Physical and Functional Properties of Purple Sweet Potato Starch
as Affected by Isolation Methods
Elisa Julianti
1,2
, Zulkifli Lubis
1
, Ridwansyah
1,2
and Era Yusraini
1,2
1
Department of Food Science and Technology, Faculty of Agriculture,
Universitas Sumatera Utara, Jalan Prof.A.Sofyan No.3 Kampus USU Medan, Indonesia
2
Centre for Tubers and Roots Crop Study, Universitas Sumatera Utara, Jalan Bioteknologi, Medan, Indonesia
Keywords: Purple Sweet Potato, Isolation Methods, Starch, Functional Properties.
Abstract: Purple sweet potato (PSP) is a guber plant which is a potential source of starch. The starch extraction using
different methods of isolation will produce starches with different physical and functional properties. In this
study, the extraction and isolation process of starches were carried out using water, sodium metabisulfite 2000
ppm, and citric acid 2000 ppm. The resulting starch was analyzed for its physical characteristics including
granule shape and size, as well as its functional characteristics including swelling power, water absorption
index, and oil absorption index, the viscosity of starch paste, and gelatinization temperature. The results
concluded that the starch produced from citric acid isolation had a smaller size of granule, and a higher water
and oil absorption index, swelling power, paste viscosity, and gelatinization temperature. Purple sweet potato
is a tuber plant which is a potential source of starch.
1 INTRODUCTION
Starch is a plant carbohydrate that is found in corn,
wheat, potatoes, rice, and others (Englyst and
Englyst, 2005). Starch granules consist of amylose
polymers that have straight chains and amylopectin
which have branched chains. Root crops such as
cassava, yam, and sweet potato are also potential
sources of starch for starch-based industries
(Moorthy, 2002). The process of extracting starch
from tubers through the process of grating, sieving,
and sedimentation or centrifugation (Daiuto et al.,
2005). Starch quality is influenced by the extraction
process and starch source (Liu et al., 2016).
Therefore, new starch sources are needed to obtain
quality starch that meets industrial requirements.
Sweet potato is the 6
th
most important food
commodity in the world (FAO, 2010). Sweet potato
plants are very tolerant of high temperatures, infertile
and dry soil conditions (Laurie et al., 2012), so they
have the potential to be developed as a source of food
raw materials. Sweet potato contains a large number
of carbohydrates, especially starch, which is between
25-30%, and 98% of the starch is easy to digest
(Antonio et al., 2011). Purple sweet potato (PSP)
contains high amounts of starch and anthocyanins
(Oswal et al., 2019). The use of purple sweet potato
as raw material for starch is still very limited, usually
it is still consumed directly with simple cooking or
used as flour.
The increasing of PSP starch utilization can be
done through suitable starch processing to produce
starch with suitable properties for industrial needs
(Jangchud et al., 2003). The physical and functional
properties of starch are important properties for its
application in food and industrial products, such as
the shape and size of starch granule, as well as the
characteristics of paste and gelatinization
(Adebowale and Lawal, 2002). The results of
previous studies indicated the isolation of starch from
yam can be carried out using alkalis or enzymes
(Wang et al., 2011), or using water, oxalic acid or
ammonium oxalate, pectin, and sodium hydroxide
(Daiuto et al., 2005). Research on the effect of the
starch isolation method on the functional
characteristics of starch has also been carried out
(Babu and Parimalavalli, 2012; Babu and
Parimalavalli, 2014; Correia et al., 2012). The current
study aimed to study the effect of the starch isolation
method on the physical and functional characteristics
of purple sweet potato starch.
166
Julianti, E., Lubis, Z., Ridwansyah, . and Yusraini, E.
Physical and Functional Properties of Purple Sweet Potato Starch as Affected by Isolation Methods.
DOI: 10.5220/0010565400003108
In Proceedings of the 6th Food Ingredient Asia Conference (6th FiAC 2020) - Food Science, Nutrition and Health, pages 166-170
ISBN: 978-989-758-540-1
Copyright
c
 2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

2 MATERIALS AND METHODS
Purple sweet potato (PSP) tubers were procuredfrom
farmers in Phak-Phak Barat Regency, North
Sumatera Province, Indonesia. Isolation agents used
in this research were distilled water, sodium
metabisulfite, and citric acid.
2.1 Isolation of PSP Starch
PSP tubers were washed and cleaned by using tap
water to remove impurities. The cleaned tuber was
peeled by using stainless steel knife, and rasping by
using a rasping machine. Starch isolation was
performed according to Tharise et al. (2014) with
modification in isolation agent of starch. The starch
isolation agents used in this study ware distilled
water, 2000 ppm sodium metabisulfite solution, and
2000 ppm citric acid solution. The blending of rasped
tubers with starch isolation agents was done at a ratio
of 1:3 until fine slurry was obtained. The slurry was
filtered using double-layered cheesecloth. The filtrate
was settled at 27-30
o
C (room temperature) for 12h.
