The Role of Plasticization in Starch Granule ... - ACS Publications

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Biomacromolecules 2000, 1, 424-432

The Role of Plasticization in Starch Granule Assembly P. A. Perry and A. M. Donald* Polymers and Colloids Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, U.K. Received February 6, 2000; Revised Manuscript Received May 26, 2000

There is a growing interest in the nonfood usage of starch-based products. This interest is predominantly motivated by the ability to produce thermoplastic materials, known as thermoplastic starch (TPS), which can be manufactured using technology already developed for the production of synthetic plastics yet which are fully biodegradable. With the strength and flexibility of TPS materials being highly dependent upon the presence of nonaqueous plasticizing solvents, the nature of the interaction between starch and plasticizer is of obvious interest. As well as interest in the industrial applications of starch, the general nature of plasticization and structure formation within native starch granules is of fundamental interest. This paper presents results from a small and wide angle scattering and calorimetric investigation of model starchplasticizer systems. Issues relevant to TPS production are addressed, and a model of lamellar assembly within starch granules facilitated by the plasticizing effect of low molecular weight solvents is proposed. The structure of dry starch can be thought of as glassy, aperiodic, and disordered on the nanometer length scale. On the molecular level, while some crystalline order is present, the crystallinity is not well defined. Upon the addition of water to dry starch a well-defined periodic lamellar arrangement, with an average lamellar repeat distance of approximately 9 nm1 develops. This is indicated by the evolution of a peak within small-angle X-ray scattering (SAXS) patterns. The overall degree of crystallinity also increases significantly, with the incorporation of water into crystalline unit cells resulting in the appearance of characteristic A-, B-, and C-type starch X-ray diffraction patterns. A representation of the currently accepted hierarchical structure of hydrated starch granules, developed by Cameron,2,3 can be found in the papers of Donald and coworkers.4,5 This model has been devised to elucidate information extracted from X-ray scattering. Real space images provide much more local information, not spatially averaged, which will therefore complement the average picture obtained from scattering. Modern advances in different types of microscopy are constantly refining the details of the internal granule morphology.6-8 Despite this knowledge of the “all or nothing” structural extremes when the granule is either completely dry or completely hydrated, little is known about the nature of the starch-water interaction, or the processes that occur during hydration. When water is replaced by nonvolatile organic solvents such as glycerol and ethylene glycol, or mixtures of these solvents and water, the situation is even more complicated and less well understood. However this situation is exactly that which pertains during the production of thermoplastic starch.9 However what is clear is that the use of solvents other than pure water significantly affects the breakdown of the granule * To whom correspondence should be addressed. Email: amd3@ phy.cam.ac.uk. Fax: +44 1223 337000.

during gelatinization, with solvents such as glycerol, and aqueous sugar solutions all leading to an increase in gelatinization temperature in a way as yet incompletely understood. To date the only attempt at an in-depth study of the structural role of water in assembling the starch granule structure at the lamellar and molecular levels was carried out using simultaneous small and wide-angle X-ray scattering (SAXS/WAXS) by Waigh and co-workers.10,11 In this earlier work the dynamic dehydration of wet starch granules was investigated; upon hydration, only the structural extremes have been experimentally accessible, due to structural changes occurring effectively instantaneously upon the addition of water to dry starch. The current work investigates the nature of the starchsolvent interaction. It deals with the nature of structural changes which occur upon solVation and the relationship between these changes and those occurring during hydration. Among other things, this work provides a test of the validity of the model of hydration and lamellar assembly proposed by Waigh and co-workers. The importance of the lamellar organization in the way it impacts on subsequent gelatinization is briefly considered, but a full discussion of the effect of changing the solvent on this process will be the subject of a subsequent paper. Experimental Section Small and Wide-Angle X-ray Scattering (SAXS/ WAXS). Simultaneous SAXS/WAXS experiments described in this study were performed on station 8.2 at the Synchrotron Radiation Source at the Daresbury Laboratory, Cheshire, U.K. Simultaneous acquisition of SAXS and WAXS allows both lamellar and molecular length scales to be probed at the same time. Performing the experiment at a synchrotron source allows SAXS and WAXS patterns from a single

