Association behavior of beta-casein

Linking variation in the casein fraction and salt composition to casein micelle size in milk of Dutch dairy goats

The casein (CN) composition, salt composition and micelle size varies largely between milk samples of individual animals. In goats, the link between those casein characteristics are unknown and could provide useful insights into goat casein micelle structure. In this study, the casein- and salt composition of 42 individual Dutch goats from 17 farms was studied and linked to casein micelle size. Micelle size, proportions of individual caseins, and protein content were associated with each other. Milk with smaller casein micelles was higher in protein content, salt content, and proportion of αs1-CN, but lower in αs2-CN and β-CN. The higher salt content in milk with small casein micelles was mainly attributed to a higher protein content, but changes in casein composition might additionally contribute to differences in mineralization. The non-sedimentable casein content in goat milk correlated with non-sedimentable fractions of β-CN and κ-CN and was independent of micelle size. Between large and small casein micelles, goat casein micelles showed more differences in casein and salt composition than bovine micelles, indicating differences in internal structure. Nevertheless, the casein mineralization in goat milk was similar to casein mineralization in bovine milk, indicating that mineralization of casein micelles follows a general principle. These results can help to better understand how composition and micelle structure in goat milk are related to each other, which may be useful to improve processing and product properties of goat milk in the future.

The effects of pH, temperature, and buffer concentration on the self-assembling behavior, secondary structure, and surface hydrophobicity of donkey and bovine β-casein

The self-assembling behavior, secondary structure, and surface hydrophobicity of purified donkey β-casein in terms of pH, temperature, and buffer concentration were investigated in comparison with commercial bovine β-casein. Critical micelle concentration of both β-caseins decreased with the lowering of pH (pH 8.0-6.0) and the increasing temperatures (25-50 °C). Critical micelle temperature of both β-caseins increased moving from pH 6.0 to 8.0 and aggregates larger than micelles formed at pH 6.0 that is close to their isoelectric point. Fluorescence spectroscopy analysis demonstrated that the maximum surface hydrophobicity was achieved at pH 6.0. The secondary structure was examined using circular dichroism spectroscopy, highlighting an increase of α-helix content and a decrease of unordered structures with the decrease of pH and increase of temperature. This work provides insights on parameters promoting molecular interactions involved in donkey β-CN self-association, useful to develop nanocarriers for encapsulating bioactive compounds in pharmaceutical and nutraceutical applications.

A Study of the Interactions of Heavy Metals in Dairy Matrices Using Fourier Transform Infrared Spectroscopy, Chemometric, and In Silico Analysis

Heavy metals are among the toxic substances longest recognized by man. Today, due to the myriad sources of exposure, such as contaminated water, food, or air, they have become a major public health problem. This work presents the effects manifested in the infrared spectrum behavior caused by the presence of Cd²⁺, Cr⁶⁺, and Pb²⁺ at different concentrations in three different matrices: water, casein, and milk; observing that the spectral modifications in the regions of different vibrational modes of nucleophilic groups such as -OH, COO- and NH₂ depending on the nature of the metal and its concentration. These findings were correlated in-silico using optimized models in Gabedit software and structural optimization was performed with MOPAC 2016 showing stable structures between the metals and Gln, Hys, Glu, and Phe of casein. By applying chemometrics (Principal Component Analysis), it was possible to observe a good correlation between the experimental data and to discriminate between the type of metal, the matrix that contains it, and the concentration could be represented through linear models that showed adjustments with a value of r² ≥ 0.95.

Structural, Binding and Functional Properties of Milk Protein-Polyphenol Systems: A Review

Polyphenols (PP) are linked to health benefits (e.g., prevention of cancer, cardiovascular disease and obesity), which are mainly attributed to their antioxidant activity. During digestion, PP are oxidised to a significant degree reducing their bio-functionality. In recent years, the potential of various milk protein systems, including β-casein micelles, β-lactoglobulin aggregates, blood serum albumin aggregates, native casein micelles and re-assembled casein micelles, to bind and protect PP have been investigated. These studies have yet to be systematically reviewed. The functional properties of the milk protein-PP systems depend on the type and concentration of both PP and protein, as well as the structure of the resultant complexes, with environmental and processing factors also having an influence. Milk protein systems protect PP from degradation during digestion, resulting in a higher bioaccessibility and bioavailability, which improve the functional properties of PP upon consumption. This review compares different milk protein systems in terms of physicochemical properties, PP binding performance and ability to enhance the bio-functional properties of PP. The goal is to provide a comprehensive overview on the structural, binding, and functional properties of milk protein-polyphenol systems. It is concluded that milk protein complexes function effectively as delivery systems for PP, protecting PP from oxidation during digestion.

