Understanding the destructuration of starch in water-ionic liquid mixtures

Article abstract of DOI:10.1039/c4gc01248h

The destructuration of native maize starch in mixtures of water and ionic liquids (ILs) containing acetate anions was studied in dynamic heating conditions, combining calorimetry, rheology, microscopy and chromatographic techniques. A phase diagram of starch in water-IL solutions was established. The phase transitions undergone by starch include the typical endothermic gelatinization phenomenon for IL-water ratios lower than 0.5, while for mixtures with a higher ionic liquid content, a complex exothermic phenomenon combining mild degradation and solubilization takes place. This results in an optimum destructuration temperature as low as 40-50 °C for an IL-water ratio close to 0.7. In addition, specific macromolecular chain breaking reactions appear to take place, depending on the nature of the cations present, resulting in different macromolecular structures of the recovered starch. These results suggest the possibility of using solvent media design for a controlled modification of starch macromolecular characteristics.

Full text of DOI:10.1039/c4gc01248h

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Understanding the destructuration of starch in
water–ionic liquid mixtures†
Cite this: DOI: 10.1039/c4gc01248h
L. S. Sciarini,*a,c A. Rolland-Sabaté,b,c S. Guilois,b,c P. Decaen,a,b E. Leroya,b and
P. Le Bailb,c
The destructuration of native maize starch in mixtures of water and ionic liquids (ILs) containing acetate
anions was studied in dynamic heating conditions, combining calorimetry, rheology, microscopy and
chromatographic techniques. A phase diagram of starch in water–IL solutions was established. The phase
transitions undergone by starch include the typical endothermic gelatinization phenomenon for IL–water
ratios lower than 0.5, while for mixtures with a higher ionic liquid content, a complex exothermic
phenomenon combining mild degradation and solubilization takes place. This results in an optimum
destructuration temperature as low as 40–50 °C for an IL–water ratio close to 0.7. In addition, specific
macromolecular chain breaking reactions appear to take place, depending on the nature of the cations
present, resulting in different macromolecular structures of the recovered starch. These results suggest
the possibility of using solvent media design for a controlled modification of starch macromolecular
characteristics.
Received 4th July 2014,
Accepted 18th August 2014
DOI: 10.1039/c4gc01248h
www.rsc.org/greenchem
philic. However, when suspended and heated in excess water,
starch undergoes an order–disorder transition called gelatini-
1. Introduction
As renewable resources become essential for industry, the crea- zation. During this phenomenon, starch granules swell and
tive design and application of innovative technology for the amylose progressively leaches out of the granules, and the
optimization of such resources is a research topic raising huge semi-crystalline structure is disrupted. Although some starch
interest in the past decade.1 Among all polysaccharides investi- molecules are readily solubilized in water, some granule rem-
gated as potential alternatives to conventional oil based plas- nants may still be present even after gelatinization has
tics, starch has attracted a large amount of attention.2 Starch occurred. Thus, starch insolubility represents a problem when
is one of the most abundant biopolymers in nature, and con- trying to obtain homogeneous amorphous materials.
