glutenin and gliadin proteins

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Cite this: DOI: 10.1039/c8fo01810c
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The thermal stability, structural changeability,
and aggregability of glutenin and gliadin proteins
induced by wheat bran dietary fiber
Sen Ma,
* Wen Han, Li Li, Xueling Zheng and Xiaoxi Wang*
Wheat bran dietary fiber (WBDF) has been reported to be responsible for the low quality of whole wheat
flour products due to its destructive effect on the gluten matrix. Glutenin and gliadin are the major components of gluten protein and contribute to a proper gluten structure. In this study, the thermostability,
surface hydrophobicity, fluorescence characteristics, free sulfhydryl contents, and molecular weight distributions of glutenin- and gliadin-rich fractions were determined after the addition of WBDF. The addition of
WBDF to glutenin resulted in an increased surface hydrophobicity and free sulfhydryl content, as well as a
red-shift of the fluorescence spectrum. However, the WBDF-modified gliadin fraction changed slightly
mainly due to its spherical conformation. Size exclusion chromatography profiles revealed increasing
Received 14th September 2018,
Accepted 9th November 2018
soluble gliadin aggregates and decreasing high molecular weight glutenin fractions as a result of WBDF
incorporation. The results from the thermostability analyses exhibited decreased weight loss and decompo-
DOI: 10.1039/c8fo01810c
sition temperatures for both glutenin and gliadin proteins at high WBDF concentration. Our results suggest
rsc.li/food-function
that changes in the gluten matrix caused by WBDF may largely rely on glutenin structure variation.
Introduction
It is well known that the wheat flour processing quality is
related to both the quantity and quality of its protein.
Fractionation and reconstitution experiments revealed that
gluten quality is responsible for bread-making differences
among wheat cultivars.1 The gluten protein can be divided
into two fractions based on its solubility in ethanol–water solution (e.g. 70% ethanol solution); the soluble fraction is gliadin,
and the insoluble fraction is glutenin. Glutenin is composed
of huge macropolymers of high- and low-molecular weight
subunits crosslinked via interchain disulfide bonds that result
in a wide range of molecular weights from 105 to 107 Da,2
while gliadin has a rather small molecular weight and can only
form intramolecular disulfide bonds and noncovalent bonds.3
It has been suggested that gliadin contributes to dough viscosity and glutenin contributes to dough elasticity and, for
this reason, both gliadin and glutenin are believed to be vital
in providing an appropriate viscous and elastic property
balance in gluten.4 In bread making, greater gas cell coalescence occurred in dough made with weak flour that had insufficient elastic gluten during processing, which was caused by
low strain hardening and poor gas cell stability.5,6 Increased
College of Food science and Technology, Henan University of Technology, Zhengzhou,
Henan 450001, China. E-mail: masen@haut.edu.cn, xxwanghaut@126.com;
Fax: +86-67758026; Tel: +86-67758026
This journal is © The Royal Society of Chemistry 2018
elasticity leads to higher loaf volumes because of more gas
entrapment.7 However, when gluten is too elastic and rigid,
bread cannot rise due to the impaired expansion of gas cells.8
Previous studies indicated that variations in glutenin and
gliadin molecular weight distributions also greatly influenced
the bread-making performance.9,10 Glutenin macropolymers
are keys to wheat quality and are strongly correlated with bread
loaf volume and dough extensibility.11 Moreover, numerous
studies reported the vital role of glutenin in producing fine
bread texture,12,13 while others contended that the gliadin fraction was more favorable and could contribute to desirable
bread loaf volumes.14,15 Therefore, glutenin and gliadin
should be well studied to better understand their contributions to gluten matrix formation and the quality of flour
products.
Wheat bran, as a good source of dietary fiber, has been
used widely in wheat flour products for nutritional purposes.
