reactivity of the chemical components 260
with ozone compared to flour treatment. 261
3.4. Dough Rheology 262
[Table 4 about here] 263
Average values of elastic modulus (G′) and viscous modulus (G″) for the dough from untreated and 264
ACP treated flours are shown in Table 4. The G′ of the dough from all the flours was greater than G”, 265
indicating a predominance of an elastic soft solid nature for the dough (Singh & Singh, 2013). Both 266
the moduli of strong wheat flour progressively increased with applied voltage and treatment time 267
(p=0.05). Statistical analysis also revealed that the interaction effect of voltage and treatment time 268
on both moduli was also significant (p=0.05). This increase might be attributed to the increased 269
viscoelasticity of the dough. However, in case of weak flour the increase was observed up to 5 min 270
exposure at 70 kV; with further increase in exposure time resulting in a reduction in both moduli. It 271
is suggested that the differences arise from the complex time and voltage dependent dynamics in 272
the plasma chemistry, involving over 75 species and 500 chemical reactions (Gordillo-Vázquez, 273
2008). 274
The tan δ (G’’/G’) is the ratio of viscous and elastic response of the material being tested. Both the 275
treated strong and weak flours did not show a large variation in tan δ, probably due to the 276
predominance of the elastic characteristics of the dough. Variation in the viscoelasticity has been 277
ascribed to the glutenin fraction (Xu, Bietz, & Carriere, 2007) and ACP treatments could change the 278
protein structure, which appears as differences in the rheological properties. Dough from the strong 279
flour treated at 60 kV for 10 min showed higher moduli and lower tan δ values as compared to 280
untreated flour. A higher moduli and lower tan δ for stronger flours was reported previously (Singh, 281
et al., 2013). On the other hand, dough from the weak flour treated with 70 kV for 5 min showed a 282
high moduli and lower tan δ as compared to its untreated counterpart. These changes are most 283
likely due to the interaction of the reactive oxygen species, mainly ozone and excited nitrogen 284
species formed in plasma. 285
3.5. FTIR Studies 286
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[Figure 3 about here] 287
Wheat flour proteins are complex and contains many molecular species with different sizes, 288
structures, and conformations (Chiang, Chen, & Chang, 2006). The α-helix, β-sheet, β-turn, and 289
aperiodic structures, when considered together, constitute the secondary structure of the protein 290
backbone (Bock, et al., 2013). FTIR spectra of dough mainly include amide I (1600-1700 cm-1
) and 291
amide II (1500-1600 cm-1
) absorption bands (Figure 3). Details on infrared absorption frequencies of 292
the protein secondary structures can be found in Dong, Huang, and Caughey (1990). Absorption 293
associated with the amide I band originates from stretching vibrations of the C=O bond of the amide, 294
while absorption associated with the amide II band derives primarily from bending vibrations of the 295
N-H bond. In the IR spectrum of gluten, the amide I band around 1650 cm-1
originates from α-helical 296
structures, and the band at 1685 cm-1
is due to β-sheets (Singh, Georget, Belton, & Barker, 2009). It 297
may be noted that, the non-repetitive C-terminal domain of gliadin proteins contains considerable 298
proportions of α-helix and β-sheet structures (Wieser, 2007). 299
The treatment time, applied voltage, and their interaction, irrespective of flour type, showed a 300
significant effect (p=0.05) on the secondary structure of flour proteins. Exposure of flour for short 301
durations of ACP caused a decrease in β-sheets + antiparallel β-sheets and increase in α-helix and β-302
turns, both in strong as well as weak wheat flour. However, strong flour showed a greater change as 303
compared to soft wheat flour. The increase in treatment beyond 60 kV for 5 min increased the β-304
sheets + antiparallel β-sheets, indicating the loss of an orderly structure of the proteins (Table 5). 305
The β-spiral structures, composed of consecutive β-turns, are reported to be the structural elements 306
contributing to the viscoelasticity of dough (Wellner, et al., 2005). The difference in ratio of β-turn + 307
β-sheets and intermolecular β-sheet + anti-parallel β-sheet between control and ozone treated flour 308
may be attributed to the variation in viscoelastic properties of their dough. The improvement in 309
viscoelasticity with plasma treatment was also clearly indicated by the increase in moduli and 310
decrease in tan δ. 311
[Table 5 about here] 312
It may be noted that the increase in viscoelasticity of the strong flour was also evident during the 313
rheological studies. The decrease in the helical structural elements in gluten of strong wheat flour, 314