Proteolysis of Micellar β-Casein by Trypsin: Secondary Structure Characterization and Kinetic Modeling at Different Enzyme Concentrations

Tryptic proteolysis of protein micelles was studied using β-casein (β-CN) as an example. Hydrolysis of specific peptide bonds in β-CN leads to the degradation and rearrangement of the original micelles and the formation of new nanoparticles from their fragments. Samples of these nanoparticles dried on a mica surface were characterized by atomic force microscopy (AFM) when the proteolytic reaction had been stopped by tryptic inhibitor or by heating. The changes in the content of β-sheets, α-helices, and hydrolysis products during proteolysis were estimated by using Fourier-transform infrared (FTIR) spectroscopy. In the current study, a simple kinetic model with three successive stages is proposed to predict the rearrangement of nanoparticles and the formation of proteolysis products, as well as changes in the secondary structure during proteolysis at various enzyme concentrations. The model determines for which steps the rate constants are proportional to the enzyme concentration, and in which intermediate nano-components the protein secondary structure is retained and in which it is reduced. The model predictions were in agreement with the FTIR results for tryptic hydrolysis of β-CN at different concentrations of the enzyme.

A pH-responsive phosphoprotein washing fluid for the removal of phenanthrene from contaminated peat moss in the cold region

Oil pollution is one of the major environmental concerns in the petroleum industry. In this study, a cheap food-grade sodium caseinate (NaCas) was used as a pH-responsive washing fluid in the remediation of phenanthrene (PHE) affected peat moss. The effects of environmental factors on the removal of PHE were systematically investigated. The results showed that increasing NaCas concentration and washing temperature improved the PHE mobilization, while high salinity and humic acid dosage displayed a negative effect. The factorial analysis revealed that three individual factors and two interactions exhibited significant effects on the washing performance. Due to the pH-responsive property of NaCas, the turbidity, total organic carbon (TOC), and chemical oxygen demand (COD) of the washing effluent were remarkably reduced by simply adjusting the solution acidity, improving the practical application of such a washing method. Significantly, the toxicity modeling proved that NaCas can reduce the binding energy between PHE and superoxide dismutase (SOD) of the selected marine organism, and thus relieve the toxicity of PHE to the organisms. Given these advantages, NaCas-assisted washing can be a viable option for the remediation of contaminated peat moss.

Micro and nano-scale compartments guide the structural transition of silk protein monomers into silk fibers

Silk is a unique, remarkably strong biomaterial made of simple protein building blocks. To date, no synthetic method has come close to reproducing the properties of natural silk, due to the complexity and insufficient understanding of the mechanism of the silk fiber formation. Here, we use a combination of bulk analytical techniques and nanoscale analytical methods, including nano-infrared spectroscopy coupled with atomic force microscopy, to probe the structural characteristics directly, transitions, and evolution of the associated mechanical properties of silk protein species corresponding to the supramolecular phase states inside the silkworm's silk gland. We found that the key step in silk-fiber production is the formation of nanoscale compartments that guide the structural transition of proteins from their native fold into crystalline β-sheets. Remarkably, this process is reversible. Such reversibility enables the remodeling of the final mechanical characteristics of silk materials. These results open a new route for tailoring silk processing for a wide range of new material formats by controlling the structural transitions and self-assembly of the silk protein's supramolecular phases.

Maillard induced glycation of β-casein for enhanced stability of the self-assembly micelles against acidic and calcium environment

Bovine β-casein (β-CN) has attracted increasingly interest as biocompatible nanocarrier for hydrophobic flavonoid due to its self-assembly ability to form micelles. This paper reported Maillard induced glycation reaction of β-CN using dextran in order to improve stability of naringenin-loaded β-CN micelles under acidic and high calcium environments. Our results showed that solubility of β-CN-graft-dextran was remarkable increased at acidic pH and the conjugation with 20 kDa dextran had the highest level of graft degree. Glycation restrained β-CN from aggregating around pH 5.0 where was close to the isoelectric point, forming spherical micelles with irregular and rough surfaces, which were significantly larger than the micelles at pH 7.0. β-CN-graft-dextran also overcame destabilization of the micelles induced by excess calcium and had no impact on the chelating ability of calcium. These findings appeared to be promising for future applications of modified β-CN-graft-dextran based on Maillard reaction as fairly stable nanocarrier under extreme condition.

