Molecular basis of the gelatinisation and swelling characteristics of waxy barley starches grown in the same location during the same season. Part II. Crystallinity and gelatinisation characteristics

Nine waxy barley samples (grown at the same site during the same season) were investigated to identify those molecular aspects of amylopectin structure and architecture which define the order and gelatinisation characteristics. Using ¹³C CP-MAS/NMR it was confirmed that the number of double helices within the starches were approximately constant although differences in crystallinity were identified by X-ray diffraction. These differences in terms of amount of crystalline order correlated well with gelatinisation temperatures. The onset (T_o), peak (T_p) and conclusion (T_c) gelatinisation temperatures were 53.4, 59.2 and 68.1 °C on average with the associated enthalpy (ΔH) of 11.0 and 13.5 J g⁻¹ on a starch and amylopectin basis. Annealing of the starches below T_o elevated T_o, T_p and T_c by +11.9, +8.2 and +5.1 °C on average and sharpened the gelatinisation range (T_c−T_o). Acid hydrolysis after annealing increased T_o, T_p and T_c (especially T_c) by +2.3, +17.4 and +34.7 °C on average. Annealing in the presence of α-amylase elevated similarly the gelatinisation parameters by +10.2, +7.1 and +2.8 °C for T_o, T_p and T_c, respectively. Crystalline lamellae lengths were found to be 5.2±0.7 and 6.2±0.4 nm using high sensitivity differential scanning micro-calorimetry and differential scanning calorimetry, respectively.     

Effect of Soaking on Wet-milling of Rice

Soaking is an essential step in wet-milling of rice flour. The effects of soaking duration and temperature (5 and 25 °C) on the properties of rice flour have been investigated. The uptake of water by rice kernels increased with temperature and reached a plateau at about 30–35%. Protein, lipid, and ash leached out during soaking. The moisture content after soaking appeared to be a key factor on loosening the structure of rice kernels, which resulted in the production of small particle flours with little starch damage. The particle size of flours did not alter the gelatinisation temperature (T_oand T_p) in DSC thermograms. Small particle and low lipid content flours appeared to have high peak viscosity measured by RVA. The change in microstructure of rice kernels during soaking was also examined by SEM.     

Retrogradation of waxy and normal corn starch gels by temperature cycling

Gelatinized waxy and normal corn starches at various concentrations (20–50%) in water were stored under temperature cycles of 4°C and 30°C (each for 1 day) up to 7 cycles or at a constant temperature of 4°C for 14 days to investigate the effects of temperature cycling on the retrogradation of both starches. Compared to starches stored only at 4°C, both starches stored under the 4/30°C temperature cycles exhibited smaller melting enthalpy for retrogradation (ΔH_r), higher onset temperature (T_o), and lower melting temperature range (T_r) regardless of the starch concentration tested. Fewer crystallites might be formed under the temperature cycles compared to the isothermal storage, but the crystallites formed under temperature cycling appeared more homogeneous than those under the isothermal storage. The effect of starch content on the retrogradation was greater when the starch gels were stored under cycled temperatures. The reduction in ΔH_r and the increase in conclusion temperature (T_c) by retrogradation under 4/30°C temperature cycles became more apparent when the starch concentration was lower (20 or 30%). Degree of retrogradation based on melting enthalpy was greater in normal corn starch than in waxy corn starch when starch content was low.     

Molecular Basis of the Gelatinisation and Swelling Characteristics of Waxy Rice Starches Grown in the Same Location During the Same Season

