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Plasticisation and Mobility in Starch-Sorbitol Films
Journal of Cereal Science 29 (1999) 273–284 Article No. jcrs.1998.0236, available online at http://www.idealibrary.com on
Plasticisation and Mobility in Starch-Sorbitol Films S. Gaudin∗†, D. Lourdin†, D. Le Botlan‡, J. L. Ilari∗ and P. Colonna† ∗Ecole Nationale d’Inge´nieurs des Techniques des Industries Agricoles et Alimentaires BP 82225, 44322 NANTES, Cedex 03, France; †Institut National de la Recherche Agronomique BP 71627, 44316 NANTES, Cedex 03, France; ‡Faculte´ des Sciences et des Techniques, UPRES A 6006, BP 82207, NANTES Cedex 03, France Received 31 March 1998
ABSTRACT The purpose of this study was to elucidate the molecular mechanism involved in the behavioural changes observed at macroscopic level in a starch-sorbitol-water system. At a low sorbitol content (<27%), maximum stress decreased and the yield at break was quite constant at around 5%. This behaviour was compared with antiplasticisation, a well-known phenomenon occurring with synthetic polymers. The study of storage loss modulus by dynamic thermal mechanical analysis (DTMA) confirmed antiplasticisation. Sorbitol at high content (>27%) acts as a plasticiser. The two relaxations observed with DTMA, a associated with glass transition (occurring at high temperature, >30 °C) and two occurring at low temperatures (<10 °C), were involved in behavioural changes. System mobility as a function of sorbitol content was determined by time domain nuclear magnetic resonance. Much of this study was performed by replacing H2O with deuterium oxide to mask the signal from water and therefore analyse only the starch and sorbitol signals. The results show the existence of two competitive effects: one associated with a decrease in mobility and the other with the enhanced mobility of the system. The relative importance of these two effects seems to be reversed, depending on sorbitol content, which could account for the changes observed at the macroscopic level. 1999 Academic Press Keywords: starch, sorbitol, plasticisation, antiplasticisation, NMR, DMTA.
INTRODUCTION Dry foods are of considerable industrial importance. Although their microbiological stability is ensured by their low aw, chemical (oxidation by oxygen and light) and structural (crispness) stabilities are not as easy to control. Polymer science is increasingly applied to elucidation of the structurefunction relationships in food systems1. Glass transition theory has been widely studied in polymer science and is a particularly useful means of improving our knowledge of the food system. The glass transition of an amorphous polymer char-
: ds=dry starch; DMTA=dynamic mechanical thermal analysis; NMR=nuclear magnetic resonance; Tg=glass transition; aw=activity of water. Corresponding author: D. Lourdin. 0733–5210/99/030273+12 $30.00/0
acterises the passage from a vitreous to a rubbery state. The structures are different in both domains. The importance of the study of glass transition temperature (Tg) has been demonstrated2 for many foods such as cookies or snacks in which the preservation of crispness is important. Products are generally stable when they are stored below the glass transition temperature, so that an understanding of this parameter is fundamental to controlling the properties, quality and stability of such products. Such foods are always complex in their formulation, being mainly composed of biopolymers (starch, proteins) that are largely responsible for the crisp texture of the final product. Addition of low molecular weight solutes (sugars, salt) causes important changes in the interactions between the different components that are responsible for the final properties of the product. In studies of structure-function relationships, a first 1999 Academic Press
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approach involves the use of binary or ternary model systems. Wheat starch, a biopolymer present in many foods, was chosen for the present study. Other elements considered were sorbitol, the starch plasticiser commonly used in the food industry, and water. Dynamic thermal mechanical analysis (DMTA) and nuclear magnetic resonance (NMR) were employed to determine the contribution of the various components to the system properties. EXPERIMENTAL Materials The starch used for film preparation was an A-type wheat starch (27% amylose, 73% amylopectin) of commercial origin (F33, Olgivie Aquitaine). Sorbitol of technical quality was obtained from Prolabo (RCS Cre´teil B 542 081 419), and D2O (99·9% D) from Euriso-top (CEA). Methods
Film preparation Films were obtained by the casting method. Native wheat starch was solubilized in a high-pressure reactor at 138 °C for 20 min using a 6% suspension in ultrapure water (resistivity 18·2 MXcm) with a precise quantity of sorbitol. This procedure was performed under a nitrogen atmosphere to avoid any risk of degradation. The solution was then spread on a thermostated plate at 70 °C. After evaporation of the solvent, a film was obtained. Some samples for NMR experimentation were prepared in H2O and others in D2O. To reduce the quantity of D2O to a minimum, an autoclave, allowing work with a 40-mL sample, was used. We must point out that the repeatability of the composition of the small amount of film (with D2O) is not as good as when larger amounts are prepared. Films were reduced to a powder using a cryogrinder. Before analysis, films were stored at constant relative humidity (57% obtained using a saturated solution of NaBr) for 72 h. Powders prepared with D2O were stored in a D2O atmosphere to avoid -OD exchange with -OH. To eliminate H2O molecules that could have been exchanged with deuterium oxide molecules during successive manipulations after film preparation, the final sample was dried several times
and rehydrated with D2O until the NMR signal was constant on two successive measurements.
Assays Determination of residual content of water and deuterium oxide after evaporation were performed by the Karl-Fisher method3 on a Mettler DL 18. Sorbitol content was determined by ionic chromatography coupled with pulsed amperometric detection. Film samples (100 mg) were washed in ultrapure water (20 mL) with vigorous stirring to achieve complete leaching of the sorbitol in the solution. After centrifugation, the sorbitol in the supernatant was analysed using a DIONEX MA1 and an ESA Coulochem II amperometrical detector (room temperature; eluant: a sodium hydroxide solution 480 m; flow rate: 0·35 mL/min; detector sensitivity: 20 lA). These assays were performed for all samples. Sorbitol content was expressed on a dry starch basis (ds). Mechanical properties Maximum stress and yield at break were measured at room temperature on an Instron 1122 universal testing machine, with stretching set to 2 mm/ min. After conditioning for 72 h at 57% relative humidity, 8 to 10 samples were cut according to the standard ASTDM D 412 method. Film thickness was measured with a micrometer. Dynamic thermal mechanical analysis Thermomechanical measurements were performed on a DMTA MKIII apparatus (Polymer Lab, Loughborough, U.K.). Vibration frequency in tension mode was set at 1 Hz, solicitation amplitude at 16 lm and the heating rate at 3 °C/min. Films were coated with a silicone-based hydrophobic grease to limit dehydration during experiments above room temperature. It was determined that a thin coating of grease had no effect on thermomechanical properties. The glass transition temperature was determined according to the method of Arvanitoyannis et al.4 from two points: the temperature corresponding to the drop in storage modulus E′ and the maximum of the tan d peak corresponding to the relaxation a. Tg was calculated as the average of these two values and recorded as Tg 1/2. Temperatures of secondary relaxations were taken at the top of tan d peaks.
