Nutritional and technological aspects of milk fat globule membrane material

Goat α(s1)-casein genotype affects milk fat globule physicochemical properties and the composition of the milk fat globule membrane

Milk fat secretion is a complex process that initiates in the endoplasmic reticulum of the mammary epithelial cell by the budding of lipid droplets. Lipid droplets are finally released as fat globules in milk enveloped by the apical plasma membrane of the mammary epithelial cell. The milk fat globule membrane (MFGM) thus comprises membrane-specific proteins and polar lipids (glycerophospholipids and sphingolipids) surrounding a core of neutral lipids (mainly triacylglycerols and cholesterol esters). We have recently described major proteins of the MFGM in the goat and we have highlighted prominent differences between goats and bovine species, especially regarding lactadherin, a major MFGM protein. Here, we show that, in the goat species, the well-documented genetic polymorphism at the α(s1)-casein (CSN1S1) locus affects both structure and composition of milk fat globules. We first evidenced that both milk fat globule size and ζ-potential are related to the α(s1)-casein genotype. At midlactation, goats displaying strong genotypes for α(s1)-casein (A/A goats) produce larger fat globules than goats with a null genotype at the CSN1S1 locus (O/O goats). A linear relationship (R(2)=0.75) between fat content (g/kg) in the milk and diameter of fat globules (μm) was established. Moreover, we found significant differences with regard to MFGM composition (including both polar lipids and MFGM proteins) from goats with extreme genotype at the CSN1S1 locus. At midlactation, the amount of polar lipids is significantly higher in the MFGM from goats with null genotypes for α(s1)-casein (O/O goats; 5.97±0.11mg/g of fat; mean ± standard deviation) than in the MFGM from goats with strong genotypes for α(s1)-casein (A/A goats; 3.96±0.12mg/g of fat; mean ± standard deviation). Two MFGM-associated proteins, namely lactadherin and stomatin, are also significantly upregulated in the MFGM from goats with null genotype for α(s1)-casein at early lactation. Our findings are discussed with regard to techno-functional properties and nutritional value of goat milk. In addition, the genetic polymorphism in the goat species appears to be a tool to provide clues to the lipid secretion pathways in the mammary epithelial cell.

Sweet buttermilk intake reduces colonisation and translocation ofListeria monocytogenesin rats by inhibiting mucosal pathogen adherence

The bovine milk fat globule membrane (MFGM) contains several antimicrobial components with proven efficacyin vitro, butin vivoevidence is scarce. The present study was performed to determine the efficacy of the bovine MFGMin vivo.Rats were fed diets based on bovine skimmed milk powder (low in MFGM) or bovine sweet buttermilk powder (high in MFGM). After dietary adaptation, rats were orally infected withSalmonella enteritidisorListeria monocytogenes.Whereas sweet buttermilk powder did not protect rats against infection withS. enteritidis, it protected againstL. monocytogenes, as shown by a lower colonisation and translocation of this pathogen. Protection coincided with higher listericidal capacity of gastric and caecal contents. The digestion products of phosphoglycerides and sphingomyelin are bactericidalin vitro.To study their role, rats were fed diets containing either 0·1 % phosphatidylcholine or sphingomyelin, or a control diet. After dietary adaptation, rats were infected withL. monocytogenes.SinceListeriacolonisation was not affected by these diets, phosphoglycerides and sphingomyelin are not involved in the protective effect of sweet buttermilk. Additionalin vitroexperiments were performed to further explore the mechanism of the beneficial effects of sweet buttermilk. Inhibition of the adherence ofL. monocytogenesto the intestinal mucosa is the most likely explanation, since sweet buttermilk powder inhibited the binding ofL. monocytogenesin both a haemagglutination assay and a Caco-2 cell adherence assay. In conclusion, sweet buttermilk powder, which is rich in MFGM, protects againstL. monocytogenesinfection in rats, probably by preventing adherence of this pathogen to the intestinal mucosa.

