Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
Effect of the starch concentration on phase transitions of starch
dispersions prepared with a glycerol solution or pure glycerol
Efeito da concentração de amido nas transições de fases de dispersões
de amido preparadas com solução de glicerol ou glicerol puro
Autores | Authors
Paulo José do Amaral SOBRAL
Universidade de São Paulo (USP)
Faculdade de Zootecnia e Engenharia de
Alimentos (FZEA)
Departamento de Engenharia de Alimentos
Caixa Postal: 23
CEP: 13635-900
Pirassununga/SP - Brasil
e-mail: [email protected]
Ana Mônica Quinta Barbosa
HABITANTE
Rosemary Aparecida de
CARVALHO
Universidade de São Paulo (USP)
Faculdade de Zootecnia e Engenharia de
Alimentos (FZEA)
Departamento de Engenharia de Alimentos
e-mail: [email protected]
[email protected]
Javier SOLORZA-FERIA
Centro de Desarrollo de Productos Bióticos
del IPN
e-mail: [email protected]
Paulo Victor Albuquerque BERGO
Universidade de São Paulo (USP)
Faculdade de Zootecnia e Engenharia de
Alimentos (FZEA)
Departamento de Engenharia de Alimentos
e-mail: [email protected]
Autor Correspondente | Corresponding Author
Recebido | Received: 22/04/2008
Aprovado | Approved: 21/09/2008
Summary
The objective of this work was to study, using a calorimetric method, the
effect of the starch concentration on the phase properties of cassava starch
dispersions, prepared using a glycerol solution and pure glycerol. Starch
dispersions (SD) were prepared with 10-70 g of starch.100 g–1 of SD using either a
20% glycerol solution (GS) or pure glycerol (PG). These dispersions were analyzed
with a differential scanning calorimeter. The water activity of these samples was
also determined at 25 °C. At low temperatures the glass transition (Tg) was well
defined in samples prepared with 10-50% of starch, but was not so for SDs with
70% starch, for both glycerol solutions. For SD prepared with GS, an increase in
starch concentration from 10 to 70% caused a reduction in Tg, but for SD with
10-50% starch and prepared with PG, the Tg remained constant, increasing
slightly for samples with 70% starch. This behaviour agreed with that of the water
activity, which increased when the starch concentration increased from 10 to 70%
for SDs prepared with GS. In the intermediate temperature domain, for the SD
with GS, an endothermal peak was visible near zero degrees due to the melting
of ice, but this transition was not seen with PG. At relatively high temperatures,
starch gelatinization was observed in all the DSC curves, while in the case of
samples with PG, an exothermal peak was also seen before the endothermal
one. The starch gelatinization temperature (Tsg) remained essentially constant for
samples with 10-50% of starch, increasing for a concentration of 70%. When the
starch dispersing agent was PG, it was observed that a decrease in the starch
concentration from 70 to 50% also caused an important reduction in Tsg. However,
in more dilute starch dispersions, the Tsg increased. Thus it was concluded that
some physical interactions may take place between the glycerol and the glucose
molecules of the cassava starch even before its gelatinization.
Key words: Glass transition; Gelatinization; Differential scanning calorimetry;
Cassava.
Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
Resumo
O objetivo deste trabalho foi o estudo do efeito da concentração de amido
nas propriedades de fases de dispersões de amido de mandioca, preparadas
usando-se uma solução do glicerol ou glicerol puro, por calorimetria. As dispersões
de amido (SD) foram preparadas com 10-70 g de amido.100 g–1 de SD usando-se
uma solução do glicerol a 20% (GS) ou glicerol puro (PG). Estas dispersões foram
analisadas com um calorímetro diferencial de varredura. Além disso, a atividade
de água dessas amostras foi determinada a 25 °C. Em baixas temperaturas, a
transição vítrea (Tg) foi bem visível nas amostras preparadas com o 10-50% de
amido, não sendo bem definido para a SD com 70%, com ambas soluções de
glicerol. Para a SD preparada com GS, o aumento da concentração de amido de
10 a 70%, causou uma redução na Tg, mas para a SD com 10-50% de amido e
preparada com PG, Tg permaneceu constante, aumentando ligeiramente para
amostras com 70% de concentração de amido. Esse comportamento está de
acordo com o da atividade de água, que aumentou quando a concentração de
amido aumentou de 10 a 70% para as SDs preparadas com GS. No domínio
intermediário de temperaturas, um pico endotérmico foi visível em torno de zero
grau, nas SDs com GS, devido ao derretimento do gelo. Esse fenômeno não foi
observado nas amostras com PG. Em temperaturas relativamente elevadas, a
gelatinização do amido foi observada em todas as curvas de DSC, enquanto no
caso das amostras com PG, um pico exotérmico foi visto antes do endotérmico.
A temperatura de gelatinização do amido (Tsg) permaneceu essencialmente
constante para amostras com 10-50% de amido, aumentando em 70% de amido.
Quando o agente dispersante do amido era PG, observou-se que uma diminuição
da concentração do amido de 70 a 50%, causava uma redução importante de Tsg.
Entretanto, em dispersões mais diluídas de amido, Tsg aumentou. Assim, pode-se
concluir que algumas interações físicas entre o glicerol e as moléculas de glicose
do amido de mandioca podem ocorrer mesmo antes de sua gelatinização.
Palavras-chave: Transição vítrea; Gelatinização; Calorimetria diferencial de
varredura; Mandioca.
www.ital.sp.gov.br/bj
Effect of the starch concentration on phase transitions of starch dispersions prepared
with a glycerol solution or pure glycerol
SOBRAL, P. J. A. et al.
1 Introduction
Starch is a biopolymer with high potential for use as
a raw material for edible and/or biodegradable film production. Starch can be made thermoplastic by de-structuring
its native component in the presence of plasticizers (e.g.
water, glycerol) under specific processing conditions that
can include the conventional plastic processing techniques (TAN et al., 2004).
The main processing techniques for the production
of starch based films are extrusion, considered to be a
dry technique, and casting, which is a moist technique. In
the extrusion process, starch is exposed to high temperatures and high shear rates, and consequently undergoes
structural changes such as gelatinization and melting
(TAN et al., 2004; RODRIGUEZ-GONZÁLEZ et al., 2004).
On the other hand, to produce flexible films by casting, it
is necessary to prepare a starch dispersion and promote
its gelatinization by thermal treatment, and then apply this
dispersion to a convenient support, followed by drying
under mild conditions (OLLETT et al., 1991; FORSSELL
et al., 1997; CHANG et al., 2000; STADING et al., 2001;
MALI et al., 2005; VICENTINI et al., 2005).
Considering that both the glass transition temperature and the melting temperature of pure dry starch are
higher than its decomposition temperature, a plasticizer is
necessary with both techniques to ensure that the starch
undergoes gelatinization rather than degradation under
the effect of heat (OLLETT et al.; 1991, CHANG et al.,
2000; FAMÁ et al., 2005). Also, the plasticizer improves
the workability of biopolymer based films and affects the
glass transition of biopolymers (SLADE and LEVINE, 1991;
GONTARD and RING, 1996; CUQ et al., 1997; SOBRAL
et al., 2002; BERGO et al., 2008).
Various studies on the phase transitions of starch
based films (GARCIA et al., 1996; FORSSELL et al., 1997;
CURVELO et al.; 2001, MALI et al., 2005; BERGO et al.,
2008) and aqueous dispersions (TAN et al., 2004; NASHED
et al., 2003; HABITANTE et al., 2008) containing glycerol
may be found in the literature, but none have investigated
all the possible phase transitions in starch dispersions
prepared with either pure glycerol or with a dilute glycerol
solution, for different starch concentrations. This may be
considered as important to a fundamental understanding
of the plasticizing effect of glycerol on starch-based products, which has not yet been fully established (HABITANTE
et al., 2008). Thus the objective of this work was to study the
effect of the starch concentration on the phase properties
of cassava starch dispersions, prepared using a glycerol
solution (20%) and pure glycerol, in both cases using a
differential scanning calorimetric method.