sidering its low cost, renewability and biodegradability, it can
be considered as a raw material for the fabrication of biologi- solvents for biopolymers has generated lots of interest. ILs are
cally degradable materials. room-temperature molten salts; since they present high
In recent years, the performance of ionic liquids (ILs) as
Starch is composed of two different glucose polymers: thermal stability and are not volatile, it has been reported that
amylose, a predominantly linear macromolecule formed from they offer an alternative to common organic solvents. For this
α(1→4) linkages with a molar mass of ∼105–106 g mol−1, and reason and because they are easily recyclable, and even some
amylopectin, a massive multiply branched polymer containing of them are biodegradable,4 they have been classified as ‘green
both α(1→4) and α(1→6) linkages with a molar mass of ∼107– solvents.’ For these reasons, they have attracted enormous
109 g mol−1. Starch is synthesized in the form of densely attention over the past decade, becoming a very important area
packed granules, containing both amorphous and crystalline of research.5 Over the last few years, the use of ILs to dissolve
regions.3 Given its granular structure, starch shows low solubi- and process starch has been reported.6–10 While the first
lity in any conventional solvent despite being highly hydro- reports focusing on ionic liquids containing chlorine anions
showed strong depolymerisation of starch, thus limiting poten-
tial applications,9 more recent works on acetate based ionic
liquids are more promising,8 despite the fact that no clear
evaluation of starch degradation in such systems has been
communicated by the authors. In the presence of chlorine
anions, the macromolecular degradation of starch has been
related to the acidic hydrolysis of glycosidic bonds.6,9,11–13
According to Mateyawa et al.8 the presence of acetate based
aLUNAM Université, CNRS, GEPEA, UMR 6144, CRTT, 37, Boulevard de l’Université,
44606 St Nazaire Cedex, France. E-mail: losciarini@agro.unc.edu.ar
bUR1268 Biopolymères Interactions Assemblages, INRA, F-44300 Nantes, France
cStructure Fédérative IBSM, INRA Nantes-Angers, Rue de la Géraudière, BP 71627,
44316 Nantes cedex 3, France
†Electronic supplementary information (ESI) available: µDSC, FTIR, 13C CP/MAS
NMR results and ESEM images are provided. See DOI: 10.1039/c4gc01248h
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ionic liquids is likely useful for avoiding such phenomena. starch. A reference cell was prepared by adding the same water
The same authors also reported an exothermic transition content as in the sample cell. The sample was stirred (50 rpm
when starch was heated in pure and concentrated 1-ethyl-3- at room temperature) for 1 h before being heated from 20 °C
methylimidazolium acetate (EMIMAc)–water solutions and it to 120 °C and cooled from 120 °C to 20 °C in the µDSC
was proposed that this exothermic transition was due to starch (µDSC7evo, Setaram, Caluire, France) at a heating/cooling rate
dissolution, without gelatinization. Stevenson et al.9 reported of 1 °C min−1. The onset (To), peak (Tp) and conclusion temp-
no enthalpic transition when analysing recovered starch, pre- eratures (Tc), and the enthalpy of the transition (ΔH) were
viously treated with 1-butyl-3-methylimidazolium chloride, determined using Calisto software (Calisto v1.32 DB v1.33). All
suggesting that after being heated in ILs, the starch is destruc- mixtures were analysed in duplicates.
tured and no further gelatinization can be observed when re-
heating in water.
2.2.2. Macromolecular characterization of samples. High
Performance Size Exclusion Chromatography coupled with
As recalled by Brennecke et al.14 one of the major advan- Multi-Angle Laser Light Scattering (HPSEC-MALLS) was used
tages of ILs is that they offer the possibility of being tailored by for characterization of the samples. The starch was suspended
modifying the chemical structure of the cation and anion moi- in the IL–water solutions and stirred for 1 h. These suspen-
eties. Consequently, in the present work, we focus on the influ- sions were then heated in an oil bath (Ministat 240, Huber,
ence of the cation by comparing the thermal destructuration Offenburg, Germany), which was used to mimic the dynamic
of starch mixed with water and two acetate based ionic liquids: heating applied by the µDSC: from 20 °C to 120 °C at 1 °C
EMIMAc and cholinium acetate. The latter presents the advan- min−1. After this thermal treatment, the samples were pre-
tage of very low toxicity, choline being an essential nutrient treated with DMSO, precipitated with ethanol, dried and solu-
and thus biocompatible.
bilized by microwave heating under pressure, as previously
described by Rolland-Sabaté et al.16 Each suspension of the
sample in water at a concentration of 0.5 g L−1 was heated for
40 s (maximal internal temperature reached: 152 °C) at 900 W.
2. Experimental
2.1. Materials
The starch solutions were then filtered through
5 µm
Durapore TM membranes (Waters, Bedford, MA, USA). Con-
centrations of the carbohydrate were determined by the sul-
phuric acid–orcinol colorimetric method described by
Planchot et al.17 Sample recoveries were calculated from the
ratio of the initial mass and the mass after filtration. Solutions
were immediately injected into the HPSEC-MALLS-system. The
equipment and the method used were the same as that
described previously.18 The SEC column was a Shodex®
KW-802.5 (8 mm ID × 30 cm) with a KW-G guard column
(6 mm ID × 5 cm) both from Showa Denko K. K. (Tokyo,
Japan). They were maintained at 30 °C. The two on-line detec-
tors were a Dawn® Heleos® MALLS system fitted with a K5
flow cell and a GaAs laser, (λ = 658 nm), supplied by Wyatt
Technology Corporation (Santa Barbara, CA, USA) and a
RID-10A refractometer from Shimadzu (Kyoto, Japan). The
eluent (Millipore water containing 0.2 g L−1 of sodium azide)
was carefully degassed and filtered on-line through Durapore
GV (0.1 µm) membranes from Millipore (Millipore, Bedford,
MA, USA), and eluted at 0.5 mL min−1. Sample recovery rates
were calculated from the ratio of the mass eluted from the
column (integration of the refractometric signal) and the
injected mass. These last values were determined using the
sulfuric acid–orcinol colorimetric method.17
Regular corn starch (Maritena 100) was purchased from Tate &
Lyle (Paris, France) with an initial moisture content of 12%.
