Food Science & Technology · Postgraduate & Research · also Food Colloids, Food Structure Design, Encapsulation Technology, Food Nanotechnology
How the arrangement of molecules and phases decides texture, stability and digestion: emulsions, foams, gels and their stability, structure-function design, encapsulation and delivery of bioactives, digestion models, and the science and regulation of nanoscale food materials.
In the exams: ICAR AIEEA-PG Code 18 Unit I (colloidal systems, gels and emulsions); ICAR AICE 35.b Unit 5 (emulsions, foams, gels; hydrocolloids)
Two foods with the same chemical composition can be completely different to eat. Milk and butter, a meringue and a raw egg white, a jelly and a glass of fruit juice with gelatin dissolved in it, differ not in their molecules but in how those molecules are arranged: as droplets, bubbles, crystals, networks and interfaces. Food structure decides texture and appearance, how a product holds together during processing and storage, how flavour is released in the mouth, and how nutrients are digested and absorbed. Designing structure deliberately, rather than discovering it by trial and error, is one of the main research directions in modern food science.
This subject covers the colloidal systems found in foods (dispersions, emulsions, foams and gels); interfacial science, including the behaviour of surfactants and proteins at interfaces; the formation of emulsions and the mechanisms of their instability; foams and aerated foods; protein and polysaccharide gels; interactions between proteins and polysaccharides; structure-function relationships and the design of food structures; microencapsulation by spray drying, coacervation and extrusion; nanoemulsions, liposomes and nanoparticles; in-vitro digestion models and bioaccessibility; nanotechnology in packaging and sensing; and the safety assessment and regulation of nanomaterials in food. It builds on chemistry-for-food-science (surface chemistry and colloids), food-chemistry-major-constituents (proteins, carbohydrates and lipids) and food-physical-properties-rheology.
These topics are examined in postgraduate entrance examinations in food science, which include colloidal systems, gels and emulsions, and in research-fellowship examinations in food safety and quality, which include emulsions, foams, gels and hydrocolloids; they are also central to doctoral research in food structure and delivery systems. The practice questions combine principles with short calculations of Laplace pressure, creaming rate, overrun, droplet surface area and encapsulation efficiency.
A colloid is a system in which one phase is dispersed in another as particles, droplets or bubbles with at least one dimension roughly between 1 nm and 1 µm, though food scientists often extend the term to structures up to tens of micrometres that behave similarly. Food colloids are classified by the states of the dispersed and continuous phases. Sols or suspensions have solid particles in a liquid, as in cocoa drinks, fruit juices with cloud and melted chocolate (sugar and cocoa particles in liquid cocoa butter). Emulsions have liquid droplets in another immiscible liquid: oil-in-water (O/W) emulsions such as milk, cream, mayonnaise, salad dressings and beverage emulsions, and water-in-oil (W/O) emulsions such as butter and margarine; multiple emulsions such as water-in-oil-in-water are studied for reduced-fat and encapsulation applications. Foams have gas bubbles in a liquid or solid: whipped cream, meringue, beer head, mousse, and solid foams such as bread, cakes, marshmallow and extruded snacks. Gels have a continuous solid network holding a large volume of liquid: jellies, set yoghurt, tofu, surimi and cheese curd. Many foods combine several: ice cream is at once a foam (air), an emulsion (partly coalesced fat), a suspension (ice and lactose crystals) and a concentrated sugar solution.
Most food colloids are thermodynamically unstable, because creating interface between two phases costs free energy, ΔG = γΔA, where γ is the interfacial tension and ΔA the increase in interfacial area. Left alone, emulsions separate and foams collapse to reduce that area. Their usefulness depends on making them kinetically stable for the required time, by slowing the processes of separation, which is the subject of much of this manual. Gels can be at true equilibrium when their network is chemically or physically cross-linked, but many food gels also change slowly with time.
The size of the dispersed elements governs many properties. As droplets or particles get smaller, the specific surface area rises: for spheres, the surface area per unit volume of dispersed phase is 6/d, so 1 cm³ of oil divided into droplets of 1 µm diameter has an interfacial area of 6 m², and into droplets of 0.1 µm, 60 m², which is why homogenisation needs much more emulsifier to cover the new surface. Size also determines appearance (droplets near the wavelength of light scatter it strongly, so emulsions with droplets around 0.1 to 1 µm are white, while nanoemulsions with droplets well below 100 nm can be transparent), creaming rate (proportional to the square of the radius), mouthfeel, and digestibility.
Because size distributions are rarely uniform, several averages are used. The surface-weighted mean diameter, d₃₂ (the Sauter mean, Σnd³/Σnd²), relates to surface area and emulsifier needs; the volume-weighted mean, d₄₃ (Σnd⁴/Σnd³), is sensitive to large droplets and therefore to coalescence and flocculation. Size is measured by static laser light scattering (diffraction), for about 0.1 µm to millimetres; by dynamic light scattering, for nanometre to about a micrometre, which reports a hydrodynamic diameter (the z-average) and a polydispersity index; by microscopy with image analysis; and by electrical sensing methods. The charge of particles is characterised by the zeta potential, measured by electrophoretic light scattering, and the structure of colloids by microscopy, rheology, scattering and nuclear magnetic resonance methods (see advanced-analytical-omics).
An interface between oil and water, or air and water, has an excess free energy per unit area, the interfacial tension γ (about 72 mN/m for air-water and roughly 30 mN/m for a pure triglyceride oil against water at room temperature). Surface-active molecules adsorb at the interface because they are amphiphilic, with a part that prefers water and a part that prefers oil or air, and they lower γ. The Gibbs adsorption equation relates the fall in interfacial tension with concentration to the amount adsorbed, and as concentration rises the interface becomes saturated; for small surfactants in water, molecules then form micelles above the critical micelle concentration.
