Casein micelle structure: Models and muddles

Progress in Protein-Based Hydrogels for Flexible Sensors: Insights from Casein

In recent years, the rapid advancement of flexible sensors as the cornerstone of flexible electronics has propelled a flourishing evolution within the realm of flexible electronics. Unlike traditional flexible devices, hydrogel flexible sensors have characteristic advantages such as biocompatibility, adhesion, and adjustable mechanical properties and have similar properties to human skin. Especially, biobased hydrogels have become the preferred substrate material for flexible sensors due to increased environmental pressures caused by the scarcity of petrochemical resources. In this regard, proteins possess advantages such as diverse amino acid compositions, adjustable advanced structures, chemical modifiability, the application of protein engineering techniques, and the ability to respond to various external stimuli. These enable the hydrogels constructed from them to have greater designability, flexibility, and adaptability. As a result, their applications in manufacturing various types of sensors have experienced rapid growth. This work systematically reviews the sensing mechanism of protein-based hydrogels, focusing on the preparation of protein-based hydrogels and the optimization of flexible sensors mainly from the perspective of a typical type of animal-derived protein casein. In addition, while the potential of casein is recognized, the limitations of casein-based hydrogels in flexible sensor applications are explored, and insights are provided into the development trends of next-generation sensors based on casein-based hydrogel materials.

The Impact of Thermal Treatment Intensity on Proteins, Fatty Acids, Macro/Micro-Nutrients, Flavor, and Heating Markers of Milk-A Comprehensive Review

Milk thermal treatment, such as pasteurization, high-temperature short-time processing, and the emerging ultra-short-time processing (<0.5 s), are crucial for ensuring milk safety and extending its shelf life. Milk is a nutritive food matrix with various macro/micro-nutrients and other constituents that are possibly affected by thermal treatment for reasons associated with processing strength. Therefore, understanding the relationship between heating strength and milk quality is vital for the dairy industry. This review summarizes the impact of thermal treatment strength on milk's nutritional and sensory properties, the synthesizing of the structural integrity and bioavailability of milk proteins, the profile and stability of fatty acids, the retention of macro/micro-nutrients, as well as the overall flavor profile. Additionally, it examines the formation of heat-induced markers, such as Maillard reaction products, lactulose, furosine, and alkaline phosphatase activity, which serve as indicators of heating intensity. Flavor and heating markers are commonly used to assess the quality of pasteurized milk. By examining former studies, we conclude that ultra-short-time-processing-treated milk is comparable to pasteurized milk in terms of specific parameters (such as whey protein behavior, furosine, and ALP contents). This review aims to better summarize how thermal treatments influence the milk matrix, guiding the dairy industry's development and balancing milk products' safety and nutritional value.

Casein-based hydrogels: Advances and prospects

Casein-based hydrogels (Casein Gels) possess advantageous properties, including mechanical strength, stability, biocompatibility, and even adhesion, conductivity, sensing capabilities, as well as controlled-releasing behavior of drugs. These features are attributed to their gelation methods and functionalization with various polymers. Casein Gels is an important protein-based material in the food industry, in terms of dairy and functional foods, biological and medicine, in terms of carrier for bioactive and sensitive drugs, wound healing, and flexible sensors and wearable devices. Herein, this review aims to highlight the importance of the features mentioned above via a comprehensive investigation of Casein Gels through multiple directions and dimensional applications. Firstly, the composition, structure, and properties of casein, along with the gelation methods employed to create Casein Gels are elaborated, which serves as a foundation for further exploration. Then, the application progresses of Casein Gels in dairy products, functional foods, medicine, flexible sensors and wearable devices, are thoroughly discussed to provide insights into the diverse fields where Casein Gels have shown promise and utility. Lastly, the existing challenges and future research trends are highlighted from an interdisciplinary perspective. We present the latest research advances of Casein Gels and provide references for the development of multifunctional biomass-based hydrogels.