However, extensive studies have revealed undesirable branenriched food quality, which was attributed to the interaction
between wheat bran dietary fiber (WBDF) and gluten fractions.16,17 Coarse bran was believed to yield an open and disaggregated gluten structure.18 Observations from standard
light microscopy also revealed damage by wheat bran dietary
fiber to gluten continuity and gas cell expansion.19 Moreover,
bran fibers could lead to gluten conformational changes and
difficulties in forming a favorable gluten network.20,21
The reduction in free thiol content and high molecular weight
glutenin subunits added to the growing evidence for changes
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in the structure and aggregation of gluten fractions induced by
the wheat bran dietary fiber.18 However, to the best of our
knowledge, fewer studies have focused on the changes in glutenin and gliadin fractions as a result of WBDF addition. As
demonstrated above, glutenin and gliadin are the most essential protein fractions to form a proper gluten matrix, which
directly determines gluten properties. Therefore, we studied
the influence of WBDF on glutenin and gliadin fractions.
This study aimed to characterize the heat, structure, and
aggregation of glutenin and gliadin induced by WBDF to add
growing evidence to the interactions between gluten fractions
and dietary fiber. For this purpose, WBDF was added at various
concentrations to glutenin and gliadin-rich fractions to study
the changes in thermal decomposition characteristics, surface
hydrophobicities, fluorescence characteristics, free sulfhydryl
contents, and molecular weight distributions. In this study, we
interpreted WBDF and gluten component interactions and
deepened the understanding of the reasons behind the changes
in gluten matrix formations induced by WBDF.
Materials and methods
Materials
Commercial wheat bran (15.32% protein, 10.17% starch,
6.64% ash, 14.21% moisture) and wheat gluten (73.64%
protein, 8.48% starch, 1.26% ash, 12.07% moisture) were purchased from a local market. The content of the components
was measured under laboratory conditions.
Wheat bran dietary fiber preparation
Wheat bran dietary fibers were prepared according to the
method from our previous study.22 Briefly, wheat bran was pulverized and blended with 10-fold distilled water in a colloid
mill. The resultant suspension was washed under running
water, and the wheat bran was dried. Next, distilled water was
blended with the wheat bran in a 10 : 1 dilution and 1.5%
(w/w) of thermostable α-amylase (40 000 U g−1) and 3% (w/w)
of alkaline protease (≥200 000 U g−1) were added at a suitable
temperature and pH. Ultimately, the solid residues were
washed and dried for 24 h at 60 °C. The prepared WBDF was
obtained after pulverizing the mixture to 150 μm of particle
size. The purity of the WBDF was assessed using a total dietary
fiber assay kit (K-TDFR, Megazyme, Bray Co., Wicklow,
Ireland). The total dietary fiber content was 90.43%, including
5.76% soluble dietary fiber and 84.67% insoluble dietary fiber.
The impurities consisted of primarily protein (4.72%) and
starch (4.12%), measured according to the Kjeldahl method
using the Kjeltec 8400 (Foss, Hoganas, Sweden) and the AACC
Method 76-11.
Glutenin and gliadin powder preparations
The preparation of gluten-WBDF dough was conducted based
on our earlier study,23 with some modifications. The WBDF
contents were 0%, 3%, 6%, 9%, 12%, and 15% (w/w) in
relation to gluten weights (using the same moisture basis).
Food Funct.
Food & Function
The gluten powder was obtained after dough-kneading, glutenwashing, freeze-drying, and grinding. The gluten and gliadin
preparations were based on the report of Khatkar et al.,24 with
some modifications.
70% ethanol solution was used to extract the gliadin-rich
fraction, and the resultant liquid–solid ratio was 1 : 30 (w/v).
