Food Hydrocolloids 25 (2011) 361e367
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Food Hydrocolloids
journal homepage: www.elsevier.com/locate/foodhyd
Micellar properties of OSA starch and interaction with xanthan
gum in aqueous solution
Veljko Krstonosi
c a, *, Ljubica Doki
c b, Jadranka Milanovi
cb
a
b
Faculty of Medicine, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia
Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 February 2010
Accepted 26 June 2010
Octenyl succinate (OSA) modified starches are used, as emulsifiers and stabilizers, in many food,
cosmetics and pharmaceutical products. The aim of this study was to determine critical micellar
concentration (CMC) of two different octenyl succinate modified waxy corn starches at 25 C, and to
examine possibility of their interactions with xanthan gum in aqueous solution. The CMC was determined by viscometry, conductometry, surface tension and dye solubilization. The CMC values for two
OSA starches (OS1 and OS2) varied from 0.050 to 0.088 g/100 cm3 and from 0.041 to 0.081 g/100 cm3
respectively, depending on applied technique. The same techniques were used for investigation of the
interactions between OSA starch and xanthan gum. The addition of xanthan gum decreases the specific
viscosity and increases surface tension and the CMC values compared to the single OSA starch solutions.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
OSA starch
Critical micellar concentration
Xanthan gum
Interaction
1. Introduction
Starch is widely used in many industrial products due to its
functional properties and nutritional value. It has been most often
used as thickening agents, because of characteristic that after
dispersing in hot water starch granules swell irreversible and form
a viscose paste (Banks & Greenwood, 1975).
However, the applications of native starches are limited due to
their storage and process instabilities. That was the reason for
developing the techniques for starch modifications. One of the
techniques is chemical modification, which is applied in production
of OSA starches by treating starches with octenyl succinic anhydride (Bhosale & Singhal, 2006).
Modification with hydrophobic octenyl succinic groups gives
the starch molecule amphiphilic nature and thus surface active
properties. Hydrophobic part of OSA starch molecule contains
a carboxylic acid which can be negatively charged (Nilsson &
Bergenstahl, 2007). The OSA starches for food applications characterize low degree of substitution (DS value), between 0.01 and
0.03, and it could be considered as weekly charged polyelectrolyte
(Shorgen, Viswanathan, Felker, & Gross, 2000). They are used in
many food, cosmetics and pharmaceutical products, as emulsifier
and stabilizer (Ortega-Ojeda, Larsson, & Eliasson, 2005; Varona,
Martin, & Cocero, 2009).
* Corresponding author. Tel.: þ381 641861004.
E-mail address: [email protected] (V. Krstonosi
c).
0268-005X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodhyd.2010.06.014
Amphiphilic molecules form the boundary layer, whereby the
hydrophobic part of molecule is oriented toward the air or lipophilic phase and hydrophilic part stays in the water. At the certain
concentration the surface is saturated and no more molecules can
enter the boundary layer. Above this concentration the monomers
form aggregates called micelles. The minimum concentration of
surfactant above which micelles are formed is critical micellar
concentration (CMC) (Varona et al., 2009; Posa et al., 2007). OSA
starches are amphiphilic molecules and CMC is one of the most
important parameters designating their application as emulsifier.
Xanthan gum is a microbial anionic heteropolysaccharide. Its
main chain is based on a linear backbone consists of 1,4-linked b-Dglucose, with a charged trisaccharide side chain at the C-3 position
on every alternate glucose residue. The trisaccharide side chain
contains D-glucuronic acid unit between two D-mannose units.
Approximately one half of terminal D-mannose unit contains a piruvatic acid residue by a ketal linkage to the O-4 and O-6 positions.
The D-mannose linked to the main chain contains an acetyl group at
position O-6 (Garcia-Ochoa, Santos, Casson, & Gomez, 2000; Rodd,
Dunstan, & Boger, 2000). It has been shown that its secondary
structure consist of a five-fold helical structure. The high molecular
weight of xanthan gum and formation of aggregates via hydrogen
bonding are the reasons why its solutions exhibit high viscosity
(Katzbauer, 1998; Viebke & Williams, 2000).
