Changes in the physical properties,solubility, and heat stability of milk protein concentrates prepared from partially acidified milk

A limiting factor in using milk protein concentrates (MPC) as a high-quality protein source for different food applications is their poor reconstitutability. Solubilization of colloidal calcium phosphate (CCP) from casein micelles during membrane filtration (e.g., through acidification) may affect the structural organization of these protein particles and consequently the rehydration and functional properties of the resulting MPC powder. The main objective of this study was to investigate the effects of acidification of milk by glucono-δ-lactone (GDL) before ultrafiltration (UF) on the composition, physical properties, solubility, and thermal stability (after reconstitution) of MPC powders. The MPC samples were manufactured in duplicate, either by UF (65% protein, MPC65) or by UF followed by diafiltration (80% protein, MPC80), using pasteurized skim milk, at either the native milk pH (~pH 6.6) or at pH 6.0 after addition of GDL, followed by spray drying. Samples of different treatments were reconstituted at 5% (wt/wt) protein to compare their solubility and thermal stability. Powders were tested in duplicate for basic composition, calcium content, reconstitutability, particle size, particle density, and microstructure. Acidification of milk did not have any significant effect on the proximate composition, particle size, particle density, or surface morphology of the MPC powders; however, the total calcium content of MPC80 decreased significantly with acidification (from 1.84 ± 0.03 to 1.59 ± 0.03 g/100 g of powder). Calcium-depleted MPC80 powders were also more soluble than the control powders. Diafiltered dispersions were significantly less heat stable (at 120°C) than UF samples when dissolved at 5% solids. The present work contributes to a better understanding of the differences in MPC commonly observed during processing.     

Storage induced changes to high protein powders: influence on surface properties and solubility

BACKGROUND: MPC 80 is a high‐protein (80%) milk powder commonly used in the food industry as a functional ingredient and valued for its nutritional quality. However, its rehydration properties decline during storage, causing more time to be required for rehydration of the powder by the end user. It is thought that changes at the surface of the powder particles contribute to this reduced solubility during storage. RESULTS: Surface composition and structural changes in milk protein concentrate (MPC) were observed during 90 days of storage at temperatures of 25 and 40 °C and relative humidities of 44, 66 and 84%. No significant changes to the surface composition (fat, protein and lactose) of the MPC powder samples occurred during storage; however, some changes in the microstructure of the powders were observed. Scanning electron microscopy analysis of the powder particles during dissolution showed the formation of a crust, consisting of a thin layer of fused casein micelles, on the surface of the stored powders. An increase in the hydrophobicity at the surface of the particles was evident by X‐ray photoelectron spectroscopy analysis of the bonding state of the elements at or near the surface and by atomic force microscopy measurements of the adherence of particles to the surface of a material. CONCLUSION: The development of this ‘crust’ is thought to contribute to the decrease in the solubility of the powder particles during storage. The increase in the hydrophobicity at the surface and the casein micelle interactions resulting in the surface crust formation appear to contribute to the decrease in the solubility of MPC during storage. Copyright © 2011 Society of Chemical Industry     

Effect of processing methods and protein content of the concentrate on the properties of milk protein concentrate with 80% protein

