Characteristics of rennet–casein-based model processed cheese containing maize starch: Rheological properties,meltabilities and microstructures

The rheological properties, meltabilities and microstructural properties of rennet–casein-based model processed cheese slices containing normal maize starch, waxy maize starch and high amylose (70%) maize starch (HAMS) were investigated as a function of the ratio of starch to protein. The storage modulus (G′) increased with increasing ratio of normal maize starch to protein in the processed cheeses, whereas G′ decreased with the addition of waxy maize starch. The tan δ values at high temperature (90 °C) decreased with the addition of both normal maize starch and waxy maize starch, but normal maize starch caused a greater reduction in the tan δ values. These properties were influenced to a lesser extent in processed cheeses containing HAMS. Processed cheeses containing normal maize starch lost meltability at ratios of starch to protein > 0.16; waxy maize starch and HAMS had less influence on the meltability. The effect of starch on the properties of the processed cheeses was attributed to phase behaviours between the protein and the starch, which was shown in confocal laser scanning micrographs.     

Temperature-dependent complexation between sodium caseinate and gum arabic

Complex formation between sodium caseinate and gum arabic as a function of temperature was investigated using dynamic light scattering, fluorescence and NMR. At neutral pH, the turbidity and the particle size increased when sodium caseinate and gum arabic mixtures were heated in situ at temperatures above a critical temperature. The increases in turbidity and particle size were reversible. This effect was considered to be due to hydrophobic interactions, leading to the formation of a complex between sodium caseinate and gum arabic. ¹H NMR spectroscopy showed that ANS, which bound to caseinate at low temperatures in caseinate solution or a caseinate-gum arabic mixture, was released at high temperatures upon formation of a caseinate or caseinate-gum arabic complex. This supported changes observed in the fluorescence of 8-anilino-1-naphthalene sulfonate upon binding to caseinate, which decreased at high temperatures for caseinate alone or when sodium caseinate was mixed with gum arabic. Light-scattering (turbidity) and dynamic light-scattering studies show that the temperature-dependent complexation between sodium caseinate and gum arabic was sensitive to the mass ratio of protein to gum arabic (greater complexation at a 1:5 ratio than a 1:1 ratio) and the pH (maximum complexation at pH 6.5).     

Interactions of fat globule surface proteins during concentration of whole milk in a pilot-scale multiple-effect evaporator

The changes in milk fat globules and fat globule surface proteins during concentration of whole milk using a pilot-scale multiple-effect evaporator were examined. The effects of heat treatment of milk at 95 degrees C for 20 s, prior to evaporation, on fat globule size and the milk fat globule membrane (MFGM) proteins were also determined. In both non-preheated and preheated whole milk, the size of milk fat globules decreased while the amount of total surface proteins at the fat globules increased as the milk passed through each effect of the evaporator. In non-preheated samples, the amount of caseins at the surface of fat globules increased markedly during evaporation with a relatively small increase in whey proteins. In preheated samples, both caseins and whey proteins were observed at the surface of fat globules and the amounts of these proteins increased during subsequent steps of evaporation. The major original MFGM proteins, xanthine oxidase, butyrophilin, PAS 6 and PAS 7, did not change during evaporation, however, PAS 6 and PAS 7 decreased during preheating. These results indicate that the proteins from the skim milk were adsorbed onto the fat globule surface when the milk fat globules were disrupted during evaporation.     

Structural changes in cow,goat, and sheep skim milk during dynamic in vitro gastric digestion

Coagulation of milk in the stomach is the first crucial step in its digestion. Using a human gastric simulator, the dynamic gastric digestion of goat and sheep skim milk were compared with that of cow skim milk, focusing particularly on their physical characteristics. The gastric contents were analyzed for changes in dry matter and microstructure, and the extent of protein digestion. The study revealed that the skim milk from all species formed a curd within the first 15 min of gastric digestion, at which time the pH was ~6.1 to 6.3. Compared with cow skim milk, the dry matter contents of the clots formed from goat and sheep skim milk were lower and higher, respectively, which was due to the differences in their total solids and protein contents. Microstructural analysis showed that, as digestion progressed, the clot structure became more cohesive, along with a decrease in moisture content, which in turn affected the breakdown and hydrolysis of caseins by pepsin; this phenomenon was similar for milk from all species. However, the extent of moisture retained in the sheep skim milk clot appeared to be lower than those of the cow and goat skim milk clots. In addition, the relative firmness of the sheep milk clot was higher than those of the cow and goat milk clots at the end of gastric digestion. The pattern of protein hydrolysis by pepsin was similar for the milk of all species, despite the differences in the proportions of different proteins. The study provided insight into the coagulation kinetics of goat and sheep skim milk under in vitro gastric digestion conditions.     