Plasticisation and mobility in starch-sorbitol films
Residual water content The measurements for residual water are indicated in Figure 1. As experimental error was very low, error bars are not shown. The value was 13·8% for the sample without sorbitol but decreased to 8·6% for a sorbitol content between 24% and 27% (ds) and then increased to 10·4% for 39·5% (ds) of sorbitol. The water content of pure sorbitol stored at room temperature and humidity was 0·47%. Mechanical properties The curves showing maximum stress and yield at break for different sorbitol contents are shown in Figure 2. System behaviour was clearly affected by addition of sorbitol, and two areas appeared showing behavioural change for a value of around 27% sorbitol: Area I: sorbitol content below 27%. In this area, films were rigid and brittle. Maximum stress
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Nuclear magnetic resonance The NMR study was performed on a low-field MINISPEC PC 120 BRUKER (0·47 T) NMR spectrometer. Free induction decay (FID) signals were obtained using the following parameters: pulse width=1·1 ls; mode detection: diode; relaxation delay: 2 s; acquisition number=128; attenuation=28; temperature=20 °C. Samples 20mm high were placed in 10-mm diameter tubes. NMR signals were sampled at 1 MHz with the TEAM 490 acquisition card of BAKKER Electronic. The fitting of the FID signals was first obtained with a home-built bigaussian fitting program which proceeded step-by-step in a visual way. These parameters were then used as starting values for the NLREG program5. The CONTIN program6 was used for continuous determination of T2∗ relaxation times. These two programs allowed the signal to be analysed in its totality. NLREG, a non-linear regressional analysis program using a combination of Gauss-Newton and Levenberg-Marquart methods, allowed discrete time values of relaxation to be obtained, with intensity expressed in arbitrary units (a.u.). CONTIN solved entire first-order linear equations as well as systems of linear algebraic equations. A distribution of relaxation times was obtained using this program.
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Figure 2 Influence of sorbitol content (ds) on mechanical properties. Β, maximum stress; Χ, yield at break.
decreased from 53 MPa for film without plasticiser to approximately 10 MPa for films containing 26·6% sorbitol. Yield at break was fairly constant at around 5%. Area II: sorbitol content above 27%. In this area, films showed a plastic behaviour. Maximum stress continued to drop, reaching 4·5 MPa for
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Figure 3 Tan d curves and the logarithm of loss modulus obtained by DMTA at different sorbitol contents (ds). – –, 0%; Φ, 0·6%; Ο, 7·2%; Β, 19%; Χ, 27·4%; Ε, 36·7%.
films containing 39·5% sorbitol. Yield at break increased rapidly from 10·6% for 30·2% sorbitol to 38·2% for 39·5% sorbitol. Thermomechanical properties Two characteristics are shown in Figure 3: tan d, an adimensional number defined as the ratio of the storage modulus to the loss modulus, and the logarithm of the storage modulus Log E′ expressed in Pa. Two relaxations were observed with these curves. They are defined by a maximum of tan d associated with a drop in storage modulus. A first relaxation designated as 1 occurred at high temperatures (>30 °C), and the second designated as 2 at low temperatures (<10 °C). For relaxation 1, only the beginning of the peak was observable, with a maximum not always very clearly apparent. The end of peak 1 was in fact disturbed by the dehydration of samples above 70 °C. This effect was more marked for materials with a low sorbitol content because of the high relaxation temperature. Nevertheless, this relaxation could be identified as a since the temperature range of this transition corresponded to the glass transition range (Tg) of such systems7. Relaxation 2 was more clearly observable (Fig. 3). Relaxation temperatures determined at the top of the peaks are
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Figure 4 State diagram for the starch-sorbitol system. Β, temperature of relaxation 2; Ο, glass transition temperature measured by DSC; Α, glass transition temperature measured by DMTA.
indicated in Figure 4 as a function of sorbitol content. Figure 4 shows the changes in Tg 1/2 measured by DMTA and Tg measured by DSC7, on the one hand, and the temperature changes for relaxation 2 as a function of sorbitol content, on the other. Water content was approximately 10%. The results obtained with the two methods were affected similarly by the addition of sorbitol (Fig. 4), but the Tg values calculated from mechanical relaxations were approximately 10 degrees above those from calorimetric measurements. In both cases, Tg was sharply lowered by the increase in sorbitol content. Tg 1/2 as measured by DMTA decreased from 70 °C for 9·4% sorbitol (ds) to 40 °C for 36% sorbitol (ds). Conversely, addition of sorbitol raised relaxation 2 temperature to its highest values. For the sample without sorbitol, relaxation temperature measured at −68 °C was concordant with b relaxation values for amylose8–10. The addition of very small quantities of sorbitol increased this relaxation temperature (−38 °C for 2% sorbitol). The temperature peak increased to 4 °C for films containing 19% sorbitol before decreasing and then stabilizing at around −3 °C at a value close to 27% sorbitol.
Plasticisation and mobility in starch-sorbitol films
1/T2∗=1/T2+1/T2 inh
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where T2 inh is a parameter representing the inhomogeneity of the magnetic field B0. To simplify the system, the signal for water and hydroxyl functions was removed by preparing samples in deuterium oxide. As these films had an excess of D2O as compared to -OH functions of starch and sorbitol, they could be neglected during analysis of the results.
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Figure 5 Influence of sorbitol contents (ds) on the logarithm of loss modulus.
FID processing The CONTIN program could not be applied to entire files in this study. The part of the signal corresponding to the solid phase displayed the PAKE phenomenon12 characterised by an increase in the signal of around 40 ls [Fig. 6(a)]. This phenomenon was taken into account by multiplying the components of the solid phase by the factor cos (xt) during discrete analysis: 2
The storage modulus value at 20 °C, i.e., the temperature at which the analysis of static mechanical properties were analysed, is shown in Figure 5 as a function of sorbitol content. At 20 °C, the value of Log (E′) was 9·2 Pa for the sample without sorbitol. The addition of sorbitol, even in very small quantities, caused an increase in the value of Log (E′) compared with the sample without sorbitol since this value reached 9·4 Pa for 4·1% sorbitol, indicating that the material was more rigid. When the sorbitol content continued to increase, Log (E′) decreased but was always greater than 9·2. Log (E′) continued to decrease with the addition of sorbitol, reaching 8·99 for 36·7% sorbitol. NMR study Time domain NMR allowed the quantitative differentiation of the phases according to their average relaxation time. The FID signal studied in this work allows access to apparent transversal relaxation times (T2∗). This technique distinguishes protons in strong interaction from those with a liquid-like behaviour11. Protons in strong interaction (not very mobile) are characterised by a short relaxation time, whereas those in weak interaction have a longer relaxation time. The relationship between true T2 (T2) and apparent T2 (T2∗) is given by the equation:
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The distribution of residuals was thus statistically scattered [Fig. 6(b)]. However, this factor could not be added to the CONTIN program and taken into account during the gaussian fitting of the signal. Thus, the part of the signal presenting the PAKE phenomenon could not be processed by this program. Accordingly, the total signal was decomposed into two parts: the solid phase signal processed by a program using a discrete method and the intermediate and liquid phase signal exploitable by CONTIN.
Starch with D2O To determine the behaviour of starch alone, a mixture of starch and D2O was prepared as described above. Thus, the signal was derived only from non-exchangeable starch protons. The results for this signal analysis (9·9% of humidity) are given in Table I, and the distribution of relaxation times for the ‘liquid’ signal in Figure 7. The ‘liquid’ phase signal (81 a.u.) had three components at 57 ls (39 a.u.), 170 ls (7 a.u.), and 750 ls (17 a.u.) and a constant of 18. The solid phase signal (1215 a.u.) also had three components at 6·6 ls (143 a.u.), 19·3 ls (725 a.u.), and 28·3 ls (347 a.u.)
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(a)
frequency impulsion. Moreover, the signal per atom gram of hydrogen (atgH) present in the sample (dry starch mass=0·8753 g) was identical to that obtained with native starch (34 265 a.u./ atgH and 34 185 a.u./atgH, respectively)13. From the quantity of atom per gram of hydrogen, it was possible to determine that 6% of the total starch signal gave a ‘liquid’ and intermediate signal. The average T2∗ of the solid phase obtained with the relationship I/T2=R (Ii/T2i) was 17 ls, a value that can be compared with the 17·2 ls, obtained with a native starch suspension exchanged by deuterium oxide13. This characteristic represents the mobility of the solid phase, which was nearly identical in both cases.