In recent years, a large increase in the production of milk protein concentrates (MPC) has occurred. However, compared with other types of milk powders, few studies exist on the effect of key processing parameters on powder properties. In particular, it is important to understand if key processing parameters contribute to the poor solubility observed during storage of high-protein MPC powders. Ultrafiltration (UF) and diafiltration (DF) are processing steps needed to reduce the lactose content of concentrates in the preparation of MPC with a protein content of 80% (MPC80). Evaporation is sometimes used to increase the TS content of concentrates before spray drying, and some indications exist that inclusion of this processing step may affect protein properties. In this study, MPC80 powders were manufactured by 2 types of concentration methods: membrane filtration with and without the inclusion of an evaporation step. Different concentration methods could affect the mineral content of MPC powders, as soluble salts can permeate the UF membrane, whereas no mineral loss occurs during evaporation, although a shift in calcium equilibrium toward insoluble forms may occur at high protein concentration levels. It is more desirable from an energy efficiency perspective to use higher total solids in concentrates before drying, but concerns exist about whether a higher protein content would negatively affect powder functionality. Thus, MPC80 powders were also manufactured from concentrates that had 3 different final protein concentrations (19, 21, and 23%; made from 1 UF retentate using batch recirculation evaporation, a similar concentration method). After manufacture, powders were stored for 6 mo at 30°C to help understand changes in MPC80 properties that might occur during shelf-life. Solubility and foaming properties were determined at various time points during high-temperature powder storage. Inclusion of an evaporation step, as a concentration method, resulted in MPC80 that had higher ash, total calcium, and bound calcium (of rehydrated powder) contents compared to concentration with only membrane filtration. Concentration method did not significantly affect the bulk (tapped) density, solubility, or foaming properties of the MPC powders. Powder produced from concentrate with 23% protein content exhibited a higher bulk density and powder particle size than powder produced from concentrate that had 19% protein. The solubility of MPC80 powder was not influenced by the protein content of the concentrate. The solubility of all powders significantly decreased during storage at 30°C. Higher protein concentrations in concentrates resulted in rehydrated powders that had higher viscosities (even when tested at a constant protein concentration). The protein content of the concentrate did not significantly affect foaming properties. Significant changes in the mineral content are used commercially to improve MPC80 solubility. However, although the concentration method did produce a small change in the total calcium content of experimental MPC80 samples, this modification was not sufficiently large enough (<7%) to influence powder solubility.     

Effect of Hydrostatic Pressure on Molecular Conformation of Tilapia (Orechromis niloticus) Myosin

ABSTRACT: Change in tilapia myosin molecular conformation due to pressurization at 50 to 200 MPa for 0 to 60 min was investigated. After a 50-MPa treatment, tilapia myosins slightly decreased their total sulfhydryl contents and exposed their hydrophobic residues. Experimental results indicated that 100- and 150-MPa treatments caused an apparent unfolding of myosins and a 1-fold increase of their surface hydrophobicity ( S _o). Myosins mainly formed intermolecular disulfide bonds with pressures of 100 to 200 MPa. In addition, increasing pressures altered the myosin conformation and decreased its Ca-ATPase activity. Myosin apparently unfolded and formed disulfide bonds and hydrophobic interactions with pressurizing at 150 MPa.     

超声波和TGase对重组草鱼肉形成过程中理化性质的影响

本研究以草鱼为原料,采用四种处理方式:对照组(只添加NaCl)、实验组1(添加NaCl和谷氨酰胺转氨酶(transglutaminase,TGase))、实验组2(添加NaCl且超声)、实验组3(添加NaCl和TGase且超声),对草鱼肉进行重组。通过测定其分子间作用力、巯基含量、浊度、溶解率等,分析重组过程中超声波和TGase对重组草鱼肉理化性质的影响。结果表明,在重组草鱼肉形成过程中,超声波和TGase对离子键、氢键、疏水相互作用、二硫键和非二硫键均有不同程度的影响,促进重组草鱼肉的形成;与此同时,在重组草鱼肉形成过程中,超声波和TGase可以加速肌原纤维蛋白巯基氧化形成二硫键,增强其表面疏水性,增加其浊度,降低重组草鱼肉蛋白质溶解率;由SDS-PAGE电泳图谱可知,在重组草鱼肉形成过程中,三个实验组在200 kDa附近的条带逐渐变淡,甚至消失,可知重组过程中超声波和TGase主要作用于肌原纤维蛋白重链。综合各试验指标结果,超声波和TGase均可促进草鱼肉蛋白质交联,且两者同时作用时交联度最高,对指导草鱼肉产业生产具有一定的参考价值。     

Extrusion modifies some physicochemical properties of milk protein concentrate for improved performance in high‐protein nutrition bars

BACKGROUND

Extruded and ground milk protein concentrate powders, specifically those with 800 g kg^–1 protein (i.e. MPC80), imparted softness, cohesion and textural stability to high‐protein nutrition (HPN) bars. The present study evaluated some physicochemical properties of extruded and conventionally produced (i.e. spray‐dried) MPC80 to explain these improvements. Protein chemical changes and aggregations within MPC80‐formulated HPN bars during storage were characterized.