Since, widespread applications in industrial products and
formulations of the polymer and surfactant combination, existence
of interactions are of huge research interest (Mata, Patel, Jian,
Ghosh, & Bahadur, 2006). According to Goddard (2002) there are
362
V. Krstonosic et al. / Food Hydrocolloids 25 (2011) 361e367
three categories of interactions between polymer and surfactant:
polyelectrolyte/oppositely charged surfactant, uncharged polymer/
charged surfactant and uncharged polymer/uncharged surfactant.
However, several authors reported existence of interactions
between hydrophobically modified polymers with surfactants of
the same charge (Bromberg, Temchenko, & Colby, 2000; Burke &
Palepu, 2001; Colby, Plucktaveesak, & Bromberg, 2001; Deo et al.,
2003).
Some of the authors reported that xanthan gum and waxy corn
starch attracted to each other in solutions (Wang, Sun, & Wang,
2001). Hydrophobically modified polymers such as OSA starches
are well known as associative thickeners due to interactions with
other polymers and surfactants in aqueous solutions (Ortega-Ojeda
et al., 2005). Ntawukulilyayo, De Smedt, Demeester, and Remon
(1996) investigated stabilization of pharmaceutical paracetamol
suspension with OSA starch and noticed the dramatic increase in
elastic modulus of suspensions after increase in OSA starch from 4%
to 6% in presence of constant concentration of xanthan gum. They
assumed that association of xanthan gum and OSA starch caused
these specific viscoelastic properties of the system.
The aim of this study was to determine CMC of two different
OSA starch representatives, and to examine their possible interaction with xanthan gum in solution. In this paper we treated OSA
starch as surfactant, due to its surface activity and we tried to find
out whether they interact with polymer of the same charge.
Knowledge of their behavior, when they are used together in
formulations, is important to predict properties of the final product.
In this paper viscometry, conductometry, dye solubilization and
tensiometry were used in order to clarify the OSA starchexanthan
gum behavior.
Viscosity measurements of OS1, OS2 and xanthan gum solutions
and their mixtures were done using Ubbelohde capillary viscometer (SCHOTT) in thermostatic bath at 25 0.1 C. Each concentration was measured in triplicates and average values were
calculated.All results were expressed as specific viscosity.
Specific viscosity (hsp):
hsp ¼
h
1
h0
(1)
2. Materials and methods
where h is solution viscosity and h0 is pure solvent viscosity.
Conductivity measurements were carried out at 25 0.1 C by
adding portions of 1 g/100 cm3 OS1 or OS2 solution into the 50 cm3
of double distilled water. For interaction determinations the adding
portions consist of mixture of 1 g/100 cm3 OS1 or OS2 and certain
xanthan gum concentration, which were added to the 50 cm3 of
xanthan gum solution of the same concentration. The solution or
blend was stirred using magnetic stirrer after addition of every
portion of OSA starch, until the steady value of conductivity was
achieved. Consort C830 multi parameter analyzer was used to
measure a specific conductance of solutions and blends.
Surface tension measurements were carried out on a Sigma
703D tensiometer (Finland) using a du Nouy ring method. All
measurements were repeated three times. In all measurements
temperature were kept constant at 25 0.1 C.
The method of dye solubilization implied the extent of water
insoluble dye Sudan III solubilization in OSA starch solutions.
Different concentrations of OSA starch solutions were prepared by
diluting 0.2 g/100 cm3 solution. 10 mg of Sudan III was dissolved in
10 ml of certain OSA starch solution and left stirring for 5 h. The
analyses were carried out using an Agilent 8453 UVeVisible spectrophotometer. Absorbance at 533 nm was monitored to determine
the extent of dye solubility.
2.1. Materials
3. Results and discussion
Octenyl succinate modified waxy corn starches OS1 and OS2 were
obtained from National Starch and Chemicals GmbH, Germany. OS1 is
recommended as good natural emulsifier and OS2 is good to use in
encapsulation. The grades of both OSA starches are for food and
pharmaceutical use. Commercial xanthan gum (Xanthural 180 CP)
was purchased from KELKO e Hamburg. Double distilled water was
used for solution preparation.
