Effects of caseins on thermal stability of bovine beta-lactoglobulin

No Glycation Required: Interference of Casein in AGE Receptor Binding Tests

For the determination of the binding of heated cow’s milk whey proteins such as β-lactoglobulin to the receptors expressed on immune cells, inhibition ELISA with the soluble form of the receptor for advanced glycation end products (sRAGE) and scavenger receptor class B (CD36) has been successfully used in the past. However, binding to heated and glycated caseins in this read-out system has not been tested. In this study, inhibition ELISA was applied to measure the binding of cow’s milk casein alone, as well as all milk proteins together, which underwent differential heat treatment, to sRAGE and CD36, and we compared those results to a dot blot read out. Moreover, binding to sRAGE and CD36 of differentially heated milk protein was measured before and after in vitro digestion. Casein showed binding to sRAGE and CD36, independent from the heat treatment, in ELISA, while the dot blot showed only binding to high-temperature-heated milk protein, indicating that the binding is not related to processing but to the physicochemical characteristics of the casein. This binding decreased after passage of casein through the intestinal phase.

Ready-to-drink protein beverages: Effects of milk protein concentration and type on flavor

This study evaluated the role of protein concentration and milk protein ingredient [serum protein isolate (SPI), micellar casein concentrate (MCC), or milk protein concentrate (MPC)] on sensory properties of vanilla ready-to-drink (RTD) protein beverages. The RTD beverages were manufactured from 5 different liquid milk protein blends: 100% MCC, 100% MPC, 18:82 SPI:MCC, 50:50 SPI:MCC, and 50:50 SPI:MPC, at 2 different protein concentrations: 6.3% and 10.5% (wt/wt) protein (15 or 25 g of protein per 237 mL) with 0.5% (wt/wt) fat and 0.7% (wt/wt) lactose. Dipotassium phosphate, carrageenan, cellulose gum, sucralose, and vanilla flavor were included. Blended beverages were preheated to 60°C, homogenized (20.7 MPa), and cooled to 8°C. The beverages were then preheated to 90°C and ultrapasteurized (141°C, 3 s) by direct steam injection followed by vacuum cooling to 86°C and homogenized again (17.2 MPa first stage, 3.5 MPa second stage). Beverages were cooled to 8°C, filled into sanitized bottles, and stored at 4°C. Initial testing of RTD beverages included proximate analyses and aerobic plate count and coliform count. Volatile sulfur compounds and sensory properties were evaluated through 8-wk storage at 4°C. Astringency and sensory viscosity were higher and vanillin flavor was lower in beverages containing 10.5% protein compared with 6.3% protein, and sulfur/eggy flavor, astringency, and viscosity were higher, and sweet aromatic/vanillin flavor was lower in beverages with higher serum protein as a percentage of true protein within each protein content. Volatile compound analysis of headspace vanillin and sulfur compounds was consistent with sensory results: beverages with 50% serum protein as a percentage of true protein and 10.5% protein had the highest concentrations of sulfur volatiles and lower vanillin compared with other beverages. Sulfur volatiles and vanillin, as well as sulfur/eggy and sweet aromatic/vanillin flavors, decreased in all beverages with storage time. These results will enable manufacturers to select or optimize protein blends to better formulate RTD beverages to provide consumers with a protein beverage with high protein content and desired flavor and functional properties.

Weighted gene co-expression network analysis identifies modules and functionally enriched pathways in the lactation process

