Rheological investigation of soy protein hydrogels induced by Maillard-type reaction

Rheological measurements were used to determine the power law relationship between gel elastic modulus and protein concentration (solid volume fraction) for soy protein hydrogels. The slope calculated from the log–log plot was used to determine gel fractal dimension. The validity of the prefactor in the power law relationship was also studied. Soy protein at 6.5–9.5% was gelled using monofunctional (glyceraldehyde) or bifunctional (glutaraldehyde) Maillard cross-linkers with or without salt.Gel fractal dimension did not vary with cross-linker type or concentration, while prefactor values increased with cross-linker concentration and when salt was used. The invariability of the fractal dimension is believed related to the absence of effect of the cross-linking agents on protein ionic repulsions, while prefactor value is believed inversely proportional to the number of inter-floc particle–particle interactions in the three-dimensional network and to floc size. The prefactor value was thus shown to increase as cross-linking degree increased and gel lacunarity decreased.     

Dynamic oscillatory rheological measurement and thermal properties of pea protein extracted by salt method: Effect of pH and NaCl

The effects of two important factors, pH (3.0-10.0) and NaCl (0-2.0 M), on pea protein gelation properties were studied using dynamic oscillatory rheometer and differential scanning calorimeter (DSC). The strongest gel stiffness was achieved at 0.3 M NaCl; higher or lower salt concentrations lead to weakening of the gel. The gelation temperature was also influenced by ionic strength; salt had a stabilization effect which inhibited pea protein denaturation at higher salt concentrations resulting in higher gelling points (p < 0.05). At a NaCl concentration 2.0 M, pea protein gelation was completely suppressed at temperatures ?100 °C. The pH also played an important role in gel formation by pea protein isolates since acid and base cause partial or even total protein denaturation. In this paper the maximum gel stiffness occurred at pH 4.0 in 0.3 M NaCl; higher or lower pH values resulted in reduced gel stiffness (p < 0.05). pH also altered the denaturation temperature of the pea protein; higher pH values resulted in higher denaturation temperatures and higher enthalpies of denaturation (p < 0.05). At pH 3 pea proteins seem like completely denatured by acid as the DSC curve showed a straight line. The gelation temperature (gelling point) peaked at pH ∼6.0 (89.1 °C). Careful adjustment of pH and NaCl concentration would enable the food industry to effectively utilize the salt-extracted pea protein isolate as a gelling agent.     

Functional Characterization of Poly(N-isopropylacrylamide) Nanohydrogels for the Controlled Release of Food Preservatives

In most active packaging systems for food applications, the preservative release is uncontrolled and frequently ineffective due to excessive or insufficient preservative concentration in the food matrix at a particular time. This work was aimed to evaluate the usefulness of pH- and temperature-sensitive poly(N-isopropylacrylamide) nanohydrogels for developing smart delivery systems to release preservatives as a response to environmental triggers. Pimaricin was used as a model preservative. Loading and release efficiencies were studied as a function of incorporation time and nanohydrogels composition, that differed in the cross-linker and acrylic acid content. Pimaricin loading efficiency was not affected by the acrylic acid. However, the cross-linking degree and incorporation time had a great influence. The lower the cross-linker content, the higher the loading efficiency, which in all cases exceeded 70 % at 12 h, approaching 100 % after 60 h of incorporation. No significant differences were observed between loading efficiencies obtained using water or methanol for pimaricin solubilisation. On the other hand, pimaricin release was dependent on the gel collapse, determined by the gel composition. Increasing the cross-linker content enhanced the nanoparticles' rigidity, and inclusion of acrylic acid as comonomer promoted strong electrostatic repulsions among the polymer chains. In both cases, the collapse was hampered, leading to a more hydrated collapsed state that favoured a sustained release of pimaricin. A mathematical model was also developed to predict the pimaricin release as a function of pH, temperature and storage time, allowing to select the most suitable nanohydrogel for a specific food application.     