EMIMAc was produced by BASF and supplied by Sigma-
Aldrich. Before use, both materials were dried with P2O5 under
vacuum at room temperature for one week. After this time, the
starch moisture was lower than 3%. Cholinium acetate
(CholAc) was synthesized by a metathesis reaction.15 Equi-
valent amounts (0.06 mol each) of cholinium chloride and pot-
assium acetate (both purchased from Aldrich) were dissolved
in absolute ethanol, and mixed and stirred for 1 hour at room
temperature. A white precipitate of potassium was formed and
removed by filtration. The ethanol was evaporated on a rotary
evaporator. The CholAc thus produced was freeze-dried prior
to use. After purification and freeze-drying, the melting point
of CholAc was 83 °C.
For analysis, the starch was suspended in aqueous solu-
tions of varying IL concentrations (from 0% w/w to 100% w/w
IL). Since different concentrations of starch were also studied,
a phase diagram was prepared.
ILs are known to be highly hygroscopic, thus the sample
preparation was carried out in a glove box under dry gas
purge.
ˉ
ˉ
ˉ
ˉ
Mn, Mw, the dispersity (Mw/Mn), and the radius of gyration,
2.2. Methodology
ˉ
RG (nm), were established using ASTRA® software from WTC
2.2.1. Micro differential scanning calorimetry (µDSC). Mix- (version 6.1 for PC), as previously described by Rolland-Sabaté
tures of 20% w/w starch in different solvents were prepared. et al.16,18 A value of 0.146 mL g−1 was used as the refractive
The solvent composition varied from 0% IL (pure water), to index increment (dn/dc) for glucans and the normalization of
100% EMIMAc or 95% CholAc (due to its high melting point, photodiodes was achieved using a low molar mass pullulan
5% water was added to CholAc; the melting point of CholAc standard (P20).
95% was 53 °C). Appropriate amounts of IL and water were
2.2.3. Rapid Visco Analyser (RVA). Viscosity properties of
weighed and thoroughly mixed before the addition of the the starch in different solutions were studied with a Rapid
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Visco Analyser (RVA-3, Newport Scientific Pty. Ltd., Australia).
Starch suspensions (7.5% w/w) were prepared by weighing the
solvent in a canister and adding starch slowly while stirring.
3. Results and discussion
3.1. Differential scanning calorimetry (µDSC)
The slurry was heated from 20 °C to 95 °C while being stirred The thermal behaviour of the starch mixed with aqueous IL
at 960 rpm for the first 10 s and then at 160 rpm until the solutions of different concentrations was monitored by µDSC
assay was completed. The heating rate was 10 °C min−1. It was (Fig. 1). Increasing concentrations of the ILs were used from
held at 95 °C for 10 min, and finally cooled to 20 °C at a the bottom to the top of the figure. When the IL concentration
cooling rate of 6.7 °C min−1. The pasting temperature (Tp), was low, the starch underwent a typical gelatinization, rep-
peak viscosity (PV), final viscosity (FV), breakdown (BD) and resented by an endothermic transition (a second endothermic
setback (SB) were obtained from the pasting curve. Samples transition can be observed at around 100 °C, and is ascribed
were assessed in duplicate.