Small-molecule surfactants used in foods include mono- and diglycerides of fatty acids, lecithins (phospholipid mixtures from soybean, sunflower or egg yolk), polysorbates, sorbitan esters, sucrose esters, polyglycerol esters, and esters of mono- and diglycerides with acids such as lactic, citric and diacetyl tartaric acid. Their balance of hydrophilic and lipophilic character is summarised by the hydrophilic-lipophilic balance (HLB): low HLB values (about 3 to 6) favour W/O emulsions and high values (about 8 to 18) favour O/W emulsions, consistent with Bancroft's rule that the continuous phase is the one in which the emulsifier is more soluble. Small surfactants adsorb quickly, lower γ strongly, and stabilise interfaces mainly by the Gibbs-Marangoni mechanism: when a film is stretched, local depletion of surfactant raises the tension there, and surfactant flows along the interface to restore it, carrying liquid with it and healing the thinning film. They form mobile, fluid interfacial layers.
Proteins are the most important natural emulsifiers and foaming agents in foods: caseins and whey proteins in dairy products, egg yolk lipoproteins and egg white proteins, soy and other plant proteins, and gelatin. They are amphiphilic polymers that adsorb at interfaces, where they partly unfold and rearrange so that hydrophobic segments face the oil or air. Flexible, disordered proteins such as β-casein adsorb and unfold quickly, forming layers with loops and tails extending into the water, which give steric stabilisation; globular proteins such as β-lactoglobulin unfold less and form dense layers in which neighbouring molecules interact, sometimes through disulphide bonds, creating viscoelastic films with high interfacial shear and dilatational moduli. Proteins usually lower γ less than small surfactants, but their films resist coalescence and bubble shrinkage well. Charged proteins also give electrostatic stabilisation, which is weakest near their isoelectric point and at high ionic strength.
Solid particles can also stabilise interfaces, forming Pickering emulsions and foams: particles partially wetted by both phases (contact angle near 90°) adsorb almost irreversibly, because the energy needed to detach a particle is many times the thermal energy, and form rigid barriers. Fat crystals stabilise W/O emulsions such as margarine and butter in this way, and particles of protein, starch, cellulose, chitin and cocoa are studied as clean-label stabilisers.
When proteins and small surfactants are both present, they compete for the interface. Small surfactants, which lower γ more, progressively displace adsorbed protein, a process imaged as 'orogenic displacement' in which surfactant domains grow and compress the protein network until it is pushed off. Mixed layers can be weaker than either alone, because the protein network is disrupted while Gibbs-Marangoni stabilisation is not yet complete. This competition is used deliberately in ice cream, where added emulsifiers displace milk proteins from fat globules so that they partly coalesce during freezing and whipping, building the fat network that stabilises the air (chapter 4), and it explains why some formulations lose foam stability when lipids or surfactants are added.
Emulsions are made by applying energy to break one liquid into droplets in another, while an emulsifier adsorbs on the new surfaces to prevent the droplets from immediately recombining. A droplet resists deformation because of its Laplace pressure, ΔP = 2γ/r, the excess pressure inside a curved interface; a droplet of 1 µm radius with γ = 10 mN/m has ΔP = 2 × 0.010/(1 × 10⁻⁶) = 20 000 Pa, and halving the radius doubles the pressure, so making smaller droplets needs much more intense shear or turbulence. Lowering γ with emulsifiers makes droplets easier to break, and fast adsorption prevents re-coalescence during homogenisation.
Equipment is chosen according to the droplet size needed. High-speed mixers and rotor-stator devices make coarse premixes and emulsions with droplets of several micrometres, as in dressings and sauces. Colloid mills shear viscous systems such as mayonnaise. High-pressure valve homogenisers force the premix through a narrow gap at pressures of tens of megapascals, generating turbulence and cavitation that break droplets to below a micrometre; they are standard in dairy, beverage and infant formula production, where two-stage homogenisation (a second, lower-pressure stage) breaks up clusters. Microfluidisers collide high-velocity jets and give very fine, narrow distributions; ultrasonic homogenisers use cavitation; and membrane emulsification presses the dispersed phase through pores into a flowing continuous phase, giving uniform droplets with low energy. The droplet size achieved depends on the energy input, the viscosity ratio of the phases, and the type and amount of emulsifier, which must be enough to cover the area created.
Emulsions break down by several mechanisms, often acting together. Gravitational separation occurs because droplets differ in density from the continuous phase: oil droplets rise (creaming) and dense droplets sink (sedimentation). The velocity of an isolated droplet follows Stokes' law, v = 2gr²(ρ₂ − ρ₁)/(9η₁), where r is the radius, ρ₂ − ρ₁ the density difference and η₁ the viscosity of the continuous phase. For a 1 µm radius oil droplet with density difference −80 kg/m³ in water (η = 0.001 Pa·s), v ≈ 1.7 × 10⁻⁷ m/s, about 15 mm per day; halving the radius cuts this fourfold. Creaming is slowed by reducing droplet size (homogenisation), matching densities (weighting agents such as ester gum or sucrose acetate isobutyrate in beverage emulsions), and thickening or gelling the continuous phase (xanthan gum, starches).
