Food Science & Technology · Food Process Engineering · also Food Dehydration, Drying Technology, Concentration of Liquid Foods, Evaporation and Drying
Removing water from liquid and solid foods: evaporator types and energy economy, drying theory and dryer design, spray, drum and freeze drying, concentration by freezing and membranes, and the quality changes that drying causes. Agricultural Engineering's post-harvest-cereals-pulses-oilseeds covers grain drying; this subject covers processed foods and powders.
In the exams: GATE XE-G Section 4 (evaporation, hot-air drying, spray and freeze drying, psychrometry); ICAR AIEEA-PG Code 18 Units II–III
Removing water is the oldest and still the most widespread way of preserving food. Sun-dried grain, fruit, fish and meat kept people alive through lean seasons long before refrigeration or canning. Today, water removal underpins whole industries: milk powder, instant coffee, dried soups, fruit-juice concentrates, tomato paste, sugar, dehydrated vegetables, infant formula and countless ingredients are made by evaporating or drying water out of a liquid or solid food. Removing water lowers water activity so that microbes cannot grow and many reactions slow down; it also cuts the weight and volume that must be packed, stored and transported, often by a factor of five to twenty.
Two families of operation are covered here. Evaporation (concentration) removes part of the water from a liquid food by boiling, leaving a concentrated liquid such as condensed milk, juice concentrate or tomato paste. Drying (dehydration) removes water further, to a solid with low moisture: powders, flakes, granules or dried pieces. Both need large amounts of energy, because every kilogram of water must be given its latent heat of vaporisation, about 2,260 kJ at 100 °C and more at lower temperatures. Much of food process engineering in this area is therefore about energy economy — multiple effects, vapour recompression, heat recovery — and about protecting quality, since heat damages flavour, colour, nutrients and the functional properties of proteins.
The subject is examined in GATE XE-G and XL-U (Food Technology), whose Section 4 names evaporation of liquid foods, hot-air drying of solids, spray and freeze drying, and psychrometry, humidification and dehumidification; in ICAR AIEEA-PG Code 18 (Units II and III: evaporation, drying, dehydration and concentration methods); and in ICAR AICE-JRF/SRF 35.a and 41. Typical questions are numerical: humidity ratio and enthalpy of air, evaporator mass and energy balances and steam economy, drying times in the constant- and falling-rate periods, and moisture-basis conversions. This manual builds on two other subjects and does not repeat them: heat-transfer fundamentals, heat exchangers and Fick's law are in food-transport-phenomena; grain drying and bin dryers are in Agricultural Engineering's post-harvest-cereals-pulses-oilseeds. Sorption isotherms and glass transition, which decide how dry a product must be and why powders become sticky, are explained in food-chemistry-major-constituents.
Most food dryers use heated air both to carry heat to the food and to carry the evaporated water away, so the properties of air–water vapour mixtures — psychrometry — are the starting point. The key quantities are defined per kilogram of dry air, because the dry-air flow stays constant through a dryer while the water content changes.
The humidity ratio (absolute humidity) W is the mass of water vapour per kilogram of dry air. From the ideal-gas law, W = 0.622 pv/(P − pv), where pv is the partial pressure of water vapour and P the total pressure; 0.622 is the ratio of the molecular weights of water and air (18.02/28.97). Relative humidity is the ratio of the actual vapour pressure to the saturation vapour pressure at the same temperature: RH = pv/ps. Air at 30 °C, where ps is about 4.25 kPa, with RH 50 per cent has pv about 2.12 kPa and, at 101.325 kPa total pressure, W = 0.622 × 2.12/(101.325 − 2.12) ≈ 0.0133 kg water per kg dry air.
The dry-bulb temperature is the ordinary air temperature. The wet-bulb temperature is the steady temperature reached by a wetted thermometer bulb in moving air: water evaporates from the wick until the heat gained from the air equals the latent heat lost, so it is lower than the dry bulb unless the air is saturated. The dew point is the temperature at which the air becomes saturated when cooled at constant humidity ratio. The specific enthalpy of moist air, per kilogram of dry air, is h ≈ 1.006 t + W(2501 + 1.86 t) kJ/kg, where t is in °C: the first term is the sensible heat of the dry air, 2501 kJ/kg is the latent heat of water at 0 °C, and 1.86 kJ/kg K the specific heat of water vapour. The humid heat, the heat needed to raise 1 kg of dry air and its vapour by 1 K, is about 1.005 + 1.88W kJ/kg K, and the humid volume, the volume of 1 kg of dry air with its vapour, is used to convert fan volumetric flow to dry-air mass flow.
The psychrometric chart plots these properties for a given pressure, usually with dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis, with curves of constant relative humidity, sloping lines of constant wet-bulb temperature and enthalpy, and lines of constant humid volume. Any two independent properties fix the state of the air, and the others can be read off.
In a hot-air dryer, ambient air is first heated at constant humidity ratio: on the chart it moves horizontally to the right, its relative humidity falls and its capacity to take up water rises. It then passes over the wet food. If no heat is added or lost other than that used for evaporation, the process is close to adiabatic saturation: the air cools and gains moisture along a line of nearly constant wet-bulb temperature (and nearly constant enthalpy) until it leaves. The water picked up per kilogram of dry air is the rise in humidity ratio, so the dry-air flow needed equals the evaporation rate divided by (W_out − W_in). For example, a dryer evaporating 100 kg of water per hour, with air entering at W = 0.010 and leaving at W = 0.030 kg/kg, needs 100/0.020 = 5,000 kg of dry air per hour.
