Gelation of Mixed Gels Containing κ-Carrageenan and Skim Milk Components

ABSTRACT: The gelation characteristics of mixed gels containing κ-carrageenan and skim milk or milk fractions (skim milk permeate or retentate) obtained by ultrafiltration were examined. Increasing the skim milk solids content of mixtures containing carrageenan increased setting temperatures and gel strength. The milk proteins contributed to gel strength but did not influence the setting temperature of mixtures. The binding of denatured whey proteins to casein micelles affected gel network formation of milk-carrageenan mixtures containing 10% milk solids. Network formation in mixed gels containing carrageenan and milk or milk fractions was initiated by the carrageenan component and dictated primarily by the ionic content of the mixtures.     

牛羊乳蛋白组分比较研究

羊乳被国际营养学界誉为"奶中之王",逐渐被人们列为日常生活的营养保健佳品。该文对羊乳和牛乳的蛋白组分(主要是酪蛋白和乳清蛋白)含量、氨基酸组成及变异体等方面的差异进行了综述,并且对两者酪蛋白胶束的差异进行了比较,为羊乳检验和加工提供理论依据。     

Solubilized micellar calcium induced low methoxyl-pectin aggregation during milk acidification

Low methoxyl (LM) pectin was combined with 3-kDa molecular weight cut-off permeates from milk subjected to pH 6.7 to 5 and 7°C or 40°C with the objective of studying the effect of solubilized micellar calcium on viscoelastic properties of LM-pectin-milk mixes. Lowering the pH of skim milk with hydrochloric acid during ultrafiltration gradually promoted permeates to exhibit gel-like behavior when combined with LM-pectin. The onset of the gel-like behavior (G′ > 1) occurred at a higher pH when permeates were obtained from milk filtered at 7°C compared with 40°C. As pH value during ultrafiltration approached 5 and regardless of temperature, G′ for permeate-pectin mixes approached the same values (∼70 Pa) as G′ for skim milk-pectin mixes. In all cases G′ was highly correlated with free calcium concentration (r > 0.95). The gradual acidification of skim milk-LM-pectin using glucono-δ-lactone, promoted a sharp increase in storage modulus as pH approached 5.2 and a maximum G′ increment (ΔG′) at pH ∼4.9. From pH 4.9 to 4, G′ continued to increase but at smaller increments. It was concluded that LM-pectin-casein micelle interaction in milk is a 2-step process: 1) solubilized micellar calcium dependent pectin-pectin interaction as pH approaches 5.0 to 4.9, and 2) pectin-casein micelle interaction in the 5.0-4.9 to 4.0 pH range.     

Comparison of casein micelles in raw and reconstituted skim milk

During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spray-drying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.     

Separation of Egg White Lysozyme by Anionic Polysaccharides

Anionic polysaccharides (Na-alginate, Na-carboxyl-methyl-cellulose, high methoxy pectin and κ-carrageenan) were evaluated for their suitability towards simple and more efficient separation of lysozyme from salted or fresh duck eggs and hen egg whites for use in foods. Only κ-carrageenan interacted and formed precipitates with lysozyme. Recovery of lysozyme from fivefold diluted salted duck egg whites was 60–65% and 78–81% from fresh duck and hen egg whites using 0.7%κ-carrageenan. The recovered lysozyme from the salted duck egg whites was stable during storage at 4°C for 35 days.     