Flocculation is the aggregation of droplets that keep their identity, caused by insufficient repulsion (low surface charge, high salt, pH near a protein's isoelectric point), by bridging of droplets by polymers adsorbed on two droplets at once, or by depletion of non-adsorbing polymers from the gap between droplets, which pushes them together. Coalescence is the merging of droplets after rupture of the thin film between them, leading to larger droplets and eventually a separate oil layer; it is resisted by thick, viscoelastic or steric interfacial layers. Partial coalescence happens when droplets contain fat crystals that pierce the film between neighbours, so that they stick together as irregular clusters without fully merging; it is essential to the structure of whipped cream, ice cream and butter making, and undesirable in liquid creams that thicken in the pack. Ostwald ripening is the growth of larger droplets at the expense of smaller ones by diffusion of the dispersed phase through the continuous phase, driven by the higher solubility of material in small, highly curved droplets; it is significant only when the dispersed phase is somewhat soluble in water, as for flavour oils such as orange oil, and is inhibited by mixing in an insoluble component such as a long-chain triglyceride. Phase inversion converts an O/W emulsion into W/O, as in butter churning. Stability is measured by following droplet size, creaming (visually or by light scattering profiles), zeta potential and rheology over time, often under accelerated conditions such as centrifugation, temperature cycling or high temperature.
Foams are dispersions of gas in a liquid or solid, and aerated foods range from beer and cappuccino to whipped cream, meringues, mousses, ice cream, marshmallows, bread and extruded snacks. Foams are made by whipping or beating (mechanical incorporation and breakage of air), by injecting gas through spargers or porous membranes, by pressure release of dissolved gas (as in aerosol creams and carbonated drinks), or by generating gas within the product (yeast or chemical leavening in doughs, steam in extrusion). The amount of gas incorporated is expressed as overrun, the percentage increase in volume over the unaerated mix: overrun = (V_foam − V_mix)/V_mix × 100, or, using the masses of equal volumes, (mass of mix − mass of foam)/mass of foam × 100. For example, if a litre of ice cream mix weighs 1100 g and a litre of ice cream weighs 550 g, the overrun is 100 per cent: half the volume of the ice cream is air.
Because gas bubbles are much larger than emulsion droplets and much lower in density, foams are less stable than emulsions. Stabilisers act at the interface and in the continuous phase. Proteins such as egg white (with ovalbumin, lysozyme and ovotransferrin forming tough films), whey proteins, caseinate, gelatin and plant proteins adsorb at the air-water interface, unfold and form viscoelastic films; small surfactants give mobile, Marangoni-stabilised films. Increasing the viscosity of the continuous phase with sugars and hydrocolloids slows drainage, and solidifying it, by heat-setting proteins in meringue and cake, gelatinising starch in bread, gelling gelatin in marshmallow, or freezing water in ice cream, fixes the foam permanently. In whipped cream and ice cream, partially coalesced fat globules adsorb at bubble surfaces and form a network that stabilises the air.
Foams break down by three main mechanisms. Drainage is the flow of liquid out of the films and channels (Plateau borders) between bubbles under gravity, thinning the films; it is slowed by higher viscosity and by interfacial layers that resist flow. Coalescence is the rupture of the thin film between bubbles, merging them; it occurs when films become too thin, when hydrophobic particles or oil droplets bridge and rupture them (the action of antifoams), or when interfacial layers are weak. Disproportionation, the foam counterpart of Ostwald ripening, is the diffusion of gas from small bubbles, where the Laplace pressure is higher, through the liquid into larger bubbles, so small bubbles shrink and disappear while large ones grow; it is rapid for gases that dissolve well in water, such as carbon dioxide, and slower for nitrogen, which is why nitrogen-flushed stouts and nitrogen-infused coffees have finer, more stable foams than carbonated beers. Elastic interfacial layers that resist shrinkage, and continuous phases that set, slow disproportionation.
Practical consequences follow. Traces of fat or egg yolk ruin the whipping of egg whites because lipids displace proteins and rupture films; copper bowls and cream of tartar (an acid) help by stabilising ovotransferrin and adjusting pH. In beer, foam proteins (including lipid transfer proteins and protein Z from barley) and hop iso-α-acids stabilise the head, while lipids and detergent residues on glasses destroy it. In bread, gluten-starch films stretch around gas cells during proofing and are set by baking, and a fine, even crumb depends on the number of gas nuclei incorporated during mixing. Antifoams, such as silicone oils or vegetable oils with hydrophobic particles, are used deliberately to prevent foaming in fermenters, during boiling of jams and in processing of potatoes and soy. Foam stability is measured by the half-life of foam volume, the rate of liquid drainage, bubble size distributions by imaging, and the rheology of the foam.
A gel is a continuous three-dimensional network of polymer molecules or particles that holds a large volume of liquid and behaves as a solid on the time scale of observation; in oscillatory rheology the storage modulus G′ exceeds the loss modulus G″ and both are nearly independent of frequency. Protein gels form when proteins unfold or change so that they aggregate into networks. Heat-set gels form from globular proteins such as whey proteins, egg white, soy proteins and myofibrillar proteins in meat and surimi: heating unfolds the molecules, exposing hydrophobic regions and reactive sulphydryl groups, and they aggregate by hydrophobic interactions and disulphide bonds. The structure depends on the balance of attraction and repulsion: far from the isoelectric point and at low ionic strength, strong repulsion gives fine-stranded, transparent gels; near the isoelectric point or at higher salt, rapid aggregation gives particulate, opaque gels with larger pores that hold water less well. Cold-set gels are formed by first heating proteins to make soluble aggregates, then inducing gelation at low temperature by adding salts such as calcium or by slow acidification, as with glucono-δ-lactone.