Gluten powder was slowly added to the ethanol solution with
stirring to avoid excessive protein aggregation. Extractions
were conducted at 20 °C for 3 h using a magnetic stirrer and
then centrifuged at 2000g for 20 min. The supernatant was
diluted with a three-fold 70% ethanol solution, and then the
pH was adjusted to 6.50 using 0.1 mol L−1 HCl and 0.1 mol L−1
NaOH. The solution was continued to be stirred with a
glass rod, and the precipitate (gliadin-rich fraction) was collected. Because glutenin demonstrates good solubility in weak
acid solutions, the glutenin-rich sediment was added to distilled water, and the pH was adjusted to 5.0. The slurry was
stirred at 20 °C for 3 h and filtered using 74 μm sieves to
remove insoluble WBDF particles. Next, the solution was
diluted with two-fold absolute ethyl alcohol and the pH was
adjusted to neutral. After three hours of stirring, the slurry was
centrifuged [2000g for 20 min], and the precipitate was
collected. Both the gliadin- and glutenin-rich fractions were
freeze-dried and pulverized to 150 μm. Glutenin (crude protein
content, 72.5%) and gliadin (crude protein content, 83.1%)
were measured according to the Kjeldahl method using the
Kjeltec 8400 (Foss, Hoganas, Sweden).
Glutenin and gliadin solution preparations
The preparation of the glutenin and gliadin solutions was performed based on the study of Han et al.22 An acetic acid solution was adopted to dissolve the gluten fractions using a magnetic stirrer. After 2 h of stirring, the solution was centrifuged
(8000g, 5 °C, 20 min) and the supernatant was preserved at
4 °C.
Surface hydrophobicity
Surface hydrophobicity (Ho) of the gluten protein was determined using 1-anilino-8-naphthalene sulfonate (ANS) as the
hydrophobic probe.25 The concentration of the glutenin and
gliadin protein solutions was measured using the Coomassie
brilliant blue method. To make the protein concentrations
within the range of 0.005–0.5 mg mL−1, the protein solution
was diluted 20, 40, 80, 100, 200, and 300 times and 50, 80, 100,
200, 500, and 800 times for glutenin and gliadin, respectively,
based on their original concentration. Twenty microliters of
ANS (8 mol L−1) were added to 4 mL of the diluted gluten solution. The fluorescence intensity of gluten was measured after
3 min, and a diluted gluten solution without ANS was used as
the zero reference. Excitation and emission wavelengths were
390 and 470 nm, respectively, and excitation and launching slit
widths were set to 5 nm. Each sample was analyzed three times.
The fluorescence characteristics of glutenin and gliadin
The fluorescence spectra of glutenin and gliadin were obtained
using a fluorescence spectrophotometer (Cary Eclipse,
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EL06063220; Agilent Technologies, CA, USA). Spectra between
300–450 nm were scanned with a 290 nm excitation wavelength and a 5 nm excitation and launching slit width.
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Free sulfhydryl (SH) content determination
Free SH (SHfree) content was determined spectrophotometrically after reacting with [5,5′-dithiobis(2-nitrobenzoic acid)]
DTNB according to Zhou et al.26 0.1 g of glutenin and gliadin
powder was dissolved in 5.5 mL of an Ellmann reagent
[ propan-2-ol, 250 mM Tris-HCl buffer ( pH 8.5) and 4 g L−1
DTNB in ethanol (5/5/1, v/v/v)] and was shaken for 10 min. The
supernatant was collected after centrifugation at 8000g for
15 min and measured at 412 nm. The results are expressed in
μmol per g of protein. The above analyses were performed in
triplicate.
Size exclusion chromatography (SEC)
The gluten solutions were separated by using an Agilent 1260
Infinity LC system (CA, USA) with a Biosep SEC-4000 column
(Phenomenex, Torrance, CA, USA). The elution parameters
were determined according to Zhao et al.,27 with slight modifications. The acetic acid solution (0.5 mol L−1) was passed
through a 0.25 μm filter that was vacuum filtered and degassed
to elute the mobile phase. The glutenin and gliadin solutions
were eluted through a 0.45 μm filter before testing. The SEC
experimental conditions were: 0.5 mL min−1 flow rate, 100 μL
injection volume, 40 °C column temperature, and UV-detection of 220 nm. The integral area of each peak was calculated
and organized using the ORIGIN software program (v. 8.5
PRO, OriginLab Corporation, USA).