3.1. Viscosity measurements
2.2. Methods
Stock solutions were prepared by dissolving 1 g of OSA starch
in 100 cm3 of double distilled water at 60 C and diluting to obtain
certain concentration.0.2 g of xanthan gum was suspended in
100 cm3 of double distilled water and left at room temperature for
two days.
Different blends of OSA starch and xanthan gum were prepared by
mixing their solutions (1 g/100 cm3 of OSA starch and 0.2 g/100 cm3
of xanthan gum) together at certain ratios and storage at room
temperature.
CMC values of OSA starches and their interactions with xanthan
gum were studied by comparing the above mentioned techniques.
In all experiments OSA starch concentration varied from 0.01 to
0.5 g/100 cm3, while during the examinations of OSA
starchexanthan gum interactions, xanthan gum concentration was
kept constant. The xanthan gum concentrations were different and
depended on the special requirements of applied methods. It was
0.002 and 0.004 g/100 cm3 for viscometry, 0.05 g/100 cm3 for dye
solubilization, 0.04 g/100 cm3 for tensiometry and 0.01, 0.05 and
0.1 g/100 cm3 in conductometric investigations. All measurements
were done 24 h after preparation of blends.
The viscosity characteristics of the dilute solutions are good indicator of polymer conformation and their changes in polymere
surfactant mixtures. The modified polymer, OSA starch, played a role
of surfactant, and xanthan gum of the polymer. Viscosity experiments
were conducted in order to understand the nature of those components and to find out the possible interactions.
The dependence of the specific viscosity on the concentration of
OS1 and OS2 in the aqueous solution is shown in Fig. 1. After certain
concentration (marked by arrow at Fig. 1) the curves of specific
viscosities decreased slope from 0.17 to 0.11 for OS2 and from 0.47
to 0.14 for OS1.
At this concentration some changes occurred in solution. Because
OSA starch has characteristics of polymer and surfactant, this point
might be a critical overlap concentration c*, which is defined as
concentration at which individual polymer molecules begin to
physically interact (Rodd et al., 2000), or critical micellar concentration which arises due to aggregation of hydrophobic groups which
tend to minimize their exposure to water (Egermayer, Karlberg, &
Piculell, 2004).
The information about molecule conformation can be obtained
by measuring the intrinsic viscosity of polymer, applying Huggins
equation which expresses the reduced viscosity of polymer as
function of concentration:
hsp
c
¼ ½h þ k½h2 c
(2)
where [h] is intrinsic viscosity, hsp/c is reduced viscosity, k is Huggins parameter and c is polymer concentration.
V. Krstonosic et al. / Food Hydrocolloids 25 (2011) 361e367
0.10
OS2
OS1
0.09
0.08
0.07
ηs p
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.0
0.1
0.2
0.3
0.4
0.5
3
c (g/100cm )
Fig. 1. Specific viscosity vs. OSA starch concentration in aqueous solutions at 25 C.
OSA starches exhibited behavior typical for polyelectrolytes
aqueous solutions, having high values of reduced viscosity at low
polymer concentrations (Yang, Chen, & Fang, 2009). Such behavior
originates from octenyl succinic groups. That was the reason for
determination of intrinsic viscosity in 0.1% and 1% NaCl aqueous
solutions at 25 C.