Food formats for effective delivery of probiotics

Probiotic bacteria are increasingly incorporated into food products intended to confer health benefits in the human gut and beyond. Little is known about how the food matrix and product formulation impacts probiotic functionality, even though such information is essential to scientific understanding and regulatory substantiation of health benefits. The food format has the potential to affect probiotic survival, physiology, and potentially efficacy, but few comparative studies in humans have been conducted. Human studies should account for the effects of the food base on human health and the bioactive components present in the foods that may augment or diminish interactions of the probiotic with the human host. Some studies show that food ingredients such as prebiotics and milk components can improve probiotic survival during the shelf life of foods, which may enhance probiotic efficacy through increased dose effects. Furthermore, there are indications that synbiotic products are more effective than either probiotics or prebiotics alone. Identification of probiotic adaptations to the food and gut environments holds promise for determining the specific cell components and potential bacterial-food interactions necessary for health benefits and determining how these factors are affected by changes in food formulation and host diet. These studies, combined with controlled human studies, are important future research activities for advancing this field.

Proteome profile and biological activity of caprine, bovine and human milk fat globules

Upon combining bidimensional electrophoresis with monodimensional separation, a more comprehensive analysis of the milk fat globule membrane has been obtained. The proteomic profile of caprine milk fat globules revealed the presence of butyrophilin, lactadherin and perilipin as the major proteins, they were also associated to bovine and human milk fat globule membranes. Xanthine dehydrogenase/oxidase has been detected only in monodimensional gels. Biological activity of milk fat globules has been evaluated in Caco2-cells, as a representative model of the intestinal barrier. The increase of cell viability was indicative of a potential nutraceutical role for the whole milk fat globule, suggesting a possible employment in milk formula preparation.

Improved solvent extraction procedure and high-performance liquid chromatography-evaporative light-scattering detector method for analysis of polar lipids from dairy materials

A normal-phase high-performance liquid chromatography-evaporative light-scattering detector method employing dichloromethane, methanol, and acetic acid/triethylamine buffer as the mobile phase was developed for analysis of polar lipids (PLs). This method was applicable for analysis of PLs from both dairy materials and soy lecithin. All of the PLs of interest such as glycolipids, phospholipids, and sphingomyelin were well separated with a total run time of 22.5 min and without necessitating the removal of neutral lipids beforehand. Peak retention times were stable, and the method was reproducible. In this study, a modified method of using solvents for extraction of PLs from dairy matrices was also investigated. The modified method offered higher extraction efficiency, consumed less time, and in some cases saved solvent use.

Filter-aided sample preparation with dimethyl labeling to identify and quantify milk fat globule membrane proteins

Bovine milk is a major nutrient source in many countries and it is produced at an industrial scale. Milk is a complex mixture of proteins, fats, carbohydrates, vitamins and minerals. The composition of the bovine milk samples can vary depending on the genetic makeup of the bovine species as well as environmental factors. It is therefore important to study the qualitative and quantitative differences of bovine milk samples. Proteins in milk can be present in casein micelles, in the serum (the water soluble fraction) or in fat globules. These fat globules have a double membrane layer with proteins being bound to or being incapsulated in the membrane layer. The identification and molecular composition of the milk proteins have gained increased interest in recent years. Proteomic techniques make it now possible to identify up to many thousands of proteins in one sample, however quantification of proteins is as yet not straightforward. We analyzed the proteins of the milk fat globule membrane using dimethyl labeling methods combined with a filter-aided sample preparation protocol. Using these methods, it is now possible to quantitatively study the detailed protein composition of many milk samples in a short period of time.

Composition, thermotropic properties, and oxidative stability of freeze-dried and spray-dried milk fat globule membrane isolated from cheese whey

The milk fat globule membrane (MFGM) was isolated from cheese whey using a recently developed novel method. The cheese-derived MFGM contained about 17-19% lipids and 65-70% protein on a dry weight basis. About 50% of the lipids in MFGM were phospholipids. Compositional analysis of the cheese whey-derived MFGM showed that it is a rich source of phosphatidylserine, sphingomyelin, and bioactive proteins CD36, butyrophilin, xanthine oxidase, and mucin 1. Utilization of MFGM in foods as a source of nutraceutical lipids depends on its oxidative stability. In this context, the impact of drying methods, namely, freeze-drying versus spray-drying, on the storage stability of MFGM was studied. Freeze-dried (FD) and spray-dried (SD) MFGM samples were morphologically very different when examined by light microscope: The thermotropic phase transition temperature (T(m)) of lipids in the FD-MFGM was 37.8 °C, and it was 48 °C in SD-MFGM. This 10 °C difference in T(m) indicated that the drying method altered the thermodynamic state of phospholipids in MFGM. At all storage temperatures studied, the zero-order rate constant of lipid oxidation, as measured by hexanal production, was 1-2 orders of magnitude greater in the spray-dried than in the freeze-dried MFGM. The results clearly indicated that the choice of drying method affects morphological characteristics, the T(m) and oxidative stability of phospholipids in MFGM.

