β-Casein–phospholipid monolayers as model systems to understand lipid–protein interactions in the milk fat globule membrane

Mechanisms for the interaction of the milk fat globule membrane with the plasma membrane of gut epithelial cells

The milk fat globule membrane (MFGM) provides infants and adults with several health benefits. These are not derived solely from its unique composition, but also from arrangement of lipids in the MFGM that, in the case of newborns, could reach the intestine partially intact. Fluorochromes associated with lipid derivatives were used to prove a fusion process between the MFGM and the cellular membrane of differentiated Caco-2 cells. To explore the mechanism of this interaction, incubations of MFGM with Caco-2 cells were carried out in the presence of fusogenic agents or compounds that block other MFGM interaction pathways with cells. Confocal fluorescence microscopy provided visual evidence of the fusion process. Lastly, determination on the lipid profile of cells after their interaction with MFGM indicated a metabolic rearrangement of lipids leading to accumulation of triacylglycerols.

Quantitative methods to detect phospholipids at the oil-water interface

Phospholipids are the main constituents of cell membranes and act as natural stabilizers of milk fat globules. Phospholipids are used in a wide range of applications, e.g. as emulsifiers in cosmetic, pharmaceutical and food products. While processed emulsion droplets are usually stabilized by a monolayer of phospholipids, cell membranes have a phospholipid bilayer structure and milk fat globules are stabilized by a complex phospholipid trilayer membrane. Despite the broad relevance of phospholipids, there are still many scientific challenges in understanding how their behavior at the fluid-fluid interface affects microstructure, stability, and physico-chemical properties of natural and industrial products. Most of these challenges arise from the experimental difficulties related to the investigation of the molecular arrangement of phospholipids in situ at the fluid-fluid interface and the quantification of their partitioning between the bulk phase and the interface, both under static and flow conditions. This task is further complicated by the presence of other surface-active components, such as proteins, that can interact with phospholipids and compete for space at the interface. Here, we review the methodologies available from the literature to detect and quantify phospholipids, focusing on oil-water interfaces, and highlight current limitations and future perspectives.

Identification and Detection of Bioactive Peptides in Milk and Dairy Products: Remarks about Agro-Foods

Food-based components represent major sources of functional bioactive compounds. Milk is a rich source of multiple bioactive peptides that not only help to fulfill consumers 'nutritional requirements but also play a significant role in preventing several health disorders. Understanding the chemical composition of milk and its products is critical for producing consistent and high-quality dairy products and functional dairy ingredients. Over the last two decades, peptides have gained significant attention by scientific evidence for its beneficial health impacts besides their established nutrient value. Increasing awareness of essential milk proteins has facilitated the development of novel milk protein products that are progressively required for nutritional benefits. The need to better understand the beneficial effects of milk-protein derived peptides has, therefore, led to the development of analytical approaches for the isolation, separation and identification of bioactive peptides in complex dairy products. Continuous emphasis is on the biological function and nutritional characteristics of milk constituents using several powerful techniques, namely omics, model cell lines, gut microbiome analysis and imaging techniques. This review briefly describes the state-of-the-art approach of peptidomics and lipidomics profiling approaches for the identification and detection of milk-derived bioactive peptides while taking into account recent progress in their analysis and emphasizing the difficulty of analysis of these functional and endogenous peptides.

Addition of buttermilk powder improved the rheological and storage properties of low-fat yogurt

Buttermilk is used widely in dairy products due to its good emulsifying and nutritional properties. In the present study, 0%-4.0% (w/w) buttermilk powder was added to low-fat yogurt with a constant protein content to investigate its efficacy on the rheological and storage properties of low-fat yogurt. Buttermilk increased the final titration acidity. Addition of buttermilk decreased the pH at the gelation point, shortened the gelation time, and thus shortened the fermentation period. Storage modulus G', yield stress, yield strain, and compact cross-links of the microstructure were enhanced greatly with addition of 1.0%-2.0% (w/w) buttermilk powder. In addition, addition of buttermilk decreased whey separation and increased the viscosity and firmness of low-fat yogurt during storage. Our findings suggest that the addition of an appropriate amount of buttermilk altered the rheological characteristics and improved the textural and storage properties of low-fat yogurt.

