Characteristics of edible films made from dairy proteins and zein hydrolysate cross-linked with transglutaminase

The main objectives of this research were to develop whey protein or casein films incorporating zein hydrolysate and also cross‐linked by transglutaminase as to well as characterize the physical and mechanical properties of the film. Zein hydrolysate decreased the solubility of the whey protein film (P < 0.05), while treatment with transglutaminase did not change the solubility of the film significantly. Electrophoresis patterns demonstrated that casein molecules were cross‐linked by transglutaminase and the extent of this cross‐linkage was further increased when zein hydrolysate was added. In addition, the use of zein hydrolysate decreased the tensile strength of the whey protein film by 35–45%. The elongation of the casein film was increased by 41% because of the action of transglutaminase and zein hydrolysate (P < 0.05). The water vapour permeability of the films was not significantly different. As the addition of zein hydrolysate and treatment with transglutaminase improved the flexibility of the films, the level of plasticizer required to maintain film flexibility could be reduced without sacrificing their water vapour permeability.     

Selenium content of goat milk and its distribution in protein fractions.

This study reports on selenium distribution in goat milk. Skim milk was found to contain the major part (94%) of total milk selenium. The selenium distribution over casein and whey protein fractions depends on the separation method used, but irrespective of these methods, skim milk selenium is mainly associated with the casein fraction (greater than 69%). Approximately 9%, 7% and 24% of selenium is removed by dialysis (molecular cutoff 10-12 kDa) from skim milk, casein and whey respectively, indicating a major association of selenium with milk proteins. This observation is confirmed by selenium analysis of individual caseins and whey proteins isolated through ion-exchange chromatography and gel filtration. Selenium concentrations of the different isolated milk proteins show considerable variation (caseins: 294-550 ng Se/g; whey proteins: 217-457 ng Se/g).     

Transglutaminase Inhibitor from Milk

ABSTRACT Cross-linking experiments of skimmed bovine milk with bacterial transglutaminase isolated from Streptoverticillium mobaraense showed only some degree of formation of high-molecular-weight casein polymers. Studies on the nature of this phenomenon revealed that bovine milk contains an inhibitor of transglutaminase activity. Removal of the casein and whey proteins from the milk resulted in a protein-poor fraction that still inhibited transglutaminase activity at cross-linking of β-casein and in several activity assays of transglutaminase. The inhibitor was partially purified by column chromatography and appeared to be a heat labile low molecular weight component. Inhibition of transglutaminase activity was observed with microbial transglutaminase, plasma transglutaminase and guinea pig liver transglutaminase. The inhibiting activity was found in bovine, goat, sheep, and human milk, but could not be detected in horse milk.     

Cross‐linking of bovine and caprine caseins by microbial transglutaminase and their use as microencapsulating agents for n‐3 fatty acids

Bovine and caprine caseins were cross‐linked with microbial transglutaminase (mTG). The mTG‐cross‐linked bovine or caprine casein dispersion, mixed with 14.5% maltodextrin (DE = 40), was used to prepare emulsions with 10.5% algae oil. Oxidative stability of emulsions was evaluated by peroxide values (PVs) and anisidine values. Adding liposoluble rosemary extract rich in carnosic acid and δ‐tocopherol lowered the formation of hydroperoxides and their subsequent decomposition products in emulsions. Emulsions stabilised with liposoluble rosemary extract rich in carnosic acid and δ‐tocopherol were spray‐dried at 180/95 °C. Algae oil microencapsulated with mTG‐cross‐linked bovine casein reduced PV by ≈ 34%, while the algae oil microencapsulated with mTG‐cross‐linked caprine casein with low levels of α_s1‐casein reduced PV by ≈ 42% at 4 weeks of storage at 30 °C. The investigation suggests that liposoluble rosemary extract rich in carnosic acid and δ‐tocopherol effectively protected algae oil during the coating process with mTG‐cross‐linked bovine and caprine caseins. The above results clearly indicated that the choice of milk caseins (bovine vs. caprine) cross‐linked with mTG impacts the oxidative stability of spray‐dried algae oil emulsions (microcapsules) enriched with n‐3 fatty acids.     