Heat Stability of Aggregated Particles of Casein Micelles and κ-Carrageenan

Abstract: Aggregated particles of casein micelles and κ-carrageenan were produced as a dried milk ingredient, then reconstituted and subjected to a heat treatment of 70 °C for 10 min. The reconstituted aggregates were found to be unstable when heated. Light scattering results showed that the aggregates dissociated partially into casein micelles. It was hypothesized that the removal of ions during ultrafiltration before spray-drying to produce the powdered ingredient significantly decreased stability upon reconstitution and heat treatment. When ions, either from whey permeate or calcium addition, were added to reconstituted aggregates, stability was greatly enhanced and the aggregates remained intact when subjected to heat. The effect of heat treatment on aggregates freshly produced with skim milk powder and κ-carrageenan was also studied. These aggregates were found to be stable during heating due to the unchanged ionic environment. Therefore, incorporation of powdered aggregates of casein micelles and κ-carrageenan into products would require the addition of whey permeate or calcium after reconstitution for stability during subsequent heating. Practical Application: Casein micelles and κ-carrageenan readily form aggregates when cooled together in solution with shearing. These aggregates have the potential to add functionality to skim milk powder; however, they have to be stable to subsequent applications. This research demonstrates that careful control of ionic balance is required to dry the aggregates, reconstitute them, and heat the reconstituted solution to pasteurization temperatures, but when controlled the intact aggregates can be maintained.     

Stability of casein micelles cross-linked by transglutaminase

In this study, caseins micelles were internally cross-linked using the enzyme transglutaminase (TGase). The integrity of the micelles was examined on solubilization of micellar calcium phosphate (MCP) or on disruption of hydrophobic interactions and breakage of hydrogen bonds. The level of monomeric caseins, determined electrophoretically, decreased with increasing time of incubation with TGase at 30°C; after incubation for 24 h, no monomeric β- or κ-caseins were detected, whereas only a small level of monomeric α_S1-casein remained, suggesting near complete intramicellar cross-linking. The ability of casein micelles to maintain structural integrity on disruption of hydrophobic interactions (using urea, sodium dodecyl sulfate, or heating in the presence of ethanol), solubilization of MCP (using the calcium-chelating agent trisodium citrate) or high-pressure treatment was estimated by measurement of the L*-value of milk; i.e., the amount of back-scattered light. The amount of light scattered by casein micelles in noncross-linked milk was reduced by >95% on complete disruption of hydrophobic interactions or complete solubilization of MCP; treatment of milk with TGase increased the stability of casein micelles against disruption by all methods studied and stability increased progressively with incubation time. After 24 h of cross-linking, reductions in the extent of light scattering were still apparent in the presence of high levels of dissociating agents, possibly through citrate-induced removal of MCP nanoclusters from the micelles, or urea- or sodium dodecyl sulfate-induced increases in solvent refractive index, which reduce the extent of light-scattering.     

Stabilization of milk proteins in acidic conditions by pectic polysaccharides extracted from soy flour

Pectic polysaccharides were extracted from soy flour at either room temperature (SPRT) or 121°C (SPH), and their abilities to stabilize milk proteins in acidic conditions were evaluated. Both SPRT and SPH were found to contain proteinaceous components that were difficult to dissociate from polysaccharide components using size exclusion chromatography, whereas the molar mass of the former was approximately twice that of the latter. Due to the higher molar mass, SPRT was expected to provide stronger steric effects to prevent aggregation between milk proteins in acidic conditions than SPH. Alkaline treatment of SPRT for breaking O-linkages between AA and monosaccharide residues decreased its molar mass by approximately 160 kDa, indicating that they contained naturally occurring conjugates of pectic and proteinaceous moieties. Particle size distributions in simulated acidified milk drink samples containing 0.2% SPRT or SPH showed monomodal distributions with median diameters of around 1.2 μm at pH 4. The presence of large protein aggregates (～5 μm) was detected at 0.2% SPRT and pH 3.2, 0.6 to 0.8% SPRT and pH 4, or 0.2% SPH and pH 3.4. The presence of excess polysaccharide molecules unbound to proteins was detected at 0.2% SPRT and pH 3.2 to 3.4, 0.4 to 0.8% SPRT and pH 4, 0.2% SPH and pH 3.4 to 3.6, and 0.4 to 0.8% SPH and pH 4. The present results suggest that molecular characteristics of pectic polysaccharides vary depending on extraction conditions and hence their functional behavior.     

