Mechanical properties of Potato Starch modified by moisture content ...

Mechanical properties of Potato Starch modified
by moisture content and addition of lubricant*
Mateusz Stasiak, Marek Molenda
Institute of Agrophysics Polish Academy of Sciences,
Doswiadczalna 4, 20-290 Lublin, Poland
Corresponding author E-mail: mstasiak@ipan.lublin.pl
Abstract
The objectives of the reported project were to determine mechanical characteristics of
potato starch of different moisture content and with addition of lubricant. Direct shear and
uniaxial compression were performed. Considerable influence of moisture content of potato
starch was found in the case of density, parameters of internal and wall friction and
flowability. Strong influence of water content was found in the case of parameters of
elasticity. Addition of magnesium stearate affected parameters of: density, flowability, internal
friction and elasticity. Bulk density varied from 604 kg·m -3 for potato starch of 18% of
moisture content to 774 kg·m -3 for 6% of moisture content. Addition of magnesium stearate
resulted in approximately 10% decrease in bulk density. Angle of internal friction obtained for
10kPa of consolidation stress decreased from 33º to approximately 24º with higher moisture
content, and to approximately 22º with addition of lubricant. Modulus of elasticity during
loading decreased from approximately 1 MPa to 0.1 MPa with an increase of moisture
content from 6 to 17% and with addition of lubricant.
Modulus of elasticity during unloading was found in a range from 19 to 42 MPa and
increased with increase of moisture content and amount of lubricant.
Keywords: potato starch, mechanical properties, friction, elasticity
1. Introduction
Potato starch, with the share of 6% in the whole world market, is one of the major
components in the diet and plays an important role in the formulation of food products, with
respect to both food functionality and nutritional quality. Up to date a lot of information is
available on chemical nature and physicochemical properties of different starches, but data
on their physical properties particularly those influenced by moisture content and addition of
lubricant are rather scarce. The objectives of the reported project were to determine
mechanical characteristics of potato starch of different moisture contents and with addition of
lubricant.
2. Material and methods
Commercial potato starch of moisture content of 6, 12 and 17.5% produced by Melvit
in Ostroleka, Poland was used as an experimental material. Moistening of the powder was
realized by humidifier in closed cubic space for 48h. Drying, to obtain lower value of moisture
content was performed in laboratory heater at 30ºC for 24h. Magnesium stearate was used
as a lubricant in amount of 1, 2 and 6% of weight of the potato starch sample and mixed for
1h in standard laboratory mixer. Direct shear tests were performed in a shear box 60 mm in
diameter. The tests followed Eurocode 1 (2006) standard procedure for consolidation
reference stresses r of 4, 6 and 10 kPa and speed of shearing V of 2 mm·min -1 . Based on
the experimental curves, the angle of internal friction , the effective angle of internal friction
, flow functions FF , wall friction coefficient µ on black, galvanized and stainless steel were
determined. Prototype uniaxial compression tester with horizontal stress control was used to
*This work is supported by the State Committee for Scientific
Research, Poland under the Grant No. N 313 141938.

