Dairy Science & Technology · Dairy Processing & Product Technology · also Membrane Technology, Dairy Ingredients Technology
MF, UF, NF and RO in milk and whey processing, the protein and lactose ingredients they make, and how their function is measured.
In the exams: ICAR AICE subject 37 Unit 4 and subject 38 Unit 3; ICAR AIEEA-PG group 17 Unit I (membrane processing in milk processing)
Membrane processes have changed dairy technology more than any other development of the past fifty years. By separating milk and whey components according to their size and charge, at moderate temperatures and without boiling, they made possible whey protein concentrates and isolates, milk protein concentrates, micellar casein, extended shelf life milk, demineralised whey for infant formula, lactose-reduced milks, high-protein yoghurts without acid whey, and the recovery of water from milk itself. A modern dairy is as much a membrane plant as a heat-treatment plant.
This manual covers membrane classification and materials; modules and plant configurations; flux, transmembrane pressure and concentration factor; concentration polarisation and fouling models; cleaning and sanitation; microfiltration for bacteria removal and micellar casein; ultrafiltration for milk protein concentrates and cheese milk; nanofiltration and electrodialysis for demineralisation; reverse osmosis for concentration and water recovery; diafiltration; the functional properties of dairy proteins; and ingredient specifications and applications. Whey-specific products (WPC, WPI, lactose, permeate) are covered from the by-product side in whey-dairy-byproducts, ESL milk in market-milk-processing, the MMV process in cheese-science-technology and drying in evaporation-drying-dairy; this manual concentrates on the membrane science and the ingredients.
ICAR AICE-JRF/SRF Dairy Technology (subject 37, Unit 4) covers classification of membrane processes, membrane hardware, plant design, UF modelling (mass transfer and resistance models), fouling, cleaning, factors affecting flux in UF and RO, energy requirements, special foods, demineralisation and the functional properties of whey proteins, casein, co-precipitates and UF retentate; Dairy Engineering (subject 38, Unit 3) covers membrane types, properties and selection. ICAR AIEEA-PG Dairy Technology names membrane processing in milk processing. Operating values in this manual are representative; supplier data and plant trials govern real design.
Membrane filtration separates the components of a liquid by passing it along a semi-permeable barrier under pressure: what passes through is the permeate, what is held back the retentate (or concentrate). Four pressure-driven processes are used in dairying, distinguished by the size of what they retain. Microfiltration (MF) has pores of roughly 0.1–10 µm (in dairying usually about 0.1–1.4 µm) and retains bacteria, spores, fat globules and, at the fine end, casein micelles, while serum proteins, lactose and salts pass. Ultrafiltration (UF) retains proteins and fat — its membranes are rated by molecular weight cut-off, commonly around 10 kDa for dairy — while lactose, salts and water pass. Nanofiltration (NF) retains lactose and divalent ions such as calcium but passes water and most monovalent ions (sodium, potassium, chloride) and small organic acids. Reverse osmosis (RO) retains virtually everything except water. The tighter the membrane, the higher the pressure needed: MF runs at low transmembrane pressures (a fraction of a bar to a few bar), UF at a few bar to around ten, NF at tens of bar and RO at several tens of bar, because the osmotic pressure of the retained solutes must be overcome. Electrodialysis, an electrically driven membrane process, is treated with NF in a later chapter.
These processes work at low or moderate temperatures without phase change, so they concentrate and fractionate milk with less energy and less heat damage than evaporation, and they can separate components by size, which heat cannot.
Dairy membranes are made from polymers or ceramics. Polymeric membranes include polysulfone and polyethersulfone (common for UF and some MF), polyvinylidene fluoride (PVDF, for MF and UF), and thin-film composite polyamide on a polysulfone support (for NF and RO); cellulose acetate was used in early plants but tolerates only a narrow pH and temperature range. Polymeric membranes are relatively cheap and are made in large spiral-wound modules, but they have limits on temperature, pH, pressure and chemical exposure — polyamide NF and RO membranes, for example, are damaged by chlorine — and they have a finite life before they must be replaced. Ceramic membranes, made from alumina, zirconia or titania on a porous support, tolerate high temperatures, extreme pH, strong cleaning chemicals and steam sterilisation, and last for many years; they are used mainly for MF (for example bacteria removal and micellar casein) but are expensive per square metre. Membrane structure may be symmetric or asymmetric, with a thin selective skin on a porous support that gives mechanical strength without adding much resistance. Choice of material balances selectivity, flux, chemical and thermal robustness, life and cost.
