David Cooke

Loss of crystalline and molecular order during starch gelatinisation: Origin of the enthalpic transition

The disruption of molecular oders which occur during the gelatinisation of starch granules has been studied by isolating dried samples from maize, waxy maize, wheat, potatoe, and tapioca starches after defined thermal pre-treatments. Residual molecular and crystalline order was quantified by 13C-c.p.-m.a.s.-n.m.r. spectroscopy and powder X-ray diffraction, respectively, and the results compared with residual gelatinisation enthalpy determined by d.s.c. For native starches, molecular (double-helical) order was significantly greater than crystalline order. Molecular and crystalline order were both found to correlated with the residual enthalpy of gelatinisation following thermal pre-treatment, indicating that both levels of structure are disrupted concurrently during gelatinisation. From the data obtained, predicted enthalpy values for the disruption of fully ordered crystalline analogues of the starches studied were calculated, and compared with values for essentially fully ordered and crystalline model material. This comparison suggests that the enthalpy of gelatinisation primarily reflects the loss of molecular (doube-helical) order.

Consequences of antisense RNA inhibition of starch branching enzyme activity on properties of potato starch

Antisense constructs containing cDNAs for potato starch branching enzyme (SBE) were introduced into potato (Solanum tuberosum L.). A population of transgenic plants were generated in which tuber SBE activity was reduced by between 5 and 98% of control values. No significant differences in amylose content or amylopectin branch length profiles of transgenic tuber starches were observed as a function of tuber SBE activity. Starches obtained from low SBE activity plants showed elevated phosphorous content. 31P n.m.r. analysis showed that this was due to proportionate increases in both 3- and 6-linked starch phosphates. A consistent alteration in starch gelatinisation properties was only observed when the level of SBE activity was reduced to below ~5% of that of control values. Starches from these low SBE activity plants showed increases of up to 5 °C in d.s.c. peak temperature and viscosity onset temperature. Studies on melting of crystallites obtained from linear (1 → 4)-α-d-glucan oligomers suggest that an average difference of double helix length of about one glucose residue might be sufficient to account for the observed differences in gelatinisation properties. We speculate that the modification of gelatinisation properties at low SBE activities is due to a subtle alteration in amylopectin branch patterns resulting in small changes in double helix lengths within granules.

Molecular order and structure in enzyme-resistant retrograded starch

Abstract Molecular features underlying the resistance to amylolytic hydrolysis in cooked and cooled gels of wheat, amylomaize V and amylomaize VII starches have been investigated using a combination of physicochemical techniques. X-ray diffraction and 13C CP/MAS NMR spectroscopy indicate levels of crystalline and double helical order to be 25–30% and 60–70%, respectively, in enzyme-resistant retrograded starches. The width of features in diffraction patterns and NMR spectra indicate smaller and/or less perfectly arranged B-type double helical aggregates than found in native potato or amylomaize VII starch. Differential scanning calorimetry in excess water shows a broad endothermic transition from below 100 to c. 170 °C which is interpreted in terms of double helix melting. Consistent with a broad melting endotherm, (linear) chain lengths present in enzyme-resistant starches cover a range of degree of polymerisation (DP) from less than 10 to c. 100 as determined by high performance anion exchange chromatography (HPAEC). This dispersion of chain lengths coincides with the range expected from previous studies for double helices (minimum required DP of 10) with no major intervening amorphous regions (maximum DP 100). HPAEC analysis also shows a periodicity in chain length for DP multiples of 6 above DP18 for all three enzyme-resistant retrograded starches. A model is proposed to account for this observation based on restricted enzyme access to potential substrates arranged in double helical aggregates. In general, enzyme-resistant retrograded starch reflects features both of aggregated/gelled amylose (high double helix content; low crystallinity, DP range from junction zones of DP 10–100), and the consequence of enzyme action on such a structure (periodicity of six units from accessibility of enzyme to aggregated substrate).

Thermal properties of polysaccharides at low moisture: 1—An endothermic melting process and water-carbohydrate interactions

Abstract The thermal properties of a broad range of polysaccharides containing 5–25% w/w water have been studied by differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA). Following room temperature conditioning, an endothermic event accompanied by material softening is observed at 45–80°C for all samples except those above their glass transition temperature. The temperature of the event is determined by thermal history and is apparently independent of polymer type or moisture content. The associated enthalpy increases with water content. Variable frequency DMTA analysis suggests a structural melting event rather than a relaxation process. The endothermic event is recovered over the days timescale after heating, and can be annealed to higher temperatures with increasing holding temperature. Results are interpreted in terms of a dynamic hydration model in which specific energetic water-carbohydrate interactions occur but with a lifetime defined by their local effective microviscosity. The observation of the endotherm below glass transition temperatures suggests that in aqueous polysaccharide glasses, enthalpic structures involving the solvent can be made and broken.

