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1.
Hydrophilic biopolymer systems reach a state of maximum plasticization at a concentration of water designated as Wg′. The thermomechanical behaviour of various extruded cereals were investigated at concentrations above and below Wg′ to determine the significance of Wg′ on the viscoelastic properties in the rubbery region above Tg. 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 Tgmay be due to the increased presence of protein and lipid. Given the apparent partly crystalline nature of these melts at moisture contents <Wg′, 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 Tgfor 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.  相似文献   

2.
The glass transition temperatures (Tg) of starch with different amylose/amylopectin ratios were systematically studied by a high-speed DSC. The cornstarches with different amylose contents (waxy 0; maize 23, G50 50 and G80 80) were used as model materials. The high heating speed (up to 300 °C/min) allows the weak Tg of starch to be visible and the true Tg was calculated by applying linear regression to the results from different heating rates. It is confirmed for the first time, that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. It was found that Tg was increased from about 52 to 60 °C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. The microstructure and phase transition were used to explain this phenomenon, in particular the multiphase transitions that occur in high-amylose starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin.  相似文献   

3.
Water uptake during cooking of ten dry bean (Phaseolus vulgaris L.) varieties was investigated. Water uptake during early stages of cooking was characteristic of the variety. Although the optimal cooking times varied widely (52–85 min), all the beans absorbed similar amounts of water when cooked for their optimal times. Cooking times were significantly correlated with W20 min (r=–0.92) and hardness index (r=0.76) of beans. Most other physical characteristics excepting the surface area of beans were generally unrelated to theW 20 min,W opt, and cooking times. No significant correlation was observed between phytate content and cooking times of beans. On cooking for their respective optimal times, all varieties absorbed nearly 1.5 times their weight of water and attained a moisture content of about 65% (wet basis).  相似文献   

4.
不同鲜食玉米灌浆期子粒主要营养成分的积累动态   总被引：1，自引：0，他引：1       下载免费PDF全文
通过理化方法对5种不同类型的14个鲜食玉米品种在不同灌浆期子粒的营养成分进行分析,研究不同类型鲜食玉米子粒主要营养成分的积累动态,筛选能够指示不同类型鲜食玉米最佳采收期的客观理化指标。结果表明,鲜食玉米灌浆期子粒的可溶性总糖、蔗糖、还原糖、蛋白质、支链淀粉、直链淀粉和含水量在不同品种间、不同灌浆期间差异极显著。随着子粒的发育,不同类型鲜食玉米品种的可溶性总糖和蔗糖含量呈动态曲线变化,变化趋势都是先上升后下降;还原糖含量和含水量呈不断下降的趋势;直链淀粉和支链淀粉含量呈不断上升的趋势;蛋白质含量变化幅度很小,变化趋势不明显。根据鲜食玉米营养成分与品质的关系,提出子粒含水量结合可溶性糖含量和粗淀粉含量可分别作为甜玉米和糯玉米最佳采收期的指标。  相似文献   

5.
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 (ΔHr), higher onset temperature (To), and lower melting temperature range (Tr) 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 ΔHr and the increase in conclusion temperature (Tc) 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.  相似文献   

6.
In the present study, blue corn flour films were developed. The cereal grain’s total composition (excluding the pericarp) is used to obtain the films. The plasticizing effects of two different polyols such as glycerol and sorbitol on the mechanical, thermal, and microstructural properties of flour films were researched. The results showed that films plasticized with sorbitol had better mechanical properties and less affinity for water than those plasticized with glycerol. The sorbitol-plasticized films were more rigid and did not lose their integrity when immersed in water. The ATR-FTIR spectra of blue corn flour plasticizer with sorbitol showed the presence of the additional band at 1745 cm−1 characteristic of the vibrational carbonyl peak, which confirms the chemical linkages between sorbitol and a polymeric matrix. The effect of the plasticizer on the glass transition temperature (Tg) was characterized using differential scanning calorimetry (DSC). Tg decreased as the plasticizer content increased. Plasticized glycerol films showed lower Tg values than those with sorbitol. SEM observations showed that it was necessary to add plasticizer to maintain film integrity. The sorbitol-plasticized flour films revealed better adhesion between phases, and these films showed a compact structure.  相似文献   