A membrane is used in a module that holds it, distributes feed over it and collects the permeate. Spiral-wound modules wrap flat membrane sheets, separated by feed spacers and permeate carriers, around a central permeate tube; they pack a large area into a small volume at low cost and dominate UF, NF and RO in dairying, but their narrow feed channels are prone to blockage by particles and require good pre-treatment, and they are harder to clean thoroughly. Spacer thickness is chosen to suit the viscosity of the feed. Tubular modules use membrane-lined tubes of around a centimetre or more in diameter; they handle viscous feeds and particles and are easy to clean, but need high flow rates and pumping energy and give little area per volume. Hollow-fibre modules contain thousands of fine fibres; they offer very high area and low cost but tolerate only low pressures and are mechanically fragile. Plate-and-frame modules stack flat sheets and suit viscous products and small plants. Ceramic membranes are usually multichannel monoliths in stainless steel housings.
Membrane plants are operated in several ways. In batch operation, feed is recirculated through the modules from a tank until the desired concentration is reached; the flux falls as concentration rises. In feed-and-bleed (continuous) operation, a recirculation loop around the modules is fed continuously and a fraction of the recirculating retentate is bled off as product; this keeps the loop at the final concentration and so at the lowest flux. Industrial plants therefore use several loops in series (multistage), each operating at a progressively higher concentration, so that most of the permeate is removed at low concentration and high flux. Cross-flow (tangential) operation, with the feed flowing parallel to the membrane at high velocity, sweeps away retained material and limits fouling; dead-end filtration is used only for clean, low-solids streams. For MF of skim milk, uniform transmembrane pressure systems circulate permeate on the permeate side in the same direction as the retentate to keep the pressure difference equal along the length of the membrane, which improves selectivity; graded-permeability ceramic membranes achieve a similar effect. Plants include feed tanks, pre-filters, feed and recirculation pumps, heat exchangers to control temperature, instrumentation for pressures, flows and concentration, and CIP systems.
Membrane plants depend on what happens before the membranes. Feed is clarified or screened to remove curd fines and particles that would block spiral-wound spacers; fat is removed by centrifugal separation because it fouls membranes and reduces protein purity; and the feed may be heat-treated to control bacteria, though heating whey too much denatures proteins and increases fouling. Cartridge or bag pre-filters protect the modules. Temperature is controlled with plate heat exchangers before and within the loops, since flux, fouling and microbial growth all depend on it. Instruments monitor pressures at each stage, flows of feed, permeate and retentate, temperature and the concentration of the retentate (often by refractometer, density or conductivity), and the control system adjusts bleed rates to hold the product specification. Long production runs require attention to microbial growth inside the plant, especially in warm operation, which is why some plants run cold or limit run length before cleaning.
Flux is the permeate flow per unit membrane area, usually given in litres per square metre per hour (L m⁻² h⁻¹, often written LMH): J = permeate flow ÷ membrane area. For example, 1,200 L/h of permeate from 80 m² is a flux of 15 LMH. Transmembrane pressure (TMP) is the average pressure difference driving permeation: TMP = (inlet pressure + outlet pressure) ÷ 2 − permeate pressure. With 4.0 bar at the inlet, 2.0 bar at the outlet and 0.5 bar on the permeate side, TMP = 3.0 − 0.5 = 2.5 bar. The pressure drop from inlet to outlet is needed to drive the cross-flow but means that TMP varies along the membrane.
The volume concentration factor (VCF) is the ratio of feed volume to retentate volume: VCF = V feed ÷ V retentate. If 10,000 L of feed gives 7,500 L of permeate and 2,500 L of retentate, VCF = 4. For a solute that is completely retained, its concentration in the retentate equals its feed concentration times VCF; milk protein at 3.3 per cent becomes 13.2 per cent at VCF 4. Rejection (retention) of a solute is R = 1 − Cp ÷ Cr, where Cp and Cr are its concentrations in permeate and retentate; a protein at 0.05 per cent in permeate and 4 per cent in retentate has R = 1 − 0.05 ÷ 4 = 0.9875. For partially retained solutes, the retentate concentration rises as Cr = Cf × VCF raised to the power R, so a solute with R = 0.1 rises by only 5 to the power 0.1, about 1.17 times, at VCF 5.
