International Journal of Food Properties
ISSN: 1094-2912 (Print) 1532-2386 (Online) Journal homepage: https://www.tandfonline.com/loi/ljfp20
Steady and dynamic shear rheology of starches
from different oat cultivars in relation to their
physicochemical and structural properties
Sukriti Singh & Maninder Kaur
To cite this article: Sukriti Singh & Maninder Kaur (2017) Steady and dynamic shear rheology of
starches from different oat cultivars in relation to their physicochemical and structural properties,
International Journal of Food Properties, 20:12, 3282-3294, DOI: 10.1080/10942912.2017.1286504
To link to this article: https://doi.org/10.1080/10942912.2017.1286504
© 2017 Taylor & Francis Group, LLC
Published online: 17 Apr 2017.
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INTERNATIONAL JOURNAL OF FOOD PROPERTIES
2017, VOL. 20, NO. 12, 3282–3294
https://doi.org/10.1080/10942912.2017.1286504
Steady and dynamic shear rheology of starches from different oat
cultivars in relation to their physicochemical and structural
properties
Sukriti Singh and Maninder Kaur
Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, India
ABSTRACT
Starches isolated from eight oat cultivars were tested for their physicochemical,
structural, and rheological properties. The isolated starches had low levels of ash
and nitrogen contents with amylose ranging from 21.8 to 32.3 g/100 g. The
amount of water released from starch gels decreased significantly (p < 0.05) with
increase in freeze thaw cycle. The scanning electron micrographs revealed the
presence of polygonal to irregularly shaped starch granules. All the starches
exhibited A-type X-Ray diffraction pattern typically characteristic of cereal
starches. The rheological properties of pastes were well described by the
Herschel-Bulkley model at a shear rate of 0–100 s−1 (R2 > 0.99). The starch pastes
behaved like a pseudoplastic fluid and exhibited shear thinning fluid characteristics with values of flow behaviour index considerably less than 1. Both the
storage and loss moduli of the pastes increased sharply initially and then
dropped after reaching the gelatinization peak. The magnitude of the dynamic
rheological parameters varied significantly (p < 0.05) between the cultivars. All
the starch pastes were highly elastic than viscous as evidenced by the lower tan
δ values. Structure property relationships were established between starches
using principal component analysis.
ARTICLE HISTORY
Received 31 August 2016
Accepted 21 January 2017
KEYWORDS
Amylose; Morphometric
properties; Oat; Rheological
properties; Scanning
electron microscopy (SEM);
Starch; X-ray diffraction
Introduction
Oats (Avena sativa L.) have been cultivated since ancient times for human consumption, animal feeding,
and industrial use. There has been an increase in human consumption of oats owing to its high β-glucan
content and antioxidant activity.[1,2] About 70 species of oats have been discovered around the world, and
among them Avena sativa L. (hulled oat) and Avena nuda L. (naked oat) are the most cultivated species.[3]
Oats are rich in phenolic compounds, protein, dietary fibre, unsaturated fatty acids, and starch (~ 60%).[4,5]
In comparison to wheat, oat grains have 2–5 times more lipids and are rich in hydrolytic enzymes.[6]
Starch is the most abundant storage reserve carbohydrate in plants. It is mainly found in seeds, fruits,
tubers, and roots and acts as a major source of energy during the period of dormancy and growth.[7]
Starches from different sources vary, particularly in their qualitative and quantitative make up as well as
in some of the physicochemical and functional properties.[8] Starch granules are synthesized and stored
in terminally differentiated plastids known as amyloplasts within oat endosperms.[9] Oat starch has
higher bound lipid content, higher relative crystallinity, and smaller amylose chain length and granule
size (6–10 µm).[10,11] In food processing, pasting and thermal properties of starch play an important role,
specifically the amylose and amylopectin ratio.[12] The increasing demand of high-quality oat as feed,
food, and industrial raw material has led to the development of many different cultivars of oat. Though
oat is not the most suitable cereal as a commercial source of starch, limited research suggests that oat
starch possesses some unique chemical, physical, and structural properties over other commercially
CONTACT Maninder Kaur
University, Amritsar, India.