7.
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 (Tp), the degree of gelatinization (%) and starch crystallinity (%) while it decreased the gelatinization temperature ranges (Tc - T0), starch gelatinization enthalpy (ΔHG) and amylose-lipid complex (%) formation. With increasing die temperature, the degree of gelatinization and amylose-lipid complex formation increased and the starch Tp, Tc-T0, ΔHG 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.  相似文献   

8.
Study of the effect of hydrothermal process conditions on pasta quality   总被引：1，自引：1，他引：1  
The effect of hydrothermal treatment on the pasting, hydration properties and colour quality of commercial fresh pasta were studied following an Instantaneous Controlled Pressure Drop treatment. This hydrothermal procedure involves a physical modification at high temperature (<144 °C) and restricted moisture content (<30%) during a very short time (15–40 s) followed by a rapid pressure drop to a vacuum (50 mbar). Two process variables (steam pressure level and processing time) were investigated using response surface methodology. Steam pressure level had the greatest effect on hydration and pasting parameters. Increased pressure resulted in elevation of mass ratio between cooked and uncooked pasta (Wi/W0)100 °C and reduction of for all treated pasta. The optimum cooking time of untreated pasta was 7 min (W/W0=2.4), whereas for the same ratio it was less than 1 min for pasta treated at pressures ranging between 2.5 and 3.5 bar. The viscosity at 10 min (V10) increased linearly when the pressure level increased. V10 was 146 cP for untreated pasta whereas it reached 2659 cP for pasta treated at 3.9 bar (144 °C) for 25 s. The cold paste viscosity and setback viscosity decreased when the pressure varied from 1 to 3.9 bar.  相似文献   

9.
The objective of this work was to evaluate the effect of alkaline cooking on the oxidative stability of oil in corn flour. A central composite design was used to study the combined effect of lime concentration (%) and steep time (h) on peroxide value (PV); specific extinction coefficients at 232 and 270 nm (K232 and K270); and FTIR absorbance at 3009 cm?1, 3444 cm?1, and 3530 cm?1 in oils from corn flour obtained by alkaline cooking. The results indicate that lime concentration and steep time affected the PV, K232, and K270. A decrease of 2.56 % was observed in the IR absorption bands, corresponding to the polyunsaturated fatty acids. The FTIR spectra also showed absorption bands related to the secondary oil oxidation products.  相似文献   

10.
Oats (Avena sativa L.) were revaluated in recent years as a promising crop for improving the nutritional quality of foods, due to their richness in many bioactive compounds, including phenols. These plant secondary metabolites are useful as radical scavenging, and also possess positive biochemical effects against cardiovascular diseases, cancer growth and age-related diseases. Twenty oat cultivars were analyzed for their soluble phenol content (SPC, ranging 0.78–1.09 gGAE/kg d.m.) and total antioxidant capacity (TAC, ranging 13.99–18.84 mmol TE/kg d.m.). In another experiment, the kinetics of SPC accumulation and TAC in the immature grains of five oat cultivars revealed a marked decrease of both parameters (−48.9% and −72.1%, respectively) from ten days after anthesis to maturity. These results could suggest a possible use of immature oat grains in human nutrition, as it was already proposed for other cereals.  相似文献   

11.
Expanded polystyrene (EPS) foams are widely used to provide protection to merchandise during shipping and handling. The disposal of EPS foams, however, causes pollution to the environment. Starch based loose fill packaging foams are commercially available to replace the EPS foams. Starch foams dissolve readily in water, and do not possess satisfactory physical and mechanical properties. To improve the functional properties and water resistance of starch foams, regular (25% amylose) and waxy corn starches blended with Mater-Bi® ZF03U (MBI) were extruded into loose-fill packaging foams using a co-rotating twin screw extruder. MBI contents of 10, 25 and 40% and starch moisture contents of 19, 22 and 25% (d.b.) were used. Waxy starch produced foam with greater radial expansion and lower unit and bulk densities than regular starch. Higher levels of MBI reduced radial expansion and increased unit and bulk densities. Foam made with regular starch had a lower water solubility index than foam made with waxy starch. The addition of MBI improved the water solubility. Higher starch moisture contents increased foam water solubility indices. Foams made from both types of starch possessed acceptable mechanical properties. Regular starch produced more rigid foams than the waxy starch as indicated by the higher compressibility values based on both single-piece and bulk samples. Moisture content affected the compressibilities. Higher moisture contents produced more rigid foams. The spring indices of bulk samples were excellent (>95%) for all MBI contents.  相似文献   

