Viscoelastic properties of oil-water interfaces covered by bovine beta-casein tryptic peptides

Interfacial mechanics of β-casein and albumin mixed protein assemblies at liquid-liquid interfaces

Protein emulsifiers play an important role in formulation science, from food product development to emerging applications in biotechnologies. The impact of mixed protein assemblies on surface composition and interfacial shear mechanics remains broadly unexplored, in comparison to the impact that formulation has on dilatational mechanics and surface tension or pressure. In this report, we use interfacial shear rheology to quantify the evolution of interfacial shear moduli as a function of composition in bovine serum albumin (BSA)/β-casein mixed assemblies. We present the pronounced difference in mechanics of these two protein, at oil interfaces, and observe the dominance of β-casein in regulating interfacial shear mechanics. This observation correlates well with the strong asymmetry of adsorption of these two proteins, characterised by fluorescence microscopy. Using neutron reflectometry and fluorescence recovery after photobleaching, we examine the architecture of corresponding protein assemblies and their surface diffusion, providing evidence for distinct morphologies, but surprisingly comparable diffusion profiles. Finally, we explore the impact of crosslinking and sequential protein adsorption on the interfacial shear mechanics of corresponding assemblies. Overall, this work indicates that, despite comparable surface densities, BSA and β-casein assemblies at liquid-liquid interfaces display almost 2 orders of magnitude difference in interfacial shear storage modulus and markedly different viscoelastic profiles. In addition, co-adsorption and sequential adsorption processes are found to further modulate interfacial shear mechanics. Beyond formulation science, the understanding of complex mixed protein assemblies and mechanics may have implications for the stability of emulsions and may underpin changes in the mechanical strength of corresponding interfaces, for example in tissue culture or in physiological conditions.

Emerging Emulsifiers: Conceptual Basis for the Identification and Rational Design of Peptides with Surface Activity

Emulsifiers are gradually evolving from synthetic molecules of petrochemical origin to biomolecules mainly due to health and environmental concerns. Peptides represent a type of biomolecules whose molecular structure is composed of a sequence of amino acids that can be easily tailored to have specific properties. However, the lack of knowledge about emulsifier behavior, structure-performance relationships, and the implementation of different design routes have limited the application of these peptides. Some computational and experimental approaches have tried to close this knowledge gap, but restrictions in understanding the fundamental phenomena and the limited property data availability have made the performance prediction for emulsifier peptides an area of intensive research. This study provides the concepts necessary to understand the emulsifying behavior of peptides. Additionally, a straightforward description is given of how the molecular structure and conditions of the system directly impact the peptides' ability to stabilize emulsion droplets. Moreover, the routes to design and discover novel peptides with interfacial and emulsifying activity are also discussed, along with the strategies to address some of their major pitfalls and challenges. Finally, this contribution reviews methodologies to build and use data sets containing standard properties of emulsifying peptides by looking at successful applications in different fields.

Interfacial dilational properties of tea polyphenols and milk proteins with gut epithelia and the role of mucus in nutrient adsorption

By interacting with nutrients, the mucus layer covering the intestinal epithelium may mediate absorption. This study aimed to determine possible interactions between epigallocatechin-3-gallate (EGCG), skim milk proteins or their complexes with human intestinal mucin films. The films were extracted from postconfluent monolayers of HT29-MTX, a human intestinal cell line, and a model system was created using drop shape tensiometry. The EGCG uptake tested in vitro on postconfluent Caco-2 cells or co-cultures of Caco-2/HT29-MTX (mucus producing) showed recovery of bioavailable EGCG only for Caco-2 cell monolayers, suggesting an effect of mucus on absorption. Interfacial dilational rheology was employed to characterize the properties of the interface mixed with mucus dispersion. Adsorption of polyphenols greatly enhanced the viscoelastic modulus of the mucus film, showing the presence of interactions between the nutrient molecules and mucus films. On the other hand, in situ digestion of milk proteins using trypsin showed higher surface activities as a result of protein unfolding and competitive adsorption of the hydrolyzed products. There was an increase of viscoelastic modulus over the drop ageing time for the mixed interfaces, indicating the formation of a stiffer interfacial network. These results bring new insights into the role of the mucus layer in nutrient absorption and the interactions of mucus and dairy products.

Friction force spectroscopy of β- and κ-casein monolayers

Friction force spectroscopy (FFS) has been applied to study the tribological properties of β- and κ-casein layers on hydrophobic substrates in aqueous solutions. Nanometer-sized imaging tips were employed. This allowed exerting and determining the high pressures needed to remove the layers and registering the topographic evolution during this process. Both β- and κ-casein layers showed similar and not particularly high initial frictional responses (friction coefficient of ∼1 when measured with a silicon nitride tip). The pressures needed to remove the layers were of the same order of magnitude for both proteins, ∼10(8) Pa, but slightly higher for those composed of β-casein. The technique has also shown to be useful in studying the two-dimensional lateral diffusion of the proteins and the wear on the layers they form.