The supernatant was poured and the starch was
collected and resuspended in isolation agent solutions
for 3h at room temperature to settle. This process was
repeated three times until the white wet starch
sediment was obtained and collected. The wet starch
was dried in a convection oven at 50
o
C for 12h and
then cooled to room temperature. The dried starch
was pulverized and sieved through an 80-mesh sieve,
packed and sealed in polyethylene plastic bags before
using for further analysis.
2.2 Determination of Starch Granule
Shape and Size
The shape and size of PSP starch granules were
studied on a Scanning Electron Microscope (SEM)
(FEI-type Quanta 650) at an accelerating voltage of
10.0 kV and magnification at 1000x. The granule size
of starch was predicted from SEM image at a
magnification of 40x by using Image J software.
2.3 Determination of Water and Oil
Absorption Index
Water absorption index (WAI) and Oil Absorption
Index (OAI) were determined by the method
described by Niba et al. (2001). Starch samples (1g)
were suspended in 5ml of water (for WAI) or 5ml
vegetable oil (for OAI) in a centrifuge tube. The
slurry was stirred for 1 min at room temperature and
centrifuged at 3000 rpm for 10 min. The supernatant
was decanted and discarded, and the sediment was
weighed, WAI and OAI were expressed as the weight
of sediment/initial weight of the starch sample (g/g).
The swelling power of starch was determined
according to Leach et al. (1957) method. 0.1g sample
was put into a weighed 50ml centrifuge tube, and
distilled water was added to give a 10 ml of total
volume, then the mixture was stirred gently by hand
for 30s at room temperature, and heated at 60
o
C for
30 min. After cooling to room temperature, the
sample was centrifuged for 30 min at 3000 rpm, and
the sediment then was weighed.
2.4 Determination of Pasting
Properties of PSP Starch
The pasting characteristics of PSP starch were
evaluated by the Rapid Visco Analyzer (RVA-Model
Tecmaster Newport Scientific, Australia). The slurry
was made at a concentration of 10% dry solids in an
aluminum container, and held for 1 minute at 50oC,
heated from 50 to 95
o
C at 6
o
C / min, and held at 95
o
C for 5 minutes. The parameters recorded were paste
temperature (PT), peak viscosity (PV), viscosity at the
end of the holding time at 95
o
C (HPV), breakdown
viscosity (BD) calculated as PV-HPV, viscosity at the
end of the holding time at 50 oC ( CPV); regression
viscosity (SB) was calculated as CPV-HPV, stability
ratio (SR) was calculated as HPV / PV, and regression
ratio (SBR) was calculated as CPV / HPV.
2.5 Statistical Analysis
All measurements were done in triplicate. All data
obtained were subjected to One Way Analysis of
Variance (ANOVA) using, and the Duncan Multiple
Range (DMR) was performed using SPSS Version
26. A Statistically significant difference was
established at p<0.05.
3 RESULTS AND DISCUSSIONS
3.1 PSP Starch Granule Size and
Shape
The starch granules shape from different isolation
methods were shown in Figure 1. The three starch
samples had a small to large sizes of granule. Most of
PSP starch granules were polygonal in shape,
however round and irregular shapes were also found.
These shapes were similar to previous studies (Ngoc
Physical and Functional Properties of Purple Sweet Potato Starch as Affected by Isolation Methods
167

et al., 2017; Soison et al., 2015; Babu and
Parimalavalli, 2014). The granule surface of starches
samples isolated by distilled water and sodium
metabisulfite looked smoother without any fissure.
This result is in line with the Babu and Parimalavalli
(2014) research results. In PSP starch isolated by
citric acid, there are some compound granules. This
may be caused by the residual protein which
undergoes gelatinization on the surface of the starch
grains and sticks to one another to form compound
granules. (Newman et al., 2007). PSP starch isolated
by distilled water had a larger number of small size
granules. Similar results were also found in Babu and
Parimalavalli (2014) results.
Figure 1: Scanning Electron Microscopy (SEM) of PSP
starch from different isolation methods : (a) Distilled water;
(b) Sodium metabisulfite; (c) Citric acid at 1000 x
magnification.
The effect of isolation methods on the size of PSP
starch granules is shown in Table 1. The size of the
starch granules varies from 7.0-27.3 µm. Granule size
of sweet potato starches from previous studies widely
varied from 2 to 60 μm (Babu and Parimalavalli,
2014; Soison et al., 2015; Ngoc et al., 2017; Babu et
al., 2015). Starch granules isolated with citric acid
tend to have a larger size, this is due to the formation
of compound granules.
Table 1: Granule Size of Sweet potato starch from different
isolation methods.
