Structure formation in casein-based gels, foams, and emulsions

Rheology of Emulsion-Filled Gels Applied to the Development of Food Materials

Emulsion-filled gels are classified as soft solid materials and are complex colloids formed by matrices of polymeric gels into which emulsion droplets are incorporated. Several structural aspects of these gels have been studied in the past few years, including their applications in food, which is the focus of this review. Knowledge of the rheological behavior of emulsion-filled gels is extremely important because it can measure interferences promoted by droplets or particle inclusion on the textural properties of the gelled systems. Dynamic oscillatory tests, more specifically, small amplitude oscillatory shear, creep-recovery tests, and large deformation experiments, are discussed in this review as techniques present in the literature to characterize rheological behavior of emulsion-filled gels. Moreover, the correlation of mechanical properties with sensory aspects of emulsion-filled gels appearing in recent studies is discussed, demonstrating the applicability of these parameters in understanding mastication processes.

Thermal Degradation and Isomerization of β-Carotene in Oil-in-Water Nanoemulsions Supplemented with Natural Antioxidants

The goal of this study was to see the impact on the retention and isomerization of encapsulated β-carotene (BC) in nanoemulsions fortified with natural antioxidants (α-tocopherol (AT) and l-ascorbic acid (AA)). The physical stability of nanoemulsion, oxidative stability, and isomerization of all-trans-β-carotene (BC) in oil-in-water (O/W) nanoemulsions were determined in the presence or absence of natural antioxidants at 25 and 50 °C at certain intervals of time by high-performance liquid chromatography (HPLC). Sodium caseinate was used as the emulsifier, and corn oil (CO) was more protective than medium-chain triglycerides (MCT) and used for isomerization studies. Mean diameters of control (without antioxidants) and AA- and AT-fortified particles were similar. Mean particle diameter of nanoemulsions increased from 10 to 25 nm at 25 °C and from 40 to 50 nm at 50 °C during 30 days of storage. The isomerization from all-trans-BC to cis-BC isomers was inhibited by antioxidants. The isomerization rates were in the following order: 13-cis-BC > 15-cis-BC > 9-cis-BC. AT had better antioxidant activities than AA in inhibiting BC degradation in O/W nanoemulsions. The results indicated that BC encapsulated in nanoemulsions supplemented with antioxidants could significantly improve BC's chemical stability.

A novel highly divergent protein family identified from a viviparous insect by RNA-seq analysis: a potential target for tsetse fly-specific abortifacients

In tsetse flies, nutrients for intrauterine larval development are synthesized by the modified accessory gland (milk gland) and provided in mother's milk during lactation. Interference with at least two milk proteins has been shown to extend larval development and reduce fecundity. The goal of this study was to perform a comprehensive characterization of tsetse milk proteins using lactation-specific transcriptome/milk proteome analyses and to define functional role(s) for the milk proteins during lactation. Differential analysis of RNA-seq data from lactating and dry (non-lactating) females revealed enrichment of transcripts coding for protein synthesis machinery, lipid metabolism and secretory proteins during lactation. Among the genes induced during lactation were those encoding the previously identified milk proteins (milk gland proteins 1-3, transferrin and acid sphingomyelinase 1) and seven new genes (mgp4-10). The genes encoding mgp2-10 are organized on a 40 kb syntenic block in the tsetse genome, have similar exon-intron arrangements, and share regions of amino acid sequence similarity. Expression of mgp2-10 is female-specific and high during milk secretion. While knockdown of a single mgp failed to reduce fecundity, simultaneous knockdown of multiple variants reduced milk protein levels and lowered fecundity. The genomic localization, gene structure similarities, and functional redundancy of MGP2-10 suggest that they constitute a novel highly divergent protein family. Our data indicates that MGP2-10 function both as the primary amino acid resource for the developing larva and in the maintenance of milk homeostasis, similar to the function of the mammalian casein family of milk proteins. This study underscores the dynamic nature of the lactation cycle and identifies a novel family of lactation-specific proteins, unique to Glossina sp., that are essential to larval development. The specificity of MGP2-10 to tsetse and their critical role during lactation suggests that these proteins may be an excellent target for tsetse-specific population control approaches.