10.1021/bm0055145 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/20/2000

Interaction between Starch and Polyols

sample to be collected in real time, so that dynamic changes can be followed. The small angle detector camera length was set between 1.5 and 3.5 m, providing a range of q values from approximately 0.009 to 0.500 Å-1. All samples were made up as starch slurries of approximately 40-45% (w/w) starch in the appropriate solution and pure solvent. These samples were investigated in aluminum DSC pans, supplied by TA instruments, which were modified in an attempt to avoid strong absorption of X-rays in the manner described by Bras.12 The temperature of the samples was controlled using a Linkam hot stage, acting as a basic heat flux DSC. This cell provides good temperature calibration, and is hermetically sealed. Problems with solvent loss are only likely to arise if temperatures much in excess of 100 °C are used; for this reason gelatinization at high temperatures, as occurs in highly concentrated glycerol solutions for instance, is not studied via this technique here. All data were corrected with respect to the incident flux, the mass of the sample, and the detector efficiencies prior to subtraction of the empty cell and liquid scattering. Corrected data were calibrated using the well-characterized scattering from wet rat tail collagen (SAXS) and high-density polyethylene (WAXS). More details about the experimental station specifications and operation can be found elsewhere.12-14 Starch samples used in the present study were waxy maize and potato, both gifts from National Starch and Chemical Company. Differential Scanning Calorimetry (DSC). A PerkinElmer power compensated DSC-7 equipped with an Intracooler II was used for all experiments, with an empty sample pan being used as a reference in all cases. Temperature and enthalpy parameters were calibrated using the melting transition of indium. The majority of samples were investigated in standard Perkin-Elmer aluminum 40 µL sample. Perkin-Elmer 60 µL large volume capsules (LVCs) which could withstand internal pressures of up to 240 atm, were used at high temperatures where the possibility of solvent loss was greatest. Sample masses were 10-25 mg. Results A comparison of the thermal behavior of waxy maize starch in 80, 85, 90, and 95% (w/w) aqueous glycerol solutions and in pure (100%) glycerol is shown in Figure 1. The familiar high temperature gelatinization endotherm can be seen to be present at high glycerol concentrations, and as expected following previous studies of the effects of sugars and polyols on starch gelatinization,15,16 it increases in temperature as the glycerol concentration increases. Additionally, the evolution of a lower temperature exothermic transition as the glycerol concentration surpasses 80% (w/ w) can be clearly seen. Leaving aside consideration of the exothermic transition for one moment, Figure 2 shows the dependence of the peak temperature of the gelatinization endotherm on the glycerol concentration of the solution. One thing which can be seen immediately from Figures 1 and 2 is that waxy maize starch can be effectively gelatinized

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Figure 1. DSC traces (vertically offset) from waxy maize starch heated (10 °C/min) in aqueous glycerol solutions of concentration 80, 85, 90, and 95% (w/w) and in 100% (pure) glycerol. System compositions were 1:3 starch-solvent. Faint arcs mark the concentration dependence of the exothermic transition and the gelatinization endotherm.

Figure 2. Variation in the peak temperature of the gelatinization endotherm upon heating (10 °C/min) waxy maize starch, over the whole glycerol compositional range. All sample compositions were 1:3 starch-solvent.

in concentrated glycerol solutions and even pure glycerol. Evans and Haisman15 proposed that polyols added to a starch-water system elevate the gelatinization temperature by reducing the activity and volume fraction of water within the system. Others have proposed17,18 that temperature elevation is due to solutes acting to preferentially bind water, reducing the amount of water available to starch during gelatinization. It can be seen that both of these proposals are inconsistent with the gelatinization behavior observed at high glycerol concentrations (low water contents) and in pure glycerol. For both these cases, gelatinization is seen to proceed even when no water is present, and without any tendency for the endotherm to split into two, as seen under “limiting water” conditions.19 These observations support the claim of Slade and Levine20 that there is no chemical requirement for water, the only requirement being for mobility, induced by some low molecular weight plasticizer. Water can be completely substituted with glycerol without eliminating, or affecting, the endotherm in any way such as its shape. All that happens is that the temperature for the onset of the gelatinization endotherm is raised. A more detailed discussion of gelatinization and the role of plasticization, mobility and the effects of added low molecular weight solutes will be presented in a future publication. From this point on this paper will concentrate on the implications of the appearance of the lower temperature exotherm only,

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Table 1. Onset Temperature, Peak Temperature, and ∆H Values for the Exothermic Transition Observed upon Heating Waxy Maize Starch in Concentrated Glycerol Solutions and Pure Glycerol DSC exotherm parameters glycerol solution concn (% (w/w))

onset temp (°C)

peak temp (°C)

enthalpy change ∆H (J/g of starch)