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 transglutaminase on the heat stability of milk: a possible mechanism

Treatment of milk with transglutaminase (TGase) affects its heat stability, but the manner in which it does so depends on whether or not the milk had been preheated before incubation and on the temperature of preheating. In raw milk, it appears that cross-link formation between the individual caseins is responsible for preventing the dissociation of kappa-casein from the micelles at pH values in the region of minimum stability. In milks preheated before incubation with TGase, denaturation of whey protein may have allowed the formation of cross-links by TGase between denatured whey proteins and the individual caseins which, in combination with cross-linking of the caseins, contributed to greatly improved heat stability at pH > 6.5. It appears from the results of this study that TGase has potential commercial applications as a food-grade additive capable of improving the heat stability of milk.     

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。     

Effect of high pressure - low temperature processing on composition and colloidal stability of casein micelles and whey proteins

The aim of this study was to identify the impact of high pressure treatments at sub-zero temperatures (high pressure - low temperature; HPLT) on milk proteins. Whey protein solutions, micellar casein dispersions and two mixtures (micellar caseins:whey proteins, 80:20 and 20:80, w/w) were pressure treated (100–600 MPa) at pH 7.0 or 5.8 at −15 °C, −35 °C and ambient temperature. Solubility data showed that whey proteins could only be affected by HPLT treatments at pH 7.0 if caseins were present, while effects could be induced at pH 5.8 without the presence of caseins. The caseins formed on the one hand large aggregates (flocs) and on the other hand the solubility was increased by the creation of smaller micelles. The formation of flocs could only be observed for HPLT treated samples, which indicates the formation of different protein interactions in milk protein based samples compared with common HP treatments.     

Composition and calcium status of acid whey from pasteurized,UHT‐treated and in‐bottle sterilized milks

Whey extracts were obtained from pasteurized, UHT‐treated and in‐bottle sterilized milks. After acidic precipitation of casein the concentration of protein, NPN, lactose, lipid, calcium, magnesium and potassium was determined. Among the parameters examined, protein content was significantly reduced in the whey extracts from UHT‐treated and in‐bottle sterilized milks compared with that from pasteurized milk, while lactose content was increased. Calcium extracted in whey was at least 80% of total calcium of the milk. The total calcium to protein ratio of whey was increased as a function of the thermal treatment of milk, while ionic calcium was about 50% of total calcium in all whey extracts. In vitro protein digestibility was found to be significantly lower in whey from UHT‐treated and in‐bottle sterilized milks than in that from pasteurized milk. Parallel estimation of the percentage of ionic calcium and of the solubility of proteins in the pH range 2–10 indicated that calcium was not involved in the pH‐dependent solubility of proteins extracted in the whey, the extent of solubility being essentially a function of the thermal treatment of milk. The results suggest that calcium was not responsible for the formation of soluble protein macroaggregates with impaired digestibility that are present in whey from milk subjected to heat treatment of increasing intensity.     

Association of Triclosan to Casein Proteins Through Solvent-Mediated High-Pressure Homogenization

ABSTRACT:  The association of triclosan (TCS), a widely used hydrophobic compound, to the bovine casein micelle is investigated in this study. The use of high-pressure homogenization (HPH) at 0, 100, 200, and 300 MPa was introduced as a method for the dissociation of casein micelles in a skim milk/ethanol solution (1: 1, v/v) in the presence of TCS at 20, 80, and 160 mg/L where ethanol evaporation served as the final step for TCS association to caseins. The majority of TCS (over 80%) was associated with the caseins regardless of initial TCS concentration or applied pressure. TCS association to caseins was enhanced by 30% with continued pressurization to 300 MPa. Micellar dissociation and reassociation was found to be an irreversible process as evidenced by microscopy images. Pressurization to 300 MPa resulted in the formation of an integrated protein network of casein proteins and noncovalently linked whey proteins where the solubility of TCS was enhanced up to 40 times its reported water solubility at the highest initial TCS level of 160 mg/L. Reformed micelles exhibited Newtonian flow behavior at all pressure levels. This study provides evidence for the solubility enhancing quality of TCS through the solvent-mediated pressure/shear-induced dissociation of casein proteins.     