Gastric aggregation of micellar casein powders induced by high hydrostatic pressure: Effect of serum Ca2+ level

Micellar casein (MC) has a unique gastric colloidal behavior in response to Ca2+ cross-linking, and its aggregation properties are closely related to pepsin and gastric acid. In this study, MC with different levels of colloidal calcium phosphate (CCP) was obtained by high hydrostatic pressure (HHP) at different pressures, followed by spray drying to obtain the powders. Different amounts of calcium chloride (exogenous Ca2+) were added to MC powders prior to in vitro simulated digestion to investigate the effect of exogenous serum Ca2+ levels on the aggregation behavior and the structure change of curds generated in gastric tract. The results revealed that HHP induced the emergence of more Ca2+-binding sites, thus Ca2+ was more likely to bind to MC matrix with low CCP levels. Meanwhile, high serum Ca2+ level provided more opportunities to form aggregates. The Highest pressure (500 MPa) with the highest Ca2+ level (5 mM) caused the lowest solubility aggregates, which were only 30% at the end of gastric digestion (120 min), half of the control sample (0 MPa with 0.15 mM Ca2+). The results of wide-angle X-ray scattering / small-angle X-ray scattering suggested that both pepsin and gastric acid-induced aggregation via Ca2+ as a bridge. For pepsin, Ca2+ cross-linked between para-κ-casein; For gastric acid, Ca2+ recombined phosphorylation sites and caused cross-linking of casein subunits.

Application of small angle scattering (SAS) in structural characterisation of casein and casein-based products during digestion

In recent years, small and ultra-small angle scattering techniques, collectively known as small angle scattering (SAS) have been used to study various food structures during the digestion process. These techniques play an important role in structural characterisation due to the non-destructive nature (especially when using neutrons), various in situ capabilities and a large length scale (of 1 nm to ∼20 μm) they cover. The application of these techniques in the structural characterisation of dairy products has expanded significantly in recent years. Casein, a major dairy protein, forms the basis of a wide range of gel structures at different length scales. These gel structures have been extensively researched utilising scattering techniques to obtain structural information at the nano and micron scale that complements electron and confocal microscopy. Especially, neutrons have provided opportunity to study these gels in their natural environment by using various in situ options. One such example is understanding changes in casein gel structures during digestion in the gastrointestinal tract, which is essential for designing personalised food structures for a wide range of food-related diseases and improve health outcomes. In this review, we present an overview of casein gels investigated using small angle and ultra-small angle scattering techniques. We also reviewed their digestion using newly built setups recently employed in various research. To gain a greater understanding of micro and nano-scale structural changes during digestion, such as the effect of digestive juices and mechanical breakdown on structure, new setups for semi-solid food materials are needed to be optimised.

Impact of Physicochemical Modifications in Casein Promoted by UV-C on the Peptide Profile of Gastric Digestion and the Transepithelial Transport of Peptides

Caseins are the main proteins in milk, and their structure and spatial conformation are responsible for their slow digestion rate. The release of bioactive and β-casomorphin peptides from casein digestion may induce allergic responses during consumption. Spectroscopic techniques were used to observe the structural changes in casein conformation induced by Ultraviolet light irradiation (UV-C). Raman spectroscopy results showed more pronounced peaks at 618 and 640 cm^-1 for phenylalanine and tyrosine moieties of the photolyzed micellar casein, respectively, suggesting changes in the micelle structure. The decrease in the intensity of Raman signals for tryptophan and tyrosine corroborates to the UV-C-induced modifications of the micelle structure. Particle size distribution showed a decrease in the average micelle size after 15 min of UV-C exposure, while low-temperature, long-time (LTLT) pasteurization led to the formation of large aggregates, as observed by atomic force microscopy. UV-C did not impact the formation or transport of peptides, as observed by using the Caco-2 cell as a model for peptide absorption. However, the absence of the opioid peptide SRYPSY from κ-casein and only 20% of the concentration of opioid peptide RYLGY were noted. This work demonstrated that UV-C can be utilized to induce the physicochemical modification of dairy products, promoting a higher digestion rate and reducing allergenicity.