2 Material and methods
This study was carried out using cassava starch
(Flor de Lotus Co., Brazil) with 14.9% moisture content
Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
and 87.6% of starch, of which 16.0% was amylose
(­VICENTINI et al., 2005). This starch was used without
prior preparation.
Starch dispersions (SD) were prepared with 10,
30, 50 and 70 g of starch.100 g–1 of SD using glycerol
solutions (GS, 20%) or pure glycerol (PG, 100%). These
components were prepared using mechanical stirring.
These two series of dispersions were analyzed using a
differential scanning calorimeter (DSC TA 2010 with a TA
5000 control unit, TA Instruments). Samples conditioned in
hermetic aluminium TA pans and weighed (~10 mg) using
a precision balance (±0.01 mg, Analytical Plus, Ohaus)
were heated at 10 °C/min between –130 and 150 °C in
an inert atmosphere (45 mL of dry N2) (HABITANTE et al.,
2008). The reference was an empty aluminium TA pan.
The phase properties were calculated using the software
Universal Analysis V1.7F (TA Instruments). Starch dispersions were also prepared with 50 g of starch.100 g–1 of
pure water and analyzed as described above.
Considering that the phase transition properties of
the samples could be affected by interactions between
either the water or the glycerol molecules with the starch
macromolecules, the water activity (a w) of the starch
dispersions was determined at 25 °C using an AquaLab
CX2 device (Decagon Devices, Inc.).
All the above mentioned tests and analyses were
run in triplicate. All polynomial regressions reported were
made using the Excel 2003 Software.
3 Results and discussion
The DSC curves of SD prepared with GS showed
phase transition phenomena in three temperature domains
(Figure 1), while those of SD prepared with PG only
showed these phenomena in two temperature domains
(Figure 2).
At very low temperatures, a phase transition was
observed as a baseline inflexion, typical of the glass transition (Tg), which was well visible in samples prepared with
10, 30 and 50% of starch. This low temperature phase transition may be attributed to the glycerol α-relaxation and
the starch β-relaxation (SOBRAL et al., 2001), also being
found in systems formed by other biopolymers and polyols
(KALICHEVSKY et al., 1993; CUQ et al., 1997; SOBRAL
et al., 2002; NASHED et al., 2003). This phenomenon,
which has been observed by other authors (HABITANTE
et al., 2008), was due to a phase separation between the
glycerol rich fraction (in solution) and the starch rich fraction (ZELEZNAK and HOSENEY, 1987). Finally, the glass
transition was hardly visible in the samples prepared with
70% of starch (Figures 1 and 2), probably because the
amount of glycerol in the liquid phase was small.
For SD prepared with GS, an increase in starch
concentration (SC) from 10 to 70% caused a reduction in
265
www.ital.sp.gov.br/bj
Effect of the starch concentration on phase transitions of starch dispersions prepared
with a glycerol solution or pure glycerol
SOBRAL, P. J. A. et al.
0.05 W.g–1
10
Exothermal heat flow
30
50
–150
–100
–50
0.05 W.g–1
0
60
0.5 W.g–1
80
100
120
140
70
–150
–100
–50
0
50
100
150
Temperature (°C)
Figure 1. DSC curves of starch dispersions prepared with a glycerol solution (20%) and various starch concentrations (values indicated). The left insert enhances visibility of the glass transitions, and the right one enhances visibility of the starch gelatinization.
