Size Separation Techniques for the Characterisation of Cross-Linked Casein: A Review of Methods and Their Applications

Structures and interactions forming stable shellac-casein nanocomplexes with a pH-cycle

Casein forms diverse structures with functionalities tunable by complexation with surfactants, and shellac is an emerging surfactant. In the present work, molecular and mesoscopic structures of shellac and micellar casein and the underlying interactions after treatment with a pH-cycle were investigated. Dispersions with 0.5 % w/v shellac and various shellac:casein mass ratios were prepared at pH 12.0 to dissolve shellac and dissociate casein micelles, followed by neutralization to pH 7.0 to form complexes. Both covalent and non-covalent (hydrogen bonding, electrostatic, and hydrophobic) interactions contributed to the complex formation. The formed complexes had an average diameter of ~80 nm. The complexation of shellac and casein prevented the precipitation of protonated shellac during neutralization, and dispersions with casein:shellac mass ratios of 2:1 and above were absent of precipitates at pH 7.0. The formed nanocomplexes may have applications for preparing novel colloidal systems and loading lipophilic bioactive compounds.

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.

Transglutaminase in Foods and Biotechnology

Stabilization and reusability of enzyme transglutaminase (TGM) are important goals for the enzymatic process since immobilizing TGM plays an important role in different technologies and industries. TGM can be used in many applications. In the food industry, it plays a role as a protein-modifying enzyme, while, in biotechnology and pharmaceutical applications, it is used in mediated bioconjugation due to its extraordinary crosslinking ability. TGMs (EC 2.3.2.13) are enzymes that catalyze the formation of a covalent bond between a free amino group of protein-bound or peptide-bound lysine, which acts as an acyl acceptor, and the γ-carboxamide group of protein-bound or peptide-bound glutamine, which acts as an acyl donor. This results in the modification of proteins through either intramolecular or intermolecular crosslinking, which improves the use of the respective proteins significantly.

Unaided efficient transglutaminase cross-linking of whey proteins strongly impacts the formation and structure of protein alginate particles

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Microbial transglutaminase (MTG) cross-linked >70% β-lactoglobulin (β-Lg) at pH 8.5.

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Initial MTG catalyzed isopeptide bond formation caused partial unfolding of β-Lg.

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>75% of whey protein cross-linked, forming hetero-polymers containing β-Lg.

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50% less alginate is needed to form particles with cross-linked than with native β-Lg.

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Cross-linked β-Lg and alginate formed suspendable hydrophobically driven particles.

There is a dogma within whey protein modification, which dictates the necessity of pretreatment to enzymatic cross-linking of β-lactoglobulin (β-Lg). Here microbial transglutaminase (MTG) cross-linked whey proteins and β-Lg effectively in 50 mM NaHCO₃, pH 8.5, without pretreatment. Cross-linked β-Lg spanned 18 to >240 kDa, where 6 of 9 glutamines reacted with 8 of 15 lysines. The initial isopeptide bond formation caused loss of β-Lg native structure with t_1/2 = 3 h, while the polymerization occurred with t_1/2 = 10 h. Further, cross-linking effects on protein carbohydrate interaction have been overlooked, leaving a gap in understanding of these complex food matrices. Complexation with alginate showed that β-Lg cross-linking decreased onset of particle formation, hydrodynamic diameter, stoichiometry (β-Lg/alginate) and dissociation constant. The complexation was favored at higher temperatures (40 °C), suggesting that hydrophobic interactions were important. Thus, β-Lg was cross-linked without pretreatment and the resulting polymers gave rise to altered complexation with alginate.

Focus on the Protein Fraction of Sports Nutrition Supplements

Increasing awareness of balanced diet benefits is boosting the demand for high-protein food and beverages. Sports supplements are often preferred over traditional protein sources to meet the appropriate dietary intake since they are widely available on the market as stable ready-to-eat products. However, the protein components may vary depending on both sources and processing conditions. The protein fraction of five commercial sports supplements was characterized and compared with that of typical industrial ingredients, i.e., whey protein concentrates and isolates and whey powder. The capillary electrophoresis profiles and the amino acid patterns indicated that, in some cases, the protein was extensively glycosylated and the supplemented amino acids did not correspond to those declared on the label by manufacturers. The evaluation by confocal laser scanning microscopy evidenced the presence of large aggregates mainly enforced by covalent crosslinks. The obtained findings suggest that, beside composition figures, provisions regarding sports supplements should also consider quality aspects, and mandatory batch testing of these products would provide more reliable information to sport dieticians.