At low TMP, flux increases roughly in proportion to pressure, as for pure water through the membrane. As TMP rises, a layer of retained solutes builds up near the membrane and flux increases less and less, reaching a limiting flux beyond which more pressure gives no more flux and only compacts the fouling layer. Flux increases with cross-flow velocity, which thins this layer, and with temperature, mainly because viscosity falls; dairy plants run either warm (around 50 °C, high flux but risk of calcium phosphate precipitation and protein denaturation) or cold (around 10 °C or below, lower flux but less bacterial growth and less mineral fouling). Flux falls as concentration rises, because the feed becomes more viscous and the polarisation layer thicker, and over time as fouling accumulates. Many plants operate at or below the critical flux, the flux below which fouling is minimal, to extend run times. Plant design uses pilot data on flux against concentration to calculate the membrane area needed: area = permeate flow ÷ average flux.
Because flux depends on the specific feed, membrane, temperature and concentration, membrane plants are designed from pilot trials rather than from theory alone. A pilot run measures flux against VCF (or concentration) at the planned temperature and cross-flow, rejection of each component, the rate of flux decline over a run, and cleaning recovery. From these data the designer calculates the average flux for each stage of a multistage plant, the membrane area per stage (area = permeate flow ÷ flux), the number of modules, pump sizes and energy use. A safety margin is added for seasonal variation in milk and whey and for membrane ageing. For example, a plant concentrating 20,000 L/h of whey ten-fold removes 18,000 L/h of permeate; at an average flux of 20 LMH it needs about 900 m² of membrane. Energy is mainly pumping energy for cross-flow and pressurisation, which is far less per kilogram of water removed than evaporation for dilute feeds.
As permeate passes through the membrane, retained solutes are carried towards the membrane surface faster than they diffuse back into the bulk, so their concentration at the surface rises above the bulk concentration. This boundary layer, concentration polarisation, is reversible: it disappears when pressure is released or flow increases. It reduces flux by adding hydraulic resistance and, for small solutes in NF and RO, by raising the osmotic pressure at the membrane surface. In protein UF, the surface concentration can reach a limit at which a gel-like layer forms. The gel-polarisation (film) model describes this limiting case: J = k ln(Cg ÷ Cb), where k is the mass transfer coefficient (which increases with cross-flow velocity and decreases with viscosity), Cg is the gel concentration and Cb the bulk concentration. For example, with k = 20 LMH, Cg = 25 per cent and Cb = 5 per cent, J = 20 × ln 5 ≈ 32 LMH. The model explains why flux falls logarithmically with concentration and reaches zero when the bulk concentration approaches the gel concentration, which sets the practical maximum concentration of UF retentates.
The resistance-in-series model writes flux as J = TMP ÷ [μ (Rm + Rf + Rc)], where μ is permeate viscosity, Rm the clean membrane resistance, Rf the fouling resistance and Rc the resistance of the polarisation or cake layer. With TMP = 2 × 10⁵ Pa, μ = 10⁻³ Pa s and Rm = 10¹² m⁻¹, the clean water flux is 2 × 10⁻⁴ m/s, or 720 LMH; if fouling adds 4 × 10¹² m⁻¹, flux falls to one-fifth, 144 LMH. In NF and RO the osmotic-pressure model applies: J = Lp (ΔP − Δπ), where Lp is the permeability and Δπ the osmotic pressure difference; as concentration rises, Δπ rises and flux falls even without fouling. Fouling proper is the less reversible accumulation of material on and in the membrane: adsorption of proteins on the membrane material, deposition of a cake or gel layer, blocking of pores (described by Hermia's models as complete, standard and intermediate pore blocking and cake filtration), precipitation of calcium phosphate (whose solubility falls with rising temperature and pH), and biofilm growth. In dairy UF the main foulants are whey proteins and calcium phosphate; in MF of milk, casein micelles and their layer on the membrane shift the effective pore size and change selectivity. Fouling is controlled by pre-treatment, choice of temperature and pH, adequate cross-flow, operating below the critical flux, and effective cleaning.