典型化学加工条件对不溶性蚕蛹蛋白凝胶特性影响

以鲜蚕蛹为原料分离出不溶性蛋白,对其氨基酸组成进行测定,并采用不同的pH、离子强度以及添加谷氨酰胺转氨酶(transglutaminase,TG酶)处理,对其进行流变学以及拉曼光谱学的分析,探究pH、离子强度以及添加TG酶对不溶性蚕蛹蛋白凝胶特性的影响规律。结果表明,不溶性蚕蛹蛋白中疏水性氨基酸比例达到40%;流变学扫描显示,经过不同pH和离子强度处理后,在pH 5.0~8.0、盐质量浓度为0~35 g/L时,不溶性蚕蛹蛋白凝胶是一种弱凝胶。在pH 6.0、盐质量浓度为35 g/L时体系的G′值和G″值最高,黏弹性最好;pH增大会引起蛋白质结构有序化增多,但不利于蛋白质网络的形成,因此在pH 8.0时的G′值和G″值最低。色氨酸残基在pH 6.0及盐浓度为35 g/L趋于包埋态,酪氨酸残基在pH 6.0及盐质量浓度为35 g/L时暴露最少,此时G′值和G″值最高,推测疏水相互作用和氢键可能是影响不溶性蛋白质凝胶网络的关键作用力。添加TG酶后的体系G′和G″模量明显增大,黏弹性增强。实验结果可为不溶性蚕蛹蛋白的加工提供一定的理论参考。     

κ-卡拉胶与魔芋胶共混凝胶的质构特性研究

为明确κ- 卡拉胶与魔芋胶共混凝胶的质构特性,探讨κ- 卡拉胶与魔芋胶配比、共混胶质量浓度、pH 值、离子强度、KCl 和CaCl2 对κ- 卡拉胶与魔芋胶共混凝胶质构特性的影响。结果表明：κ- 卡拉胶与魔芋胶通过分子间力产生交互作用,对共混凝胶性能(如硬度、弹性和黏聚性)等具有良好的协同增效性。形成共混凝胶较好的条件为：共混凝胶质量浓度为2.0g/100mL、pH3.0～6.0、离子强度0.3～0.4mol/L、KCl 0.1～0.4g/100mL、CaCl20.1g/100mL。     

漂烫条件对脱水苋菜中VC和粗蛋白的影响

研究了苋菜脱水工艺中的漂烫工序,对漂烫时温度、时间、pH、食盐浓度、蔗糖浓度和不同金属离子等因素对苋菜中VC保存率和粗蛋白含量的影响进行了分析。研究结果表明:提高食盐浓度可以更好的保存VC,但会引起粗蛋白的损失;蔗糖可提高VC的保存率,低浓度的蔗糖溶液可导致粗蛋白的损失,高浓度的蔗糖溶液可提高粗蛋白的含量。     

Multilayer emulsions as delivery systems for controlled release of volatile compounds using pH and salt triggers

Multilayer oil-in-water (M-O/W) emulsions were compared to primary oil-in-water (P-O/W) emulsions as carriers for volatile organic compounds (VOCs) under various environmental conditions (pH and salt). The M-O/W emulsion consisted of soy oil coated with β-lactoglobulin (βLG) and pectin layers. The release of VOCs with different physiochemical properties from aqueous solutions and emulsion systems was measured using static and dynamic headspace methods. The partition coefficients (K) calculated by the phase ratio variation (PRV) method, showed different volatile release profiles between the emulsion types. An increase in VOC release was found for the unstable P-O/W emulsion at pH 5, whereas M-O/W emulsions were stable at the same pH and retained the hydrophobic VOCs. Hydrophobic interactions and hydrogen bonds with the secondary dense layer of pectin may be responsible for the improved retention. Increasing pH and ionic strength acts as a VOC release trigger to detach the pectin from the interface. The release rates from initial dynamic curves support the results under equilibrium conditions. The results of this study demonstrate the capability of using M-O/W emulsions for controlled release of VOCs, as well as an alternative system to create stable emulsions with similar VOC release profiles.     

Effect of alginate concentrations on survival of microencapsulated Lactobacillus casei NCDC-298

This study reports the tolerance of Lactobacillus casei NCDC-298 encapsulated in different alginate concentrations (2%, 3% or 4%), to low pH (1.5), high bile salt concentration (1% or 2%) and heat processing (55, 60 or 65 °C for 20 min). The release of encapsulated cells in simulated aqueous solution of colonic pH was also assessed. The survival of encapsulated L. casei was better at low pH, high bile salt concentration and during heat treatment as compared to free cells. The survival increased proportionately with increasing alginate concentrations without affecting the release of entrapped cells in solution of colonic pH.     