© 2017 Taylor & Francis Group, LLC
mandyvirk@rediffmail.com
Department of Food Science and Technology, Guru Nanak Dev
INTERNATIONAL JOURNAL OF FOOD PROPERTIES
3283
available starches.[11] Starches from different oat cultivars have been found to differ in their physicochemical, thermal, and rheological properties. [6,10,11,13–15] Oat starch utilization has been extensively
investigated except in case of food applications.[16] Oat starch can be used as a raw material to
manufacture thin starch films.[17] Oat starch in combination with oat hydrolysate or with xanthan
gum has been used for thickening of sweet and sour sauces.[18] The properties of starch pastes that play
the important role in commercial application involve transparency, texture, viscosity, tendency to
retrograde, and resistance to shear.[18]
The science of rheology studies the flow and deformation of substances during food processing and
handling. The method and intensity of various food processing operations such as cooking and mixing
determine the structural, sensory, and functional properties of final starch-based products.[19] Rheological
characteristics are mostly based on stress (force per area) and strain (deformation per length). The
relationship between stress and strain is analysed for loss modulus, complex shear modulus, and dynamic
viscosity.[20] Few models are used to explain the flow of fluids involved in food industries, such as linear
(Newtonian or Bingham), power law (Ostwald-de-Waele), power law with yield stress (Herschel-Bulkley)
and Casson models.[21] For Non Newtonian fluids, power law model is most widely used to describe the
flow properties of liquids in theoretical analysis as well as in practical engineering applications.[22] The
rheological properties of starch pastes or gels may be investigated as flow behaviour, viscoelasticity, creep,
and gel strength.[23,24]
The significance of specific adjustment of rheological properties is in regulation of production
processes and optimization of stability applicability and sensory properties of food product.[25]
Rheological properties determine the use of starch as gelling agent or thickeners.[13] The most
basic rheological characteristic of starch paste is viscosity, which changes upon applied shearing in
a broad range. In many cases, viscosity of food products depends not only on shear rate but is also
changing in time.[13]
Flow behaviour of oat starches both under dynamic and shear conditions has not been the subject
of the detailed study. Keeping this in mind, the present study was undertaken to investigate
physicochemical, morphological, and rheological properties of starch from different oat cultivars
for the selection of appropriate cultivar to see its suitability in different processes.
Materials and methods
Materials
Representative samples of eight Indian oat cultivars, OL-9, OL-10, Kent (PAU, Ludhiana.), OS-6, OS-7,
OS-364, HFO-114 (CCS HAU, Hisar), and PLP-1 (CSK HPKV, Palampur), were procured. Seeds were
cleaned for dirt, damaged seeds, and foreign material and stored at 20°C until further use. All the reagents
used in the study were of analytical grade. Oat starch was isolated using the method of Hoover and
Vasanthan with some modifications.[15] Oat grains (500 g) were steeped in water (50°C) for 3 h and then
ground in a laboratory blender at low speed for 5 min. The resultant slurry was then passed through a
nylon cloth and centrifuged at 5000g for 15 min. The supernatant was discarded, and the sediment was
suspended in excess 0.02 g/100 g NaOH. After standing for 1 h, the supernatant was removed. This
procedure was repeated four times. The final sediment was suspended in distilled water and centrifuged
in wide mouthed cups at 3000 rpm for 5 min. The upper non-white layer was scrapped off. The white
layer was suspended in distilled water and centrifuged for 3–4 times. The resultant starch was collected
and dried in an oven at 40°C for 12 h.
Physicochemical properties
The moisture, ash, and protein contents of oat starches were estimated using standard methods.[26] The
amylose content of oat starches was determined by the method of Williams et al. [27] Swelling power and
solubility was determined at 90°C using the method of Leach et al. [28] Light transmittance of starch
3284
S. SINGH AND M. KAUR
pastes was determined using method given by Craig et al.
determined by method of Hoover & Ratnayake.[30]
[29]
Freeze–thaw stability of starches was
X-ray diffraction and morphological properties
The crystalline order in starches was determined using X-ray diffractometer (Shimadzu-7000, Japan)
operated at 40 kV and 30 mA. The diffraction curves were obtained from 4° (2θ) to 30° (2θ) with the
scanning speed of 5°/min. For measurement of morphological properties, starch suspension (1%) in ethanol
was applied on an aluminium stub using double-sided adhesive tape. The sample was then coated with a
thin gold palladium film using sputter coat (Ion sputter JFC-1100, JEOL, USA) and was observed in JSM6100 Scanning Microscope (JEOL, USA) at an accelerating potential of 15 kV.