The exponential growth in knowledge has resulted in a better understanding of the lactation process in a wide variety of animals. However, the underlying genetic mechanisms are not yet clearly known. In order to identify the mechanisms involved in the lactation process, various mehods, including meta-analysis, weighted gene co-express network analysis (WGCNA), hub genes identification, gene ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment at before peak (BP), peak (P), and after peak (AP) stages of the lactation processes have been employed. A total of 104, 85, and 26 differentially expressed genes were identified based on PB vs. P, BP vs. AP, and P vs. AP comparisons, respectively. GO and KEGG pathway enrichment analysis revealed that DEGs were significantly enriched in the "ubiquitin-dependent ERAD" and the "chaperone cofactor-dependent protein refolding" in BP vs. P and P vs. P, respectively. WGCNA identified five significant functional modules related to the lactation process. Moreover, GJA1, AP2A2, and NPAS3 were defined as hub genes in the identified modules, highlighting the importance of their regulatory impacts on the lactation process. The findings of this study provide new insights into the complex regulatory networks of the lactation process at three distinct stages, while suggesting several candidate genes that may be useful for future animal breeding programs. Furthermore, this study supports the notion that in combination with a meta-analysis, the WGCNA represents an opportunity to achieve a higher resolution analysis that can better predict the most important functional genes that might provide a more robust bio-signature for phenotypic traits, thus providing more suitable biomarker candidates for future studies.

Casein as a Modifier of Whey Protein Isolate Gel: Sensory Texture and Rheological Properties

The objective of this study was to determine if casein could be used to adjust the structure of whey protein gels and alter targeted textural properties. Secondarily, we sought to determine if specific structural and mechanical properties were associated with sensory texture terms. Heat set gels were made from whey proteins alone or combined with casein in micellar or dispersed form at pH 6.0 and 5.5. Replacing the whey protein with casein produced a gel breakdown pattern that was more cohesive during mastication with increased moisture retention. Additionally, casein addition reduced gel strength but minimally altered recoverable energy (an indicator of elasticity). Structural breakdown patterns were shifted from brittle- to ductile-like fracture for gels containing dispersed casein at pH 5.5 or micellar casein at pH 6.0. Shifts in microstructure observed by confocal microscopy could not explain the changes in mechanical or sensory textures. The differentiating sensory attributes among treatments were adhesiveness, cohesiveness of mass, tackiness, firmness, fracturability, and deformability. Most notably, adding casein increased cohesiveness while maintaining water holding properties. Sensory texture properties could be explained by a combination of macroscopic structural changes (appearance), fracture properties, and postfracture breakdown pattern. Overall, it was demonstrated that casein can be used to alter whey protein gel structure such that sensory firmness and fracturability are decreased and cohesiveness is increased, while preventing a large increase in moisture release. PRACTICAL APPLICATION: There is a current desire to use alternative sources of protein in a variety of food applications, which requires the ability to design food structures with specific textural properties. Whey protein gels were used as a model soft solid structure with textural attributes of low cohesiveness and water release, and high firmness and fracturability. It was shown that adding casein modified the structure such that cohesiveness increased, firmness and fracturability decreased, and water holding ability was maintained. Using a second source of protein to modify a primary protein network appears to be a viable way to adjust textural properties.

Colloidal properties of protein complexes formed in β-casein concentrate solutions as influenced by heating and cooling in the presence of different solutes

Monomeric bovine β-casein self-associates into micelles under appropriate conditions of protein concentration, serum composition and temperature. The present study investigated self-association characteristics of a β-casein concentrate (BCC) prepared from milk at pilot-scale using membrane filtration. The BCC had a casein:whey protein ratio of 77:23, with ∼95% of casein consisting of β-casein, and the remainder being mostly κ-CN. BCC was reconstituted to 1.2% protein (a typical level in infant formula) in various liquid media at pH 6.8 and incubated at different temperatures from 4 to 63 °C for 30 min. Self-association of β-casein on heating was thermo-reversible in deionised water, lactose (4, 6 or 8%) or calcium (9 mM) solutions. In most serum phases, BCC became highly opaque after incubation at 63 °C, but clarified rapidly during cooling to 25 °C. However, in simulated milk ultrafiltrate (SMUF), which has a high ionic strength and is supersaturated in calcium phosphate (CaP), BCC remained opaque during cooling to 25 °C, and retained residual turbidity after 15 h of holding at 4 °C; if SMUF was prepared without phosphate then turbidity development in BCC solutions was markedly reduced. The complexes responsible for this turbidity development were successfully dissociated with 50 mM trisodium citrate. Analysis of pH during heating and holding at 60 °C indicated that SMUF acidified continuously under the period of study, while acidification in BCC/SMUF mixtures terminated after a short period, indicating that the type of CaP formed on heating is altered in the presence of BCC. This study demonstrates that BCC ingredients exhibit pronounced temperature-dependant changes in colloidal properties that are strongly affected by the presence of minerals commonly found in nutritional product formulations.

