John R. Mitchell

Organisation of the external region of the starch granule as determined by infrared spectroscopy

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) was used to study the external regions of starch granules. Native starches (wheat, potato, maize, waxy maize and amylomaize) were analysed and compared to gelatinised and acid-hydrolysed starches. The IR spectra of potato and amylomaize starches were closer to that of highly ordered acid-hydrolysed starch than the other starches. FTIR was not able to differentiate between A- and B-type crystallinity so the difference observed between starches was not related to this factor. The variation between starch varieties was interpreted in terms of the level of ordered structure present on the edge of starch granules with potato and amylomaize being more ordered on their outer regions. This could explain the high resistance of both these starches to enzyme hydrolysis.

Structure and behaviour in hydrophilic matrix sustained release dosage forms: 3. The influence of pH on the sustained-release performance and internal gel structure of sodium alginate matrices

Abstract The effect of pH and drug solubility on the release kinetics of sodium alginate matrices has been studied. Release of a highly soluble model drug, chlorpheniramine maleate, was significantly faster in simulated gastric fluid (SGF) than in simulated intestinal fluid (SIF), whereas the opposite effect was observed for hydrochlorothiazide, a drug of poor solubility. These results could be explained in terms of the internal microscopic structure of the hydrated surface layer formed on matrix hydration and by the different hydration kinetics of the polymer in these two media. Cryogenic electron microscopy revealed the hydrated surface layer formed by alginate matrices in SGF to be particulate and porous in nature, in contrast to the highly hydrated continuous gel layer formed in SIF. Drug release mechanisms were discussed with respect to drug solubility and the structure and properties of the surface layers formed by alginate matrices when hydrated in different pH media.

A computer simulation of the dynamics of bubble growth and shrinkage during extrudate expansion

Abstract A model describing the dynamics of bubble growth in starchy extrudates is proposed. The model takes into account heat and moisture loss to the surroundings from an expanding cell. A combination of a power law model with the Williams-Landel-Ferry (WLF) equation is used to describe the rheological behaviour of the extruded fluid. Numerical calculations predict that a bubble cell first grows by rapid vaporization of the superheated moisture and subsequently shrinks. This post-extrusion shrinkage arises from the cooling of vapour, which produces a negative pressure difference. Qualitatively the predicted effect of initial temperature and moisture from the model is in accord with observations taken from the literature. Variations of the melt temperature and its glass transition temperature with time reveal that the structure is fixable at a temperature of about 30 K above the glass transition temperature. The effect of other model parameters, including surface tension, initial bubble radius, rate of heat loss and constants in the power law viscosity model, are investigated. The limitations of this approach are discussed.

The retrogradation of waxy maize starch extrudates: effects of storage temperature and water content.

The effects of water content and storage temperature on the kinetics of the retrogradation of nonexpanded waxy maize starch extrudates were studied using 1H pulsed NMR and wide‐angle X‐ray diffraction. The increase in crystallinity observed by XRD was accompanied by a decrease in the relaxation times of the solid‐like component of the NMR free induction and the spin‐echo decays, and an increase in the contribution of the solid‐like component to the total signal. The dependence of the rate of starch retrogradation on the storage temperature showed the typical “bell‐shaped” behavior, which was successfully modeled using the Lauritzen–Hoffman theory of crystallization of chain‐folded polymers. This theory was extended to model the effect of water content on the rate of isothermal crystallization by exploiting the ten‐Brinke and Karasz, and the Flory equations to describe the dependence of the glass‐transition and the melting temperatures on water content.

Understanding the texture of low moisture cereal products: mechanical and sensory measurements of crispness

The texture of extruded starch products has been measured sensorially and instrumentally. The instrumental data were analyzed using different approaches such as fractal analysis and peak analysis. The data showed useful correlations between sensory crispness and crackliness and a parameter obtained from the application of a newly developed fractal method to analyze the force deformation curve obtained from a penetration test of the cereal samples. This suggests that fractal analysis of the force-deformation plots could be used instead of sensory analysis. Moreover, it was observed that an increase in water content altered the texture of starch based samples from crispy to crackly, whereas the texture of starch-sucrose samples remained unchanged in the same hydration range. © 2000 Society of Chemical Industry

Xanthan/locust bean gum interactions at room temperature

Abstract Rheological and ultracentrifugation studies have been conducted on heated and unheated mixtures of xanthan with whole locust bean gum, and temperature fractions of the latter possessing different mannose: galactose ratios. The results suggest that xanthan and galactomannans may interact by two distinct mechanisms. One takes place at room temperature, gives weak elastic gels, and has little dependence upon the galactose content of the galactomannan, whilst the second requires significant heating of the polysaccharide mixture, gives stronger gels, and is highly dependent upon galactomannan composition. The results are discussed with reference to existing models proposed for the xanthan/galactomannan interaction.