RESULTS

Extruded MPC80 powders had broader particle size distribution (P < 0.05) and smaller volume‐weighted mean diameter (P < 0.05) than the spray‐dried control. Loose, tapped and particle densities increased (P < 0.05) and correspondingly occluded and interstitial air volumes decreased (P < 0.05) after extruding and milling MPC80. Extrusion decreased water holding capacity (P < 0.05) and solubility (P < 0.05), yet improved the wettability (P < 0.05) of MPC80. MPC80 free sulfhydryl (P < 0.05) and free amine (P < 0.05) concentrations decreased after extrusion. Sulfhydryl and amine concentrations changed (P < 0.05) and disulfide‐linked and, more prominently, Maillard‐induced aggregates developed during HPN bar storage.

CONCLUSION

Extrusion and milling together changed the physicochemical properties of MPC80. Chemical changes and protein aggregations occurred in HPN bars prepared with either type of MPC80. Thus, the physicochemical properties of the formulating powder require consideration for desired HPN bar texture and stability. © 2017 Society of Chemical Industry     

不同溶剂中玉米醇溶蛋白的聚集状态和结构性质

本文采用紫外光谱(UV)、傅里叶红外光谱(FT-IR)、荧光光谱和扫描电镜(SEM)等技术研究不同溶解环境下(甲醇、乙醇、异丙醇、乙酸和丙酮)zein蛋白的溶解、聚集和结构性质,用静态接触角研究zein蛋白膜的表面疏水性。结果显示Zein在100%乙酸和80%乙醇和异丙醇溶液中呈现出良好的溶解状态和较高的透光率;其次,在70%乙酸溶液中粒径最大,为5230.16 nm;在80%乙醇溶液中的粒径最小,为25.52nm。异丙醇使zein表现出强荧光吸收。80%乙酸和乙醇溶解的zein蛋白与容器接触一侧的接触角为88.90°和86.60°,表现出中性润湿性,适合稳定油水界面;从80%甲醇溶液中形成的蛋白膜的空气侧接触角最大,为72.10°。综上分析表明,溶剂浓度差异影响蛋白质与水的相互作用力导致蛋白聚集程度不同。此外,溶剂极性和电负性差异通过影响分子间疏水相互作用和氢键以及二硫键的形成,进而影响蛋白粒径、表面疏水性和空间构象。     

Solubility of commercial milk protein concentrates and milk protein isolates

High-protein milk protein concentrate (MPC) and milk protein isolate (MPI) powders may have lower solubility than low-protein MPC powders, but information is limited on MPC solubility. Our objectives in this study were to (1) characterize the solubility of commercially available powder types with differing protein contents such as MPC40, MPC80, and MPI obtained from various manufacturers (sources), and (2) determine if such differences could be associated with differences in mineral, protein composition, and conformational changes of the powders. To examine possible predictors of solubility as measured by percent suspension stability (%SS), mineral analysis, Fourier transform infrared (FTIR) spectroscopy, and quantitative protein analysis by HPLC was performed. After accounting for overall differences between powder types, %SS was found to be strongly associated with the calcium, magnesium, phosphorus, and sodium content of the powders. The FTIR score plots were in agreement with %SS results. A principal component analysis of FTIR spectra clustered the highly soluble MPC40 separately from the rest of samples. Furthermore, 2 highly soluble MPI samples were clustered separately from the rest of the MPC80 and MPI samples. We found that the 900 to 1,200 cm⁻¹ region exhibited the highest discriminating power, with dominant bands at 1,173 and 968 cm⁻¹, associated with phosphate vibrations. The 2 highly soluble MPI powders were observed to have lower κ-casein and α-_S1-casein contents and slightly higher whey protein contents than the other powders. The differences in the solubility of MPC and MPI were associated with a difference in mineral composition, which may be attributed to differences in processing conditions. Additional studies on the role of minerals composition on MPC80 solubility are warranted. Such a study would provide a greater understanding of factors associated with differences in solubility and can provide insight on methods to improve solubility of high-protein milk protein concentrates.     

