Milk-based Gels Made with κ-Carrageenan

ABSTRACT: The gel strength of κ‐carrageenan (0.1 to 0.4% w/w) ‐ reconstituted skim milk (2.5 to 20% w/w milk solids) mixtures was influenced by the concentrations of milk solids, κ‐carrageenan and cations. Particle size measurements showed that particle interactions in diluted skim milk‐κ‐carrageenan mixtures were dependent on the conformation of the κ‐carrageenan. Heat treatment of milk, resulting in alteration of the casein micelle, did not affect the interaction of κ‐carrageenan with casein in dilute solutions or the gel strength of milk‐κ‐carrageenan mixtures. κ‐Carrageenan must be available in the random coil form in solution, prior to cooling to its coil to helix transition temperature, for effective gelation of milk‐κ‐carrageenan mixtures.     

Small and large deformation rheology and microstructure of κ-carrageenan gels containing commercial milk protein products

The rheological behaviour of commercial milk protein/κ-carrageenan mixtures in aqueous solutions was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate, and whey protein isolate were considered. As seen by confocal laser microscopy, mixtures of κ-carrageenan with skim milk powder, milk protein concentrate, and sodium caseinate showed phase separation, but no phase separation was observed in mixtures containing whey protein isolate. For κ-carrageenan concentrations up to 0.5 wt%, the viscosity of the mixtures at low shear rates increased markedly in the case of skim milk powder and milk protein concentrate addition, but did not change by the addition of sodium caseinate or whey protein isolate. For κ-carrageenan concentrations from 1 to 2.5 wt%, small and large deformation rheological measurements, performed on the milk protein/κ-carrageenan gels, showed that skim milk powder, milk protein concentrate or sodium caseinate markedly improved the strength of the resulting gels, but whey protein isolate had no effect on the gel stength.     

Sodium Caseinate and Skim Milk Gels Formed by Incubation with Microbial Transglutaminase

Several suspensions and emulsions containing commercial sodium caseinate or skim milk were gelatinized by Ca²⁺-independent microbial transglutaminase treatment. The characteristics of the gels were largely affected by the enzyme concentrations employed. For caseinate gels generally the higher enzyme concentration gave steep decreases in breaking strength, strain and cohesiveness of the gels. The creep tests on emulsified gels prepared to two different enzyme concentrations showed that the gel made with a higher enzyme concentration was the more viscoelastic. For skim milk gels, the enzyme treatment in higher concentration caused substantial increase of the breaking and hardness while the strain and cohesiveness had little or no changes.     

MILK GEL STRUCTURE. XI. ELECTRON MICROSCOPY OF GLUCONO-δ-LACTONE-INDUCED SKIM MILK GELS1

Gels were prepared from fresh and reconstituted skim milk (10–30% total solids) using glucono-δ-lactone as the acid precursor within the pH range of 4 to 6. Firmness of glucono-δ-lactone-induced skim milk gels increased with (a) decrease in pH of skim milk having the same total solids content, (b) increase in total solids at a given pH, (c) pre-heating skim milk to 90°C, and (d) adding glucono-δ-lactone at a higher temperature. Electron microscopy revealed that in condition (a) larger sizes of casein micelles (particles) were associated with a higher firmness whereas in conditions (b), (c), and (d) the increased firmness was correlated with decreased micelle sizes. The relation between rheological behavior and microstructure of glucono-δ-lactone-induced skim milk gels was more dependent upon the micellar arrangement than upon the size and shape of the casein micelles. Conditions which led to large casein micelle clusters and chains in conditions (b), (c), and (d) resulted both in severe syneresis and in weaker gels. An unusual phenomenon was observed in skim milk gels made at pH 5.5, particularly at the lower total solids content (10%): the casein particles consisted of a solid core surrounded by an outer lining, 0.03–0.05 μm thick, which resembled a membrane-line structure; there was a free annular space of 0.05 to 0.08 μm between the lining and the core. The existence of this structure was confirmed by thin-sectioning and freeze-fracturing techniques.     