Application of immunological methods for the detection of species adulteration in dairy products

A number of enzyme‐linked immunosorbent assays (ELISAs) have been developed for the detection of milk adulteration in dairy products. Target antigens have been caseins, lactoglobulins, immunoglobulins and other whey proteins. Polyclonal and monoclonal antibodies have been used in a variety of formats including direct, indirect, competitive and sandwich ELISAs. ELISAs have been successfully applied to the detection of cows' milk adulteration of sheep, goat and buffalo milk. Goat milk adulteration of sheep milk has also been detected. A number of ELISAs have also been applied to cheese. It is recommended that ELISA should be used in combination with PCR to ensure compliance with current legislation.     

Kinetic description of heat‐induced cross‐linking reactions of whey protein‐free casein solutions

Heat treatment of milk products induces several physico‐chemical changes that can compromise rennetability and milk protein functionality. The aim of this study was to investigate the kinetic parameters of heat‐induced polymerisation of whey protein‐free casein micelles in the native environment of skim milk, which were produced by microfiltration and ultrafiltration in a diafiltration mode. The intra‐ or intermolecular polymerisation of casein monomers by covalent cross‐linking of amino acids was analysed by gel‐permeation chromatography. The degree of polymerisation and the amounts of polymerisation products such as lysinoalanine and histidinoalanine leading to cross‐linking of proteins could be described as zero‐order reaction.     

Addition of crude tiger nut protein extract affects stiffness of enzymatically cross‐linked dairy proteins

The influence of crude tiger nut protein extract on the gel properties of enzymatically cross‐linked dairy proteins was investigated. Enzymatic cross‐linking of dairy proteins in the presence of crude tiger nut proteins caused the formation of larger casein polymers and increased the degree of polymerisation. Gel stiffness of acidified products containing whey proteins was higher when cross‐linking occurred in the presence of crude tiger nut proteins. The results are relevant for improving the textural characteristics of acidified aqueous tiger nut extract (tiger nut milk) enriched in dairy proteins.     

Formation and potential uses of milk proteins as nano delivery vehicles for nutraceuticals: A review

The aim of this review was to highlight the progress achieved in the use of milk protein as nano vehicles for nutraceuticals. Reassembled casein and β‐casein micelles and core/shell nanoparticles from casein with other biopolymers have been prepared. Also, cross‐linking of casein micelles has developed stable nanoparticles. Nanogels of whey proteins (WP), β‐lactoglobulin (β‐LG) and lactoferrin (Lf) have been prepared by controlled thermal treatment, and several core/shell nanoparticles have been developed from WP or β‐LG with several polysaccharides. The developed caseins and WP nanoparticles have been used as carriers for several nutraceuticals. Examples have been presented and discussed.     

LTQ-Orbitrap 液- 质联用技术对水牛奶酪蛋白的鉴定

建立一种快速、高效的分析酪蛋白组分氨基酸序列的LTQ-Orbitrap质谱方法。为了研究水牛奶酪蛋白、乳牛奶酪蛋白和山羊奶酪蛋白的氨基酸序列组成及氨基酸的替换现象,采用LTQ-Orbitrap液-质联用技术分别对乳源酪蛋白的4种主要组分进行分析,搜索数据库获得4种组分的氨基酸全序列。与水牛奶4种酪蛋白组分进行比对。结果表明,乳牛奶的4种酪蛋白发生氨基酸替换的部位和比率明显小于山羊奶。这意味着与水牛奶酪蛋白相比,乳牛奶酪蛋白氨基酸的稳定性优于山羊奶酪蛋白。     

Commercial Milk Enzyme‐Linked Immunosorbent Assay (ELISA) Kit Reactivities to Purified Milk Proteins and Milk‐Derived Ingredients