85 90 95 100

29 ( 1 32.5 ( 0.5 55.2 ( 0.5 81.7 ( 0.7

41 ( 1 52.7 ( 0.6 71 ( 1 93.4 ( 0.5

-8 ( 2 -29 ( 3 -43 ( 3 -57 ( 2

and its relationship with ideas based on mobility as in the Slade and Levine picture alluded to above. Returning now to consideration of this transition, it is observed that its temperature and enthalpy change both increase with increasing glycerol content within the solvent, as shown in Table 1. The presence of an exothermic DSC transition has previously been reported by van Soest et al.,21 in a study of thermoplastic starch production using potato starch and glycerol. The transition was attributed to glycerol having a greater “interaction” with starch polymers than water. However, the nature of this “interaction” and the origin of the exothermic transition were not addressed. More information about the origin of the exothermic transition and the concentrated solution regime is provided by real time simultaneous small and wide-angle X-ray scattering (SAXS/ WAXS) results. Figure 3 shows the variation in small angle scattering patterns and SAXS peak intensity from waxy maize starch in 80, 85, 90, and 95% (w/w) glycerol solutions and in pure glycerol. Figure 3 shows that at concentrations of 80% (w/w) (and below, not shown) the SAXS peak is fully evolved at room temperature. At these concentrations no exothermic transition is observed upon heating. As the glycerol solution concentration is increased to 85% (w/w), Figure 3b, the room temperature SAXS peak intensity is reduced by comparison; only upon heating does this peak intensity to its maximum (this is seen more clearly in Figure 3e). A small exotherm is present in the DSC trace from this sample, the peak temperature of which relates to the point at which the rate of increase of peak intensity slows. Further increasing the glycerol solution concentration to 90% (w/w), results in the SAXS peak intensity recorded at room-temperature being much reduced (Figure 3, parts c and e) and at glycerol concentrations of 95% (w/w) (Figure 3d) and above there is no SAXS peak present at all at room temperature. SAXS patterns from these samples are found to be almost identical to those exhibited by “dry”, unhydrated starch, with the SAXS peak now only appearing upon heating. The plot of the peak intensity shown in Figure 3e shows that the upturn in peak intensity is correlated with the DSC peak position, marked by the dashed vertical lines. Figure 3e also indicates that the rate of peak evolution increases upon increasing the glycerol concentration, and this is consistent with the apparent “sharpening” of the exothermic DSC transition, which occurs upon increasing glycerol concentration (see Figure 1). Each of these high glycerol content systems exhibits a prominent exothermic transition

which becomes enlarged (increased enthalpy change) and moves to higher temperatures with increasing solution concentration. In each case there is a correlation between the peak temperature of the exothermic transition and the slowing of the SAXS peak evolution rate. The exothermic transition appears to relate directly to processes occurring during the evolution and maturation of the SAXS peak. The same qualitative behavior is observed with other starch cultivars, with the temperature at which the SAXS peak first appears and the rate of peak evolution both increasing with increasing glycerol concentration. As a further examples and an example of a starch with a native B type structures data for potato starch in solutions with different glycerol concentrations is shown in Figure 3f (once again the dashed vertical lines show the position of the corresponding DSC exotherm peak). It can be seen that the qualitative behavior is exactly the same as for waxy maize, but the appearance of the exotherm first occurs at lower glycerol concentrations. In general this concentration dependence of the SAXS peak evolution is the only significant difference between starches. Thus, as Figure 3f shows, the SAXS peak from potato starch is completely evolved at room temperature only for concentrations of approximately 70% (w/w) and below. In waxy maize starch the upper concentration boundary for full room temperature SAXS peak evolution is approximately 80-85% (w/w). In turn, at the same glycerol concentration, potato starch requires heating to a higher temperature than waxy maize starch to initiate peak evolution. The behavior exhibited by starch in glycerol is not unique; in other pure nonaqueous organic solvents, there is observed to be no SAXS peak at room temperature upon initial mixing. Heating these samples results in SAXS peak evolution and the eventual loss of SAXS peak intensity during gelatinization. Dynamic SAXS peak intensity data from a range of starch cultivars in a selection of solvents is shown in Figure 4. Figure 4 shows that the temperature at which SAXS peak evolution is initiated is dependent upon the starch cultivar and the plasticizing solvent. For instance, it can be seen that when combined with the same solvent (ethylene glycol or glycerol) potato starch requires heating to higher temperature then waxy maize starch to initiate SAXS peak evolution while maize and waxy maize starches show effectively identical behavior when mixed with the same solvent (butane-1,4-diol). In each case the peak temperature of the exothermic transition correlates with the slowing of the rate of SAXS peak evolution. Using this temperature ramp the rate of lamellar assembly is observed to be effectively the same, regardless of starch or solvent. Wide-angle X-ray scattering (WAXS), collected simultaneously with SAXS reveals that the crystallinity development within starch exhibits the same concentration and temperature dependencies as SAXS peak evolution upon heating in concentrated glycerol solutions, pure glycerol, and other pure nonaqueous solvents. As an example, the behavior of waxy maize starch in concentrated glycerol solutions and pure glycerol is shown in Figure 5. Again, only the location of concentration boundaries show a significant variation between different starch cultivars.