Partial Purification and Characterization of Exo-Polygalacturonase Produced by Penicillium oxalicum AUMC 4153

Pectinase enzymes are important industrial enzymes having considerable applications in several industries, especially in food processing. Pectinases contribute 25% of global food enzyme sales. Therefore, the demand for a commercial enzyme with desirable characteristics and low production costs has become one of the great targets. Hence, this study aims to produce exo-polygalacturonase (exo-PG) using local fungal isolate Penicillium oxalicum AUMC 4153 by utilizing sugar beet manufacturing waste (sugar beet pulp) as a sole raw carbon source under shaken submerged fermentation, which is purified and characterized to optimize enzyme biochemical properties for industrial application. The purity of the obtained exo-PG was increased by about 28-fold, and the final enzyme yield was 57%. The partially purified enzyme was active at a broad range of temperatures (30–60 °C). The optimum temperature and pH for the purified exo-PG activity were 50 °C and pH 5. The enzyme was stable at a range of pH 3 to 6 and temperature 30–50 °C for 210 min. The values for K_m and V_max were 0.67 mg/mL, with polygalacturonic acid as substrate and 6.13 µmole galacturonic acid/min/mg protein, respectively. It can be concluded that purified exo-PG production by P. oxalicum grown on sugar beet waste is a promising effective method for useful applications.

Hemp globulin forms colloidal nanocomplexes with sodium caseinate during pH-cycling

Seed from industrial hemp (Cannabis sativa L.) contains around 25% protein (mainly globulins) which is easily digested, but the low solubility of hemp globulins (HG) limits their application in many food systems. In this study, the solubility of HG was improved by blending HG with sodium caseinate (SC) and treating with a pH-cycling process. The pH-cycling involved adjusting the pH to 12 and reacting for 1 hr, followed by neutralisation to pH 7. Nanoparticles composed of HG and SC (Z-average diameter ≈ 130 nm) were formed after the pH-cycling, and the solubility of HG increased to > 80% when there was more than 1% of SC for 1% of HG. These HG|SC nanoparticles were monodisperse (PDI < 0.17) and ζ-potential was ≈ -17 mV. Hydrogen bonding is the main forces that assembles HG|SC nanoparticles because the nanoparticles dissociated by heat treatment (up to 60 °C) or urea, which is an effective hydrogen bond breaker. HG|SC nanoparticles will aggregate irreversibly above 60 °C, possibly due to thiol-disulphide exchange. The nanoparticles were heat-stable as the Z-average diameter was only 229 nm after heating (90 °C, 30 min). N-ethylmaleimide blocked free thiol groups on HG and resulted in less disulphide-linked HG aggregation after pH- cycling, which in turn lead to smaller HG|SC nanoparticles and a bimodal particle size distribution, indicating the importance of disulphide bond for the formation of monodisperse HG|SC nanoparticles. The soluble and heat-stable HG|SC nanoparticles could be used to increase the hemp protein content in beverages and emulsions.

Size Modulation of Enzymatically Cross-Linked Sodium Caseinate Nanoparticles via Ionic Strength Variation Affects the Properties of Acid-Induced Gels

Enzymatic cross-linking by microbial transglutaminase is a prominent approach to modify the structure and techno-functional properties of food proteins such as casein. However, some of the factors that influence structure-function-interrelations are still unknown. In this study, the size of cross-linked sodium caseinate nanoparticles was modulated by varying the ionic milieu during incubation with the enzyme. As was revealed by size exclusion chromatography, cross-linking at higher ionic strength resulted in larger casein particles. These formed acid-induced gels with higher stiffness and lower susceptibility to forced syneresis compared to those where the same number of ions was added after the cross-linking process. The results show that variations of the ionic milieu during enzymatic cross-linking of casein can be helpful to obtain specific modifications of its molecular structure and certain techno-functional properties. Such knowledge is crucial for the design of protein ingredients with targeted structure and techno-functionality.