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Figure 6 Multigaussian fitting of the NMR signal sample at 36% sorbitol (ds) and 10·3% H2O: (a) (b) scattering of residues. For times greater than measurements were averaged and the signal to noise then greater.
from a signal, 100 ls, ratio is
that certainly corresponded to a wide distribution. The standard deviation of the fitting was quite satisfactory since it was 0·5 a.u. for an initial signal of 1296 a.u. (representing 0·045%) and only slightly superior to that measured before the radio
Exploitation of FID. Films were prepared with D2O to mask the signal for water and hydroxyl functions, which came solely from non-exchangeable starch and sorbitol protons. The different sample components are shown in Table I. The signal calculated per atom gram of hydrogen (/atgH) (Table II) present in each sample was constant, which demonstrates the good quality of the fitting applied to these signals [34 500 a.u./atgH ±600 (1·7%)]. It was possible, given the quantity of atom per gram of hydrogen of the sorbitol in samples, to calculate the part of the residual signal corresponding to sorbitol and then deduce the sorbitol content present in ‘liquid’ and intermediate phases. Results obtained for the six samples (Table II) show that this percentage was practically constant between 50 and 56% for amounts of sorbitol between 17·5 and 48·1% (ds). The changes in distributions of relaxation times corresponding to the intermediate and ‘liquid’ part of the signal show modifications in the behaviour of these curves (Fig. 8) which broaden after around 30% sorbitol. The processing of the difference in signals between two films prepared with D2O at 45·8% (ds) and 32·9% (ds) (0·458 and 0·329 g/g dry starch) confirmed this broadening of the curves. This signal corresponded to the T2∗ distribution of the 12·9 sorbitol points between 32·9% and 45·8% (Fig. 9). The processing indicated that sorbitol distribution in the solid as well as liquid and intermediate phases provided values in agreement with those obtained by the previous method. In fact, the average T2∗ of the solid phase was 22 ls,
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Processing of FID containing different sorbitol contents Analysis of ‘liquid’ and intermediate part of the signal
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Figure 7 T2∗ distribution of the liquid signal of a starch sample containing 9·9% D2O.
and that of the liquid phase 110 ls, with a distribution of these two populations around 50%. The peak corresponding to the solid part was clearly determined, whereas that corresponding to the ‘liquid’ and intermediate part was very broad, as in the preceding processing.
Comparison with the sample without sorbitol. Signals from the sample without sorbitol and the one containing 18·5% of sorbitol (ds), i.e., the two prepared in D2O, are superimposed in Figure 10. The comparison of average relaxation times for the ‘liquid’ and intermediate phases (Table I) of the two samples shows that this value was greater for the sample without sorbitol (105 ls and 95 ls, respectively). The mobility of most mobile parts of starch thus seemed to be reduced by the addition of sorbitol. This reduction of mobility was observed until 32·9% (average T2∗:68 ls). Beyond this value, the average relaxation time of the ‘liquid’ and intermediate phase increased until 94 ls for a sample containing 45·8% sorbitol but is always inferior to the value of the sample without sorbitol.
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Figure 8 Changes in the T2∗ distribution of ‘liquid’ and intermediate phases of samples prepared in D2O as a function of the amount of sorbitol added (ds). — - - - —, 18·5% sor, 32·9% sorbitol; - - - - - - , 45·8% sorbitol. bitol;
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Figure 9 T2∗ distribution of the 12·9 points of sorbitol added between 45·8% and 33·4% sorbitol (ds) obtained by subtraction of the FID signals of the two samples.
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Figure 10 Superposition of two signals prepared in D2O. -----, sample without sorbitol; · · · , sample containing 18·5% (ds) sorbitol.
samples containing the same sorbitol content, one prepared in H2O and the other in deuterium oxide. For two samples containing 34% sorbitol (0·460 g sorbitol/g dry starch), the processing of the resulting file by a discrete method gave the values indicated in Table III. This indicated that 55% of the water was in the ‘liquid’ phase, with an average T2∗ of 92 ls. The ‘solid’ water in strong interaction with solid starch and sorbitol showed an average T2∗ of 16·3 ls. A comparison of relaxation time distributions for the file obtained previously (Fig. 11) with that of a sample without sorbitol (both with the same water content) (Fig. 12) indicates the difference in water behaviour with or without sorbitol. This comparison was performed for relaxation times between 92 ls and 630 ls. The distribution of water relaxation times (Fig. 11) displayed a continuous distribution, which was not the case for the sample without sorbitol in which the peaks are clearly separated. This shows that the behaviour of water changed in the presence of sorbitol. DISCUSSION
Water behaviour The behaviour of water was determined by processing the difference in signals between two
The macroscopic behaviour of the starch-water system shows considerable variations depending on the amount of sorbitol added. The results obtained by mechanical and thermomechanical
Plasticisation and mobility in starch-sorbitol films
Table III
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Analysis of the difference between two samples containing 34% sorbitol, one prepared with H2O and the other with D2O Processing of ‘liquid’ and intermediate part of signal
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Figure 11 T2∗ distribution of water obtained by the difference between two signals with the same sorbitol content, one prepared with H2O and the other with D2O.