Effect of sodium alginate on the yogurt stability was dependent on the thickening effect and interaction between casein micelles and sodium alginate

The effect of sodium alginate (SA) on the yogurt stability and the related mechanisms were investigated. It was found that low-concentration SA (≤0.2 %) increased the yogurt stability, while high-concentration SA (≥0.3 %) decreased the yogurt stability. Sodium alginate increased the viscosity and viscoelasticity of yogurt and this effect was positively correlated with its concentration, suggesting that SA worked as the thickening agent in yogurt. However, addition of ≥0.3 % SA damaged the yogurt gel. These results suggested that interaction between milk protein and SA might play an important role in the yogurt stability besides the thickening effect. Addition of ≤0.2 % SA did not change the particle size of casein micelles. However, addition of ≥0.3 % SA induced aggregation of casein micelles and increased the size. And the aggregated casein micelles precipitated after 3 h storage. Isothermal titration calorimetry analysis showed that casein micelles and SA were thermodynamically incompatible. These results suggested that the interaction between casein micelles and SA induced aggregation and precipitation of casein micelles, which was critical in the destabilization of yogurt. In conclusion, the effect of SA on the yogurt stability was dependent on the thickening effect and the interaction between casein micelles and SA.

Caseins: Versatility of Their Micellar Organization in Relation to the Functional and Nutritional Properties of Milk

The milk of mammals is a complex fluid mixture of various proteins, minerals, lipids, and other micronutrients that play a critical role in providing nutrition and immunity to newborns. Casein proteins together with calcium phosphate form large colloidal particles, called casein micelles. Caseins and their micelles have received great scientific interest, but their versatility and role in the functional and nutritional properties of milk from different animal species are not fully understood. Caseins belong to a class of proteins that exhibit open and flexible conformations. Here, we discuss the key features that maintain the structures of the protein sequences in four selected animal species: cow, camel, human, and African elephant. The primary sequences of these proteins and their posttranslational modifications (phosphorylation and glycosylation) that determine their secondary structures have distinctively evolved in these different animal species, leading to differences in their structural, functional, and nutritional properties. The variability in the structures of milk caseins influence the properties of their dairy products, such as cheese and yogurt, as well as their digestibility and allergic properties. Such differences are beneficial to the development of different functionally improved casein molecules with variable biological and industrial utilities.

Genetically Encoded Self-Assembling Protein Nanoparticles for the Targeted Delivery In Vitro and In Vivo

Targeted nanoparticles of different origins are considered as new-generation diagnostic and therapeutic tools. However, there are no targeted drug formulations within the composition of nanoparticles approved by the FDA for use in the clinic, which is associated with the insufficient effectiveness of the developed candidates, the difficulties of their biotechnological production, and inadequate batch-to-batch reproducibility. Targeted protein self-assembling nanoparticles circumvent this problem since proteins are encoded in DNA and the final protein product is produced in only one possible way. We believe that the combination of the endless biomedical potential of protein carriers as nanoparticles and the standardized protein purification protocols will make significant progress in "magic bullet" creation possible, bringing modern biomedicine to a new level. In this review, we are focused on the currently existing platforms for targeted self-assembling protein nanoparticles based on transferrin, lactoferrin, casein, lumazine synthase, albumin, ferritin, and encapsulin proteins, as well as on proteins from magnetosomes and virus-like particles. The applications of these self-assembling proteins for targeted delivery in vitro and in vivo are thoroughly discussed, including bioimaging applications and different therapeutic approaches, such as chemotherapy, gene delivery, and photodynamic and photothermal therapy. A critical assessment of these protein platforms' efficacy in biomedicine is provided and possible problems associated with their further development are described.