Cellular uptake of β-carotene from protein stabilized solid lipid nanoparticles prepared by homogenization-evaporation method

With a homogenization-evaporation method, β-carotene (BC) loaded nanoparticles were prepared with different ratios of food-grade sodium caseinate (SC), whey protein isolate (WPI), or soy protein isolate (SPI) to BC and evaluated for their physiochemical stability, in vitro cytotoxicity, and cellular uptake by Caco-2 cells. The particle diameters of the BC loaded nanoparticles with 0.75% SC or 1.0% WPI emulsifiers were 75 and 90 nm, respectively. Mean particle diameters of three BC loaded nanoparticle nanoemulsions increased less than 10% at 4 °C while they increased more at 25 °C (10-76%) during 30 days of storage. The oxidative stability of BC loaded nanoparticles encapsulated by proteins decreased in the following order: SC > WPI > SPI. The retention rates of BC in nanoparticles were 63.5%, 60.5%, and 41.8% for SC, WPI, and SPI, respectively, after 30 days of storage at 25 °C. The BC's chemical stability was improved by increasing the concentration of protein. Both the rate of particle growth and the total BC loss at 25 °C were larger than at 4 °C. The color of BC loaded nanoparticles decreased with increasing storage in the dark without oxygen, similar to the decrease in BC content of nanoparticles at 4 and 25 °C. Almost no cytotoxicity due to BC loaded nanoparticles cellular uptake was observed, especially when diluted 10 times or more. The uptake of BC was significantly improved through nanoparticle delivery systems by 2.6-, 3.4-, and 1.7-fold increase, respectively, for SC, WPI, and SPI, as compared to the free BC. The results of this study indicate that protein stabilized, BC loaded nanoparticles can improve stability and uptake of BC.

How does oil type determine emulsion characteristics in concentrated Na-caseinate emulsions?

Macroscopic properties and ensemble average diffusion of concentrated (dispersed phase 50-60 wt%) Na-caseinate-stabilised emulsions for three different oils (soybean oil, palm olein and tetradecane) were explored. On a volume fraction basis, pulsed gradient stimulated echo (PGSTE)-NMR data show that droplet dynamics for all three systems are similar within a region of the emulsion morphology diagram. The exact limits of the emulsion space depend however on which oil is considered. The reduced solubility of tetradecane in water, and Na-caseinate in tetradecane, result in the stabilisation of flocs during formulation. Floc formation is not observed when soybean oil or palm olein is used under identical emulsion formulation conditions. Linear rheology experiments provide indirect evidence that the local structure and the properties of the thin film interfacial domain of tetradecane emulsions vary from those of soybean oil and palm olein emulsions. Collectively these data indicate that protein/oil interactions within a system dominate over specific oil droplet structure and size distribution, which are similar in the three systems.

Physico-chemical stability of astaxanthin nanodispersions prepared with polysaccharides as stabilizing agents

The emulsification and stabilization ability of four selected polysaccharides, namely, gum Arabic, xanthan gum, pectin and methyl cellulose, in the preparation of water-dispersible astaxanthin nanoparticles using the emulsification-evaporation technique was investigated in this study. The chemical and molecular structure of polysaccharides had significant effects (p < 0.05) on the physicochemical properties of the prepared astaxanthin nanodispersions. Among all prepared nanodispersions, sample produced and stabilized using gum Arabic showed the smallest average particle size (295 nm) and highest physical stability. The observed considerable degradation of astaxanthin in the resulting nanodispersions during processing (24-70% w/w) and storage at 10 °C for 30 d (86-96% w/w) illustrated the limited chemical stability of polysaccharide-stabilized nanodispersions.