dynamic measurements indicate that there are two types of behaviour in starch containing materials plasticised by sorbitol. A change in the properties of these materials was observed for a sorbitol content of around 27%, allowing the differentiation of two areas, referred to here as low or high content areas, depending on whether sorbitol is present in amounts less than or greater than 27%. At low sorbitol content, films are rigid and brittle, and sorbitol does not have the classical effect of a plasticiser on the yield at break. This behaviour shows some similarities to antiplasticisation, a phenomenon observed in some synthetic polymers such as polycarbonate14–17. On the other hand, maximum stress decreases with yield at break at low plasticiser content, which is
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different from the classical antiplasticisation effect. This phenomenon, which has been observed in the case of starch-glycerol18,19 at low plasticiser content (<15%), was also comparable to antiplasticisation, though a different effect on maximum stress was noted. In this work, DMTA was probably more suitable for brittle materials than simple measurement of stress-strain, allowing us to demonstrate an increase of the modulus and therefore enhanced rigidity in materials containing low amounts of plasticiser. This characteristic has also been found in synthetic polymer-antiplasticiser20–23 systems (polycarbonate, PVC). In the area of high sorbitol content, changes in mechanical properties (stress-strain) were similar to those observed with current plasticised materials. The compatible low molecular weight compound decreased the glass transition temperature of the
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polymer to room temperature, thereby producing a plastic material. According to data in previous work7, the glass transition temperature of starch is close to room temperature when sorbitol content is approximately 27%. However, DMTA data clearly indicated that glass transition was not the only relaxation involved in the behavioural change of the material. In fact, relaxation 2 led to an important drop in loss modulus, which was all the more marked when sorbitol content was greater. For high plasticiser contents, the temperature of this relaxation stabilized at −3 °C, which corresponds to the glass transition of pure sorbitol24. This behaviour of relaxation 2 temperature was also observed with the starch-glycerol-water system8. Indeed, after an increase in temperature to −43 °C for 18·4% glycerol (wt%), relaxation 2 temperature decreased and stabilized at about −50 °C for around 25% glycerol (wt%). A temperature of −50 °C is very close to that of glass transition of pure glycerol (−52 °C as determined by dielectrical measurements19). At the molecular level, the data provided by NMR showed changes in mobility of the two phases. The average T2∗ of the ‘liquid’ and intermediate phase decreased for low plasticiser contents and then increased for high contents. This difference in the mobility of the ‘liquid’ and intermediate phase does not appear to be attributable to sorbitol distribution between the two phases. In fact, the distribution of sorbitol in the two phases (solid on the one hand and ‘liquid’ and intermediate on the other) did not really change as a function of sorbitol content (an increase of the ‘liquid’ and intermediate phase from 50 to 56%). The results show that 6% of the total signal of a starch sample prepared in D2O had ‘liquid’ and intermediate behaviour (average T2∗ of 105 ls). This indicates that a part of the starch protons (those not belonging to hydroxyl functions) had greater mobility. The protons responsible for this greater mobility could not be clearly identified by this method. The mobility of this phase was changed by addition of sorbitol since a decrease in mobility was noted (average T2∗ of the ‘liquid’ and intermediate phase of 95 ls for a sample containing 18·5% sorbitol). This decrease continued up to 32·9% sorbitol (average T2∗:68 ls). After this sorbitol content, the average T2∗ increased but was always inferior to the value obtained with the sample without sorbitol. These results indicate that sorbitol binds strongly to starch from the moment the first sorbitol molecules
are added, inducing a decrease in the mobility of the system. At the same time, a broadening of T2∗ distributions was observed with NMR, notably for high plasticiser content. This increasingly broader distribution was indicative of the appearance of new interactions in this study, i.e., H-bond type sorbitol-sorbitol and starch-sorbitol-sorbitol interactions. The results show the existence of two competitive effects: on the one hand, sorbitol molecules bound to the most mobile parts of starch and were associated with a loss of system mobility, and on the other hand ‘clusters’ of sorbitol associated with greater system mobility. The respective importance of these two effects would appear to be reversed between the areas of low and high plasticiser content. The molecular mechanisms seem to be identical in both cases, but their relative importance produces different macroscopic behaviour. The work of Liu et al.16 on the polycarbonatedi-n-butyl phtalate system helps substantiate this hypothesis. These authors used the low-resolution NMR technique to study longitudinal relaxations of polycarbonate protons (the diluent signal was masked by deuteration). This study indicated that clusters of diluent appeared when more than 10% was used. Thermodynamically, these ‘clusters’ would not be large enough to form a new phase in the real sense of the term. This feature might be the cause of controversy about the existence or not of phase separation in such systems7,25. In the starch-sorbitol system, when sorbitol content is high, sorbitol-sorbitol and starch-sorbitol interactions become quite numerous and enhance the system mobility observed at macroscopic level. NMR provided data about water mobility in the system. An analysis of water behaviour was only performed for a sample containing a large amount of sorbitol (34% ds). Concerning the solid part of the signal, strong interactions with starch as well as with sorbitol were noted (average T2∗: 92 ls). Concerning the ‘liquid’ part, the difference in the distribution profiles of relaxation times between a sample containing sorbitol and another without sorbitol (but with the same water content) indicated that water behaves differently in the presence or absence of sorbitol. The broadening of peaks in the case of the sample containing sorbitol was indicative of interactions between sorbitol and water, probably of the H-bond type. For high contents, a coexistence of several types of interactions was found. The phenomenon ob-
Plasticisation and mobility in starch-sorbitol films
served in Figure 1 has already been seen in other systems7,26. A hypothesis that increased amounts of water in areas of high plasticiser contents was based on the appearance of interactions between water and excess plasticiser. This hypothesis would seem to be confirmed for the starch-sorbitol system. Interactions appearing in the ‘liquid’ part with sorbitol could have been responsible for the increased water content observed.
5. 6.
7. 8.
CONCLUSION In this study, the addition of sorbitol induced important behavioural changes in starch films. The techniques employed indicate the nature of these differences and of the molecular phenomenon involved in the behavioural changes observed at the macroscopic level. Sorbitol, up to 27%, induced an antiplasticisation effect. Beyond this value, sorbitol acted as a plasticiser. These changes could be due to two competitive effects whose relative importance is reversed as a function of the amount of sorbitol added. DMTA, which is more sensitive than simple measurements of static mechanical properties, is certainly more suitable for the study of brittle material and allows the detection of variations not apparent with measurements in static mode. Time domain NMR provides a quantitative and qualitative approach to system mobility. Thus, it is possible to study samples containing elements with relaxation times corresponding to solid as well as liquid behaviour. NMR does not allow a component to be identified with a particular group of protons but provides new information concerning the functioning of the system, especially with respect to behavioural changes. REFERENCES 1. Slade, L. and Levine, H. Beyond water activity: recent advances on a alternative approach to the assessment of food quality and safety. Critical Review of Food Science and Nutrition 30 (1991) 115–360. 2. Slade, L. and Levine, H. Water and the glass transition— dependence of the glass transition on composition and chemical structure. Special implication for flour functionality in cookie baking. Journal of Food Engineering 22 (1994) 143–188. 3. Dalbert, R. and Tranchant, J. Le dosage de l’eau par la me´thode de Fisher. In Chimie et Industrie 68 (1952) 871–879. 4. Arvanitoyannis, I., Kalichevsky, M., Blanshard, J.M.V. and Psomiadou, E. Study of diffusion and permeation of gases in undrawn and uniaxially drawn films made from
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potato and rice starch conditioned at different humidities. Carbohydrate Polymers 24 (1994) 1–15. Dennis, J.E., Gay, D.M. and Welch, R.E. An additive nonlinear least-squares algorithm. ACM Transactions on Mathematical Software 7 (1981) 3. Provencher, S.W. CONTIN: A general purpose constrained regularization program for inverting noisy linear algebraic and integral equations. Computer Physics Communications 27 (1982) 229–242. Lourdin, D., Coignard, L., Bizot, H. and Colonna, P. Glass transition temperature versus equilibrium relative humidity. Polymer 38 (1997) 5401–5406. Bradley, S.A. and Carr, S.H. Mechanical loss processes in polysaccharides. Journal of Polymer Science 14 (1976) 111–124. Nishinari, K. and Fukada, E. Viscoelastic, dielectric, and piezoelectric behaviour of solid amylose. Journal of Polymer Science 18 (1980) 1609–1619. Shogren, R.L. Effect of moisture and various plasticisers on the mechanical properties of extruded starch. In ‘Biodegradable Polymers and Packaging’, (C. Ching, D. Kaplan and E. Thomas, eds), Technomic Publishing, Lancaster (1993) pp 43–51. Le Botlan, D. and Ouguerram, L. Spin-spin relaxation time determination of intermediate states in heterogeneous products from free induction decay nuclear magnetic resonance signals. Analytica Chimica Acta 349 (1997) 339–347. Abragam, A. Chapter IV: Dipolar line width in a rigid lattice. In ‘The Principles of Nuclear Magnetism’, (N.F. Mott and D.H. Wilkinson, eds), University Press, Oxford (1961) pp 97–132. Rugraff, Y., Desbois, P. and Le Botlan, D. Quantitative analysis of wheat starch-water suspensions by pulsed NMR spectroscopy measurements. Carbohydrate Research 295 (1996) 185–194. Jackson, W.J. and Caldwell, J.R. Antiplasticisation. II. Characteristics of antiplasticisers. Journal of Applied Polymer Science 11 (1967a) 211–226. Jackson, W.J. and Caldwell, J.R. Antiplasticisation. III. Characteristics and properties of antiplasticisable polymers. Journal of Applied Polymer Science 11 (1967b) 227–244. Liu, Y., Jones, A.A., Inglefield, P.T. and Ogden, P. An NMR study of antiplasticisation of a polymeric glass. Macromolecule 239 (1990) 968–977. Ngai, K.L., Rendell, R.W., Yee, A.F. and Plazek, D.J. Antiplasticisation effects on a secondary relaxation in plasticised glassy polycarbonates. Macromolecule 24 (1991) 61–67. Lourdin, D., Bizot, H. and Colonna, P. ‘Antiplasticisation’ in starch-glycerol films? Journal of Applied Polymer Science 63 (1997) 1047–1053. Lourdin, D., Bizot, H. and Colonna, P. Correlation between static mechanical properties of starch-glycerol materials and low-temperature relaxation. Macromolecular Symposya 114 (1997) 179–185. Guerrero, S.J. Antiplasticisation and crystallinity in poly(vinyl) chloride. Macromolecule 22 (1989) 3480–3485. Pezzin, G. Dynamic-mechanical study of the secondary transition of poly(vinyl)chloride. Journal of Applied Polymer Science 11 (1967) 2553–2566. Robeson, L.M. The effect of antiplasticisation on secondary loss transitions and permeability of polymers. Polymer Engineering and Science 9 (1968) 277–281.
