Sandra E. Hill

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.

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.

The mechanisms of hydrothermal deconstruction of lignocellulose: New insights from thermal–analytical and complementary studies

Graphical abstract Highlights ► Thermal analysis provides real-time data on hydrothermal reactions under realistic conditions. ► From DSC, hemicellulose hydrolysis has low enthalpy change but xylose degradation is exothermic. ► Hydrothermal exothermic degradation reactions may be similar to early stage biomass pyrolysis. ► DMTA shows that the polymeric structure of lignin in biomass is degraded at high temperatures. ► Hydrothermal reactions are effective at greater than 50% solids content.

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.

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.

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.

Structural reorganisation of cellulose fibrils in hydrothermally deconstructed lignocellulosic biomass and relationships with enzyme digestibility

BackgroundThe investigation of structural organisation in lignocellulose materials is important to understand changes in cellulase accessibility and reactivity resulting from hydrothermal deconstruction, to allow development of strategies to maximise bioethanol process efficiencies. To achieve progress, wheat straw lignocellulose and comparative model wood cellulose were characterised following increasing severity of hydrothermal treatment. Powder and fibre wide-angle X-ray diffraction techniques were employed (WAXD), complemented by enzyme kinetic measurements up to high conversion.ResultsEvidence from WAXD indicated that cellulose fibrils are not perfectly crystalline. A reduction in fibril crystallinity occurred due to hydrothermal treatment, although dimensional and orientational data showed that fibril coherency and alignment were largely retained. The hypothetical inter-fibril spacing created by hydrothermal deconstruction of straw was calculated to be insufficient for complete access by cellulases, although total digestion of cellulose in both treated straw and model pulp was observed. Both treated straw and model pulps were subjected to wet mechanical attrition, which caused separation of smaller fibril aggregates and fragments, significantly increasing enzyme hydrolysis rate. No evidence from WAXD measurements was found for preferential hydrolysis of non-crystalline cellulose at intermediate extent of digestion, for both wood pulp and hydrothermally treated straw.ConclusionsThe increased efficiency of enzyme digestion of cellulose in the lignocellulosic cell wall following hydrothermal treatment is a consequence of the improved fibril accessibility due to the loss of hemicellulose and disruption of lignin. However, incomplete accessibility of cellulase at the internal surfaces of fibrillar aggregates implies that etching type mechanisms will be important in achieving complete hydrolysis. The reduction in crystalline perfection following hydrothermal treatment may lead to an increase in fibril reactivity, which could amplify the overall improvement in rate of digestion due to accessibility gains. The lack of preferential digestion of non-crystalline cellulose is consistent with the existence of localised conformational disorder, at surfaces and defects, according to proposed semicrystalline fibril models. Cellulases may not interact in a fully selective manner with such disordered environments, so fibril reactivity may be considered as a function of average conformational states.

Effects of L-ascorbic acid on the conversion of cassava starch

Abstract Studies have previously shown that the inclusion of ascorbic acid with starches, which are then gelatinised in excess water, causes differences in the amount of starch conversion. Also, incorporation of ascorbic acid in dilute pastes has been shown to decrease the viscosity of the paste when stored at a range of temperatures. In this study the pasting behaviour of cassava starch in the presence of ascorbic acid has been followed using a Rapid Visco Analyser. Decreases in peak and final viscosity were apparent, with the largest reductions being with the highest amounts of ascorbic acid addition (0.1%). In contrast with other published work the effects of ascorbic acid on heating pastes made with a high starch content were studied. The amount of conversion of cassava starch was estimated by assessing the volume occupied by the swollen granules and the amount of soluble material released. Viscosity measurements of the starch polysaccharides after solubilisation in alkali were also established. Pasting behaviour of the samples that had previously been heated in the presence of ascorbic acid and buffer was also assessed. Ascorbic acid inclusion seemed to affect starch conversion with the results for peak and final viscosity correlating with the alkaline viscosity estimates. The inclusion of ascorbic acid also alters the appearances of the starch sample. Ascorbic acid inclusion made the samples lighter in colour and they did not hold their shape after processing.

Evidence for sulfite induced oxidative reductive depolymerisation of starch polysaccharides

Abstract The influence of low levels of sodium sulfite on the viscosity of 0.90 g/100 mL dispersions of wheat and potato starch pasted at 95 °C has been studied. It is shown that, if pasting is carried out in 0.01 g/100 mL sulfite, the viscosity of the starch following solubilisation in 0.5 M KOH is lower than a control pasted in the absence of sulfite. The starch intrinsic viscosity is also reduced after pasting in sulfite, indicating that some polysaccharide degradation has occurred. It is suggested that this is the reason for the increased release of polysaccharide from the starch granule when pasted in the presence of sulfite. For both starches the intrinsic viscosities show that there is a reduction in degradation when the sulfite level is increased or when the polar antioxidant propyl gallate is included in addition to sulfite indicating that the mechanism is oxidative reductive depolymerisation of the starch polysaccharides. It is also suggested that oxidative degradation may have an important influence on the integrity of the starch granule following pasting and may be a factor in the functional behaviour of oxidising agents in baked products. Low levels of sodium sulfite and sodium chloride both dramatically reduce the viscosity of potato starch as a result of a non-specific ionic effect.