Presence Of Casein Micelles Research Articles

Effect of green tea extract on heat-induced protein interactions and rennet coagulation in milk

The impact of green tea extract (GTE) on heat-induced milk protein interactions and enzymatic coagulation of milk was investigated. GTE addition affected the renneting process, more apparent for heat-treated samples. The enzymatic phase of rennet coagulation was negatively affected by GTE, with a significantly lower amount of κ-casein hydrolysis for intense heat-treated samples. Gel characteristics of raw milk were unaffected by GTE, but mild heat-treated samples with GTE displayed decreased gel firmness and increased water holding capacity compared with control samples. Model studies conducted with purified β-lactoglobulin A and κ-casein revealed that GTE caused a significant impact on interactions between denatured β-lactoglobulin A and κ-casein. Heat-induced large aggregate/complex formation between β-lactoglobulin A and κ-casein in milk samples was not significantly affected, highlighting the importance of the reaction medium. The presence of casein micelles in milk, which are strong candidates for phenolic binding, may ultimately impact protein–protein interactions.

“Casein micelle -nanoparticle double cross-linking” triggered stable adhesive, tough CA/MWCNT/PAAm hydrogel wearable strain sensors, for human motion monitoring

Flexible hydrogels have emerged as highly-desirable materials for wearable strain sensors. However, pristine biomass hydrogel systems are limited by their lack of stretchability, self-adhesion, and sensitivity. Here, a novel CA/MWCNT/PAAm double-network conductive hydrogel was developed through integrating casein (CA) micelles and multi-walled carbon nanotubes (MWCNT) into the polyacrylamide (PAAm) network. The resulting hydrogel displayed desired properties such as adhesiveness, toughness, self-healing, and near-infrared photothermal response. In this hybrid system, MWCNT were uniformly dispersed in the presence of casein micelles through hydrogen bonding and electrostatic interactions, favoring its role of nano reinforcement. Moreover, based on the “casein micelle-nanoparticle double cross-linking" mechanism and its double network structure, the prepared hydrogel showed high extensibility (2288 % ± 63 %), fast responsiveness (273 ± 5.13 ms), high sensitivity (GF = 12.46 ± 0.35), and a wide strain range (1–1000 %). Through consistent and repeated electrical inputs, this hydrogel was able to detect including large and small human movements, such as hand, leg, and swallowing motions. The results from this study provide a new way to fabricate bio-based hydrogel sensors with excellent mechanical and electrical properties.

Determination of hydro-colloidal characteristics of milk protein aggregates using Asymmetrical Flow Field-Flow Fractionation coupled with Multiangle Laser Light Scattering and Differential Refractometer (AF4-MALLS-DRi)

During heat treatment, whey proteins (WP) are denatured and form different kinds of whey protein aggregates (WPA) depending on the physico-chemical conditions. In the presence of casein micelles, mixed aggregates (MA), resulting from the interaction of κ-casein and WP, are formed. The aim of this study is to understand the effect of pH on the WPA structure and the impact of the casein/whey proteins (Cas/WP) ratio on MA morphology. In this work, WP solutions were heated at pH 5.8 or 7 to produce WPA. On the other hand, aqueous mixtures containing three Cas/WP ratios (82/18, 56/44 and 30/70) were heated at 80 °C during 1 h at pH 6.3 to obtain MA. Milk protein aggregates were analyzed by Transmission Electron Microscopy (TEM) and Asymmetrical Flow Field-Flow Fractionation coupled with Multiangle Laser Light Scattering and Differential Refractometer (AF4-MALLS-DRi). Dense and spherical WPA were formed at pH 5.8 whereas branched and fractal aggregates were obtained at pH 7. Regarding MA, results suggested that they were mainly produced with the 82/18 Cas/WP ratio whereas a majority of WPA was obtained with the 30/70 ratio. At intermediate ratio, the mixture was composed of small MA and WPA that did not interact with κ-casein. Moreover, WP seem to interact preferentially with larger casein micelles. Thereby, AF4-MALLS-DRi proved to be a powerfull technique to characterize the complex structure of milk protein aggregates and an interesting alternative to size exclusion chromatography especially for MA and casein micelles which interact with the stationary phase and are retained in the column.

Interactions in casein micelle - Pea protein system (Part II): Mixture acid gelation with glucono-δ-lactone

In our preceding study (Part I), the thermal denaturation and aggregation of enriched pea protein fractions, namely vicilin/convicilin 7S (Vic) and legumin 11S (Leg), were investigated in the absence or in presence of casein micelles (CM) at pH ≈ 7.1. The present report (Part II) focuses on the glucono-δ-lactone (GDL) acid-induced gelation of either co-heated proteins in admixture (namely route 1) or thermally-aggregated pea proteins mixed with unheated CM (route 2), while applying a pea protein (either non fractionated PP or Vic or Leg) - to - CM weight ratio of 1: 1 and total protein concentration of 3.6 wt%. The pea protein thermal aggregates obtained by route 1 were of lower size and less soluble than those yielded in isolation. For route 1, gelation of the heated CM – Vic mixture was triggered at a higher pH value and led to higher final storage modulus G′ than corresponding protein samples in isolation. In contrast, the presence of large and sedimentable aggregates in the case of the CM – PP and CM – Leg mixtures impaired gelation. Concerning route 2, either the PP or Vic aggregates mixed with unheated CM resulted in rapid gelation and higher final G’ values than those measured for their single-protein sample counterparts. Viscoelastic properties of the mixed gel depended on the pea protein fraction used, thermal aggregation route, extent of physical interactions between pea proteins during acidification and further involvement of CM. Hence, route 2 would be more reliable than route 1 to produce a “mixed” dairy-like gelled product containing pea proteins with improved texture properties.