–90
10
Temperature (ºC)
Exothermal heat flow
–80
30
50
0.5 W.g–1
–100
–110
70
–150
–100
–50
0
50
100
150
200
Temperature (°C)
–120
0
20
40
60
Starch concentration (%)
80
Figure 2. DSC curves of starch dispersions prepared with pure
glycerol and various starch concentrations (values indicated).
Figure 3. Glass transition temperatures (Tg) of starch dispersions prepared with a glycerol solution () or pure glycerol ()
for various starch concentrations (SC).
the glass transition temperature (Tg) from –82.0 ± 0.4 to
–93.8 ± 0.4 °C following a second order polynomial trend
(Equation 1, R2 = 0.986) (Figure 3). A reduction in Tg is
usually considered as a plasticization phenomenon and
may be due to an increase in the concentration of the
plasticizer agent, such as, for example, the water molecule. With respect to the behaviour of the water activity
of these SD (Figure 4), it can be seen that this remained
essentially constant (0.95-0.96) for samples with starch
concentrations between 10 to 50%, with a slight decrease
(0.91 ± 0.01) in the 70% starch samples. This suggests
that these dispersions may have been concentrated
as a consequence of water adsorption by the cassava
starch granules. In this case, glycerol/starch interactions
favoured the entrance of water into the starch granules,
giving place to a higher glycerol concentration in the liquid
phase, which provoked the reduction in Tg of the SD.
Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
Tg = –3.7 x 10–3 SC2 + 0.1SC – 82.9
(1)
It is also worth noting that the glass transition
observed for the SD with 10, 30 and 50% of starch and
prepared with PG, also occurred at a practically constant
temperature (≈–100 °C) (Figure 3), being different from that
shown with a 70% starch concentration, which was –93.5 ±
266
www.ital.sp.gov.br/bj
Effect of the starch concentration on phase transitions of starch dispersions prepared
with a glycerol solution or pure glycerol
SOBRAL, P. J. A. et al.
aw = 1.0 x 10–4 SC2 – 2.0 x 10–3 SC + 0.09
(2)
In the intermediate temperature domain, for the
SD with GS, an endothermal peak was seen near zero
degrees, due to the melting of ice, and this phenomenon
also became less visible in samples with 70% of starch
because of the small amount of water in the liquid phase
(Figure 1). No melting of ice was observed with PG
(Figure 2).
The ice melting temperature (Tm) calculated as
the onset temperature of the peak, observed solely in
SD prepared with GS, was essentially constant, staying
around –14 °C for SC varying between 10 and 50%
(Figure 5). An increase to 70% starch in the SC caused
an important decrease in Tm (–24.8 ± 3.2 °C), possibly
due to substantial water absorption by the starch granules, such that the resultant solution showed a greater
glycerol concentration, inducing a depressing effect on
the Tm (FRANKS and MATHIAS, 1983). This behaviour
is consistent with the trend shown by the values for the
water activities of these samples, as explained previously
(Figure 4).
∆hm = –2.5SC + 199.2
In the case of GS, the starch gelatinization temperature (Tsg), calculated as the peak temperature, remained
essentially constant (70-71 °C) when the starch concentration varied between 10 and 50%, but for 70% starch the
Tsg increased to 129.4 ± 6.1 °C (Figure 7), in agreement
with the behaviour of both the glass transition temperature
1.0
0
0.8
–5
0.6
0.4
0.2
0.0
(3)
At relatively high temperatures, another endothermal peak appeared in SD prepared with GS and PG
(Figures 1 and 2) due to starch gelatinization, while in the
case of samples with PG, an exothermal peak appeared
before the endothermal one. The gelatinization of starch
granules occurs as a cooperative process, due to
constraints induced in the crystallites by the amorphous
areas (SEOW and TEO, 1993; GARCIA et al., 1996). When
starch is heated in an excess of water, its granules swell
and the amylose starts leaching out, characterizing starch
gelatinization, which is considered as a first order phase
transition (DONOVAN and MAPES, 1980; LUND, 1984;
LELIEVRE and LIU, 1994). The peak observed in samples
prepared with 10 and 30% of starch was typical of dilute
systems, being present as a single endothermal peak
(DONOVAN and MAPES, 1980; BILIADERIS et al., 1986).