Membranes must be cleaned at least daily (more often for some MF and whey plants) to restore flux and maintain hygiene. A typical cleaning-in-place sequence for dairy polymeric membranes is: rinse with water to remove product; alkaline wash with sodium hydroxide at a pH around 11–12 and a moderate temperature, often with surfactants and chelating agents, to remove protein and fat; rinse; acid wash with nitric or phosphoric acid, or citric acid, to dissolve calcium phosphate and other minerals; rinse; and sanitise. Enzymatic cleaners (proteases, sometimes lipases) are used for polymeric membranes that cannot take strong alkali or high temperature, often as the first stage. The order (alkali or acid first) is chosen by the fouling: whey UF is commonly cleaned with enzyme or alkali first, while milk UF with heavy mineral fouling may start with acid. Sanitisers depend on membrane chemistry: chlorine-tolerant polysulfone and PES may be sanitised with hypochlorite within limits, whereas polyamide NF and RO membranes need non-chlorine sanitisers such as hydrogen peroxide or peracetic acid. Ceramic membranes can be cleaned with strong alkali and acid at high temperatures and even steam-sterilised.
Cleaning effectiveness is checked by measuring the clean water flux (or normalised water permeability) at a standard pressure and temperature after cleaning and comparing it with the value for a new membrane or a reference; plants set a minimum recovery, and falling recovery signals irreversible fouling or membrane damage. Microbiological swabs, ATP tests and checks of permeate quality complement flux tests. Integrity tests (pressure-hold or bubble-point tests, conductivity of permeate) detect damaged membranes or seals that would let bacteria or proteins pass. Every membrane has limits on pH, temperature, pressure and chemical concentration, and exceeding them shortens its life; polymeric spiral elements are typically replaced after a limited number of years, a significant operating cost. Water quality matters too: hard water deposits scale, and chlorine in water destroys polyamide membranes. Good cleaning programmes are developed with membrane suppliers and validated in the plant.
Microfiltration with ceramic membranes of about 1.4 µm pore size can remove most bacteria and spores from skim milk while letting casein micelles, serum proteins and solutes pass, because bacteria (around 0.4–2 µm and larger) are bigger than micelles (around 0.1–0.2 µm average). Fat globules overlap bacteria in size, so whole milk cannot be microfiltered this way: milk is separated, the skim is microfiltered, and the cream is heat-treated separately (often at high temperature) before being remixed. The bacteria-rich MF retentate, a small stream, is sterilised and returned or discarded. The process removes a very high proportion of bacteria and spores (several log units), and is used for extended shelf life milk (with pasteurisation after remixing), for cheese milk (against late blowing and for raw-milk-style cheese where regulations allow MF as part of the process), and for milk destined for infant formula and powders with strict spore limits. Uniform transmembrane pressure or graded-permeability membranes are essential to keep selectivity high and avoid the micelle layer closing the pores.
Tighter microfiltration, with membranes of about 0.1 µm, retains casein micelles while letting serum (whey) proteins, lactose and salts pass. Applied to skim milk, with diafiltration to wash out more serum protein, it produces micellar casein concentrate (MCC), a high-protein ingredient in which casein remains in its native micellar form, and a permeate called 'native whey' or 'ideal whey', a clear stream of undenatured serum proteins free of caseinomacropeptide, starter culture and cheese fines, which can be concentrated by UF into very high-quality whey protein. MCC is used to standardise cheese milk, in high-protein beverages and yoghurts, and in nutritional products; its casein-to-whey protein ratio is higher than milk's. MF of whey (rather than milk) with similar membranes removes residual fat, phospholipids and bacteria before UF, which is the first step in making whey protein isolate by membrane processing. The selectivity of 0.1 µm MF depends strongly on temperature (warm operation, around 50 °C, gives better serum protein passage) and on the polarisation layer of micelles.
Ultrafiltration of skim milk retains casein and whey proteins together while lactose, soluble minerals and water pass into the permeate. Concentrating and diafiltering skim milk and then spray-drying the retentate produces milk protein concentrates (MPC), named by their protein content in the dry matter (for example MPC 42, 56, 70, 80 and 85), and milk protein isolates (MPI) at around 90 per cent protein. As the protein content rises, lactose and soluble minerals fall, and the colloidal calcium phosphate associated with casein is retained, so high-protein MPCs are rich in calcium. MPCs are used in cheese making, processed cheese, yoghurts, high-protein beverages, nutrition bars, clinical and sports nutrition, and as a partial replacement for milk powder or caseinates. A recognised problem is the loss of solubility of high-protein MPC powders during storage, attributed to casein interactions at the particle surface forming an insoluble skin; it is reduced by partial replacement of calcium with sodium or potassium (by ion exchange or by injecting carbon dioxide or adding salts before drying), by controlling drying conditions and storage temperature, and by high-shear rehydration.