Figure 3. Dynamic SAXS data from waxy maize starch heated at 2 °C/min in concentrated glycerol solutions. SAXS from starch in glycerol solutions of concentration (w/w): (a) 80%; (b) 85%; (c) 90%; (d) 95%. (e) Variation in SAXS peak intensity at a q vector of 0.064 Å-1. Dotted lines mark the peak temperatures of the exothermic transitions determined by DSC. (f) Variation in SAXS peak intensity at the same q vector of 0.064 Å-1 for potato starch heated at 2 °C/min in concentrated glycerol solutions and pure glycerol (100%). The dotted lines mark the peak temperatures of the exothermic transitions determined by DSC.

The left-hand plot in Figure 5 shows that increasing glycerol solution concentration at room temperature leads to reduced definition of the characteristic A-type WAXS diffraction peaks, centered around approximately 16, 18, and 23.5° 2θ, from waxy maize starch. Starch mixed with pure glycerol exhibits WAXS patterns with a lack of definition characteristic of “dry”, unhydrated starch. The right plot shows that upon heating to 85 °C (a temperature at which the SAXS peak is fully evolved) diffraction peaks centered about approximately 18 and 23.5° 2θ (marked with arrows) become more prominent and sharpen. Each WAXS pattern develops into the form observed from hydrated waxy maize starch at room temperature, just as with SAXS. This illustrates that in addition to water, concentrated glycerol solutions and pure glycerol can facilitate crystallization of starch. Variation in crystallinity occurring upon solvation was probed further by investigation of the development of the 100 (5.5° 2θ) inter-helix diffraction peak from potato (Btype) starch. Figure 6 shows the form of the 100 peak, as well as the 9 nm lamellar repeat peak, upon heating to 75 °C potato starch (40% (w/w) starch) mixed with ethylene glycol. 100 peaks shown in Figure 6 were modeled as simple Gaussians using the Kaleidagraph data analysis and package.

Values of the best-fit peak height, position, and full width at half-maximum (fwhm) are given in Table 2. Table 2 shows that the 100 peak increases in intensity, sharpens and moves to lower values of q upon maturation at elevated temperatures. This indicates that crystalline helical ordering is enhanced upon holding at elevated temperatures; ordering is characterized by increased mean inter-helix distance (shift in the 100 peak to lower q values) and increased crystalline size/perfection (sharper peak). Simple Bragg analysis reveals that the inter-helix d spacing increases from approximately 1.51 to 1.57 nm. The latter value is very close to a value of 1.6 nm, which is observed for hydrated potato starch. This finding during solVation is in accord with that of Cleven et al.22 who reported that diffraction peaks for B-type starch shifted toward greater lattice spacings with increasing hydration of the sample. It is proposed, based upon the side-chain liquid crystalline analogy of Waigh10,11 that development of the 100 peak represents the transformation from a dry nematic arrangement of amylopectin helices, to plasticized and mobile smecticlike helical ordering. Thus, the crystalline lamellae are identified with smectic layers. When plasticized, helices move apart and take up the same ordered positions on the B-type crystalline lattice whether the solvent is water or

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Figure 4. Variation of SAXS peak intensity upon heating (2 °C/min) a range of starch-solvent mixtures (1:3 starch-solvent). The plots show the effect of varying solvent on waxy maize starch (top left) and potato starch (the effect of glycerol on waxy maize starch is also shown for comparison) (top right) and the effect of butane-1,4-diol on maize and waxy maize starch (bottom). Dotted lines mark peak temperatures of the DSC exothermic transitions.

Figure 5. WAXS patterns from waxy maize starch mixed with 85, 90 and 95% (w/w) glycerol solutions and pure (100%) glycerol at 25 °C (left) and after heating to 85 °C (right). Arrows indicate peaks which exhibit the greatest concentration and temperature dependence.

another nonaqueous plasticizer of the starch granule. The enthalpic bonus derived from crystallization gives rise to the exothermic transition observed by DSC. It is not clear from the current data and cannot be ascertained using these techniques whether nonaqueous solvent molecules are incorporated into the crystalline unit

cell of A- and B-type starch, and/or whether they act only to increase polymer chain mobility, allowing the adoption of enthalpically favorable crystalline arrays. This uncertainty is not surprising, however, as even the role which water plays in determining crystallinity in native starch granules is still a matter of some debate.23-25 The apparent equality of water