Other protein gels are formed by acid or enzymes. Acid gels form when casein micelles lose charge as pH falls towards their isoelectric point of about 4.6, as in yoghurt and fresh cheese; heat treatment of milk before acidification makes the gel firmer because denatured whey proteins join the network. Rennet gels form when chymosin removes the hairy caseinomacropeptide layer of κ-casein, allowing the micelles to aggregate in the presence of calcium (see fermentation-food-biotechnology). Transglutaminase cross-links glutamine and lysine residues, forming covalent networks used to bind meat and fish pieces and to firm yoghurts and plant-protein products. Gelatin, derived from collagen, forms thermoreversible gels on cooling as chains re-form triple helices; gelatin gels melt near body temperature, which gives their characteristic melt-in-the-mouth quality.
Polysaccharides gel by forming junction zones between chains, often through ordered helices and ion bridges. Agar, from red seaweeds, forms strong gels at low concentration with marked thermal hysteresis: it sets at about 32 to 40 °C but melts only at about 85 °C or above, useful for gels that must withstand warm conditions. Carrageenans differ: κ-carrageenan forms firm, brittle gels with potassium ions, ι-carrageenan forms soft, elastic gels with calcium ions, and λ-carrageenan does not gel but thickens; κ-carrageenan also interacts specifically with κ-casein and is used at low levels to stabilise milk drinks. Alginates, from brown seaweeds, form cold-set gels with calcium ions bridging blocks of guluronic acid in an 'egg-box' arrangement, used to make restructured foods, fruit pieces and beads, with gelation controlled by the rate of calcium release. Pectins gel in two ways: high-methoxyl pectin needs high sugar (soluble solids above about 55 per cent) and low pH (about 2.8 to 3.5) to reduce water activity and charge so that chains associate by hydrogen bonding and hydrophobic interactions, as in jams; low-methoxyl pectin gels with calcium in an egg-box structure and suits low-sugar products. Gellan gum forms clear gels with mono- or divalent cations. Starch forms gels on cooling of gelatinised pastes through retrogradation of amylose, followed more slowly by amylopectin. Some non-gelling polysaccharides gel together: xanthan gum with locust bean gum or konjac glucomannan forms thermoreversible gels through synergistic interaction.
Gels are characterised by small-deformation rheology (G′, G″, the gel point, and the temperatures of setting and melting), by large-deformation tests that measure fracture stress and strain (distinguishing brittle from elastic gels, which matters for the sensory perception of firmness and how a gel breaks down in the mouth), and by water-holding capacity and syneresis, the expulsion of liquid as the network contracts, which is a defect in yoghurt and some desserts but desired in cheese making. Mixing gelling agents creates composite gels whose properties depend on whether the networks interpenetrate, phase separate or one fills the other, the basis of much texture design in desserts, confectionery and plant-based foods.
When proteins and polysaccharides are mixed in water, as they are in most foods, they interact in ways that can either separate or join them. In segregative interactions, the two polymers repel each other, usually because they have similar charges or because mixing is thermodynamically unfavourable (thermodynamic incompatibility), so above certain concentrations the mixture separates into two aqueous phases, one rich in protein and the other rich in polysaccharide. Phase diagrams map the compositions at which this happens. Such water-in-water emulsions, for example of gelatin and maltodextrin or of milk proteins and certain polysaccharides, can be structured by shear and then fixed by gelation, creating particles, fibres and composite gels with designed textures; uncontrolled phase separation, however, causes defects such as whey separation in dairy desserts thickened with incompatible polysaccharides.
In associative interactions, the polymers attract each other, most commonly by electrostatic attraction between a positively charged protein (below its isoelectric point) and a negatively charged polysaccharide such as gum arabic, pectin, carrageenan, alginate or carboxymethylcellulose. Depending on charge density, ratio, pH, ionic strength and concentration, they form soluble complexes or insoluble complexes that separate as a dense, polymer-rich liquid phase, a process called complex coacervation. Coacervation occurs in a pH window between the isoelectric point of the protein and the pKa of the polysaccharide's acidic groups, where the two carry opposite charges; maximum yield is usually near the pH at which the charges balance. Salt screens the charges and suppresses complex formation. The classic system is gelatin and gum arabic, which forms coacervates at about pH 4 that deposit around oil droplets and are hardened by cross-linking, the basis of a widely used encapsulation process (chapter 8).
Controlled interactions solve many practical problems. Acidified milk drinks, such as drinking yoghurts and milk-juice blends at pH about 3.5 to 4.2, would sediment because casein aggregates near its isoelectric point; high-methoxyl pectin or carboxymethylcellulose adsorbs on the positively charged casein particles, giving them a negatively charged, steric coat that prevents aggregation. The amount must be enough to cover the particles; too little causes bridging flocculation, and excess non-adsorbed pectin can cause depletion flocculation. Similarly, κ-carrageenan at low concentration forms a weak network with κ-casein in chocolate milk that keeps cocoa particles suspended.
Covalent conjugates formed by the Maillard reaction between protein amino groups and polysaccharide reducing ends, under controlled dry heating, combine the surface activity of the protein with the steric stabilisation of the polysaccharide, giving emulsifiers that are stable to heat, low pH and salt; they occur naturally in gum arabic, whose small protein fraction attached to a highly branched arabinogalactan makes it an excellent emulsifier for beverage emulsions. Layer-by-layer deposition of oppositely charged polymers on emulsion droplets builds multilayer interfaces that protect encapsulated oils from oxidation and control their digestion. Electrostatic complexes are also used to form fat replacers that mimic the size and mouthfeel of fat droplets, to stabilise foams, to form gel particles (microgels) for texture and delivery, and to make fibrous structures in plant-based meat analogues. In each case the key variables are pH relative to the protein's isoelectric point, the charge density of the polysaccharide, the mixing ratio, ionic strength, temperature and the order of mixing.