Compositional and structural properties of whey proteins of sweet,acid and salty whey concentrates and their respective spray dried powders

The influence of physiochemical characteristics of whey concentrates obtained by ultrafiltration of acid and salty whey streams on the surface composition, particle organisation, secondary structures and protein interactions of the respective spray dried whey powders was investigated. Their properties were compared with those of native and sweet whey. Acid whey concentrate demonstrated characteristically low surface charge, high surface hydrophobicity, high average particle size and high thiol activity compared with sweet and native whey concentrates. Salty whey concentrate was characterised by low surface hydrophobicity, high thiol activity and low average particle size. Surface characterisation of whey powders revealed protein-rich surfaces for all whey powders while those in salty whey were highly hydrophobic. Protein characteristics of native and sweet whey powders largely followed those of concentrates. In contrast, protein characteristics of the acid and salty whey powders largely changed from those of the concentrates.     

Forces Involved in Soy Protein Gelation: Effects of Various Reagents on the Formation, Hardness and Solubility of Heat-Induced Gels Made from 7S, 11S, and Soy Isolate

The effects of various reagents on the formation, hardness and solubility of heat-induced gels of soybean 7S, 11S globulins and isolate were studied. Gels were formed in 30 mM Tris HCl buffer (pH 8.0) with or without reagents by heating at 80°C for 30 min. The results indicated that electrostatic interactions and disulfide bonds are involved in the formation of 11S globulin gels; mostly hydrogen bonding in 7S globulin gels and hydrogen bonding and hydrophobic interactions in soy isolate gels. Analyses of the proteins solubilized from the gels indicated that the basic subunits of 11S globulin interact with 7S globulin in soy isolate gels. The contribution of certain acidic subunits to network formation in US soy isolate gels is limited     

The kinetics of heat-induced structural changes of beta-lactoglobulin

Heat-induced structural changes of beta-lactoglobulin were studied at temperatures ranging from 67.5 to 82.5 degrees C, and at pH 7.5. These changes were monitored by measurement of surface hydrophobicity, thiol availability, and protein solubility. Kinetic studies were conducted to quantitatively describe the contribution of hydrophobic and SH/SS interchange reactions to the thermal structural changes of beta-lactoglobulin. Results indicate that beta-lactoglobulin is sensitive to heat-induced interchange reactions with consequences for protein solubility. The extent of changes measured by the increase in surface hydrophobicity and the decrease in slow-reacting SH groups content could be described by a first-order fractional conversion model and were characterized by activation energy values of 233.9 +/- 8.6 and 148.2 +/- 6.7 kJ/mol, respectively. The break in the Arrhenius plot suggested in literature for beta-lactoglobulin denaturation was confirmed in this study only for the kinetics of exposed SH groups.     

Correlations between the solubility and surface characteristics of milk protein concentrate powder particles