Bean phenolic compound changes during processing: Chemical interactions and identification

The common bean (Phaseolus vulgaris L.) represents one of the main crops for human consumption, due to its nutritional and functional qualities. Phenolic compounds have beneficial health effects, and beans are an essential source of these molecules, being found mainly in the seed coat and its color depends on the concentration and type of phenolic compounds present. The bean during storage and processing, such as cooking, germination, extrusion, and fermentation, undergoes physical, chemical, and structural changes that affect the bioavailability of its nutrients; these changes are related to the interactions between phenolic compounds and other components of the food matrix. This review provides information about the identification and quantification of phenolic compounds present in beans and the changes they undergo during processing. It also includes information on the interactions between the phenolic compounds and the components of the bean's cell wall and the analytical methods used to identify the interactions of phenolic compounds with macromolecules.

A highly selective two-way purification method using liquid chromatography for isolating αS2-casein from goat milk of five different breeds

The main challenges in the purification of α_S2-casein are due to the low quantity in milk and high homology with other casein subunits, i.e., α_S1-casein, β-casein, and κ-casein. To overcome these challenges, the aim of this study was to develop a two-step purification to isolate native α_S2-casein in goat milk from five different breeds; British Alpine, Jamnapari, Saanen, Shami, and Toggenburg. The first step of the purification was executed by anion-exchange chromatography under optimal elution conditions followed by size exclusion chromatography. Tryptic peptides from in-gel digestion of purified α_S2-casein were sequenced and analyzed by LC-ESI-MS/MS. From 1.05 g of whole casein, the highest yield of α_S2-casein (6.7 mg/mL) was obtained from Jamnapari and the lowest yield (2.2 mg/mL) was from Saanen. A single band of pure α_S2-casein was observed on SDS-PAGE for all breeds. The α_S2-casein showed coverage percentage of amino acid sequence from 76.68 to 92.83%. The two-step purification process developed herein was successfully applied for isolating native α_S2-casein from goat milk with high purity, which will allow for future in vitro studies to be conducted on this protein.

Efficient protein extraction for shotgun proteomics from hydrated and desiccated leaves of resurrection Ramonda serbica plants

Resurrection plant Ramonda serbica is a suitable model to investigate vegetative desiccation tolerance. However, the detailed study of these mechanisms at the protein level is hampered by the severe tissue water loss, high amount of phenolics and polysaccharide, and possible protein modifications and aggregations during the extraction and purification steps. When applied to R. serbica leaves, widely used protein extraction protocols containing polyvinylpolypyrrolidone and ascorbate, as well as the phenol/SDS/buffer-based protocol recommended for recalcitrant plant tissues failed to eliminate persistent contamination and ensure high protein quality. Here we compared three protein extraction approaches aiming to establish the optimal one for both hydrated and desiccated R. serbica leaves. To evaluate the efficacy of these protocols by shotgun proteomics, we also created the first R. serbica annotated transcriptome database, available at http://www.biomed.unipd.it/filearrigoni/Trinity_Sample_RT2.fasta . The detergent-free phenol-based extraction combined with dodecyl-β-D-maltoside-assisted extraction enabled high-yield and high-purity protein extracts. The phenol-based protocol improved the protein-band resolution, band number, and intensity upon electrophoresis, and increased the protein yield and the number of identified peptides and protein groups by LC-MS/MS. Additionally, dodecyl-β-D-maltoside enabled solubilisation and identification of more membrane-associated proteins. The presented study paves the way for investigating the desiccation tolerance in R. serbica, and we recommend this protocol for similar recalcitrant plant material.