Six waxy rice samples (grown at the same site during the same season) were investigated to identify those aspects of amylopectin structure and granule composition which differentiate the starches into high (T_p 77·8 °C on average) or low (T_p 66·2 °C on average) gelatinisation temperature. All starches contained less than 2·5% amylose with negligible lipid. Granule dimensions were similar (mean diameter 4·9–5·7 μm) as was the β-amylolysis limit (59·9–63·0%). However, the structural elements of amylopectins within the two groups varied. The low gelatinisation temperature starches contained two major chain fractions upon debranching with iso-amylase, F3 (DP 16) and F1 (DP 51) on average while the high gelatinisation temperature starches contained three fractions, F3 (DP 16), F2 (DP 19) and F1 (DP 40). The proportion of F1/(F2+F3) for the low and high gelatinisation starches representing on average 20·2 and 10·5% respectively. Hence, the higher proportion of exterior chains and especially those with DP 19 appear to confer the higher gelatinisation temperature characteristic of these starches. Debranched β-limit dextrins were prepared from these starches. Regardless of their origin, (high or low gelatinisation temperature), they contained three fractions: F_3β (DP 5), F_2β (DP 13) and F_1β (DP 19). Hence, the high versus low gelatinisation characteristic was imparted by the exterior chains. Using 13-C CP-MAS/NMR it was confirmed that the number of double helices within the starches were approximately constant although small differences in crystallinity were identified by X-ray diffraction. Retrogradation of the solubilised native high and low gelatinisation temperature amylopectin molecules confirmed that the ‘high temperature gelatinisation amylopectin’ molecules form higher dissociation temperature and enthalpy crystalline domains. This confirms that the higher proportion of short and especially relatively longer short chains promotes crystallite formation.     

Swelling and Enzymatic Hydrolysis of Starch in Low Water Systems

The matrix effects of starch to water ratio, temperature and time on the gelatinisation, swelling and enzymatic (alpha -amylase) hydrolysis of five starches (waxy maize, maize, wheat, tapioca and potato) were investigated. It was established that by controlling the extent of gelatinisation with appropriate incubation conditions, some residual order could be retained. With starch to water ratios of 1:1, the amount of residual order represented 67·5, 40·9, 37·7, 30·3 and 15·1 for waxy maize, maize, wheat, tapioca and potato starches respectively. Whilst gelatinisation was controlled by the availability of water together with temperature and time, the effect on swelling was more marked. This reflects the fact that although granule order may be (just) lost under limiting (water) conditions, the expansion of amorphous material is more extensively moderated when water is restricted. Under the conditions investigated, the amount of hydrolysis exceeded the swelling factor which shows that although the granules may be only partially swollen, they are susceptible to amylase hydrolysis and hence potentially digestion. However, the amount of hydrolysis is also intimately related to the extent of gelatinisation which has important implications to food, alcohol production and other industrial processes.     

Effect of extrusion cooking on chemical structure,morphology, crystallinity and thermal properties of sorghum flour extrudates

The effect of feed moisture content (10, 14 and 18%) and die temperature (110 and 160 °C) on functional properties, specific mechanical energy (SME), morphology, thermal properties, X-ray diffraction pattern (XRD), Fourier transform infrared spectroscopy (FTIR) and amylose-lipid complex formation of extruded sorghum flour was investigated. Results showed that the extrusion cooking significantly changed the functional properties of extruded sorghum flour. Increasing feed moisture increased the peak gelatinization temperature (T_p), the degree of gelatinization (%) and starch crystallinity (%) while it decreased the gelatinization temperature ranges (T_c - T₀), starch gelatinization enthalpy (ΔH_G) and amylose-lipid complex (%) formation. With increasing die temperature, the degree of gelatinization and amylose-lipid complex formation increased and the starch T_p, T_c-T₀, ΔH_G and crystallinity decreased. The FTIR spectra also showed that the extrusion cooking did not create new functional groups or eliminate them in sorghum protein, whereas the sorghum extrudate protein had random coil conformation.     

Genetic and Environmental Variation in Sorghum Starch Properties

Starch was isolated from eight local Zimbabwean landrace varieties, an improved cultivar (SV2) and a hybrid (DC-75) of sorghum grown in four environments. Amylose content, pasting (peak (PV), hot-paste (HPV), cool-paste (CPV) viscosity), textural and thermal (gelatinisation peak temperature (T_p) and gelatinisation energy (ΔH)) properties of the starches were determined. The F -tests from analyses of variance detected significant (p<0·001) differences among genotypes and growing environments for the starch properties measured. The results indicate that a range of genetic and environmental variability exists for these traits in sorghum genotypes although the latter could be greater than varietal effects. Hybrid DC-75 largely differed in starch amylose content, pasting PV, and gel hardness from the local landrace varieties. Environments used for local landrace varieties caused significant differences in starch properties, hence selection and monitoring of growing conditions is essential if a particular genotype is to maintain minimum variation in the desired pasting, textural or thermal properties. Genotype×environment interactions indicate that in breeding programmes, selection for starch properties at a single location would be misleading.     