Recirculating part of the exhaust air raises the inlet humidity but recovers heat, and is used where the exhaust is still far from saturation. Leaving the exhaust too humid, on the other hand, reduces the driving force for drying and risks condensation in ducts.
Food plants also humidify and dehumidify air. Humidification — adding water by sprays, steam injection or wetted pads — is used in ripening rooms, cold stores for fresh produce, dough proofers and some storage rooms where products must not lose moisture. Evaporative (adiabatic) humidification also cools air along a wet-bulb line, which is the principle of evaporative coolers.
Dehumidification is used to dry the air entering low-temperature dryers, to keep hygroscopic powders and confectionery from absorbing moisture, and to prevent condensation. Air can be dehumidified by cooling it below its dew point so that water condenses on a cold coil, and then reheating it to the required temperature, or by passing it through a desiccant (for example a rotating silica-gel or lithium chloride wheel) that adsorbs water and is regenerated with hot air. Dehumidified-air dryers allow drying at low temperatures, which protects heat-sensitive products, and heat-pump dryers combine the two functions by using the cold side of a heat pump to condense water and the hot side to reheat the air, recovering the latent heat.
Evaporation concentrates a liquid food by boiling off part of its water. It is used to reduce bulk before storage and transport (fruit-juice concentrates, condensed milk), as a step before drying, since removing water by evaporation costs much less energy per kilogram than removing it in a spray dryer, and to produce products whose character depends on concentration (tomato paste, jam, sweetened condensed milk, sugar syrups before crystallisation). An evaporator has two essential parts: a heat exchanger (the calandria or heating element), in which steam condenses on one side and the liquid boils on the other, and a separator, in which the vapour is disengaged from the concentrate. The vapour is then condensed in a condenser, and a vacuum system keeps the pressure low.
Most food evaporators work under vacuum. Lowering the pressure lowers the boiling point — water boils at about 60 °C at 20 kPa absolute — which protects heat-sensitive products and increases the temperature difference available from a given steam supply. The overall mass balance is simple: feed F with solids fraction xF gives concentrate P with solids fraction xP and vapour V, with F xF = P xP and V = F − P = F(1 − xF/xP). Concentrating 10,000 kg/h of juice from 12 to 48 per cent solids removes 7,500 kg/h of water and leaves 2,500 kg/h of concentrate.
A solution boils at a higher temperature than pure water at the same pressure, because dissolved solids lower the vapour pressure of the water. This boiling-point elevation (BPE) reduces the useful temperature difference between the steam and the boiling liquid. For dilute ideal solutions, BPE = Kb × m, where m is the molality of solute and Kb, the ebullioscopic constant of water, is about 0.51 K kg/mol; a solution containing 1 mol of sugar per kilogram of water boils about 0.5 K above pure water. Real food concentrates, especially sugar-rich juices and salt solutions at high concentration, show much larger elevations than the dilute formula predicts.
Dühring's rule states that the boiling temperature of a solution plotted against the boiling temperature of pure water at the same pressure gives a straight line for a given concentration. Dühring lines are therefore used to find the boiling point of a concentrate at any pressure from its boiling point at one pressure. In evaporator calculations the vapour leaving a boiling solution is superheated by the BPE, but its latent heat and saturation temperature for the next effect are taken at the pressure of the vapour space. Hydrostatic head in tall tubes also raises the boiling point at the bottom of the liquid column, which is one reason thin-film evaporators are favoured for heat-sensitive products.
The heat transferred in an evaporator is Q = U A ΔT, where U is the overall heat transfer coefficient, A the heating area and ΔT the difference between the condensing steam temperature and the boiling temperature of the liquid (after allowing for BPE). U combines the film coefficients of the condensing steam (very high), the tube wall and the boiling liquid, plus any fouling. For dilute, low-viscosity liquids U can be high, in the range of thousands of W/m² K; for viscous concentrates it falls sharply, because the boiling-side film coefficient depends on viscosity and on how vigorously the liquid moves. Fouling — the deposition of protein, minerals or caramelised sugar on the heating surface — is a major problem with milk and fruit juices; it lowers U, raises the product temperature at the wall and forces cleaning.
The heat balance links steam use to evaporation. For a single effect, S λs = F cp (T1 − TF) + V λv, where S is the steam rate, λs its latent heat, cp the specific heat of the feed, T1 the boiling temperature, TF the feed temperature and λv the latent heat of the vapour. If the feed enters below its boiling point, part of the steam heat raises it to the boiling point; if it enters above, it flashes and some water evaporates without steam. Heat transfer theory, film coefficients and heat exchanger design are covered fully in food-transport-phenomena; here they are applied to the boiling liquid.
The steam economy of an evaporator is the mass of water evaporated per mass of steam used. In a single effect it is slightly less than one — commonly about 0.8 to 0.9 when the feed enters below its boiling point — because some steam heat goes to warming the feed and because latent heat increases as temperature falls, so a kilogram of steam condensing at a higher temperature cannot quite evaporate a kilogram of water at a lower one. A single effect is therefore expensive to run on a large scale.
In a multiple-effect evaporator, the vapour from the first effect is used as the heating medium of the second, which operates at a lower pressure so that its liquid boils at a lower temperature; the vapour from the second heats the third, and so on, until the vapour from the last effect goes to the condenser. Each kilogram of live steam is thus used repeatedly, and as a rough rule the economy rises nearly in proportion to the number of effects: about 1.6 to 1.8 for two effects and about 2.4 to 2.7 for three, depending on feed temperature and arrangement. Large milk and juice evaporators commonly use four to seven effects, often with vapour recompression as well.