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23. Robeson, L.M. and Faucher, J.A. Secondary loss transitions in antiplasticised polymers. Polymers Letters 7 (1969) 35–40. 24. Quinquenet, S., Gabrielle-Madelmont, C., Ollivon, M. and Serpelloni, M. Polymorphism of hydrated sorbitol. Thermochimica Acta 125 (1988) 125–140. 25. Kalichevsky, M.T., Blanshard, J.M.V. and Marsh, R.D.L. Applications of mechanical spectroscopy to the
study of glassy biolymers and related systems. In ‘The Glassy State in Foods’, ( J.M.V. Blanshard and P.J. Lillford, eds), University Press, Nottingham (1993) pp 133–156. 26. Guo, J.H. Effects of plasticisers on water permeation and mechanical properties of cellulose acetate: antiplasticisation in slightly plasticised polymer film. Drug Development and Industrial Pharmacy 19 (1993) 1541–1555.
Plasticisation and Mobility in Starch-Sorbitol Films S. Gaudin∗†, D. Lourdin†, D. Le Botlan‡, J. L. Ilari∗ and P. Colonna† ∗Ecole Nationale d’Inge´nieurs des Techniques des Industries Agricoles et Alimentaires BP 82225, 44322 NANTES, Cedex 03, France; †Institut National de la Recherche Agronomique BP 71627, 44316 NANTES, Cedex 03, France; ‡Faculte´ des Sciences et des Techniques, UPRES A 6006, BP 82207, NANTES Cedex 03, France Received 31 March 1998
ABSTRACT The purpose of this study was to elucidate the molecular mechanism involved in the behavioural changes observed at macroscopic level in a starch-sorbitol-water system. At a low sorbitol content (<27%), maximum stress decreased and the yield at break was quite constant at around 5%. This behaviour was compared with antiplasticisation, a well-known phenomenon occurring with synthetic polymers. The study of storage loss modulus by dynamic thermal mechanical analysis (DTMA) confirmed antiplasticisation. Sorbitol at high content (>27%) acts as a plasticiser. The two relaxations observed with DTMA, a associated with glass transition (occurring at high temperature, >30 °C) and two occurring at low temperatures (<10 °C), were involved in behavioural changes. System mobility as a function of sorbitol content was determined by time domain nuclear magnetic resonance. Much of this study was performed by replacing H2O with deuterium oxide to mask the signal from water and therefore analyse only the starch and sorbitol signals. The results show the existence of two competitive effects: one associated with a decrease in mobility and the other with the enhanced mobility of the system. The relative importance of these two effects seems to be reversed, depending on sorbitol content, which could account for the changes observed at the macroscopic level. 1999 Academic Press Keywords: starch, sorbitol, plasticisation, antiplasticisation, NMR, DMTA.
INTRODUCTION Dry foods are of considerable industrial importance. Although their microbiological stability is ensured by their low aw, chemical (oxidation by oxygen and light) and structural (crispness) stabilities are not as easy to control. Polymer science is increasingly applied to elucidation of the structurefunction relationships in food systems1. Glass transition theory has been widely studied in polymer science and is a particularly useful means of improving our knowledge of the food system. The glass transition of an amorphous polymer char-
: ds=dry starch; DMTA=dynamic mechanical thermal analysis; NMR=nuclear magnetic resonance; Tg=glass transition; aw=activity of water. Corresponding author: D. Lourdin. 0733–5210/99/030273+12 $30.00/0
acterises the passage from a vitreous to a rubbery state. The structures are different in both domains. The importance of the study of glass transition temperature (Tg) has been demonstrated2 for many foods such as cookies or snacks in which the preservation of crispness is important. Products are generally stable when they are stored below the glass transition temperature, so that an understanding of this parameter is fundamental to controlling the properties, quality and stability of such products. Such foods are always complex in their formulation, being mainly composed of biopolymers (starch, proteins) that are largely responsible for the crisp texture of the final product. Addition of low molecular weight solutes (sugars, salt) causes important changes in the interactions between the different components that are responsible for the final properties of the product. In studies of structure-function relationships, a first 1999 Academic Press
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approach involves the use of binary or ternary model systems. Wheat starch, a biopolymer present in many foods, was chosen for the present study. Other elements considered were sorbitol, the starch plasticiser commonly used in the food industry, and water. Dynamic thermal mechanical analysis (DMTA) and nuclear magnetic resonance (NMR) were employed to determine the contribution of the various components to the system properties. EXPERIMENTAL Materials The starch used for film preparation was an A-type wheat starch (27% amylose, 73% amylopectin) of commercial origin (F33, Olgivie Aquitaine). Sorbitol of technical quality was obtained from Prolabo (RCS Cre´teil B 542 081 419), and D2O (99·9% D) from Euriso-top (CEA). Methods
Film preparation Films were obtained by the casting method. Native wheat starch was solubilized in a high-pressure reactor at 138 °C for 20 min using a 6% suspension in ultrapure water (resistivity 18·2 MXcm) with a precise quantity of sorbitol. This procedure was performed under a nitrogen atmosphere to avoid any risk of degradation. The solution was then spread on a thermostated plate at 70 °C. After evaporation of the solvent, a film was obtained. Some samples for NMR experimentation were prepared in H2O and others in D2O. To reduce the quantity of D2O to a minimum, an autoclave, allowing work with a 40-mL sample, was used. We must point out that the repeatability of the composition of the small amount of film (with D2O) is not as good as when larger amounts are prepared. Films were reduced to a powder using a cryogrinder. Before analysis, films were stored at constant relative humidity (57% obtained using a saturated solution of NaBr) for 72 h. Powders prepared with D2O were stored in a D2O atmosphere to avoid -OD exchange with -OH. To eliminate H2O molecules that could have been exchanged with deuterium oxide molecules during successive manipulations after film preparation, the final sample was dried several times
and rehydrated with D2O until the NMR signal was constant on two successive measurements.