The reduced viscosity vs. concentration of OS1 in NaCl solutions at
25 C are shown in Fig. 2. It is obvious that after certain OS1
concentration reduced viscosity decreases. Calculation of the specific
viscosity values for measurements done in NaCl showed the same
slope change tendencies as in pure water. The change of slopes for
specific viscosities in NaCl happened after the concentrations at
which reduced viscosities reached maximum (Fig. 2). These
concentrations, for both OSA starches, were lower that the concentrations in pure aqueous solution due to presence of NaCl, and
decreased with increase in NaCl concentration. The decrease in
reduced viscosity occurred because of reduction in the hydrodynamic
size of molecule (Rochefort & Middleman, 1987), which is influenced
by presence of hydrophobic groups. Nilsson, Thuresson, Hansson, and
Lindman (1998) reported that the hydrophobic tails of
0.7
0.5
0.6
3
ηsp/c (100cm /g)
0.4
0.3
3
0.5
0.4
ηsp/c (100cm /g)
0.1% NaCl
1% NaCl
0.3
0.2
0.1
0.0
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
c (g/100cm3)
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
363
hydrophobically modified polymers in dilute solutions associate
intra-molecularly in order to minimize contact with water. For this
reason, after certain concentration the polymer chains probably start
to curl and shrink and this concentration is critical micellar concentration. The micelles of OSA starches are different than those formed
by small surfactant molecules, and it is obvious that hydrodynamic
size of molecules became smaller after CMC. Intrinsic viscosity for
OSA starches was estimated before the CMC (Fig. 2).
The Huggins parameter k gives information about solvent
quality. Values between 0.3 and 0.7 have been suggested for perfect
solutions, and for k 1 formation of aggregates is encouraged
(Braga, Azevedo, Marques, Menossi, & Cunha, 2006; Millard,
Dintzis, Willett, & Klavons, 1997). Parameters k for OS1 and OS2
were 61.97 and 28.70 in 0.1% NaCl and 98.53 and 59.18 in 1% NaCl
respectively, which are much higher than 1, meaning that OSA
starches have high tendencies to self-aggregation.
The obtained results for intrinsic viscosities for OS1 and OS2
were 0.1864 100 cm3/g and 0.1005 100 cm3/g in 0.1% NaCl and
0.1585 100 cm3/g and 0.0726 100 cm3/g in 1% NaCl respectively. The
increase in NaCl concentration present in OSA starch solutions, led
to increase in Huggins parameters and to decrease in intrinsic
viscosities of determined OSA starches, due to salt effect on
polyelectrolyte.
According to Hormnirum, Sirivat, and Jamieson (2000) the
critical overlap concentration can be estimated as c* ¼ 1/[h].
Therefore the critical overlap concentrations for OS1 and OS2
would be 5.36 g/100 cm3 and 9.95 g/100 cm3 in 0.1% NaCl and
6.31 g/100 cm3 and 13.77 g/100 cm3 in 1% NaCl respectively.
The change of slope in plot of specific viscosities vs. OSA starch
concentration in aqueous solutions (Fig. 1) for OS1 and OS2
occurred at 0.088 g/100 cm3 and 0.081 g/100 cm3 respectively.
Those values are lower that the values for critical overlap concentrations calculated above, which is a confirmation for the conclusion that values obtained from Fig. 1 are points of the critical
micellar concentrations.
Intrinsic viscosity is related to the molecular weight, M, through
the MarkeHouwink equation (Casas, Santos, & Garcia-Ochoa, 2000;
Chuah, Lin-Vien, & Soni, 2001):
½h ¼ K$M a
(3)
where K and a are specific constants to the solvent and temperature
used in measurements.
Since the information for K and a for OSA starches are not available, it is not possible to calculate molecular weight. Due to the fact
that both OSA starches, determined in this work, have the same origin
and DS values (0.01e0.03 because they are recommended for food
and pharmaceutical use), they have the same values for constants
K and a. So it could be concluded that OS1 has higher molecular
weight than OS2, because it has higher value of intrinsic viscosity.
In the presence of constant xanthan gum concentration the
specific viscosity of xanthan gumeOSA starch blends decreased for
low OSA starch concentration. At concentration, determined as
CMC of OSA starch (Fig. 1), the viscosity of the blends reached the
minimum (Fig. 3).
This is unexpected behavior, and could be indicator of some
conformational changes in presence of xanthan gum molecules,
considering that the specific viscosity of single OSA starch solutions
increased with the concentration increase (Fig. 1.).