Thermal treatment to form a complex surface layer of sodium caseinate and gum arabic on oil–water interfaces

This study reported an alternative approach to electrostatic interaction to generate a biopolymer complex surface layer on emulsion droplets. By increasing the temperature, a complex surface layer of sodium caseinate (CN) and gum arabic (GA) on emulsion droplets could be formed either through the adsorption of complexes formed in the solution or through direct complexation of CN and GA with the surface. Mixtures of CN and GA were heated at pH 7 and then used to form oil-in-water emulsions at high temperatures. Changes in the average particle size and the ζ-potential of the emulsions indicated that complexes of GA and CN adsorbed to the interface at temperatures above 60 °C. A thick complex surface layer was also observed using confocal laser scanning microscopy. The addition of GA or CN to emulsions made with CN or GA resulted in an apparent binding of GA or CN to the emulsion droplets that depended on the sequence of addition. This temperature-induced formation of a complex surface layer was considered to be due to hydrophobically driven complexation between CN and the protein fraction of GA. The formation of the complex surface layer was dependent on the concentration, the temperature, and the addition order of the second component. This finding may imply an alternative option for the formation of biopolymer multilayers or complex surface layers on colloidal particles.     

Gastric digestion of milk protein ingredients: Study using an in vitro dynamic model

The coagulation behavior and the kinetics of protein hydrolysis of skim milk powder, milk protein concentrate (MPC), calcium-depleted MPC, sodium caseinate, whey protein isolate (WPI), and heated (90°C, 20 min) WPI under gastric conditions were examined using an advanced dynamic digestion model (i.e., a human gastric simulator). During gastric digestion, these protein ingredients exhibited various pH profiles as a function of the digestion time. Skim milk powder and MPC, which contained casein micelles, formed cohesive, ball-like curds with a dense structure after 10 min of digestion; these curds did not disintegrate over 220 min of digestion. Partly calcium-depleted MPC and sodium caseinate, which lacked an intact casein micellar structure, formed curds at approximately 40 min, and a loose, fragmented curd structure was observed after 220 min of digestion. In contrast, no curds were formed in either WPI or heated WPI after 220 min of digestion. In addition, the hydrolysis rates and the compositions of the digesta released from the human gastric simulator were different for the various protein ingredients, as detected by sodium dodecyl sulfate-PAGE. Skim milk powder and MPC exhibited slower hydrolysis rates than calcium-depleted MPC and sodium caseinate. The most rapid hydrolysis occurred in the WPI (with and without heating). This was attributed to the formation of different structured curds under gastric conditions. The results offer novel insights about the coagulation kinetics of proteins from different milk protein ingredients, highlighting the critical role of the food matrix in affecting the course of protein digestion.     

DN100气体超声流量计管道配置对计量性能影响研究

随着气体超声流量计技术水平日趋成熟,国内外相关标准对管道配置的要求经历了数次变化.由于现场可能存在阻流件干扰,为研究不同管道配置对小口径超声流量计的计量性能影响,考察了不同直管段长度在复杂工艺条件下对不同型号超声流量计的适用性,开展了D N 100超声流量计与标准装置的测试实验.结果表明,安装流动调整器后,在前直管段为...     

Structure and stability of heat-treated concentrated dairy-protein-stabilised oil-in-water emulsions: A stability map characterisation approach

Emulsion instabilities such as depletion flocculation, coalescence, aggregation and heat-induced protein aggregation may be detrimental to the production of sterilised food emulsions. The type and the amount of protein present in the continuous phase and at the oil–water interface are crucial in the design of emulsions with appropriate stability. In this study, four oil-in-water model emulsion systems (pH 6.8–7.0) were formulated, characterised and categorised according to the potential interactions between protein-coated or surfactant-coated emulsion droplets and non-adsorbed proteins present in the continuous phase. The heat stability, the creaming behaviour and the flow behaviour of the model emulsions were influenced by both the emulsifier type and the type of protein in the continuous phase. The results suggest that this stability map approach of predicting droplet–droplet, droplet–protein and protein–protein interactions will be useful for the future design of heat-stable emulsion-based beverages with good creaming stability at high protein concentrations.