Imad A. Farhat

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.

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.

Characterisation of starches from West African yams

Starches from four varieties of West African yams were extracted and characterised. The physicochemical properties investigated (granule size and morphology, amylose content, crystal form, gelatinisation and pasting behaviour) depended strongly on the yam variety. The starch granules extracted from water yam (Dioscorea alata), white yam (D rotundata) and yellow yam (D cayensis) varieties showed mononodal particle size distributions centred between 31 and 35 µm, while the bitter yam (D dumetorum) exhibited a binodal size distribution of starch granules centred at 4.5 and 9 µm. Light microscopy confirmed the variation in starch granule size and shape with yam variety. The X-ray diffractogram of yellow yam was of the B type, while bitter yam showed an A pattern. The starches extracted from the white and water yams were of the intermediate C-type patterns. The temperatures of onset of gelatinisation were derived from DSC and RVA measurements; values of 69.4 and 75.0 °C for the yellow yam, 71.5 and 78.2 °C for the white yam, 76.5 and 79.8 °C for the water yam and 78.1 and 83.1 °C for the bitter yam were obtained. © 1999 Society of Chemical Industry

FTIR study of state and phase transitions of low moisture sucrose and lactose.

Mid-infrared spectra of freeze-dried sucrose and lactose systems were acquired over a range of temperatures (30-200 degrees C) and water contents (0-6.3%). Starting from the glassy state, the experimental conditions were selected to cover the main thermal transitions: the glass-rubber transition, the crystallisation and, for some samples, the subsequent melting. The FTIR spectra were very sensitive to the physical state. While subtle but systematic spectral differences between the glassy and rubbery states were detectable throughout the spectrum, a very pronounced increase in spectral resolution was observed as crystallisation occurred and was followed by the expected spectral broadening during melting. The temperatures at which these changes occurred were in satisfactory agreement with the transition temperatures measured by differential scanning calorimetry (DSC). The increase in molecular mobility as a result of increasing temperature or plasticisation by water led to a significant shift of the O-H stretching band to higher wavenumbers indicating a weakening of hydrogen bonding. This shift reached a maximum as the DSC measured crystallisation temperature range was approached. As expected, the crystallisation led to a highly effective hydrogen bonding network. This was more significant for lactose than for sucrose. No significant step change in hydrogen bonding was observed at Tg. As anticipated, the temperature at which these transitions occurred decreased with increasing water content but overlapped when observed in the context of the shifted temperature (T-Tg).

Effect of the Extent of Conversion and Retrogradation on the Digestibility of Potato Starch

The effect of starch conversion on the susceptibility of potato granules to α-amylase was studied by direct sampling at different pasting times corresponding to different points on the RVA profile of a 6.4% (w/w) suspension of starch in distilled water. Native granules showed high resistance to α-amylase with 8.6 ± 0.4% digestibility for a 6 h incubation period with the enzyme. When the suspension was heated to 60 °C, the digestibility increased to 53.5 ± 0.7% although, at this temperature, there was still no noticeable increase in the measured viscosity (≤ 0.040 Pa.s). The material sampled after a pasting time corresponding to the RVA peak viscosity showed a digestibility of 88.4 ± 0.5%. This suggested, owing to the expected retrogradation of amylose on cooling, the quasi-total susceptibility of amylopectin to enzymatic digestion at this pasting stage. The effect of ions on the swelling of potato starch was used to assess whether the decrease of the swelling of the granules in the presence of NaCl was paralleled by an increase in resistance to α-amylase. A small (∼ 6.1%) but significant decrease in the digestibility of pasted starch was observed in the presence of salt. Finally, the effect of the retrogradation of the amylopectin fraction on its digestibility was assessed in extruded potato starch ribbons containing 35% (w/w) water and stored at different temperatures. After 14 days of storage, the digestibility decreased from 77.0 ± 0.9% in the freshly extruded samples to between 28.0 ± 1.7% and 42.1 ± 0.3%, depending on the storage temperature. This suggested a measurable difference in the α-amylase susceptibility between the A and B polymorphs of retrograded amylopectin.