Microstructure of acid–induced skim milk–locust bean gum–xanthan gels

The microstructure of acid skim milk gels (14% w/w milk protein low heat powder) with or without addition of locust bean gum (LBG), xanthan gum (XG) and LBG/XG blends was determined by transmission electron microscopy (TEM), phase-contrast light microscopy (PCLM) and scanning electron microscopy (SEM). Three polysaccharide concentrations (0.001%, 0.02% and 0.1%, w/w) were used for binary mixtures. In the case of ternary mixtures, three LBG/XG weight ratios were used (4/16, 11/9 and 16/4) at 0.02% total polysaccharide concentration. Control acid skim milk gels were structured by a homogeneous network of casein particles (0.1–0.7 μm in diameter) and clusters immobilizing whey in small pores (1–5 μm in diameter). Filamentous structures and small aggregates were observed at the surface of casein particles. Low concentration of LBG or XG (0.001% w/w) did not affect markedly the microstructure of acid skim milk gels. Conversely, LBG or XG at 0.02 or 0.1% concentration and LBG/XG blends at the three ratios selected had a great influence on the gel microstructure. Although the size and surface structure of the casein particles were not modified by the presence of polysaccharides, the primary casein network appeared very compact with a decrease of pore size and a large increase in the porosity of the network at the supramolecular level (sponge-like morphology). The effect is stronger for gels containing LBG and XG used at higher concentration and less apparent for gels containing LBG/XG blends. Skim milk/XG gels were highly organized into fibrous structures whereas skim milk/LBG gels were more heterogeneous. These structures were discussed in the light of volume-exclusion effects (demixing) and specific interactions between casein micelles and polysaccharides. At the three weight ratios, skim milk/LBG/XG gels displayed both jagged “coral-like”, “veil-like” and filamentous structures. These structures could originate from a secondary network constituted by the known LBG/XG synergistic interactions.     

Dynamic rheology of renneted milk gels containing fat globules stabilized with different surfactants

Anhydrous milk fat was emulsified with alpha s1-CN (casein), alpha s2-CN, beta-CN, kappa-CN, alpha-lactalbumin, beta-lactoglobulin, Tween 80, or phosphatidylcholine to produce a 30% fat cream in a 0.1 M imidazole pH 7 buffer. The creams were mixed with skim milk to yield a fat content of 3.4% and the viscoelastic properties of the recombined milks clotted with chymosin were measured. Recombined milk containing globules coated with the more amphipathic and phosphorylated alpha s2-CN and beta-CN clotted faster but gel firmness increased more slowly and weaker gels were formed. Gel firmness increased more rapidly for milks containing globules coated with of alpha s1-CN and kappa-CN that possess more uniformly distributed hydrophobic domains.     

Formulation,process conditions,and biological evaluation of dairy mixed gels containing fava bean and milk proteins: Effect on protein retention in growing young rats

Food formulation and process conditions can indirectly influence AA digestibility and bioavailability. Here we investigated the effects of formulation and process conditions used in the manufacture of novel blended dairy gels (called “mixed gels” here) containing fava bean (Vicia faba) globular proteins on both protein composition and metabolism when given to young rats. Three mixed dairy gels containing casein micelles and fava bean proteins were produced either by chemical acidification (A) with glucono-δ-lactone (GDL) or by lactic acid fermentation. Fermented gels containing casein and fava bean proteins were produced without (F) or with (FW) whey proteins. The AA composition of mixed gels was evaluated. The electrophoretic patterns of mixed protein gels analyzed by densitometry evidenced heat denaturation and aggregation via disulfide bonds of fava bean 11S legumin that could aggregate upon heating of the mixtures before gelation. Moreover, fermented gels showed no particular protein proteolysis compared with gel obtained by GDL-induced acidification. Kinetics of acidification were also evaluated. The pH decreased rapidly during gelation of GDL-induced acid gel compared with fermented gel. Freeze-dried F, A, and FW mixed gels were then fed to 30 young (1 mo old) male Wistar rats for 21 d (n = 10/diet). Fermented mixed gels significantly increased protein efficiency ratio (+58%) and lean mass (+26%), particularly muscle mass (+9%), and muscle protein content (+15%) compared with GDL-induced acid gel. Furthermore, F and FW formulas led to significantly higher apparent digestibility and true digestibility (+7%) than A formula. Blending fava bean, casein, and whey proteins in the fermented gel FW resulted in 10% higher leucine content and significantly higher protein retention in young rats (+7% and +28%) than the F and A mixed gels, respectively. Based on protein gain in young rats, the fermented fava bean, casein, and whey mixed proteins gel was the most promising candidate for further development of mixed protein gels with enhanced nutritional benefits.     