Assays Determination of residual content of water and deuterium oxide after evaporation were performed by the Karl-Fisher method3 on a Mettler DL 18. Sorbitol content was determined by ionic chromatography coupled with pulsed amperometric detection. Film samples (100 mg) were washed in ultrapure water (20 mL) with vigorous stirring to achieve complete leaching of the sorbitol in the solution. After centrifugation, the sorbitol in the supernatant was analysed using a DIONEX MA1 and an ESA Coulochem II amperometrical detector (room temperature; eluant: a sodium hydroxide solution 480 m; flow rate: 0·35 mL/min; detector sensitivity: 20 lA). These assays were performed for all samples. Sorbitol content was expressed on a dry starch basis (ds). Mechanical properties Maximum stress and yield at break were measured at room temperature on an Instron 1122 universal testing machine, with stretching set to 2 mm/ min. After conditioning for 72 h at 57% relative humidity, 8 to 10 samples were cut according to the standard ASTDM D 412 method. Film thickness was measured with a micrometer. Dynamic thermal mechanical analysis Thermomechanical measurements were performed on a DMTA MKIII apparatus (Polymer Lab, Loughborough, U.K.). Vibration frequency in tension mode was set at 1 Hz, solicitation amplitude at 16 lm and the heating rate at 3 °C/min. Films were coated with a silicone-based hydrophobic grease to limit dehydration during experiments above room temperature. It was determined that a thin coating of grease had no effect on thermomechanical properties. The glass transition temperature was determined according to the method of Arvanitoyannis et al.4 from two points: the temperature corresponding to the drop in storage modulus E′ and the maximum of the tan d peak corresponding to the relaxation a. Tg was calculated as the average of these two values and recorded as Tg 1/2. Temperatures of secondary relaxations were taken at the top of tan d peaks.
Plasticisation and mobility in starch-sorbitol films
Residual water content The measurements for residual water are indicated in Figure 1. As experimental error was very low, error bars are not shown. The value was 13·8% for the sample without sorbitol but decreased to 8·6% for a sorbitol content between 24% and 27% (ds) and then increased to 10·4% for 39·5% (ds) of sorbitol. The water content of pure sorbitol stored at room temperature and humidity was 0·47%. Mechanical properties The curves showing maximum stress and yield at break for different sorbitol contents are shown in Figure 2. System behaviour was clearly affected by addition of sorbitol, and two areas appeared showing behavioural change for a value of around 27% sorbitol: Area I: sorbitol content below 27%. In this area, films were rigid and brittle. Maximum stress
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Nuclear magnetic resonance The NMR study was performed on a low-field MINISPEC PC 120 BRUKER (0·47 T) NMR spectrometer. Free induction decay (FID) signals were obtained using the following parameters: pulse width=1·1 ls; mode detection: diode; relaxation delay: 2 s; acquisition number=128; attenuation=28; temperature=20 °C. Samples 20mm high were placed in 10-mm diameter tubes. NMR signals were sampled at 1 MHz with the TEAM 490 acquisition card of BAKKER Electronic. The fitting of the FID signals was first obtained with a home-built bigaussian fitting program which proceeded step-by-step in a visual way. These parameters were then used as starting values for the NLREG program5. The CONTIN program6 was used for continuous determination of T2∗ relaxation times. These two programs allowed the signal to be analysed in its totality. NLREG, a non-linear regressional analysis program using a combination of Gauss-Newton and Levenberg-Marquart methods, allowed discrete time values of relaxation to be obtained, with intensity expressed in arbitrary units (a.u.). CONTIN solved entire first-order linear equations as well as systems of linear algebraic equations. A distribution of relaxation times was obtained using this program.
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Figure 2 Influence of sorbitol content (ds) on mechanical properties. Β, maximum stress; Χ, yield at break.
decreased from 53 MPa for film without plasticiser to approximately 10 MPa for films containing 26·6% sorbitol. Yield at break was fairly constant at around 5%. Area II: sorbitol content above 27%. In this area, films showed a plastic behaviour. Maximum stress continued to drop, reaching 4·5 MPa for
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Figure 3 Tan d curves and the logarithm of loss modulus obtained by DMTA at different sorbitol contents (ds). – –, 0%; Φ, 0·6%; Ο, 7·2%; Β, 19%; Χ, 27·4%; Ε, 36·7%.
films containing 39·5% sorbitol. Yield at break increased rapidly from 10·6% for 30·2% sorbitol to 38·2% for 39·5% sorbitol. Thermomechanical properties Two characteristics are shown in Figure 3: tan d, an adimensional number defined as the ratio of the storage modulus to the loss modulus, and the logarithm of the storage modulus Log E′ expressed in Pa. Two relaxations were observed with these curves. They are defined by a maximum of tan d associated with a drop in storage modulus. A first relaxation designated as 1 occurred at high temperatures (>30 °C), and the second designated as 2 at low temperatures (<10 °C). For relaxation 1, only the beginning of the peak was observable, with a maximum not always very clearly apparent. The end of peak 1 was in fact disturbed by the dehydration of samples above 70 °C. This effect was more marked for materials with a low sorbitol content because of the high relaxation temperature. Nevertheless, this relaxation could be identified as a since the temperature range of this transition corresponded to the glass transition range (Tg) of such systems7. Relaxation 2 was more clearly observable (Fig. 3). Relaxation temperatures determined at the top of the peaks are
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Figure 4 State diagram for the starch-sorbitol system. Β, temperature of relaxation 2; Ο, glass transition temperature measured by DSC; Α, glass transition temperature measured by DMTA.
indicated in Figure 4 as a function of sorbitol content. Figure 4 shows the changes in Tg 1/2 measured by DMTA and Tg measured by DSC7, on the one hand, and the temperature changes for relaxation 2 as a function of sorbitol content, on the other. Water content was approximately 10%. The results obtained with the two methods were affected similarly by the addition of sorbitol (Fig. 4), but the Tg values calculated from mechanical relaxations were approximately 10 degrees above those from calorimetric measurements. In both cases, Tg was sharply lowered by the increase in sorbitol content. Tg 1/2 as measured by DMTA decreased from 70 °C for 9·4% sorbitol (ds) to 40 °C for 36% sorbitol (ds). Conversely, addition of sorbitol raised relaxation 2 temperature to its highest values. For the sample without sorbitol, relaxation temperature measured at −68 °C was concordant with b relaxation values for amylose8–10. The addition of very small quantities of sorbitol increased this relaxation temperature (−38 °C for 2% sorbitol). The temperature peak increased to 4 °C for films containing 19% sorbitol before decreasing and then stabilizing at around −3 °C at a value close to 27% sorbitol.
Plasticisation and mobility in starch-sorbitol films
1/T2∗=1/T2+1/T2 inh
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Figure 5 Influence of sorbitol contents (ds) on the logarithm of loss modulus.
FID processing The CONTIN program could not be applied to entire files in this study. The part of the signal corresponding to the solid phase displayed the PAKE phenomenon12 characterised by an increase in the signal of around 40 ls [Fig. 6(a)]. This phenomenon was taken into account by multiplying the components of the solid phase by the factor cos (xt) during discrete analysis: 2
The storage modulus value at 20 °C, i.e., the temperature at which the analysis of static mechanical properties were analysed, is shown in Figure 5 as a function of sorbitol content. At 20 °C, the value of Log (E′) was 9·2 Pa for the sample without sorbitol. The addition of sorbitol, even in very small quantities, caused an increase in the value of Log (E′) compared with the sample without sorbitol since this value reached 9·4 Pa for 4·1% sorbitol, indicating that the material was more rigid. When the sorbitol content continued to increase, Log (E′) decreased but was always greater than 9·2. Log (E′) continued to decrease with the addition of sorbitol, reaching 8·99 for 36·7% sorbitol. NMR study Time domain NMR allowed the quantitative differentiation of the phases according to their average relaxation time. The FID signal studied in this work allows access to apparent transversal relaxation times (T2∗). This technique distinguishes protons in strong interaction from those with a liquid-like behaviour11. Protons in strong interaction (not very mobile) are characterised by a short relaxation time, whereas those in weak interaction have a longer relaxation time. The relationship between true T2 (T2) and apparent T2 (T2∗) is given by the equation:
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The distribution of residuals was thus statistically scattered [Fig. 6(b)]. However, this factor could not be added to the CONTIN program and taken into account during the gaussian fitting of the signal. Thus, the part of the signal presenting the PAKE phenomenon could not be processed by this program. Accordingly, the total signal was decomposed into two parts: the solid phase signal processed by a program using a discrete method and the intermediate and liquid phase signal exploitable by CONTIN.