Thermogravimetric analyzer
Thermogravimetry (TG) measurements were performed using
a thermogravimetric analyzer (Q50, Waters LLC, New Castle,
Delaware USA) according to Nawrocka et al.28 Freeze-dried
gluten samples within 10 mg were placed in oxide aluminum
pans and heated from 40 to 600 °C at a heating rate of 20 °C
min−1. The degradation temperatures (Td) and weight loss of
the gliadin and glutenin samples were recorded. The TG tests
were performed in replicate.
Statistical analysis
Statistical analyses were performed with ANOVA (level of significance, P < 0.05) followed by Duncan’s test with SPSS software (version 19.0, SPSS Inc., Chicago, IL, USA).
Results and discussion
Surface hydrophobicity
The surface hydrophobicity of glutenin and gliadin upon
WBDF addition is shown in Fig. 1. ANS was used as a hydrophobic probe due to its sensitivity to polarity and its ability to
amplify fluorescence emission signals to compensate for low
protein quantum yields.22 Observations from the Ho intensities of the two protein fractions suggested that gliadin has
This journal is © The Royal Society of Chemistry 2018
Fig. 1 Surface hydrophobicity of glutenin (Glu) and gliadin (Gli) upon
WBDF addition. ■ Glu fraction and ● Gli fraction. The error bars show
the corresponding standard deviation and the y-axis on the left and
right is assigned to Glu and Gli samples, respectively.
higher surface hydrophobicity than glutenin and the addition
of WBDF brought subtle changes to gliadin surface hydrophobicity. This could be ascribed to the spherical conformation of
the gliadin proteins, which make it harder for WBDF to
destroy such compact particles and reveal the hydrophobic
amino acids that are buried in the protein core during the
gluten matrix formation. However, with increasing WBDF
content, the surface hydrophobicity of glutenin increased from
2611.2 ± 77.07 to 4883.55 ± 68.94. According to Laligant
et al.,29 one of the possible reasons that contributed to the
higher surface hydrophobicity might have been the dissociation of protein subunits and the extension of peptide
chains. Hence, it seems that addition of WBDF leads to structural changes of the glutenin filaments.
Fluorescence characteristics
The gluten protein has fluorescence characteristics owing to
the presence of aromatic amino acids including tryptophan,
tyrosine, and phenylalanine. According to our earlier study,22
tryptophan fluorescence was regarded to be a useful tool to
study protein structural changes not only because of its
responsiveness to the microenvironment polarity but also
because of its greatest fluorescence intensity. Tryptophan exhibits broad fluorescence absorption centered around 350 nm
with an approximate width of 60 nm. The stretch of gluten proteins exposes tryptophan residues to the polar environment
and thus leads to a red-shift, or a blue-shift in the fluorescence
spectrum.30 One study assigned the fluorescence spectra of
tryptophan to five spectral classes (A, S, I, II and III).22 In the
current study, we show that both glutenin and gliadin exhibited fluorescence absorption peaks that centered around
350 nm, and according to the fluorescence spectra mentioned
above, were assigned to spectra III (Fig. 2). The spectra III fluorescence peak corresponds to indole chromophore emissions
on the protein surface that is in contact with free water mole-
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Free sulfhydryl content
Fig. 2 Fluorescence characteristic of glutenin (Glu) and gliadin (Gli)
induced by increasing WBDF (wheat bran dietary fiber) levels from 0 to
15%. Spectra between 300–450 nm were scanned with a 290 nm excitation wavelength and a 5 nm excitation and launching slit width.
cules, which is rare in normal proteins but is common in
stretched proteins. Such findings were in line with our previous work in which the gluten protein was studied and exhibited a similar fluorescence peak.22 According to these studies,
it can be deduced that the gluten protein fractions are more
inclined to stretch in acetic acid solution.