to the melting of amylose–lipid complexes). The fact that the
2.2.4. Microscopy. The microstructure of the starch sus- gelatinization shifts to higher temperatures with increasing IL
pended in different IL solutions before and after the enthalpic concentration is consistent with the previously described effect
transitions was analysed with a light microscope, LEICA due to the presence of different salts in aqueous solution.19,20
DMRD. Light, polarized light and differential interference con- The effect of the salts on starch gelatinization has been found
trast images were obtained. Suspensions of the starch were to follow the Hofmeister series, with kosmotropes (structure
prepared in the same way as the samples analysed by µDSC. making, salting-out) delaying gelatinization and chaotropes
The appropriate amount of IL, water (when required) and (structure breaking, salting-in) accelerating it. Acetate is a well-
starch were weighed. These mixtures were stirred for 1 h and known kosmotrope, so it was expected to produce a shift of
then heated in an oil bath (Ministat 240, Huber, Offenburg, gelatinization toward higher temperatures. Nevertheless, a
Germany) at 1 °C min−1 until the corresponding temperature further increase in IL concentration (up to 50% for EMIMAc
was reached. Samples were then removed from the oil bath and 60% for CholAc) led to a decrease in the gelatinization
and cooled in an ice bath. The samples were immediately temperature. This trend will be discussed below (see Macromole-
observed under the microscope.
cular characteristics).
2.2.5. Statistical analysis. The data obtained were statisti-
If we now consider high IL concentrations (from the top to
cally treated by variance analysis, while means were compared the bottom, Fig. 1) an exothermic transition is present. The
by the Fisher LSD test at a significance level of 0.05 (INFOSTAT highest concentration of CholAc studied was 95%, but these
statistical software, Facultad de Ciencias Agropecuarias, Uni- results were not included in the figure since the exothermic
versidad Nacional de Cordoba, Argentina).
peak was not complete at 120 °C, which is the upper limit for
Fig. 1 Micro differential scanning calorimetry thermograms for regular corn starch heated in EMIMAc–water (left) and CholAc–water (right)
solutions.
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Table 1 µDSC results for regular corn starch heated in IL–water solutionsa
Solvent
Transition
Endo
ΔH (J g−1
)
To (°C)
Tp (°C)
Tc (°C)
0% IL (pure water)
11.9 0.2a
60.3 0.1b
67.2 0.0b
73.3 1.4d
10% CholAc
20% CholAc
30% CholAc
50% CholAc
60% CholAc
70% CholAc
80% CholAc
90% CholAc
95% CholAc
Endo
Endo
Endo
Endo
Endo
Exo + endo
Exo
13.7 0.0b
14.3 0.9bc
16.2 0.0d
16.2 0.5d
14.4 0.9bc
nd
39.5 1.4B
67.2 12.7C
nd
72.5 0.5c
77.5 0.2de
80.3 0.5e
73.9 0.4cd
58.7 4.7b
46.7 0.2B
68.2 2.4E
85.9 1.1F
97.8 2.5G
78.4 0.1c
82.7 0.1d
85.7 0.6d
78.7 0.2c
65.1 3.4b
nd
78.4 1.4CD
103.7 2.7E
nd
84.6 0.4ef
86.7 3.4fg
89.8 0.1g
83.1 0.1e
69.6 2.6c
66.9 0.8b
88.4 1.9CD
114.3 3.4E
nd
Exo
Exo
10% EMIMAc
20% EMIMAc
30% EMIMAc
50% EMIMAc
60% EMIMAc
70% EMIMAc
80% EMIMAc
90% EMIMAc
100% EMIMAc
Endo
Endo
Endo
Endo
None
Exo
Exo
Exo
Exo
14.0 0.3b
14.7 0.4bc
15.5 0.3cd
12.5 0.6a
nd
17.3 4.5A
63.3 2.3C
110.8 5.3D
180.7 20.8E
70.7 1.0c
73.1 0.4cd
72.9 0.4c
51.5 0.3a
nd
36.2 0.8A
48.9 1.7B
56.2 0.5C
62.3 0.6D
76.6 0.9c
78.4 0.2c
77.7 0.5c
56.4 0.3a
nd
46.7 0.9A
65.0 1.9B
74.8 0.5C
80.1 0.3D
82.9 0.8d
84.8 0.2ef
83.7 0.3ef
62.9 0.9a
nd
52.2 0.6A
72.5 0.8B
85.7 0.8C
92.4 0.3D
a ΔH: transition enthalpy; To: onset temperature; Tp: peak temperature; Tc: conclusion temperature. Values followed by different lowercase letters
in the same column are significantly different (p < 0.05). Values followed by different uppercase letters in the same column are significantly
different (p < 0.05).