Treatment of dairy effluent by shear-enhanced membrane filtration: The role of foulants

Treatment of dairy effluent by membrane filtration permits to recover both reusable water and nutrients, but its advantages are weakened by concentration polarization and subsequent membrane fouling as they cause flux decline and permeate quality deterioration. In order to control flux decline, model dairy effluent was treated by shear-enhanced membrane filtration in this study, and the role of foulants was analyzed. For ultrafiltration (UF) of dairy effluent, protein adsorption and deposition caused flux decline and hydrophilic modification of membranes can reduced this fouling. While for nanofiltration (NF), flux decline was mitigated by the presence of casein micelles, and when the caseins were removed by UF pretreatment, whey proteins would agglomerate through calcium bridges, resulting in severe flux decline during NF concentration. Even if most of proteins were retained by UF pretreatment, free calcium ions increased due to the absence of casein micelles, also inducing flux decline at high concentration factor. Thus, the two-step UF/NF process was not found advantageous as it did not improve the average permeate flux nor permeate quality, while increasing significantly the process energy consumption. This conclusion was valid not only for rotating disk module (RDM) but also for vibratory shear-enhanced process (VSEP), which is available in large membrane area with a low energy consumption.

Growth of milk fermentative bacteria in the presence of chitosan for potential use in cheese making

The effects of different molecular weight chitosans alone and their complexes with casein on the growth of three representative milk fermentative bacteria (Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus thermophilus and Propionibacterium freudenreichii) were compared. In nutrient broth, all chitosans showed a dose dependent inhibition of S. thermophilus and L. delbrueckii ssp. bulgaricus growth. Chitosan of high and low molecular weight, but not chitosan oligosaccharides, showed a dose dependent inhibition of P. freudenreichii that was dependent on the initial bacterial load. The inhibiting effects of chitosan were greatly reversed when the biopolymers were incubated with milk before interaction with bacteria. Under these conditions a cheese like product was made with high molecular weight chitosan as the coagulating agent. The product had lower firmness than the respective cheese control, probably because of a higher water retention. The effect of chitosan on milk fermentative processes appeared to be dependent not only on its molecular weight and concentration, but also on the presence of casein micelles or milk fat, that, acting in a competitive manner, could prevent the inhibitory activity of these biopolymers on bacterial growth.

Gelation of casein-whey mixtures: effects of heating whey proteins alone or in the presence of casein micelles.

The aim of the present work was to investigate the role of whey protein denaturation on the acid induced gelation of casein. This was studied by determining the effect of whey protein denaturation both in the presence and absence of casein micelles. The study showed that milk gelation kinetics and gel properties are greatly influenced by the heat treatment sequence. When the whey proteins are denatured separately and subsequently added to casein micelles, acid-induced gelation occurs more rapidly and leads to gels with a more particulated microstructure than gels made from co-heated systems. The gels resulting from heat-treatment of a mixture of pre-denatured whey protein with casein micelles are heterogeneous in nature due to particulates formed from casein micelles which are complexed with denatured whey proteins and also from separate whey protein aggregates. Whey proteins thus offer an opportunity not only to control casein gelation but also to control the level of syneresis, which can occur.

Influence of whey protein denaturation on κ-carrageenan gelation

Response surface methodology was applied to study the effect of different heating temperature/time treatments on whey protein denaturation and its effect on κ-carrageenan gelation in milk. The path of gel formation was followed using small deformation rheology and the extent of whey protein denaturation was determined by gel permeation chromatography. κ-Carrageenan did not influence the rate of whey protein denaturation and it was unlikely that whey protein denaturation played a significant role on κ-carrageenan gelation in milk. In skim milk serum or skim milk ultrafiltrate the path of gel formation and gel strength were not influenced by the severity of heat treatment but increasing the concentration of whey proteins enhanced the gel strength. Heat treatment became important for carrageenan gelation in skim or recombined milks (i.e. in the presence of casein micelles) by influencing the gelation temperature and gel strength. Increasing the concentration of whey proteins in the recombined milks had a beneficial effect on gel strength.

Transmission electron microscopy of human saliva

Globular, or micelle-like, structures have been demonstrated in human parotid saliva by photon correlation spectroscopy and transmission electron microscopy (TEM, negative staining). However, preparation of the TEM specimens may lead to artefact formations. To verify the presence of micelle-like structures in human parotid saliva, the application of cryo-transmission electron microscopy may prove valuable. The aim of this study was therefore to examine human parotid saliva using cryo-TEM and to correlate these results with parallel TEM examinations of parotid saliva prepared with negative staining. Whole saliva was also examined for the presence of these structures, as they may be of importance in oral physiology. Human and bovine milk were included as the presence of casein micelles in these fluids is well established. Freshly collected parotid saliva was prepared for cryo-TEM by instantaneous vitrification in liquid nitrogen. Specimens were also prepared with negative staining (2% ammonium molybdate) from freshly collected parotid saliva, from 0.45 μm filtered parotid saliva, from glutaraldehyde fixed parotid saliva, and from clarified whole saliva. Specimens of human and bovine milk were prepared from freshly obtained milk as well as from glutaraldehyde fixed milk samples. Cryo-TEM demonstrated globular structures in parotid saliva, mostly in the size range about 100–350 nm, appearing as single structures or in clusters. Similar globular structures were observed in negatively stained samples of parotid and whole saliva, and appeared to be multi-globular. TEM of human and bovine milk showed numerous multi-globular casein micelles with a structure similar to the globular structures observed in human saliva. The study demonstrates globular structures in human parotid and whole saliva similar to the casein micelles of milk, indicating the presence of salivary micelle-like structures as an important fraction of human saliva and that the negative staining technique may be suitable for TEM examinations of ultra-structural phenomena in saliva.