A study of lipolysis induced by adjuncts from edible Aspergillus sp. solid culture products on ripened semi-hard cheese

BACKGROUND

Aspergillus sp. has been used in traditional Japanese fermented foods. Protease-containing culture products of A. oryzae have been applied as the adjunct enzyme source to enrich the flavor in ripened cheese. Although proteolysis was stimulated, the increase of free fatty acids (FFA) was recognized in some products. Since an excess amount of FFA accumulation can cause rancidity in cheese products, the assessment of lipase activity was considered to be essential for the cheese adjunct preparation.

RESULTS

Although an equal lipase activity from the adjunct materials of A. kawachii NBRC 4308, A. luchuensis RIB 2604 and A. oryzae AHU 7139 was applied to semi-hard cheese, the FFA level was significantly higher in A. oryzae cheese than in the others. Furthermore, the profiles of volatile components were different in experimental cheeses. An in vitro study with experimental curds demonstrated that the high FFA might not depend on the lipase retainability on curds. On the contrary, the pronounced activation of the lipases occurred in A. oryzae after incubation with the curds. Moreover, incubation of the insoluble lipase that had been attached to the cells with skim milk curd extracts allowed the release of lipases from the cells into the medium with remarkable activation.

CONCLUSION

A. oryzae AHU 7139 possessed a complex lipolytic system comprising extracellular and cell-binding lipases that were attributed to the increase in FFA in A. oryzae cheese. © 2022 Society of Chemical Industry.

Formulation of infant formula with different casein fractions and their effects on physical properties and digestion characteristics

This study investigated whether casein (CN) fractions exhibit better physical properties and digestibility than native casein micelles presently used in the production of infant formula. The structural performance of native casein micelles (micellar casein concentrates, MCC), β-CN, κ-CN and β + κ-CN were explored, and physical properties and digestion characteristics (i.e., digestibility, particle size, zeta potential and microscopic morphology) of the infant formula with MCC, β-CN, κ-CN or β + κ-CN were determined to elucidate the applicability of these casein types in infant formula. Results indicated that the β + κ-CN infant formula solution had the largest particle size with the most unstable potential. Moreover, both β-CN and κ-CN infant formula showed high solubility, while κ-CN displayed the lowest foaming capacity and high foaming stability. β-CN infant formula expressed an effective digestibility property, however, it possessed the largest particle size after gastrointestinal digestion. Therefore, β-CN fraction infant formula showed better digestibility than casein infant formula, and thus this work provides a theoretical basis for the development of infant formula.

Production and storage stability of concentrated micellar casein

Concentrated micellar casein (CMC) is a high-protein ingredient that can be used in process cheese product formulations. The objectives of this study were to develop a process to produce CMC and to evaluate the effect of sodium chloride and sodium citrate on its storage stability. Skim milk was pasteurized at 76°C for 16 s and cooled to ≤4°C. The skim milk was heated to 50°C using a plate heat exchanger and microfiltered with a graded permeability (GP) ceramic microfiltration (MF) membrane system (0.1 μm) in a continuous feed-and-bleed mode (flux of 71.43 L/m² per hour) using a 3× concentration factor (CF) to produce a 3× MF retentate. Subsequently, the retentate of the first stage was diluted 2× with soft water (2 kg of water: 1 kg of retentate) and again MF at 50°C using a 3× CF. The retentate of the second stage was then cooled to 4°C and stored overnight. The following day, the retentate was heated to 63°C and MF in a recirculation mode until the total solids (TS) reached approximately 22% (wt/wt). Subsequently, the MF system temperature was increased to 74°C and MF until the permeate flux was <3 L/m² per hour. The CMC was then divided into 3 aliquots (approximately 10 kg each) at 74°C. The first portion was a control, whereas 1% of sodium chloride was added to the second portion (T1), and 1% of sodium chloride plus 1% of sodium citrate were added to the third portion (T2). The CMC retentates were transferred hot to sterilized vials and stored at 4°C. This trial was repeated 3 times using separate lots of skim milk. The CMC at d 0 (immediately after manufacturing) contained 25.41% TS, 21.65% true protein (TP), 0.09% nonprotein nitrogen (NPN), and 0.55% noncasein nitrogen (NCN). Mean total aerobic bacterial counts (TBC) in control, T1, and T2 at d 0 were 2.6, 2.5, and 2.8 log cfu/mL, respectively. The level of proteolysis (NCN and NPN values) increased with increasing TBC during 60 d of storage at 4°C. This study determined that CMC with >25% TS and >95% casein as percentage of TP can be manufactured using GP MF ceramic membranes and could be stored up to 60 d at 4°C. The effects of the small increase in NCN and NPN, as well as the addition of sodium chloride or sodium citrate in CMC during 60 d of storage on process cheese characteristics, will be evaluated in subsequent studies.