The solubility of high-protein milk protein concentrate (MPC) may decrease significantly during storage, particularly at relatively high temperatures and humidity. The objective of this study was to seek correlations between the solubility loss of MPC during storage and various surface characteristics determined on the basis of simultaneous nanoscale topographical imaging and nanomechanical mapping of MPC particle surfaces using atomic force microscopy. A control MPC and a calcium-depleted MPC were stored at 45°C and 66% relative humidity for up to 60 d. The solubility of the control MPC was 56% at the beginning of the storage and gradually decreased to 10% at the end of the 60-d storage. The calcium-depleted MPC exhibited more rapid decreases from almost 100% at the beginning of the storage to 18% after storage for 45 d, after which we observed no significant difference in solubility between the control and calcium-depleted MPC. Averaged or root mean squared roughness values calculated using topographical images were found to have no correlation with the solubility. Deformation, Derjaguin-Muller-Toropov modulus, and adhesion images revealed the presence of individual casein micelles and larger clusters of aggregated casein micelles at MPC particle surfaces, whereas we observed no correlation between the solubility and averaged values of these nanomechanical properties. Furthermore, Derjaguin-Muller-Toropov modulus and adhesion images showed that the peripheral edges of individual casein micelles and their clusters had significantly higher values of the corresponding nanomechanical properties than other regions in the images, indicating the occurrence of the fusion of casein micelles. The surface area coverage or the percent area of the fused regions in an image revealed significant negative linear correlations with the solubility for both the control and calcium-depleted MPC. The present results support the hypothesis that the fusion of casein micelles at MPC powder particle surfaces is a causative factor for the solubility loss of MPC during storage and in turn suggest that the solubility loss may be alleviated by inhibiting the formation of a crust or skin on powder particle surfaces.     

Functionality of milk protein concentrate 80 with emulsifying salts and its applications in analogue cheeses

Milk protein concentrate 80 (MPC80) was prepared with different emulsifying salts (ES). The effects on particle size (D50), solubility, and surface hydrophobicity (H₀) of MPC80 were then observed after production. The molecular weight and secondary structure of MPC80 protein were also investigated through sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Fourier Transform Infrared (FTIR) spectrometry. Particle size (D50) was reduced from 31.37 to 20.67 μm following the addition of sodium phosphate (SPS). The solubility of MPC80 fortified with sodium citrate salt (SCS), SPS, and sodium pyrophosphate (SPP) was improved by 80.32, 78.23, and 55.80%, respectively. The SDS-PAGE pattern showed no significant difference between the control and single-ES-fortified samples, but major protein bands (α_s-CN, β-CN, κ-CN, BSA, and β-LG) had a stronger intensity than binary-ES-fortified MPC80. The FTIR results showed that the β-sheet content of ES-fortified MPC80 was relatively higher than that in the control. Compared with analogue cheeses made by MPC80 with and without ES, SCS–SPP (92:8)-modified MPC80 presented a better fluid mass and homogeneous colour.     

纳米鱼骨对鲢鱼肌球蛋白凝胶性能的影响

本实验以鲢鱼骨为原料制成纳米鱼骨（nano-scaled fish bone，NFB）、微米鱼骨（micro-scaled fish bone，MFB），并以CaCl2为对照，将其加入至鲢鱼肌球蛋白中，通过静态流变学、动态流变学、表面疏水性分析以及活性巯基浓度和溶解度的测定，比较不同钙源对肌球蛋白凝胶性能的影响。结果表明：各组肌球蛋白样品均具有剪切稀化现象，NFB和CaCl2的添加均会增加肌球蛋白的表观黏度，MFB则相反，这与NFB和CaCl2释放出的钙离子促进了蛋白质分子间的相互作用密切相关，而MFB由于颗粒较大，干扰了蛋白质分子间相互作用。加热过程中，NFB和CaCl2的添加进一步促进肌球蛋白通过形成二硫键和疏水相互作用，使肌球蛋白黏弹性升高。NFB对肌球蛋白胶凝特性的提升作用与CaCl2相近，明显高于MFB组，这为今后将NFB应用于鱼糜制品中提供了一定的数据支撑。     

Changes of Molecular Forces During Thermo-Gelling of Protein Isolated from PSE-Like Chicken Breast by Various Isoelectric Solubilization/Precipitation Extraction Strategies