A Multi-stages Biosynthetic Pathway in Starch Granules Revealed by the Ultrastructure of Maize Mutant Starches

Our previous work indicated that starches containing B-type crystallites show low susceptibility to amylolysis and suggested that B-type crystallites have an effect on starch granule organisation. To elucidate granular ultrastructure, double wxae and aedu maize mutant starches containing A- (30 and 50% respectively) and B-type (70 and 50% respectively) crystallites were treated with porcine pancreatic alpha -amylase. The surface structure of the native and degraded starches was studied by scanning electron microscopy, and the internal ultrastructure by transmission electron microscopy after staining with PATAg reagents. The results confirm the influence of B-type crystallites on granule organisation and indicate that starches containing B-type crystallites show an amylolysis attack pattern with minor exocorrosion and major endocorrosion. The granule organisation of A- and B-type starches proposed is not consistent with an onion ring model¹and may account for the different behaviour of these starches to amylolysis. Transmission electron microscopy showed that most native wxae and aedu starch granules are composed of a core with a classical alternating structure and a peripheral ring. The peripheral ring in wxae starch was ordered and resistant to amylolysis, whereas that of aedu was disordered and degradable. It is proposed that these two specific forms of granule organisation are attributable to variations in the enzymatic activities of specific starch synthesising enzymes during the course of starch biosynthesis.     

Thermomechanical Glass Transition of Extruded Cereal Melts

Cereal melts were produced via extrusion cooking using a twin-screw extruder at in-barrel moisture contents ranging from 25 to 42% (wb). Dynamic mechanical thermal analysis was performed to determine the plasticizing effect of water on the glass transition (T_g) for melts obtained from degermed corn meal, oat flour, pregelatinized corn starch and pregelatinized waxy maize. For degermed corn meal, T_gwas found to decrease with increasing water contents up to about 30% (wb). Above 30% moisture content, T_gwas found to be essentially constant and approximately equal to 270K. Thus, W_g′ was estimated to be 30% moisture content. For samples of moisture content less than W_g′, loss tangent vs. temperature plots in the glass transition region were observed to be broader and of smaller amplitude than those above W_g. Glass transitions ranged from 270 to 321K for sample moisture contents ranging from 36·3 to 22·2% (wb), respectively. For an oat flour, T_gvalues ranging from 270 to 303K were observed for moisture contents of 39·2 to 26·1% (wb), respectively. Melts formed from pregelatinized corn starch and pregelatinized waxy maize had glass transitions similar to corn meal at a similar moisture content of 27%. The significance of these results, in terms of extrusion cooking of cereals, is discussed in this work.     

Determination of thermal and retrogradation properties of rice starch using near-infrared spectroscopy

Starch is a major component of rice grain and thus plays an important role in grain quality. For breeding rice with improved quality, the thermal and retrogradation properties of starch may be routinely measured. Since direct measurement is time-consuming and expensive, rapid predictive methods based on near-infrared spectroscopy (NIRS) can be applied to measure these quality parameters. In this study, calibration models for measurement of thermal and retrogradation properties were built from the spectra of grain and flour samples. The results indicated that both grain and flour spectra could give similar accuracy (r²=∼0.78) in determining the peak temperature (T_p) and conclusion temperature (T_c) of gelatinization. However, flour spectra (r²=0.80) were superior to the grain spectra (r²=0.73) in measuring onset temperature (T_o). Furthermore, the thermal properties of width at half peak height (ΔT_1/2) and enthalpy of gelatinization (ΔH_g), and retrogradation properties of enthalpy of retrogradation (ΔH_r) and retrogradation percentage (R%) could only be successfully modeled with the flour spectra. The models reported in the present study are usable for routine screening of a large number of samples in early generation of selection in breeding programs. For accurate assay of the thermal and retrogradation properties, however, direct instrumental measurement should be employed in later generations.     