Mya, Jamieson, and Sirivat (1999) reported the same results for
Triton X-100 and polyacrilamide. They explained those results by
following equation (3):
3
c (g/100cm )
Fig. 2. Reduced viscosity vs. OS1 concentration in 0.1% and 1% NaCl at 25 C.
hsp ¼
2:5NA c
Vh
M
(4)
V. Krstonosic et al. / Food Hydrocolloids 25 (2011) 361e367
364
0.20
argument that xanthan gum and OSA starch may interact through
formation of inclusion complexes between OSA tails and xanthan
gum helix.
0.19
0.18
0.17
3.2. Conductivity measurements
0.16
CMC
ηsp
0.15
0.14
0.13
0.12
CMC
0.11
3
0,004 g/100cm xanthan gum + OS2
3
0,004 g/100cm xanthan gum + OS1
0.10
0.09
0.0
0.1
0.2
0.3
0.4
0.5
0.6
3
c (g/100cm )
Fig. 3. Specific viscosity of OSA starchexanthan gum blends vs. OSA starch concentration for constant xanthan gum concentration (0.004 g/100 cm3) at 25 C.
where NA is Avogadro’s number, c is concentration of molecules in
solution, M is molecular weight and Vh is polymer hydrodynamic
volume.
They concluded that consequence of the interactions between
Triton X-100 and polyacrilamide was decrease in number of
particles in solution which reflected as decrease in specific
viscosity.
The same observation can be used to explain our results.
The reason why minimum was reached at CMC was formation of
mixed micelles between OSA starch and xanthan gum molecules.
After that concentration, increase in viscosity with increase in OSA
starch concentration occurred. Upon further increase in OSA starch
concentration, the viscosity continued to increase, suggesting
formation complexes between formed mixed micelles. Examinations were done with two different xanthan concentrations
(0.002 g/100 cm3 and 0.004 g/100 cm3) and the same results were
obtained for both concentrations. Deo et al. (2003) reported similar
results for hydrophobically modified polyelectrolyte and surfactant
sodium dodecyl sulfate (SDS) of the same charge, and also
concluded that they formed mixed micelles. According to Wang
et al. (2001) waxy corn starch and xanthan gum are attracted to
each other in solution. Besides that, formation of mixed micelles is
90
90
A
80
70
70
60
60
50
50
k (μS/cm)
k (μS/cm)
80
Conductometric measurements are widely used for CMC
determinations (Fuguet, Rafols, Roses, & Bosch, 2005; Onesippe &
Lagerge, 2008; Sovilj & Petrovi
c, 2006). The specific conductance
increases linearly with increase in concentration of ionic surfactant,
in this work OSA starch, up to the CMC. After that concentration,
specific conductance continues to increase, but with a lower slope
than before the CMC. The point after which curve changes the slope
represents CMC value (Fig. 4). The CMC point for OS1 and OS2
determined by conductivity measurements were 0.073 g/100 cm3
and 0.062 g/100 cm3 respectively.
In the presence of xanthan gum, which concentration was
constant, the specific conductivity dependence on OSA starch (OS1)
concentration is presented on Fig. 5.
The conductivity of surfactant with added polymer usually
shows three regions. The first break point, at the end of the first
region, below CMC, is related to beginning of surfactantepolymer
association and it is called critical aggregation concentration (CAC).
The second break point, after second region, over CMC, is polymer
saturation point (PSP) by the surfactant (Sovilj & Petrovi
c, 2006).
The break points CAC and PSP of surfactant and polymer of the
same charge (Burke & Palepu, 2001) are less significant than for the
charged surfactant/uncharged polymer (Sovilj & Petrovi
c, 2006), as
well as for surfactant and oppositely charged polymer (Onesippe &
Lagerge, 2008).
The results obtained by determination of specific conductance of
OSA starchexanthan gum blends (Fig. 5) indicated existence of OSA
starchexanthan gum interactions. Namely, it is noticeable that after
CAC the curve changed slope, which is the result of interaction
between those two components. For 0.1 g/100 cm3 of xanthan gum
concentration more significant changes occurred after CAC. The
specific conductance showed non-linear dependence on OSA starch
concentration until PSP was reached. In our recent work
(Krstonosic, Dokic, Dokic, & Dapcevic, 2009) we reported that critical overlap concentration for xanthan gum was 0.082 g/100 cm3.