Improved Stability and In Vitro Anti-Arthritis Bioactivity of Curcumin-Casein Nanoparticles by Ultrasound-Driven Encapsulation

Curcumin possesses beneficial biological functions, namely anti-inflammation and anti-diabetic functions. However, due to its low solubility and crystallinity, its applications are limited. In this work, curcumin was encapsulated in casein micelles in order to form curcumin-casein nanoparticles by ultrasound treatment (5 min). The ultrasound treatment induced the entry of the hydrophobic groups to the inner micelles and the polar sulfydryl groups to the surface of the micelles in order to form compact curcumin-casein nanoparticles of an appropriate size (100-120 nm) for cellular endocytosis. The product exhibited excellent stability during 8 months of cold storage, 6 days at room temperature, and 2 days at body temperature. Advanced in vitro experiments demonstrated that curcumin-casein nanoparticles displayed significantly greater inhibitory activity against the proliferation and proinflammatory cytokines of human fibroblast-like synoviocyte-osteo arthritis (HFLS-OA) cells and HFLS-rheumatoid (RA) cells than native curcumin due to better cellular uptake as a result of the low crystallinity and the appropriate nano-size of the nano-form. The results provide a reference for the use of ultrasound treatment to encapsulate other drug molecules and curcumin-casein nanoparticles as potential treatment for arthritis.

Structure And Biological Functions Of Milk Caseins

Caseins, which are contained in milk, play a significant role in the development of clinical symptoms of allergic reactions in adults and children. To date, the properties of caseins have been studied, their primary structure has been identified. However, despite available scientific reviews and original articles, an issue of the structural organization of milk casein micelles and their biological functions is still very controversial. In this regard, the proposed review is relevant, since it most fully reflects current information about various types of caseins, their physicochemical and immunobiological properties, and analyzes in detail the existing theories on the structural organization of casein micelles. Of particular interest in our review are the data on the comparative analysis of the structure and properties of caseins in both human breast milk and cow’s milk, as well as the section dealing with the allergenic activity of cow’s milk caseins and their cross-reactivity with milk proteins of other mammalian species. The objective of this review is to summarize current data on the structure and properties of casein proteins, and to determine their role in the formation of sensitization to dairy products.

Milk proteins: Processing, gastric coagulation, amino acid availability and muscle protein synthesis

It is well-known that the postprandial muscle protein synthetic response to protein ingestion is regulated on various levels, including dietary protein digestion and amino acid (AA) absorption, splanchnic AA retention, the availability of dietary protein-derived AA in the circulation, delivery of AA to the muscle, uptake of AA by the muscle, and intramuscular signaling. AA availability after consumption of dairy products is primarily determined by the rate of gastric emptying of milk proteins, which is mainly linked to coagulation of milk proteins in the stomach. Caseins form gastric coagula, which make their gastric emptying and subsequent postprandial aminoacidemia notably slower than that of whey proteins. Only recently, the role of processing, food structure, preservation and matrix on coagulation herein has been getting attention. In this review we describe various processes, that affect gastric coagulation of caseins and therewith control gastric emptying, such as the conversion to caseinate, heat treatment in the presence of whey proteins, conversion to stirred yoghurt and enzymatic hydrolysis. Modulating product characteristics by processing can be very useful to steer the gastric behavior of protein, and the subsequent digestion and AA absorption and muscle anabolic response to maintain or increase muscle mass.

Resveratrol Stabilization and Loss by Sodium Caseinate, Whey and Soy Protein Isolates: Loading, Antioxidant Activity, Oxidability

The interaction of protein carrier and polyphenol is variable due to their environmental sensitivity. In this study, the interaction between resveratrol and whey protein isolate (WPI), sodium caseinate (SC) and soy protein isolate (SPI) during storage were systematically investigated from the aspects of polyphenol loading, antioxidant activity and oxidability. It was revealed that resveratrol loaded more in the SPI core and existed both in the core of SC micelles and on the particle surface, while WPI and resveratrol mainly formed in complexes. The loading capacity of the three proteins ranked in order SC > SPI > WPI. ABTS assay showed that the antioxidant activity of the protein carriers in the initial state was SC > SPI > WPI. The results of sulfhydryl, carbonyl and amino acid analysis showed that protein oxidability was SPI > SC > WPI. WPI, with the least oxidation, improved the storage stability of resveratrol, and the impact of SC on resveratrol stability changed from a protective to a pro-degradation effect. Co-oxidation occurred between SPI and resveratrol during storage, which refers to covalent interactions. The data gathered here suggested that the transition between the antioxidant and pro-oxidative properties of the carrier is the primary factor to investigate its protective effect on the delivered polyphenol.