Alteration of surface pressure of macromolecular monolayer at the air-water interface and electrochemical impedance characteristics

Proteins are very important biological macromolecules, specific in their functions essential for many biological activities. Proteins are among the most vital components of all living matters with thousands of different types, and they are very specific in their nature which makes them important for usage in many biotechnological, biomedical, and food processing applications. Langmuir monolayer studies provide the characteristics of monolayer at the air-liquid interface. Langmuir monolayer of protein is affected by the molecular weight, macromolecular structure, denaturation and its purity. The interaction of the protein with other molecules greatly affects the behaviour of monolayer. Coconut oil layer at the air-water interface was prepared with different sub-phases including casein protein and whey protein. Wilhelmy plate measurement technique has been used to measure the surface pressure of the monolayer at the air-liquid interface. Experiments were carried out to study the stability using electrochemical impedance measurements. The electrostatic forces dominated the coconut oil - protein interaction, and the interface of macromolecules were controlled by expansion and compression. The macromolecules were adsorbed with a preferential orientation at the interface, and the stability of colloidal dispersions was governed by the intermolecular interactions. The stability using electrochemical impedance spectroscopy on the selected protein macromolecules along with other analysis were performed by varying various parameters.

Preparation of Non-Surface-Active Langmuir Trough Subphases from Milk

Milk polar lipid interfacial behavior continues to be analyzed using Langmuir trough experiments, but the reported Langmuir trough subphases commonly used are not fully representative of milk. A method to transform liquids of biological origin, such as milk, into appropriate Langmuir trough subphases does not currently exist, which hinders the applicability of Langmuir trough experiments to nature. Here, a procedure to manufacture milk-derived Langmuir trough subphases with insignificant amounts of surfactants from bovine milk is presented. Ultrafiltration is used to remove the bulk of protein surfactants from milk followed by the creation of solvent-induced emulsions that remove trace proteins and lipids from collected skim milk permeates. Change in surface tension upon compression of resulting washed permeates from the surfactant removal process was ≤0.1 mN/m, terming the permeates non-surface-active (NSA). NSA permeates (72.4 ± 0.2 mN/m) had a surface tension similar to that of ultrapure water (72.6 ± 0.1 mN/m), but their pH, conductivity, percent total solids, Brix percentage, alkalinity, and hardness were not the same, with NSA permeates being more compositionally similar to skim milk than water. The lift-off points of milk ganglioside GM3 monolayer surface pressure-area isotherms spread on NSA permeates and ultrapure water subphases were significantly different when compared for the same sample, indicating that surface tension measurements obtained on ultrapure water are not the same as with NSA milk permeate. Overall, surfactants were removed from bovine milk without the addition of exogenous compounds, allowing for the production of a NSA solution derived from milk that can be used in the Langmuir trough experiment to more realistically resemble the natural environment of milk polar lipids. The procedure described here was able to produce NSA solutions for other dairy beverages aside from milk, indicating that it can be applicable to other biological fluids.

The association of lipophilic phospholipids with native bovine casein micelles in skim milk: Effect of lactation stage and casein micelle size

A known biological role of casein micelles is to transport calcium from mother to young and provide amino acids for growth and development. Previous reports demonstrated that modified casein micelles can be used to transport and deliver hydrophobic probes. In this study, the distribution of lipid-soluble phospholipids, including sphingomyelins (SM) and phosphatidylcholines (PC), was quantified in whole raw milk, skim raw milk, and casein micelles of various sizes during early, mid, and late lactation stages. Low-pressure size exclusion chromatography was used to separate casein micelles by size, followed by hydrophobic extraction and liquid chromatography-mass spectrometry for the quantification of PC and SM. Results showed that the SM d18:1/23:0, d18:1/22:0, d18:1/16:0, d16:1/22:0, d16:1/23:0, and d18:1/24:0 and the PC 16:0/18:1, 18:0/18:2, and 16:0/16:0 were dominating candidates appearing in maximum concentration in whole raw milk obtained from late lactation, with 21 to 50% of total SM and 16 to 35% of total PC appearing in skim milk. Of the total SM and PC found in skim milk, 35 to 46% of SM and 22 to 29% of PC were associated with the casein micelle fraction. The highest concentrations of SM d18:1/22:0 (341 ± 17 µg/g of casein protein) and PC 16:0/18:1 (180 ± 20 µg/g of casein protein) were found to be associated with the largest casein micelles (diameter = 149 nm) isolated in milk from late lactation, followed by a decrease in concentration as the casein micelle size decreased.

