Microstructure and rheology of phase-separated gels of gelatin + oxidized starch

We report on small-deformation rheology and confocal microscopy at 24 °C of high-sugar gel samples based on an aqueous mixture of gelatin + oxidized (non-gelling) starch exhibiting thermodynamic incompatibility. Using a standard protocol involving the heating of previously rapidly quenched samples at a fixed holding temperature (24–60 °C) for a fixed time (from 20 s to 30 min), gel samples of different storage modulus and degrees of spinodal-type phase separation were systematically generated. The developing time-dependent modulus of the gelatin gel was found to be sensitive to the extent of gelatin crosslinking as influenced by the nature of the thermal processing conditions. Under identical processing conditions, a gradual increase in starch content gave gels of lower elastic modulus and increased degree of microscopic phase separation. Taken all together, our results indicate that the gel rheology is determined by a subtle combination of overall system composition and temperature/time processing history. We conclude that the mechanical properties of this mixed biopolymer system are not directly deducible from the extent of phase separation observed microscopically without detailed additional information concerning sample preparation and treatment conditions.     

Morphology of starch in surimi gels

 The purpose of this work was to study the changes undergone by starch during heat-induced surimi gel preparation either with or without added egg white, and their effects on the structure of gels using light and scanning electron microscopy. Gels were made from SA-grade Alaska pollack (Theragra chalcogramma) surimi with: (1) salt (3%, w/w); (2) salt and waxy corn starch (3% and 5%, respectively w/w); or (3) salt, waxy corn starch and egg white (3%, 5% and 5%, respectively, w/w). Final moisture was adjusted to 73% or 83%. The gels were prepared by prior setting (40°C, 30 min, followed by 90°C, 30 min) or cooking (90°C, 30 min). The prepared gel was frozen and stored at –20°C (±1°C) until analysis. Samples were observed by light and scanning electron microscopy. The results show that the starch granules alter according to the processing conditions, with the predominance of crystalline or amorphous morphology depending upon the availability of heat and water. Large cavities formed in the protein gel matrix during setting can trap water; as a result, water availability is limited for starch to swell and gelatinize even in the high-moisture gel. Received: 13 March 1997     

Isomaltodextrin strengthens model starch gels and moderately promotes starch retrogradation

Isomaltodextrin (IMD) is a novel highly branched α-glucan with emerging applications in foods due to its nutritional functionalities. In this study, the impact of IMD on the viscoelasticity and microstructure of starch gels and starch retrogradation were investigated. IMD solutions displayed typical Newtonian behaviour. Their viscosities had a positive correlation with concentration and a negative correlation with temperature. When maintained at relative humidity of 95%, IMD absorbed 40% water of its original weight. The addition of IMD to normal corn starch increased strength of gels as indicated by higher storage moduli (G′) and resulted in a denser and smaller pore structure, but there were no changes in elasticity. Upon storage for 7 days, addition of IMD at high concentration (starch: IMD = 2:1) significantly increased the degree of starch retrogradation. This study provided perspectives underlying applications of IMD in starchy foods as a novel ingredient with textural functionality.     

Phenomenological viscoelasticity of some rice starch gels

The applicability of a powerful but still easy to use technique, based on a phenomenological theory of viscoelasticity, for processing and analyzing dynamic mechanical data of some rice gels was investigated. Based on this theory a continuous relaxation spectra was generated by application of Tikhonov regularization procedure on continuous Maxwell model. Interpretation of relaxation spectra in terms of number of peaks, its peak intensity H(λ) and appearance of its main distribution peak and magnitude of equilibrium elasticity modulus (G_e) of continuous Maxwell model was found to appropriately reflect main peculiarities of the viscoelastic behavior of rice starch gels. An increase in number of peaks in the relaxation spectra was observed for starch gels having higher amylose content indicating the creation of more heterogeneous structure. H(λ) and G_e values also increased with increase in amylose content demonstrating a transition of the system to more stable state like a gel.     