which is specific for hydration capacity, indicated a possible lowering of gluten hydration capacity. In 315
the case of ACP treated weak wheat flour, an increase in the α-helix of protein molecules was 316
observed, along with β-sheet conformation. These results indicate that an ordering of protein 317
molecule and enhancement of hydrogen-bond strength has occurred in weak wheat flour. Thus, the 318
secondary structure of protein became more stable in ACP treated weak wheat flour. Results 319
presented were in agreement with those of Safonova, Kholodova, and Golota (2011) who explored 320
the effects of ozone on wheat flour. 321
4. Conclusions 322
A dielectric barrier discharge induced atmospheric pressure cold plasma was explored as a means to 323
change the structural and functional properties of strong and weak wheat flours. ACP treatments 324
were found to result in a voltage and treatment time dependent increase in the viscoelasticity of the 325
dough. An improvement in the dough strength and optimum mixing time for both weak and strong 326
wheat flours was observed. The elastic and viscous moduli of strong wheat flour increased with 327
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applied voltage and treatment time. Following treatment, a significant variation in the tan δ was not 328
found for either type of flour. FTIR spectroscopy revealed the alteration of the secondary structure 329
of gluten proteins following ACP exposure. These changes resulted from the action of reactive 330
chemical species of the ACP, as observed through ozone measurements and optical emission 331
spectroscopy of the discharge. Future studies will focus on assessment of ACP induced changes to 332
the starch in flour. 333
5. Acknowledgements 334
NN remains highly thankful to the Irish Research Council for the Embark Fellowship while AK to CSIR, 335
New Delhi for financial assistance. 336
6. References 337
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Amjad, M., Salam, Z., Facta, M., & Ishaque, K. (2012). A Simple and Effective Method to Estimate the 344
Model Parameters of Dielectric Barrier Discharge Ozone Chamber. Instrumentation and 345
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Attri, P., Arora, B., & Choi, E. H. (2013). Utility of plasma: a new road from physics to chemistry. RSC 347
Advances, 3(31), 12540-12567. 348
Bloksma, A. H. (1972). The Relation Between the Thiol and Disulfide Contents of Dough and its 349
Rheological Properties. Cereal Chemistry, 49, 104-118. 350
Bock, J. E., & Damodaran, S. (2013). Bran-induced changes in water structure and gluten 351
conformation in model gluten dough studied by Fourier transform infrared spectroscopy. 352
Food Hydrocolloids, 31(2), 146-155. 353
Chiang, S.-H., Chen, C.-S., & Chang, C.-Y. (2006). Effect of wheat flour protein compositions on the 354
quality of deep-fried gluten balls. Food Chem, 97(4), 666-673. 355
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Table 1 Ozone concentrations measured inside the packages. 1
Voltage [kV] Time [min] Ozone [ppm]
60 5 100 ± 20
60 10 200 ± 40
70 5 160 ± 40
70 10 240 ± 20
2
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Table 2 Mixographic properties of ACP treated wheat flour
Sample PT (min) LPV (%) RPV (%) PW (%) RPW (%) SLOPES (%) 8V (%) 8W (%) INTEGRAL
(%*min)
Strong control 4.07±1.86 57.50±1.86 58.99±1.31 36.76±0.49 28.92±0.12 1.03±0.51 56.06±1.56 24.92±1.91 201±3.61
60-5 4.22±0.01 55.61±0.10 57.51±0.60 37.03±2.0 31.97±2.92 3.37±3.14 55.10±0.32 24.96±1.81 208±3.10
60-10 4.76±0.64 57.15±1.22 58.10±1.91 37.59±3.0 27.88±0.99 1.17±1.86 56.99±0.38 26.75±1.18 239.57±3.13
70-5 4.1±0.18 55.99±0.42 58.99±2.75 35.47±0.18 27.25±0.35 2.77±0.01 55.31±1.71 25.20±1.15 204.29±7.28
70-10 5.43±0.05 58.48±0.24 58.48±3.21 35.70±1.97 32.67±4.03 1.66±0.18 56.86±2.26 27.25±3.43 273.54±9.37
Weak control 2.35±0.05 47.33±2.00 47.09±2.69 33.76±0.48 23.16±0.49 2.42±0.16 41.65±2.12 18.79±1.12 100.26±2.62
60-5 2.83±0.23 49.35±0.68 50.04±0.55 38.50±4.92 30.21±2.40 3.25±0.60 44.47±1.29 21.52±0.62 125.28±1.17
60-10 2.84±0.69 49.40±1.17 50.66±0.56 44.20±0.51 30.75±4.31 1.39±4.31 1.39±1.27 45.48±1.76 22.35±2.67
70-5 2.72±0.11 49.27±1.50 49.02±1.62 38.94±15.4 28.36±3.13 3.54±2.96 43.15±1.74 19.73±1.87 127.32±1.84
70-10 3.80±0.69 50.14±0.75 49.18±2.13 33.54±1.83 35.79±1.23 0.16±0.24 44.93±1.79 24.91±1.01 200±1.14
Peak Time: PT, optimum dough development time; Left peak value: LPV, Dough consistency before 1mins of optimum dough development; Right
peak value; RPV, Dough consistency after 1mins of optimum dough development; Peak width: PW, Optimum mixing tolerance; Right peak width:
RPW, Mixing tolerance after 1mins of optimum dough development; Slope: Value at the end of mixing cycle: 8V, Dough consistency at the end of
mixing cycle; Width at the end of mixing cycle: 8W, Mixing tolerance at the end of mixing cycle; Integral: Area under midline curve for the first 8
min of mixing, dough strength.