Evaluation of Alternative Methods to Increase Calcium Retention in Cottage Cheese Curd

The effect of several alternative methods including addition of rennet, addition of carrageenan and use of 2:1 (v/v) preconcentrated skim milk by ultrafiltration (UF) upon calcium retention, yield, composition and sensory properties of dry curd cottage cheese was investigated. Although each of the processing methods resulted in the manufacture of dry curd cottage cheese with different compositions and properties, none of them was satisfactory for increasing calcium retention. Added carrageenan bound additional whey proteins, added rennet interfered with curd syneresis and whey expulsion during cooking and use of UF preconcentrated skim milk resulted in an increase in yield, total solids and protein of the curd.     

Phase behaviour,rheology and microstructure of mixture of meat proteins and kappa and iota carrageenans

The phase behaviour of mixtures of salt soluble meat proteins, kappa (κ) and iota (ι) carrageenan in non-gelling conditions (45 °C) were determined at pH 5.6, 6.2 and 7.1. The concentration of meat proteins ranged from 0.1 to 1.0 percent and that of κ-carrageenan and ι-carrageenan from 0.02 to 0.3 percent in the mixtures. Mixtures separated under gravity to form soluble/liquid and gelled/complex phases. For meat proteins- κ-carrageenan mixtures, phase separations at all meat protein/carrageenan ratios were observed. For meat protein-ι-carrageenan mixtures, soluble complexes were formed at low meat protein to ι-carrageenan ratios and gels at higher ratios. The yield of the complex/gels increased with the increase in the concentration of the meat proteins and carrageenans and decreased with increase in the pH of the initial mix. The complex/gels formed became stronger with the increase in carrageenan in the mix and with κ-carrageenan compared to ι-carrageenan. Chemical analyses and scanning electron and phase contrast microscopy indicated that in phase separated mixtures, the bulk of the meat proteins and carrageenan were found in the gel compared to the liquid phase; and that meat protein interacted with carrageenan in the gel and formed soluble complexes with carrageenan in the liquid phase. SDS-PAGE showed that the meat proteins that interacted to form the complex/gels with carrageenan included myosin heavy chain, α-actinin, actin, myosin light chains and proteins with molecular weights around 150 and 50 kD. The outcomes of the present study could be used in the formulation of multi-component foods with a range of consistencies containing meat proteins.     

Retention of β-galactosidase activity in crude cellular extracts from Lactobacillus delbrueckii ssp. bulgaricus 11842 upon drying

Crude cellular extracts (CCEs) containing active β-galactosidase from Lactobacillus delbrueckii ssp. bulgaricus 11842 were spray-dried at three different outlet air temperatures (45, 55 or 65°C) or freeze-dried, with or without whey proteins, casein, whey or skim milk as drying adjuncts. The use of whey or skim milk resulted in significantly ( P   <  0.05) higher β-galactosidase activity retention in comparison to all other CCEs. This effect was not related to the initial total solids (TS) content (4–10%) of the feedstock solutions, but was presumably caused by the presence of lactose in the whey or skim milk CCE preparations.     

