Retrogradation of Starch in Cooked Wheat

Retrogradation phenomenon in cooked wheat grain was investigated by means of Differential Scanning Calorimetry (DSC) and mechanical compression tests. The extent of starch retrogradation as measured by endotherm increased with time. More importantly, the onset, peak and conclusion temperatures are dependent on the storage temperature and are significantly lower for grain stored at lower temperatures. The firmness of the grain due to starch crystallization as indicated by the elastic modulus also increased with time and was higher at lower storage temperatures. Lowered storage temperature not only accelerates starch recrystallization and thus firming process of the cooked grain, but also seems to generate a different organization in the ordering of the starch.     

Vane Yield Stress of Starch Dispersions

ABSTRACT: Yield stresses of 5% (wt/wt) cross-linked waxy maize, tapioca, and Amioca starch dispersions (SDs) were measured with the vane method at different rotational speeds ( N ). The static (σ_0S) and dynamic (σ_0d) yield stress values of each SD were measured before and after breaking down its structure under continuous shear, respectively. The difference (σ_0S - σ_0d) was associated with the stress required to break the internal bonds (σ_b). Values of total yield stress (σ₀=σ_0S) and σ_b, and shear rate were related by power law relationships. The contribution of viscous and network stresses were estimated from an energy balance model. Textural characteristics of the SDs were analyzed in a texture map.     

Degradation of Wheat Starch in a Single-Screw Extruder: Mechano-Kinetic Breakdown of Cooked Starch

Wheat starch was processed in a 19 mm diameter split-barrel laboratory extruder under varying conditions of barrel temperature (79–121^oC), screw speed (50, 100 rpm) and moisture content (25, 30%). Final product and samples removed from along the barrel length were examined for changes in molecular size by dilute solution viscometry. Final products were also examined for paste viscosity and estimated degree of cook. A mathematical model was developed which relates the residence time of the starch in the barrel, the nominal shear stress acting on the starch and the degree of cook of the product to the extent of molecular degradation of the starch. Solution viscometry was found to be more rapid than gel permeation methods previously used to determine the extent of molecular changes.     

Rheological Properties of Rice Starch Dispersions in Dimethyl Sulfoxide

The steady and dynamic shear rheological properties of rice starches dispersed in dimethyl sulfoxide (DMSO) solution (90% DMSO‐10% water) were evaluated at various concentrations (7, 8, 9 and 10%, w/w). Rice starch dispersions in DMSO solution at 25°C showed a shear‐thinning flow behavior (n=0.44–0.60) and their consistency index (K) and apparent viscosity (η_a,100) increased with the increase in concentration. The apparent viscosity over the temperature range of 25–70°C obeyed the Arrhenius temperature relationship, indicating that the magnitudes of activation energy (E_a) were in the range of 11.7–12.7 kJ/mol. The Carreau model provided better fit on the shear rate‐apparent viscosity data than the Cross model. Dynamic frequency sweep test showed that both storage modulus (G′) and loss modulus (G′′) of rice starch dispersions increased with the increase in concentration. G′′ showed a higher dependence on frequency (ω) compared to G′ due to the higher G′′ slopes. All rice starch dispersions showed the plateau of G′ at high frequencies. Intrinsic viscosity of rice starch dispersions in DMSO was 104.1 mL/g.     

Surface Energy of Extruded and Jet Cooked Starch

Contact angles of water and other liquids were measured on films and extruded ribbons of normal, high amylose and waxy cornstarch using a Chan dynamic contact angle analyzer. The contact angles were then used to calculate the critical surface energy of the starch via a Zisman plot. Attempts were also made to use the contact angles to calculate the surface energy of starch via the harmonic-mean method. No differences were found for the critical surface energy due to starch type. Using a composite of all contact angles for the three different starches, an average critical surface energy of 40 dynes/cm was calculated. This value was close to the literature value of 39 dynes/cm found for starch. The surface energy of starch, calculated from the harmonic-mean method, was more variable and ranged from 35-42 dynes/cm. The harmonic-mean method not only calculates the surface energy, but it also estimates the polar and non-polar components of surface energy. The estimate calculated for the polar portion for the surface energy of starch was too low for a polar solid like starch. Values for the polar component range from 0.3 to 10 dynes/cm. A theoretical argument is made for the surface energy of starch being greater than the 39 dynes/cm now reported in the literature.     