Numerous commercial enzyme‐linked immunosorbent assay (ELISA) kits exist to quantitatively detect bovine milk residues in foods. Milk contains many proteins that can serve as ELISA targets including caseins (α‐, β‐, or κ‐casein) and whey proteins (α‐lactalbumin or β‐lactoglobulin). Nine commercially‐available milk ELISA kits were selected to compare the specificity and sensitivity with 5 purified milk proteins and 3 milk‐derived ingredients. All of the milk kits were capable of quantifying nonfat dry milk (NFDM), but did not necessarily detect all individual protein fractions. While milk‐derived ingredients were detected by the kits, their quantitation may be inaccurate due to the use of different calibrators, reference materials, and antibodies in kit development. The establishment of a standard reference material for the calibration of milk ELISA kits is increasingly important. The appropriate selection and understanding of milk ELISA kits for food analysis is critical to accurate quantification of milk residues and informed risk management decisions.     

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.     

Aggregation and Dissociation of Milk Protein Complexes in Heated Reconstituted Concentrated Skim Milks

Heating milk at 120°C at pH 6.55 or pH 6.85 caused the denaturation of whey proteins and increased their association with the casein micelles. The dissociation of K -, β-, and α_s-caseins (in that order by extent) from the casein micelles increased with severity of heat treatment. The effect was greater at higher pH. Gel filtration chromatography followed by gel electrophoresis of fractions showed the dissociated protein was composed of disulfide-linked k -casein/β-lactoglobulin complexes of varying composition, casein aggregates of varying sizes and some monomeric protein. When reconstituted concentrate was prepared from NFDM made from heated milk the non-sedimentable (88,000 ± g for 90 min) caseins or whey proteins/heating time profiles were altered and the rate of aggregation, as measured by turbidity of heated milks, was significantly reduced.     

The effect of formulated goats' milk on calcium bioavailability in male growing rats

BACKGROUND: There are two main proteins in milk; whey and casein. Casein contains casein phosphopeptides (CPP), which are released on digestion of the milk. These may increase calcium solubility by binding calcium in the small intestine. Thus increasing casein in the diet may help to stimulate bioavailability of calcium and increase bone density. The present study tested this hypothesis in growing male rats fed diets containing three different concentrations of casein from goat milk. RESULTS: Rats fed the diet containing no casein had significantly lower calcium absorption when compared to rats fed the diets that contained 80% and 57% of goat milk protein as casein; however, no significant difference was observed between rats fed diets with 80% and 57% casein. The varying amounts of casein had no effect on mineral uptake or retention in the femur. Biomechanical testing and mineral analysis of the femurs showed no differences between diet groups. The mechanism to explain this lack of retention remains unclear. CONCLUSION: The diets containing 80% and 57% of goat milk protein as casein delivered increased calcium absorption compared to the diet containing no casein, suggesting a minimum level of casein is needed to optimize calcium absorption from goat milk. Copyright © 2009 Society of Chemical Industry     

Chemical changes in bovine milk fat globule membrane caused by heat treatment and homogenization of whole milk

The effects of heat treatment and homogenization of whole milk on chemical changes in the milk fat globule membrane (MFGM) were investigated. Heating at 80 degrees C for 3-18 min caused an incorporation of whey proteins, especially beta-lactoglobulin (beta-Ig), into MFGM, thus increasing the protein content of the membrane and decreasing the lipid. SDS-PAGE showed that membrane glycoproteins, such as PAS-6 and PAS-7, had disappeared or were weakly stained in the gel due to heating of the milk. Heating also decreased free sulphydryl (SH) groups in the MFGM and increased disulphide (SS) groups, suggesting that incorporation of beta-Ig might be due to association with membrane proteins via disulphide bonds. In contrast, homogenization caused an adsorption of caseins to the MFGM but no binding of whey proteins to the MFGM without heating. Binding of caseins and whey proteins and loss of membrane proteins were not significantly different between milk samples that were homogenized before and after heating. Viscosity of whole milk was increased when milk was treated with both homogenization and heating.