Multiple effects improve economy but not capacity. The total temperature difference available — from the live-steam temperature to the boiling temperature in the last effect, set by the condenser and vacuum — is shared among the effects, and further reduced by the BPE in each. If the same total ΔT is divided among n effects, each has roughly 1/n of it, so each effect of a given area transfers about 1/n of the heat a single effect would; with n effects of equal area, the total evaporation is about the same as that of one effect of the same area using the whole ΔT. The benefit is that this evaporation needs about 1/n of the steam. Multiple effects thus trade capital (n times the heating area) for running cost (steam), and the optimum number of effects is set by the balance between the two, the value of steam and the heat sensitivity of the product.
Calculations for a multiple-effect evaporator solve the mass and heat balances of every effect together with Q = U A ΔT for each, usually assuming equal areas and finding the temperature distribution by iteration. In exam problems the effects are often idealised: equal heat transfer, negligible BPE and feed at its boiling point, so that each kilogram of steam evaporates about one kilogram in each effect.
In forward feed, the liquid flows in the same direction as the vapour, from the first (hottest, highest-pressure) effect to the last. No pumps are needed between effects because the liquid moves to lower pressures, and the most concentrated liquid is in the coolest effect, which suits heat-sensitive products; but the concentrated, viscous liquid boils at the lowest temperature, so heat transfer is poorest where viscosity is highest. In backward feed, fresh liquid enters the last (coolest) effect and the concentrate leaves from the first (hottest); pumps are needed to move liquid to higher pressures, but the viscous concentrate is handled at the highest temperature where its viscosity is lowest, giving better heat transfer and often better economy when the feed is cold. Mixed feed combines the two, and parallel feed sends fresh feed to every effect, as in salt crystallisation. The choice depends on feed temperature, the viscosity curve of the product and its heat sensitivity; most milk evaporators use forward feed with falling-film effects.
The vapour leaving an evaporator still carries almost all the latent heat supplied to it; in a simple plant this heat is thrown away in the condenser. Vapour recompression raises the pressure, and hence the condensing temperature, of some or all of this vapour so that it can be used again as the heating medium in the same or an earlier effect. Because only a small rise in saturation temperature is needed — enough to give a working ΔT of a few kelvin — the energy needed to compress the vapour is much less than the latent heat it delivers. A simple comparison shows the scale: condensing a kilogram of vapour releases over 2,000 kJ, while raising its saturation temperature by a few kelvin needs only a small fraction of that as compression work. Recompression therefore pays wherever large quantities of water are evaporated continuously, as in dairy and juice plants, and it is now the main route to reducing the energy used in evaporation.
In thermal vapour recompression (TVR), a steam-jet ejector (thermocompressor) uses high-pressure motive steam, expanded through a nozzle, to entrain part of the low-pressure vapour from an effect and compress the mixture to an intermediate pressure. The mixture then heats the first effect. TVR has no moving parts, is cheap and reliable, and adding one thermocompressor typically gives a gain in economy similar to adding one more effect. It needs a supply of steam at a pressure well above that of the heating side, and its efficiency falls if operating conditions move away from the design point. The excess vapour not recompressed passes on to later effects or the condenser. In a typical milk evaporator a thermocompressor takes vapour from the first or second effect, so the live steam drives the first effect with a mixture of fresh and recycled vapour; the economy of a three-effect plant with TVR can approach that of a four- or five-effect plant without it.
In mechanical vapour recompression (MVR), an electrically driven compressor or high-speed fan compresses essentially all of the vapour from the evaporator and returns it to the heating side of the same unit. Live steam is then needed only for start-up and small make-up, and the equivalent steam economy can be very high, commonly quoted as equal to many effects of a conventional plant. Because a fan can raise the saturation temperature only modestly, MVR evaporators work with small temperature differences — often in the range of about 5 to 10 K — and so need large heating areas, and they suit products with low boiling-point elevation; high BPE eats into the small ΔT. MVR is widely used for milk, whey and other dilute dairy streams and wherever electricity is cheap relative to steam, and is also attractive where the plant aims to cut fossil-fuel use. Combined plants, with MVR on the early effects and TVR or a finisher on the concentrated end, are common. The choice between TVR, MVR and more effects is an economic one, depending on the prices of steam and electricity, capital cost and the product's properties.
The simplest evaporators are open pans, heated by steam jackets or coils, still used for jam, jaggery and small-scale products where the cooked flavour and colour are wanted; they work at atmospheric pressure and give long residence times. The short-tube (calandria or Robert) evaporator has a bundle of short vertical tubes in a steam chest with a large central downcomer; liquid boiling in the tubes rises and returns down the downcomer by natural circulation. It is robust and cheap but gives long residence times and poor heat transfer with viscous liquids, and is now used mainly for sugar syrups.
In a rising-film (climbing-film) evaporator, liquid enters the bottom of long vertical tubes, starts boiling, and the expanding vapour drives the liquid up the tube walls as a thin film. Heat transfer is good and residence time short, but a sufficient temperature difference is needed to generate the vapour that drives the film, and it does not suit very viscous liquids.