Rheological properties
A small amplitude oscillatory rheological measurement was made for starches with a Modular Compact
Rheometer (Anton Paar, Austria) equipped with parallel plate system (4 cm diameter). The gap size was
set at 1000 µm. The strain and frequency was set at 2% and 10 Hz, respectively, for all determinations.
The dynamic rheological properties, such as storage modulus (G’), loss modulus (G’’), and loss factor
(tan δ), were determined for starches. Starch suspension of 15% (w/w) concentration was loaded onto the
ram of the rheometer and covered with a thin layer of low density silicon oil (to minimize evaporation
losses). The starch samples were subjected to temperature sweep testing and were heated from 45–95°C
at the rate of 2°C/min.[31] For frequency sweep measurement, the starch slurry (15%) was prepared,
manually stirred, and then heated at 85°C in a water bath followed by 3 min stirring. The sample was
allowed to cool at room temperature and then loaded on the ram of rheometer. Frequency sweep tests
from 0.1–100 rad/s were performed at 25°C. The storage modulus (G’), loss modulus (G’’), and loss
tangent (tan δ) were derived at 25°C.
Steady shear properties were determined using the method described by Park et al with slight
modifications.[32] The starch suspension (10%) was sheared continuously from 1 to 1000s−1. To describe
the variation in rheological properties of samples under steady shear, the data was fitted to the well-known
Herschel-Bulkley (Eq 1) model:
σ=σ0+Κ.γn →
(1)
Where σ is the shear stress (Pa), γ is the shear rate(s−1), Κ is the consistency index (Pa.sn), n is
flow behaviour index (dimensionless), and σ0 is yield stress. TA rheometer Data Analysis software
(version VI. 1.76) was used to obtain the experimental data and to calculate storage modulus (G’),
loss modulus (G’’), and complex viscosity (ƞ*).
Statistical analysis
The data reported in all tables are average of triplicate observations. The data were subjected to statistical
analysis using Minitab Statistical Software version. 13 (Minitab Inc., USA), and Pearson correlation
coefficient between the properties analysed was established. Principal component analysis of measured
properties was carried out to provide difference and similarities among different oat cultivars.
Results and discussion
Physicochemical properties
Differences in the starch physicochemical characteristics have significant impact on their functional and
rheological behaviour, affecting their suitability for specific use.[33] The isolated starches from different oat
cultivars had moisture, ash, and nitrogen content in the ranges of 6.43 – 10.22, 0.11–0.81, and 0.02–0.5 g/
INTERNATIONAL JOURNAL OF FOOD PROPERTIES
3285
Table 1. Proximate composition, amylose content, swelling power, and solubility of starches from different oat cultivars.
Cultivars
OL-9
OL-10
Kent
OS-6
OS-7
OS-346
HFO-114
PLP-1
Moisture (g/100g)
10.22
8.13
6.91
9.02
7.45
6.43
10.13
9.52
±
±
±
±
±
±
±
±
0.1d
0.2bc
0.8ab
0.3c
0.4b
0.1a
0.3d
0.2cd
Ash
(g/100g)
0.51
0.61
0.81
0.19
0.11
0.25
0.19
0.56
±
±
±
±
±
±
±
±
0.06d
0.04d
0.05e
0.05ab
0.02a
0.05b
0.04ab
0.06cd
Nitrogen (g/100g)
0.03
0.04
0.05
0.04
0.05
0.05
0.04
0.02
±
±
±
±
±
±
±
±
0.002b
0.001c
0.002d
0.001c
0.003d
0.004d
0.003c
0.002a
Amylose (g/100g)
29.7
21.8
27.1
30.3
30.2
31.1
28.7
32.3
±
±
±
±
±
±
±
±
0.35cd
0.42a
0.43b
0.52d
0.49d
0.29de
0.37c
0.36e
SP (g/g)
13.1
16.1
15.3
14.1
19.1
16.2
18.7
17.0
±
±
±
±
±
±
±
±
0.90a
0.62bc
0.91b
0.54ab
0.76d
0.73bc
0.53d
0.81c
Solubility
(g/100g)
12.4
18.2
20.1
19.1
21.1
19.1
22.0
21.0
±
±
±
±
±
±
±
±
1.2a
0.91b
0.81c
0.93bc
0.23cd
0.42bc
0.76d
0.54cd
Values are an average of triplicate observations; ±standard deviation.