Figure 6. Evolution of the 9 nm lamellar repeat peak (left) and the 100 inter-helix peak (right) in potato starch mixed with ethylene glycol, heated to 75 °C. Table 2. Gaussian Fit Parameters from the 100 Peak from Potato Starch in Ethylene Glycol peak height peak position peak width χ2 (×10-4) (×10-1 Å-1) (×10-2 Å-1) (×10-8) 20 °C 75 °C (5 min) 75 °C (10 min)

2.1 ( 0.3 3.5 ( 0.2 4.3 ( 0.2

4.15 ( 0.05 4.03 ( 0.02 3.99 ( 0.02

4.9 ( 0.2 4.0 ( 0.1 3.6 ( 0.1

4.8 6.5 5.3

R 0.97 0.99 0.99

and low molecular weight organic solvents in producing crystallinity is an area which would benefit from further investigation. As well as arising from heating, it was found that SAXS peak evolution and enhanced crystallinity could occur upon room-temperature storage of starch mixed with ethylene glycol and glycerol. In addition, it is observed that the exothermic DSC transition which occurs upon heating starch mixed with ethylene glycol and glycerol (see Figure 1 for example) loses definition, decreases in size and shifts to lower temperatures. Figure 7 presents real time data showing room-temperature SAXS peak evolution from waxy maize starch in ethylene glycol. The time scale of this process at room temperature is found to be highly dependent upon the identity of the plasticizing solvent and the starch cultivar. SAXS peak evolution in waxy maize starch mixed with glycerol occurs only upon prolonged room-temperature storage (data not shown). In this case the onset of peak evolution took upward of 4 days and full maturation of the peak occurred over a further period of 1-2 days. Again, whereas complete SAXS peak evolution and crystallization occur in less than 2 h in waxy maize starch mixed with ethylene glycol, potato starch requires over 12 h. This retardation in time (compared to waxy maize starch) correlates with the retardation in temperature observed upon heating (see Figures 3f and 4 for example). A final important point to note is that evolution of the SAXS peak, enhanced crystallinity and the exothermic DSC transition, which are proposed to characterize the formation of an ordered, crystalline starch structure were found to be irreversible with respect to time and temperature in all cases. Model of Plasticization and Lamellar Assembly. It is proposed that dry, unsolvated starch granules are disordered, having no periodic lamellar structure and little defined

Figure 7. (a) Variation of the intensity of the SAXS peak (q ) 0.065 Å-1) with temperature for waxy maize starch mixed with ethylene glycol (40% (w/w) starch) at room temperature, as derived from the SAXS pattern shown in part b.

crystallinity. Amorphous granular regions are unplasticized and are low in mobility, and this includes the amorphous regions between the potentially crystalline lamellae. This lack of molecular mobility within the granule thwarts the adoption of enthalpically favorable conformations. Systems in which starch is mixed with concentrated glycerol solutions and pure nonaqueous solvents are composed of suspensions of “dry”, unsolvated and unplasticized granules suspended in a distinct fluid phase. The absence of a SAXS peak is due to the lack of periodic lamellar structure within the granule. With the structure of starch being viewed as being analogous to that of a side-chain liquid-crystalline polymer10,11 it is proposed

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that the semicrystalline growth ring regions are disordered, with lamellae being present as collapsed glassy nematic (Btype starch such as potato) or isotropic (A-type starch, as typefied by waxy maize) structures. In other words, the amylopectin is not exhibiting much crystallinity in this state, as reflected by the WAXS patterns in Figures 5 and 6. Considering the nature of small angle scattering from lamellar systems an alternative scenario can be imagined in which the absence of a room-temperature SAXS peak is due to coincidental contrast matching of distinct periodic lamellar regions within solvated, plasticized starch granules. This possible explanation however has been ruled out on the basis of a contrast variation small-angle neutron scattering experiment,26 which showed that varying the scattering density of glycerol in a starch-glycerol mixture did not result in the appearance of a room-temperature small angle lamellar repeat peak. Upon heating or prolonged room-temperature storage starch granules become solvated, with amorphous regions of the starch granule becoming plasticized. Plasticization, either due to the presence of plasticizing solvent or the input of thermal energy, increases the mobility and entropy of the amylopectin chains within the amorphous lamellar regions of the semicrystalline growth ring. Mobility within these regions allows enthalpically driven assembly of the lamellar structure to be initiated, with amylopectin double helices moving into register, assuming a smectic-like order. Glycerol and other nonaqueous solvents, as with water, plasticize the amorphous lamellar regions, allowing the helices within the crystalline lamellae to take up enthalpically favorable crystalline arrangements. Plasticization results in the formation of a periodic lamellar structure within a system which is immobile and aperiodic at room temperature. The driving force for the assembly process is the enthalpic bonus derived from crystallization. This enthalpic bonus gives rise to the exothermic DSC transition and is the explanation of the starch-glycerol “interaction” proposed by van Soest et al.21 The presence of a low molecular weight plasticizer is necessary and, in the absence of heating, is sufficient alone to bring about lamellar assembly, given long enough. Heating in the presence of a plasticizing solvent brings about far more rapid lamellar assembly due to the highly cooperative action of thermal and solvent plasticization. There is an effective solVent-temperature-time superposition governing lamellar and crystalline assembly. (This can be considered as an extension of the moisture-temperature-time superposition which has been proposed by other authors27,28 as the determinant of biopolymer functionality.) Increasing the temperature of “dry” samples leads to solvation induced by the combination of reduced steric constraints, reduced solvent viscosity, and increased ingress rates. Without the cooperative combined effect of heat and solvent, room temperature rates of lamellar assembly are far more representative of simply the ability of the solvent to enter and plasticize the starch granule. It is proposed that a critical minimum level of plasticization and entropy of the amorphous lamellar regions is needed before structural changes on the lamellar and molecular length scales can be initiated. The temperature or storage