determine bulk density , bulk density of compacted material comp , modulus of elasticity E
and Poisson’s ratio Maximum consolidation stress of 10kPa was applied as recommended
by Eurocode1.
3. Results
Moisture content was found to influence strongly density of potato starch. Poured and
consolidated density decreased with an increase of moisture content from 774 to 604 kg·m -3
and 895 to 788 kg·m -3 , respectively. Similar tendency was noted when magnesium stearate
was added to potato starch of 6% m.c. and resulted in decrease of poured density to 726 kg
m -3 . For consolidated density addition of lubricant resulted in increase in this parameter up to
904 kg·m -3 .
Experimental relationships obtained with direct shear tester are presented in Figure 1.
Strong slip stick effect was observed in the case of 6% m.c. potato starch for consolidation
stresses of 4, 6 and 10kPa. The highest amplitude of oscillations was observed for
consolidation of 10kPa.
Shear stress [kPa]
7
6
5
4
3
2
1
Moisture content
6%
12%
17.5%
Consolidation stress 10 kPa
Consolidation stress 6 kPa
Consolidation stress 4 kPa
Shear stress [kPa]
7
Addition of magnesium stereate
0%
6 1%
Consolidation stress 10 kPa
2%
5 6%
4
3
2
1
Consolidation stress 6 kPa
Consolidation stress 4 kPa
0
0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09
l/D
*This work is supported by the State Committee for Scientific
Research, Poland under the Grant No. N 313 141938.
0
0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08
FIGURE 1: Shear stress – relative displacement l/D relationship of potato starch for
different moisture content and addition of magnesium stearate.
Increase in moisture content resulted in cessation of slip stick effect. Addition of magnesium
stearate resulted in lower values of maximum stable values of shear stresses. Moreover
even addition of 1 % of lubricant resulted in disappearance of the slip stick effect.
Angle of internal friction was found strongly dependant on all considered factors
(Fig. 2). Average values of decreased sharply with an increase in moisture content, from
approximately 25° for 6% of moisture content to approximately 16° for 12% of moisture
content. No considerable decrease was observed for further increase in moisture content.
Probable reason of such behavior was that the powder reaches state of saturation and water
acts as lubricant.
28
26
24
22
20
18
16
14
12
10
F(2, 46)=57,408, p=,00000
6 12 17
Moisture content [%]
φ
32
30
28
26
24
22
20
18
16
14
12
10
8
l/D
F(3, 46)=74,429, p=0,0000
0 1 2 6
Addition of magn. stereate [%]

FIGURE 2: Relationships between angle of internal friction of different moisture content
and addition of magnesium stearate. Points denote mean values and vertical bars
denote the 0.95 confidence interval; F, p - analysis of variance parameters.
In the case of 1% and 2% addition of lubricant a decrease in angle of internal friction from
about 28° to 16° was observed. The lowest value of angle of internal friction was obtained for
higher than 6% addition of lubricant
Cohesion increased with consolidation stress from approximately 0.25 kPa to
approximately 0.5 kPa with an increase in consolidation stress. No influence of moisture
content on cohesion was observed (Fig. 3).
0,7
0,6
0,6
Cohesion [kPa]
0,6
0,5
0,4
0,3
0,2
F(2, 46)=11,802, p=,00007
Cohesion [kPa]
0,5
0,4
0,3
0,2
F(2, 46)=,97926, p=,38328
Cohesion [%]
0,5
0,4
0,3
0,2
0,1
0,1
0,1
0,0
4 6 10
Consolidation stress [kPa]
0,0
6 12 17
Moisture content [%]
0,0
0 1 2 6
Addition of magn. stereate [%]
FIGURE 3: Relationships between: a) cohesion and consolidation stress, b)cohesion and
moisture content and c) cohesion and addition of the lubricant. Points denote mean
values and vertical bars the 0.95 confidence intervals; F, p - analysis of variance
parameters.
Addition of lubricant in an amount of 1% resulted in decrease in cohesion and angle of
internal friction. Reaching 6% concentration cohesion increased almost to the value obtained
when no lubricant was present.
Significant differences in FF of examined samples were observed as shown in
Figure 4. Values of FF were characteristic for free and easy flowing materials.
2,4
2,2
17.5% m.c.
POTATO STARCH
2,0
easy flowing 12% m.c.
1,8
Unconfined yield strenght [kPa]
1,6
1,4
1,2
1,0
0,8
0,6
0,4
2% add. of magn. stereate
0,2 free flowing
0,0
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Major consolidation stress [kPa]
FIGURE 4: Flow functions of examined powders.
*This work is supported by the State Committee for Scientific
Research, Poland under the Grant No. N 313 141938.
6% m.c.
1% add. of magn. stereate
6% add. of magn. stereate
Values of FF increased by 50% with an increase in moisture content from
approximately 0.9 kPa to 1.4 kPa for the lowest consolidation stress and from approximately
1.6 to 2.2 kPa for consolidation stress of 10 kPa. In the case of addition of magnesium