12.
A sensory profiling analysis of heat-processed oats (Avena sativaL.) was performed. The oat grains were of three varieties (Kapp, Mustang and Svea), stored at different relative humidities (30, 55 or 80%) for 3·5 or 15·5 months and heat-treated either with or without prior dehulling. The main differences in sensory attributes were related to the various thermal processes. Processed dehulled samples, particularly of theMustangvariety, had the highest intensities of oat odour, oat flavour, fresh odour and fresh flavour. Samples processed with hulls, in particular the varieties Kapp and Svea had the highest intensities of odour and flavour, rancid odour and flavour and bitterness. The sensory profile of processed oats were described partly by variations in phenolic compounds of low molecular weight, fat acidity and moisture content after processing. Twenty-nine per cent of the variation in odour and flavour attributes was explained by the 11 phenolic compounds analysed. Water content and the phenolic compoundsp-coumaric acid, vanillin,p-hydroxybenzaldehyde and coniferyl alcohol were significantly correlated (P<0·05) to high levels of rancidity, flavour intensity and bitterness and low levels of freshness, oat odour and flavour. The avenanthramides were related mainly to low levels of flavour intensity and rancid odour and flavour. Caffeic acid and fat acidity were related to low intensities of sweetness and aftertaste.  相似文献   

13.
燕麦粉不含面筋蛋白,使燕麦全粉面条煮熟后易浑汤、蒸煮损失率大,目前只能温水浸泡复水食用,是限制燕麦更广泛推广的难题之一。为了满足市场对燕麦全粉面条的需求,以利用二级挤压技术生产的燕麦全粉挤压面条为试材,对成型面条进行了调湿老化、蒸制老化及冷冻老化处理,比较了不同老化方式对燕麦全粉挤压面条蒸煮品质的影响。结果表明,调湿老化处理的面条蒸煮损失率最小(2.26%~3.34%),蒸制老化次之(4.19%~5.19%),冷冻老化最大(5.16%~7.97%)。不同老化处理的面条感官评分差异不显著,以调湿老化处理24h的面条感官评分最高(87.2分);调湿老化处理面条的淀粉相对结晶度(12.47%~16.10%)、热焓值(1.044 0~1.056 0J·g-1)及回生值(519.5~558.0cP)均最大,蒸制老化次之,冷冻老化最小。综合分析可知,对燕麦挤压面条进行淀粉老化处理,能够显著降低燕麦全粉挤压面条的蒸煮损失率,最优方式为调湿老化24h。  相似文献   

14.
The objective of this research was to investigate the effects of different ingredients (reducing sugars, leavening agents, citric acid), processing conditions (feed moisture content, exit die temperature), and extrusion cooking methods (with/without CO2 injection) on furfural and 5-hydroxymethylfurfural (HMF) formation in corn-based extrudates. The decrease in furfural levels was 16% upon addition of sodium-bicarbonate, while the increase was 12% upon addition of ammonium-bicarbonate for extrudates produced at 150 °C and feed moisture content of 22%. The furfural concentration of extrudates produced at 150 °C and feed moisture content of 22% without leavening agents was 810 μg/100 g and increased to 11,901 μg/100 g upon citric acid addition (about a 15-fold increase). Furthermore, both low feed moisture and high exit die temperature had promoting effects on furfural and HMF contents. The CO2 injection method did not have considerable effect on furfural and HMF levels of the extrudates; however, it positively affected the physical properties of extrudates.  相似文献   