Stabilising emulsion-based colloidal structures with mixed food ingredients

The physical scientist views food as a complex form of soft matter. The complexity has its origin in the numerous ingredients that are typically mixed together and the subtle variations in microstructure and texture induced by thermal and mechanical processing. The colloid science approach to food product formulation is based on the assumption that the major product attributes such as appearance, rheology and physical stability are determined by the spatial distribution and interactions of a small number of generic structural entities (biopolymers, particles, droplets, bubbles, crystals) organised in various kinds of structural arrangements (layers, complexes, aggregates, networks). This review describes some recent advances in this field with reference to three discrete classes of dispersed systems: particle-stabilised emulsions, emulsion gels and aerated emulsions. Particular attention is directed towards explaining the crucial role of the macromolecular ingredients (proteins and polysaccharides) in controlling the formation and stabilisation of the colloidal structures. The ultimate objective of this research is to provide the basic physicochemical insight required for the reliable manufacture of novel structured foods with an appealing taste and texture, whilst incorporating a more healthy set of ingredients than those found in many existing traditional products.

Enzymatically structured emulsions in simulated gastrointestinal environment: impact on interfacial proteolysis and diffusion in intestinal mucus

Fundamental knowledge of physicochemical interactions in the gastrointestinal environment is required in order to support rational designing of protein-stabilized colloidal food and pharmaceutical delivery systems with controlled behavior. In this paper, we report on the colloidal behavior of emulsions stabilized with the milk protein sodium caseinate (Na-Cas), and exposed to conditions simulating the human upper gastrointestinal tract. In particular, we looked at how the kinetics of proteolysis was affected by adsorption to an oil-water interface in emulsion and whether the proteolysis and the emulsion stability could be manipulated by enzymatic structuring of the interface. After cross-linking with the enzyme transglutaminase, the protein was digested with use of an in vitro model of gastro-duodenal proteolysis in the presence or absence of physiologically relevant surfactants (phosphatidylcholine, PC; bile salts, BS). Significant differences were found between the rates of digestion of Na-Cas cross-linked in emulsion (adsorbed protein) and in solution. In emulsion, the digestion of a population of polypeptides of M(r) ca. 50-100 kDa was significantly retarded through the gastric digestion. The persistent interfacial polypeptides maintained the original emulsion droplet size and prevented the system from phase separating. Rapid pepsinolysis of adsorbed, non-cross-linked Na-Cas and its displacement by PC led to emulsion destabilization. These results suggest that structuring of emulsions by enzymatic cross-linking of the interfacial protein may affect the phase behavior of emulsion in the stomach and the gastric digestion rate in vivo. Measurements of ζ-potential revealed that BS displaced the remaining protein from the oil droplets during the simulated duodenal phase of digestion. Diffusion of the postdigestion emulsion droplets through ex vivo porcine intestinal mucus was only significant in the presence of BS due to the high negative charge these biosurfactants imparted to the droplets. This implies that the electrostatic repulsion produced can prevent the droplets from being trapped by the mucus matrix and facilitate their transport across the small intestine mucosal barrier.

Behaviour of protein-stabilised emulsions under various physiological conditions

Emulsion forms a major part of many processed food formulations. During the past few decades, the physico-chemical properties of oil-in-water emulsions under various food processing conditions have been extensively studied. However, over the recent years, interest has turned to understanding the behaviour of emulsions during consumption, i.e. physiological processing. In general, on ingestion, an emulsion is exposed to a relatively narrow range of physical (e.g. shear and temperature) and biochemical (e.g. dilution, pH, pepsin, pancreatin, mucins and bile salts) environments as it passes through the mouth into the stomach and then the intestines. There is currently limited knowledge of the physico-chemical and structural changes, which an emulsion may undergo when it passes through the physiologically active regime. A better understanding of the gastro-intestinal processing of emulsions would allow manipulation of physico-chemical and interfacial properties to modulate lipid ingestion, improve bioavailability of lipid soluble nutrients and reduce absorption of saturated fats, cholesterol and trans fats. Food emulsions are commonly stabilised by proteins, as they are not only excellent emulsifiers but also provide nutritional benefits to the product. The effects of digestion conditions on interfacial protein structures are complicated because of potential breakdown of these structures by proteolytic enzymes of the gastrointestinal tract. Studies dealing directly with the behaviour of protein-based emulsions under digestion conditions are very limited. This paper provides an overview of the behaviour of oil-in-water emulsions stabilised with globular proteins, namely lactoferrin and β-lactoglobulin. Recent advances in understanding the interactions between interfacial proteins on oil droplets and various physiological materials (e.g. enzymes and bile salts) in in vitro digestion systems are considered. Major emphasis is placed on the recent work carried out in our laboratory at Massey University on the behaviour of milk protein based emulsions (lactoferrin or β-lactoglobulin) during their passage through the gastro-intestinal tract.