Nanosecond structural dynamics of intrinsically disordered β-casein micelles by neutron spectroscopy

β-casein undergoes a reversible endothermic self-association, forming protein micelles of limited size. In its functional state, a single β-casein monomer is unfolded, which creates a high structural flexibility, which is supposed to play a major role in preventing the precipitation of calcium phosphate particles. We characterize the structural flexibility in terms of nanosecond molecular motions, depending on the temperature by quasielastic neutron scattering. Our major questions are: Does the self-association reduce the chain flexibility? How does the dynamic spectrum of disordered caseins differ from a compactly globular protein? How does the dynamic spectrum of β-casein in solution differ from that of a protein in hydrated powder states? We report on two relaxation processes on a nanosecond and a sub-nanosecond timescale for β-casein in solution. Both processes are analyzed by Brownian oscillator model, by which the spring constant can be defined in the isotropic parabolic potential. The slower process, which is analyzed by neutron spin echo, seems a characteristic feature of the unfolded structure. It requires bulk solvent and is not seen in hydrated protein powders. The faster process, which is analyzed by neutron backscattering, has a smaller amplitude and requires hydration water, which is also observed with folded proteins in the hydrated state. The self-association had no significant influence on internal relaxation, and thus, a β-casein protein monomer flexibility is preserved in the micelle. We derive spring constants of the faster and slower motions of β-caseins in solution and compared them with those of some proteins in various states (folded or hydrated powder).

Unveiling the structure of the primary caseinate particle using small-angle X-ray scattering and simulation methodologies

The low-resolution structure of casein (CN) clusters in sodium caseinate (NaCas) solution and its conformational dynamics were obtained by size-exclusion chromatography (SEC), analytical ultracentrifugation (AUC), small-angle X-ray scattering (SAXS), and molecular dynamics (MD) simulations. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and native PAGE revealed that the casein clusters consisted predominantly of α- and β-CN complexes, and a trace amount of κ-CN. The AUC analysis indicated that the casein clusters were composed of 34.6% of casein monomers, 19.2%, 20.4%, and 25.8% of complexes with molar weight (M_w) of ~50.3, ~70.6, and ~133 kDa, respectively. The volume fractions of components in casein clusters were quantified as 64.3% of α_s1-β-α_s2-CN, 22.3% of α_s1-CN, 8.5% of α_s2-CN, and 4.4% of α_s1-α_s2-CN, respectively. The ensemble optimization method (EOM) gave a fitting result where α_s1-β-α_s2-CN species coexisted in ~35.3% under compact conformation and ~64.7% in elongated conformation in solution. The three-dimensional structures of α_s1-β-α_s2-CN from EOM showed a good overlay on the casein clusters ab initio model obtained from DAMMIN and DAMMIX program. MD simulations revealed that α_s1-β-α_s2-CN underwent a conformational change from the elongated state into the compact state within the initial 200 ns of simulations. The addition of nonionic surfactants affected little the backbone-to-backbone interactions in the formation of the casein clusters. We propose that α_s1-CN, β-CN, α_s2-CN, and κ-CN associated in consecutive steps into casein clusters, and a trace of κ-CN may be located at the surface of the assemblies limiting the growth of casein aggregates.

Casein Micelles as an Emerging Delivery System for Bioactive Food Components

Bioactive food components have potential health benefits but are highly susceptible for degradation under adverse conditions such as light, pH, temperature and oxygen. Furthermore, they are known to have poor solubilities, low stabilities and low bioavailabilities in the gastrointestinal tract. Hence, technologies that can retain, protect and enable their targeted delivery are significant to the food industry. Amongst these, microencapsulation of bioactives has emerged as a promising technology. The present review evaluates the potential use of casein micelles (CMs) as a bioactive delivery system. The review discusses in depth how physicochemical and techno-functional properties of CMs can be modified by secondary processing parameters in making them a choice for the delivery of food bioactives in functional foods. CMs are an assembly of four types of caseins, (α_s1, α_s2, β and κ casein) with calcium phosphate. They possess hydrophobic and hydrophilic properties that make them ideal for encapsulation of food bioactives. In addition, CMs have a self-assembling nature to incorporate bioactives, remarkable surface activity to stabilise emulsions and the ability to bind hydrophobic components when heated. Moreover, CMs can act as natural hydrogels to encapsulate minerals, bind with polymers to form nano capsules and possess pH swelling behaviour for targeted and controlled release of bioactives in the GI tract. Although numerous novel advancements of employing CMs as an effective delivery have been reported in recent years, more comprehensive studies are required to increase the understanding of how variation in structural properties of CMs be utilised to deliver bioactives with different physical, chemical and structural properties.