柠檬黄素在淀粉凝胶内的扩散特性研究

测定了在4℃和35℃条件下柠檬黄素在3种浓度的小麦﹑小米淀粉凝胶内的扩散系数。结果表明,在相同条件下,与小麦淀粉凝胶相比,柠檬黄在小米淀粉凝胶内的扩散较慢。扩散系数随着凝胶浓度的减小而增大,随着温度升高而升高。与改变浓度相比,降低温度更能有效地降低柠檬黄在淀粉凝胶内的扩散速度。     

微量成分对葛根淀粉凝胶特性的影响

研究了蛋白质、脂肪、灰分、葛根素等微量成分对葛根淀粉凝胶特性的影响。通过TPA测定,发现人的咀嚼并不能识别出微量成分含量对其质构的差异,结合感官评定得出微量成分含量越高的葛根淀粉凝胶所具有的葛根清香和葛根汁色泽也越明显。葛根素的添加量在1%以下对凝胶的硬度和黏度影响较大,但葛根素无色无味,是特殊的功能性成分,适合在新产品中作为特征性物质存在。以上研究结果均为新产品开发和葛根淀粉应用提供理论依据。      

Effect of oxidised starch on calcium pectinate gels

Small-deformation oscillatory measurements have been used to study the effect of low-viscosity oxidised starch on the rheology of calcium pectinate gels formed by controlled cooling from the sol state at high temperature. Large reductions in modulus (G′; 0.5% strain; 10°C) were observed at starch concentrations below the minimum critical concentration for gelation of oxidised starch alone under the same conditions (32 wt%). This behaviour is tentatively ascribed to thermodynamic incompatibility between the two polymers causing incipient precipitation of calcium pectinate within the gel network.     

Microstructure and elastic modulus of mixed gels of gelatin + oxidized starch: Effect of pH

The microstructure and elastic shear modulus of cold-set gels formed from high-sugar aqueous mixtures of gelatin (7 wt%) + oxidized starch (0-6 wt%) were investigated as a function of pH. Samples prepared at 90 °C, with citric acid added to adjust the pH, were rapidly quenched to ∼1 °C, subjected to a standard thermal treatment (40 °C for 10 min), and then investigated by confocal microscopy and small-deformation rheology at 24 °C. Under ‘natural’ conditions of pH ≈ 5.2 (no citric acid addition), the samples exhibited phase separation with a characteristic spinodal-type morphology. The spatial extent of the structural heterogeneity, expressed by a single length-scale parameter, was found to increase with starch concentration. Gradual acidification led to a reduction in this length-scale parameter, leading to complete inhibition of phase separation below a certain characteristic pH value in the range 4.5-4.9 (depending on starch content). Over the investigated pH range, the effect of starch addition was to reduce the storage modulus of the resulting gel. This reduction was more pronounced for the phase-separated samples. The pH of maximum rigidity was found to decrease from pH_max ≈ 4.6 for 0 wt% starch to pH_max ≈ 4.2 for 6 wt% starch. Taken all together, these observations can be understood in terms of the effects of pH on the cross-linking behaviour of the gelatin and the nature of the gelatin-starch electrostatic interactions. The microscopy results are consistent with a transition in behaviour from thermodynamic incompatibility (segregative interactions) at high pH to soluble complexation (associative interactions) at low pH.     

变性淀粉在速冻食品中的应用

随着人们生活节奏的加决,生活水平的提高和饮食结构的变化,人们对食品的需求有了明显的变化。减轻家务劳动,日常食用些既美味又营养的食品。在讲究高效的今天,已成为普遍现象。进入90年代速冻食品的消费出现了新的热点,一些传统的食品的行列,如:速冻汤圆、速冻包子、速冻春卷等备受青眯。速冻食品一般需要经过预冷、冷冻、冷藏和使用     

Manufacture of acid gels from skim milk using high-pressure homogenization

The effect of high-pressure homogenization (HPH) alone or in combination with a thermal treatment (TT) was investigated for the manufacture of acid gels from skim milk. Raw skim milk was subjected to HPH (0 to 350 MPa) or a TT (90°C, 5 min), or both, in the following processing combinations: 1) HPH, 2) HPH followed by TT, 3) TT followed by HPH, 4) TT, and 5) raw milk (control). After treatments, L* (lightness) values were measured, and then skim milk was acidified with 3% glucono-δ-lactone and rheological properties (G′ and gelation time), and whey holding capacity was evaluated. Treatments in which HPH and TT were combined showed greater L* values than those in which just HPH was applied. In all treatments, the L* values decreased as the pressure was increased up to 300 MPa with little change afterward. Gelation times were lower when HPH was combined with TT compared with the acid skim milk gels that were just pressure treated. The final G′ in gels obtained from skim milk subjected to the combined process (HPH and TT) was greater and pressure-dependent compared with all other gels. A maximum G′ (∼320 Pa) was observed with skim milk subjected to a combination of thermal processing before or after HPH at 350 MPa. Acid gels obtained from HPH milk at 350 MPa showed a linear decrease in whey holding capacity over time, retaining 20% more whey after centrifugation for 25 min compared with samples treated at lower pressures and all other treatments. Our results suggest that HPH in combination with TT can be used to improve the rheological properties and stability of yogurt, thus decreasing the need for additives.     