Partial Digestibility of Cooked Amylomaize Starch in Humans and Mice

A part of ingested high-amylose starch was indigestible and appeared in feces in both humans and mice. Starch from ordinary corn was not found in feces of either humans or mice. Fecal high-amylose starch was birefringent, showed no microscopic evidence of enzymic corrosion and had the red polarization color characteristic of high-amylose starch. The amylose content of fecal starch in both humans and mice was in the range 40 to 49% compared with an initial value of 70% in the amylomaize meal. Depending on the amount of amylomaize eaten (in baked muffins), human feces contained from 6 to 11% of high-amylose starch. Approximately 11% of the high-amylose starch ingested with 70 g of amylomaize meal (in baked muffins) appeared in the stool.     

Rheology of Okenia hypogaea Starch Dispersions in Aqueous Solution of DMSO

The aim of this work was to study the rheological behavior of Okenia hypogea starch dispersions (OSD) in aqueous solution (90/10, v/v) of dimethyl sulfoxide (DMSO). Okenia starch dispersions with 3 and 7 % (w/v) total solids were prepared at 20 °C and rheological tests were undertaken at 20, 40 and 60 °C, using a shear rate controlled rotational viscometer. The data fitted better the power law than the Casson model. Flow curves and flow behavior index indicated shear‐thinning behavior. All rheological parameters were affected by the solids concentration and the measurement temperature. As regards amylose and amylopectin content okenia starch is grouped among “ordinary” (non‐waxy) starches. Overall, OSD behaved similarly to potato starch dispersions and corn starch dispersions used as controls.     

Flow Behavior and Gelatinization of Cowpea Flour and Starch Dispersions

The flow behavior of a 25% cowpea slurry with 8% oil held at 70°C showed shear-thinning behavior and an Arrhenius temperature relationship. Cowpea flour (8%) and starch (2.5%) slurries heated for less than 1 min at 70–87°C exhibited shear-thickening while those heated longer times exhibited shear-thinning behavior. Maximum viscosities attained due to heat-induced gelatinization showed a power relationship with temperature of heating. Starch gelatinization kinetics followed a first-order equation and the temperature dependence of the rate constant followed the Arrhenius relationship with an activation energy of about 47.4 kcal/mol. Heating the slurries for >200 min above 80°C resulted in loss of viscosity.     

Dialdehyde Starch Hydrazones: Preparation and Properties of Cationic Dispersions

Cationic dialdehyde starch (DAS) dispersions have been prepared in water at 15% concentration by a new cooking procedure. A stable fluid form of cationic DAS, a wet- and dry-strength agent for paper, should facilitate commercial use. Cationic DAS dispersions of pH 3.0 stored in polyethylene containers at 25°C or below were unchanged after 6 months; other storage conditions were also evaluated. An unexpected benefit of the new preparative conditions was a 40% reduction of cationizing agent, betaine hydrazide hydrochloride, without loss of effectiveness. Further savings may be possible by blending cationic DAS with unmodified starch; wet- and dry-strength improvements were maintained in paper with a blend containing 33% unmodified starch and 67% cationic DAS.     

Characterization of Resistant Starch from Autoclaved Wheat Starch

The nature of a particular type of in vitro undigestible starch, i.e. resistant starch (RS), was studied. This type of undigestible starch is found in products processed at relatively high moisture contents, such as baking and autoclaving. The RS was concentrated by enzymic digestion of available starch and protein and recovered by centrifugation. RS from autoclaved wheat starch was evaluated by various methods. The crystallinity was studied by X-ray diffraction and DSC. The degree of polymerization (DP) was evaluated with gel permeation chromatography and iodine binding and the starch content and linearity by enzymic methods. The results indicate that RS consist of crystallized, linear α-glucans of relatively low molecular weight (DP approx. 65). The recovery of RS after various drying procedures, and the introduction of a thermostable amylase (Termamyl®) in the RS assay was also studied. Oven-drying of the fibre residues used in the analysis had no significant effect on RS recovery, whereas freeze-drying was accompanied by a slight increase in RS content. The use of Termamyl®, in the assay of RS in the fibre residues, decreased the yield of RS by 10–15%.     

木薯淀粉、谷朊粉、复合磷酸盐与瓜尔胶对冷冻熟面品质的影响

该研究使用质构仪、布拉班德粉质仪、低场核磁共振和电镜扫描研究改良剂-木薯淀粉、谷朊粉、复合磷酸盐与瓜尔胶,对冷冻熟面(frozen cooked noodles,FCN)品质、加工性能、冻融稳定性和内部结构的影响,同时探究4种改良剂的最佳复配添加量。结果表明:木薯淀粉降低硬度使面条变黏,谷朊粉提高面条的硬度和回复性,复合磷酸盐提高面条的内聚性和回复性,瓜尔胶对硬度和回复性影响先升高后降低;谷朊粉和复合磷酸盐强化面筋网络,提高面粉加工性能,瓜尔胶抑制冰晶生长,保护面条品质。     