The sheep milk fat globule membrane proteome

Milk fat globule membranes (MFGM) are three-layered structures that enclose fat droplets, and are composed by an internal monolayer of endoplasmic reticulum origin, surrounded by a bilayer derived from the apical membrane of the lactating cell. In this work, an optimized protein extraction method was applied to sheep MFGM, and extracts were subjected to SDS-PAGE separation followed by shotgun LC tandem mass spectrometry (GeLC-MS/MS) for identification and characterization. In total, 140 unique sheep MFGM proteins (MFGMPs) were identified. All protein identification data were subjected to Gene Ontology (GO) classification for localization and function. Moreover, the relative abundance of all identified MFGMPs was estimated by means of the normalized spectral abundance factor (NSAF) approach, and GO abundance classes were obtained. The data gathered in this work provide a detailed picture of the proteome expressed in healthy sheep MFGs, and lay the foundations for future studies on sheep lactation physiology and on its alterations in pathological conditions.

Human milk fat globules: polar lipid composition and in situ structural investigations revealing the heterogeneous distribution of proteins and the lateral segregation of sphingomyelin in the biological membrane

Although human milk fat globules (MFG) are of primary importance since they are the exclusive lipid delivery carriers in the gastrointestinal tract of breast-fed infants, they remain the poorly understood aspect of milk. The objectives of this study were to investigate these unique colloidal assemblies and their interfacial properties, i.e. composition and structure of their biological membrane. In mature breast milk, MFG have a mean diameter of 4-5 microm, a surface area of about 2m(2)/g fat and an apparent zeta potential ζ=-6.7 ± 0.5 mV at 37°C. Human MFG contain 3-4mg polar lipids/g fat as quantified by HPLC/ELSD. The main polar lipids are sphingomyelin (SM; 36-45%, w/w), phosphatidylcholine (19-23%, w/w) and phosphatidylethanolamine (10-15%, w/w). In situ structural investigations of human MFG have been performed using light and confocal microscopy with adapted fluorescent probes, i.e. Nile Red, the extrinsic phospholipid Rh-DOPE, Fast Green and the lectin WGA-488. This study revealed a spatial heterogeneity in the human milk fat globule membrane (MFGM), with the lateral segregation of SM in liquid-ordered phase domains of various shapes and sizes surrounded by a liquid-disordered phase composed of the glycerophospholipids in which the proteins are dispersed. The glycocalyx formed by glycoproteins and cytoplasmic remnents have also been characterised around human MFG. A new model for the structure of the human MFGM is proposed and discussed. The unique composition and lateral organisation of the human MFGM components could be of metabolic significance and have health impact for the infants that need to be further explored.

Characterization of Lactobacillus reuteri interaction with milk fat globule membrane components in dairy products

A set of methods has been developed to study the adhesion between four Lactobacillus reuteri strains and the milk fat globule membrane (MFGM) components in dairy products. By combining sucrose density gradient (SDG) centrifugation and bacterial DNA quantification it was found which strains of L. reuteri were more strongly associated with the dairy products, and the results were corroborated by direct binding rate and force measurements made with optical tweezers. It was determined that strong binding was associated with hydrophobicity of the bacteria and that this hydrophobicity is correlated with the presence of LiCl-extractable protein on the surface of the bacteria. Confocal laser scanning microscopy (CLSM) allowed for the visualization of interactions between bacteria and MFGM. This study demonstrates that these methods can be used in combination to characterize, both qualitatively and quantitatively, the adhesion of lactic acid bacteria strains in dairy products.