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Table 3 Correlation coefficients between mixograph parameters
Parameter PT (min) LPV (%) RPV (%) PW (%) RPW (%) SLOPES (%) 8V (%) 8W (%) INTEGRAL
(%*min)
PT (min) 1
LPV (%) 0.897** 1
RPV (%) 0.758** 0.936** 1
PW (%) -0.264 -0.086 0.007 1
RPW (%) 0.293 0.252 0.307 0.369 1
SLOPES (%) 0.115 -0.012 -0.062 -0.417 0.082 1
8V (%) 0.845** 0.974** .981** -0.073 0.28 -0.012 1
8W (%) 0.834** 0.803** 0.807** -0.121 0.445* 0.214 0.831** 1
INTEGRAL (%*min) 0.995** 0.923** 0.790** -0.244 0.281 0.068 0.871** 0.823** 1
**Correlation is significant at the 0.01 level.
* Correlation is significant at the 0.05 level.
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Table 4. Rheological properties of dough prepared from ACP treated wheat flour
Wheat type Treatment G' (Pa) G" (Pa) Tan δ
Strong Control 37893 ± 3383 12383 ± 1407 0.326 ± 0.008
60 kV-5 min 31387 ± 1755 10147 ±1557 0.322 ± 0.031
60 kV-10 min 45090 ± 1414 14002 ± 704 0.310 ± 0.005
70 kV-5 min 46841 ± 1359 14687 ± 2689 0.313 ± 0.048
70 kV-10 min 51454 ± 1419 15586 ± 887 0.303 ± 0.026
Weak Control 52693 ± 1974 17442 ± 1413 0.332 ± 0.039
60 kV-5 min 45757 ± 1418 14998 ± 1835 0.327 ± 0.029
60 kV-10 min 54432 ± 2776 17086 ± 795 0.314 ± 0.001
70 kV-5 min 72028 ± 2711 22908 ± 1411 0.319 ± 0.031
70 kV-10 min 67152 ± 1405 20315 ± 1272 0.302 ± 0.012
Table 5 Secondary structures of ACP treated dough proteins.
Wheat type Treatment Intermolecular
β-sheets + antiparallel
β-sheets (%)
α-helix
(%)
β-turn+
β-sheets (%)
Strong Control 38.96±0.12 54.56±2.71 18.92±0.56
60-5 32.83±0.02 45.06±2.22 19.72±1.11
60-10 32.63±1.43 46.67±3.58 16.24±0.47
70-5 31.35±1.78 48.45±0.37 16.12±1.58
70-10 40.41±1.36 45.62±4.38 12.05±0.06
Weak Control 33.07±0.21 47.81±2.18 19.12±2.39
60-5 35.88±0.80 45.58±0.51 18.53±1.02
60-10 33.04±3.05 49.82±0.18 19.13±0.80
70-5 33.08±1.29 49.82±0.13 17.18±1.67
70-10 33.03±3.66 48.13±0.98 19.63±0.33
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Figure 1 FTIR spectra of H2O, D2O and H2O-D2O
Figure 2 Optical Emission Spectra of the discharge inside an empty package 70 kV (RMS)
applied voltage.
Figure 3 FTIR Spectra of (A) Control weak flour, (B) Treated (70 kV, 10 min) wheat flour, (C)
Control strong wheat flour, and (D) Treated (70 kV, 10 min) strong wheat flour.
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Figure 1:
Figure 2:
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Figure 3:
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Atmospheric pressure cold plasma (ACP) treatment of wheat flour
N.N. Misra1, Seeratpreet Kaur
2, Brijesh K Tiwari
3, Amritpal Kaur
2, Narpinder Singh
2 and P.J. Cullen
1,4
1BioPlasma Research Group, School of Food Science and Environmental Health, Dublin Institute of Technology, Dublin 1, Ireland
2Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 05, India
3Department of Food Biosciences, Teagasc Food Research Centre, Dublin, Ireland
4School of Chemical Engineering, University of New South Wales, Sydney, Australia
Supplementary Material
Figures provided below are representative mixographs of the control and ACP treated weak and strong wheat flour. For all the mixographs, the
abscissa represents the mixing time, while the ordinate-axis represents the consistency (%).
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Sample: Weak Flour, Control
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Weak Flour, Treated at 60 kV for 5 min
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Weak Flour, Treated at 60 kV for 10 min
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Weak Flour, Treated at 70 kV for 5 min
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Weak Flour, Treated at 70 kV for 10 min
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Sample: Strong Flour, Control
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Strong Flour, Treated at 60 kV for 5 min
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Strong Flour, Treated at 60 kV for 10 min
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Strong Flour, Treated at 70 kV for 5 min
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Strong Flour, Treated at 70 kV for 10 min