Production of Liquid Milk Protein Concentrate with Antioxidant Capacity, Angiotensin Converting Enzyme Inhibitory Activity, Antibacterial Activity, and Hypoallergenic Property by Membrane Filtration and Enzymatic Modification of Proteins

Liquid milk protein concentrate with different beneficial values was prepared by membrane filtration and enzymatic modification of proteins in a sequential way. In the first step, milk protein concentrate was produced from ultra-heat-treated skimmed milk by removing milk serum as permeate. A tubular ceramic-made membrane with filtration area 5 × 10−3 m2 and pore size 5 nm, placed in a cross-flow membrane house, was adopted. Superior operational strategy in filtration process was herein: trans-membrane pressure 3 bar, retention flow rate 100 L·h−1, and implementation of a static turbulence promoter within the tubular membrane. Milk with concentrated proteins from retentate side was treated with the different concentrations of trypsin, ranging from 0.008–0.064 g·L−1 in individual batch-mode operations at temperature 40 °C for 10 min. Subsequently, inactivation of trypsin in reaction was done at a temperature of 70 °C for 30 min of incubation. Antioxidant capacity in enzyme-treated liquid milk protein concentrate was measured with the Ferric reducing ability of plasma assay. The reduction of angiotensin converting enzyme activity by enzyme-treated liquid milk protein concentrate was measured with substrate (Abz-FRK(Dnp)-P) and recombinant angiotensin converting enzyme. The antibacterial activity of enzyme-treated liquid milk protein concentrate towards Bacillus cereus and Staphylococcus aureus was tested. Antioxidant capacity, anti-angiotensin converting enzyme activity, and antibacterial activity were increased with the increase of trypsin concentration in proteolytic reaction. Immune-reactive proteins in enzyme-treated liquid milk protein concentrate were identified with clinically proved milk positive pooled human serum and peroxidase-labelled anti-human Immunoglobulin E. The reduction of allergenicity in milk protein concentrate was enzyme dose-dependent.

Optimization of Protein Extraction Method for 2DE Proteomics of Goat's Milk

Two-dimensional electrophoretic (2DE)-based proteomics remains a powerful tool for allergenomic analysis of goat’s milk but requires effective extraction of proteins to accurately profile the overall causative allergens. However, there are several current issues with goat’s milk allergenomic analysis, and among these are the absence of established standardized extraction method for goat’s milk proteomes and the complexity of goat’s milk matrix that may hamper the efficacy of protein extraction. This study aimed to evaluate the efficacies of three different protein extraction methods, qualitatively and quantitatively, for the 2DE-proteomics, using milk from two commercial dairy goats in Malaysia, Saanen, and Jamnapari. Goat’s milk samples from both breeds were extracted by using three different methods: a milk dilution in urea/thiourea based buffer (Method A), a triphasic separation protocol in methanol/chloroform solution (Method B), and a dilution in sulfite-based buffer (Method C). The efficacies of the extraction methods were assessed further by performing the protein concentration assay and 1D and 2D SDS-PAGE profiling, as well as identifying proteins by MALDI-TOF/TOF MS/MS. The results showed that method A recovered the highest amount of proteins (72.68% for Saanen and 71.25% for Jamnapari) and produced the highest number of protein spots (199 ± 16.1 and 267 ± 10.6 total spots for Saanen and Jamnapari, respectively) with superior gel resolution and minimal streaking. Six milk protein spots from both breeds were identified based on the positive peptide mass fingerprinting matches with ruminant milk proteins from public databases, using the Mascot software. These results attest to the fitness of the optimized protein extraction protocol, method A, for 2DE proteomic and future allergenomic analysis of the goat’s milk.

Preparation and potential applications of casein-polysaccharide conjugates: a review

Glycation of casein and caseinates with polysaccharides via Maillard reaction is a simple and environmentally safe way to prepare new food ingredients of improved functional properties. Sodium caseinate has been used mainly to prepare conjugates with several polysaccharides particularly maltodextrins and dextrans. The functional properties of these conjugates are influenced by the used polysaccharides and heating conditions. Under optimal heating conditions substantial improvements have been evident in their emulsification and foam properties of these conjugates. Casein-polysaccharide conjugates have several potential applications in food processing and microencapsulation. This article gives an overview on their formation and potential uses. © 2019 Society of Chemical Industry.