The significant behavior of specific conductance at 0.1 g/100 cm3 of
xanthan gum may be because OSA starch molecules build into
xanthan gum network, became slower, which reflected on specific
conductance.
40
30
20
B
40
30
20
10
10
CMC
0
0.00
0.05
0.10
0.15
0.20
3
c (g/100cm )
0.25
0.30
CMC
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
3
c (g/100cm )
Fig. 4. Determination of the CMC of OSA starches. Plot of specific conductance vs. A) OS1, B) OS2 concentrations in aqueous solutions at 25 C.
V. Krstonosic et al. / Food Hydrocolloids 25 (2011) 361e367
365
76
200
180
PSP
160
140
OS2
OS1
72
CAC
68
100
γ (mN/m)
k (μS/cm)
120
80
60
xanthan
concentration (%)
0
0,01
0,05
0,1
40
20
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
64
60
56
52
0.35
48
0.00
3
c (g/100cm )
0.02
0.04
0.06
0.08
0.10
0.12
3
It is obvious that CAC increases with increase in xanthan gum
concentration, and PSP values were independent on xanthan gum
concentration (Table 1). The similar results for CAC dependence on
polymer concentration were reported by Onesippe and Lagerge
(2008) for chitosan and SDS.
3.3. Surface tension measurements
A surface tension measurement is another method to study
surfactant micellarization and possible interactions in solution.
Below the CMC surface tension strongly decreases while surfactant
concentration increases. After reaching the CMC surface tension
remains constant. Therefore the CMC can be determined as
concentration after which surface tension does not change. The
CMC was determined by measuring the surface tension of different
concentration of OS1 and OS2 (Fig. 6), and obtained values were
0.050 g/100 cm3 and 0.041 g/100 cm3 respectively.
Varona et al. (2009) reported higher CMC values for OSA starch
derived from waxy maze, but there were no information about its
molecular mass and DS values which may have the influence on
CMC. Minimum surface tension about 54 mN/m was reached for
OS1 and 60 mN/m for OS2 which is in agreement with data
reported by Varona et al. (2009).
Shorgen and Biresaw (2007) investigated surface properties of
water soluble starches, and they reported that the surface tension
of OSA starch (DS 0.02e0.04) declined to 42e43 mN/m which is
a slightly lower value than we reported above. They did not
determine CMC, but from figure they presented it is obvious that
after 2.5e3 g/ml of OSA starch, surface tension remains constant
which is in agreement with our results.
Several authors reported reduction of surface tension after
adding polyelectrolyte to the surfactant solution due to their
Table 1
Comparison of CAC and PSP values obtained by conductometric titration for OS1 and
OS2.
Xanthan gum concentration
(g/100 cm3)
0.01
0.05
0.1
CAC (g/100 cm3)
PSP (g/100 cm3)
OS1
OS2
OS1
OS2
0.028
0.030
0.048
0.030
0.038
0.052
0.199
0.199
0.199
0.186
0.186
0.186
Fig. 6. Determination of the CMC values for OSA starches. Plot of surface tension vs.
OSA starch concentration in aqueous solutions at 25 C.
synergistic effect (Deo et al., 2003; Mata et al., 2006; Onesippe &
Lagerge, 2008), but Fig. 7 presents opposite effect. Namely, addition of xanthan gum increase the surface tension and CMC value of
OSA starch solution.
Prud’homme and Long (1983) reported that xanthan gum
aqueous solutions do not show the surface activities for low
concentrations (0.1 g/100 cm3), which was confirmed by Secouard,
Malhiac, and Grisel (2006). Prud’homme and Long (1983) and
Benichou, Aserin, Lutz, and Garti (2007), suggested also, a dramatic
decrease of surface tension at 1 g/100 cm3 xanthan concentration.