Obviously, there are two groups of fluorescence spectra with
different fluorescence intensities that were assigned to glutenin and gliadin, respectively. For the glutenin filaments, a
high probability of exposure of tryptophan residues was
expected, which resulted in a higher fluorescence intensity.
Unlike glutenin, most of the gliadins are globular and encase
tryptophan residues within their core. Therefore, gliadin
protein has fewer exposed tryptophan residues compared with
glutenin and thus exhibited fluorescence spectra with low
intensity. Furthermore, a significant red-shift was observed in
the gliadin samples, indicating a stronger gliadin tryptophan
polarity compared with that of glutenin, which may be due to
the fluorescence quantum yield of gliadin that mostly
comes from tryptophan on the particle surfaces, rather than in
the cores.
The addition of WBDF led to a gradual decrease in the fluorescence quantum yield of the glutenin and gliadin proteins.
We also noticed that the tryptophan microenvironment of the
glutenin samples was more sensitive to WBDF as evidenced by
a significant reduction in the glutenin fluorescence intensity
after WBDF was added. Such a finding corresponds well to the
surface hydrophobicity changes in the two proteins induced by
WBDF. Besides, a slight red-shift was observed in both protein
samples, suggesting that the tryptophan residues had
increased polarity. The WBDF polysaccharide is enriched with
polar hydroxyl groups, which could enhance the polarity of
the microenvironment around tryptophan when it interacts
with proteins.
Food Funct.
Fig. 3 shows the effect of WBDF on the accessible thiol contents
in glutenin and gliadin proteins during dough mixing. In agreement with the literature, we believe that changes in free sulfhydryl (SH) levels are persuasive indicators of SS bond variations
and the degrees to which protein molecular chains aggregate.31,32
The results in Fig. 3 showed a significantly lower SH content of
gliadin compared with that of glutenin. It is universally known
that glutenin subunits develop ordered fibrous macromolecular
polymers with intermolecular disulfide linkages, while gliadins
only form intramolecular disulfide linkages. According to a
previous study, the intermolecular disulfide linkages are more
readily cleaved than intrachain bonds, which are likely to be
buried in the core of the native molecule protected from chemical reduction.33 Therefore, it is possible that the low reduction
reaction probability of intermolecular disulfide bonds in gliadin
protein is responsible for its low SH content.
Fig. 3 shows the significant differences between glutenin
and gliadin samples in the accessible thiol content as WBDF
is supplemented from 0 to 15%. For WBDF enriched glutenin,
the SH glutenin content increased from 5.27 μmol g−1 to
12.24 μmol g−1, which implied that WBDF might be involved
in the alteration of the glutenin tertiary structure to account
for the thiol/disulfide exchange. A strong correlation between
the SH content and the formation of a proper gluten network
has been observed in many studies.3,31 The molecular weight
distribution of glutenin has been recognized as one of the
main determinants of dough quality and is governed by the
state of the disulfide structure.3 Hence, the replacement of
gluten with WBDF could hinder glutenin aggregation by
increasing the SH content. However, it appears that intermolecular disulfide bonds in gliadin are more stable as indicated by the subtle changes in SH content as the WBDF was
increased. This may be ascribed to the compact globular structure of gliadin that reduced the possibility to contact with the
Fig. 3 Effect of the WBDF level on the free sulfhydryl content of glutenin (Glu) and gliadin (Gli). The error bars show the corresponding standard deviation.
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WBDF polysaccharide. Consequently, it is possible that the
impact of WBDF on the gluten SH content was largely caused
by changes in the glutenin SH content.