Values followed by similar superscripts in a column do not differ significantly (p < 0.05).
100g, respectively (Table 1). Small amount of nitrogen in starches is due to lyophospholipids, the
endosperm storage proteins that are located within the starch granules.[10] Amylose content is responsible
for number of factors that affect the swelling power, solubility, and gel forming properties of starch.[34]
Amylose content of oat starches was in the range of 21.8–32.3 g/100g agreeing with the previously reported
values of 19–32.3 g/100g[10] and 28.5–28.7 g/100g[11] in oat starches. The lowest and the highest amylose
content for OL-10 and PLP-1 were observed.
Swelling power (SP) is used to study the bonds between the crystalline portions of starch. Starch granules
with large crystalline area and strong bond swell less in cold water.[35] SP of oat starches ranged between 13.1
and 19.1 g/g with the highest value shown by OS-7 cv. starch. Starch swelling occurs concomitantly with loss
of birefringence and precedes solubilization.[36] Solubility is used as an indicator of destruction within the
starch components.[35] Starch solubility was found to be the highest for HFO-114 (22.04 g/100g). Among all
the cultivars, OL-9 cv. starch showed the lowest SP and solubility values. The low swelling power of starches
may be attributed to the presence of a large number of crystallites formed by the association between long
amylopectin chains.[36] Ratnayke et al concluded that amylose content primarily influences solubility while
amylopectin influences the SP.[37] Lipid-amylose complexes, starch chains in amylopectin ratio, molecular
weight, and relative distribution of amylose and amylopectin are reported to affect the SP and solubility of
starch.[38]
The Light transmittance value of starch pastes from different cultivars is presented in Fig. 1. All
the starch pastes showed a decrease in light transmittance values with the increase in storage
duration. The decrease in light transmittance can be attributed to the retrogradation of starches
while storage.[39] OS-346 cv. starch exhibited highest transmittance values, whereas OS-7 starch
showed the lowest values at different duration of storage.
Freeze thaw stability
Freeze thaw stability of different oat starches after five freeze thaw cycles is presented in Fig. 2. Freeze thaw
stability is an important property of starch in food industry as it helps to evaluate the ability of starch to
withstand the physical changes that occur during freezing and thawing. The amount of water released on
reassociation of starch chains during freeze thaw cycles determines the stability of starch gel.[15] The amount
of water released from starch gels decreased significantly with increase in freeze thaw cycle. OS-346 cultivar
was significantly (p < 0.05) different from other cultivars in exhibiting higher freeze thaw stability after five
freeze thaw cycles, whereas OS-7 showed least freeze thaw stability.
X- ray diffraction
X-ray diffractograms of starches from different oat cultivars are shown in Fig. 3. All the starches
showed A-type X-ray diffraction pattern typically found in many cereal starches. In the diffraction
spectra, there were strong diffraction peaks at 15 and 23° (2θ) and an unresolved doublet at 17 and
3286
S. SINGH AND M. KAUR
Figure 1. Transmittance (Abs.) of starches from different oat cultivars.
Figure 2. Freeze thaw stability of starches from different oat cultivars.
18° (2θ). All the starches showed similar diffraction patterns but varied in their values of diffraction
intensity. The intensity of the major diffractions peaks was the highest in OS-6 cv. starch. Studies by
Hoover et al.[10] on Canadian oat starch reported that no peak was observed at 2θ of 20°, which is
due to the presence of highly crystalline structure of amylose–lipid complexes.[10] It has been proved,
using solid-state NMR, that amorphous amylose–lipid complexes are present in native maize, rice,
and oat starches.[40]
Morphological properties
Scanning electron micrographs (SEM) of starches from different oat cultivars is shown in Fig. 4. SEM
showed that the starch granules were irregular to polygonal in their shape. The surface of the granules
appeared to be smooth with no evidence of pores. These results are in agreement with that reported
earlier for Canadian oat starch.[41] The starches separated from different oat cultivars differed
INTERNATIONAL JOURNAL OF FOOD PROPERTIES
3287
PLP-1
HFO-114
OS-346
Cv.