the µDSC; the peak onset temperature is presented in Table 1. treatment recoveries were between 95% and 100% for all
This exotherm starts at lower temperatures when water is samples. DMSO pre-treatment is known to remove the poly-
added and the heat released (ΔH) is also decreased. There is a saccharide oligomers with degree of polymerization (DP) lower
critical concentration (depending on the IL used) at which than 12. Thus, this high recovery percentage shows that
both transitions (exo- and endothermic) take place: 70% the heating of samples in different IL solutions does not
CholAc and 60% EMIMAc. In the former case, both transitions induce the apparition of sugars with DPs smaller than 12. The
can be observed, but in the latter both phenomena seem to solubilisation recovery rates and the elution recoveries of the
happen at very close temperatures thus probably cancelling starches were higher than 90%. The high sample recovery
one another.
values obtained here indicate that the fractionation response
The same behaviour was also observed by Mateyawa et al.8 was quantitative for all the samples. Overall, this solubilisation
working with EMIMAc and by Koganti et al.21 using N-methyl procedure was thus considered to enable the structural charac-
morpholine N-oxide (NMMO). These authors attributed the terization of these samples.
exothermic transition to starch dissolution in these solvents.
Fig. 2a presents chromatograms for starch heated in pure
Enthalpy values for the exotherm of normal corn starch in water and in EMIMAc solutions. When considering starch
NMMO were 17.5 J g−1 (no enthalpy change was observed heated in pure water, two peaks were observed for the differen-
when increasing NMMO concentration from 70 to 78%), tial refractive index signal (corresponding to the concentration
whereas Mateyawa et al.8 did not provide any ΔH value. In the of the chains). The first and bigger one (peak I, 5.8 mL) corre-
present study, ΔH of the exothermic transition ranged between sponds to amylopectin population, while the second, and
17.3 J g−1 (70% EMIMAc) and 180.7 J g−1 (100% EMIMAc) smaller, to amylose (peak II, 6.6 mL). When analysing the
(Table 1). Moreover, when heated at low rates (0.1 °C min−1), starch–EMIMAc 100% chromatogram, two peaks are also
two peaks were clearly observed by µDSC (data presented in observed; nevertheless, some important features can be high-
the ESI, Fig. S1†); this finding indicates that more than a lighted: (1) the first peak started to elute at higher volumes,
single phenomenon would be responsible for the exothermic indicating a lower size for these molecules as the elution
transition. The same trends were observed at different starch– volume is inversely proportional to the molecular size, (2) the
solvent ratios (data not shown).
second peak is bigger than the first one, and (3) no evident
shift in peak II is observed. In addition the molar mass is
clearly lower for each fraction of starch–EMIMAc 100% com-
3.2. Macromolecular characterization of treated starches
In order to understand the phenomena underlying the pared to starch–pure water solutions. Overall, these features
exothermic transition, the macromolecular properties of starch indicate that amylopectin is depolymerized when heated in
treated with ILs were studied using the HPSEC-MALLS system. EMIMAc, which explains the shift of the amylopectin peak
To this end, the starch was treated with different IL–water solu- (which accounts for a smaller size), while there is co-elution of
tions, recovered and pre-treated with DMSO. The DMSO pre- the depolymerisation products and amylose, thus explaining
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Table 2 Weight-average molar mass (M¯ w), dispersity (M¯ w/M¯ n) and
z-average radius of gyration (R¯G) determined by HPSEC-MALLS for corn
starch samples treated in ILs solutions
Whole population
Mw (107) (g mol−1
)
Mw/Mn
ˉ
ˉ
ˉ
ˉ
RG (nm)
Solvent
Pure water
44.36 0.63
31.43 2.93
23.08 0.15
8.78 0.02
33.13 0.18
33.57 2.69
32.3 0.07
8.11 0.00
7.56 0.49
10.59 0.31
6.65 1.37
7.10 0.04
11.09 0.37
8.44 0.89
5.05 0.06
4.48 0.05
302.8 1.8
275.5 10.7
247.6 1.4
225.8 1.1
279.2 0.6
268.6 9.5
282.4 2.3
208.5 0.0
EMIMAc 50%
EMIMAc 70%
EMIMAc 100%
CholAc 60%
CholAc 70%
CholAc 80%
CholAc 95%
EMIMAc 100%, respectively, compared to starch heated in
pure water. Interestingly, when treated with CholAc, the
ˉ
reduction in Mw is non-linear, and reductions are 25%, 24%,
27% and 81% for CholAc 60%, CholAc 70%, CholAc 80% and
ˉ
CholAc 95%, respectively. The same trend is observed for RG.