Importance of casein micelle size and milk composition for milk gelation

The economic output of the dairy industry is to a great extent dependent on the processing of milk into other milk-based products such as cheese. The yield and quality of cheese are dependent on both the composition and technological properties of milk. The objective of this study was to evaluate the importance and effects of casein (CN) micelle size and milk composition on milk gelation characteristics in order to evaluate the possibilities for enhancing gelation properties through breeding. Milk was collected on 4 sampling occasions at the farm level in winter and summer from dairy cows with high genetic merit, classified as elite dairy cows, of the Swedish Red and Swedish Holstein breeds. Comparisons were made with milk from a Swedish Red herd, a Swedish Holstein herd, and a Swedish dairy processor. Properties of CN micelles, such as their native and rennet-induced CN micelle size and their ζ-potential, were analyzed by photon correlation spectroscopy, and rennet-induced gelation characteristics, including gel strength, gelation time, and frequency sweeps, were determined. Milk parameters of the protein, lipid, and carbohydrate profiles as well as minerals were used to obtain correlations with native CN micelle size and gelation characteristics. Milk pH and protein, CN, and lactose contents were found to affect milk gelation. Smaller native CN micelles were shown to form stronger gels when poorly coagulating milk was excluded from the correlation analysis. In addition, milk pH correlated positively, whereas Mg and K correlated negatively with native CN micellar size. The milk from the elite dairy cows was shown to have good gelation characteristics. Furthermore, genetic progress in relation to CN micelle size was found for these cows as a correlated response to selection for the Swedish breeding objective if optimizing for milk gelation characteristics. The results indicate that selection for smaller native CN micelles and lower milk pH through breeding would enhance gelation properties and may thus improve the initial step in the processing of cheese.     

Effects of Cation Properties on Sol-gel Transition and Gel Properties of κ-carrageenan

The phase transition temperatures, rheological properties and gel‐network characteristics for gelation of κ‐carrageenan‐salt (NaCl, KCl and CaCl₂) solutions and their aged gels were investigated. The effectiveness of increasing gelling and gel‐melting temperatures at the salt concentrations examined followed the sequence of K⁺ > Ca2⁺ > Na⁺. This sequence was also true for the gel strength and the melting enthalpy (DH) of the most crosslinked junction zones of aged gels at low salt concentrations. Nonetheless, a different order (Ca⁺⁺ > K⁺ and Na⁺) was found for increasing storage modulus and gelation rate during early‐stage gelation, thermal hysteresis and the DH of aged gels in some salt‐carrageenan systems.     

Effect of high pressure treatments combined with various levels of κ-carrageenan on cold-set binding in restructured pork

This study investigated the effects of high pressure processing and κ-carrageenan on cold-set binding in restructured pork meat. High pressure (>200 MPa) combined with carrageenan level higher than 1.5% caused increased breaking force and tensile strength ( P  < 0.05). Increasing the pressure level caused a continuous κ-carrageenan network in the junctions between meat particles, as confirmed by scanning electron microscopy imaging. With regard to the water binding properties, all treatments showed moisture content ranging between 74% and 76%. Increases in pressure levels and holding times tended to improve the water-holding capacity (WHC) of restructured pork; however, the κ-carrageenan concentration showed no relation with WHC. Cooking loss decreased significantly ( P  < 0.05) with increases in both pressure level and κ-carrageenan concentration. By increasing pressure levels, both the L ^*- and a ^*-values increased, but the b ^*-value showed no significant difference ( P  > 0.05) among the treatments, with the exception of 100 MPa for 30 min. The addition of κ-carrageenan had no effect on colour ( P  > 0.05). Therefore, the results indicate that pressure above 200 MPa and addition of 1.5% carrageenan has potential use in cold-set meat restructuring.     