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Molecular weight distributions
Fig. 4 shows the SEC profile of glutenin and gliadin protein
fractions induced by different levels of WBDF. The elution
profile shows that glutenin and gliadin are composed of two
fractions, where the elution time ranges from 8 min to
13.5 min ( peak 1) and 13.5 to 20 min ( peak 2). Integral peak
areas were calculated to represent the corresponding gluten
fraction contents and are shown in Table 1. Observations from
the SEC profiles demonstrated the differences in the molecular
weight distributions of the gluten fractions. Gliadin exhibited
a remarkably higher integral area content in fraction 1 compared with that in fraction 2, while the two glutenin fractions
( peak 1 and peak 2) exhibited similar contents. The A1 and A2
values of the gliadin samples were significantly higher compared with those of the glutenin samples. There was a positive
correlation between the peak area and the UV absorption
intensity, which implicitly suggests a higher solubility of the
gliadin fractions in acetic acid solution. A previous study
observed a better solubility and dispersity of gliadin at low pH
due to higher electrostatic interactions on the surface of the
gliadin particles.34 The higher the gliadin nanoparticle
content, the more likely the aggregate structures will be
Paper
Table 1 Integral areas of the peaks corresponding to the SEC profiles
of WBDF modified glutenin (Glu) and gliadin (Gli) samples
Sample
WBDF
content (%)
A1
A2
a
Gli
0
3
6
9
12
15
1134.64 ± 3.10
1219.93 ± 17.58b
1246.62 ± 3.49b
1440.99 ± 4.81c
1438.58 ± 14.34c
1427.90 ± 18.26c
676.91 ± 17.83a
698.86 ± 5.32a
661.92 ± 11.89a
618.83 ± 1.95b
615.15 ± 19.47b
622.56 ± 4.78b
Glu
0
3
6
9
12
15
283.93 ± 11.32ab
286.84 ± 18.50ab
295.47 ± 12.62a
279.46 ± 0.24ab
265.08 ± 17.10ab
256.94 ± 5.92b
302.23 ± 13.74a
283.03 ± 13.59ab
264.70 ± 13.04bc
239.26 ± 12.92cd
234.94 ± 3.38d
236.65 ± 1.22d
Mean values followed by different letters in the same column are
significantly different (p < 0.05). Gli, gliadin protein; Glu, glutenin.
formed.35 As indicated by the peak 1 elution time of the glutenin and gliadin samples, the two eluted fractions had similar
molecular weights. It is well known that gliadin has a smaller
molecular weight than glutenin. However, owing to the better
solubility and aggregability of the gliadin protein in solution,
the protein fraction corresponding to peak 1 in the gliadin
eluate profile may represent soluble aggregates.
Upon the addition of WBDF, the A2 value of glutenin and
gliadin decreased, which implied that this protein fraction was
depolymerized. However, the A1 value showed different trends
as WBDF increased from 0 to 15%. The content of soluble
aggregates in gliadin samples increased from 1134.64 ± 3.10 to
1427.90 ± 18.26, suggesting enhanced aggregation induced by
WBDF. As indicated from another study, the high molecular
mass of gliadin (>97 kDa) was also observed in dough containing dietary fiber including highly methoxylated pectin,
xanthan gum, and locust bean gum.36 These aggregates were
believed to be formed by gliadins linked through noncovalent
interactions.37 Therefore, it can be inferred that WBDF causes
disaggregation of glutenin fractions, but yields high molecular
weight gliadin aggregates.
Thermostability
Fig. 4 SEC profiles of changes in glutenin (Glu) and gliadin (Gli) with
the addition of different amounts of WBDF.
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Thermogravimetry (TG) measurements were applied to determine the dependence of gluten samples on temperature, and
to provide information about the structural changes of the
gluten fractions. Owing to the limited information obtained
from the TG curves, DTG curves were combined to observe
decomposition temperatures. The so-called DTG curve was the
first derivative of the TG curve versus temperature. The degradation temperatures (Td) and decreases in gliadin and glutenin
sample weights are presented in Table 2. The thermal degradation temperatures of gliadin and glutenin without WBDF
were 319.81 ± 0.97 °C and 299.06 ± 0.82 °C, respectively,
suggesting a higher thermal stability of gliadin than glutenin.