In a falling-film evaporator, the liquid is distributed at the top of long vertical tubes and flows down the inside walls under gravity as a thin film, with vapour flowing downwards in the centre and separating at the bottom. Because flow does not depend on vapour generation, falling-film evaporators work well with very small temperature differences, which makes them ideal partners for MVR and multiple effects. Residence time is short — seconds to a minute or so per pass — and there is little hydrostatic head, so product temperature stays close to the low boiling point. These properties make the falling-film evaporator the standard for milk, whey, fruit juices and coffee extract. Its weak points are the need for an even distribution of liquid to every tube, since dry patches cause burn-on and fouling, and a minimum wetting rate that may require recirculation. Distributors at the top of each calandria, careful control of feed rate and temperature, and preheating the feed to its boiling point before it enters the tubes are the usual remedies.
In a forced-circulation evaporator, a pump drives the liquid at high velocity through the heating tubes, often under enough pressure that boiling is suppressed in the tubes and the liquid flashes in the separator. High velocity gives good heat transfer with viscous liquids and reduces fouling and scaling, so forced circulation is used for viscous, crystallising or scaling products such as tomato paste, salt solutions and some finishing stages; the cost is pumping power and longer residence time.
Plate evaporators use gasketed or welded plates, like a plate heat exchanger, with steam on one side and the boiling film on the other. They are compact, have short residence times and can be opened for cleaning and extended by adding plates, so they suit heat-sensitive products and smaller plants. Agitated (wiped or scraped) thin-film evaporators spread the product on the inside of a heated cylinder with rotating blades; they handle very viscous and heat-sensitive materials — fruit pastes, concentrated extracts — at the cost of high capital and running costs and small capacity. Centrifugal (conical) evaporators spread the liquid as a very thin film on rotating cones, giving residence times of about a second for the most heat-sensitive products. Selection balances viscosity, heat sensitivity, fouling tendency, capacity and cost.
Moisture content is expressed on a wet basis (water per total mass) or a dry basis (water per dry solids). Drying calculations use the dry basis X because the dry-solids mass stays constant: X = M_wb/(1 − M_wb) and M_wb = X/(1 + X). A product at 80 per cent wet basis has X = 4.0 kg water per kg solids; drying it to 10 per cent wet basis (X = 0.111) removes 3.89 kg water per kg of solids. The equilibrium moisture content Xe is the moisture at which the food stops gaining or losing water in air of given temperature and humidity; it comes from the sorption isotherm, and only the free moisture X − Xe can be removed in that air.
When a wet food is dried in air of constant conditions, the drying curve (X against time) and the drying-rate curve (rate against X) usually show three stages. In a short initial adjustment period the surface heats or cools to a steady temperature. In the constant-rate period, the surface is covered with free water and evaporation proceeds as from a free water surface: the rate is set by external conditions, and the surface temperature stays near the wet-bulb temperature of the air. When the moisture falls to the critical moisture content Xc, the surface begins to dry out and the falling-rate period begins, in which the rate is controlled by the movement of water inside the food and falls as the food dries.
In the constant-rate period, heat transferred from the air to the surface is used entirely to evaporate water. The drying rate per unit area is Rc = h (Ta − Tw)/λw, where h is the convective heat transfer coefficient, Ta and Tw the dry- and wet-bulb temperatures, and λw the latent heat at Tw; equivalently, it can be written from the mass transfer coefficient and the humidity difference between saturated air at the surface and the bulk air. The rate increases with air velocity (which raises h), air temperature and dryness, so these are the control variables in this period. Because the product surface stays at the wet-bulb temperature, the product is protected from heat damage while surface water remains, even in hot air — a principle exploited in spray drying.
The time for the constant-rate period follows from a mass balance: tc = Ms (X0 − Xc)/(A Rc), where Ms is the dry-solids mass and A the drying area. For example, with Ms/A = 10 kg solids per m², X0 = 4.0, Xc = 1.5 and Rc = 2.5 kg/m² h, tc = 10 × 2.5/2.5 = 10 h. Many foods, particularly cellular materials such as fruits and vegetables, show little or no true constant-rate period, because their surface water is limited; their drying is controlled internally almost from the start.
Below the critical moisture content, water must move from inside the food to the surface by liquid diffusion, capillary flow and vapour diffusion, and the rate falls. In the simplest model, the rate falls linearly with free moisture from Rc at Xc to zero at Xe, and integration gives the falling-rate time tf = [Ms (Xc − Xe)/(A Rc)] ln[(Xc − Xe)/(X − Xe)]. With Ms/A = 10, Rc = 2.5, Xc = 1.5, Xe = 0.05 and a final X of 0.15, tf = (10 × 1.45/2.5) × ln(1.45/0.10) = 5.8 × 2.674 ≈ 15.5 h. The falling-rate period therefore often takes longer than the constant-rate period even though less water is removed.
A more physical model treats internal moisture movement as diffusion with an effective diffusivity Deff, using Fick's second law. For a slab of thickness 2L dried from both faces, at long times the moisture ratio MR = (X − Xe)/(X0 − Xe) ≈ (8/π²) exp(−π² Deff t/(4L²)). The drying time thus increases with the square of thickness — halving the thickness of slices cuts falling-rate drying time to about a quarter — and Deff increases with temperature roughly following an Arrhenius relation. Thin-layer empirical models (Lewis, Page, Henderson–Pabis and others) are fitted to experimental data in research and dryer design; these, and the details of grain drying, are covered in post-harvest engineering.
Cabinet (tray) dryers are insulated chambers in which trays of food are dried by heated air blown across or through them. They are simple and flexible, suitable for small batches, pilot work and products such as fruit, vegetables, herbs and spices, but drying is often uneven between trays near the air inlet and outlet, and labour is high.