However, for samples prepared with 50 and 70% of starch,
the endothermal peak was typical of starch samples at
intermediate concentration levels, with two endothermic
transitions. Various authors (DONOVAN and MAPES, 1980;
BILIADERIS et al., 1986; GARCIA et al., 1996), working at
this level of starch concentration, evidenced a two-stage
melting process as a result of starch crystallite disorganization. A similar DSC curve was observed for the sample
with 50% of starch, prepared with pure water (Figure 6).
Temperature (ºC)
Water activity
The enthalpy associated with ice melting, which
reflects the amount of water melted in the sample,
decreased from 160.2 ± 0.6 to 7.8 ± 2.7 J.g–1 when the
starch concentration increased from 10 to 70%. This
reduction could be considered as linear (Equation 3,
R2 = 0.930). Similar behaviour with respect to ice melting
enthalpy has been observed in studies on freeze dried
foods and fruits (ROOS, 1995) and on cassava starch
aqueous dispersions containing glycerol (HABITANTE
et al., 2008).
200
180
160
140
–10
120
100
–15
80
–20
60
40
–25
0
20
40
60
80
Starch concentration (%)
Figure 4. Water activity (aw) of starch dispersions prepared with
a glycerol solution (), or pure glycerol () with various starch
concentrations (SC).
Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
–30
Ice melting enthalpy (J.g–1)
2.0 °C. Also, an inverse behaviour was observed for the
water activity data for SD prepared with PG (Figure 4): the
water activity (aw) increasing from 0.08 ± 0.01 to 0.44 ±
0.00 when the starch concentration increased from 10 to
70% (Equation 2, R2=0.996), probably due to strong interactions between the glycerol molecules and the starch
macromolecules.
20
0
20
40
60
Starch concentration (%)
80
0
Figure 5. Ice melting temperature () and enthalpy (∆hm, ♦) of
starch dispersions prepared with glycerol solution and various
starch concentrations (SC).
267
www.ital.sp.gov.br/bj
Effect of the starch concentration on phase transitions of starch dispersions prepared
with a glycerol solution or pure glycerol
SOBRAL, P. J. A. et al.
When the starch dispersing agent was PG, it was
observed that an increase in the starch concentration
from 10 to 50% caused a reduction in Tsg from 134.4 ±
0.4 to 111.2 ± 1.6 °C (Figure 7). However, in the more
concentrated starch dispersion (70%), the Tsg increased,
reaching a value of 139.9 ± 0.9 °C, close to that observed
for SD with 70% starch prepared with GS (Figure 7). This
behaviour was probably due to the hypertonic effect of
pure glycerol, which produced a certain dehydration of
the starch granules. That interaction between glycerol and
the starch granules could also be confirmed by the results
for the water activity of these dispersions (Figure 4). In
general it has been considered that an increase in glycerol content increases the starch gelatinization onset
temperature (Van SOEST et al., 1996, NASHED et al.,
2003), which agrees with the results of this study for SD
with 10-50% of starch.
The peak temperature determined for the exothermal
peaks of samples prepared with 10, 30 and 50% of starch
and with pure glycerol (Figure 2), presented a similar
behaviour to the respective Tsg (Figure 7). This phenomenon could be associated with some crystallization in the
starch granules, but further studies are needed to better
explain these results. No previous similar results could be
found in the literature.