Milk fat globules and associated membranes: Colloidal properties and processing effects

The composition and physical-chemical properties of the milk fat globule membrane (MFGM) is a subject that has gained increased interest in the field of food colloids, mainly because the nutritional and technological value of the MFGM. In fact, related changes in integrity and structure during milk processing pose a huge challenge as far as efforts directed to isolate the components of the fat globule membrane. MFGM characteristics and potential utilization are subjects of dissension. Thus, the effects of processing and the colloidal interactions that exist with other milk constituents need to be better understood in order to exploit milk fat and MFGM, their functionality as colloids as well as those of their components. These are the main subjects of this review, which also reports on the results of recent inquiries into MFGM structure and colloidal behavior.

Organization of lipids in milks, infant milk formulas and various dairy products: role of technological processes and potential impacts

The microstructure of milk fat in processed dairy products is poorly known despite its importance in their functional, sensorial and nutritional properties. However, for the last 10 years, several research groups including our laboratory have significantly contributed to increasing knowledge on the organization of lipids in situ in dairy products. This paper provides an overview of recent advances on the organization of lipids in the milk fat globule membrane using microscopy techniques (mainly confocal microscopy and atomic force microscopy). Also, this overview brings structural information about the organization of lipids in situ in commercialized milks, infant milk formulas and various dairy products (cream, butter, buttermilk, butter serum and cheeses). The main mechanical treatment used in the dairy industry, homogenization, decreases the size of milk fat globules, changes the architecture (composition and organization) of the fat/water interface and affects the interactions between lipid droplets and the protein network (concept of inert vs active fillers). The potential impacts of the organization of lipids and of the alteration of the milk fat globule membrane are discussed, and technological strategies are proposed, in priority to design biomimetic lipid droplets in infant milk formulas.

Milk sphingomyelin domains in biomimetic membranes and the role of cholesterol: morphology and nanomechanical properties investigated using AFM and force spectroscopy

Milk sphingomyelin (MSM) and cholesterol segregate into domains in the outer bilayer membrane surrounding milk fat globules. To elucidate the morphology and mechanical properties of theses domains, supported lipid bilayers with controlled molar proportions of MSM, dioleoylphosphatidylcholine (DOPC) and cholesterol were produced in buffer mimicking conditions of the milk aqueous phase. Atomic force microscopy imaging showed that (i) for T < 35 °C MSM segregated in gel phase domains protruding above the fluid phase, (ii) the addition of 20 mol % cholesterol resulted in smaller and more elongated l(o) phase domains than in equimolar MSM/DOPC membranes, (iii) the MSM/cholesterol-enriched l(o) phase domains were less salient than the MSM gel phase domains. Force spectroscopy measurements furthermore showed that cholesterol reduced the resistance of MSM/DOPC membrane to perforation. The results are discussed with respect to the effect of cholesterol on the biophysical properties of lipid membranes. The combination of AFM imaging and force mapping provides unprecedented insight into the structural and mechanical properties of milk lipid membranes, and opens perspectives for investigation of the functional properties of MSM domains during milk fat processing or digestion.

Adsorption of bile salts to milk phospholipid and phospholipid-protein monolayers

The adsorption of bile salts to milk phospholipid and phospholipid-protein monolayers at the air-water interface was studied under simulated intestinal conditions using a Langmuir trough, epifluorescence microscopy, and atomic force microscopy. Surface pressure changes were affected by temperature, initial surface pressure, and bile composition. The rate of addition of bile salts and the initial surface pressure of the monolayers had an impact on the microstructure of the mixed monolayers. The presence of proteins in monolayers at different ratios did not affect the surface pressure change upon addition of bile. However, at 20 °C, the addition of bile to phospholipid and phospholipid-protein monolayers led to different features with branching and clustering of liquid-ordered domains and possible formation of bile salt-rich areas within liquid-ordered domains. This study provides a basic understanding of the interfacial changes occurring at the surface of milk fat globules and milk phospholipid liposomes during their passage in the duodenum.

Cryo-SEM images of native milk fat globule indicate small casein micelles are constituents of the membrane

Schematic representation of the native milk fat globule membrane: casein micelles and whey proteins on the surface of the membrane.