OS-7
OS-6
Kent
OL-10
OL-9
0
5
10
15
20
25
30
2 Theta
Figure 3. X-ray diffraction of starches from different oat cultivars at 2θ (4–30°).
significantly in their granule size which was determined by taking averages of 20 measurements. OS-7 cv.
starch showed the presence of large size granules with granule length and width ranges of 4.2–11.3 µm
and 3.9–11.3 µm, respectively. The smallest granule length (3.9–7.3 µm) and width (3.1–7.6 µm) was
observed in OS-346 cv. starch. The variation in granule morphology may be attributed to climatic and
agronomic conditions during processing. The complexity of starch biosynthesis results in natural
variability in amylose and amylopectin molecules, which is reflected in diversity of granules.[39]
Rheological properties
Steady shear properties
The plots of shear stress (σ) versus shear rate (γ) for starch pastes from different oat cultivars are depicted
in Fig. 5a. From the graph, it can be seen that with increase in shear rate, shear stress increased rapidly
and then levelled off as shear rate further increased. This type of shear thinning behaviour is a typical
characteristic of pseudoplastic fluids. The consistency index (K), flow behaviour (n) index, and the yield
stress (σ0) were derived by fitting the flow curves by Herschel-Bulkley model (Table 2). The data fitted
well with the model with high values of coefficients of determination (R2=0.999). The consistency and
flow behaviour indices ranged from 0.06 to 3.08 Pa.sn and 0.59 to 0.94, respectively (Table 2). Starch paste
from OS-6 cv. was significantly (p < 0.05) different from all other cultivars in exhibiting the highest K and
the lowest n value, whereas OS-7 starch paste exhibited the lowest K and the highest n value. The higher
K value indicates higher structural strength resulting in a less thixotropic behaviour.[39] The n value,
which indicates the extent of shear thinning behaviour as it deviates from 1, ranged from 0.59 to 0.94. The
shear thinning behaviour can be explained by breaking of an entangled polysaccharide molecule network
during shearing.[32]Yield stress is the minimum stress required to initiate flow and indicates that there is a
cross-linked or other interactive structure in a material that must be broken down before flow can occur
at an appreciable rate.[42,43] All the investigated starch pastes were pseudoplastic fluids that differed in
their values of Herschel-Bulkley yield stress. Starch paste from OS-6 cv. was significantly different from
others in its lowest n value. This indicates its more shear thinning behaviour with high magnitude of yield
3288
S. SINGH AND M. KAUR
A
B
D
E
G
C
F
H
Figure 4. Scanning electron micrograph of starches from different oat cultivars: (A) OL-9, (B) OL-10, (C) Kent, (D) OS-6, (E) OS-7, (F)
OS-346, (G) HFO-114, and (H) PLP-1.
stress and consistency coefficient. A decrease in viscosity with increase in shear rates irrespective of starch
cultivars was observed.
Dynamic shear properties
The data regarding dynamic shear properties of starch suspensions at temperatures ranging from
40°C to 95°C are shown in Table 3. It is well known that the effect of temperature on rheological
properties needs to be documented because a wide range of temperatures is encountered during
processing and storage of fluid food.[44] The rheological properties of the starches measured using
a dynamic rheometer showed significant variation in the peak G’ (Storage dynamic modulus, G’’
(Loss modulus) and peak tan δ values. G’ is the amount of energy stored in material and
recovered from it per cycle while G’’ is the amount of energy dissipated or lost per cycle of
sinusoidal deformation.[45] The maximal G’ value showed considerable variation with the highest
value of 5998 Pa for OS-7 and the lowest value of 3284 Pa for OL-9 cv., respectively. G’ and G’’
values of all starch suspensions increased to a maximum and then decreased with continuous
heating (Fig. 5b). G’ increased with increasing temperature due to swelling of starch granules
which come into contact with each other and occupy greater portion of the suspension leading to
higher system rigidity. The increase in G’’ can be accounted for leaching of amylose molecules
from swollen granules and forming a continuous network and behaving in a viscous manner.[39]
The reduction in G’ and G’’ with further heating is the result of extended rupture and disintegration of starch granules and weakening of inter-chain interactions due to increase in molecular mobility and melting of crystallites.[46] G’ was greater than G’’ in all starches showing their
more elastic than viscous behaviour. The higher dynamic moduli (G’ and G’’) of starch may be
attributed to its highest swelling power as viscoelasticity of gelatinized starch granules dispersion
is governed by swelling behaviour of starch granules. tanδ which is ratio of G’’/G’ for all starches
was less than 1. tan δ <1 indicates predominantly elastic behaviour, while a tan δ >1 indicates
INTERNATIONAL JOURNAL OF FOOD PROPERTIES
3289
Figure 5. (a) Flow curves of starch pastes (10%) from different oat cultivars. (b) Changes in storage modulus (G′) and changes
in Loss modulus (G′′) of 15% oat starch suspensions during heating: (A) OL-9 (B) OL-10 (C) Kent (D) OS-6 (E) OS-7 (F) OS-346
(G) HFO-114 (H) PLP-1. (c) Changes in storage modulus (G′) and changes in Loss modulus (G′′) of 15% oat starch suspensions
during a frequency sweep (0.1–100 rad/s at 25ºC): (A) OL-9 (B) OL-10 (C) Kent (D) OS-6 (E) OS-7 (F) OS-346 (G) HFO-114 (H)
PLP-1.