15.
《Field Crops Research》2002,74(1):67-79
Field experiments were conducted during two rainy seasons to study the effect of soil moisture deficit on total biomass, pod yield, harvest index (HI) and drought tolerance index (DTI) in groundnut (Arachis hypogaea L.) cultivars possessing a wide range of specific leaf area (SLA, 144–241 cm2 g−1). There were three soil moisture regimes: adequate irrigation (W1), drought simulated under rain-out-shelter (W2) and rain-fed (W3). This experiment had two parts, in one, five cultivars were exposed to W1, W2 and W3, and in a second, seven cultivars were exposed to W1 and W3. Using the same set of seven cultivars, pot-culture experiments were conducted to study relative water content (RWC), stomatal conductance (gs) and single leaf carbon exchange rate (CER) during increasing moisture-deficit in two contrasting (rainy and summer) seasons. Variation in DTI was significant, and low SLA types had greater DTI under both W2 and W3. The ranking of SLA among cultivars was consistent between experiments conducted during the two seasons. The rate of reduction in leaf RWC during the progressive moisture-deficit was related directly to SLA (r=0.78, P<0.01). The coefficient of determination of the slopes calculated between RWC and soil moisture during the experimental period was more in the summer (r2=0.82) than the rainy (r2=0.54) season.Under increasing moisture-deficit, the low SLA types were able to maintain higher RWC, CER and gs in both seasons. The relationships between RWC and CER (r=0.91, P<0.01), and RWC and gs (r=0.65, P<0.01) were significant.It is suggested that under water-limited conditions there is a significant inverse relationship between SLA and RWC. The low SLA types (water use efficient) were found to be drought tolerant in terms of total dry matter production in the field studies, and maintenance of higher RWC under drought like situations in pot-culture experiments. Thus the ability of the low SLA types (higher water use efficiency, WUE) to maintain higher RWC may form the basis for the differences in drought tolerance vis a vis WUE in groundnut cultivars differing in SLA. Suggestions are made to select parents for drought tolerance or WUE, and to initiate breeding to combine traits like high HI, and WUE in terms of lower SLA. Ultimately, selection for both WUE (measured in terms of SLA) and yield traits (HI) should result in cultivars with improved performance in rain-fed agriculture.  相似文献   

16.
Dielectric relaxations of wheat doughs with different water contents and effects of sucrose, NaCl, and their mixture on relaxation temperatures were investigated using dielectric analysis (DEA). All ingredients were dissolved in distilled water used to prepare wheat flour doughs to optimum consistency. Before analysis, samples were stored at room temperature in vacuum desiccators over aw range of 0·225–0·753. Dynamic DEA measurements were made at a heating rate of 2 °C/min from 40 °C below and above the observed relaxation zone. The frequencies used were 0·1, 0·5, 1, and 5 Hz. Steady state water contents varied from 3·21 to 10·89 g H2O/100 g dm over aw range used for the plain dough (flour+water). Added ingredients increased sorption of doughs. The tan δ of DEA showed an α-relaxation (glass transition) in all doughs at all frequencies used. The relaxation peak temperature, taken as Tg, increased with increasing frequency. Added sucrose decreased the Tg of doughs, as well as added NaCl. A dramatic depressing effect of NaCl on Tg was probably due to an increase in conductivity of doughs.  相似文献   

17.
Granular particles (crumbs) of gluten, wheat starch, and waxy maize starch were obtained by extrusion in a twin-screw cooker extruder and conditioned to a range of moisture contents (4–18%). The textures of the granules were assessed by a small sensory panel and the sounds produced during crushing (acoustic emission) measured. Solid, essentially amorphous, bars of gluten, wheat starch and waxy maize starch were produced by heat setting under pressure. The glass transition temperatures were measured by differential scanning calorimetry and dynamic mechanical thermal analysis, crystallinity by X-ray diffraction and the fracture behaviour examined using a three-point bend test. Heat-set gluten failed via elastic (brittle) fracture below its glass transition temperature, whereas, as the moisture content was increased, the glassy gelatinised starches underwent a brittle-ductile transition while still in the glassy state. This change in fracture mechanism explains why extruded gluten granules exhibited crispness at higher moisture contents than extruded starch granules, despite gluten having a lower glass transition temperature. The results suggest that the prediction of brittle textures (crisp, crunchy) from a knowledge of the glass transition temperature alone is not possible because bipolymers can exhibit different fracture mechanisms when in the glassy state.  相似文献   