Neutral glycosphingolipid content of ovine milk

Milk glycosphingolipids (GSL) have been reported to participate in the newborn's defense against pathogens. Taking this into account, in this study we determined the neutral GSL content of ovine milk, including its fatty acid profile. Its role in bacterial adhesion was also addressed by immunodetection of separate GSL in a high-performance thin-layer chromatography overlay assay. Ovine milk has a neutral GSL pattern similar to human milk and includes lactosylceramide (LacCer; 45.7%), monohexosylceramide (glucosylceramide and galactosylceramide, 31.2%), globotriaosylceramide (Gb3; 19.1%), and globotetraosylceramide (Gb4; 3.5%). Globotriaosylceramide and Gb4 are present in human but not bovine milk. Neutral GSL contained C23:0 and C24:0 as the most abundant fatty acids, a finding consistent with its high content of very long chain fatty acids (longer than C20). Most fatty acids were saturated and had a low content of polyunsaturated fatty acids. Bovine enterotoxigenic Escherichia coli strains bound strongly to LacCer and showed a weak binding to monohexosylceramide. The K99 strain also bound strongly to Gb3, and F41 to Gb4. Lactosylceramide, monohexosylceramide, and Gb3 were also observed to bind to human uropathogenic E. coli strains. The results reported here show the ability of neutral GSL in ovine milk to bind to E. coli strains. These compounds could be used as an alternative and available source to supplement infant or bovine formulas with a view to preventing bacterial infections.

Qualitative and quantitative profiling of the bovine milk fat globule membrane proteome

Milk is a biological fluid of unique quality and complexity. It has co-evolved with mammals and mankind to nourish offspring and contains macro- and micronutrients for growth and development of the newborn. The milk fat globule membrane (MFGM) represents an important milk fraction, which is rich in bioactive proteins. In order to better understand functionality of milk fractions and, thereby, enhance the benefits of milk products, detailed qualitative and quantitative protein knowledge of fractions such as MFGM is required. We report the qualitative and quantitative profiling of two MFGM-enriched milk fractions, a whey protein concentrate (WPC) and a buttermilk protein concentrate (BMP), as derived from three different analytical workflows. First, an LC-MS/MS-based shotgun approach revealed 244 protein identities in WPC and 133 in BMP, respectively, and provided an extensive characterisation of the protein content in those two fractions. Second, label-free profiling resulted in rapid and efficient semi-quantitative comparison and yielded valuable protein fingerprints. Third, absolute quantification of selected MFGM proteins was achieved by stable isotope dilution (SID)-MS, in combination with multiple reaction monitoring (MRM) detection. In summary, we provide new information on composition, quantity and possible health benefits of two MFGM-enriched milk fractions highly valuable for future nutritional applications.

Significance of the nonvolatile minor compounds of the neutral lipid fraction as markers of the origin of dairy products

In the past few years several researchers have approached the problem of traceability of dairy products mainly by examining the volatile compounds that could be transferred from forage into milk and cheese. Our research focused on the study of the composition of the nonvolatile minor components of the neutral lipid fraction in mountain dairy products, obtained from animals feeding on pasture, and in milk and cheese samples produced from cows under intensive breeding, fed with concentrates and silages. Hydrocarbons were separated by silica gel column chromatography from the whole lipid matrix and analyzed by GC/MS. Among all the compounds detected, 1-phytene, 2-phytene, neophytadiene, and to a lesser extent the esters of phytol with C16 and C18 fatty acids seem to be promising tools for the recognition of the feeding system. The value of the sum of isoprenoid hydrocarbons (summation operator-hyd) of mountain dairy products (12.3-34.0 mg/kg) was always higher than that obtained from plain samples (1.3-6.4 mg/kg).

Quantification of milk fat globule membrane proteins using selected reaction monitoring mass spectrometry

Although some of the physiological roles of milk fat globule membrane (MFGM) proteins are still unclear, there is increasing evidence that the consumption of bovine MFGM proteins has significant nutritional health benefits for humans; therefore, it may be important to be able to estimate the MFGM proteins in complex ingredients. In this study, the absolute quantification (AQUA) technique, which is typically used for the quantification of proteins in proteomic studies, was applied for the quantification of bovine MFGM proteins in butter milk protein concentrate. Six MFGM proteins (fatty acid binding protein, butyrophilin, PAS 6/7, adipophilin, xanthine oxidase, and mucin 1) were simultaneously quantified using high-resolution selected reaction monitoring mass spectrometry. Samples were rehydrated in 6.7 M urea buffer prior to dilution to 2.2 M before tryspin digestion. Direct rehydration in 2.2 M urea buffer or 2.2 M urea/20% acetonitilrile buffer reduced peptide yield digestion. Isotopically labeled peptides were used as internal standards. The coefficient of variation ranged from 5 to 15%, with a recovery of 84-105%. The limit of detection was in the range of 20-40 pg.