Microwave irradiation differentially affect the physicochemical properties of waxy and non-waxy hull-less barley starch

Waxy and non-waxy hull-less barley kernels and their isolated starches were irradiated under different microwave conditions (power 640, 720, and 800 W, time 60, 120 and 180 s). Changes in physicochemical properties were studied to investigate the effects of microwave irradiation (MWI) on in-kernel starches and isolated starches. For in-kernel starch, microwave reduced the ratio of 1047/1022 cm⁻¹ wavelengths, gelatinization enthalpy (ΔH_g) and relative crystallinity (RC), indicating that microwave of starch within the cells disrupted the crystalline regions. For isolated starch, microwave decreased the ratio of 1047/1022 cm⁻¹ wavelengths but increased ΔH_g of isolated starch, indicating that microwaving resulted in disruption of amorphous structure and an increase in the amount of remaining double helix structure. Moreover, viscosities of in-kernel starches decreased as microwave power and time increased, but this was not observed in isolated starches. Microwave treatment induced an enhancement of gelatinization temperature for non-waxy starches (NWS) but decreased in waxy starches (WS). Microwave had a greater effect for swelling power and solubility on in-kernel MWI-WS than MWI-WS, whereas the reverse results were found between in-kernel MWI-NWS and MWI-NWS. The results indicated that amylose plays a profound role in the properties of isolated and in-kernel starches during microwaving.     

Amylose and Lipid Contents,Amylopectin Structure,and Gelatinisation Properties of Waxy Wheat (Triticum aestivum) Starch

Starch was isolated from the endosperm of three recently developed waxy wheat lines and their parents. Their amylose and lipid contents, amylopectin structures and gelatinisation properties were evaluated. The endosperm starch from waxy wheat lines is composed essentially of amylopectin. The apparent amylose (1·2–2·0 g/100 g) and lipid contents (0·12–0·29 g/100 g) are much lower than the apparent amylose (26·0–28·4 g/100 g) and lipid contents (1·05–1·17 g/100 g) of their non-waxy parents. The amylopectin of waxy wheat lines is structurally identical to that of the parents. The peak gelatinisation temperature and gelatinisation enthalpy for waxy starch are significantly higher than for non-waxy starch, but the gelatinisation enthalpy for the amylopectin fraction of waxy starch is nearly identical to that of non-waxy starch.     

Analysis of quantitative trait loci for some starch properties of rice (Oryza sativa L.): thermal properties, gel texture and swelling volume

To understand the genetic basis of gelatinization temperature (GT), gel textural traits and flour swelling volume, quantitative trait loci (QTL) were mapped for these traits using a doubled haploid (DH) population derived from a cross between indica variety Zai-Ye-Qing 8 (ZYQ8) and japonica variety Jing-Xi 17 (JX17). The results indicated that the starch property parameters were continuously distributed among the DH lines, and some DH lines showed transgressive segregation for all the parameters. A total of 16 QTLs were identified for seven traits. A major QTL, the alk gene on chromosome 6 was significant for the three GT traits, onset temperature (T_o), peak temperature (T_p), and completion temperature (T_c). This locus could explain 49.4, 38.9, and 28.3% of the total variance, respectively, indicating that GT parameters were substantially controlled by the alk gene which has previously been identified for alkali spreading value (ASV). The additive effects of alk on T_o,T_p and T_c were from ZYQ8. Another two QTLs on chromosomes 1 and 7 were also identified for the three GT parameters, whose positive effects were contributed from JX17. Another QTL on chromosome 10 with the positive effects coming from ZYQ8 was significant only for T_c. However, the enthalpy (ΔH) of gelatinization was controlled by two minor QTLs on chromosomes 1 and 7. A major QTL, the Wx gene on chromosome 6 was identified for gel hardness and flour swelling volume which explained 58.8 and 36.6% of the total variances, respectively. Another two QTLs were also detected for flour swelling volume, one of which (qSV-7) could explain 21.4% of the total variance. However, gel cohesiveness was controlled by a major QTL located between alk and Wx on chromosome 6.     