Effect of Pre-Heating Prior to Low Temperature 0.1 µm-Microfiltration of Milk on Casein-Whey Protein Fractionation

During skim milk microfiltration (nominal pore size of 0.1 µm) at 10 °C, the whey protein purity in the permeate is reduced by an enhanced serum casein permeation, primarily of β-casein. To decrease casein permeation, the possibility of a pre-heating step under pasteurization conditions before the filtration step was investigated, so as to shift the equilibrium from soluble serum casein monomers to impermeable micellar casein. Immediately after the pre-heating step, low temperature microfiltration at 10 °C was conducted before the casein monomers could diffuse into the serum. The hypothesis was that the dissociation of β-casein into the serum as a result of a decreasing temperature takes more time than the duration of the microfiltration process. It was found that pre-heating reduced the β-casein permeation during microfiltration without significantly affecting the flux and whey protein permeation, compared with a microfiltration at 10 °C without the pre-heating step. Furthermore, the addition of calcium (5 and 10 mM) not only reduced the casein permeation and thus increased the permeate purity, defined as a high whey protein-to-casein (g L^-1/g L^-1) ratio, but also decreased the filtration performance, possibly due to the structural alteration of the deposited casein micelle layer, rendering the deposit more compact and more retentive. Therefore, the possible combination of the addition of calcium and pre-heating prior to microfiltration was also investigated in order to evidence the potential increase of whey protein (WP) purity in the permeate in the case of Ca²⁺ addition prior to microfiltration. This study shows that pre-heating very close to low temperature microfiltration results in an increased purity of the whey protein fraction obtained in the permeate.

Effect of Temperature, Added Calcium and pH on the Equilibrium of Caseins between Micellar State and Milk Serum

Micellar casein and casein monomers in milk serum are in a dynamic equilibrium. At temperature below 15–20 °C a considerable amount of casein monomers, β-casein in particular, is released from the casein micelle into the aqueous serum phase. This study investigates the effects of added calcium and related variations of pH on this peculiar equilibrium in order to minimize the amount of caseins in the serum and to better understand the casein permeation during microfiltration. The pH was varied in the range of 6.3 to 7.3 and the content of calcium was increased up to 7.5 mM by adding CaCl₂. Upon equilibration, the milk was separated by ultracentrifugation and the amounts of protein in the supernatant were analyzed. It was shown that the addition of low amounts of calcium shifts the equilibrium towards the micellar casein phase and can, thus, lower the serum casein content induced at low temperatures. Relative to that, the adjustment of pH separately from the CaCl₂ addition had a minor effect on casein concentration and composition in the serum.

Revisiting the dynamics of proteins during milk powder hydration using asymmetric flow field-flow fractionation (AF4)

The dynamics of β-casein and casein micelles in the reconstitution of skim milk were revisited in this study. β-casein migrates into casein micelles upon an increase in temperatures due to an increase in the hydrophobic effect and lower calcium-phosphate cluster solubility. This process can be reversed upon cooling. These phenomena are well known in fresh milk and are not yet clearly established for reconstituted milk powder. As milk powder is commonly used as a functional ingredient in food products, it is of interest to investigate the migration of casein micelle β-casein to and from the serum phase in reconstituted milk. This study aimed to use asymmetrical flow field flow fractionation (AF4) in combination with various detectors to revisit the dynamics of β-casein when reconstituting skim milk at different temperatures. Fluorescence-labelled β-casein was added to fresh and reconstituted skim milk and rapid transport of β-casein into the outer shell of the casein micelles could be observed already after 5 ​min of reconstitution at 50 ​°C. This process stabilized after approximately 5 ​h, which indicates that an equilibrium of β-casein between the serum and the micellar phase was reached. Similar results were found for fresh milk. The apparent density of the casein micelles in the skim milk samples was also found to increase during reconstitution at 50 ​°C. During cold reconstitution of milk powders, the migration of β-casein to the serum was not observed. The results suggest that β-casein was already present in the serum phase upon reconstitution at 6 ​°C. When a sample was reconstituted for 180 ​min at 50 ​°C, the migration of β-casein back into the serum was observed upon cooling the same sample to 6 ​°C. The size of casein micelles in reconstituted milk at 6 ​°C was larger compared to reconstitution at 50 ​°C. With AF4 and the multi-detector approach, the change in concentration and size of casein micelles can be readily investigated and the migration of β-casein can be tracked simultaneously. Therefore, the method is a valuable tool for studies of the properties and changes in various milk samples.