Role of maltodextrins in the staling of starch gels

The possible role of maltodextrins with a low degree of polymerisation (DP) as agents that retard bread staling was studied by following the staling process on a model system consisting of starch gels by means of textural and calorimetric assays. Maltodextrins of DP 2 to 7 were added at concentrations of 0.5% (starch base) to 15% (w/w) starch gels. Textural and calorimetric results were analysed by the Avrami equation. The addition of maltodextrins promoted a slight decrease in the initial firmness of the starch gels. All the samples with maltodextrins showed a lower rate of firming. The same effect was observed on the rate of amylopectin retrogradation, except for the maltose sample under short and intermediate storage times. It was concluded that low DP maltodextrins might be responsible for the anti-staling effect obtained by using α-amylase addition in the breadmaking process.     

7种淀粉凝胶应力松弛特性研究

将7种淀粉采用相同方法制备出凝胶块作为实验试样,经质构仪进行压缩和应力松弛实验得到流变学特性参数。利用统计分析软件进行非线性回归拟合,建立了广义Maxwell力学模型。结果表明:淀粉凝胶具有良好的压缩黏弹性力学性质,五元件广义Maxwell模型适用于模拟淀粉凝胶的应力松弛现象。通过主成分分析,应力松弛参数可归纳为两类,即凝胶粘弹特性和凝胶松弛时间。      

淀粉原料3D打印特性

3D打印作为一种新兴的加工方法,在食品制造领域受到越来越多的关注。该研究以马铃薯、小麦和玉米淀粉为原料,对3D打印样品的特性进行了研究。采用外观形态、颜色、碘吸收光谱、快速黏度分析仪(rapid visco analysis,RVA)、质构分析、傅里叶变换红外光谱(fourier transform infrared spectroscopy,FT-IR)和扫描电镜(scanning electron microscope,SEM)对产品质量进行了评价。结果表明,所有淀粉凝胶均可获得完整的3D打印模型。用小麦淀粉制作的3D打印样品的尺寸最接近CAD设计模型。小麦淀粉凝胶具有较低的黏度、较好的挤出性和贮藏性能。此外,小麦淀粉制作的3D打印样品的微观结构比马铃薯淀粉和玉米淀粉具有更规则的网状结构。为3D打印技术在食品生产中的应用提供了理论依据,并为实际生产提供了技术支持。     

Microstructure of suwari and kamaboko sardine surimi gels

In a previous work it was suggested that the texture of kamaboko (set and cooked) gels made from sardine surimi under varying setting conditions was predetermined by the specific matrix forming in each suwari (set) gel. This paper describes the microstructure of the networks formed in suwari and kamaboko gels set at 25, 35 and 40 °C for 30 or 60 min as examined by scanning electron microscopy (SEM). Cooking conditions for kamaboko gels were fixed at 90 °C for 30 min; other preparation conditions were invariable. At low magnification (≤×500) the gel matrixes were compact, with practically no differences among lots. At higher magnification (×20 000), the suwari gel matrixes formed at low temperature consisted of globules. At higher temperatures the globules joined up to form fibrillar structures (fibres) and zones of disordered globule aggregation (coagula); at longer setting times, lateral bonding of the fibres became apparent. Kamaboko gels produced from unstructured globular matrixes exhibited only a few fibrillar zones and large areas of coagula. Where there was already an incipient fibrous formation, these developed into individual fibres or bundles of fibres that correlated with the best texture characteristics. Suwari gels with extensive lateral bonded fibres gave rise to kamaboko gels with a highly compact appearance under SEM; this correlated with a decline in texture values. These different structures suggest that the protein–protein bonds in the suwari networks have different levels of stability to heat, and these levels determine whether or not the proteins can subsequently be reorganised when the kamaboko gel forms. © 1999 Society of Chemical Industry     

Effect of wheat fibre in frozen stored fish muscular gels

The technological effect of wheat fibre, as a healthy ingredient in which consumers are interested, was studied in surimi gel products. Three and six per cent wheat fibre with different particle sizes was added. Functional properties of wheat fibre, such as water retention capacity (WRC), swelling (SW) and fat adsorption capacity (FAC), were determined. Elastic modulus (G′) of surimi with 6% wheat fibre was lower throughout thermal gelling. Under scanning electronic microscopy (SEM), gel samples with added fibre presented uneven distribution of fibre. This induced the formation of a non-homogeneous protein net associated with lower gel strength, cohesiveness and water binding capacity (WBC). The sensory panel found no differences in appearance but differences in texture among samples containing different proportions and types of fibre. Fibre with large particle sizes protected surimi from loss of gel strength and hardness during freezing. There were very few variations during frozen storage of different samples.     