Perry and Donald

Figure 8. Schematic representation of the proposed model of lamellar assembly in B-type starch.

time required to produce the minimum level of mobility is a function of the starch cultivar as well as the plasticizing solvent. For example, ethylene glycol fully plasticizes waxy maize starch at room temperature in approximately 2 h, while replacing ethylene glycol with glycerol (increased molecular weight and viscosity) increases this time to several days. As an example of the dependence on starch cultivar, it is found that effective plasticizationsand lamellar organizationsof potato starch can take more than five times as long as waxy maize starch upon storage at room temperature. It is proposed that the minimum required mobility level is the same for any given starch cultivar whatever the plasticizing solvent, but that the minimum level differs between starches. The conditions necessary to attain this minimum level of mobility for any given starch are then dependent upon the temperature and the nature of the plasticizing solvent. This model of lamellar assembly in B-type starch is represented schematically in Figure 8. This model is completely in accord with that proposed by Waigh and coworkers10,11 with dynamic solvation data strongly supporting the model proposed, based upon results from dynamic dehydration. It should be noted that the equivalent schematic picture for an A type starch would have a more isotropic structure in the dry state, with denser intercrystalline packing. Dependence on the Nature of the Plasticizing Solvent. Figure 3 shows that increasing the concentration of aqueous glycerol solution leads to an elevation of the temperature at which lamellar assembly and crystallization are initiated. Figure 4 illustrates that increasing the molecular weight of pure plasticizing solvent (moving from ethylene glycol (Mw ) 62) to glycerol (Mw ) 92) also has the effect of elevating the temperature or storage time required to initiate lamellar assembly. This is consistent with a higher molecular weight solvent being less able to penetrate the starch granule as well as being less able to increase the free volume of amorphous regions, hence being a less effective plasticizer. Increased viscosity (and possibly to a lesser extent increased molecular size and reduced diffusion rates) due to increasing solvent


molecular weight will act to retard the entry of solvent into the starch granule. It is also apparent from Figure 4 that molecular weight is not the only determinant of the temperature/time at which solvation, plasticization and lamellar assembly are initiated. It can be seen that butane-1,4-diol (Mw ) 90) induces lamellar assembly in waxy maize starch at a higher temperature than the slightly higher molecular weight solvent, glycerol. It seems clear that hydrogen bonding also plays a major role. Hydrogen bonding is a major structural element in the starch granule and hence it is also a major determinant for the effectiveness of a plasticizer of starch. The greater the hydrogen bonding capacity of a molecule, the more effective a plasticizer of the starch granule the solvent would be expected to be. The higher temperature at which lamellar assembly is initiated in butane-1,4-diol as compared to glycerol is consistent with the lower density of -OH groups and the corresponding reduced hydrogen bonding capacity. Extrapolating these results, it is proposed that the structural changes and the chronology of processes occurring during hydration and lamellar assembly in starch mixed with water are the same as observed during solVation and lamellar assembly in starch mixed with nonaqueous plasticizers. It is considered that only the temperature/time at which the assembly process is initiated is dependent upon the identity of the solvent. With water acting as the solvent, it is proposed that hydration, plasticization and resulting lamellar assembly and crystallization occur effectively instantaneously at room temperature, due to the highly effective plasticizing ability of water. The applicability of the lamellar assembly model to hydration is supported by the observation, reported elsewhere16,26 and to be published more fully later, that there are no significant differences between the interactions and behavior of starch in water and starch in a range of nonaqueous plasticizers. The solvated, plasticized structure of the granule and the loss of granular order which occurs during gelatinization are the same no matter whether the solvent is pure water, glycerol, ethylene glycol, or a range of higher molecular weight sugar solutions. The only variation in these systems is the temperature at which the gelatinization process is initiated, which always increases. Working from the hypothesis that there must be sufficient solvent ingress to cause swelling in the amorphous growth ring regions, followed by destabilization of the semicrystalline stack,29 it follows that any factor which reduces the ingress of solvent will impede gelatinization. Following the progress of self-assembly due to solvent ingress and plasticization, as recorded in this paper, provides a measure of the ease with which the solvent can enter the granule, and hence lead to subsequent gelatinization. It therefore follows that, although the self-assembly may not in itself be a necessary prerequisite for gelatinization, it is only after it has occurred that enough solvent has entered the granule to cause the destabilization of the structure which is gelatinization. Therefore, the mechanistic picture presented here for the effect of solvents upon the organization within the granule, provides a good indicator for the qualitative trends of gelatinization temperature in the same selection of