Food properties emerge from structure at several length scales: molecules (composition, conformation), supramolecular assemblies (helices, micelles, crystals, interfacial layers), microstructure (droplets, bubbles, cells, crystals, network strands, on micrometre scales) and macrostructure (pieces, layers, pores visible to the eye). Texture depends on how these structures resist and break down under the forces of biting and chewing; flavour release depends on how aroma compounds and tastants partition between phases and diffuse out of the matrix; stability depends on mobility, which is governed by the glass transition and water activity (see shelf-life-stability); and digestion depends on how easily enzymes reach their substrates. The materials-science approach to foods treats them as soft condensed matter and applies concepts such as phase diagrams, glass transitions, percolation and fracture mechanics to explain and predict these properties.
Examples show the principle. The snap, gloss and melting of chocolate come from a dense network of cocoa butter crystals in the correct polymorph (form V) surrounding sugar and cocoa particles; the spreadability of margarine comes from a network of β′ fat crystals holding liquid oil and water droplets; the creaminess of mayonnaise from a concentrated, flocculated emulsion of oil droplets that behaves as a soft solid; the crispness of a biscuit from a glassy, porous solid foam; and the juiciness of meat from water held within the myofibrillar network. Changing composition without changing structure rarely reproduces a product, which is why reformulation to reduce fat, sugar or salt is a structural problem as much as a compositional one.
Structure design uses processing to build the arrangements that give desired properties. In reduced-fat products, emulsion gels, in which droplets are bound into a gel network, and microgel particles of protein or polysaccharide can replace some fat while keeping viscosity and creaminess; water-in-oil-in-water emulsions place water inside oil droplets, lowering fat content. Oleogels structure liquid oils into solid-like materials with ethylcellulose, waxes, monoglycerides or phytosterol mixtures, offering alternatives to saturated and trans fats in spreads and bakery; bigels combine oleogel and hydrogel phases. For salt and sugar reduction, structures that concentrate tastants where they are released first, inhomogeneous distributions (for example layered or particulate salt), and faster-dissolving particles increase perceived intensity with less of the ingredient. Air can replace mass: aerated chocolate and confectionery reduce calories per volume.
Plant-based meat analogues rely on structuring proteins into fibres: high-moisture extrusion cools and aligns proteins in a long cooling die, shear cells deform protein blends between a rotating and a stationary surface, and spinning and freeze structuring are also used, with anisotropy (difference in properties along and across fibres) as a measure of meat-likeness. Three-dimensional food printing deposits pastes, gels or melts layer by layer to build personalised shapes and textures, for example for people with swallowing difficulties, who need foods of defined texture classes. Structures are also designed for health: emulsions that slow lipid digestion or gels that delay gastric emptying may increase satiety; intact plant cell walls slow starch and lipid digestion, so the degree of cell rupture during processing changes the glycaemic response to legumes and cereals; and encapsulation targets the release of bioactives (chapters 8 and 9). Design is iterative: build a structure, characterise it by microscopy, rheology and scattering, test its function (sensory, stability, digestion), and adjust the formulation and process.
Encapsulation surrounds or embeds a core material (an active ingredient) within a wall or matrix to protect it, control its release, mask its taste or odour, separate reactive ingredients, or convert a liquid into an easily handled powder. Microcapsules range from about 1 to 1000 µm; particles below about 1 µm, and especially below 100 to 200 nm, are often called nanocapsules or nanoparticles (chapter 9). Structures include reservoir (core-shell) capsules, in which a distinct core is surrounded by a shell, and matrix (monolithic) particles, in which the active is dispersed throughout. Typical cores are flavour oils, fish and algal oils rich in polyunsaturated fatty acids, vitamins, carotenoids, polyphenols, probiotics, enzymes, minerals such as iron, and acids and leavening agents. Wall materials must be food-grade and suited to the process and release: carbohydrates (maltodextrins, gum arabic, octenyl succinic anhydride-modified starches, cyclodextrins, alginate, chitosan, pectin, cellulose derivatives), proteins (whey proteins, caseinates, gelatin, soy and pea proteins, zein), and lipids (fats, waxes, phospholipids).
Spray drying is the most widely used encapsulation method for flavours and oils, because it is continuous, economical and uses existing equipment. An emulsion of the core in a solution of wall material is atomised into hot air, typically with inlet temperatures around 150 to 220 °C and outlet temperatures around 70 to 90 °C; water evaporates in seconds, forming a dry skin around the droplets. The droplet temperature stays near the wet-bulb temperature while water evaporates, protecting heat-sensitive cores. Volatile flavours are retained better than expected from their volatility because, as the surface dries, water diffuses through the skin faster than larger flavour molecules (selective diffusion). Good results need a stable feed emulsion with small droplets, high solids content, wall materials with good film-forming and emulsifying properties (such as gum arabic or OSA starch with maltodextrin), and correct drying conditions. Spray chilling or spray cooling atomises a molten lipid containing the active into cold air, forming solid lipid particles that release on melting, used for minerals, vitamins and acids in bakery; fluidised-bed coating sprays a coating onto suspended particles to build controlled layers.