Application of high-resolution ultrasonic spectroscopy for detection of the plasmin activity toward β-casein

High-resolution ultrasonic spectroscopy (HR-US) was applied for precise detection of plasmin activity towards β-casein in buffer at pH 7.8 and 37 °C. The evolution of ultrasonic velocity and ultrasonic attenuation measured at 15.5 MHz is related to the concentration of peptide bonds hydrolyzed and loss of β-casein aggregates, respectively. The ultrasonic assay presents sensitive and direct activity-based quantification of plasmin levels in milk. The variation in plasmin concentration between HR-US and ELISA method owed to the differing detection principles. The real-time ultrasonic profiles of hydrolysis were utilized to describe the kinetic aspect of plasmin activity. The non-linear activity curve was fitted with classic and inverse Michaelis-Menten type models. Within 1-8.6 mg·mL^-1 β-casein, the V_max and K_M obtained were (6.30 ± 2.21) × 10^-5 mol.kg^-1·min^-1 and 10.33 ± 3.50 mg·mL^-1, respectively. The maximum peptide bond cleaved was 5-6 (2.7% degree of hydrolysis) achieved at 1 mg·mL^-1 β-casein.

Effect of enzymatic cross-linking of naringenin-loaded β-casein micelles on their release properties and fate in in vitro digestion

The microbial transglutaminase (mTG) was used to improve the stability of the naringenin-loaded β-casein micelles (CNMs). The formation of cross-linked CNMs was confirmed by SDS-PAGE electrophoresis, showing a decrease in monomeric β-CN levels with increasing crosslinking time. Dynamic light scattering (DLS) showed that after crosslinking the particle size distribution did not change upon dilution, suggesting occurrence of intra-crosslinking. Fluorescence spectroscopy and circular dichroism (CD) showed that crosslinking induced only minor changes in the structure. Finally, release of naringenin in buffer at pH 7.4 demonstrated a slower release from the cross-linked micelles compared to the untreated micelles. In addition, the cross-linked micelles exhibited a partial resistance to pepsin enzyme. We conclude that crosslinking with mTG is a suitable method to modulate naringenin release kinetics from β-CN micelles and improves the potential of these micelles as delivery systems targeted to the small intestine.

Advancement of food-derived mixed protein systems: Interactions, aggregations, and functional properties

Recently, interests in binary protein systems have been developed considerably ascribed to the sustainability, environment-friendly, rich in nutrition, low cost, and tunable mechanical properties of these systems. However, the molecular coalition is challenged by the complex mechanisms of interaction, aggregation, gelation, and emulsifying of the mixed system in which another protein is introduced. To overcome these fundamental difficulties and better modulate the structural and functional properties of binary systems, efforts have been steered to gain basic information regarding the underlying dynamics, theories, and physicochemical characteristics of mixed systems. Therefore, the present review provides an overview of the current studies on the behaviors of proteins in such systems and highlights shortcomings and future challenges when applied in scientific fields.

Nature-Assembled Structures for Delivery of Bioactive Compounds and Their Potential in Functional Foods

Consumers are demanding more natural, healthy, and high-quality products. The addition of health-promoting substances, such as bioactive compounds, to foods can boost their therapeutic effect. However, the incorporation of bioactive substances into food products involves several technological challenges. They may have low solubility in water or poor stability in the food environment and/or during digestion, resulting in a loss of their therapeutic properties. Over recent years, the encapsulation of bioactive compounds into laboratory-engineered colloidal structures has been successful in overcoming some of these hurdles. However, several nature-assembled colloidal structures could be employed for this purpose and may offer many advantages over laboratory-engineered colloidal structures. For example, the casein micelles and milk fat globules from milk and the oil bodies from seeds were designed by nature to deliver biological material or for storage purposes. These biological functional properties make them good candidates for the encapsulation of bioactive compounds to aid in their addition into foods. This review discusses the structure and biological function of different nature-assembled carriers, preparation/isolation methods, some of the advantages and challenges in their use as bioactive compound delivery systems, and their behavior during digestion.