This indicates that both ILs have a different response in the
presence of water, with rather small quantities of water
(20%) significantly reducing the depolymerisation caused by
CholAc.
ˉ
ˉ
The dispersity (Mw/Mn) decrease for the samples treated
with ILs (from 7.56 to 7.10 and 4.48 for EMIMAc 100% and
CholAc 95%, respectively) is linked to the reduction of the
overall peak broadness, and explained by the reduction of the
amylopectin molar mass.
Although the two acetate based ionic liquids tested do not
completely avoid starch depolymerisation, the reductions of
the molar masses found in this study are very different to
Fig. 2 Chromatograms of regular corn starch treated with EMIMAc (a)
and CholAc (b) solutions. Differential refractive index (DRI) and molar
mass (Mw) versus elution volume.
the increased area of the second peak. Finally, no evidence of those obtained after treating starches with halide based imida-
amylose depolymerisation is found (no shift of the peak II). zolium IL, where reductions of 1–3 orders of magnitude can
For the samples treated with EMIMAc 70%, amylopectin also be observed.6,9,12
eluted at higher volumes, although the overall profile and
From Table 2 it can be observed that a slight depolymerisa-
molar mass distribution are more similar to that of pure water. tion takes place when treating starch with EMIMAc 50% and
For EMIMAc 50%, no shift of the amylopectin peak was CholAc 60%, even though no exothermic transition was
observed, but the area of peak II is bigger than for starch observed by µDSC. This finding may explain why the gelatini-
treated with water, indicating the presence of amylopectin zation shifts to lower temperatures and ΔH decreases when
depolymerisation products. Nevertheless, since mild depoly- the starch is heated in µDSC with these IL solutions, since this
merisation occurs under these conditions (Table 2) the detec- mild depolymerisation may facilitate starch swelling, shifting
tor response to amylopectin is still high.
gelatinization toward lower temperatures.
For the starch heated in 95% CholAc, only one peak could
Moreover, the significantly lower molar masses observed
be clearly detected, while the amylopectin fraction is rep- for the amylopectin populations (peak I, Fig. 2) in EMIMAc
resented by a shoulder (Fig. 2b) and the molar mass is smaller 100% and particularly in CholAc 95% treated samples com-
for each elution volume. This accounts for the depolymerisa- pared to starch–pure water solutions account for a less dense
tion of amylopectin by CholAc as well. When water was added structure (as these fractions exhibit the same size because
to CholAc, a shift of amylopectin elution toward higher elution volume is proportional to size in HPSEC). One can
volumes is still present, but again the overall behaviour is deduce that the original amylopectin population is linearized
more similar to that of pure water.
after heating in the ILs, and further in CholAc.
ˉ
ˉ
Table 2 shows Mw and RG values obtained by integrating the
To summarize, it is clear that the starch is depolymerized
signals for the whole population of molecules present in the when heated in IL and that the depolymerization pattern varies
ˉ
sample. A progressive and linear reduction in Mw is observed according to the cation nature, not only to anion characteristics.
when the EMIMAc concentration is increased, with a reduction Though at present it is not possible to propose a mechanistic
of 29%, 48% and 80% for EMIMAc 50%, EMIMAc 70% and explanation for this differential behavior, these results
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suggest the possibility of tailoring the ionic liquid for a con- cular species was monitored by FTIR and NMR. No significant
trolled modification of the macromolecular characteristics of changes were found between starch heated in water or the ILs.
starch through mild depolymerisation during destructuration.
The possible interaction between the starch and the IL
These results are presented in the ESI (Fig. S2 and S3†).
It is also possible that the mechanism involves not only
during heating that could lead to the formation of new mole- starch and the IL, but also water molecules. For future studies,
Fig. 3 Light, polarized-light and differential interference contrast images for starch treated in ILs. (a) Light and polarized-light images of starch
heated in EMIMAc solutions (To); (b) differential interference contrast and polarized-light images of starch heated in EMIMAc solutions (Tc), and (c)
differential interference contrast and polarized-light images of starch heated in CholAc solutions (Tc). Bar scale: 20 µm.
