J.J.G. van Soest

Short-range structure in (partially) crystalline potato starch determined with attenuated total reflectance Fourier-transform IR spectroscopy

A fast and direct method, based on infrared spectroscopy, for quantitative determination of starch short-range structure has been developed. The IR spectrum of starch is sensitive to changes in short-range structure in the C—C and C—O stretching region at 1300-800 cm−1. The IR absorbance band at 1047 cm−1 is sensitive to the amount of ordered or crystalline starch and the band at 1022 cm−1 is characteristic of amorphous starch. The ratios (R) of the heights of the bands at 1047 and 1022 cm−1, which expresses the amount of ordered starch to amorphous starch, and 1047 and 1035 cm−1, which is a measure of the amount of ordered starch, showed a linear regression with the amount of potato starch having B-type crystallinity as determined with wide-angle X-ray diffractometry. The IR spectrum and thus the short-range order is also sensitive to water content. In particular, the band at 994 cm−1, which is related to intramolecular hydrogen bonding of the hydroxyl group at C-6, is water sensitive. It is possible to quantify the IR data in terms of short-range order (SIR) over a range of 10–50% water. The method has been applied to quantify the changes in short-range structure during the melting of potato starch with 18 or 26% (w/w) water. The amount of short-range structure and the changes during melting in the (partially) destructured starch samples concur with differential scanning calorimetry and wide-angle X-ray diffractometry measurements.

Influence of glycerol and water content on the structure and properties of extruded starch plastic sheets during aging

The properties of starch plastic sheets were investigated by stress—strain measurements in relation with starch crystallinity. Granular potato starch was plasticized with different amounts of glycerol and water by extrusion. The materials were amorphous directly after processing. During aging above the glass transition temperature at various humidities single helical (V and E-type) and double helical (B-type) crystallinity was formed. The rate of crystallization is a function of water and glycerol content. The amorphous rubbery materials were soft and weak with high elongations. During aging the materials became less flexible with higher elastic modulus and tensile stress. The changes are related to changes in water content and glass transition temperature and to changes in B-type crystallinity. The changes in stress—strain properties are explained by the formation of helical structures and crystals, which results in a reinforcement of the starch network by physical crosslinking.

The influence of starch molecular mass on the properties of extruded thermoplastic starch

Abstract The mechanical properties of a low and a high molecular mass thermoplastic starch (TPS) were monitored at water contents in the range of 5–30% (w/w). The granular starches were plasticized by extrusion processing with glycerol and water. The low molecular mass starch was prepared by partial acid hydrolysis of potato starch. The extruded TPS materials were stored at 60% relative humidity for 12 months to level out differences in starch structure due to retrogradation. The water content was then varied by an additional storage period at various humidities. The average molecular masses of the TPS materials, composed of native starch or of hydrolysed starch, were 37 000 and 1900 kg mol−1, respectively. The apparent amylose contents of the high and low molecular mass materials were 25% and 11%, respectively. Differences were observed in thermal properties and crystallinity between the two types of materials, as a function of water content but not as a function of molecular mass. The stress-strain properties of the materials were dependent on the water content. The materials showed a viscoelastic behaviour characteristic of a semicrystalline polymer. Materials containing less than 9% water were glassy with an elastic modulus between 400 and 1000 MPa. For the materials a transition from brittle to ductile behaviour occurred at a water content in the range of 9–10%, which is in accordance with the observed glass transition temperature at this water content. The rubbery materials, with a water content of 9–15%, were tough and an optimum in ultimate elongation was observed. Above a water content of 15% the materials became weak and soft and the strain at break decreased. No significant differences in brittle-to-ductile transition as a function of water content were observed between the low and high molecular mass TPS materials. In the rubbery state with 14% water, the elongations at break of the high and low molecular mass materials were 100–125% and 30–50%, respectively. The tearing energy of the materials showed a maximum at a water content of 9–10%. The energies at this maximum of the high and low molecular mass materials were 0.15 and 0.1 J mm−2, respectively. The lower strain and tearing energy of the low molecular mass materials in the rubbery state were attributed to the reduced amylose chain length as well as the molecular mass and the degree of branching of the amylopectin molecules. This resulted in a material with a less effective entangled starch matrix. The entanglements were described as a complex network of the linear amylose chains and the outer chains of the amylopectin molecules in which hydrogen bonding plays an important role.