Gel-Forming Characteristics of Milk Proteins, 2. Roles of Sulfhydryl Groups and Disulfide Bonds

The gel-forming characteristics of milk proteins were investigated by employing skim milk either nonpreheated or preheated at 80°C, and coagulating them at 70 or 80°C with glucono-delta-lactone. The solubility of each gel in the phosphate buffer (pH 7.0) containing urea and 2- mercaptoethanol was examined. The disulfide bonds played a more important role in the gel coagulated at 80°C from skim milk preheated at 80°C than in the gel coagulated at 70°C from nonpreheated skim milk.The effect of the reduction treatment with 2-mercaptoethanol was more pronounced on preheated skim milk than on nonpreheated skim milk. Sulfhydryl groups and disulfide bonds, which were buried in the molecules of whey proteins in their native state, were rendered accessible following heat treatment at 80°C.The gel prepared from skim milk pretreated with oxidizing agent, hydrogen peroxide (i.e., skim milk has no accessible sulfhydryl groups as a result), or the gel prepared from skim milk pretreated with reducing agent, 2-mercaptoethanol, (i.e., skim milk has few disulfide bonds as a result), displayed weak gel formation. But the gel prepared from the mixture of these skim milks with appropriate ratio displayed higher gel firmness. These findings suggest that intermolecular disulfide bonds are formed by the exchange reaction between sulfhydryl groups and disulfide bonds during gel formation.     

Amylose–Potassium Oleate Inclusion Complex in Plain Set‐Style Yogurt

Health and wellness aspirations of U.S. consumers continue to drive the demand for lower fat from inherently beneficial foods such as yogurt. Removing fat from yogurt negatively affects the gel strength, texture, syneresis, and storage of yogurt. Amylose–potassium oleate inclusion complexes (AIC) were used to replace skim milk solids to improve the quality of nonfat yogurt. The effect of AIC on fermentation of yogurt mix and strength of yogurt gel was studied and compared to full‐fat samples. Texture, storage modulus, and syneresis of yogurt were observed over 4 weeks of storage at 4 °C. Yogurt mixes having the skim milk solids partially replaced by AIC fermented at a similar rate as yogurt samples with no milk solids replaced and full‐fat milk. Initial viscosity was higher for yogurt mixes with AIC. The presence of 3% AIC strengthened the yogurt gel as indicated by texture and rheology measurements. Yogurt samples with 3% AIC maintained the gel strength during storage and resulted in low syneresis after storage for 4 wk.     

A novel technique for differentiation of proteins in the development of acid gel structure from control and heat treated milk using confocal scanning laser microscopy

The incorporation of caseins and whey proteins into acid gels produced from unheated and heat treated skimmed milk was studied by confocal scanning laser microscopy (CSLM) using fluorescent labelled proteins. Bovine casein micelles were labelled using Alexa Fluor 594, while whey proteins were labelled using Alexa Fluor 488. Samples of the labelled protein solutions were introduced into aliquots of pasteurised skim milk, and skim milk heated to 90 degrees C for 2 min and 95 degrees C for 8 min. The milk was acidified at 40 degrees C to a final pH of 4.4 using 20 g glucono-delta-lactone/l (GDL). The formation of gels was observed with CSLM at two wavelengths (488 nm and 594 nm), and also by visual and rheological methods. In the control milk, as pH decreased distinct casein aggregates appeared, and as further pH reduction occurred, the whey proteins could be seen to coat the casein aggregates. With the heated milks, the gel structure was formed of continuous strands consisting of both casein and whey protein. The formation of the gel network was correlated with an increase in the elastic modulus for all three treatments, in relation to the severity of heat treatment. This model system allows the separate observation of the caseins and whey proteins, and the study of the interactions between the two protein fractions during the formation of the acid gel structure, on a real-time basis. The system could therefore be a valuable tool in the study of structure formation in yoghurt and other dairy protein systems.     