UF of cheese milk at a low concentration factor standardises its protein content to reduce seasonal variation and increase the capacity of each vat; at a high concentration factor it produces a pre-cheese with the composition of the final cheese, which is coagulated without draining whey, as in the MMV process used for cast Feta-type cheese, quark and cream cheese (see cheese-science-technology). UF is also used for high-protein, lactose-reduced fluid milks (often combined with diafiltration and lactase), for protein-fortified yoghurts and strained-style products without acid whey (see fermented-milks-technology), and for concentrating milk before transport. The permeate produced in all these processes is a lactose-rich stream that must be used; its utilisation is covered in whey-dairy-byproducts. Retentate composition is predicted with mass balances using VCF and rejection: completely retained components concentrate by VCF, while freely permeating ones stay at feed concentration, so protein in dry matter rises with concentration and further with diafiltration.
Membranes underpin a growing range of 'filtered' fluid milks. In one common approach, skim milk is ultrafiltered to remove part of the lactose and water, the retentate is diafiltered or recombined with water and the separated cream, and lactase hydrolyses the remaining lactose; the product has less sugar and more protein than ordinary milk, and does not taste as sweet as milk in which all the original lactose has been hydrolysed. Other schemes use NF to remove some minerals or RO to adjust solids. Filtered milks may also be microfiltered for longer shelf life. The composition must be calculated carefully to meet label claims for protein, sugar and lactose, and to meet the national definition of milk or of a milk product, since adding or removing components may change what the product can be called.
Nanofiltration membranes are tighter than UF but looser than RO. They retain lactose and other molecules above a few hundred daltons and divalent ions such as calcium, magnesium, phosphate and sulphate, but pass water and much of the monovalent ions (sodium, potassium, chloride) and small organic acids such as lactic acid. This makes NF useful for simultaneous concentration and partial demineralisation of whey: as whey is concentrated, salts pass with the water, and with diafiltration NF can remove a substantial part (commonly up to about 30–50 per cent) of the ash, especially the monovalent salts that give whey its salty taste. NF-treated whey is used for demineralised whey powders, for infant formula base, and as a feed to further demineralisation. NF is also used to concentrate and deacidify acid whey (removing lactic acid), to recover sodium hydroxide from spent CIP solutions for reuse, and in the production of lactose-reduced milks. Because NF membranes are usually polyamide composites, they are sensitive to chlorine and require careful cleaning chemistry.
For deeper demineralisation, electrodialysis (ED) and ion exchange are used. In electrodialysis, the liquid flows between alternating cation-exchange and anion-exchange membranes in a stack, with an electric field applied across it. Cations move towards the cathode through cation-exchange membranes and anions towards the anode through anion-exchange membranes, so that alternate compartments become depleted of salts (the product, here whey) and enriched (the brine). ED can remove most of the minerals of whey, though energy use and processing time rise steeply as the whey becomes less conductive at high demineralisation, and membranes are fouled by proteins and calcium salts. Ion exchange uses columns of cation- and anion-exchange resins that exchange the whey's minerals for hydrogen and hydroxide ions; it reaches very high demineralisation (around 90 per cent and above) but uses regeneration chemicals and produces saline effluent. Industrial D90 whey for infant formula is commonly produced by combining NF with ED or ion exchange. The choice depends on the degree of demineralisation required, product quality, chemical and energy costs, and effluent rules.
Reverse osmosis membranes pass essentially only water, so RO concentrates all the solids of milk or whey without separating them. Because the osmotic pressure of the retained solutes rises as they concentrate, RO requires high pressures and reaches practical limits at moderate solids, for example around 18–25 per cent total solids for whey and somewhat higher for milk, beyond which flux becomes too low. RO is used to pre-concentrate whey and milk before evaporation, which saves energy because removing water through a membrane is cheaper than boiling it in early evaporator stages; to concentrate whey at small cheese plants for cheaper transport to a central processor; to concentrate milk on farms or at collection centres to reduce haulage; and in combination with NF and UF in fractionation schemes. Calculations use the solids balance: concentrate mass = feed solids ÷ concentrate solids fraction, water removed = feed − concentrate. For example, 10,000 L of whey at 6 per cent solids concentrated to 18 per cent gives about 3,333 L of concentrate and 6,667 L of RO permeate (water).