18.
The aim of this study was to investigate the effects of different variables on lysine loss as determined by furosine content in corn-based extrudates. Three formulations were prepared to study the effects of different chemical leavening agents, processing conditions (feed moisture content: 22, 24 or 26%; exit die temperature: 110 or 150 °C), and extrusion cooking methods (with/without CO2 injection) on furosine formation. Furosine levels of extrudates from both extrusion methods decreased around 20% when feed moisture content was increased from 22% to 26%. Amadori compounds (precursor of furosine) are formed in the early stages of the Maillard reaction, and later they are converted to further products. Consequently, furosine contents of extrudates significantly decreased as exit die temperature increased from 110 to 150 °C. Furosine contents of extrudates produced with sodium- and ammonium-bicarbonate at 150 °C exit die temperature significantly decreased, while the ones produced at 110 °C significantly increased. This may be due to accelerated formation of fructosyllysine at higher pH values followed by early degradation at 150 °C. The CO2 injection method did not have a significantly different effect on furosine content of extrudates than that produced by the conventional extrusion method, but had a positive effect on the physical properties of extrudates.  相似文献   

19.
Plant responses to water deficit need to be monitored for producing a profitable crop as water deficit is a major constraint on crop yield. The objective of this study was to evaluate physiological responses of cotton (Gossypium hirsutum) to various environmental conditions under limited water availability using commercially available varieties grown in South Texas. Soil moisture and variables of leaf gas exchange were measured to monitor water deficit for various varieties under different irrigation treatments. Lint yield and growth variables were also measured and correlations among growth parameters of interest were investigated. Significant differences were found in soil moisture, leaf net assimilation (An), stomatal conductance (g), transpiration rate (Tr), and instantaneous water use efficiency (WUEi) among irrigation treatments in 2006 while no significant differences were found in these parameters in 2007. Some leaf gas exchange parameters, e.g., Tr, and leaf temperature (TL) have strong correlations with An and g. An and WUE were increased by 30–35% and 30–40%, respectively, at 600 μmol (CO2) m−2 s−1 in comparison with 400 μmol (CO2) m−2 s−1. Lint yield was strongly correlated with g, Tr, WUE, and soil moisture at 60 cm depth. Relative An, Tr, and TL started to decrease from FTSW 0.3 at 60 cm and FTSW 0.2 at 40 cm. The results demonstrate that plant water status under limited irrigation management can be qualitatively monitored using the measures of soil moisture as well as leaf gas exchange, which in turn can be useful for describing yield reduction due to water deficit. We found that using normalized An, Tr, and TL is feasible to quantify plant water deficit.  相似文献   

20.
Organically modified montmorillonite (C8PPh-MMT) was obtained using the ion exchange reaction between Na+-montmorillonite (Na+-MMT) and 1-octenyltriphenyl phosphonium chloride (C8PPh-Cl−). Polystyrene nanocomposites were then prepared by in-situ free-radical polymerization of the styrene containing intercalated C8PPh-MMT. The resulting polystyrene hybrids with various organoclay contents were investigated with FT-IR, which confirmed that PS hybrids were successfully prepared via the reaction of styrene monomer in the interlayers of the clays. The variations of the thermal behaviors of the hybrids with increases in the organoclay content from 0 to 8 wt% were determined. The glass transition temperatures (Tg) and initial thermal degradation temperatures (TD i) of the PS hybrids were found to increase linearly with increases in the organoclay loading. Regardless of the organoclay content of the hybrids, the clay was found to be dispersed homogeneously in the matrix polymer. This is direct evidence that the PS hybrids formed nanocomposites. This result was confirmed with XRD and TEM.  相似文献   

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