Effect of buttermilk made from creams with different heat treatment histories on properties of rennet gels and model cheeses

Although many studies have reported negative effects on cheese properties resulting from the use of buttermilk in cheese milk, the cause of these effects has not been determined. In this study, buttermilk was manufactured from raw cream and pasteurized cream, as well as from a cream derived from pasteurized whole milk. Skim milks with the same heat treatments were also manufactured to be used as controls. Compositional analysis of the buttermilks revealed a pH 4.6-insoluble protein content approximately 10% lower than that of the skim milk counterparts. Milk fat globule membrane (MFGM) proteins remained soluble at pH 4.6 in raw cream buttermilk; however, when heat was applied to cream or whole milk before butter making, MFGM proteins precipitated with the caseins. Rennet gel characterization showed that MFGM material in the buttermilks decreased the firmness and increased the set-to-cut time of rennet gels, but this effect was amplified when pasteurized cream buttermilk was added to cheese milk. The microstructure of gels was studied, and it was observed that gel appearance was very different when pasteurized cream buttermilk was used, as opposed to raw cream buttermilk. Model cheeses manufactured with buttermilks tended to have a higher moisture content than cheeses made with skim milks, explaining the higher yields obtained with buttermilk. Superior retention of MFGM particles was observed in model cheeses made from pasteurized cream buttermilk compared with raw cream buttermilk. The results from this study show that pasteurization of cream and of whole milk modifies the surface of MFGM particles, and this may explain why buttermilk has poor coagulation properties and therefore yields rennet gels with texture defects.     

Distribution and carbohydrate structures of high molecular weight glycoproteins, MUC1 and MUCX, in bovine milk

High molecular weight glycoproteins, MUC1 and MUCX, originating from bovine milk, were compared with regard to their distribution in milk fat and skim milk fractions and for presence of carbohydrate structures. Polymorphic MUC1, which migrated into 6% resolving SDS-PAGE gels, was found in both milk fat globule membrane and skim milk phases of bovine milk. In contrast, MUCX, appearing as a non-polymorphic single band in 3% polyacrylamide stacking gels, was present only in the skim milk fraction. Peptide-N-glycosidase F digestion studies indicated that MUC1 and MUCX possessed N-glycans with MUC1 containing more N-glycans than MUCX. Exoglycosidase digestion studies revealed the existence of abundant terminal sialic acid residues in both MUC1 and MUCX. Lectin-binding studies showed that MUCX likely possessed more complex carbohydrate structures than MUC1. The complex carbohydrate structures carried by both MUC1 and MUCX suggest that they may have potential to bind a wide spectrum of pathogenic microorganisms. If that proves to be the case in vivo, such structures could have a role in preventing or reducing some infectious diseases.     

不同添加剂对鸡肉盐溶蛋白质热诱导凝胶性质的影响

采用质构分析法、动态流变分析法、差示扫描微量热法研究大豆分离蛋白、花生蛋白、卡拉胶对鸡胸肉和鸡腿肉盐溶蛋白热诱导凝胶性质的影响。试验结果表明:不同添加剂可在不同程度上改善鸡胸肉和鸡腿肉盐溶蛋白热诱导凝胶特性,其中卡拉胶添加效果最好。由动态流变学分析可知,鸡胸肉和鸡腿肉盐溶蛋白形成凝胶存在不同作用机理。热稳定性分析表明,鸡肉混合蛋白热诱导凝胶形成主要与肌球蛋白有关,肌动蛋白作用效果不明显。不同添加剂可增强鸡胸肉和鸡腿肉盐溶蛋白的变性温度和变性热,其中大豆分离蛋白添加效果最显著。     

Effect of milk solids concentration on the gels formed by the acidification of heated pH-adjusted skim milk