stearate significant decrease in FF value was observed. For consolidation stress of 4 kPa
values of unconfined yield strength decreased from 0.9 kPa to approximately 0.4 kPa for
each concentration of magnesium stearate. For the highest value of consolidation stress
values of FF decreased roughly by 25% for addition of 1% of magnesium stearate. For 2%
and 6% of lubricant addition FFs decreased by about 50%.
Modulus of elasticity during loading decreased from approximately 1 MPa to 0.1 MPa
with an increase of moisture content from 6% to 17% and with addition of lubricant (Fig.5).
Loading Modulus of Elasticity [MPa]
Loading modulus of Elasticity [MPa]
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
F(2, 19)=3,8505, p=,03946
6 12 17
Moisture content [%]
F(3, 19)=5,3000, p=,00797
0 1 2 6
Addition of magn. stereate [%]
Unloading Modulus of Elasticity [MPa]
Unloading Modulus of Elasticity [MPa]
50
45
40
35
30
25
20
15
65
60
55
50
45
40
35
30
25
20
15
10
F(2, 19)=,25379, p=,77845
6 12 17
Moisture content [%]
F(3, 19)=7,5383, p=,00161
0 1 2 6
Addition of magn. stereate [%]
FIGURE 5. Modulus of elasticity for loading and unloading as a function of powder
moisture content and amount of lubricant addition. Points denote mean values and
vertical bars denote the 0.95 confidence intervals; F, p - analysis of variance parameters.
No influence of m.c. was observed for unloading modulus of elasticity, however increase
was observed, from about 20 to 45MPa, with addition of magnesium stearate.
Average values of friction coefficient are presented in Figure 6. The highest values of
friction coefficient were obtained in the case of black steel, while values obtained at
galvanized and stainless stell were nearly equal. No differences in friction coefficient were
observed at various consolidation stresses. Coefficient of friction increased by approximately
40% with increasing moisture content.
*This work is supported by the State Committee for Scientific
Research, Poland under the Grant No. N 313 141938.

0,50
0,50
0,45
0,45
F(2, 98)=30,178, p=,00000
Firiction coeffiecient
0,40
0,35
0,30
F(2, 98)=2,6511, p=,07563
Friction coefficient
0,40
0,35
0,30
0,25
0,25
0,20
4 6 10
Consolidation stress [kPa]
0,20
6 12 17
Moisture content [%]
0,50
0,50
0,45
F(3, 98)=23,351, p=,00000
0,45
F(2, 98)=10,605, p=,00007
Friction coefficient
0,40
0,35
0,30
0,25
Friction coefficient
0,40
0,35
0,30
0,25
0,20
0 1 2 6
Magnesium stereate addition [%]
0,20
black steel
stainless steel
galvanized steel
FIGURE 6. Coefficient of wall as dependent on consolidation stress, moisture content,
amount of lubricant addition and type of steel. Points denote mean values and vertical
bars denote the 0.95 confidence interval; F, p - analysis of variance parameters.
4. Conclusions
Values of mechanical parameters of potato starch are strongly influenced by moisture
content of the material and amount of lubricant added.
Slip stick effect observed during experiments decreased with an increase of m.c. and
addition of magnesium stearate. Angle of internal friction decreased with an increase in water
content in whole range of moisture content and even minimum addition of lubricant, while
practically no considerable influence was observed in the case of cohesion. Values of FF
increased with increase in moisture content and decreased with addition of lubricant.
Modulus of elasticity during loading decreased with increase in moisture content and addition
of lubricant. The unloading modulus of elasticity showed opposite trend.
The highest friction coefficient was observed for black steel. No influence of
consolidation stress on the friction coefficient was observed. Strong increase in friction
coefficient with an increase in m.c. was noted. Addition of magnesium stearate resulted in
an increase in friction coefficient.
5. References
Eurocode 1, Part 4. (2006). Basis of design and actions on structures. Actions in silos and
tanks. EN 1991-4.,
*This work is supported by the State Committee for Scientific
Research, Poland under the Grant No. N 313 141938.