Changes in the mechanical properties of thermoplastic potato starch in relation with changes in B-type crystallinity

The influence of crystallization on the stress-strain behaviour of thermoplastic potato starch has been monitored. Potato starch has been processed by extrusion with glycerol and water added as plasticizers. The thermoplastic starch consists of a molecular network of semicrystalline amylose and amylopectin with some granular fragments. A rapid increase in B-type crystallinity and a change in mechanical properties is observed within 2 weeks at 70 and 90% relative humidities. An increase in B-type crystallinity from 5 to 30% compared to native potato starch leads to an increased elastic modulus (from 10 to 70 MPa) and tensile stress (from 3 to 7 MPa) for thermoplastic starch materials with a water content of circa 15%. The elongation is decreased from 105 to 55%. For materials with more than 30% B-type crystallinity relative to native potato starch, a sharp decrease in elongation is observed and the materials form cracks. The effects are explained by an increase in physical cross-links by amylose and amylopectin intermolecular double helix formation and crystallization resulting in a reinforced network. A further intramolecular crystallization of the amorphous amylopectin reduces the intermolecular interactions of amylopectin and induces internal stress within the network of semicrystalline amylose and amylopectin. The increased internal stress at the highly crystalline amylopectin sites finally leads to cracking of the materials.

Mechanical properties of thermoplastic waxy maize starch

The properties of thermoplastic amylopectin materials were investigated by stress-strain and relaxation measurements as a function of water content and crystallinity. Granular waxy maize starch was plasticized by extrusion with water and glycerol. The materials are amorphous after processing. The sharp fall in modulus at a water content of 10% is characteristic of a glass to rubber transition of an amorphous polymer. The materials are brittle below their glass transition temperature with a modulus of approximately 1000 MPa and an elongation of less than 20%. The amorphous rubbery materials are soft and weak with moduli of 0-10 MPa and tensile strengths of 0-2 MPa. The materials are viscoelastic, show plastic flow, and form a highly entangled polymer matrix, resulting in high values of elongation (500%), due to the high amylopectin molecular mass. Above glass transition temperature the amylopectin forms inter- and intramolecular double helices, crystallizing in a B-type crystal. The initial increase in modulus, tensile strength, and relaxation time is the result of the lower mobility of the amylopectin molecules and the reinforcement of the network by physical crosslinking. The drastic reduction of the elongation and the formation of cracks is the result of intramolecular crystallization. At crystalline junction zones the internal stress is increased and the interaction between molecules is reduced.

Structure and properties of compression‐molded thermoplastic starch materials from normal and high‐amylose maize starches

The structural and mechanical properties of compression-molded normal and high-amylose maize starches were studied as a function of processing water content and ageing time. Rubbery thermoplastic starches were produced by compression molding of four maize starches with differences in amylose content and amylopectin structure. Glycerol (30% on the basis of dry starch) and water (between 10 and 35% on the basis of total mass) were used as plasticizers. After processing, the amorphous thermoplastic starch materials crystallized during ageing. The semicrystalline materials contained both E-type and V-type, as well as B-type crystallinity. The properties of the thermoplastic starch materials are dependent on water content during processing, starch source, and ageing time. The normal maize starch materials are highly flexible with elongations between 56 and 104%. The elongations of the high-amylose maize starch materials were between 5–35%. The tensile stress and E-modulus of the normal maize starch materials were in the range of 3.9–6.7 and 27–131 MPa, respectively. The tensile stress and E-modulus of the high-amylose maize starch materials increased from approximately 0.5 to 23 and 5 to 700 MPa, respectively, with increasing water content during processing from 10 to 35%. The differences in mechanical properties of the normal and high-amylose materials were explained by differences in the structure of the amylose and amylopectin structure. It was concluded that both lead to differences in the starch network.

Interaction between dry starch and plasticisers glycerol or ethylene glycol, measured by differential scanning calorimetry and solid state NMR spectroscopy

Abstract The interaction of crystalline amylose and of crystalline and amorphous amylopectin with the plasticisers glycerol or ethylene glycol in the absence of water was studied, by using differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Upon heating starch freshly mixed with plasticisers, a strong exothermal interaction enthalpy of ΔH∼−35 J/g was detected by DSC. At room temperature glycerol interacts mainly with the amorphous starch regions, the interaction taking 8 days to reach equilibrium. For ethylene glycol the interaction is faster, taking four days to reach equilibrium, and the rate is not affected by crystallinity. Ethylene glycol interacts in a more ordered manner with amorphous than with crystalline material, resulting in a narrower ethylene glycol cross-polarisation magic angle spinning (CP/MAS) signal when equilibrium is reached at room temperature. Upon heating, more glycerol or ethylene glycol is immobilised, but in a less ordered manner than upon storage at room temperature. This results in a more intense, but broader plasticiser CP/MAS signal upon heating. Interaction in a more ordered manner probably implies interaction with more of the hydroxy groups of the plasticiser. The polysaccharide mobility is increased more when the plasticiser interacts in a more ordered manner, as observed by small starch signals in HP/DEC spectra.