Tunnel dryers are long chambers through which trucks of trays move, semi-continuously, while heated air flows along the tunnel. In concurrent (parallel-flow) tunnels, the hottest, driest air meets the wettest product: rapid initial drying with the product protected by evaporative cooling, low risk of scorching, but the dry product leaves in contact with cool, humid air, so the final moisture cannot be very low and case hardening (a dry, impermeable surface over a moist interior) can occur. In countercurrent tunnels, the hottest air meets the driest product: low final moisture and good heat economy, but the dry product may be overheated, and wet product entering meets cool, humid air, slowing the start and risking spoilage or condensation. Two-stage tunnels combine a concurrent first stage with a countercurrent finishing stage, and centre-exhaust designs do the same in one tunnel.
Conveyor (belt) dryers carry a layer of product on a perforated belt through zones of air flowing up or down through the bed, with temperatures and velocities set separately for each zone. They are the workhorse of large-scale vegetable and fruit dehydration, giving continuous operation and good control.
In a fluidised-bed dryer, air is blown upward through a bed of particles at a velocity high enough to suspend them, so that the bed behaves like a boiling liquid. Every particle is surrounded by moving air, so heat and mass transfer are fast and temperatures uniform; drying times are short. Fluidised beds suit small, fairly uniform particles such as peas, diced vegetables, granules, grains and salt, and are widely used as second-stage dryers and agglomerators for spray-dried powders. Vibrated beds help to fluidise sticky or irregular materials. Sizing is governed by the minimum fluidisation velocity, above which the bed fluidises, and the terminal velocity of the smallest particles, above which they are carried out. Pneumatic (flash) dryers carry fine wet solids in a fast hot-air stream for seconds, suitable for starch and flour; rotary dryers tumble the product in a rotating inclined cylinder with hot air, used for robust granular materials. Solar dryers, direct or indirect, are important for small producers in sunny regions and improve on open sun drying by protecting the product from dust, insects and rain.
In a drum (roller) dryer, a slurry, purée or paste is applied as a thin layer on the outside of a slowly rotating hollow metal drum heated inside by steam, typically at temperatures above 120 °C. The film dries within less than one rotation — often within seconds to a minute — and is scraped off by a fixed knife as a sheet or flakes. Single-drum dryers use applicator rolls or dip feeding; double-drum dryers have two drums rotating towards each other with the feed in the nip between them, the gap setting the film thickness. Vacuum drum dryers enclose the drum to dry heat-sensitive products at lower temperatures.
Drum drying has high thermal efficiency because heat is conducted directly from the drum to a thin film, and it handles viscous materials that cannot be atomised. Its disadvantages are the high product temperature — the film reaches temperatures close to that of the drum as it dries — which can cause cooked flavours, browning and protein denaturation, and the limited capacity. Drum-dried milk powder, for example, is less soluble than spray-dried powder but is valued in chocolate making for its flavour. Products include instant mashed potato flakes, infant cereals, pregelatinised starches, fruit and vegetable flakes and some animal feeds; drum drying gelatinises starch, which is why it suits instant cereals.
Spray drying converts a liquid feed — usually a concentrate from an evaporator — into a powder in a single step. The feed is atomised into fine droplets in a large chamber, where it meets hot air; water evaporates in seconds, and the dried particles are separated from the air by cyclones, bag filters or both. Inlet air temperatures are commonly in the range of about 150 to 220 °C for food products, with outlet temperatures of about 70 to 100 °C, depending on product and design. Despite the hot air, the droplets stay near the wet-bulb temperature while they hold free water, and the particles leave at temperatures related to the outlet air, so heat damage is limited: spray drying is the main method for milk powders, infant formula, whey, instant coffee, egg powder, flavour encapsulates and many ingredients.
Air and spray can flow co-currently (the usual arrangement for heat-sensitive foods, as the hottest air meets the wettest droplets), counter-currently (for heat-tolerant materials and denser powders) or in mixed flow. Modern plants are multi-stage: the main chamber removes most of the water, leaving the powder slightly moist, and an integrated or external fluidised bed completes drying at lower temperature and cools the powder. This saves energy and gives better powder properties than single-stage drying to the final moisture. Because evaporating water in a spray dryer costs much more energy than in an evaporator, feeds are concentrated as far as their viscosity allows before atomisation.
Atomisation creates the large surface area that makes fast drying possible: a litre of liquid divided into droplets of 50 µm diameter has a surface of about 120 m². Three main atomisers are used. Rotary atomisers (wheels or discs spinning at high peripheral speed) fling the liquid off as droplets; droplet size is controlled mainly by wheel speed, they handle viscous and abrasive feeds and wide flow ranges, and they need wide chambers because the spray travels radially. Pressure (hydraulic) nozzles force the liquid at high pressure through a small orifice with a swirl insert, producing a hollow-cone spray; they give coarser, denser particles in tall, narrow chambers and are common for milk and for producing agglomerated powders, but they wear and block with abrasive or particle-laden feeds. Two-fluid (pneumatic) nozzles break up the liquid with high-velocity air or steam, giving fine droplets from viscous feeds at low liquid pressure, at the cost of compressed-air energy. Droplet size distribution, together with feed solids, decides particle size and bulk density.
The quality of a powder depends on its moisture content and water activity (low enough for stability; milk powders are commonly dried to a few per cent moisture), its particle size distribution, bulk density, flowability, solubility and the reconstitution properties together called instant properties: wettability (how readily particles are wetted and sink), sinkability, dispersibility (how readily they separate in water) and solubility. Fine single particles wet poorly and form lumps; agglomerated particles, larger and more porous, wet and disperse quickly.