Complex coacervation (chapter 6) disperses oil in a solution of gelatin and gum arabic above the gelling temperature, lowers the pH to about 4 so that the polymers form a coacervate that deposits around the droplets, cools to gel the shell, and cross-links it, traditionally with glutaraldehyde in non-food uses and with transglutaminase or plant phenolics for food; it gives high payloads (often well above 50 per cent oil) and capsules that release under pressure or heat, used for fish oils and flavours. Extrusion methods include melt extrusion, in which flavour is mixed into a molten carbohydrate mass that is extruded and cooled to a glass, protecting flavours against oxidation by the low permeability of the glassy matrix; and extrusion-dripping, in which a solution of alginate containing the active is dripped or sprayed into a calcium chloride bath, gelling instantly into beads, widely used for probiotics, cells and enzymes and for 'popping' spheres in drinks. Other methods include inclusion complexes with β-cyclodextrin, whose hydrophobic cavity holds small molecules such as flavour compounds and reduces their volatility and degradation; freeze drying of emulsions or suspensions for heat-sensitive materials; liposomes (chapter 9); and electrospraying and electrospinning for very fine particles and fibres.
Performance is described by several measures. Encapsulation efficiency is the fraction of the core that is inside the capsules rather than on the surface: EE (%) = (total core − surface core)/total core × 100. For example, a spray-dried fish oil powder with 30 g of total oil per 100 g and 1.5 g of surface (extractable) oil has an encapsulation efficiency of 95 per cent; surface oil oxidises quickly, so high efficiency matters for shelf life. Loading or payload is the core content of the powder. Release can be triggered by diffusion through the wall, dissolution of the wall in water (most carbohydrate walls), melting (lipid walls, at a designed temperature), swelling, pH change (for example alginate or chitosan systems), enzymatic degradation in the gut, or mechanical rupture during chewing. Probiotic encapsulation aims to protect cells during processing, storage and passage through the acid stomach, with release in the intestine; viability must be shown by counts throughout shelf life. Encapsulated ingredients must also behave well in the final product, without grittiness, off-colours or interactions with other ingredients.
Nanoemulsions are emulsions with droplet radii typically below about 100 nm (definitions vary, some using diameters up to 200 nm). Because their droplets are much smaller than the wavelength of light, they scatter little light and can be transparent or slightly turbid, which allows oil-soluble flavours, colours, vitamins and bioactives to be added to clear beverages. Their small size also makes them very resistant to gravitational separation, because Brownian motion outweighs creaming, and less prone to flocculation and coalescence; they can increase the bioaccessibility of poorly water-soluble compounds such as carotenoids and curcumin by speeding lipid digestion and micelle formation. They remain thermodynamically unstable, however, and Ostwald ripening is often their main instability when the oil has some water solubility, as with flavour oils; adding an insoluble oil such as a long-chain triglyceride to the dispersed phase suppresses it.
Nanoemulsions are made by high-energy methods, high-pressure homogenisation, microfluidisation and ultrasonication, which need intense energy and enough emulsifier to cover the large area, or by low-energy methods, which use the physical chemistry of surfactant-oil-water systems: spontaneous emulsification, in which an organic phase containing oil and a water-miscible surfactant or solvent is mixed into water and droplets form as the surfactant diffuses; the phase-inversion temperature method, which uses the change of non-ionic surfactants from hydrophilic to lipophilic with temperature; and the phase-inversion composition method. Low-energy methods usually need high surfactant levels, often synthetic surfactants, which limits their acceptance in food. Microemulsions differ fundamentally: they are thermodynamically stable, transparent systems of oil, water and surfactant (often with a co-surfactant) that form spontaneously, with structures of a few to tens of nanometres; they need very high surfactant concentrations and break down on dilution, so their food uses are limited.
Liposomes are spherical vesicles formed by one or more phospholipid bilayers enclosing an aqueous core. They can carry water-soluble compounds in the core and fat-soluble compounds in the bilayer. They are classed as small unilamellar vesicles (tens of nanometres), large unilamellar vesicles (about 100 nm to 1 µm) and multilamellar vesicles. They are prepared by hydrating a dried lipid film followed by sonication or extrusion through membranes of defined pore size to reduce and unify size, by ethanol injection, by reverse-phase evaporation, or by microfluidisation, which suits scale-up. Liposomes from food lecithins are used to deliver vitamins, minerals, enzymes in cheese ripening and antimicrobials; their limitations are physical instability (aggregation, fusion, leakage), oxidation of unsaturated phospholipids, and cost. Stabilisation includes cholesterol or phytosterols in the bilayer, coating with chitosan or other polymers, and drying with sugars such as trehalose.
Solid lipid nanoparticles are made from lipids that are solid at body temperature; their crystalline matrix can protect and slow the release of actives, but crystallisation into more ordered polymorphs during storage can expel the active, which nanostructured lipid carriers, mixing solid and liquid lipids, are designed to reduce. Biopolymer nanoparticles are formed from proteins and polysaccharides: zein, the prolamin of maize, is dissolved in aqueous ethanol and precipitated by adding water (antisolvent precipitation), forming hydrophobic nanoparticles that carry curcumin, essential oils and other compounds; chitosan forms nanoparticles by ionic gelation with tripolyphosphate; and protein-polysaccharide complexes form nanogels. All these systems are characterised by size and polydispersity (by dynamic light scattering, which reports a z-average diameter and a polydispersity index, with values below about 0.2 to 0.3 indicating fairly uniform dispersions), zeta potential, morphology (electron microscopy), encapsulation efficiency, stability over time and under processing, and release behaviour. Their behaviour in real foods and in the gut, and their safety (chapter 12), must be demonstrated before use.
The health value of a nutrient or bioactive compound depends not only on how much a food contains but on how much is released and absorbed. Bioaccessibility is the fraction of a compound released from the food matrix during digestion and made available for absorption, for example carotenoids transferred into mixed micelles in the small intestine. Bioavailability is the fraction that is absorbed and reaches the systemic circulation or the site of action, which also depends on uptake, metabolism and excretion. Food structure affects both: carotenoids in raw carrot cells are poorly bioaccessible, while cooking, homogenisation and the presence of fat increase their release; nanoemulsions and small droplets increase lipid digestion and the bioaccessibility of fat-soluble compounds; and intact plant cell walls slow starch and protein digestion.