Pressure-aided proteolysis of beta-casein

Beta-casein, which is present in the form of micelles at atmospheric pressure, has been hydrolyzed during pressure treatment to improve the accessibility of the protein. Two proteolytic enzymes with different specificities were used. Trypsin was aimed at mainly hydrolyzing hydrophilic segments of beta-casein and chymotrypsin at hydrolyzing hydrophobic segments of beta-casein. Measurements on aggregation during hydrolysis at atmospheric pressure showed that probably not micelle disruption, but disruption of much larger aggregates, occurs in the process. Peptide profiles were measured via reversed-phase chromatography. Measurements on enzyme activity after pressure treatments showed that trypsin was inactivated by pressure, which could explain all differences in peptide profiles compared to atmospheric experiments. Pressure did not influence the reaction mechanism, probably because the hydrophilic part of beta-casein is sufficiently accessible. However, chymotryptic proteolysis under pressure yielded new peptides that could not be explained by a change in enzyme activity. Here, pressure altered the mechanism of hydrolysis, by changing either enzyme specificity or substrate accessibility, which led to different peptides that can have different properties.

The contribution of Aeromonas salmonicida extracellular products to the induction of inflammation in Atlantic salmon (Salmo salar L.) following vaccination with oil-based vaccines

Intraperitoneal injection of Atlantic salmon with oil-based vaccines often results in severe side effects. Aeromonas salmonicida subspecies salmonicida, a primary antigen in the vaccines, produces extracellular products (ECPs) that are included in the formulation but the role of ECPs in inducing side effects is not well understood. In the present study, we evaluated the contribution of ECPs to early inflammatory reactions since early events determine the outcome of inflammation. Five groups of Atlantic salmon pre-smolts were injected intraperitoneally with one of the following preparations: (1) A. salmonicida water-in-oil (w/o) containing standard amounts of ECPs; (2) A. salmonicida (w/o) with ECPs concentrated five times; (3) A. salmonicida (w/o) without ECPs (ECPs were removed by washing and re-suspension of the bacteria prior to formulation); (4) w/o only (without antigens), and (5) physiological saline. Tissue sections of the injection site (pyloric caeca and surrounding areas) were collected at monthly intervals for 4 months in phosphate buffered formalin and processed for light microscopy. Computer-assisted microscopy with the help of Image Pro analysis program was used to measure the area of inflammation on H&E stained sections. Differential cell counts of leucocytes involved in the inflammatory reaction were also done based on morphology. Overall results show that fish injected with vaccines containing concentrated amounts of ECPs displayed a higher average area of inflammation compared to all other groups. In contrast, washed preparations induced mild reactions compared to vaccines containing either standard or concentrated ECPs. Mild, non-persistent reactions were observed in the group injected with oil adjuvant only. Neutrophils were persistent in inflammations induced by all preparations except w/o only. No inflammatory reactions were observed in the group injected with PBS. The results suggest that ECPs are pro-inflammatory in Atlantic salmon. It is anticipated that ECPs are more readily exposed to inflammatory cells than the bacterial cells themselves during early stages of inflammation because of their orientation at the water-oil interface. The results indicate that ECPs of A. salmonicida play an important role in the induction of early inflammatory reactions. It is also documented that the combination of antigens with oil adjuvants, and not the adjuvants alone, is the inducer of strong inflammatory reactions in Atlantic salmon.

Interfacial properties of acidified skim milk

The purpose of this study is to investigate the tension properties and dilatational viscoelastic modulus of various skim milk proteins (whole milk, EDTA-treated milk, beta-casein, and beta-lactoglobulin) at an oil/water interface at 20 degrees C. Measurements are performed using a dynamic drop tensiometer for 15,000 s. The aqueous bulk phase is a skim milk simulated ultrafiltrate containing 11 x 10(-3) g L(-1) milk protein. At pH 6.7, beta-casein appears as the best to decrease the interfacial tension, whereas beta-lactoglobulin leads to the highest interfacial viscoelastic modulus value. Whole milk was almost as surface-active as individual beta-casein in terms of the final (steady-state) lowering of the interfacial tension, but the rate of tension lowering was smaller. EDTA treatment improved the rate of tension lowering of whole milk. The acidification of milk, from previous measurements, would lead to the enhancement of surface activity. At t=15,000 s, the order of effectiveness is pH 4.3 > pH 5.3 = pH 5.6 > pH 6.7 whole milk, suggesting that pH 4.3 whole milk is the best surface active. As compared to pH 6.7 whole milk, the use of pH 5.3 and pH 5.6 milk as surface active would result in the use of milk containing more free beta-casein born of pH-dissociated casein micelles.