Starch with D2O To determine the behaviour of starch alone, a mixture of starch and D2O was prepared as described above. Thus, the signal was derived only from non-exchangeable starch protons. The results for this signal analysis (9·9% of humidity) are given in Table I, and the distribution of relaxation times for the ‘liquid’ signal in Figure 7. The ‘liquid’ phase signal (81 a.u.) had three components at 57 ls (39 a.u.), 170 ls (7 a.u.), and 750 ls (17 a.u.) and a constant of 18. The solid phase signal (1215 a.u.) also had three components at 6·6 ls (143 a.u.), 19·3 ls (725 a.u.), and 28·3 ls (347 a.u.)
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(a)
frequency impulsion. Moreover, the signal per atom gram of hydrogen (atgH) present in the sample (dry starch mass=0·8753 g) was identical to that obtained with native starch (34 265 a.u./ atgH and 34 185 a.u./atgH, respectively)13. From the quantity of atom per gram of hydrogen, it was possible to determine that 6% of the total starch signal gave a ‘liquid’ and intermediate signal. The average T2∗ of the solid phase obtained with the relationship I/T2=R (Ii/T2i) was 17 ls, a value that can be compared with the 17·2 ls, obtained with a native starch suspension exchanged by deuterium oxide13. This characteristic represents the mobility of the solid phase, which was nearly identical in both cases.
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Figure 6 Multigaussian fitting of the NMR signal sample at 36% sorbitol (ds) and 10·3% H2O: (a) (b) scattering of residues. For times greater than measurements were averaged and the signal to noise then greater.
from a signal, 100 ls, ratio is
that certainly corresponded to a wide distribution. The standard deviation of the fitting was quite satisfactory since it was 0·5 a.u. for an initial signal of 1296 a.u. (representing 0·045%) and only slightly superior to that measured before the radio
Exploitation of FID. Films were prepared with D2O to mask the signal for water and hydroxyl functions, which came solely from non-exchangeable starch and sorbitol protons. The different sample components are shown in Table I. The signal calculated per atom gram of hydrogen (/atgH) (Table II) present in each sample was constant, which demonstrates the good quality of the fitting applied to these signals [34 500 a.u./atgH ±600 (1·7%)]. It was possible, given the quantity of atom per gram of hydrogen of the sorbitol in samples, to calculate the part of the residual signal corresponding to sorbitol and then deduce the sorbitol content present in ‘liquid’ and intermediate phases. Results obtained for the six samples (Table II) show that this percentage was practically constant between 50 and 56% for amounts of sorbitol between 17·5 and 48·1% (ds). The changes in distributions of relaxation times corresponding to the intermediate and ‘liquid’ part of the signal show modifications in the behaviour of these curves (Fig. 8) which broaden after around 30% sorbitol. The processing of the difference in signals between two films prepared with D2O at 45·8% (ds) and 32·9% (ds) (0·458 and 0·329 g/g dry starch) confirmed this broadening of the curves. This signal corresponded to the T2∗ distribution of the 12·9 sorbitol points between 32·9% and 45·8% (Fig. 9). The processing indicated that sorbitol distribution in the solid as well as liquid and intermediate phases provided values in agreement with those obtained by the previous method. In fact, the average T2∗ of the solid phase was 22 ls,
Plasticisation and mobility in starch-sorbitol films
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Processing of FID containing different sorbitol contents Analysis of ‘liquid’ and intermediate part of the signal
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Figure 7 T2∗ distribution of the liquid signal of a starch sample containing 9·9% D2O.
and that of the liquid phase 110 ls, with a distribution of these two populations around 50%. The peak corresponding to the solid part was clearly determined, whereas that corresponding to the ‘liquid’ and intermediate part was very broad, as in the preceding processing.
Comparison with the sample without sorbitol. Signals from the sample without sorbitol and the one containing 18·5% of sorbitol (ds), i.e., the two prepared in D2O, are superimposed in Figure 10. The comparison of average relaxation times for the ‘liquid’ and intermediate phases (Table I) of the two samples shows that this value was greater for the sample without sorbitol (105 ls and 95 ls, respectively). The mobility of most mobile parts of starch thus seemed to be reduced by the addition of sorbitol. This reduction of mobility was observed until 32·9% (average T2∗:68 ls). Beyond this value, the average relaxation time of the ‘liquid’ and intermediate phase increased until 94 ls for a sample containing 45·8% sorbitol but is always inferior to the value of the sample without sorbitol.
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Figure 8 Changes in the T2∗ distribution of ‘liquid’ and intermediate phases of samples prepared in D2O as a function of the amount of sorbitol added (ds). — - - - —, 18·5% sor, 32·9% sorbitol; - - - - - - , 45·8% sorbitol. bitol;
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Figure 9 T2∗ distribution of the 12·9 points of sorbitol added between 45·8% and 33·4% sorbitol (ds) obtained by subtraction of the FID signals of the two samples.
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Figure 10 Superposition of two signals prepared in D2O. -----, sample without sorbitol; · · · , sample containing 18·5% (ds) sorbitol.
samples containing the same sorbitol content, one prepared in H2O and the other in deuterium oxide. For two samples containing 34% sorbitol (0·460 g sorbitol/g dry starch), the processing of the resulting file by a discrete method gave the values indicated in Table III. This indicated that 55% of the water was in the ‘liquid’ phase, with an average T2∗ of 92 ls. The ‘solid’ water in strong interaction with solid starch and sorbitol showed an average T2∗ of 16·3 ls. A comparison of relaxation time distributions for the file obtained previously (Fig. 11) with that of a sample without sorbitol (both with the same water content) (Fig. 12) indicates the difference in water behaviour with or without sorbitol. This comparison was performed for relaxation times between 92 ls and 630 ls. The distribution of water relaxation times (Fig. 11) displayed a continuous distribution, which was not the case for the sample without sorbitol in which the peaks are clearly separated. This shows that the behaviour of water changed in the presence of sorbitol. DISCUSSION
Water behaviour The behaviour of water was determined by processing the difference in signals between two
The macroscopic behaviour of the starch-water system shows considerable variations depending on the amount of sorbitol added. The results obtained by mechanical and thermomechanical
Plasticisation and mobility in starch-sorbitol films
Table III
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Analysis of the difference between two samples containing 34% sorbitol, one prepared with H2O and the other with D2O Processing of ‘liquid’ and intermediate part of signal
Processing of solid part of the signal
T2∗ (ls)
I (a.u.)
Average T2∗ (ls)
I (a.u.)