Isolation
Methods
Granule Size (µm)
Mean Minimum Maximum
D
istilled Wate
r
16.6 7.0 24.0
Sodium
M
etabisulfite
19.4 13.6 21.7
Citric Aci
d
20.2 14.5 27.3
3.2 Functional Properties of PSP Starch
The water absorption index (WAI) of sweet potato
starch was in the range of 0.72-1.03 ml/g. Table 2
showed that PSP starch isolated with citric acid tends
to have a higher WAI. WAI is related to the
interactive forces among starch components, weak
interactive forces result in high WAI. WAI is an
important parameter that determines starch viscosity
(Oswal et al., 2019).
Table 2: Effect of isolation methods on functional
properties of PSP starch.
Parameters Isolation Methods
Distillation
Wate
r
Sodium
Metabisulfite
Citric Acid
Water
Absorption
Index
(g
/
g)
0.90±0.17
b
0.72±0.12
c
0.73±0.04
a
Oil
Absorption
Index
(g
/
g)
1.09±0.14
c
1.21±0.09
b
1.46±0.07
a
Swelling
Power
(g
/
g)
7.69±0.18
c
9.58±1.09
b
13.35±2.47
a
a)
Value reported as the mean ± Std. Dev. of three
replications
b)
Means followed by same letter superscripts within a row
are not significantly different (p<0.05)
Oil absorption index (OAI) of PSP starch as
affected by isolation methods ranged from 1.09-1.46
ml/g, and it is was higher than the results of Babu and
Parimalavalli (2013) research which found that the
OAI values ranged from 0.52-0.82 ml/g. The Higher
OAI might be due to greater hydrophobic tendency
than hydrophilic tendency of isolated starches.
The swelling power (SP) of starch at 60
o
C is
ranged from 7.69-13.35 g/g. The previous research
found that the swelling power of sweet potato
starches was 5.23-16.38g/g with temperature range of
65- 95
o
C (Huang et al., 2010).
3.3 Pasting Properties of PSP Starch
Pasting properties determined by RVA are shown in
Table 3. There are no significant differences in
pasting properties of starch with different isolation
methods. Table 3 shows that the viscosity of starch
isolated with citric acid tends to be higher than other
isolation methods. Balasubramanian et al. (2014)
reported that acid-modified starch would have a
higher peak viscosity.
(
a
)
(
b
)
(
c
)
6th FiAC 2020 - The Food Ingredient Asia Conference (FiAC)
168

Table 3: Effect of isolation methods on pasting temperature
of PSP starch.
Parameters Isolation Methods
Distillation
Water
Sodium
Metabisu
lfite
Citric
Acid
Peak
Viscosity
(cp)
3786.00±
350.53
a
3664.33
±129.40
a
3992.00
±458.75
a
Hot Paste
Viscosity
(cp)
2300.00±
271.81
a
2078.67
±79.96
a
2419.00
±466.13
a
Final
Viscosity
(cp)
3184.67±
316.25
a
2860.33
±127.34
a
3414.67
±694.32
a
Breakdown
Viscosity
(cp)
1486.00±
112.30
a
1585.67
±187.43
a
1573.00
±311.17
a
Setback
(cp)
884.64±
45.09
a
781.67±
48.00
a
995.67±
228.23
a
Gelatinizati
on
Temperatur
e (
o
C)
68.88± 0.49
a
69.23±
0.38
a
70.05±
1.06
a
a)
Value reported as the mean ± Std. Dev. of three
replications
b)
Means followed by same letter superscripts within a row
are not significantly different (p<0.05)
Pasting properties in this research are following the
results of previous studies (Tsakama et al., 2010; Babu
and Parimalavalli, 2014). Pasting properties of starch
is determined by granule size and structure, amylose
content, and amylopectin structure. PSP starch
isolated with citric acid has a larger granule size so
that it has a higher viscosity.
The pasting temperature is the minimum
temperature required to cook a starch sample.
Although statistically there was no difference in the
pasting temperature of the three isolation methods,
the pasting temperature of PSP of starch isolated with
citric acid also tended to be higher. The lower pasting
temperature indicates that the starch can be cooked
faster and requires less energy (Babu and
Parimalavalli, 2014).
4 CONCLUSIONS
Each isolation method had its own physical and
functional characteristics, which affect the end-use
quality of starch-based foods. Starch isolated with
sodium metabisulfite exhibited lower peak viscosity
in addition to lower swelling. It may be concluded
that sodium metabisulfite could be used to isolate
starch with desirable properties, suitable for many
food products.
ACKNOWLEDGEMENTS
The authors are very grateful for the financial
supported by The Directorate General of Research
Strengthening and Development, Ministry of
Research and Technology, National Agency for
Research and Innovation through “Penelitian Terapan
2019-2020” project.
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