D. de Wit

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.

Crystallinity in starch bioplastics

Thermoplastic starch (TPS) materials have been prepared by kneading, extrusion, compression moulding and injection moulding of several native starches with the addition of glycerol as a plasticizer. Two types of crystallinity can be distinguished in TPS directly after processing: (i) residual crystallinity: native A-, B- or C-type crystallinity caused by incomplete melting of starch during processing; (ii) processing-induced crystallinity: amylose VH−, VA− or EH−type crystallinity which is formed during thermomechanical processing The amount of residual crystallinity is related to processing conditions like processing temperature or applied shear stress. The composition of the mixture indirectly influences the amount of residual crystallinity. Lower amounts of glycerol cause a reduction in residual crystallinity. This effect is attributed to the increase in melt viscosity at decreasing plasticizer content, which causes an enhancement of shear stress on the melt. It has been concluded that composition and processing parameters are interrelated. Processing-induced crystallinity, also influenced by processing parameters, is caused by the fast recrystallization of amylose into single-helical structures. Increasing the screw speed during extrusion or increasing residence time during kneading causes an increase in single helical type crystallinity. The amount of crystallized amylose is proportional to the amount of amylose. The addition of complexing agents like calciumstearate or the presence of lysophospholipids cause the crystallization of amylose into these type of structures. In waxy starches, containing no amylose, obviously no V- or E-type crystallinity is formed.

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.

Chemical and physical transitions of periodate oxidized potato starch in water

Abstract Physical and chemical transitions of dialdehyde starch in aqueous suspensions have been determined as a function of temperature and pH. The pH conditions were restricted to neutral or lower, since alkaline degradation is commonly recognized. Physical alterations were determined by measuring viscosity transitions at room temperature and upon heating. Chemical degradation was surveyed by controlling the amount of aldehydes, the production of carboxylic acids and the decrease in molecular weight. At neutral conditions and upon heating at 90 °C extensive chemical degradation occurred. This was manifested by a gradual conversion of aldehydes into carboxylic acids and alcohols, accompanied by a decrease in molecular weight. Degradation upon heating was minimized by reducing the pH to 3. At ambient temperature, the granular integrity was lost beyond pH 5 or 6 depending on the degree of oxidation of the material.

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.

The gelation of dialdehyde starch

The rheological behaviour of periodate oxidized potato starches has been investigated in both dilute and concentrated aqueous dispersions. On heating, low viscosity solutions were obtained at low concentrations and viscoelastic gels at high concentrations. During the gelation process, two types of gel structure were distinguished. First a particle gel is formed, and on continued heating and shearing, deformation and disruption of the granules is observed, resulting in the formation of a polymer network. The increasing stability of the particle gels with increasing degree of oxidation has been attributed to an increase in particle rigidity. The aldehyde groups in dialdehyde starch are supposed to reinforce the granular structures, as a result of intragranular hemiacetalization. The gel stability could be improved by adding native potato starch to the premix.

Criteria for mutation analysis in MEN 1‐suspected patients: MEN 1 case‐finding

Multiple endocrine neoplasia type 1 (MEN 1) is an autosomal, dominantly inherited cancer syndrome, with tumours in various endocrine glands. In 1997 the responsible tumour suppressor gene was identified. MEN1 gene germ‐line mutations are detected in the vast majority of MEN 1 patients, however, with regard to case‐finding, unfortunately only at a very low frequency in patients with apparently sporadic MEN 1‐related tumours. In order to increase the detection rate of disease gene carriers among patients with apparently sporadic MEN 1‐related tumours, clinical criteria were needed.

An improved kinetic model for the periodate oxidation of starch

Abstract The kinetics of the periodate oxidation of starch to dialdehyde starch have been studied thoroughly based on a reliable high-performance liquid chromatographic analysis. Investigation of the early stage of reaction has revealed that it proceeds initially following second-order kinetics. Later on, however, the resulting second-order model deviates from the experimental data. In order to get a model for the course of the total reaction, the kinetics have been approached from a different angle. Based on elementary kinetic principles, an appropriate model has been derived that can be used to simulate the oxidation process. An oxidation process performed semi-continuously has been modelled and compared with a batch process.