Average T2∗ (ls)
443·0 242·0 119·0 48·0
67·0 168·0 156·0 212·0
92·0
201·0
16·3
5
2.0
4 1.5
P (T2*)
P (T2*)
3
2
1.0
0.5 1
0
0 0
100
200 300 400 500 Relaxation time (T2*) (µs)
600
700
Figure 11 T2∗ distribution of water obtained by the difference between two signals with the same sorbitol content, one prepared with H2O and the other with D2O.
dynamic measurements indicate that there are two types of behaviour in starch containing materials plasticised by sorbitol. A change in the properties of these materials was observed for a sorbitol content of around 27%, allowing the differentiation of two areas, referred to here as low or high content areas, depending on whether sorbitol is present in amounts less than or greater than 27%. At low sorbitol content, films are rigid and brittle, and sorbitol does not have the classical effect of a plasticiser on the yield at break. This behaviour shows some similarities to antiplasticisation, a phenomenon observed in some synthetic polymers such as polycarbonate14–17. On the other hand, maximum stress decreases with yield at break at low plasticiser content, which is
0
Figure 12 with H2O.
500 1000 Relaxation time (T2*) (µs)
1500
T2∗ distribution of a starch sample prepared
different from the classical antiplasticisation effect. This phenomenon, which has been observed in the case of starch-glycerol18,19 at low plasticiser content (<15%), was also comparable to antiplasticisation, though a different effect on maximum stress was noted. In this work, DMTA was probably more suitable for brittle materials than simple measurement of stress-strain, allowing us to demonstrate an increase of the modulus and therefore enhanced rigidity in materials containing low amounts of plasticiser. This characteristic has also been found in synthetic polymer-antiplasticiser20–23 systems (polycarbonate, PVC). In the area of high sorbitol content, changes in mechanical properties (stress-strain) were similar to those observed with current plasticised materials. The compatible low molecular weight compound decreased the glass transition temperature of the
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polymer to room temperature, thereby producing a plastic material. According to data in previous work7, the glass transition temperature of starch is close to room temperature when sorbitol content is approximately 27%. However, DMTA data clearly indicated that glass transition was not the only relaxation involved in the behavioural change of the material. In fact, relaxation 2 led to an important drop in loss modulus, which was all the more marked when sorbitol content was greater. For high plasticiser contents, the temperature of this relaxation stabilized at −3 °C, which corresponds to the glass transition of pure sorbitol24. This behaviour of relaxation 2 temperature was also observed with the starch-glycerol-water system8. Indeed, after an increase in temperature to −43 °C for 18·4% glycerol (wt%), relaxation 2 temperature decreased and stabilized at about −50 °C for around 25% glycerol (wt%). A temperature of −50 °C is very close to that of glass transition of pure glycerol (−52 °C as determined by dielectrical measurements19). At the molecular level, the data provided by NMR showed changes in mobility of the two phases. The average T2∗ of the ‘liquid’ and intermediate phase decreased for low plasticiser contents and then increased for high contents. This difference in the mobility of the ‘liquid’ and intermediate phase does not appear to be attributable to sorbitol distribution between the two phases. In fact, the distribution of sorbitol in the two phases (solid on the one hand and ‘liquid’ and intermediate on the other) did not really change as a function of sorbitol content (an increase of the ‘liquid’ and intermediate phase from 50 to 56%). The results show that 6% of the total signal of a starch sample prepared in D2O had ‘liquid’ and intermediate behaviour (average T2∗ of 105 ls). This indicates that a part of the starch protons (those not belonging to hydroxyl functions) had greater mobility. The protons responsible for this greater mobility could not be clearly identified by this method. The mobility of this phase was changed by addition of sorbitol since a decrease in mobility was noted (average T2∗ of the ‘liquid’ and intermediate phase of 95 ls for a sample containing 18·5% sorbitol). This decrease continued up to 32·9% sorbitol (average T2∗:68 ls). After this sorbitol content, the average T2∗ increased but was always inferior to the value obtained with the sample without sorbitol. These results indicate that sorbitol binds strongly to starch from the moment the first sorbitol molecules
are added, inducing a decrease in the mobility of the system. At the same time, a broadening of T2∗ distributions was observed with NMR, notably for high plasticiser content. This increasingly broader distribution was indicative of the appearance of new interactions in this study, i.e., H-bond type sorbitol-sorbitol and starch-sorbitol-sorbitol interactions. The results show the existence of two competitive effects: on the one hand, sorbitol molecules bound to the most mobile parts of starch and were associated with a loss of system mobility, and on the other hand ‘clusters’ of sorbitol associated with greater system mobility. The respective importance of these two effects would appear to be reversed between the areas of low and high plasticiser content. The molecular mechanisms seem to be identical in both cases, but their relative importance produces different macroscopic behaviour. The work of Liu et al.16 on the polycarbonatedi-n-butyl phtalate system helps substantiate this hypothesis. These authors used the low-resolution NMR technique to study longitudinal relaxations of polycarbonate protons (the diluent signal was masked by deuteration). This study indicated that clusters of diluent appeared when more than 10% was used. Thermodynamically, these ‘clusters’ would not be large enough to form a new phase in the real sense of the term. This feature might be the cause of controversy about the existence or not of phase separation in such systems7,25. In the starch-sorbitol system, when sorbitol content is high, sorbitol-sorbitol and starch-sorbitol interactions become quite numerous and enhance the system mobility observed at macroscopic level. NMR provided data about water mobility in the system. An analysis of water behaviour was only performed for a sample containing a large amount of sorbitol (34% ds). Concerning the solid part of the signal, strong interactions with starch as well as with sorbitol were noted (average T2∗: 92 ls). Concerning the ‘liquid’ part, the difference in the distribution profiles of relaxation times between a sample containing sorbitol and another without sorbitol (but with the same water content) indicated that water behaves differently in the presence or absence of sorbitol. The broadening of peaks in the case of the sample containing sorbitol was indicative of interactions between sorbitol and water, probably of the H-bond type. For high contents, a coexistence of several types of interactions was found. The phenomenon ob-
Plasticisation and mobility in starch-sorbitol films
served in Figure 1 has already been seen in other systems7,26. A hypothesis that increased amounts of water in areas of high plasticiser contents was based on the appearance of interactions between water and excess plasticiser. This hypothesis would seem to be confirmed for the starch-sorbitol system. Interactions appearing in the ‘liquid’ part with sorbitol could have been responsible for the increased water content observed.
5. 6.
7. 8.
CONCLUSION In this study, the addition of sorbitol induced important behavioural changes in starch films. The techniques employed indicate the nature of these differences and of the molecular phenomenon involved in the behavioural changes observed at the macroscopic level. Sorbitol, up to 27%, induced an antiplasticisation effect. Beyond this value, sorbitol acted as a plasticiser. These changes could be due to two competitive effects whose relative importance is reversed as a function of the amount of sorbitol added. DMTA, which is more sensitive than simple measurements of static mechanical properties, is certainly more suitable for the study of brittle material and allows the detection of variations not apparent with measurements in static mode. Time domain NMR provides a quantitative and qualitative approach to system mobility. Thus, it is possible to study samples containing elements with relaxation times corresponding to solid as well as liquid behaviour. NMR does not allow a component to be identified with a particular group of protons but provides new information concerning the functioning of the system, especially with respect to behavioural changes. REFERENCES 1. Slade, L. and Levine, H. Beyond water activity: recent advances on a alternative approach to the assessment of food quality and safety. Critical Review of Food Science and Nutrition 30 (1991) 115–360. 2. Slade, L. and Levine, H. Water and the glass transition— dependence of the glass transition on composition and chemical structure. Special implication for flour functionality in cookie baking. Journal of Food Engineering 22 (1994) 143–188. 3. Dalbert, R. and Tranchant, J. Le dosage de l’eau par la me´thode de Fisher. In Chimie et Industrie 68 (1952) 871–879. 4. Arvanitoyannis, I., Kalichevsky, M., Blanshard, J.M.V. and Psomiadou, E. Study of diffusion and permeation of gases in undrawn and uniaxially drawn films made from
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