This higher thermal stability could be related to the spherical
construction of the gliadin proteins, which limited the ability
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Table 2 Thermogravimetric parameters, weight loss at 600 °C and
degradation temperature (Td), for WBDF-modified glutenin (Glu) and
gliadin (Gli) samples
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Sample
WBDF
content (%)
Td (°C)
Weight loss (%)
a
Gli
0
3
6
9
12
15
319.81 ± 0.97
319.76 ± 1.32a
319.49 ± 1.03a
319.83 ± 1.85a
319.87 ± 1.79a
314.8 ± 1.12b
68.07 ± 0.57a
72.05 ± 0.49b
72.32 ± 0.84b
71.35 ± 0.42b
72.28 ± 0.63b
72.52 ± 0.65b
Glu
0
3
6
9
12
15
299.06 ± 0.82a
299.38 ± 0.58a
299.89 ± 0.97a
300.10 ± 1.69a
296.79 ± 1.81b
297.57 ± 1.52b
71.67 ± 0.31a
71.19 ± 0.58a
72.82 ± 0.41ab
72.11 ± 0.36a
72.36 ± 0.49ab
73.34 ± 0.29b
Mean values followed by different letters in the same column are
significantly different (p < 0.05). Gli, gliadin protein; Glu, glutenin.
rich fractions induced by WBDF. The addition of WBDF to glutenin yielded increasing surface hydrophobicity and free sulfhydryl content, as well as a red-shift in the fluorescence spectrum. However, little changes occurred in WBDF-modified
gliadin fractions, which implied that gliadin structures were
more stable than glutenin structures. The SEC profiles revealed
an increase in soluble gliadin aggregates, and a decrease in
high molecular weight glutenin fractions after WBDF was
added, indicating the disaggregation of the glutenin fraction
upon WBDF addition. It can be inferred from the above findings that WBDF yields a weaker and looser glutenin structure,
while the gliadin protein structure was hardly affected owing
to its spherical conformation. Therefore, changes in the gluten
protein network induced by WBDF may be mostly due to the
interactions of WBDF with glutenin.
Conflicts of interest
There are no conflicts to declare.
of the protein molecular chains to stretch and the chemical
bonds to break. During the thermal decomposition stage, the
covalent peptide bonds, the disulfide bridges, and the O–N
and O–O linkage breaks lead to the decomposition of gluten
proteins.38 It was reported that gliadin intramolecular disulfide bonds tended to get buried inside the gliadin particles
which made them difficult to break during heating.33 The
observations of the gliadin and glutenin SH content in our
study also provide indirect evidence for this. The weight loss in
the gliadin and glutenin samples without WBDF was 68.07 ±
0.57 and 71.67 ± 0.31%, respectively, which further confirmed
the higher thermal stability of gliadin than glutenin.
As seen from Table 2, the addition of WBDF caused similar
changes in weight loss and degradation temperatures for glutenin and gliadin samples. According to Khatkar et al.,38 the
weight loss changes provided information about the changes
in the gluten structure with an increase in weight loss,
suggesting the formation of a more open and weak gluten
structure. The addition of WBDF led to a gradual increase in
weight loss for both gliadin and glutenin which indicated
looser gluten fractions induced by WBDF. However, the temperature at which glutenin and gliadin degrade decreased only
at high WBDF content levels. The Td values remained
unchanged at low WBDF content, which indicated that the
gluten proteins are thermally stable in the presence of small
amounts of WBDF. Similar results were obtained by
Nawrocka et al.,39 who also studied the thermal properties of
fiber-modified gluten. It can be concluded that high WBDF
levels contribute to a looser gluten structure that impairs the
thermostability of the gluten fractions.
Conclusion
The present study investigated the thermostability, structural
changes, and aggregation behavior of glutenin- and gliadin-
Food Funct.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Henan Provincial Colleges and Universities in Henan
University of Technology (no. 2018RCJH08), the Henan
Province Colleges and Universities Young Backbone Teacher
Plan (no. 2016GGJS-070), the Key Scientific and Technological
Project of Henan Province (no. 172102110008), and the
National Natural Science Foundation of China (no. 31571873).
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Food & Function
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