Agglomeration (instantising) is done by returning fines from the cyclones to the atomisation zone so that they stick to moist droplets (straight-through agglomeration), or by re-wetting the powder with steam or water spray and re-drying it in a fluidised bed. Lecithin may be applied to whole-milk powder to make the fat-coated surface wettable.
Stickiness is the main limitation for products rich in sugars and organic acids, such as fruit juices and honey: their low glass transition temperatures mean the particles are rubbery and sticky at outlet conditions, so they adhere to the chamber walls. Remedies include adding drying aids such as maltodextrin to raise the glass transition temperature, using lower outlet temperatures and dehumidified air, cooling the chamber walls, and pre-crystallising lactose in whey concentrates before drying so that it does not form a hygroscopic amorphous glass. Spray dryers also carry a risk of dust fires and explosions; plants are fitted with explosion vents, suppression systems and careful control of deposits.
Freeze drying (lyophilisation) removes water from a frozen food by sublimation — direct conversion of ice to vapour — under vacuum, followed by desorption of the remaining bound water. Because the product stays frozen and then rigid during most of the process, its structure is preserved, shrinkage is minimal, and the dried product is porous, rehydrates rapidly and keeps most of its colour, flavour, aroma and nutrients. Freeze-dried coffee, fruit pieces for breakfast cereals, soups, military and space rations, probiotic cultures, and high-value ingredients are typical products.
Sublimation is possible only below the triple point of water, 0.01 °C and about 611 Pa, where solid, liquid and vapour coexist. Below this pressure, ice heated to its sublimation temperature passes directly to vapour. Freeze dryers therefore work at chamber pressures of tens to a few hundred pascals, with a condenser held at a temperature well below that of the product (often −40 °C to −80 °C) so that the vapour pressure of ice on the condenser is lower than at the product and vapour flows from product to condenser. The latent heat of sublimation, about 2,840 kJ/kg, is larger than the heat of vaporisation, and the low pressures and long cycle times make freeze drying the most expensive common drying method, several times the cost of hot-air drying.
A freeze-drying cycle has three stages. First, the product is frozen, either before loading or on the shelves. The freezing rate controls ice crystal size: slow freezing gives large crystals, which leave large pores and allow faster sublimation but may damage cellular structure; fast freezing gives small crystals and smaller pores. Second, primary drying removes the ice by sublimation: heat is supplied by heated shelves (by conduction through the tray) and by radiation, and a sublimation front moves from the surface into the product, leaving a porous dried layer behind. Heat must travel through the frozen layer or the dried layer, and vapour must escape through the dried layer, whose low thermal conductivity and resistance to vapour flow limit the rate; drying time rises steeply with product thickness. Third, secondary drying removes much of the unfrozen, sorbed water by desorption at higher shelf temperatures, bringing moisture down to low levels, commonly a few per cent or less.
Cycle design sets shelf temperature, chamber pressure and the duration of each stage. A higher chamber pressure improves heat transfer to the product (more gas conduction) but reduces the vapour-pressure driving force; there is an optimum for each product. The end of primary drying is detected by product thermocouples approaching shelf temperature, by pressure-rise tests (closing the valve to the condenser and watching pressure) or by comparing pressure gauges of different types.
During primary drying, the product temperature at the sublimation front must stay below a critical temperature. For amorphous systems, this is the collapse temperature, close to (usually a few degrees above) the glass transition temperature of the maximally freeze-concentrated phase, Tg′; for systems in which solutes crystallise, it is the eutectic temperature. If the product rises above this temperature, the freeze-concentrated matrix around the ice crystals softens and flows, the pores close, and the product collapses: it shrinks, becomes dense and sticky, dries slowly, retains more water and rehydrates poorly; volatile retention and appearance also suffer. Sugar-rich foods such as fruit juices have low Tg′ values and must be dried at very low product temperatures, or formulated with higher-molecular-weight solids such as maltodextrins that raise Tg′. Collapse temperatures are measured by freeze-drying microscopy and differential scanning calorimetry. Most of the skill in freeze-drying cycle design lies in running primary drying as warm as possible, for speed, while keeping the product safely below collapse.
Osmotic dehydration removes part of the water from pieces of fruit, vegetable, meat or fish by immersing them in a concentrated solution of sugar, salt or a mixture. The cell membranes act as partly selective barriers, and water moves out of the tissue towards the more concentrated solution while some solute moves in. Two counter-current flows result: water loss, usually the larger, and solids gain. A third, small flow of natural solutes such as acids, sugars, vitamins and minerals leaks out into the solution. Osmotic dehydration can remove up to about half of the water of fruit pieces without a phase change and at mild temperatures, but it cannot by itself make a shelf-stable product; it is used as a pre-treatment before air drying, freeze drying or freezing, or as part of a combined process.
The rate and balance between water loss and solids gain depend on solute type and concentration (sucrose for fruit, salt or sugar–salt mixtures for vegetables and fish), temperature, immersion time, piece size and shape, agitation and the ratio of solution to product. Larger solute molecules such as sucrose penetrate less than smaller ones such as glucose or salt, giving more water loss relative to solids gain. Benefits include better colour, flavour and texture after drying, reduced browning (sugar coating limits oxygen contact), less energy in the following drying step and partial removal of acid, giving a sweeter product. The spent solution is a problem: it must be reconcentrated and reused, or used in other products.