Changes in protein intermolecular interactions during thermo-gelling were measured to compare the gelation properties of isoelectric solubilization/precipitation (ISP)-isolated protein extracted (solubilized at 3.5 and 11.0, precipitated at 5.5 and 6.2) from pale, soft, and exudative (PSE)-like chicken breast meat with raw meat paste. The solubility of both in water and salt (0.6 M NaCl) decreased significantly after ISP treatments. Protein profile analysis revealed that precipitation pH showed little influence on protein profile. Under pH 3.5 and pH 11.0 solubility conditions, the recovered protein at pH 6.2 showed significantly higher gel hardness than that at pH 5.5, which can be induced by hydrophobic change. Surface hydrophobicity (SH₀) and hydrophobic interactions at 25 °C presented similar results, indicating that the soluble protein at pH 11.0 exhibited a higher value after precipitation at pH 6.2 than that at pH 5.5, and the hydrophobicity of pH 3.5 isolates was higher than that of the pH 11.0 groups. However, the maximum hydrophobicity upon heating was inconsistent with initial tendencies. Given that the hydrophobic residues were exposed sufficiently during the ISP process, the ISP-treated proteins, particularly the samples extracted at pH 3.5, might be less susceptible to heat-induced exposure. The gelation behavior of the ISP-treated proteins had been modified on the basis of the intermolecular bonds during heating. In conclusion, the precipitation condition demonstrated excellent relevance for product development based on functionality.     

The soluble recombinant N-terminal domain of HMW 1Dx5 and its aggregation behavior

This study seeks to clarify and determine the fundamental properties of N-terminal domain of high molecular weight glutenin subunits (HMW-GS) 1Dx5 (1Dx5-N). 1Dx5-N was expressed in E. coli and its solubility was measured by spectrophotometry. Effects of edible salts (NaCl, Na₂CO₃), disulfide bond reductant dithiothreitol (DTT) and hydrophobic interactions of denaturant sodium dodecyl sulfonate (SDS) on 1Dx5-N polymer were investigated by native polyacrylamide gelelectrophoresis (PAGE), nonreducing/reducing SDS-PAGE, intrinsic fluorescence, size exclusion chromatography (SEC), dynamic light scattering (DLS) and circular dichroism (CD). Results showed that 1Dx5-N formed a soluble aggregate in aqueous solutions by native-PAGE, clarifying the role of N-terminal of HMW-GS in the insolubility of the whole subunits. Meanwhile, the hydrophobic interaction was more potent in promoting the aggregation of 1Dx5-N in aqueous solutions from the results of SEC, DLS and CD. Edible salts, NaCl and Na₂CO₃, could improve the polymer formation of 1Dx5-N through disulfide bonds. Moreover, Na₂CO₃ at high concentrations (> 200 mM) greatly favored polymer formation by disulfide bonds, and it induced other types of cross-links between amino acids in 1Dx5-N according to nonreducing/reducing SDS-PAGE and fluorescence spectrum. Moreover, the formation of covalent bonds was reinforced by hydrophobic interactions between 1Dx5-N. Therefore, these results provide much novel information on the N-terminal domain of HMW-GS to facilitate the understanding of its functional properties in wheat flour.     

Effect of manufacture and reconstitution of milk protein concentrate powder on the size and rennet gelation behaviour of casein micelles

Samples of raw skim milk, ultrafiltration/diafiltration retentate, concentrated retentate and milk protein concentrate powder (MPC80) from a single commercial production run were analysed using photon correlation spectroscopy. Measurements revealed insignificant differences in casein micelle size between the samples. In addition, there was no discernable difference between raw skim milk and MPC powder dissolved at 60 °C in the amount of casein remaining in supernatants from centrifugation at either 25,000 × g or 174,200 × g. Casein micelles did not appear to be altered during manufacture of MPC. The rennet gelation behaviour of reconstituted MPC was compared with raw skim milk. Reconstituted MPC did not coagulate unless supplemented with approximately 2 mm calcium chloride, which was attributed to the mineral removal during ultrafiltration/diafiltration. Addition of sufficient calcium could restore rennet coagulation kinetics and gel strength of reconstituted MPC to approximately that of raw skim milk.     