solvents. This picture will be further developed in the context of gelatinization in a subsequent paper. A final point to note is that the implications which this may have for the production of thermoplastic starch.9 As well as gelatinization (or destructurisation) which takes place upon heating to high temperatures, the exothermic solvationassembly process must be considered in aqueous systems with a high glycerol content. The higher the glycerol concentration, the higher the temperature to which the starch-solvent mixture must be heated to produce a solvated, plasticized, homogeneous system, which is subsequently disrupted. Dependence on the Starch Cultivar. In addition to the dependence on the nature of the plasticizer it is found (see Figure 4 for example) that in any given solvent, potato starch requires heating to a higher temperature than waxy maize starch to initiate lamellar assembly and crystallization. It is proposed that the temperature at which solvent first enters the starch granule and lamellar assembly is initiated is dependent upon the structure of the starch granule and the extent to which it is permeable to solvent. It is proposed that the amorphous lamellae of potato starch are less permeable to glycerol than in waxy maize starch. This is in direct agreement with molecular density data determined using small-angle neutron scattering26,30 which showed potato starch has a greater density of carbohydrate and lower density of water within the amorphous lamellar regions than waxy maize starch. It was concluded that the amorphous lamellar regions of potato starch were less “open” and permeable to water (and presumably other solvents) than the corresponding regions in waxy maize starch. This attribution is supported by Figure 4, which shows that waxy maize and maize starches, two cultivars which have been shown26 to have effectively identical intragranular compositions, show no significant difference in lamellar assembly initiation temperature when combined with the same solvent (butane-1,4diol). Conclusions This work has provided new information about the general nature of the starch-plasticizer interaction and the dependence of this interaction on the nature of the plasticizer and the structure of the starch. It has been demonstrated that starch granules can be solvated and effectively plasticized by a variety of nonaqueous solvents. For many of these solvents complete plasticization may require prolonged storage and/or heating. In the absence of ingress of the solvent, the lamellar and crystalline organization is imperfect. Self-assembly upon solvation transforms the disordered structure of dry starch into an ordered smecticlike lamellar system with pronounced crystallinity. Increasing the molecular weight or decreasing the hydrogen bonding capability of the solvent leads to an elevation of the time/temperature required for lamellar assembly. There is a solVent-temperature-time superposition governing lamellar and crystalline assembly within starch. It is proposed that there is a minimum level of mobility within the amorphous lamellar regions necessary to initiate