Intermediate-moisture foods (IMF) are foods with enough moisture to be soft and palatable without rehydration but with water activity low enough, together with other hurdles, to be shelf-stable without refrigeration or full sterilisation. Definitions vary, but IMFs are commonly described as having water activity of about 0.6 to 0.9 and moisture contents of about 15 to 50 per cent. Traditional examples include dried fruits such as dates, figs and raisins, jams and jellies, honey, fruit cakes, sweetened condensed milk, many traditional sweets, salted and dried fish, jerky and some dry sausages; modern IMFs include soft pet foods and many snack bars.
Water activity is lowered by removing water (partial drying), by adding humectants that bind water — sugars, salt, glycerol, sorbitol, propylene glycol where permitted — or by both. Because many bacteria are inhibited below about aw 0.90, while moulds and osmophilic yeasts can grow to about 0.61 to 0.80, IMFs at the upper end of the range need additional protection: low pH, preservatives such as sorbate and benzoate, mild heat treatment and good packaging. The combination is the practical application of hurdle technology. IMFs face their own quality problems: non-enzymatic browning and lipid oxidation have high rates at intermediate water activity, and textural changes such as hardening or sugar crystallisation occur during storage.
Designing an IMF starts from the target water activity and the sorption isotherm of the product. The amount of humectant needed can be estimated from Raoult's law for simple systems, from empirical equations such as Ross's equation (the aw of a mixture approximated as the product of the aw values each component would give alone in the same water), or by measurement. Salt is a very effective humectant per gram but limited by taste; glycerol is effective and imparts softness but gives a sweet-bitter taste at high levels; sugars are acceptable in sweet products. Formulations are then checked by challenge testing with relevant spoilage organisms and pathogens, and by storage trials at the expected temperatures. Moisture migration between components with different water activities — such as a moist filling and a crisp shell — must be prevented by matching water activities or using barriers, because water moves from higher to lower aw regardless of moisture content.
Freeze concentration removes water as ice rather than as vapour. A liquid food is cooled below its freezing point so that pure ice crystals form, leaving a more concentrated solution; the ice crystals are then separated, usually in a wash column in which the ice bed is washed with melted ice to recover adhering concentrate. Because no heating is involved and there is no vapour–liquid interface for volatile compounds to escape across, freeze concentration gives the best retention of delicate aromas of any concentration method, and no heat damage. It is used for high-value products such as premium fruit juices, coffee and tea extracts, and some beers and wines.
Its limitations are cost and concentration range. Growing large, pure ice crystals that can be separated cleanly requires careful control of nucleation and crystal growth, usually in a scraped-surface heat exchanger followed by a ripening (recrystallisation) vessel in which small crystals dissolve and large ones grow. The maximum concentration is limited by viscosity, which rises steeply as the solution approaches the freeze-concentrated state, commonly to around 40 to 50 per cent solids. Refrigeration costs, though lower per kilogram than evaporation in terms of latent heat (the heat of fusion, about 334 kJ/kg, is much smaller than the heat of vaporisation), and high capital costs restrict its use. Block and progressive freeze concentration, in which ice grows as a layer on a cooled surface, are simpler alternatives under development.
Membrane processes concentrate liquid foods by forcing water through a semi-permeable membrane under pressure, without phase change and at low temperatures. Reverse osmosis (RO) uses dense membranes that pass water but retain almost all solutes, including sugars, salts and many aroma compounds. The applied pressure must exceed the osmotic pressure of the concentrate, which, by van't Hoff's equation π = i c R T, rises with solute concentration; because osmotic pressure becomes very high at high sugar concentrations, RO is practical for juices only to about 25 to 30 per cent solids, and is used to pre-concentrate milk, whey, juices and maple sap before evaporation, and to recover water.
Nanofiltration passes water and monovalent ions but retains sugars and larger molecules, and is used to partially demineralise whey. Ultrafiltration retains proteins and fat while passing water, lactose and salts; it is a fractionation process rather than a simple concentration, used to standardise milk protein, make whey protein concentrates and in cheese making. Microfiltration removes bacteria and fat globules. Membrane concentration uses much less energy than evaporation per kilogram of water removed and avoids heat damage, but is limited by the maximum concentration reachable, by fouling and concentration polarisation (build-up of retained solutes at the membrane surface, which reduces flux), by cleaning needs and by membrane cost. The principles of these membrane processes are covered in mechanical-unit-operations; here their role as concentration steps is the focus.
The choice among evaporation, freeze concentration and membranes balances product quality, final concentration, energy and cost. Evaporation reaches high concentrations (65 per cent or more for juices and pastes) at reasonable cost, especially with multiple effects and vapour recompression, but loses volatile aromas, which are often recovered separately by stripping the first vapour and returning the essence to the concentrate, and causes some heat damage. Membrane processes are cheapest in energy but limited in concentration; freeze concentration gives the best quality at the highest cost. Hybrid schemes are common: reverse osmosis to pre-concentrate, followed by evaporation to the final concentration, or ultrafiltration combined with evaporation in dairy plants. Energy and quality must be judged together: the cheapest route per kilogram of water removed is worthless if it loses the aroma that gives a premium juice its price, and the best-quality route is unaffordable for a commodity concentrate. The decision is therefore made product by product, from the value of the product, the required final concentration and the energy prices of the site.
Drying changes the structure of foods. Shrinkage occurs as water leaves and the solid matrix contracts; it is large in air-dried fruits and vegetables and small in freeze-dried foods, whose rigid frozen structure is preserved. Case hardening is the formation of a hard, dry, relatively impermeable surface layer while the interior is still moist; it is caused by drying the surface too fast — high air temperature and low humidity at the start — and by migration of dissolved solids to the surface, and it slows further drying and can trap moisture inside. Porosity, bulk density and texture depend on the drying method: drum-dried flakes are dense, freeze-dried pieces very porous, puffed products expanded by rapid release of vapour. Loss of crispness or caking during storage occurs when dried foods absorb moisture and pass through their glass transition. Dried foods are also fragile and liable to break during handling, which creates fines and dust that must be screened out.