Penetration Studies of Blood Globin Gels

Studies were made on the effect of temperature, pH and protein and salt concentration on the penetration force withstood by globin gels. The registered force increased with heating temperature (60 - 95°C) and protein concentration (1.4 - 5.0%). The gelation pH was dependent on both protein and salt concentration; the higher the protein or salt concentration the lower the gelation pH. At 3% protein concentration globin formed a gel around pH 5 - 6. At 0.7% concentration and higher, the presence of salt weakened the gel strength, while the addition of plasma increased the gelation pH of salt-containing globin gels. A substantially higher penetration force was measured for bovine globin gels than porcine globin gels. Further concentration and spray-drying decreased the gel strength of globin gels.     

Soy protein cold-set hydrogels as controlled delivery devices for nutraceutical compounds

The release of riboflavin from soy protein hydrogels of filamentous or particulate microstructure was investigated under gastric and intestinal conditions in the presence or absence of digestive proteases. Microscopic examination showed riboflavin arranged into crystals dispersed randomly throughout the gels. Rheological analysis revealed that the riboflavin load weakened the gel networks, which led to gel disintegration during hydro-swelling tests at pH 7.5. Dissolution tests showed that riboflavin release was faster at pH 7.5 than at pH 1.2 for both gels. Initial release was faster from particulate gels, due to their higher porosity and eroding granular texture. The release mechanisms involved in riboflavin release were diffusion and matrix degradation for both gels at pH 1.2 and pH 7.5. In the presence of pepsin at pH 1.2, both gel types provided good protection of riboflavin for at least 6 h, while both were digested in the presence of pancreatin at pH 7.5. These results suggest that both gels might be useful for transporting bioactive molecules through the gastrointestinal tract and delivering them in the small intestine. Considering their non-synthetic nature, they should be of great interest to developers of innovative functional foods.     

Fat crystal-stabilized water-in-oil emulsions as controlled release systems

Fat crystal-stabilized water-in-oil emulsions were developed as a controlled release matrix for the delivery of salt. Glycerol monostearate (GMS), glycerol monooleate (GMO) and polyglycerol polyricinoleate (PGPR) were used as emulsifiers and hydrogenated canola oil (HCO) was added as a solid fat. Salt release towards an external aqueous phase was measured via conductivity as a function of temperature. Following 2 h of release at room temperature, the GMS emulsion had the highest encapsulation efficiency followed by the PGPR-HCO, PGPR-only and GMO–HCO emulsions, respectively. The GMS crystals formed Pickering shells around the water droplets that effectively prevented salt transport whereas in the GMO–HCO emulsion, the presence of partial interfacial HCO crystal coverage resulted in lower retention. All crystal-stabilized emulsions showed rapid release of their salt load upon melting of the surrounding solid fat, while little temperature effect was observed with the PGPR-based emulsions. However, these emulsions were sensitive to the presence of a salt concentration gradient whereas the fat crystal-stabilized emulsions showed little response. Overall, this study demonstrated that the spatial distribution of the stabilizing fat crystals (i.e., interfacial vs. continuous phase) as well as the emulsifier type were critical factors controlling salt release patterns.     

Formation and stability of α-lactalbumin–lysozyme spherical particles: Involvement of electrostatic forces

The formation and the subsequent stability of spherical microparticles resulting from the self-assembly between two oppositely charged proteins, lysozyme (LYS) and apo α-lactalbumin (apo α-LA), at pH 7.5 and 45 °C were studied under different physico-chemical conditions—ionic strength, type of salts, type of buffer. Increasing the ionic strength reduced the ability of the two proteins to interact together and to form microspheres. The formation of such microparticles was completely abolished at an ionic strength of 100 mM. Increasing salt concentration also allowed destabilisation and dissociation of formed microspheres in salt-dependent manner as assessed by turbidity experiments and microscopic observations. Microparticles were destabilised with either NaCl, MgCl₂ or CaCl₂, but the latter showed the greatest destabilising effect. The higher efficiency of calcium ions in the destabilisation experiment could be attributed to the presence of specific calcium binding site on α-LA. Interestingly, complete disappearance of formed particles was not reached even after adding salt concentrations as high as 250 mM of NaCl or 7.5 mM of divalent cations. Our results suggest that electrostatic interactions are clearly involved in the first events of the two-protein assembly and spherical particles building. Moreover, non-electrostatic forces are also involved in maintaining the integrity and stability of formed microparticles.     