Association of Enzymatically and Nonenzymatically Functionalized Caseins Analyzed by Size-Exclusion Chromatography and Light-Scattering Techniques

The influence of covalent protein modifications resulting from the Maillard reaction (glycation) of casein and lactose on the noncovalent association behavior of the protein was studied. Nonenzymatic cross-linking with methylglyoxal (MGO) and glutaraldehyde (GTA) as well as enzymatic cross-linking with microbial transglutaminase (mTG) was investigated in comparison. Molar mass, particle size, and conformational characteristics of nonmicellar casein associates as well as the extent of intraparticle protein cross-linking were examined utilizing size-exclusion chromatography (SEC) combined with UV detection and static and dynamic light scattering. Cross-linking resulted in the stabilization of a certain fraction of casein associates, with particle sizes of approximately 30 nm in radius of gyration (R_g), and promoted an incorporation of further casein molecules into those particles, yielding molar masses (M_w) of 1.0-1.2 × 10⁶ g/mol. When caseins were additionally conjugated with lactose during the early Maillard reaction, a further growth of the associates up to approximately 50 nm in R_g with a M_w of 2.1 × 10⁶ g/mol was observed. Furthermore, glycation reactions induced a transition from slightly elongated, random-coil structures toward more anisotropic conformations. Associates consisting of caseins cross-linked with GTA appeared to preserve the original particle conformation.

Formation of Stable Supramolecular Structure with β-Lactoglobulin-Derived Self-Assembling Peptide f1-8 and Bovine Micellar Caseins

The development of stable macromolecular structures with tailored functional properties in the dairy industry using innovative stabilizers is of great interest. The self-assembling peptide f1-8 (Pf1-8) derived from β-lactoglobulin was found to interact with whey proteins, consequently changing their physicochemical properties. The objective of the present work was to evaluate the interaction between Pf1-8 and micellar casein (CN) and the changes in their physicochemical properties and stability at different pH values (6.6-2.6) on model solutions containing CN and Pf1-8 at various ratios (1:1, 5:1, and 10:1) using spectrofluorimetry, TEM, SEC-HPLC, and SDS-PAGE analyses. No CN precipitation occurred for the solution at the 1:1 ratio even at pH values below 4.6. In all samples, CN was completely dissociated to primary casein particles (PCP) to form stable supramolecular structures strongly bound to peptide gels via hydrophobic interactions. Thus, a novel milk-protein-derived peptide responsible for stabilizing complex structures composed of CN was discovered.

Acid-Induced Gelation of Caseins Glycated with Lactose: Impact of Maillard Reaction-Based Glycoconjugation and Protein Cross-Linking

During food processing or storage, milk proteins can react with reducing sugars via the Maillard reaction (glycation), which may alter their techno-functional properties. The aim of this study was to investigate the relationship between molecular changes of casein occurring during different stages of the Maillard reaction and its acid-induced gelling properties. Therefore, sodium caseinate was heated in a dry state at 60 °C in the presence of lactose and analyzed for structural modifications by determining Amadori compounds (glycoconjugation) indirectly as furosine, the total lysine modification, and the extent of protein cross-linking. For techno-functional characterization, acid-induced gels were prepared by the addition of glucono-δ-lactone and evaluated by measuring pH kinetics during gel formation, gel strength, and water holding capacity. The time to reach pH 4.6 during the gelation process was significantly delayed with increasing extent of the Maillard reaction. Glycation with lactose also led to a significant increase in gel strength and water holding capacity. The increase in gel stability was rather independent from the amount of sugars covalently bound to the proteins during the early phase of the Maillard reaction but strongly correlated to the degree of protein polymerization. Small- and medium-sized casein oligomers, formed during advanced stages of the Maillard reaction, contributed considerably to the formation of stronger gels with higher water holding capacity, whereas a sharp increase in the relative amount of the polymer fraction observed during prolonged cross-linking processes caused a spontaneous destabilization of the gel network. Knowledge about structure-function relationships on a molecular level can provide useful information to control food texture by raw material quality.