脂质过氧自由基诱导兔肉肌浆蛋白聚集机制

为全面理解兔肉品质的形成过程,利用偶氮二异丁脒盐酸盐（2,2’-azobis(2-amidinopropane)dihydrochloride,AAPH）的热分解作用生成过氧自由基（ROO·）诱导兔肉肌浆蛋白氧化,通过AAPH处理后兔肉肌浆蛋白中高铁肌红蛋白相对含量、超铁肌红蛋白浓度、羰基含量、总巯基含量以及肌浆蛋白表面疏水性、粒度、浊度和溶解性测定,肌浆蛋白十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（sodium dodecyl sulfate-polyacrylamide gel electrophoresis,SDS-PAGE）和内源荧光光谱分析,研究ROO·胁迫肌浆蛋白氧化聚集机制。结果表明,随着AAPH浓度的升高,高铁肌红蛋白和超铁肌红蛋白相对含量显著增加（P＜0.05）,蛋白质羰基含量显著增加（P＜0.05）,总巯基含量显著下降（P＜0.05）。内源荧光光谱和表面疏水性分析结果表明ROO·可以引起肌浆蛋白的去折叠和疏水基团暴露。SDS-PAGE分析结果表明ROO·可以导致肌浆蛋白二硫键交联。此外,随着AAPH浓度的增大,肌浆蛋白粒度和浊度呈现出逐渐增大的趋势,表明ROO·可以引发肌浆蛋白的聚集。疏水相互作用和二硫键交联是ROO·胁迫肌浆蛋白聚集的主要作用力。ROO·不仅可以直接引发肌浆蛋白聚集,还可以通过影响肌红蛋白诱导氧化体系促进肌浆蛋白交联聚集。     

Rheological properties of acid-induced soy protein-stabilized emulsion gels in the absence and presence of N-ethylmaleimide

The storage modulus (G′) and fracture properties of the non-treated and NEM-treated emulsion gels were investigated in the absence and presence of unadsorbed soy protein aggregates (USPA). In the absence of USPA, a decrease in the G′ of emulsion gels was observed after NEM treatment. However, in the presence of USPA, the addition of NEM only slightly decreased the G′ (p < 0.05). For both non-treated and NEM-treated emulsions, a ∼63-folds higher G′ value was obtained after the USPA addition. These results revealed the presence of sulphydryl group – disulfide bond interchange reactions at ambient temperature and under acidic conditions. In the absence of USPA, the sulphydryl group – disulfide bond interchange reactions was the main interactions responsible for the higher G′ values of non-treated emulsion gels, but for the emulsions with USPA presented, the large quantity of non-covalent interactions (e.g. hydrophobic group & hydrogen bonds) is the main interactions responsible for the aggregation and gelation of emulsion droplets. In the presence of USPA, the sulphydryl group – disulfide bond interchange reactions formed in the non-treated emulsion gels did not increase the final G′ but increased the stability of network. A power law relation was observed between the USPA concentration and the final G′, as well as between the oil volume fraction and the fracture stress/strain.     

On quantifying the dissolution behaviour of milk protein concentrate

Milk protein concentrate (MPC) is a newly developed dairy powder with wide range of applications as ingredients in the food industry, such as cheese, yogurt, and beverage. MPC has relatively poor solubility as a result of their high protein content (40–90 wt%), with distinct dissolution behaviour in comparison to skim milk or whole milk powders. Here, a focused beam reflectance measurement (FBRM) was used to monitor the dissolution process of an MPC powder, with the data used to develop a kinetic dissolution model based on the Noyes–Whitney equation. The model was used to estimate the dissolution rate constant k and the final particle size in suspension d_∞, describing dynamic dissolution behaviours and final solubility respectively of a particular powder. In this work, the effects of dissolution temperature, storage duration and storage temperature on dissolution properties of an MPC powder were also investigated. A quantitative understanding of relationship between process and storage conditions with powder functionality could be achieved from k and d_∞ profiles. This approach can potentially be applied to predict the dissolution behaviour of specific dairy powders in a more robust manner than conventional solubility tests.