Dairy plants use large amounts of water, and membrane processes also produce water-rich streams. RO permeate from milk or whey concentration, and the vapour condensate from evaporators (often called 'cow water' because it originates in the milk), are relatively clean but contain traces of organic matter and volatile compounds. RO 'polishers' treat these streams to a quality suitable for reuse — for boiler feed, cleaning, cooling towers, and in some cases, with further treatment and regulatory approval, for product-contact uses. Water reuse reduces abstraction and effluent volumes, and plants increasingly aim to be net water producers. The reuse category must match the water quality: national rules and food safety management systems define where recovered water may be used, and monitoring for microbial and chemical quality is essential.
Concentration alone cannot purify a retained product beyond a certain point: in UF, lactose and soluble salts remain in the retentate liquid at roughly their feed concentration, so the protein content in the dry matter is limited, and the retentate becomes too viscous to concentrate further. Diafiltration (DF) adds water (or a buffer) to the retentate and removes the same or a larger volume as permeate, washing out the permeable solutes while keeping the retained ones. It is used to reach high-protein WPC and MPC grades, to make WPI, MCC and MPI, to remove lactose for lactose-reduced products, and in NF to remove monovalent salts. The cost is water use, longer processing and more permeate to handle, so the amount of diafiltration water is optimised.
In continuous (constant-volume) diafiltration, water is added at the same rate as permeate is removed, so the retentate volume stays constant. For a solute that passes freely through the membrane (zero rejection), its concentration falls exponentially with the number of diavolumes N (the volume of diafiltration water divided by the retentate volume): C ÷ C0 = e^(−N). Three diavolumes leave e^−3, about 5 per cent, of the solute (a 95 per cent reduction); reducing a solute to 1 per cent of its initial concentration needs about ln(100), roughly 4.6 diavolumes. For a solute with rejection R, the expression becomes C ÷ C0 = e^(−N(1 − R)), so partly retained solutes wash out more slowly. In discontinuous (sequential dilution) diafiltration, the retentate is diluted with water and reconcentrated in repeated steps; each step reduces the permeable solute by the dilution ratio, and it uses more water than continuous diafiltration for the same result. Plants combine an initial concentration step (to reduce the volume to be washed), diafiltration at that intermediate concentration, and a final concentration step, which minimises water and membrane area.
The value of a dairy protein ingredient depends on how it behaves in a food. Solubility is the starting point, because most other functions require dissolved or well-dispersed protein. It is measured as nitrogen solubility index or protein solubility (protein in the supernatant after centrifugation under standard conditions, as a percentage of total protein) at specified pH and temperature. Native whey proteins are soluble across the pH range, including at their isoelectric points around pH 4.5–5.2, which allows clear acidic whey protein drinks; heat denaturation lowers solubility, especially near the isoelectric point. Caseins are insoluble near pH 4.6 but soluble at neutral pH as micelles or caseinates. Heat stability (resistance to aggregation on heating, measured for example by heat coagulation time or by turbidity after heating) matters for UHT beverages and retorted nutrition products; it is affected by pH, mineral balance (calcium destabilises) and protein concentration.
Proteins adsorb at oil–water and air–water interfaces, lowering interfacial tension and forming viscoelastic films that resist coalescence of droplets and bubbles. Emulsifying properties are measured as emulsifying activity index (interfacial area stabilised per gram of protein, estimated from turbidity of a standard emulsion) and emulsion stability (change on storage, centrifugation or heating). Caseinates are excellent emulsifiers because their flexible, amphiphilic molecules adsorb rapidly; whey proteins form stronger, more cohesive interfacial films. Foaming properties are measured as overrun or foam expansion (the volume of foam formed) and foam stability (drainage or collapse over time). Whey protein isolates foam well, which is used in aerated products and meringue replacers; residual fat and phospholipids in WPC strongly reduce foaming, which is one reason for removing them by microfiltration. pH, salts, sugar, heat treatment and partial hydrolysis all modify emulsifying and foaming behaviour.
Whey proteins form gels on heating above their denaturation temperature at sufficient concentration: denatured molecules unfold, expose reactive groups and aggregate through hydrophobic interactions and disulphide bonds into a network. Gel properties depend on protein concentration (above a minimum gelling concentration), pH (fine-stranded, transparent gels far from the isoelectric point; particulate, opaque gels near it), ionic strength and calcium (which bridge proteins and give coarser gels), and heating rate. Gel strength is measured by texture analysis (breaking force, hardness) or by dynamic rheology (storage modulus). Caseins form gels by different routes: acid gels (as in yoghurt) and rennet gels (as in cheese). Water binding and viscosity contribute to texture and yield in meat products, bakery and dairy desserts. Many functional properties can be tailored by processing — controlled heat treatment, pH adjustment, microparticulation, hydrolysis, cross-linking with transglutaminase — and by choosing the right ingredient: WPC for heat-set gels, WPI for clear acidic drinks and foams, caseinate for emulsions, MCC for heat-stable high-protein drinks.