Our results of xanthan gum surface activity showed that xanthan
gum did not significantly change water surface tension between
0.01 and 0.1 g/100 cm3. That is the reason why addition of xanthan
gum raises the surface tension and CMC values of OSA starch water
solutions. As xanthan molecules do not adsorb at airewater interface, OSA starch molecules were brought into bulk from airewater
surface, through their interactions with xanthan gum. After certain
OSA starch concentration (0.06 g/100 cm3 for OS1 from Fig. 7) the
76
72
OS1
OS1+xanthan gum
68
γ (mN/m)
Fig. 5. Specific conductance of OS1exanthan gum blends vs. OS1 concentration for
constant xanthan gum concentration in aqueous solutions at 25 C.
c (g/100cm )
64
60
56
52
48
0.00
0.02
0.04
0.06
0.08
0.10
0.12
3
c (g/100cm )
Fig. 7. Surface tension vs. OS1 concentration for OS1 aqueous solutions and for
OS1exanthan gum mixtures while xanthan concentration maintained constant (0.04%)
at 25 C.
V. Krstonosic et al. / Food Hydrocolloids 25 (2011) 361e367
366
surface tension values for OSA starchexanthan gum blends became
constant and the values of surface tension for blends and for single
OSA starch solutions overlapped.
3.4. Dye solubilization measurements
Sudan III is water insoluble, organic, azo dye, which solubility is
possible in surfactant aqueous solutions. It was expected that solubility of Sudan III rapidly increase after OSA starch micelle formation
in aqueous solutions. Fig. 8 represents solubility of Sudan III as
function of OSA starch concentration. It is obvious; from Fig. 8 that
CMC for both investigated OSA starches was around 0.080 g/100 cm3.
The solubilization process is influenced by several different factors.
Some of them are: structure of the surfactant and organic compound,
temperature, presence of polymer, etc (Burke & Palepu, 2001). In
order to determine OSA starchexanthan gum interaction, we investigated dependence of Sudan III solubilization as function of OSA
starch concentration in presence of constant xanthan gum concentration (0.05 g/100 cm3). The absorbance for those systems did not
show any significant differences from absorbance for pure OSA starch
solutions. That was because the method does not have adequate
sensitivity for those investigations.
Summarizing the results, it is obvious for all applied techniques
that the CMC values for OS1 were higher than for OS2. It was
because OS1 has higher molecular weight, thus its molecules
contain more hydrophilic groups, and they have higher tendency to
stay in the water than OS2 molecules.
The results obtained for CMC are different for all applied
methods, because the CMC is moderately method-dependent
(Moulik, 1996). According to Moulik (1996) the most frequently
used methods for CMC determination are tensiometry, conductometry and fluorometry. Ghosh and Banerjee (2002) studied
interaction between SDS and globular protein trypsin using
tensiometric, conductometric, calorimetric, fluorimetric, viscometric, and circular dichroism techniques. For determination of the
CMC values of SDS they used only tensiometric, conductometric
and calorimetric method. Viscometry is effective method in
examination of conformational and rheological changes (Ghosh &
Banerjee, 2002), which we used in determination of CMC values.
In view of previous information we concluded that the values of
CMC obtained, in this work, by tensiometric and conductometric
methods are the most reliable.
0.45
0.40
OS1
OS2
0.35
A
0.30
0.25
0.20
0.15
0.10
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
3
c (g/100cm )
Fig. 8. Absorbance of solubilized Sudan III dye vs. OSA starch concentration.
4. Conclusions
In this paper four different techniques were used to determine
critical micellar concentration of two variety of octenyl succinate
modified waxy corn starches in aqueous solution. The applied techniques provide comparable results of CMC. OSA starch molecules
formed micelles with smaller hydrodynamic size of single molecules
than before CMC. They curl and shrink at CMC in order to minimize
contact between OSA groups and water molecules. The interactions
between OSA starches and xanthan gum were investigated using the
same techniques. The results indicated that the interactions existed,
except results obtained by dye solubilization method which was not
enough sensitive for those investigations. The addition of xanthan
gum decreases the specific viscosity and increases surface tension
and the CMC compared to the single OSA starch solutions. OSA
starchexanthan gum interaction, reported in this paper, confirm the
assumption, previously reported by Ntawukulilyayo et al. (1996), that
interaction between those two components existed.
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