Analysis of the Factors Affecting Static In Vitro Pepsinolysis of Food Proteins

In this meta-analysis, we collected 58 publications spanning the last seven decades that reported static in vitro protein gastric digestion results. A number of descriptors of the pepsinolysis process were extracted, including protein type; pepsin activity and concentration; protein concentration; pH; additives; protein form (e.g., ‘native’, ‘emulsion’, ‘gel’, etc.); molecular weight of the protein; treatment; temperature; and half-times (HT) of protein digestion. After careful analysis and the application of statistical techniques and regression models, several general conclusions could be extracted from the data. The protein form to digest the fastest was ‘emulsion’. The rate of pepsinolysis in the emulsion was largely independent of the protein type, whereas the gastric digestion of the native protein in the solution was strongly dependent on the protein type. The pepsinolysis was shown to be strongly dependent on the structural components of the proteins digested—specifically, β-sheet-inhibited and amino acid, leucine, methionine, and proline-promoted digestion. Interestingly, we found that additives included in the digestion mix to alter protein hydrolysis had, in general, a negligible effect in comparison to the clear importance of the protein form or additional treatment. Overall, the findings allowed for the targeted creation of foods for fast or slow protein digestion, depending on the nutritional needs.

Characterization of Structural Changes of Casein Micelles at Different PH Using Microscopy and Spectroscopy Techniques

This study aimed to evaluate the influence of pH changes on morphometric parameters of casein micelles and a general overview of their conformational structure through microscopy techniques, Raman spectroscopy and multivariate analysis. It was found that casein micelles morphology and protein secondary structure depend strongly upon pH. The changes of arithmetic average roughness (Ra), size, and shape of casein micelles at different pH are properly characterized by atomic force and cryo-transmission electron microscopy. Morphometric changes of casein micelles were correlated correctly with folding and unfolding of casein molecules as evaluated by Raman spectroscopy when the pH was varied. The novelty of this contribution consists in demonstrating that there is a close structure-functionality relationship between the morphometric parameters of proteins and their secondary structure. Knowledge about casein micelles can help improve their use of its diverse applications.

Casein Microgels as Benzydamine Hydrochloride Carriers for Prolonged Release

This research aims to investigate the properties of nano- and micro-sized casein hydrogels crosslinked by sodium tripolyphosphate as drug delivery systems. Benzydamine hydrochloride was chosen as a model hydrophilic drug. The gels were synthesized by varying different parameters: casein concentration, casein/crosslinking ratio, and addition of ethanol as a co-solvent. The electrostatic attractive interactions between the casein and the sodium tripolyphosphate were confirmed by FTIR spectroscopy. The particle sizes was determined by dynamic light scattering and varied in the range between several hundred nanometers and several microns. The yield of the gelation process was high for all investigated samples and varied between 55.3% and 78.3%. The encapsulation efficiency of the particles was strongly influenced by the casein concentration and casein/crosslinker ratio and its values were between 4.6% and 22.4%. The release study confirmed that casein particles are useful as benzydamine carriers and ensured prolonged release over 72 h.

The in-vitro digestion behaviors of micellar casein acting as wall materials in spray-dried microparticles: The relationships between colloidal calcium phosphate and the release of loaded blueberry anthocyanins

Micellar casein (MC) is a natural carrier for delivering various bioactive substances, and its gastrointestinal digestion behavior has an important impact on the loaded materials. Studies have shown that the digestion behavior of MC is dominated by colloidal calcium phosphate (CCP) in micelle structure. In this paper, The MCs with different CCP levels were used as the carriers to prepare spray-dried microparticles loaded with blueberry anthocyanins (ACNs), then the release of ACNs during digestion was investigated. The results found that the microparticles with less CCP showed the faster dissolution and quicker protein hydrolysis, which caused weaker curd ability. The coagulation was believed as the critical issue to influence the digestion and release behaviors. Therefore, lowering CCP resulted in significantly more ACNs released. This study demonstrated the possibility of using CCP levels to control MC digestion behaviors, which can further determine the release of loaded bioactive substances in casein-based delivery systems.