Comparative NMR relaxometry of gels of amylomaltase-modified starch and gelatin

With the aim of generating gelatin-like starch gel functionality, starches extracted from normal potato, high amylose potato, maize, waxy maize, wheat and pea and oxidized potato starch were modified with amylomaltase (AM) (4-α-glucanotransferase; E.C. 2.4.1.25) from Thermus thermophilus. Gel characteristics after storage for 1 and 10 days at 20 °C of 12.0% gels were assessed by monitoring proton relaxation for the resulting 51 enzyme-modified starches and two gelatins using low-field ¹H nuclear magnetic resonance (LF NMR) relaxometry. Discrete and distributed exponential analysis of the Carr–Purcell–Meiboom–Gill (CPMG) LF NMR relaxation data revealed that the pastes and gels contained one water component and that the spin–spin relaxation time constants (T₂) and distributions differed with respect to starch type and enzyme modification. Typically, AM modification resulted in starches with decreased T₂ relaxation time and a more narrow T₂ distribution indicating a more homogeneous water population. In contrast, treatment with a branching enzyme (BE) (EC 2.4.1.18) combined with AM increased T₂ relaxation time and a broadened T₂ distribution. As evaluated by the principal component analysis (PCA), long chains of amylopectin generated hard gels and decreased T₂ relaxation time at both day 1 and day 10. Especially at day 10, T₂ relaxation time could be predicted from the amylopectin chain length (CL) distribution. Reconstructed amylopectin CL distribution required to emulate gelatin LF NMR data suggest the importance of combined fractions of long (DP 60–80) and short (DP 10–25) amylopectin chains.     

Anti-oxidant activities of mung bean starch and starch gels prepared from whole and hulled seeds

Anti-oxidant activities of ethanol extracts from whole and hulled mung bean starches (WMS and HMS) isolated using distilled water, and gels (WMSG and HMSG) were investigated. Total chlorophyll contents of WMS and WMSG were 4.61 and 2.14 μg/mL, respectively. WMS at 0.29 mg of gallic acid/g of MS and 0.54 mg of quercetin/g of MS had higher total phenolic compounds and flavonoid contents than HMS at 0.11 mg of gallic acid/g of MS and 0.28 mg of quercetin/g of MS. Total phenolic contents of WMSG and HMSG increased by 2.14x and 1.64x, respectively. Total flavonoid contents decreased by 0.98x and 0.57x. The 1,1-diphenyl-2-picrylhydrazyl and 2,2’-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) diammonium salt radical scavenging activities and reducing powers of WMS and WMSG were higher than for HMS and HMSG. Cheongpomuk made using WMS exhibits increased anti-oxidant activities of gels and is a good food for improvement in health functionality.     

Thermal and viscoelastic properties of starch gels from maize varieties

The aim of this work was to determine the thermal, functional and rheological properties of maize (Zea mays) starch isolated from seven varieties. Chemical analysis was undertaken in all starch samples. The gelatinization and retrogradation temperature at different storage times, as well as the enthalpy of the isolated starches, were determined using differential scanning calorimetry (DSC). Swelling and solubility were also measured in individual samples. Dynamic oscillatory tests (amplitude and frequency sweeps) were undertaken on starch samples with 10% (w/v) of total solids during a cycle of three stages (kinetics) of heating/cooling, using a strain‐controlled rheometer. The samples presented an amylose content which ranged from 22% to 29%, typical in normal starches, the lipid values were under 1%, while the protein contents were just over 1%. The calorimetric profile for the studied starches showed a peak temperature (gelatinization) over the temperature range from 72.5 to 75.7 °C and enthalpy values between 13.68 and 17.58 J g^?1. Four starches presented enthalpy values of the retrogradation transition that increased with the storage time, showing differences among the starches analysed. Maximum swelling and solubility were usually found at the second stage of the above‐mentioned cycle. The rheological profile showed that the gels formed during the first stage of the above‐mentioned kinetics presented the behaviour of weak viscoelastic gels with the storage or elastic modulus (G′) higher than the loss or viscous modulus (G″) over the applied strain and frequency ranges. All samples showed a more elastic character as the kinetics progressed. Starches isolated from diverse maize varieties showed differences in their characteristics studied, and might produce different functional properties in the products where they are used. Copyright © 2006 Society of Chemical Industry