Kinetics of starch digestion and functional properties of twin-screw extruded sorghum

The time-course of starch digestion in twin-screw extruded milled sorghum grain was investigated using an in-vitro procedure based on glucometry. The sorghum grains were hammer-milled, and extruded at three levels each of moisture and screw speed. Irrespective of the extrusion conditions, extruded and non-extruded milled sorghum grain exhibited monophasic digestograms, and the modified first-order kinetic and Peleg models adequately described the digestograms. Extrusion increased the rate of digestion by about ten times compared with non-extrudates. Starch gelatinisation varied in the extrudates, and microscopy revealed a mixture of raw, gelatinised and destructured starch and protein components in the extrudates. Starch digestion parameters significantly (p < 0.05) correlated with extruder response and various functional properties of the extrudates. Extrusion conditions for maximum starch gelatinisation in milled sorghum grain for fastest digestion as an efficient animal feed were interpolated, as well as the conditions for directly-expanded extrudates with potential for human food, where minimum starch digestion is desired.     

Towards an optimal process for gelatinisation and hydrolysis of highly concentrated starch–water mixtures with alpha-amylase from B. licheniformis

The enzymatic hydrolysis of starch is usually carried out with 30–35 w/w% starch in water. Higher substrate concentrations (50–70 w/w%) were reached by using a twin-screw extruder for gelatinisation and for mixing enzyme with gelatinised starch prior to enzymatic hydrolysis in a batch reactor. The aim of this study was to determine which parameters are important for gelatinisation of wheat starch and to investigate the effects of different extrusion conditions on the enzymatic hydrolysis. After extrusion, the degree of gelatinisation was measured. During hydrolysis, the carbohydrate composition, the dextrose equivalent (DE) and the alpha-amylase activity were measured. Gelatinisation measurements showed that mechanical forces lowered the temperature required for complete gelatinisation. During hydrolysis experiments, high DEs were observed even if starch was not completely gelatinised during extrusion. Due to high substrate concentrations, the residual alpha-amylase activity remained high throughout enzymatic hydrolysis, although high temperatures were used. Increased substrate concentrations did not affect the carbohydrate composition of the product. Furthermore, the time required for the batch hydrolysis step could be varied by choosing a different enzyme-to-substrate ratio. This article provides a basis for detailed optimisation of this process to develop an industrial-scale process at high substrate concentrations.     

Viscoelastic Properties of Extruded Cereal Melts

Hydrophilic biopolymer systems reach a state of maximum plasticization at a concentration of water designated as W_g′. The thermomechanical behaviour of various extruded cereals were investigated at concentrations above and below W_g′ to determine the significance of W_g′ on the viscoelastic properties in the rubbery region above T_g. Cereal melts were produced via extrusion cooking using a twin-screw extruder at in-barrel moisture contents ranging from 25 to 42% (wb). Dynamic mechanical thermal analysis (DMTA) was used to determine the viscoelastic properties for melts obtained from degermed corn meal, oat flour, pregelatinized corn starch and pregelatinized waxy maize. All of the extruded melts displayed reductions in the storage modulus of about two orders of magnitude from the glassy to rubbery regimes. At temperatures greater than 320K, another transition is apparent for extruded oat flour. These losses may be attributed to a reduction in physical crosslinks and/or entanglements. With respect to the other melts investigated, differences in the oat flour melts at temperatures above T_gmay be due to the increased presence of protein and lipid. Given the apparent partly crystalline nature of these melts at moisture contents <W_g′, the most appropriate representation of the viscoelastic properties may be isotimic plots of the viscoelastic functions. These contour plots provide a more comprehensive guide to material behaviour than the ‘traditional’ state diagrams found throughout the literature. An important point is that prediction of viscoelastic properties above T_gfor polymers where the internal structure changes with temperature or moisture content, cannot be predicted using the same methods of reduced variables as for amorphous polymers. Thus rather than using these methods, experimental data, such as given here, may prove to be necessary to design or understand processes involving cereal melts based on their viscoelastic properties.     