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a possibly fruitful approach for trying to understand the inter-
Interestingly, starch suspended in EMIMAc 70% and
actions between these three components and their influence CholAc 80% is completely destructured at 56 °C and 92 °C,
on the destructuration mechanism would be the use of mole- respectively, and under these conditions, only mild depolymer-
cular simulation. A recent paper showed the particular interest isation occurs (Table 2).
in this tool for understanding the interactions in the case of
When EMIMAc 100% and CholAc 95% were used, a few gas
cellulose dissolution in IL–water and IL–DMSO mixtures.22 It bubbles were observed under the microscope. These gas
would also be interesting to study the destructuration of starch bubbles may indicate the formation of volatile products, but
in IL–DMSO mixtures, since these simulations show that the could not be identified.
co-solvent nature plays an important role in cellulose dissol-
ution by IL.22
These results are supported by images obtained with an
Environmental Scanning Electron Microscope (ESEM) for
starch heated in pure water, pure EMIMAc and CholAc 95%. In
these images, the destructuration/solubilisation process is
evidenced (Fig. S4†).
3.3. Microstructure of starch suspensions
Fig. 3 presents some representative images of corn starch
heated in the IL solutions under light and polarized-light
microscopy (Fig. 3a), and differential interference contrast and
3.4. Rapid Visco Analyser (RVA)
polarized-light microscopy (Fig. 3b and c). Fig. 3a presents RVA is an empirical study commonly performed on starch slur-
images of the samples before the enthalpic transition ries to follow the viscosity behaviour as the sample is heated.
(endothermic or exothermic depending on the water content) While heated, the starch granules start to retain solvent and
for the starch suspensions heated in EMIMAc solutions where swell, which results in a concomitant increase in viscosity, i.e.
a well-defined granular structure can be observed (similar viscosity onset temperature. The viscosity of the suspension
images were obtained for CholAc, data not shown). Images of increases to the point where the number of swollen intact
the starch suspensions after the enthalpic transition when starch granules is maximal. The peak viscosity (PV) is indica-
heated in EMIMAc and CholAc solutions are presented in tive of the solvent-binding capacity. When the temperature
Fig. 3(b and c). From these images, it can be observed that, increases and the granule absorbs as much solvent as to
when heated in pure water, starch granules are gelatinized: achieve its rupture point, the viscosity decreases to
a
swelled and deformed with no remaining crystallinity. minimum. This decrease in viscosity is called breakdown (BD).
However, when EMIMAc is added (30 and 50%), gelatinization When the starch suspension cools, the amylose retrogrades
is not complete, since some granules are still birefringent as a (re-crystallizes), resulting in an increase in viscosity named
result of their crystallinity. When the amount of EMIMAc setback (SB), until a gel is formed at the end of the test. In this
reached 70%, however, no polarization was evident after study, the viscosity onset temperature correlated with the
heating, nor the presence of granular remnants; the same was µDSC onset temperature, although the former was higher than
observed for EMIMAc 100%. The absence of granular rem- the latter (Tables 1 and 3).
nants may indicate that starch is not only depolymerized in
It has been established that the viscosity onset temperature
concentrated EMIMAc solutions, but is also solubilized. Both is higher than the gelatinization onset temperature,23 since
phenomena, leading to an overall starch destructuration, may different techniques detect starch transitions in different ways,
account for the exothermic transitions observed by µDSC. giving slight differences in the determined parameters.
Fig. 3c shows the same behaviour when CholAc was used, but
Fig. 4 presents the viscosity profiles of starch in EMIMAc
80% of CholAc was necessary to achieve complete destructura- (Fig. 4a) and CholAc (Fig. 4b) solutions. The ILs alone were
tion, since at a lower concentration (70%) granular remnants also analysed: their viscosity was near zero and no change in
were present.