The role of bile salts in digestion

Bile salts (BS) are bio-surfactants present in the gastrointestinal tract (GIT) that play a crucial role in the digestion and absorption of nutrients. The importance of BS for controlled release and transport of lipid soluble nutrients and drugs has recently stimulated scientific interest in these physiological compounds. BS are so-called facial amphiphiles showing a molecular structure that is very distinct from classical surfactants. This peculiar molecular structure facilitates the formation of dynamic aggregates able to solubilise and transport lipid soluble compounds. The detergent nature of BS has been studied in the literature, mostly concentrating on the self-assembly behaviour of BS in solution but also in relation to protein denaturation and its effect on improving proteolysis. In contrast, the affinity of BS for hydrophobic phases has received less attention and studies dealing directly with the interfacial behaviour of BS are very limited in the literature. This is despite the fact that the interfacial activity of BS plays a vital role in fat digestion since it is closely involved with lypolisis. BS adsorb onto fat droplets and can remove other materials such as proteins, emulsifiers and lipolysis products from the lipid surface. The unusual surface behaviour of BS is directly related to their intriguing molecular structure and further knowledge could provide an improved understanding of lipid digestion. This review aims to combine the new insights gained into the surface properties of BS and their role in digestion. A better understanding of surface activity of BS would allow manipulation of physico-chemical and interfacial properties to modulate lipid digestion, improve bioavailability of lipid soluble nutrients and reduce absorption of saturated fats, cholesterol and trans fats.

Formation of protein-rich coatings around lipid droplets using the electrostatic deposition method

The purpose of this study was to determine whether protein-rich coatings could be formed around lipid droplets using an electrostatic deposition method. These coatings were assembled using two methods: (i) beta-lactoglobulin was adsorbed to beta-lactoglobulin-pectin-coated lipid droplets; (ii) beta-lactoglobulin-pectin complexes were adsorbed to beta-lactoglobulin-coated lipid droplets. Composite particles, consisting of lipid droplets with protein-rich biopolymer coatings, could be formed using both approaches (e.g., at pH 4, the protein surface load could be increased from 3 to 59 mg m(-2)). These composite particles could be made small (d < 500 nm) and relatively stable to gravitational separation at certain protein concentrations. Nevertheless, aggregation and sedimentation occurred at sufficiently high protein concentrations because of charge neutralization. The composite particles remained stable after they were heated above the thermal denaturation temperature of the globular proteins at pH 4. When the heated composite particles were adjusted to pH 7, where beta-lactoglobulin and pectin are both negatively charged, some of the pectin and beta-lactoglobulin became detached from the droplet surfaces but the protein surface load was still higher than in a nontreated sample. These composite particles may be useful for increasing the protein concentration in biopolymer coatings surrounding lipid droplets, which potentially has practical applications in the food industry (e.g., in protecting omega-3 oils from oxidation or in developing natural weighting agents).

Macroporous polymer from core-shell particle-stabilized Pickering emulsions

Poly(styrene-co-N-isopropylacrylamide) (PS-co-PNIPAM) core-shell particles were synthesized and used as particulate emulsifiers in the preparation of particle-stabilized (Pickering) emulsions. Highly concentrated oil-in-water emulsions with an internal phase up to 80 vol % can be produced using PS-co-PNIPAM core-shell particles along as the emulsifiers in emulsions. The core-shell particles are adsorbed at the liquid interface, acting as a barrier against oil droplet coalescence. In addition, it is likely that excess particles simultaneously form a gel in the continuous phase to trap oil droplets in the gel matrix, in turn inhibiting creaming and phase inversion. Evaporation in air of such a core-shell particle-stabilized emulsion directly leads to porous membranes in the absence of chemical reactions. The pore walls of the final structures are densely packed with layers of the core-shell particles. This provides great flexibility to prepare functionalized porous materials for opening up new applications.