3290
S. SINGH AND M. KAUR
Table 2. Herschel-Bulkley model parameters for starch pastes from different oat cultivars.
σo (Pa)
1.78
0.45
2.10
2.80
2.50
1.00
2.40
2.80
Cultivars
OL-9
OL-10
Kent
OS-6
OS-7
OS-346
HFO-114
PLP-1
K (Pa.sn)
1.69 ± 0.08
2.09 ± 0.09
0.45 ± 0.01
3.08 ± 0.10
0.06 ± 0.02
0.67 ± 0.13
2.19 ± 0.01
0.49 ± 0.07
n(-)
0.67 ± 0.01
0.66 ± 0.01
0.77 ± 0.01
0.59 ± 0.01
0.94 ± 0.01
0.69 ± 0.01
0.60 ± 0.01
0.89 ± 0.01
R2
0.999
0.987
0.999
0.999
0.992
0.993
0.999
0.998
Values are an average of triplicate observations; ± standard deviation.
K: Consistency index, n: Flow behaviour index, σo: Yield stress.
Table 3. Dynamic rheological properties of starches from different oat cultivars.
Cultivars
OL-9
OL-10
Kent
OS-6
OS-7
OS-346
HFO-114
PLP-1
G’ (Pa)
3284
3893
5744
3964
5998
4218
3777
4042
±
±
±
±
±
±
±
±
140a
135bc
127d
136bc
149e
137c
146b
130bc
G’’ (Pa)
315
769
434
766
498
957
340
977
±
±
±
±
±
±
±
±
20a
18d
19b
23d
27c
29e
33a
37e
Breakdown in G’
421
806
794
219
±
±
±
±
–
130 ±
–
444 ±
Tanδ
30c
19d
17d
19b
0.09
0.19
0.07
0.19
0.08
0.22
0.09
0.24
21a
22c
±
±
±
±
±
±
±
±
0.01a
0.02b
0.02a
0.01b
0.03a
0.02bc
0.01a
0.02c
TG’max (°C)
87.6
85.1
87.6
87.6
87.6
87.6
90.1
87.6
±
±
±
±
±
±
±
±
0.05b
0.08a
0.09b
0.08b
0.07b
0.06b
0.04c
0.09b
Values are an average of triplicate observations; ±standard deviation.
Values followed by similar superscripts in a column do not differ significantly (p < 0.05).
Table 4. Storage modulus (G’), loss modulus (G’’), complex viscosity (ƞ*), and tan δ for oat starch pastes at 6.28 rad/s at 25°C.
Cultivars
OL-9
OL-10
Kent
OS-6
OS-7
OS-346
HFO-114
PLP-1
G’ (Pa)
3449 ± 30g
3502 ± 20h
2119 ± 32b
3099 ± 40f
646 ± 16a
2383 ± 42c
3005 ± 32e
2575 ± 19d
G’’ (Pa)
186 ± 15c
464 ± 10d
152 ± 14b
179 ± 17bc
105 ± 9a
209 ± 16c
147 ± 11b
591 ± 12e
tan δ
0.05 ± 0.01a
0.13 ± 0.02b
0.07 ± 0.01a
0.05 ± 0.03a
0.16 ± 0.01bc
0.09 ± 0.02ab
0.04 ± 0.01a
0.22 ± 0.01c
549
562
338
494
104
380
478
420
ƞ*
± 12f
± 9f
± 14b
± 13e
± 17a
± 20c
± 19e
± 15d
Values are an average of triplicate observations; ± standard deviation.