The influence of various small plasticisers and malto-oligosaccharides on the retrogradation of (partly) gelatinised starch

Ageing of gelatinised and partly gelatinised potato starch and wheat starch were investigated in the presence of plasticisers with increasing size and number of OH groups (ethylene glycol, glycerol, threitol, xylitol, glucose, and for potato starch also maltose). The influences of these plasticisers and of granular remnants (ghosts) on recrystallisation were determined by using X-ray diffraction. Recrystallisation of potato starch samples in the presence of plasticisers resulted in crystallinity indices of ∼0.5. The largest reduction in potato starch recrystallisation is found for threitol (4 OH) and xylitol (5 OH). In the plasticiser range examined, the crystallisation inducing effect of granular potato starch remnants is reduced better when the plasticiser contains more OH groups. Wheat starch recrystallises to a lesser extent than potato starch, resulting in crystallinity indices of ∼0.4. The results for wheat starch do not show clear trends for the influences of plasticiser size and of ghosts. The difference in behaviour of the two starches is probably caused by wheat starch having shorter amylopectin chains. Resulting from these shorter amylopectin chains, the remaining structure in wheat starch ghosts may resemble A-type crystallinity, making it more difficult to form B-type crystals. Alternatively, the trends as found for potato starch may occur, but are less manifest for wheat starch, due to the lower total extent of recrystallisation. Solid state CP/MAS NMR spectra of the wheat starch samples containing ethylene glycol were obtained, in order to compare completely and partly gelatinised systems. The spectra were identical, confirming that the ghost structures do not influence wheat starch recrystallisation. Apparently, wheat starch ghosts do not act as nuclei for crystallisation. Similarly, the influence of various malto-oligosaccharides in combination with granular remnants (ghosts) was investigated on wheat starch ageing. Gelatinised and partly gelatinised wheat starch were plasticised with maltose, maltotriose, maltotetraose, maltopentaose or maltohexaose. This resulted in crystallinity indices of ∼0.2, with the largest reduction in recrystallisation for maltotriose and maltotetraose. No trend was found for the influence of ghosts. The presence of ghosts did not influence the 13C solid state HP/DEC NMR spectra. Less recrystallisation took place than with the previously mentioned smaller plasticisers that resulted in crystallinity indices of ∼0.4. The finding that maltose was able to reduce retrogradation better than glucose could be of practical importance.

The influence of glycerol on structural changes in waxy maize starch as studied by Fourier transform infra-red spectroscopy

The influence of glycerol on the retrogradation kinetics for waxy maize starch-water systems was monitored by Fourier transform infra-red spectroscopy. The spectra showed the C---C and C---O stretching region (1300-800 cm−1) to be sensitive to the retrogradation process. A multistage kinetic process in terms of structural changes on a molecular level was observed during the retrogradation of waxy maize in a 10% (w/w) gel. For the first two stages (formation of helices and induction time for helix aggregation) no significant kinetic differences were observed in the gels with various glycerol contents. Stage three is described as the primary aggregation and crystallization step. In the final stage, syneresis of water occurs. The calculated rate constants clearly show decreasing retrogradation of waxy maize starch with increasing glycerol content. The effects are explained in terms of starch-glycerol interactions and stabilization of water resulting in a decreased mobility of the starch chains.

Effect of glycerol on the morphology of starch–sunflower oil composites

The presented study involves the encapsulation of sunflower oil in starch by casting emulsions of oil in aqueous starch solutions. Glycerol was used as a plasticizer and lecithin was used as an emulsifier, to improve the emulsion stability. Increasing glycerol concentration in the samples resulted in different dispersed phase morphologies in the starch-sunflower oil composites. It was observed that increasing glycerol concentration resulted in a decrease in the particle sizes and polydispersity of the oil droplets. The two determining parameters on the formation of the dispersed phase during emulsification; viscosity and interfacial tension between two phases, were investigated to evaluate the effect of glycerol on the system. Since glycerol was found not to affect the viscosity of the aqueous starch phase, it was concluded that glycerol affects the dispersed phase morphology due to its effect on interfacial tension between the oil and aqueous starch phase, during emulsification.