In vivo digestion of bovine milk fat globules: effect of processing and interfacial structural changes. I. Gastric digestion

The aim was to study the in vivo gastric digestion of fat globules in bovine cream from raw, pasteurised or pasteurised and homogenised milk. Fasted rats were gavaged once and chyme samples were collected after 30, 120 and 180 min post-gavage. Proteins from raw (RC) and pasteurised (PC) creams appeared to be digested faster and to a greater extent. Free fatty acids (FAs) increased throughout the 3h postprandial period. Short and medium chain FAs were released more rapidly than long chain FAs which were hydrolysed to a greater degree from PC. The size of the fat globules of all creams increased in the stomach. Protein aggregates were observed in pasteurised and homogenised cream chyme. Protrusions, probably caused by the accumulation of insoluble lipolytic products, appeared at the surface of the globules in RC and PC chyme. Overall, PC proteins and lipids appeared to be digested to a greater extent.

In vivo digestion of bovine milk fat globules: effect of processing and interfacial structural changes. II. Upper digestive tract digestion

The aim of this research was to study the effect of milk processing on the in vivo upper digestive tract digestion of milk fat globules. Fasted rats were serially gavaged over a 5h period with cream from raw, pasteurised, or pasteurised and homogenised milk. Only a few intact dietary proteins and peptides were present in the small intestinal digesta. Significantly (P<0.05) more longer chain (C≥10) fatty acids were present in the digesta of rats gavaged with raw (448 mg g(-1) digesta dry matter (DDM)) and homogenised creams (528 mg g(-1) DDM), as compared to pasteurised and homogenised cream (249 mg g(-1) DDM). Microscopy techniques were used to investigate the structural changes during digestion. Liquid-crystalline lamellar phases surrounding the fat globules, fatty acid soap crystals and lipid-mucin interactions were evident in all small intestinal digesta. Overall, the pasteurised and homogenised cream appeared to be digested to a greater extent.

Storage stability and physical characteristics of tea-polyphenol-bearing nanoliposomes prepared with milk fat globule membrane phospholipids

The objective of this work was to better understand the functional properties of milk phospholipids when used as ingredients to prepare liposomes. Liposomal dispersions (10%) were prepared using high-pressure homogenization, and their physical properties as well as their ability to encapsulate tea polyphenols were investigated. The extent of encapsulation, measured by HPLC, increased with tea polyphenol concentration up to about 4 mg·mL(-1). At polyphenol concentrations ≥ 6 mg·mL(-1), the liposome dispersions were no longer stable. The influence of pH (3-7), storage temperature (room temperature or refrigeration), and addition of sugars (0-15%) were studied for liposomes containing 4 mg·mL(-1) polyphenols. The liposomal dispersions were also stable in the presence of peptides. The storage stability of the systems prepared with milk phospholipids was compared to that of liposomes made with soy phospholipids. Soy liposomes were smaller in size than milk phospholipid liposomes, the encapsulation efficiency was higher, and the extent of release of tea polyphenols during storage was lower for milk phospholipid liposomes compared to soy liposomes. The results suggest that milk phospholipids could be employed to prepare tea-polyphenol-bearing liposomes and that the tea catechins may be incorporated in the milk phospholipid bilayer more efficiently than in the case of a soy phospholipid bilayer.

Stability of milk fat globule membrane proteins toward human enzymatic gastrointestinal digestion

The milk fat globule membrane (MFGM) fraction refers to the thin film of polar lipids and membrane proteins that surrounds fat globules in milk. It is its unique biochemical composition that renders MFGM with some beneficial biological activities, such as anti-adhesive effects toward pathogens. However, a prerequisite for the putative bioactivity of MFGM is its stability during gastrointestinal digestion. We, therefore, subjected MFGM material, isolated from raw milk, to an in vitro enzymatic gastrointestinal digestion. Sodium dodecyl sulfate PAGE, in combination with 2 staining methods, Coomassie Blue and periodic acid Schiff staining, was used to evaluate polypeptide patterns of the digest, whereas mass spectrometry was used to confirm the presence of specific MFGM proteins. Generally, it was observed that glycoproteins showed higher resistance to endogenous proteases compared with non-glycosylated proteins. Mucin 1 displayed the highest resistance to digestion and a considerable part of this protein was still detected at its original molecular weight after gastric and small intestine digestion. Cluster of differentiation 36 was also quite resistant to pepsin. A significant part of periodic acid Schiff 6/7 survived the gastric digestion, provided that the lipid moiety was not removed from the MFGM material. Overall, MFGM glycoproteins are generally more resistant to gastrointestinal digestion than serum milk proteins and the presence of lipids, besides glycosylation, may protect MFGM glycoproteins from gastrointestinal digestion. This gastrointestinal stability makes MFGM glycoproteins amenable to further studies in which their putative health-promoting effects can be explored.