Relevance of amylose-lipid complexes to the behaviour of thermally processed starches

Maize grits of 50% moisture content (db) were thermally converted at 140 °C without shear. Analysis of the samples using X-ray diffraction, differential scanning calorimetry, optical and scanning electron microscopy and by measurement of hot water absorption and solubility indices indicated that maize grits heated for 15 min or longer had undergone virtually complete melting of crystalline structure without loss of granular integrity and amylose-lipid complexes were formed. Rapid visco analysis (RVA) profiles of these samples were characterised by decreasing gelatinisation peak and final viscosities with longer heating times, but no cold swelling peak was observed for any heat treatment. Similar RVA patterns were also obtained for cereal starches (maize, wheat, rice) as well as for cassava starch in the presence of glycerol monostearate (GMS), whereas cassava, potato, waxy maize starch and waxy maize starch in the presence of GMS, all characterised by low lipid or amylose contents, showed cold swelling behaviour. It was concluded that the presence of amylose-lipid complexes in the granular starch particles played a key role in restriction of water penetration into the particles as well as starch granule swelling and solubilisation in cold water.

The molecular weight distribution and conformation of citrus pectins in solution studied by hydrodynamics

Abstract The molecular weight distribution of a citrus pectin has been analysed by a combined approach using gel-permeation chromatography with low-speed sedimentation equilibrium. (1) A pectin preparation from citrus fruit was fractionated on Sepharose CL-2B/Sepharose CL-4B. (2) Weight average molecular weights of the fractions were determined by low speed sedimentation equilibrium in multichannel cells. (3) An absolute calibration for the column for this material was thereby defined. (4) The (lognormal) molecular weight distribution thus obtained is consistent with a weight average of (90000±10000) g/mol, obtained separately on unfractionated material, and consistent with a distribution obtained on the same material but using light scattering as the molecular weight probe. The conformation of the pectin fractions in solution was studied in terms of: (1) the Wales-Van Holde parameter, k s [η] ; (2) Mark-Houwink-Kuhn-Sakurada plots of sedimentation coefficient and intrinsic viscosity data versus molecular weight; (3) rod models and (4) worm-like-coil models. The sedimentation data is consistent with a rod model (or a worm-like-coil with a large persistence length) with mass per unit length ∼ 430 g mol−1 nm−1. The intrinsic viscosity data is also consistent with a rod model but shows some anomalous features which may be suggestive of worm-like-coil behaviour at higher molecular weight, although it is not possible to fit this data with a realistic value of the mass per unit length.

Milling—A Further Parameter Affecting the Rapid Visco Analyser (RVA) Profile

ABSTRACT Rapid Visco Analyser (RVA) profiles were recorded for raw maize grits and two extruded nonexpanded pellets based on wheat and maize. Large differences were found between the profiles obtained when an impeller mill was used to prepare the samples compared with a disk mill. The differences were related to differences in particle properties of the ground products (particle-size distribution, particle shape, and protein content). Generally, milling the samples with the impeller mill resulted in greater starch conversion than with a disk mill. For raw maize grits, this was shown by X-ray diffraction, differential scanning calorimetry (DSC), and alkaline viscosity measurements. Several other laboratory mills were tested and all produced particulates with a sieve range of 125–212 μm that had substantially differing RVA profiles. Cooling the sample during milling did not nullify the milling effects. All the laboratory mills produced <20% of the particulates of the size range required for the RVA analysis...

Viscosity of galactomannans during high temperature processing: influence of degradation and solubilisation

The rheological properties of guar gum (GG) and locust bean gum (LBG), in response to high temperature treatments, were measured using a rheometer equipped with a high pressure cell. This has allowed the viscosity to be assessed at temperatures above 100°C and as the polymer suspension is heated from 20 to 121°C and then cooled back to ambient temperature to simulate a food sterilisation cycle. Activation energies for depolymerisation estimated from viscosity changes with time at a series of constant temperatures were estimated as 63 kJ/mol for GG and 98, 104, 110 kJ/mol for three different samples of LBG. A model was developed to interpret the viscosity change through the simulated sterilisation cycle. This took into account the degradation of the polysaccharide and the change in viscosity due to thermal motion. Estimations of molecular weight changes during the heating process suggest that GG is more susceptible to thermal degradation than LBG. It is suggested that this is due to the greater ability of the latter to associate in solution.