Recent Advances on the Development of Protein-Based Adhesives for Wood Composite Materials-A Review

The industrial market depends intensely on wood-based composites for buildings, furniture, and construction, involving significant developments in wood glues since 80% of wood-based products use adhesives. Although biobased glues have been used for many years, notably proteins, they were replaced by synthetic ones at the beginning of the 20th century, mainly due to their better moisture resistance. Currently, most wood adhesives are based on petroleum-derived products, especially formaldehyde resins commonly used in the particleboard industry due to their high adhesive performance. However, formaldehyde has been subjected to strong regulation, and projections aim for further restrictions within wood-based panels from the European market, due to its harmful emissions. From this perspective, concerns about environmental footprint and the toxicity of these formulations have prompted researchers to re-investigate the utilization of biobased materials to formulate safer alternatives. In this regard, proteins have sparked a new and growing interest in the potential development of industrial adhesives for wood due to their advantages, such as lower toxicity, renewable sourcing, and reduced environmental footprint. This work presents the recent developments in the use of proteins to formulate new wood adhesives. Herein, it includes the historical development of wood adhesives, adhesion mechanism, and the current hotspots and recent progress of potential proteinaceous feedstock resources for adhesive preparation.

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.

Influence of Chymosin on Physicochemical and Hydrolysis Characteristics of Casein Micelles and Individual Caseins

The effects of chymosin on the physicochemical and hydrolysis characteristics of casein micelles and individual caseins were investigated. Adding 0.03 units of chymosin/mL led to the casein micelles in skim milk coagulating after a 3 h incubation period at 30 °C. SDS–PAGE investigation showed that β-CN, κ-CN, α_s-CN, and a portion of β-lactoglobulin (β-LG) in the milk supernatant fraction (MSF) were precipitated into the milk pellet fraction (MPF). The mean particle size of the MSF with chymosin decreased from 254.4 nm to 179.2 nm after a 3 h incubation period. Mass spectrometry and SDS–PAGE analysis suggested that chymosin hydrolyzed individual β-CN, κ-CN, and α_s-CN, but not β-LG. Chymosin hydrolysis led to a decrease in the molecular weights of the hydrolyzed β-CN, κ-CN, and α_s-CN. Particle size analysis indicated that there was no difference in the particle size distribution of hydrolyzed β-CN and α_s-CN. Moreover, our outcomes demonstrated that the hydrolysis of κ-CN by chymosin occurs before that of β-CN and α_s-CN.

An Extensive Review of Natural Polymers Used as Coatings for Postharvest Shelf-Life Extension: Trends and Challenges

Global demand for minimally processed fruits and vegetables is increasing due to the tendency to acquire a healthy lifestyle. Losses of these foods during the chain supply reach as much as 30%; reducing them represents a challenge for the industry and scientific sectors. The use of edible packaging based on biopolymers is an alternative to mitigate the negative impact of conventional films and coatings on environmental and human health. Moreover, it has been demonstrated that natural coatings added with functional compounds reduce the post-harvest losses of fruits and vegetables without altering their sensorial and nutritive properties. Furthermore, the enhancement of their mechanical, structural, and barrier properties can be achieved through mixing two or more biopolymers to form composite coatings and adding plasticizers and/or cross-linking agents. This review shows the latest updates, tendencies, and challenges in the food industry to develop eco-friendly food packaging from diverse natural sources, added with bioactive compounds, and their effect on perishable foods. Moreover, the methods used in the food industry and the new techniques used to coat foods such as electrospinning and electrospraying are also discussed. Finally, the tendency and challenges in the development of edible films and coatings for fresh foods are reviewed.

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.