Bovine β-casein binding studies of a Schiff base ligand: fluorescence and circular dichroism approaches

In this study, a Schiff base derived from a heterocyclic moiety was synthesized and characterized. The in vitro binding behaviour of this ligand with β-casein (β-CN) was investigated using biophysical techniques. For evaluation, thermodynamics variables of interactions between the Schiff base ligand and β-CN, such as fluorescence at different temperatures, were measured. The results showed that the Schiff base ligand possessed considerable associated binding to β-CN and that the procedure was enthalpy driven. The β-CN conformation was also changed to give a further unfolded structure. Fluorescence resonance energy transfer was used to estimate the interval between donor (β-CN) and acceptor (Schiff base ligand). All these experimental results proposed that β-CN might act as carrier protein for the Schiff base ligand to deliver it to the target molecules.

Synthesis and Characterization of Cobalt(II) and Manganese(II) Schiff Base Complexes: Metal Effect on the Binding Affinity with β-Casein

In this study, two metal Schiff base complexes (cobalt(II) and manganese(II), were synthesized and characterized by ¹H-NMR and FT-IR analyses. The in vitro binding behavior of this complexes with β-Casein (β-CN) was investigated by using biophysical techniques. For evaluation the thermodynamics parameters of interaction between Schiff base complexes and β-CN,the fluorescence data at different temperatures were done. The results showed that the intrinsic fluorescence of the β-CN was quenched by increasing the complexes through the dynamic quenching mechanism. Also, these complexes demonstrated a considrable binding affinity to β-CN and the process is mainly entropy driven. Fluorescence resonance energy transfer was used in order to estimating the distance between donor (β-CN) and acceptor (complex). Results demonstrated that the sequence of tendency of the complexes to β-CN was as follows: Mn-Schiff base complex > Co-Schiff base complex. All these experimental results propose that β-CN might act as carrier protein for Co(II) and Mn (II) complexes to deliver it to the target molecules.

Application of high-resolution ultrasonic spectroscopy for real-time monitoring of trypsin activity in β-casein solution

High-resolution ultrasonic spectroscopy (HR-US) was applied for real-time monitoring of β-casein hydrolysis by trypsin at various conditions for the first time. The technique is based on the precision measurement of hydration changes proportional to the number of peptide bond hydrolyzed. As HR-US exhibits ultrasonic transparency for most solution, the analysis did not require optical transparency like for 2,4,6-trinitrobenzenesulfonic acid (TNBS) assay. Appropriate enzymatic models were fitted with degree of hydrolysis (d_h) profiles to provide kinetic and mechanistic description of proteolysis in terms of initial hydrolysis rate, r⁰, and rate constant of hydrolysis, k_h, and enzyme inactivation, k_d. Maximal r⁰ and d_h were obtained at 45 °C and pH 8. The exponential dependence of kinetic parameters allowed determination of the activation (E_A = 50.3 ± 7 kJ/mol) and deactivation (E_D = 62.23 ± 3 kJ/mol) energies of hydrolysis. The ultrasonic assay provided rapid detection of trypsin activity even at sub-nanomolar concentration.

A comparison of β-casein complexes and micelles as vehicles for trans-/cis-resveratrol

Bovine β-casein is an amphiphilic protein that exists as a monomer and self-organizes into micelles in aqueous solution. The protein has been used as natural vehicles for bioactives. Trans-resveratrol has received significant attention due to its vast health benefits and conversion to cis-isomer during processing and storage. However, cis-isomer has not yet gained as much attention as that of trans-isomer. In this study, the interaction of β-casein with trans- and cis-resveratrol was characterized. Trans-resveratrol exhibited a higher affinity for β-casein than cis-isomer, and β-casein could bind two isomers simultaneously to form protein-diligand complexes. Both trans- and cis-isomers could be encapsulated into β-casein micelles with encapsulation efficiencies of ~69% and ~57%, respectively. The β-casein micelles could delay photo-isomerization of trans-isomer to cis-isomer, while β-casein-ligand complex showed a better protective effect for both isomers during storage than β-casein micelles. These results might be useful for the development of protein-based carriers for the polyphenols.

Small-Angle Neutron Scattering Study of High Fat Fish Oil-In-Water Emulsion Stabilized with Sodium Caseinate and Phosphatidylcholine

We report on small-angle neutron scattering (SANS) investigations of separate phase domains in high fat (70%) oil-in-water emulsions emulsified with the combination of sodium caseinate (CAS) and phosphatidylcholine (PC). The emulsion as a whole was studied by contrast variation to identify scattering components dominated by individual emulsifiers. The emulsion was subsequently separated into the aqueous phase and the oil-rich droplet phase, which were characterized separately. Emulsions produced with 1.05% (w/w) CAS and PC fraction which varies between 1.75% (w/w) and 0.35% (w/w) provided droplets between 10 and 19 μm in surface weighted mean in 70% fish oil-in-water emulsions. At least two-third of the overall CAS is associated with the interface, while the rest remains with the aqueous phase. Six percent of PC formed a monolayer in the interface, while the rest of the PC remains in the droplet phase in the form of multilayers. When the separated components were resuspended, the resuspended emulsion showed similar characteristics compared to the original emulsion in terms of droplet size distribution and neutron scattering. Instead, CAS in the aqueous phase separated from the emulsion shows aggregation not present in the corresponding CAS-in-D₂O system.