乳蛋白胶粒的稳定性研究及应用

根据蛋白质一级结构原理,选用了一种碱性蛋白质—鱼精蛋白,研究了牛乳蛋白胶粒在适口酸性条件下的稳定性问题。确定出乳蛋白饮料的最佳工艺条件:牛奶含量(v/v)35%,白砂糖12.0%(m/v),柠檬酸含量0.2%(m/v),调酸温度在20℃之内,去离子水定容;以及鱼精蛋白的添加顺序和含量:在调酸前添加稳定效果最佳,含量(m/v)在0.3%即可达到稳定效果。     

Rheological properties of concentrated skim milk: importance of soluble minerals in the changes in viscosity during storage

Properties of condensed milks prior to spray drying dictate to a large extent the functionality of the resulting milk powder. Rheological properties of concentrated skim milk, with total solids content of 45% but different mineral content, were studied as a function of shear rate and storage time at 50 degrees C. These milks are proposed as a model to study the effects of minerals on rheology and age gellation of condensed milk prior to drying. During storage of the concentrated milk, the apparent viscosity, particularly after 4 h, increased markedly at all shear rates studied. The yield stress also increased steeply after 4 h of storage at 50 degrees C. The changes in apparent viscosity of concentrated milk stored for up to 4 h were largely reversible under high shear, but irreversible in samples stored for longer time. The appearance of yield stress suggested the presence of reversible flocculation arising from weak attraction between casein micelles, with a transition from reversible to irreversible aggregation during storage. Particle size analysis confirmed irreversible aggregation and fusion of casein micelles during storage. Gradual reduction of mineral content of concentrated milks resulted in a marked decrease in the apparent viscosity and casein micelle aggregation during storage, while addition of minerals to milk had the opposite effect. The results demonstrated that the soluble mineral content is very important in controlling the storage-induced changes in the rheology of concentrated milks.     

Calcium release from milk concentrated by ultrafiltration and diafiltration

The present work studied the solubilization of Ca during acidification in milk concentrated by ultrafiltration (UF) and diafiltration (DF). The effect of heating milk at 80°C for 15 min was also evaluated. In addition to measuring buffering capacity, the amount of Ca released as a function of pH was determined. The area of the maximum peak in buffering capacity observed at pH ~5.1, related to the presence of colloidal Ca phosphate, was significantly affected by casein volume fraction but did not increase proportionally with casein concentration. In addition, a lower buffering capacity and less solubilized Ca were measured in 2× DF milk compared with 2× UF milk. Heat treatment did not change the buffering capacity or Ca release in 1× and 2× concentrated milk. On the other hand, at a higher volume fraction (4×), more Ca was present in the soluble phase in heated 4× UF and DF milk compared with unheated milk. This is the first comprehensive study on the effect of concentration, distinguishing the effect of UF from that of DF, before and after heating, on Ca solubilization.     

牛羊乳酪蛋白制备及电泳分析比较

采采用新鲜和复原的牛羊乳为原料,等电点沉淀,通过洗涤、干燥等步骤分别制得牛乳和羊乳酪蛋白,用凯氏定氮法对其进行定量检测,应用十二烷基硫酸钠聚丙烯酰胺凝胶电泳（SDS—PAGE）分析比较牛羊乳酪蛋白组分差异。结果表明：牛乳酪蛋白得率为86．75％,羊乳酪蛋白得率为90．55％,高含量的酪蛋白主要集中在电泳图谱的中分子量组,可分为ds,CN、as2-CN、β-CN和K—CN,牛羊乳as2-CN分子量羊乳大于牛乳,牛乳a-CN含量比较多,羊乳β—CN含量比较多,鲜乳与复原乳全蛋白主要组分在电泳图谱中除酪蛋白差别外,上端的高分子量组清蛋白区羊乳IgG重链的分子量比牛乳小。应用蛋白质电泳分析技术可以区分羊乳和牛乳的蛋白质组分,等电点沉淀法制备酪蛋白的方法简单,易于操作。