Starch Retrogradation and Texture of Cooked Milled Rice During Storage

The effect of storing cooked Bengal and Cypress milled rice at -13, 3, 20, and 36 °C on texture and degree of starch retrogradation was investigated. Cooked rice firmness increased, while stickiness decreased, during storage at -13 and 3 °C. Starch retrogradation, measured with a differential scanning calorimeter, was observed for both cultivars during storage at -13 and 3 °C, but not at 36 °C. At 20 °C, retrogradation occurred in Cypress, but not in Bengal. Starch retrogradation showed positive linear trends with firmness for both cultivars at all storage temperatures (R²= 0.80) and with stickiness for Bengal stored at -13 and 3 °C and for Cypress stored at 3 and 20 °C (R2 = 0.88).     

水肥耦合对小麦产量及淀粉特性的影响

利用干旱棚,以洛阳1971～1999年20年的降雨量为依据,以每月占年平均降雨量的加权值进行分配,模拟不同水肥条件下对小麦产量及淀粉特性的影响.结果表明:不同降雨年型不同施肥组合有以下规律:N、P水平相同的情况下,水分高的处理产量高;在欠水年和丰水年,P肥相同的情况下,N肥高的产量低;在平水年,不同施肥水平呈现出不同变化,氮肥N105＞N0＞N210,氮肥相同水平下P84 ＞P168 ＞P0.水分是产量的主导因素.小麦产量的提高可通过协调成产3因素来实现.淀粉的峰值黏度、终结黏度、稀懈值及反弹值受水肥影响较大,而糊化温度和低谷黏度受水肥影响较小,在平水年以N105P84处理的淀粉糊化特性各参数值较高,在一定程度上改善了面条品质,而在丰水年氮肥较高的各参数的值较低.     

Components of Vane Yield Stress of Structured Food Dispersions

The vane technique was used to determine the static (σ_os) and dynamic (σ_0d) yield stresses of 6 commercial foods: mayonnaise, mustard, ketchup, applesauce, and 2 brands of tomato puree (samples TD and TR). The samples, other than TR, exhibited shear‐thinning behavior. For those samples, based on a model proposed earlier, the contributions of bonding (σ‐_b), network (σ‐_n), and viscous (σ_v to yield stress of the products were determined. For the homogenized products, σ_b > σ_n, whereas for those processed in a finisher, σ_n > σ_b. Because sample TR exhibited shear‐thickening behavior, the model to estimate yield stress components was not applicable. The structural conditions necessary for the validity of the proposed model are established. Equilibrium stress (σ_∞) values obtained by extrapolation of vane torque data to infinite time were comparable to σ_0d values.     

Adsorption of Wheat alpha-Amylase Isoenzymes to Wheat Starch

Two groups of α-amylase isoenzymes have been identified in germinated wheat, but only one group would adsorb on to undamaged starch granules. This group represented 84.2% of the total α-amylase activity. The non-adsorbing isoenzymes were electrophoretically identical to the α-amylases of immature wheat grain.     

Alkali Gelatinisation of Wheat Starch

Alkali gelatinisation of wheat starch was found to depend on starch and alkali concentration and was faster at higher temperatures and higher alkali/starch ratios. Viscosity of an alkaline starch slurry continued to increase after both in vitro digestibility and soluble amylose reached maximum values. Despite extensive granule swelling, deformation and bursting, birefringence was retained to some degree indicating differences in mechanism between alkali and heat gelatinisation.     

Wheat Starch Modification Through Biotechnology

Natural mutations that affect the amylose/amylopectin ratio in starch are unlikely to develop naturally in wheat due to its allohexaploid genome (2n=6x; AABBDD). One of the strategies to modify wheat starch structure involves identification of germplasms with null alleles for starch biosynthetic genes, followed by exchange of functional alleles with the identified null alleles through classical plant breeding. This technique has successfully been used to combine the three null alleles for granule-bound starch synthase I (GBSSI) to develop a wheat line that produces amylopectin-rich (>95%) starch (waxy starch). Another strategy to alter expression levels of starch biosynthetic genes employs recent advances in molecular biology and genetic engineering of wheat. For this approach, various monocot vectors have been developed that drive expression of wheat starch branching enzyme I (SBEI) cDNA sequences in the anti-sense orientation. Several of the wheat cell lines transformed with the anti-sense vectors express branching enzyme (BE) activity at a significantly lower level than non-transformed cells. One transgenic wheat plant expressing the anti-sense SBEI RNA produces a ten-fold lower level of BE activity in kernels than wild-type wheat. Analysis of starch produced from the transgenic plant shows that starch structure and properties have been altered.