大豆蛋白凝胶制备及其性质研究

研究以NaCl作为凝固剂,将大豆分离蛋白(SPI)80℃预热处理10 h,在不同浓度、离子强度及pH值条件下,通过95℃加热蛋白液30min使蛋白变性来制备大豆蛋白凝胶,并对凝胶的质构及凝胶特性进行了分析。结果表明:pH值7.0、80℃预热处理10 h的大豆蛋白溶解性明显小于普通SPI,其他pH值条件下两者溶解性差异不大;pH值2.0、80℃预热处理10 h的蛋白凝胶情况明显优于普通SPI,在一定范围内,随着蛋白浓度及离子强度的增加,凝胶情况越来越好。     

EFFECT OF MgCl2/SODIUM PYROPHOSPHATE ON CHICKEN BREAST MUSCLE MYOSIN SOLUBILIZATION AND GELATION

Sodium chloride (0.29 M) at pH 7 solubilized about 24% of the myosin of washed, minced chicken breast muscle. At a similar pH, 0.2 M sodium chloride in the presence of 10 mM sodiumpyrophosphate and 10 mM magnesium chloride solubilized almost 60% of the myosin. In spite of the greater solubility of myosin under the latter conditions, when gels were prepared with these concentrations of salt at pH 7, the gels without the sodium pyrophosphate and magnesium chloride were slightly superior in both stress (39.3 kPa vs 28.3 kPa) and true strain (2.3 vs 2.0) values. Gels made at a lower pH (6.1–6.5) made much poorer gels. This was true whether the low pH was obtained naturally in the preparation of the sample or re‐adjusted after bringing the mince to a neutral pH. It appears that conditions of pH and salt content that cause solubilization of myosin at more dilute conditions does not contribute to gel quality, but the neutral pH is an important factor for obtaining good gels at ionic strengths <0.3.     

Effects of pH and ionic strength of NaCl on the stability of diacetyl and (-)-α-pinene in oil-in-water emulsions formed with food-grade emulsifiers

The aim of the present study was to assess the influence of pH (3, 7 and 9) as well as ionic strength of NaCl (0mmol/l, 100mmol/l, 200mmol/l and 400mmol/l) on the retention and release characteristics of diacetyl and (-)-α-pinene in oil-in-water (o/w) emulsions prepared with dried egg yolk (DEY) or starch sodium octenylsuccinate (SOE). Increase of pH from 3 to 9 progressively enhanced retention of diacetyl in emulsions prepared with both DEY and SOE. Whereas, in samples flavoured with (-)-α-pinene, the highest and lowest retention time-courses were detected at pH 9 and pH 7 as well as pH 7 and pH 3 regarding emulsions prepared with DEY as well as SOE, respectively. With increasing salt concentration, the retention of diacetyl was decreased, irrespectively of the applied emulsifier type, whereas generally opposite effects were observed in the case of (-)-α-pinene. The parameters of release characteristics i.e., release rate constants (k) and release mechanism factors (n), were calculated using Avrami's equation. ANOVA revealed significant effects (p<0.001) of pH and NaCl concentration on k parameters, whereas relationship between applied environmental conditions and n factors was dependent on hydrophobicity of the odourant. Sensory evaluation revealed higher correlation values between odour intensity and aroma compound concentration in emulsions aromatized with (-)-α-pinene than with diacetyl.     

Absorption of whey protein isolated (WPI)-stabilized β-Carotene emulsions by oppositely charged oxidized starch microgels

β-Carotene is a natural antioxidant that is beneficial for human health. β-Carotene is well known for its aqueous insolubility and its sensitivity to environmental stimuli. To achieve the targeted delivery of β-Carotene to human intestine, a microgel system was developed. The microgel is made of oxidized potato starch polymers, which are obtained by chemical cross-link process by sodium trimetaphosphate (STMP). At pH < pI (WPI: pH 5), as in the acidic condition in stomach, the positively charged WPI emulsified β-Carotene nanoemulsion droplets were absorbed by the negatively charged starch microgel particles, which prevented the early release of β-Carotene in stomach. At pH > pI, as in the intestinal condition, WPI–β-Carotene nanoemulsion droplets and microgel particles both carry negative charges. Under this condition, β-Carotene will be released from microgel. In this study, we investigated the absorption capacity of β-Carotene nanoemulsion droplets by oxidized starch microgel particles of various degrees of oxidation (DO) and the cross-link density (R_{cross-linker/polymer(w/w)}) is investigated as a function of pH and salt concentrations. We found that DO30% with R_{cross-linker/polymer(w/w)} = 0.1 was the optimal gel type for nanoemulsion droplets absorption, and pH 3 and ionic strength of 0.06 M were the optimal conditions for nanoemulsion droplets absorption. The swelling capacity of the microgel particles rather than its ζ-potential was dominant in governing the absorption capacity. The in vitro release experiment under stimulated gastrointestinal fluids suggested that β-Carotene emulsions droplets remained stable at the gastric condition and were majorly released under the intestinal condition. The results suggested that the oxidized starch microgel could be used to prevent the early release of β-Carotene in the stomach and target delivery of them to the intestine.     