Effect of water partitioning on the glass-transition behaviour of phase separated amylopectin-gelatin mixtures

The glass transition behaviour of concentrated 1:1 and 3:1 amylopectin‐gelatin mixtures (water contents between 18 and 36% dry basis) was studied for different compositions using FTIR microspectroscopy and DMTA. The thermograms of these mixtures exhibited 2 transitions indicating the existence of 2 phases. This was in agreement with the FTIR microspectroscopy results where 50‐80mm amylopectin-rich regions could be clearly identified in the continuous gelatin matrix. The assignment of the DMTA transitions was ascertained by varying the gelatin/amylopectin ratio. The prediction of the Tg for each of the 2 phases based on the existing values for the binary systems was best accomplished when the water partitioning between the polysaccharide and the protein was accounted for using the water vapour sorption/ desorption isotherms of the individual biopolymer components of the blend. Finally, typical glass transition ‐ “water partitioning” phase diagrams of phase separated amylopectin‐gelatin mixtures were constructed. q 1999 Elsevier Science Ltd. All rights reserved.

On the extrapolation of the melting temperature of dry starch from starch-water data using the Flory-Huggins equation

Abstract The experimental results of several workers reported in the scientific literature have been re-analysed using the Flory-Huggins equation. The melting temperatures of various starches were re-calculated from data obtained at different water contents, accepting the possibility of an interaction between the biopolymer and the diluent (χ1 > 0); the results were compared to those obtained assuming no interaction. The melting temperatures determined by this analysis were between 50 and 100 K higher than those reported in the original papers.

A pulsed 1H NMR study of the hydration properties of extruded maize-sucrose mixtures

Abstract The molecular mobility in maize and maize-sucrose extudates was studied as a function of the degree of hydration (up to 35% w/w dry basis) using proton spin-spin relaxation NMR. The extrusion of maize systems was found to enhance the molecular mobility of the constituents generating the rigid ‘solid-like’ and the mobile ‘liquid-like’ components of the NMR free induction decay signal. As the moisture content was increased, the amplitude of the normalised NMR signal arising from the rigid component decreased dramatically for the samples containing sucrose, suggesting an increased mobility and thus a solution type behaviour of the sugar. This hypothesis was supported by the observation of shorter T 2 values for the liquid component of the signal recorded for the samples containing sugar. Sucrose was found to be miscible with the polysaccharide at moisture contents below 15%, but progressively dissolved into the additional water at higher moisture contents. This suggested the occurrence of phase separation over a short distance scale as the water content of the maize-sucrose system was modified. The results are discussed in terms of unequal water partitioning between the maize and the sugar components.

The influence of carrageenan on molecular mobility in low moisture amorphous sugars

Abstract The influence of less than 1% of κ-carrageenan on the mobility of glucose syrup was studied in the context of the glass–rubber transition using proton NMR relaxometry. Glass-transition temperatures, ( T g ) were measured by differential scanning calorimetry (DSC) on glucose syrup samples containing 0 or 0.9% κ-carrageenan, between 0 and 1.4% KCl, and at water contents from 3.5 to 16% (wwb). Potassium chloride was added to vary the extent of gelation of the carrageenan in order to assess the effect of the biopolymer network on molecular mobility. Contrary to the reported increase of the rheologically determined glass-transition temperature, in the presence of gelling agents, the addition of 0.9% κ-carrageenan to glucose syrup with and without KCl, had no effect on the DSC measured T g . In addition, there was no effect on molecular mobility in the glassy region. The presence of carrageenan only significantly affected the mobile part of the NMR free induction decay at relatively high temperatures.

Effect of Sugars on Retrogradation of Waxy Maize Starch-Sugar Extrudates

ABSTRACT The effect of sucrose, fructose, and xylose on the retrogradation of waxy maize starch extrudates at relatively low moisture contents (20–50 g of water/ 100 g of dry solid) at 277–353 K was investigated using X-ray diffraction and 1H nuclear magnetic resonance (NMR) relaxometry. The role of the sugar depended on the type of sugar and its concentration, but most importantly on the retrogradation conditions (water content and storage temperature). For the isothermal retrogradation at 313 K, fructose considerably increased the rate of retrogradation over the range of water contents investigated, and the increase was proportional to the sugar concentration. The behavior of xylose and sucrose was more complex. At 10% sugar content, both sugars enhanced starch retrogradation, while at 30% sugar content, xylose accelerated the process at <35% (mc, dsb) but decreased the rate constant when more water was present. This crossover took place at 42% mc for sucrose. Sugars enhanced the transformation of the A...