In-vitro digestion models simulate the gastrointestinal tract to study these effects without human or animal studies, at least as a first step. Because results depend strongly on conditions, an international network of researchers (INFOGEST) published a harmonised static protocol in 2014, revised in 2019. In outline, food is mixed with simulated salivary fluid containing α-amylase for 2 minutes at pH 7 (the oral phase); then with simulated gastric fluid containing pepsin (and, in the revised protocol, gastric lipase) at pH 3 for 2 hours (the gastric phase); and then with simulated intestinal fluid containing pancreatin (or individual enzymes) and bile salts at pH 7 for 2 hours (the intestinal phase), all at 37 °C with defined electrolyte concentrations and enzyme activities. Static models use fixed conditions in each phase; they are simple and reproducible and allow comparison between laboratories, but they do not reproduce gradual acidification, secretion, gastric emptying or mechanical forces.
Dynamic models add realism: computer-controlled systems simulate gradual changes of pH, secretion of enzymes and bile, peristaltic mixing and the emptying of the stomach into the intestine, and some reproduce the shape and contractions of the human stomach; others include absorption through dialysis membranes. They are more physiologically relevant but complex, expensive and less standardised. Semi-dynamic protocols, including a semi-dynamic version of the INFOGEST method, bridge the two.
Measurements during and after digestion quantify what happens. Lipid digestion is followed by the pH-stat method: as lipase releases free fatty acids from triglycerides at constant pH, alkali is added to neutralise them, and the volume of alkali gives the extent and rate of lipolysis; smaller droplets, with more surface area for lipase, are digested faster. Protein digestion is followed by the release of free amino groups, peptide profiles by electrophoresis and mass spectrometry, and digestibility; starch digestion is classified by the Englyst method into rapidly digestible starch (released within 20 minutes), slowly digestible starch (20 to 120 minutes) and resistant starch (not digested in 120 minutes), which relates to glycaemic response. Bioaccessibility of lipophilic compounds is measured by centrifuging the intestinal digesta and analysing the micellar fraction. Coupling digestion with cell models, particularly Caco-2 cell monolayers that resemble intestinal enterocytes, adds a measure of uptake and transport, and microbial fermentation models simulate the colon.
In-vitro results are valuable for comparing products and mechanisms, but they must be interpreted with caution. They omit absorption feedback, hormonal regulation, the full variety of the microbiota, inter-individual variation and the complex physiology of infants, older adults and people with disease, for whom modified protocols exist. Where possible, they are validated against human data; for example, correlations between in-vitro and in-vivo protein digestibility support the use of in-vitro methods in protein-quality assessment. Designs for health effects should eventually be tested in well-conducted human studies.
Nanotechnology is applied to food packaging more than to food itself. Polymer nanocomposites incorporate small amounts, often a few per cent by weight, of nanoscale fillers into plastics or biopolymers. Layered silicate clays such as montmorillonite, dispersed as thin platelets, force gas molecules to follow a tortuous path around them, reducing oxygen, carbon dioxide and water-vapour permeability and improving mechanical and thermal properties; they are used in beverage bottles, films and coatings. Nanocellulose, in the form of cellulose nanocrystals and nanofibrils from wood, agricultural residues or bacteria, strengthens bio-based films and coatings and improves their barrier to oxygen and grease, supporting the replacement of fossil plastics. Other fillers include nano-silica and carbon-based materials.
Active packaging releases or absorbs substances to extend shelf life (see food-packaging), and nanomaterials are studied for several active functions. Silver nanoparticles release silver ions that are antimicrobial, and they have been incorporated into storage containers and films in some markets; zinc oxide and titanium dioxide nanoparticles absorb ultraviolet light and have antimicrobial or photocatalytic activity; and nanoscale iron and other materials are investigated as oxygen scavengers. The benefits must be weighed against the possibility that nanoparticles or ions migrate into food: migration depends on the polymer, the particle, the food simulant and the conditions, and regulators require migration testing and safety assessment, with some jurisdictions restricting or not authorising particular nanomaterials for food-contact use. Coatings applied directly to foods, such as edible coatings on fruit containing nanoemulsions of essential oils, are regulated as food components.
Intelligent packaging monitors the condition of the food or its environment and communicates it. Nanomaterials enable sensitive, low-cost indicators and sensors. Gold and silver nanoparticles change colour dramatically when they aggregate or when their surface changes, because of shifts in their surface plasmon resonance; this is used in colorimetric tests that detect pathogens, toxins, pesticides or spoilage amines by visible colour change. Freshness indicators respond to compounds released during spoilage, such as volatile amines from fish and meat, carbon dioxide from microbial growth or hydrogen sulphide, through pH-sensitive dyes, often immobilised on nanostructured supports that increase sensitivity. Time-temperature indicators record cumulative thermal exposure through chemical, enzymatic, polymer or nanoparticle-based reactions whose colour change matches the kinetics of quality loss in the food (see shelf-life-stability). Gas sensors based on nanostructured metal oxides or conducting polymers detect oxygen, ethylene or volatile spoilage compounds in packs and storage rooms, and radio-frequency identification tags with printed sensors combine identification with data.