Values followed by similar superscripts in a column do not differ significantly (p < 0.05).
predominantly viscous behaviour. OL-9 cv. starch was significantly different from all other
cultivars in its lowest G’, G’’, and tan δ values. The temperature (TG’max) at which the G’ of
starch samples recorded the highest value ranged from 85.08°C to 90.08°C for OL-10 and HFO114 cv., respectively. Breakdown in G’ is the difference between peak G’ at TG’max and the
minimum G’ at 90°C.[8] Maximum breakdown was observed in OL-10 cv., while it was lowest in
OS-346 cv. starch. The difference in breakdown values among the oat starches may be attributed
to the difference in morphological characteristics of starch granules and peak G’ value.
Frequency sweeps have been used widely to provide further insight into the structure of
biomaterials.[47] In the present study, the starch pastes were subjected to frequency sweep tests
over a frequency range of 0–100 Hz to examine their viscoelastic nature (Fig. 5c). From the curves, it
can be seen that with the increase in angular frequency (ω) there was increase in the values of G’ and
G’’ while those of Complex viscosity (ƞ*) decreased showing a low frequency dependency. ƞ* is a
measure of the overall resistance to flow. Table 4 shows the values of G’, G’’, tan δ, and ƞ* values of
starch pastes from different oat cultivars at 6.28 rad/s and at 25°C. The values of G’ were much
greater than G’’ at all frequency values showing the strong elastic behaviour of starch samples. In
Figure 6. (a) Principal component analysis: loading plot of first principal component (PC1) and second principal component (PC2) describing the variation among measured properties of starches
from different oat cultivars. (b) Principal component analysis: score plot of PC1 and PC2 describing the overall variation among the properties of starches from different oat cultivars.
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S. SINGH AND M. KAUR
case of an ideal elastic behaviour, G’ is expected to be independent of frequency and much higher
than G’’ (G’≫ G’’).[48] The value of tan δ were in the range of 0.04–0.22 (Table 4), indicating that all
the samples were more elastic than viscous. The starch pastes also differed significantly in their
complex viscosity values ranging from 338 to 549 Pa.s.
Principal component analysis
The measured properties of the starches from different oat cultivars were analysed by principal
component analysis (PCA) to learn about similarities and differences of the samples. The results of
PCA of starches are shown in Fig. 6a and 6b. The loading plot provided information about
correlations between measured starch properties (Fig. 6a). Amylose content was observed to be
positively correlated with yield stress (r = 0.657, p < 0.05). A highly significant positive correlation of
G’ with n (r = 0.661, p < 0.05) and a negative correlation with K (r = −0.670, p < 0.05) and ƞ* (r =
−0.888, p < 0.05) was observed. G’’ showed a significant positive correlation with tan δ (r = 0.957, p
< 0.05), whereas n showed a negative correlation with K (r = −0.865, p < 0.05) and ƞ*(r = −0.771, p <
0.05). K was observed to be positively correlated to ƞ*(r = 0.750, p < 0.05). The first principal
component (PC1) and the second principal component (PC2) were described 40.1% and 20.5% of
the variance, respectively. Together, the first two principal components represented 60.6% of the
total variability. PCA showed that OS-7 cv. was located at far left of the score plot with a large
negative score in PC1, and its starch was significantly different from all other cultivars in terms of its
measured properties (Fig. 6b). PLP-1 and Kent cv. had small negative score in PC1. All other
cultivars were located at the right of score plot with a positive score in PC1.
Conclusion
Significant differences among the physicochemical, morphological, and rheological properties of
starches from eight oat cultivars were observed which could be attributed to the varietal
differences. The starch granules were polygonal to irregular in shape and showed A-type X-ray
diffraction pattern. All the starch pastes showed shear thinning behaviour, with typical characteristic of pseudoplastic fluid G’>G’’ in all starches showing their more elastic than viscous
behaviour. There was increase to maximum in G’ and G’’ of starch suspensions and then decrease
on continuous heating in all the oat cultivars. Correlation analysis data indicated a significant
interdependence of physicochemical with structural and rheological properties of oat starches.
OS-7 cv. starch was significantly different from all other cultivar in its measured properties as
revealed by PCA.
Acknowledgement
The authors would also like to thank Mr Mohinder Singh (Punjab University, Chandigarh, India) for his technical
assistance. The authors have declared no conflict of interest.
Funding
The author S Singh would like to acknowledge the UGC-UPE fellowship scheme for their financial support.
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