viscosity was observed during the heating and cooling pro-
Table 3 RVA parameters for regular corn starch heated in IL–water solutionsa
Pasting
temperature (°C)
Solvent
Peak viscosity (cP)
1444 119a
Trough (cP)
1049 42ab
Breakdown (cP)
374 9b
Final viscosity (cP)
2673 271b
Setback (cP)
1623 229a
0% IL (pure water)
72.5 0.5c
30% CholAc
50% CholAc
70% CholAc
95% CholAc
2700 41b
5473 399d
nd
2264 18d
4754 339e
nd
nd
nd
nd
nd
1740 27a
4332 721c
nd
nd
nd
nd
nd
90.9 0.8f
84.9 0.1e
nd
nd
nd
nd
nd
30% EMIMAc
50% EMIMAc
70% EMIMAc
100% EMIMAc
3568 76c
8249 20e
3071 95b
2849 391b
1387 38bc
1548 20c
815 16a
2181 38c
6701 0d
2256 79c
168 22a
3994 76c
4289 62c
3868 36.8c
9939 308.6d
2607 38b
2742 42b
3053 21c
7255 105d
80.6 0.5d
62.6 0.4b
55.7 0.9a
78.8 2.1d
2681 412d
a Values followed by different lowercase letters in the same column are significantly different (p < 0.05).
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During the cooling stage, the viscosity increases significantly,
probably as a consequence of the interaction between the pro-
ducts of depolymerisation, which are smaller and less
branched than amylopectin, favouring their association, and
also as a consequence of amylose retrogradation. As water is
added (EMIMAc 70%), the viscosity onset temperature and
peak viscosity are lower (in agreement with µDSC results,
Table 1). The decrease in the viscosity onset temperature is
related to the lower viscosity of the solvent when compared to
pure EMIMAc, and its diffusion into the granule would be
faster facilitating depolymerisation. However, starch depoly-
merisation is lower in EMIMAc 70%, explaining the lower vis-
cosity value during heating when compared to pure EMIMAc
(Table 3). On increasing the water content to 50% (EMIMAc
50%), solvent diffusion into the granules increases and the
amount of water is enough to gelatinize the starch. In
addition, a slight depolymerisation is also observed with
EMIMAc 50% (Table 2). Both phenomena may explain the
higher viscosity shown by this sample (Table 3). When
EMIMAc 30% is used, the pasting behaviour is closer to that of
starch in pure water, although the overall viscosity is higher.
These results are in good agreement with Mateyawa et al.8
Fig. 4b shows that the starch in CholAc solutions has a
different behaviour than in EMIMAc solutions. When heated
in the concentrated CholAc solution (CholAc 95%), two peaks
are present: at the beginning of the test, CholAc is in the solid
state – explaining the high viscosity of the sample at this point
– but as the temperature increases, it melts; a second increase
in the viscosity is observed during the cooling period. The
Fig. 4 RVA curves for starch heated in EMIMAc (a) and CholAc (b) exothermic peak (observed by µDSC) for this sample starts at
solutions.
around 97 °C (Table 1); this may explain the absence of a vis-
cosity peak during heating. However, some depolymerisation
may have occurred during the heating at 95 °C, forming
smaller and more linear molecules from amylopectin, explain-
ing the slight increase in viscosity during cooling. CholAc 70%
could not be analysed since the viscosity exceeded the RVA
limit (10 000 cP).
cesses. From Fig. 1 it can be seen that the starch heated in con-
centrated EMIMAc solutions (100% and 70%) undergoes an
exothermic transition, related to starch depolymerisation/solu-
bilisation; when the amount of EMIMAc is 50% or lower, an
endothermic transition – ascribed to gelatinization – takes
place. Fig. 4a shows that when EMIMAc 100% is used, the vis-
cosity increases as depolymerisation/solubilisation take place.
For CholAc 50% and 30%, an increase in the viscosity was
observed between 85 and 90 °C (Table 3), and no viscosity
Fig. 5 Phase diagrams for starch treated in solution with different IL concentrations. (a) EMIMAc treated starch, (b) CholAc treated starch.
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breakdown was found. The viscosity increase started late
during heating, while the maximum temperature reached by
the RVA is 95 °C. This temperature may not be sufficient to
completely disrupt the starch granular structure, although
granules may swell and some amylose may leach out, resulting
in a viscosity increase.
Notes and references
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The results presented in this study show that two different
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Acknowledgements
Authors gratefully thank the Région des Pays de la Loire for
financial support. We would also like to thank Bérénice Houin-
sou-Houssou for her kind technical support, and Brigitte
Bouchet, Bénédicte Bakan and Guillaume Roelens for their
support in microscopy, FTIR and ESEM analysis, respectively.
This journal is © The Royal Society of Chemistry 2014
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