Progress in micellar casein concentrate: Production and applications

Micellar casein concentrate (MCC) is a novel ingredient with high casein content. Over the past decade, MCC has emerged as one of the most promising dairy ingredients having applications in beverages, yogurt, cheese, and process cheese products. Industrially, MCC is manufactured by microfiltration (MF) of skim milk and is commercially available as a liquid, concentrated, or dried containing ≥9, ≥22, and ≥80% total protein, respectively. As an ingredient, MCC not only imparts a bland flavor but also offers unique functionalities such as foaming, emulsifying, wetting, dispersibility, heat stability, and water-binding ability. The high protein content of MCC represents a valuable source of fortification in a number of food formulations. For the last 20 years, MCC is utilized in many applications due to the unique physiochemical and functional characteristics. It also has promising applications to eliminate the cost of drying by producing concentrated MCC. This work aims at providing a succinct overview of the historical progress of the MCC, a review on the manufacturing methods, a discussion of MCC properties, varieties, and applications.

Approaches in Animal Proteins and Natural Polysaccharides Application for Food Packaging: Edible Film Production and Quality Estimation

Natural biopolymers are an interesting resource for edible films production, as they are environmentally friendly packaging materials. The possibilities of the application of main animal proteins and natural polysaccharides are considered in the review, including the sources, structure, and limitations of usage. The main ways for overcoming the limitations caused by the physico-chemical properties of biopolymers are also discussed, including composites approaches, plasticizers, and the addition of crosslinking agents. Approaches for the production of biopolymer-based films and coatings are classified according to wet and dried processes and considered depending on biopolymer types. The methods for mechanical, physico-chemical, hydration, and uniformity estimation of edible films are reviewed.

Investigating casein gel structure during gastric digestion using ultra-small and small-angle neutron scattering

This study aimed to understand the structural devolution of 10% w/w rennet-induced (RG) and transglutaminase-induced acid (TG) gels in H₂O and D₂O under in vitro gastric conditions with and without pepsin. The real-time devolution of structure at a nano- (e.g. colloidal calcium phosphate (CCP) and micelle) and micro- (gel network) level was determined using ultra-small (USANS) and small-angle neutron scattering (SANS) with electron microscopy. Results demonstrate that gel firmness or elasticity determines disintegration behaviour during simulated mastication and consequently the particle size entering the stomach. Shear of mixing in the stomach, pH, and enzyme activity will also affect the digestion process. Our results suggest that shear of mixing primarily results in erosion at the particle surface and governs gel disintegration behaviour during the early stages of digestion. Pepsin diffusivity, and hence action, occur more readily in the latter stages of gastric digestion via access to the particle interior. This occurs via the progressively larger pores of the looser gel network and channels created within the larger, less dense casein micelles of the RG gels. Gel firmness and brittleness were greater in the D₂O samples compared to H₂O, facilitating gel disintegration. Despite the higher strength and elasticity of RG compared to TG, the protein network strands of the RG gels become more compact when exposed to the acidic gastric environment with comparatively larger pores observed through SEM imaging. This led to a higher degree of digestibility in RG gels compared to TG gels. This is the first study to examine casein gel structure during simulated gastric digestion using scattering and highlights the benefits of neutron scattering to monitor structural changes during digestion at multiple length scales.

A regenerated fiber from rennet-treated casein micelles

Casein as the major protein of milk is a promising protein source for biopolymer fibers. Current casein-based fibers are fabricated by dissolving caseins in alkaline media and wet spinning in a coagulation bath containing harsh chemicals. In milk, casein is present in so-called casein micelles (CMs). Based on the rennet-induced aggregation, we developed a process that can be applied for the spinning of micellar casein fibers in a sustainable way without the use of harsh chemicals. Fabricated fibers show a surface with a characteristic microstructure, which can also be detected embedded in a network structure inside the fiber. The fibers are stable under acidic and neutral conditions and decompose in alkaline media down to aggregates with sizes comparable to the characteristic microstructure. The so far reached tensile properties of the micellar fiber are between low and mid double-digit percentage range compared to casein azlons.