Concerning the gelatinization enthalpy (∆hsg), in
both starch dispersions prepared using GS (Equation 4,
R2 = 0.959) and PG (Equation 5, R2 = 0.999), a second
order polynomial behaviour with a maximum value was
observed (Figure 8). Similar behaviour was observed for
the crystallization enthalpy (∆hc) of SDs prepared with PG
(Equation 6, R2 = 0.944). The enthalpy of the gelatinization
of SDs prepared with GS and PG varied between 11 and
13 J.g–1 of starch and 7 and 12 J.g–1 of starch, respectively.
These values agreed with those determined for various
starch types (7-17 J.g–1 of starch) (BILIADERIS et al.,
1986; FUJITA et al., 1993). However in this case it may
be considered that the presence of glycerol did not affect
these properties, because in the case of SD prepared
with pure water (Figure 6), the gelatinization enthalpy was
13.0 ± 0.5 J.g–1 of starch.
∆hsg = –2.6 x 10–3 SC2 + 0.17 SC + 11.7
(4)
∆hsg = –5.8 x 10–3 SC2 + 0.44 SC + 4.5
(5)
∆hc = –7.3 x 10–3 SC2 + 0.59 SC – 3.25
(6)
150
120
Temperature (ºC)
(Figure 3) and the ice melting temperature (Figure 5).
It could be observed that the presence of glycerol in
the dispersing solution affected starch gelatinization,
promoting a slight increase in Tsg, which was 65.2 ± 0.2 °C
for SD prepared with pure water (Figure 6).
90
60
30
0
0
20
40
60
80
Starch concentration (%)
Figure 7. Starch gelatinization (, ) and crystallization (x)
temperatures of starch dispersions prepared with a glycerol
solution () or pure glycerol (, x) with various starch concentrations (SC).
16
Gelatinization enthalpy
(J.g–1 of starch)
Exothermal heat flow
14
0.02 W.g
–1
12
10
8
6
4
2
0
20
40
60
80
100
120
Temperature (°C)
Figure 6. DSC curve of starch dispersion (50%) prepared with
pure water.
Braz. J. Food Technol., v. 11, n. 4, p. 263-270, out./dez. 2008
0
20
40
60
80
Starch concentration (%)
Figure 8. Gelatinization (∆hsg, , ) and crystallization (∆hc, x)
enthalpy of starch dispersions prepared with a glycerol solution
(), or pure glycerol (, x) with various starch concentrations
(SC).
268
www.ital.sp.gov.br/bj
Effect of the starch concentration on phase transitions of starch dispersions prepared
with a glycerol solution or pure glycerol
SOBRAL, P. J. A. et al.
4 Conclusions
It can be concluded from this work, that the glycerol molecules interact with the cassava starch granules
before the starch gelatinization takes place, affecting all
phase transitions of the starch dispersions, but depending
on the starch concentration. At very low temperatures, all
phenomena related with the glass transition were visible
in most of the starch dispersions, except for those with
the highest starch concentration (70%). Melting of ice was
observed only for starch dispersions prepared with the
glycerol solution. No ice melting signal was observed for
SD prepared using pure glycerol. At higher temperatures,
cassava starch gelatinization occurred, being affected by
both the starch and the glycerol concentration.
According to the results obtained in this work, it is
important to observe that to prepare a film forming solution
based on cassava starch using the casting technique,
the thermal treatment of the starch dispersion should be
different according to whether the glycerol was added
to the film forming solution before or after this treatment.
In addition, for the production of films using a dry technique such as extrusion, the use of pure glycerol should
be preferred. Although it is possible to attain similar
gelatinization temperatures using a more dilute glycerol
solution, the presence of a great amount of water in the
samples may allow for expansion of the material at the
outlet of the extruder.
Acknowledgments
To FAPESP for the research grants (04/08771-7,
05/57781-8), for the PD fellowship awarded to PVAB
(05/54688-7) and the PV fellowship to JSF (05/54952-6),
and to CNPq for the PQI fellowship awarded to PJAS. JSF
acknowledges the Instituto Politécnico Nacional in Mexico.
This work was part of the Project CYTED XI.20.
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