Drying and storage of dried foods cause several chemical changes. Non-enzymatic (Maillard) browning, which peaks at intermediate water activities, darkens fruits and vegetables and damages flavour and lysine availability; it is favoured by high temperatures towards the end of drying, when water activity passes through the critical range. Sulphiting (with sulphur dioxide or sulphites, where permitted and labelled) has traditionally been used to prevent browning in dried apricots and other fruits and to protect vitamin C, while blanching before drying inactivates enzymes responsible for enzymatic browning and off-flavours in vegetables. Lipid oxidation is a problem in dried foods containing fat, such as milk powders and dried meat, especially at very low water activities and with large surface areas exposed to air; packaging under inert gas and antioxidants are used against it. Pigments such as carotenoids and chlorophylls degrade with heat, light and oxygen.
Water-soluble and heat-sensitive vitamins, especially vitamin C and thiamin, are partly lost, more so in slow, hot-air drying than in freeze drying or rapid spray drying. Volatile flavour compounds are lost with the evaporating water; the selective diffusion theory explains why spray and freeze drying retain more aroma than expected: once the surface dries, it becomes much less permeable to the larger aroma molecules than to water, so aroma is held inside while water continues to escape. Protein denaturation reduces the solubility of milk and egg powders, which is why low outlet temperatures and gentle second-stage drying are used for powders that must reconstitute well.
Most dried foods are rehydrated before use, and the speed and completeness of rehydration are key quality attributes. The rehydration ratio — mass after rehydration divided by dry mass — and the coefficient of rehydration, which compares the water taken up with the water originally removed, express how well a product recovers. Rehydration is never complete for air-dried vegetables, because drying causes irreversible changes: loss of cell turgor, collapse of tissue, denaturation of proteins and changes in starch and cell-wall polymers reduce the ability to reabsorb water. Freeze-dried and porous products rehydrate fastest; case-hardened and collapsed products slowest. Pre-treatments, drying conditions and particle size all influence rehydration.
The shelf life of dried foods depends on water activity, temperature, oxygen, light and packaging. Packaging with good moisture and oxygen barriers is essential, because a dried food in a permeable package will gradually take up water until it reaches equilibrium with the surrounding air, losing crispness, caking and allowing faster browning or even mould growth. Shelf-life prediction from sorption isotherms and package permeability is covered in shelf-life-stability and food-packaging. Nitrogen flushing, oxygen absorbers and moisture-proof laminates are routine for milk powders, dried fruit and dehydrated vegetables.
A student uses this subject for the calculations that recur in GATE and ICAR papers — reading the psychrometric chart, finding humidity ratios and air flows, balancing evaporators and computing steam economy, converting wet and dry moisture bases, and estimating drying times in the constant- and falling-rate periods. In the laboratory the same student dries slices in a cabinet dryer, plots drying and drying-rate curves, identifies the critical moisture content, measures rehydration ratio and compares the quality of sun-dried, air-dried and freeze-dried samples.
Five years into a dairy, juice or ingredient plant, a process technologist runs evaporators and spray dryers to target concentrate solids, powder moisture and bulk density; tunes inlet and outlet temperatures to balance throughput against solubility and stickiness; schedules cleaning when fouling cuts evaporator capacity; investigates caking, lumping or poor wettability complaints; and tracks steam, electricity and water per tonne of product. In a vegetable dehydration plant, the same role sets belt-dryer zone temperatures, blanching and sulphiting where permitted, and checks colour, moisture and rehydration of every lot.
A senior engineer or plant head chooses the technology for new capacity — how many effects, TVR or MVR, plate or falling film, single- or multi-stage spray drying, whether a freeze dryer can pay for itself — on the basis of energy prices, product value and quality requirements. They lead energy-reduction programmes, heat recovery from dryer exhaust, dust-explosion safety, and the qualification of new products on existing lines, and they negotiate specifications with customers for powder properties such as solubility index, bulk density and particle size.
A research leader works on modelling and optimising drying and concentration: simultaneous heat and mass transfer in porous foods, glass-transition-based control of stickiness and collapse, encapsulation of bioactives and probiotics by spray and freeze drying, novel methods such as refractance-window, microwave–vacuum and electrohydrodynamic drying, and low-energy concentration by membranes and freeze concentration. They advise industry and regulators on energy and quality trade-offs, supervise doctoral research and set examination questions that test understanding rather than recall. They also judge the evidence behind claims of energy savings and quality improvement made for new equipment.
This manual does not give a full psychrometric chart or steam tables (questions supply the values they need), nor the iterative design of multiple-effect evaporators with unequal areas, or compressor power calculations for MVR. Grain drying, bin and recirculating dryers are in Agricultural Engineering's post-harvest-cereals-pulses-oilseeds; membrane process theory is in mechanical-unit-operations; heat exchanger design is in food-transport-phenomena; shelf-life calculations for dried foods are in shelf-life-stability. Emerging drying methods (refractance window, microwave–vacuum, electrohydrodynamic) are mentioned only in passing. Operating temperatures quoted for spray dryers and other equipment are typical ranges, not specifications.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; property values such as saturation pressures and latent heats are given in each question.
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