籽粒苋蛋白质功能特性的理论研究(II)—籽粒苋蛋白质发泡性的理论研究

采用碱性提取和等电点法分离出籽粒苋蛋白质 ,测定了不同浓度、pH值、离子强度、蔗糖浓度条件下的蛋白质发泡力和泡沫稳定性 ,探讨了籽粒苋蛋白质在不同条件下发泡性的变化规律 ,为籽粒苋蛋白质在食品加工中的应用提供了一定的理论依据。     

Role of pH and Ionic Strength on Water Relationships in Washed Minced Chicken-breast Muscle Gels

ABSTRACT The relationship between pH, ionic strength, and water balance of chicken-breast muscle gels was investigated. An increase in gel pH (pH 6.4 to 7.4) without added NaCl led to dramatic increases in water-holding capacity and water uptake (P < 0.05). Gels at 150 mM NaCl exhibited less ability to adsorb water than salt-free gels (P < 0.05 at pH 6.8 to 7.4) and had lower water-holding capacities (P < 0.05) and fold scores at and below pH 7. Varying salt concentration of the gel-bathing solutions had dramatic effect on the water uptake of the gels. The results show that strong water-absorbing gels can be produced at low ionic strengths and suggest that the negative charge of the muscle proteins is the driving force for water uptake and retention.     

Retention of aroma compounds in food matrices of similar rheological behaviour and different compositions

Two matrices with a similar rheological behaviour but with a different composition have been developed: one containing carbohydrates (d-glucose, pectin and starch) and in the second one, called complex matrix, a lipid (triolein) was added. The release of six aroma compounds is quantified by using the measurements of partition coefficients at thermodynamic equilibrium. The role of lipid (triolein) on the retention of all the aroma compounds was pointed out. The effect of carbohydrates was more complex: in comparison with water, ethyl hexanoate and trans-2-hexenal were more retained whereas diacetyl, 2-pentanone and cis-3-hexenol were “repulsed” from the matrix. The kinetic study of the release from these matrices had shown a decrease of the initial rate of release by reference with water. From carbohydrates matrix, the decrease of the release for three compounds (ethyl acetate, ethyl hexanoate, 2-pentanone) seemed to arise from the variation of diffusion and/or retention by carbohydrates. For the three other aroma compounds (diacetyl, cis-3-hexenol and trans-2-hexenal) no variation of the initial rate was registered. The comparison of the release rates from carbohydrate and complex matrices indicated the role of lipids and the comparison of the release rates from water and complex matrix showed the combined effects of texture and lipids. The decrease of initial release rate was more important in presence of lipids than in presence of carbohydrates. The most important decrease was observed with the most hydrophobic compound.     

明胶与海藻酸钠的静电复合机制及热动力学分析

为探明明胶（gelatin，GEL）与海藻酸钠（sodium alginate，AG）静电作用机制及影响因素，对不同GEL和AG质量比（r）、不同pH值的液体体系进行常温浊度滴定、Zeta电位测定及亚甲基蓝吸收光谱实验等，研究复合物形成的影响因素，并利用等温滴定微量热技术对GEL与AG结合过程进行热动力学分析。结果表明，常温下GEL与AG结合是以强烈的静电相互作用为主的焓驱动放热过程；NaCl通过静电屏蔽作用，在低浓度（＜0.02?mol/L）时促进GEL与AG的结合，高浓度时抑制其结合。环境pH值通过调整GEL与AG所带的净电荷从而影响其静电作用，低pH值时形成不溶性复合物，高pH值时形成可溶性复合物；提高r或者提高GEL和AG总浓度可以促进其相互作用，r=7∶3时，可以在较高pH值下形成稳定的复合物悬浊液，pH值在2.5～3.5变化对悬浊液影响较小。本实验为GEL-AG基生物材料的开发及其在食品、医药领域应用提供理论依据。