Chaperone-like food components: from basic concepts to food applications

The significance of chaperones in preventing protein aggregation including amyloid fibril formation has been extensively documented in the biological field, but there is limited research on the potential effect of chaperone-like molecules on food protein functionality and food quality.

Controlling the Ratio between Native-Like, Non-Native-Like, and Aggregated β-Lactoglobulin after Heat Treatment

The amount of heat-denatured whey protein is typically determined by pH 4.6 precipitation. Using this method, a significant amount of nondenatured protein was reported even after long heating times. Apparently, a fraction of the unfolded protein refolds into the "native" state rather than form aggregates. This fact is known and has been explained using kinetic models. How the conditions affect the refolding and aggregation is, however, not fully understood. Therefore, this study investigates the unfolding, refolding, and aggregation process of β-lactoglobulin using circular dichroism and size-exclusion chromatography to characterize different folding variants and to quantify their content. The proteins remaining in solution at pH 4.6 were confirmed to be native-like. The nonaggregated fraction contains proteins with a native-like and two types of non-native-like conformations. The nonaggregated fraction increased with decreasing temperature (60-90 °C) and concentration (1-50 g/L) and increasing electrostatic repulsion (pH 7-8; 0-50 mM). The native-like fraction in the nonaggregated fraction was independent of pH, ionic strength, and concentration but increased with decreasing temperature.

Nanoscale understanding of thermal aggregation of whey protein pretreated by transglutaminase

Nanoscale structures of whey protein isolate (WPI) pretreated by microbial transglutaminase (mTGase) and subsequent heating were studied in this work and were correlated to zeta-potential, surface hydrophobicity, thermal denaturation properties, and macroscopic turbidity and viscosity. Dispersions of 5% w/v WPI were pretreated by individual or sequential steps of preheating at 80 °C for 15 min and mTGase, used at 2.0-10.2 U/g WPI for 1-15 h, before adjustment of the pH to 7.0 and to 0-100 mM NaCl for heating at 80 °C for 15 and 90 min. The zeta potential and surface hydrophobicity of WPI increased after all pretreatment steps. Preheating increased cross-linking reactivity of WPI by mTGase, corresponding to significantly increased denaturation temperature. Particle size analysis and atomic force microscopy revealed that structures of sequentially pretreated WPI remained stable after heating at 100 mM NaCl, corresponding to transparent dispersions. Conversely, WPI pretreated by one step aggregated at only 100 mM NaCl and resulted in turbid dispersions. Besides reporting a practical approach to produce transparent beverages, nanoscale phenomena in the present study are important for understanding whey protein structures in relevant applications.

Preservation of (-)-epigallocatechin-3-gallate antioxidant properties loaded in heat treated β-lactoglobulin nanoparticles

(-)-Epigallocatechin-3-gallate (EGCG) was loaded in heat treated β-lactoglobulin (β-Lg) for the preservation of antioxidant activity. The effects of pH (2.5-7.0), the heating temperature of β-Lg (30-85 °C), the molar ratio of β-Lg to EGCG (1:2-1:32), and the β-Lg concentration (1-10 mg/mL) on the properties of β-Lg-EGCG complexes were studied. All four factors significantly influenced the particle size, the ζ-potential, and the entrapment efficiency of EGCG and EGCG loading in β-Lg particles. A stable and clear solution system could be obtained at pH 6.4-7.0. The highest protection of EGCG antioxidant activity was obtained with β-Lg heated at 85 °C and the molar ratio of 1:2 (β-Lg: EGCG). β-Lg-EGCG complexes were found to have the same secondary structure as native β-Lg.