Dairy protein ingredients are sold against specifications agreed between supplier and user. Typical items are: protein content (total nitrogen × 6.38 for dairy proteins, often stated on a dry basis), moisture, fat, lactose, ash, pH, solubility, bulk density, particle size, scorched particles, heat stability, and specific functional tests relevant to the application (gel strength, foam overrun, emulsifying capacity, viscosity). Microbiological criteria commonly include total plate count, coliforms or Enterobacteriaceae, yeasts and moulds, pathogens (Salmonella, Listeria, Staphylococcus aureus, Cronobacter for infant formula ingredients), and spore counts (mesophilic, thermophilic and heat-resistant), which are critical for ingredients used in UHT and retorted products and infant formula. Chemical contaminants, allergen status and, for some markets, halal or kosher certification and origin declarations are also specified. Names such as WPC 80 or MPC 70 indicate nominal protein; buyers should confirm whether the figure is on a dry basis or as is.
Membrane-derived ingredients are used across the food industry. Cheese makers use MPC and MCC to standardise milk and raise yield; yoghurt makers use them for protein fortification and strained-style products without acid whey; processed cheese and analogue makers use MPC and caseinates; beverage makers use WPI for clear, acidic high-protein drinks and MCC or MPC for neutral, heat-stable ready-to-drink products; bakeries use WPC for structure and browning; meat processors use proteins for water binding and emulsification; confectioners use them in bars and aerated products; and infant formula manufacturers use demineralised whey, WPC, alpha-lactalbumin-enriched fractions, lactoferrin and MFGM ingredients to approach the composition of human milk. Clinical and sports nutrition rely on rapidly digested whey proteins, slowly digested casein and hydrolysates. Selecting an ingredient means matching its composition and functionality to the process and product, and checking regulatory status and labelling rules in the target market, since names, permitted uses and claims differ between countries.
How a membrane-derived ingredient may be named and used depends on the market. Some countries have standards of identity for products such as whey protein concentrate or milk protein concentrate; others regulate them through general food law, ingredient approval systems or generally recognised safety status. Use in standardised foods such as cheese, yoghurt and milk is sometimes restricted: the cheese standards of some countries limit or define the use of ultrafiltered milk and milk protein concentrate, and milk standards may restrict adding protein. Infant formula ingredients are tightly regulated for composition, contaminants and microbiology. Claims such as 'high protein' or 'lactose-free' must meet national definitions. Exporters and formulators should check the current rules in each target market before choosing an ingredient or a process.
A student uses this subject to classify membrane processes, calculate flux, TMP, VCF, rejection and diafiltration, explain fouling models and cleaning sequences, and relate protein functionality to ingredient choice. It supports AICE Dairy Technology Unit 4 questions and practical sessions on UF and RO pilot plants.
A membrane plant operator or ingredients technologist five years in runs UF, NF and RO loops, sets temperature, pressure and diafiltration to hit WPC, MPC or demineralisation specifications, monitors flux decline and cleaning effectiveness, replaces membranes, and troubleshoots solubility or functionality complaints from customers. This subject provides the models, calculations and functional tests behind those tasks.
A plant head decides on investment in membrane capacity and module types, the product portfolio of protein ingredients and permeate uses, water recovery schemes, and customer specifications. They weigh energy, membrane replacement and effluent costs against ingredient prices.
A professor or research director works on membrane fouling mechanisms, novel membranes, fractionation of minor proteins, protein functionality and its modification, and process integration of membranes with evaporation and drying. They examine AICE Dairy Technology and Engineering on these topics and advise industry on ingredient development.
This manual does not yet give: membrane supplier operating limits for specific products; detailed plant design calculations for multistage loops; quantitative energy comparisons with evaporation; full standard methods for each functional test; standards of identity for MPC, MCC and WPC in specific countries; and detailed electrodialysis stack design. Pore sizes, pressures, temperatures and demineralisation percentages are representative ranges, and model examples use illustrative parameters.
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