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lamellar assembly and crystallization. The actual temperature (or storage time) at which lamellar assembly is initiated is dependent upon the starch cultivar (due to variations in structure and permeability) and the plasticizing solvent (due to variations in plasticizing ability and ingress kinetics). On the basis of these results a universal model has been proposed of lamellar assembly brought about upon the addition of low molecular weight plasticizing solvents. This model emphasizes the important role of plasticization and molecular mobility within amorphous granular regions in determining the lamellar and crystalline structure within starch granules. The role played by polyols during the production of thermoplastic starch can now be better understood with a sound knowledge of the starch-plasticizer interaction. Acknowledgment. Drs. B. U. Komanscheck and R. K. Heenan, respectively of the SRS at the Daresbury Laboratory and ISIS at the Rutherford Appleton Laboratories, are gratefully acknowledged for technical and scientific assistance during and after experimental beamtime. Dr. T. A. Waigh is thanked for many invaluable discussions and he, along with Drs. R. E. Cameron and P. J. Jenkins, is acknowledged for motivating the physical study of starch structure and functionality in the first place. Finally, the authors would like to thank the Biotechnology and Biological Sciences Research Council and Nestle plc. for financial support of this project and CASE sponsorship of P.A.P. References and Notes (1) Jenkins, P. J.;Cameron, R. E.; Donald, A. M. Sta¨ rke 1993, 45, 41720. (2) Cameron, R. E.; Donald, A. M. Polymer 1992, 33, 2628-35. (3) Cameron, R. E.; Donald, A. M. Carbohydr. Res. 1993, 244, 225236. (4) Jenkins, P. J.; Cameron, R. E.; Donald, A. M.; Bras, W.; Derbyshire, G. E.; Mant, G. R.; Ryan, A. J. J. Polym. Sci., Phys. Ed. 1994, 32, 1579-83. (5) Jenkins, P. J.; Donald, A. M. Int. J. Biol. Macromol. 1995, 17, 31521.

Perry and Donald (6) Atkin, N. J.; Abeysekera, R. M.; Cheng, S. L.; Robards, A. W. Carbohydr. Polym. 1998, 36, 173-192. (7) Baldwin, P. M.; Adler, J.; Davies, M. C.; Melia, C. D. J. Cereal Sci. 1998, 27, 255-265. (8) Oostergetel, G. T.; Bruggen, E. F. J. v. Carbohydr. Polym. 1993, 21, 7-12. (9) Tomka, I. In Thermoplastic starch; Tomka, I., Ed.; Plenum: New York, 1991; pp 627-637. (10) Waigh, T. A.; Riekel, C.; Gidley, M.; Donald, A. M. Macromolecules 1998, 31, 7980-7984. (11) Waigh, T. A. The structure and side chain liquid crystalline polymeric properties of starch. Ph.D. thesis, Cambridge University, 1997. (12) Bras, W.; Derbyshire, G. E.; Devine, A.; Clark, S. M.; Cooke, J.; Komanschek, B. E.; Ryan, A. J. J. Appl. Crystallogr. 1995, 28, 2632. (13) Bras, W.; Derbyshire, G. E.; Ryan, A. J.; Mant, G. R.; Belton, F.; Lewis, R. A.; Hall, C. J.; Greaves, G. M. Nucl. Instrum. Methods Phys. Res., Sect. A 1993, 326, 587-591. (14) Bras, W.; Derbyshire, G. E.; Ryan, A. J.; Mant, G. R.; Manning, P.; Cameron, R. E.; Mormann, W. J. Phys. IV 1993, 3, 447-450. (15) Evans, I.; Haisman, D. Sta¨ rke 1982, 34, 224-231. (16) Perry, P. A.; Donald, A. M. Abstr. Pap. Am. Chem. Soc. 1998, 215, 236. (17) Labuza, T. P., Labuza, T. P., Eds.; Academic Press: New York, 1975. (18) Derby, R. I.; Miller, B. S.; Miller, B. F.; Trimbo, H. Cereal Chem. 1975, 52, 702. (19) Donovan, J. Biopolymers 1979, 18, 263-75. (20) Slade, L., Levine, H., Slade, L., Levine, H., Eds.; Butterworth: London, 1988; p 115. (21) van Soest, J. J. G.; Hulleman, S. H. D.; de Wit, D.; Vliegenthart, J. F. G. Ind. Crops Prod. 1996, 5, 11. (22) Cleven, R.; van der Berg, C.; van der Plas, L. Starch/Sta¨ rke 1978, 30, 223. (23) Eisenhaber, F.; Schulz, W. Biopolymers 1992, 32, 1643-1664. (24) Duprat, F.; Gallant, D.; Guilbot, A.; Mercier, C.; Robin, J. P. In Les Polymeres Vegetaux; Monties, B., Ed.; Gauthier-Villars: Paris, 1980. (25) Imberty; A Buleon, A; Tran, V.; Perez, S. Starch 1991, 43, 37584. (26) Perry, P. A. Plasticisation and thermal modification of starch. Ph.D. Thesis, Cambridge University, 1999. (27) Levine, H.; Slade, L. Carbohydr. Polym. 1986, 6, 213. (28) Lillie, M. A.; Gosline, J. M. Biopolymers 1990, 29, 1147. (29) Jenkins, P. J.; Donald, A. M. Carbohydr. Res. 1998, 308, 133-147. (30) Perry, P. A.; Donald, A. M. Int. J. Biol. Macromol. 2000, in press.

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The Role of Plasticization in Starch Granule ... - ACS Publications

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