Reconstituted skim milk of 10–25% total solids was adjusted to pH values between about 6.2 and 7.1 and heated at 80 °C for 30 min. Gels were formed from the heated milks by slow acidification to pH 4.2 and the gelation process and final gels were analyzed for their rheological properties. At each milk concentration, the final acid gel firmness (final G′) and breaking stress could be changed markedly by manipulation of the pH during heating. The final gel firmness and breaking stress could also be modified by changing the concentration of the milk solids prior to heating and acidification. The results indicated that similar gel firmness and breaking stress could be achieved over a range of milk concentrations by control of the pH of the milk during heating. When expressed as a percentage change in final G′ or breaking stress relative to that obtained at the natural pH, plots of the change in final G′ or breaking stress versus pH fell close to a single curve, indicating that the same mechanism may influence the gelation properties at all milk concentrations. The final G′ and breaking stress were related to the denaturation and interaction of the whey proteins with the casein micelles, and the formation of non-sedimentable casein when the milk was heated.     

The effect of the addition of thiol reagents to heated milk on protein interactions and acid gelation properties

Low levels of β-mercaptoethanol (β-ME) or N-ethylmaleimide (NEM) were added to preheated skim milk (SM) and preheated whey protein-enriched skim milk (WPE-SM). Addition of NEM did not affect the heat-induced interactions between the proteins in heated SM and WPE-SM; addition of β-ME reduced most disulphide bonds of κ-casein. Neither NEM nor β-ME affected the distribution of proteins between the colloidal and serum phases. The heated then treated SM and WPE-SM were acidified to form acid gels. Acid gels containing β-ME had higher yield stress and G′ values than those made from control heated milk. In contrast, adding NEM to heated SM and WPE-SM lowered the yield stress values of the acid gels, but affected the final G′ values only slightly. The rheological results suggested that thiol-disulphide exchange reactions occurred during acid gelation. However, the newly formed disulphide bonds influenced only the yield properties, not the G′ values of the gels.     

High Pressure Effects on Gelation of Surimi and Turkey Breast Muscle Enhanced by Microbial Transglutaminase

High pressure effects on the strength (stress) and elasticity/deformability (strain) of surimi and turkey breast meat gels containing microbial transglutaminase (TGase) were evaluated. Pressurization of muscle proteins at 4°C prior to incubation at 25°C or 40°C (setting) increased gel strength 2–3 fold in uncooked surimi gels, but not in uncooked turkey gels. However, pressurization at 40°C or 50°C prior to setting increased the strength of turkey gels. Similar effects of prior pressurization, but of lesser magnitude, occurred in gels formed by directly or subsequently (following setting) cooking at 90°C. SDS-PAGE confirmed that myosin crosslinking occurred due to TGase activity during the setting treatment, which had survived prior pressure treatment. High pressure rendered protein substrates more accessible to TGase thereby enhancing intermolecular cross-link formation and gel strength.     

Manufacture of nonfat yogurt from a high milk protein powder.

Nonfat yogurts were manufactured from skim milk fortified with a new high milk protein powder. The powder, containing approximately 84% milk protein, was added to skim milk to obtain 5.2 to 11.3% total protein, 11.1 to 15% total solids, and 1.6 to 7.9% lactose in the yogurt mix. Mixes were homogenized, pasteurized at 90 degrees C for 10 min, and fermented with a yogurt culture at 42 degrees C to pH 4.6. Controls were made from the same skim milk fortified with NDM to approximately 14% total solids. Yogurts made with the protein powder and containing 5.6% protein were similar in firmness to the control and had good flavor when fresh and after 2 wk of storage. Yogurts with more than 5.6% protein were too firm and had an astringent flavor. Acetaldehyde content of all yogurts was comparable with that of the control, and fat content ranged from .18 to .33%. As the protein content of yogurts increased, the porosity of yogurts, as seen by scanning electron microscopy, decreased. Good quality nonfat yogurts can be produced by supplementing skim milk with a high milk protein powder up to 5.6% protein. The added protein assists in providing a firm body and minimal whey separation without the use of stabilizers.