(Homo)polymer-mediated colloidal stability of micellar solutions

Despite their wide range of applications, there is a remarkable lack of fundamental understanding about how micelles respond to other components in solution. The colloidal stability of micellar solutions in presence of (homo)polymers is investigated here following a theoretical bottom-up approach. A polymer-mediated micelle-micelle interaction is extracted from changes in the micelle-unimer equilibrium as a function of the inter-micelle distance. The homopolymer-mediated diblock copolymer micelle-micelle interaction is studied both for depletion and adsorption of the homopolymer. The fluffy nature of the solvophilic domain (corona) of the micelle weakens the depletion-induced destabilization. Accumulation of polymers into the corona induces bridging attraction between micelles. In fact, both depletion and adsorption phenomena are regulated by the coronal thickness relative to the size of the added polymer. Penetration of guest compounds into the coronal domain of crew-cut micelles, with a narrower yet denser corona, is less pronounced as for starlike micelles (with a more diffuse corona). Therefore, crew-cut micelles are less sensitive to the effect of added compounds, and hence more suitable for applications in multicomponent systems, such as industrial formulations or biological fluids. The trends observed for the colloidal stability of crew-cut micelles qualitatively match with our experimental observations on aqueous dispersions of polycaprolactone-polyethylene glycol (PCL-PEO) micellar suspensions with added PEO chains.

Different motilities of microtubules driven by kinesin-1 and kinesin-14 motors patterned on nanopillars

Kinesin is a motor protein that plays important roles in a variety of cellular functions. In vivo, multiple kinesin molecules are bound to cargo and work as a team to produce larger forces or higher speeds than a single kinesin. However, the coordination of kinesins remains poorly understood because of the experimental difficulty in controlling the number and arrangement of kinesins, which are considered to affect their coordination. Here, we report that both the number and spacing significantly influence the velocity of microtubules driven by nonprocessive kinesin-14 (Ncd), whereas neither the number nor the spacing changes the velocity in the case of highly processive kinesin-1. This result was realized by the optimum nanopatterning method of kinesins that enables immobilization of a single kinesin on a nanopillar. Our proposed method enables us to study the individual effects of the number and spacing of motors on the collective dynamics of multiple motors.

The influence of temperature- and divalent-cation-mediated aggregation of β-casein on the physical and microstructural properties of β-casein-stabilised emulsions

The objective of this study was to assess the influence of self-association of β-casein (β-CN) induced by both increasing temperature (5-55 °C) and divalent cation addition (Ca²⁺ or Mg²⁺) on the properties of β-CN-stabilised emulsions. The particle size of 0.5% (w/w) β-CN in 10 mM imidazole/HCl buffer (pH 6.8) was determined as a function of temperature and addition of divalent cations. Addition of CaCl₂ caused a greater increase in protein particle size than MgCl₂. Oil-in-water emulsions stabilised with 0.5% (w/w) β-CN, β-CN with added CaCl₂ or MgCl₂ (β-CN/Ca and β-CN/Mg, respectively) were also investigated as a function of temperature using light scattering, analytical centrifugation, rheology and confocal laser scanning microscopy (CLSM). Emulsions prepared with β-CN/Ca flocculated after incubation at 55 °C for 20 min and displayed significantly different physical properties (p < 0.05) compared to emulsions stabilised with β-CN or β-CN/Mg in the temperature range 5-55 °C. Based on CLSM analysis and analysis of the interfacial protein load, this flocculation was attributed to the interaction of adsorbed β-CN between droplets and the interaction of adsorbed and non-adsorbed β-CN aggregates in the aqueous phase via calcium bridges. Furthermore, the flocculation of β-CN/Ca emulsions was reversible upon cooling, which is similar to that of β-CN/Ca in solution. In conclusion, the temperature-dependent behaviour of β-CN-stabilised emulsions correlated to the temperature-induced aggregation of β-CN, particularly in the presence of Ca²⁺. Hence, the stability of β-CN-stabilised emulsions can be predicted from the extent of β-CN aggregation in aqueous solution (i.e., aggregate size).