Tamarind seed polysaccharide: preparation, characterisation and solution properties of carboxylated, sulphated and alkylaminated derivatives

Abstract A range of derivatives of tamarind seed polysaccharide has been prepared, characterised and selected solution properties examined. Following oxidation of terminal sidechain galactose residues with galactose oxidase, subsequent oxidation and reductive amination have been used to prepare a range of carboxylated and alkylaminated derivatives respectively. Sulphated derivatives have been prepared by reaction with a sulphur trioxide-pyridine complex in dimethylformamide. The nature and extent of substitution have been characterised by potentiometric titration, infrared and 1H and 13C NMR spectroscopy. From the dependence of intrinsic viscosity on ionic strength (Smidsrod & Haug, 1971), carboxylated and sulphated derivatives are found to have characteristically stiffened backbones as found previously for the native polysaccharide (Gidley et al., 1991). Binding of divalent cations to carboxylated derivatives is shown to be relatively weak, although polymer precipitation was noted in the presence of Pb2+. Alkylaminated polysaccharides show only a modest decrease in surface and interfacial tension compared with the native polysaccharide, although significant foam formation and stabilisation was found for a nonylaminated sample. Following enzymic depolymerisation, this material showed a marked decrease in surface and interfacial tension suggesting that interfacial activity in alkylaminated tamarind polysaccharide is only apparent under disruptive solution conditions. The results of 1H NMR line width and T1 measurements before and after depolymerisation suggests that this is due to solution viscosity rather than specific interaction effects.

Effect of isolation procedures on the molecular composition and physical properties of Eucheuma cottonii carrageenan

Direct treatment of Eucheuma cottonii with hot potassium hydroxide leads to a solid residue (alkali treated weed) which was found to contain predominantly carrageenan and cellulose. Following dispersion and heating iri water, separation into polymer-containing solution and an insoluble particulate phase was readily achieved. Chemical analysis of solubilized polymers showed that the predominant carrageenan present in native weed was a mu/kappa-carrageenan which was readily converted into kappa with hot alkali. The particulate phase from both materials was predominantly cellulose; characterization by X-ray diffraction showed the presence of partial cellulose type I crystallinity. Solid state C-13-NMR spectra showed that this crystalline fraction was similar to 53% of the total cellulose and was comprised of similar to 40% type I alpha and similar to 60% type I beta. The cellulosic part seemed unaffected by the hot alkali treatment. A carrageenan type polysaccharide was found associated with the cellulosic fraction; which underwent the same conversion as the mu/kappa carrageenan on alkali treatment. Further refining of solubilized carrageenan by isopropyl alcohol precipitation, gel pressing or freeze-drying, resulted in only minor differences in chemical composition. Hot solution viscosity and gelation properties in the presence of potassium chloride were measured showing no significant differences in gelling behaviour between alkali treated weed and the different carrageenan isolates but a higher solution viscosity for the alkali treated weed.

Thermal properties of polysaccharides at low moisture: II. Molecular order and control of dissolution temperature in agar

Differential scanning calorimetry (DSC) has been used to probe ordered structures and glassing behaviour for a range of agars containing < 25% w/w water. Most commercial agars are supplied in an ordered (double-helical) state, show an endothermic helix-to-coil transition above 100‡C at low-moisture, and require 90–100‡C for solubilisation in excess water. Agars dried from the coil (single-chain) state show no corresponding endothermic transitions and only require a minimum of 45‡C for aqueous dissolution. Evidence from helix-to-coil transition enthalpies, equilibrium water content as a function of relative humidity, and solid-state13C NMR spectroscopy suggests that water molecules are associated enthalpically with double-helical agar. Single-chain agar is apparently not obtained in a glassy state by direct drying from solution, but in common with double-helical forms, exhibits rubber/glass transition behaviour following heating (in a DSC pan) to 180‡C.

Thermal properties of polysaccharides at low moisture: Part 3 — Comparative behaviour of guar gum and dextran

The behaviour of polysaccharides under conditions of limited water is poorly understood. By contrast, properties of polysaccharides in dilute aqueous environments are reasonably well understood. In particular, generalities of rheological behaviour exist for a large number of polysaccharides (Morris et al., 1981; Morris, 1995) consistent with a random coil configuration and structuring via volume occupancy augmented (at sufficiently high concentrations) by interpolymer entanglement (Ferry, 1970; Graesley, 1974). More specific interactions between polymers in aqueous environments can result in a range of more permanent structures such as gels (Morris, 1995; Gidley and Robinson, 1990). Dextran and guar gum have very different chemical structures (backbones based on (1→6) linked α-d-glucose and (l→4) linked β-d-mannose, respectively), but are both neutral polymers with solution rheologies characteristic of random coils (Morris et al., 1981). The main difference between them lies in the chain stiffness (persistence) which is markedly greater for guar due to the two-bond equatorial-equatorial linkage between monomers compared with the three-bond linkage for dextran (Rees et al., 1982; Burton and Brant, 1983). Detailed rheological characterisation of guar gum solutions (Robinson, Ross-Murphy and Morris, 1982; Richardson and Ross-Murphy, 1987) shows that the system is sufficiently ideal to be related quantitatively to molecular shape parameters. The only deviation from this ideal behaviour is in the consequence of entanglement on viscosity which is somewhat greater for guar than for other random coil polymers (Morris et al., 1981; Robinson, Ross-Murphy and Morris, 1982) probably due to short lived interpolymer associations (Goycoolea, Morris and Gidley, 1995).