Interaction between β-casein and whey proteins as a function of pH and salt concentration

The effectiveness of β-casein as a chaperone in the aggregation of whey proteins was investigated. β-Casein altered heat-induced aggregation as shown by a reduction in turbidity of β-lactoglobulin, α-lactalbumin, and bovine serum albumin (BSA) solutions. The pH of the mixtures greatly affected how much β-casein reduced the turbidity of the solutions; the maximum reductions in turbidity were observed at pH 6.0. Reducing the pH decreased the effectiveness of β-casein as a chaperone. An increase in ionic strength by the addition of NaCl or CaCl(2) also decreased the effectiveness of the chaperone. The addition of CaCl(2) had a larger effect than the addition of NaCl. The chaperone effect was seen at temperatures up to 145 °C. Differential scanning calorimetry (DSC) showed that β-casein did not alter the denaturation temperature of β-lactoglobulin. The kinetics curves for loss of native protein and turbidity development showed that β-casein did not function by slowing the aggregation process. It was concluded that β-casein competes with whey protein in the aggregate process and the aggregates formed in the presence of β-casein are smaller in size than those formed during whey protein self-aggregation. The formation of smaller aggregates gives rise to less turbid, more soluble protein solutions.

Alpha casein micelles show not only molecular chaperone-like aggregation inhibition properties but also protein refolding activity from the denatured state

Casein micelles are a major component of milk proteins. It is well known that casein micelles show chaperone-like activity such as inhibition of protein aggregation and stabilization of proteins. In this study, it was revealed that casein micelles also possess a high refolding activity for denatured proteins. A buffer containing caseins exhibited higher refolding activity for denatured bovine carbonic anhydrase than buffers including other proteins. In particular, a buffer containing α-casein showed about a twofold higher refolding activity compared with absence of α-casein. Casein properties of surface hydrophobicity, a flexible structure and assembly formation are thought to contribute to this high refolding activity. Our results indicate that casein micelles stabilize milk proteins by both chaperone-like activity and refolding properties.

Molecular defects of the glycine 41 variants of alanine glyoxylate aminotransferase associated with primary hyperoxaluria type I

G41 is an interfacial residue located within the alpha-helix 34-42 of alanine:glyoxylate aminotransferase (AGT). Its mutations on the major (AGT-Ma) or the minor (AGT-Mi) allele give rise to the variants G41R-Ma, G41R-Mi, and G41V-Ma causing hyperoxaluria type 1. Impairment of dimerization in these variants has been suggested to be responsible for immunoreactivity deficiency, intraperoxisomal aggregation, and sensitivity to proteasomal degradation. However, no experimental evidence supports this view. Here we report that G41 mutations, besides increasing the dimer-monomer equilibrium dissociation constant, affect the protein conformation and stability, and perturb its active site. As compared to AGT-Ma or AGT-Mi, G41 variants display different near-UV CD and intrinsic emission fluorescence spectra, larger exposure of hydrophobic surfaces, sensitivity to Met53-Tyr54 peptide bond cleavage by proteinase K, decreased thermostability, reduced coenzyme binding affinity, and catalytic efficiency. Additionally, unlike AGT-Ma and AGT-Mi, G41 variants under physiological conditions form insoluble inactive high-order aggregates (approximately 5,000 nm) through intermolecular electrostatic interactions. A comparative molecular dynamics study of the putative structures of AGT-Mi and G41R-Mi predicts that G41 --> R mutation causes a partial unwinding of the 34-42 alpha-helix and a displacement of the first 44 N-terminal residues including the active site loop 24-32. These simulations help us to envisage the possible structural basis of AGT dysfunction associated with G41 mutations. The detailed insight into how G41 mutations act on the structure-function of AGT may contribute to achieve the ultimate goal of correcting the effects of these mutations.