The practical uptake of such systems depends on cost per pack, reliability over the product's life, avoiding false alarms, consumer understanding, compatibility with recycling, and regulatory approval of materials in contact with food. In the European Union, active and intelligent materials have specific rules requiring that active substances are authorised and that intelligent components do not migrate into food or mislead consumers; other jurisdictions apply general food-contact rules case by case. Detection devices for laboratories and processing lines, such as nanomaterial-enhanced biosensors and surface-enhanced Raman spectroscopy using metal nanostructures to amplify signals, are a further application, reaching very low detection limits for contaminants but still needing validation in complex food matrices (see advanced-analytical-omics). Life-cycle and end-of-life considerations, including whether nanocomposites interfere with recycling streams and how nanomaterials behave in the environment, are part of responsible development.
Materials at the nanoscale can behave differently from the same substances in bulk or dissolved form. Their very high surface area per unit mass increases reactivity; their size can allow uptake across biological barriers, such as the intestinal epithelium, and distribution to organs; they may persist as particles rather than dissolving; and properties such as shape, surface charge, coating, crystallinity and agglomeration influence their behaviour and toxicity. A substance known to be safe as a conventional ingredient or additive therefore may not be automatically safe in nanoform, and conversely, nanoscale structures are not hazardous merely because they are small. Many foods naturally contain nanoscale structures, such as casein micelles, fat globule membranes and plant cell components, and processes such as homogenisation create droplets in the nanoscale range from digestible food materials; these are generally distinguished from persistent, engineered nanomaterials.
Regulators therefore focus on materials that retain nanoscale properties in the food and in the body. The European Food Safety Authority's guidance on risk assessment of nanomaterials in the food and feed chain (2018, updated in 2021) sets out a tiered approach. The material must be characterised in detail (particle size distribution, shape, surface area, surface chemistry, crystallinity, impurities) as manufactured, in the food and in biological fluids; its dissolution or degradation in conditions simulating digestion is tested, and if it dissolves quickly into non-nanoscale forms, conventional assessment of the dissolved substance may suffice. If it persists as particles, studies of toxicokinetics (absorption, distribution, accumulation, excretion), in-vitro and in-vivo genotoxicity tested with nanoparticle-appropriate methods, and repeated-dose toxicity are required. Characterising nanoparticles in complex food matrices, and choosing realistic exposure scenarios, remain technically difficult.
Definitions of nanomaterial differ between jurisdictions. The European Commission's recommended definition (revised in 2022) refers to materials of solid particles in which half or more of the particles in the number-based size distribution have at least one external dimension between 1 and 100 nm; food legislation in the European Union uses the term 'engineered nanomaterial' and requires that ingredients present as engineered nanomaterials be labelled in the ingredient list with the word 'nano' in brackets after the name, and novel-food and additive authorisations specify the form assessed. In the United States, the regulator has issued guidance that considers whether a product involves nanotechnology, without a separate legal category, assessing safety case by case within existing frameworks such as food additive petitions and generally recognised as safe notifications. Other countries, including Canada, Australia and New Zealand, several Asian countries and others, have their own approaches, and Codex has no specific standard for nanomaterials in food, while international organisations such as the OECD coordinate testing guidance.
The case of titanium dioxide (E 171), a white colouring used widely in confectionery, bakery decorations and sauces, illustrates how assessments can change. E 171 consists of particles of which a fraction are in the nanoscale range. In 2021 the European Food Safety Authority concluded that it could no longer be considered safe as a food additive, mainly because concerns about genotoxicity could not be ruled out, and the European Union withdrew its authorisation as a food additive from 2022. Authorities in some other jurisdictions reviewed the same evidence and reached different conclusions, keeping it authorised while monitoring, which shows how regulatory judgements can differ on uncertain evidence. Silicon dioxide (E 551) and other additives containing nanoscale fractions have also been re-evaluated. For developers of nano-enabled foods, packaging and supplements, the practical lessons are to characterise materials thoroughly, to prefer digestible, food-grade materials that do not persist as particles, to engage early with the relevant regulator, and to communicate openly with consumers, whose acceptance of nanotechnology in food is often cautious.
A student uses this subject to explain and control the structure of products made in the laboratory and pilot plant: making and characterising emulsions by droplet size and creaming, setting gels with different hydrocolloids, whipping foams and measuring overrun, spray-drying a flavour emulsion and measuring encapsulation efficiency, and running an INFOGEST digestion. Calculations of Laplace pressure, Stokes velocity, overrun, specific surface area and encapsulation efficiency are standard examination items.
Five years into product development, a technologist chooses emulsifiers and stabilisers for dressings, beverages and dairy desserts, solves creaming, syneresis and foam problems, designs reduced-fat or reduced-sugar structures, specifies encapsulated ingredients from suppliers and checks their stability in products, and works with regulatory colleagues on the status of nano-sized ingredients.
A senior R&D leader with twenty-five years' experience sets the structure-design strategy for a portfolio, decides on investment in encapsulation and structuring technologies such as high-moisture extrusion, evaluates delivery systems for bioactives and their evidence, and manages regulatory and consumer risks around nanotechnology and clean-label reformulation.
A researcher at this level develops new colloidal structures and delivery systems, studies interfacial and digestion mechanisms with advanced characterisation, models structure-property relationships, and contributes to the scientific basis of nanomaterial risk assessment and to harmonised digestion protocols.
This manual covers food colloids, structure design, encapsulation and nanotechnology at postgraduate level. It does not give formulations for commercial products, detailed equipment design, or the current authorisation status of every hydrocolloid, emulsifier and nanomaterial in each jurisdiction, which changes and must be checked. Numerical values (interfacial tensions, gelling conditions, spray-drying temperatures, size thresholds) are typical and vary with materials and methods. Rheology is treated in food-physical-properties-rheology, packaging in food-packaging and analytical characterisation in advanced-analytical-omics.
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