Contributions from microstructural changes to the rheological behavior of casein dispersions during drying

In several applications, a protein such as casein in dispersion form undergoes multiple processing steps including drying. In this work, the rheological and microstructural features of casein dispersions concentrated by evaporation of the solvent (drying dispersions) were studied in comparison with those of equal concentrations of the as-prepared dispersions without drying. The molecular assembly of casein is affected by drying along with the conformational composition changes in the secondary structures such as α-helix, β-sheets, turns and random structures of the protein. Modeling of the rheological data indicates that these changes also affect the packing of casein molecular assemblies and these molecular assemblies in alkaline dispersions can behave as soft deformable particles. During drying, casein dispersions show prominent shear thinning for concentrations higher than 20 wt% along with the prevalence of α-helices and β-sheets. In comparison, the as-prepared dispersions show different microstructural features, and therefore different rheological responses. A detailed analysis shows that alkalinity changes during drying is the crucial factor controlling the microstructural changes of the soft casein particles and hence the rheology.

Effect of Potato Pulp Pectic Polysaccharide on the Stability of Acidified Milk Drinks

In order to broaden the application of potato pulp pectic polysaccharide (PPP) in stabilizing acidified milk drinks (AMDs) and investigate the stabilizing effect and physical properties of AMDs prepared with PPP, a comparative study was made among PPP, commercial high methoxyl pectin (HMP) and low methoxyl pectin (LMP). The zeta potential, rheology, particle size and serum separation of AMDs were evaluated after preparing with PPP, HMP and LMP, respectively. Results indicated that PPP led to lower serum separation than LMP (14.65% for AMDs prepared with 0.5% PPP compared to 25.05% for AMDs prepared with 0.5% LMP), but still higher than HMP (9.09% for AMDs prepared with 0.5% HMP). However, narrower particle size distribution and lower viscosity of AMDs was achieved by PPP than by LMP and HMP. PPP can electrostatically adsorb on the surface of casein and its abundant neutral sugar side chains would provide steric hindrance to prevent casein flocculation in AMDs. Our results might provide some new ideas for the application of PPP in improving the stability of AMDs.

Study on β-Casein Depleted Casein Micelles: Micellar Stability, Enzymatic Cross-Linking, and Suitability as Nanocarriers

β-Casein is an amphiphilic protein and thus considered as multilaterally bound in casein micelles. Its polar molecule part, in particular the phosphoserine residues, can interact electrostatically with colloidal calcium phosphate (CCP) to form nanoclusters and its nonpolar molecule part enhances micellar stability by forming hydrophobic bonds to other caseins. Because cooling weakens hydrophobic interactions, a substantial portion of β-casein can be irreversibly removed from the casein micelle by repeated depletion steps, including cooling and subsequent ultracentrifugation. Although this effect of cooling on the micellar β-casein concentration has been well known for decades, the influence of depletion on the main characteristics of casein micelles has been less investigated yet. Therefore, we aimed to analyze the consequences of β-casein depletion on the stability as well as the functionality of casein micelles to evaluate the suitability of depleted compared to native casein micelles as nanocarriers. Up to 43.2% of the total β-casein was irreversibly sequestered from native casein micelles by repeated cooling and ultracentrifugation steps. Depletion showed no effect on size distribution as well as polydispersity and particle concentration of micelle suspensions as measured via dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA), respectively. Furthermore, the stability of the micelles against ethanol or the chelating agent ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was not influenced by β-casein depletion. Notwithstanding, depleted micelles were less susceptible to enzymatic cross-linking by microbial transglutaminase (mTG), indicating narrowed water channels due to depletion. Additionally, loading experiments showed that depleted micelles could be loaded with linoleic acid (LA) as intensively as native micelles, whereupon LA displaces up to 81.3% of β-casein from native micelles. Our results confirm that depletion does not enhance the ability of the casein micelle to act as a nanocarrier for hydrophobic substances but could support the understanding of the casein micelle structure. Based on the observed unchanged stability against EGTA, the hindered enzymatical cross-linking, and the efficient displacing of β-casein by LA, we suggest that the major portion of micellar β-casein is hydrophobically incorporated into the micelle structure without impact on the formation of calcium phosphate nanoclusters. The main role of β-casein for the casein micelle structure, therefore, might be to facilitate the high hydration of the interior and thus the high permeability of casein micelles.