Starch Properties of a Bread Wheat (Triticum aestivum L.) Mutant with an Altered Flour-pasting Profile

Ethyl methanesulphonate treatment of seeds from cultivar Kanto 107 lacking Wx-A1 and Wx-B1 proteins induced a mutant, named K107Afpp4, showing an altered flour-pasting profile, i.ea markedly increased viscosity at a low temperature and a decreased peak time in Rapid Visco Analyser measurement. The colour of the iodine complex of the isolated starch from the mutant remained bluish-purple but its λ_maxdecreased to 584 nm from 601 nm shown by the wild type Kanto 107. The absorbance at 680 nm (blue value) of the mutant starch decreased to 0·206 from 0·324 for Kanto 107. The amylose content of K107Afpp4 starch was 12·7% measured by concanavalin A method, compared to 21·9% for the wild type. The apparent amylose content of the mutant starch measured by amperometric titration was 15·9% while that of Kanto 107 was 25·3%. Gel permeation chromatography showed that the mutant starch has a decreased amount of lower molecular weight fractions than starch from the wild type. Differential scanning calorimetry indicated that mutant starch has a higher gelatinisation peak temperature, 61·8 °C, than the wild type, 59·5 °C. The mutant will be useful as a genetic resource for altering wheat starch/flour properties and for clarifying the relationship between amylose content and wheat quality.     

Investigation of the high-amylose maize starch gelatinization behaviours in glycerol-water systems

The effect of glycerol on gelatinization behaviours of high-amylose maize starch was evaluated by confocal laser scanning microscopy (CLSM), scanning electronic microscope (SEM), differential scanning calorimetry (DSC), texture analyzer (TPA) and rheometer. Gelatinization of the high-amylose maize starches with glycerol content of 10% (w/w) began at 95.4 °C (T_o), peaked at 110.3 °C (T_p), and completed at 118.9 °C (T_c). The birefringence began to disappear at around 100 °C and finished at 120 °C which corresponded well to the onset and conclusion temperatures obtained by DSC. The high-amylose maize starch granules maintained original morphological structure at 100 °C and swelled to a great degree at 110 °C. The high-amylose maize starch paste formed at 100 °C showed the lowest hardness (39.92 g), while at 120 and 130 °C, showed the highest hardness (610.89 g and 635.43 g, respectively). It should be noted that in going from 100 °C to 110 °C there is a significant increase in the viscosity of the slurry solution. The identical apparent viscosity was observed when the shear rate exceed 100 s⁻¹, resulting from the high-amylose maize starch granules were completely gelatinized at 120 °C, which was consistent with DSC analysis.     

Effect of soaking temperature on commingled rice properties

Parboiling involves soaking, steaming, and drying, and soaking is important in achieving desired parboiled rice properties. This study investigated the effects of soaking temperature and commingling on rice properties prior to steaming. Rough rice of four cultivars (Taggart, CL151, XL753, and CL XL745) and their combinations at 1:1 wt ratio were soaked at 65, 70 or 75 °C for 3 h, and dried. Both soaking temperature and difference in onset gelatinization temperature (T_o) of individual cultivars in commingled rice affected milling and physicochemical properties. The head brown rice yield was greater when the soaking temperature was below but close to the T_o for individual rice cultivars, but became difficult to predict for commingled rice. Commingled rice consisting of high T_o rice cultivars required higher soaking temperatures to reduce chalkiness during soaking. The color attributes of commingled rice was predominately affected by the cultivar that exhibited the most change. The gelatinization properties were governed by the low-T_o cultivar, whereas the pasting properties were more influenced by the high-T_o cultivar for the commingled rice. Therefore, using commingled rice with a wide range of gelatinization temperature as a feedstock may lead to inconsistent quality of parboiled rice.     

Study on structure and molecular dynamics of starch/poly(sodium acrylate)-grafted superabsorbent by <Superscript>13</Superscript>C solid state NMR

The domain-structure of samples containing a series of starch/poly(sodium acrylate)-grafted superabsorbents, pure starch, pure poly(sodium acrylate), and blend of starch/poly(sodium acrylate) has been studied by high-resolution solid-state ¹³C NMR spectroscopy at room temperature. The result shows that the crystallinity of starch decreases greatly in the grafted and blended samples. The values of ¹H spin-lattice relaxation time in rotating frame T _1ρ and ¹H spin-lattice relaxation time T ₁ shows that starch and poly(sodium acrylate) components in both grafted and blended samples have good compatibility in nanometer scale. In the ¹³C cross-polarization/magic-angle-spinning (CP/MAS) spectra, the chemical shift of the carbonyl group of poly(sodium acrylate) depends on the composition of the grafting samples, which indicates that the starch and the poly(sodium acrylate) components of the grafting samples exhibit better compatibility with each other than that of blended samples at molecular level.