Dairy Science & Technology · Dairy Engineering & Plant Operations · also Dairy Engineering II, Dairy Equipment
Construction, operation, care and sizing of the machines in a milk plant: tanks, separators, homogenisers, pasteurisers, sterilisers and fillers.
In the exams: ICAR AIEEA-PG group 17 Unit II (homogeniser, pasteuriser, steriliser, mixing and agitation); ICAR AICE subject 38 Unit 5
Every dairy product passes through a sequence of machines: tanks and silos, separators, homogenisers, heat exchangers and sterilisers, mixers, churns, cheese vats, freezers and fillers. A dairy technologist must understand how each works, how it is built to be hygienic, how it is sized and operated, what goes wrong with it and how it is cared for. Equipment choice also shapes product quality: a homogeniser valve, a plate pack, a dasher blade or a filler seal can make the difference between a good product and a defective or unsafe one.
This manual covers the sanitary design and materials of dairy equipment; milk storage tanks and silos; centrifugal separators and clarifiers; homogenisers; plate heat exchangers within the HTST pasteuriser; tubular and scraped-surface heat exchangers; UHT plants; retorts and in-container sterilisers; mixing and agitation equipment; butter churns and continuous butter makers; cheese vats, curd makers and moulding systems; ice cream freezers; and filling and packaging machines. The processes these machines carry out are covered in the product subjects (market-milk-processing, fat-rich-dairy-products, cheese-science-technology, frozen-dairy-desserts), evaporators and dryers in evaporation-drying-dairy, and cleaning and hygienic design principles in cip-sanitation-hygienic-design and the food technology track's food-plant-sanitation; this manual refers to them.
ICAR AIEEA-PG Dairy Technology (group 17, Unit II) asks about the materials and sanitary features of dairy equipment, the homogeniser (theory, triplex pump, lubrication, care, accessories and standards), the pasteuriser (construction, materials, HTST, care), sterilisers, mixing and agitation equipment and dairy plant layout; ICAR AICE-JRF/SRF Dairy Engineering (subject 38, Unit 5) covers dairy processing equipment, aseptic processing and packaging equipment and the design of storage tanks and heat exchangers. Values given are representative; manufacturers' specifications govern real equipment.
Dairy equipment must be made of materials that do not react with milk, do not taint it, withstand cleaning chemicals and heat, and can be cleaned to a microbiologically safe state. Austenitic stainless steel is the standard material for product-contact surfaces. Type 304 (about 18 per cent chromium and 8 per cent nickel, hence '18/8') is used for most milk equipment; type 316, which adds molybdenum, resists chloride corrosion better and is chosen for salty or acidic products such as brine, whey, cheese and cultured products, and for equipment exposed to chlorinated sanitisers; low-carbon grades (304L, 316L) resist corrosion at welds. Stainless steel resists corrosion because chromium forms a thin, self-repairing passive oxide layer; nitric acid cleaning helps keep it passive, while chlorides, especially at high temperature and low pH, cause pitting and stress corrosion cracking. Copper and brass are avoided because copper catalyses fat oxidation and gives oxidised flavours; mild steel corrodes; aluminium is attacked by alkaline cleaners. Elastomers for seals and gaskets include EPDM (good for hot water and steam, not for fats), nitrile rubber (good for fats), fluoroelastomers (good chemical and heat resistance) and silicone; PTFE is used for some seals. Plastics in contact with food must be food-grade and suitable for the temperatures and chemicals used.
Sanitary (hygienic) design means equipment can be cleaned effectively, preferably in place, and does not harbour product residues or micro-organisms. Product-contact surfaces are smooth, with a surface roughness commonly specified at an arithmetic mean roughness of 0.8 µm or better for most dairy uses; welds are ground and polished flush. Internal corners have generous radii rather than sharp angles; surfaces drain completely (pipes and tanks slope towards drains); there are no crevices, threads, lap joints or blind holes in product zones; dead legs (branches where product or cleaning solution cannot flow) are avoided or kept very short; and seals are designed so that no gaps form between components. Pipe connections use sanitary unions or clamp couplings with hygienic gaskets, and valves are of hygienic designs (diaphragm, butterfly, single- and double-seat mix-proof valves). Equipment exteriors are also designed to be cleanable, with no ledges that collect dirt. Detailed hygienic design principles, the 3-A Sanitary Standards and the EHEDG guidelines are covered in cip-sanitation-hygienic-design; this manual applies them to individual machines.
Milk moves between machines through stainless steel pipework driven by pumps. Centrifugal pumps, with an impeller spinning in a casing, are the commonest: they are cheap, hygienic and suited to low-viscosity liquids such as milk, whey and cleaning solutions, and their flow falls as discharge pressure rises, so they can run safely against a partly closed valve for short periods. Positive-displacement pumps — rotary lobe, gear, piston, progressive cavity and diaphragm types — deliver a nearly constant flow regardless of pressure and are used for viscous products (cream, concentrates, yoghurt, curd), for gentle handling of products with particles or delicate structure, and for metering, as with the timing pump of an HTST pasteuriser; like homogenisers, they need relief valves to prevent damage if the outlet is closed. Pump selection considers flow, head, viscosity, shear sensitivity, cleanability and the net positive suction head available, since cavitation damages pumps and products. Pipework is sized for product velocity (low enough to avoid shear and foaming during production) and cleaning velocity (high enough for turbulent CIP flow), sloped for drainage and supported to avoid sagging pockets.
Raw milk is received into insulated storage tanks or silos, and processed milk, cream and intermediate products are held in process and buffer tanks. Large vertical silos, often holding tens to hundreds of thousands of litres, stand outdoors with only the lower part inside the building to save space; they are double-walled, with insulation between the inner stainless steel shell and an outer cladding, so that chilled milk warms only slowly. Smaller horizontal tanks are used indoors. Silos are fitted with a manhole (often at the bottom, inside the building), an agitator, a temperature sensor, level measurement (by pressure transducers, radar or ultrasonic sensors), high- and low-level alarms, a CIP inlet feeding spray devices at the top, an air vent with filter (so that the tank is not damaged by vacuum or pressure during filling, emptying or cleaning), a sampling valve and an outlet valve. Process tanks for cultured products and ice cream mix may have jackets for heating or cooling and more powerful agitators.
Milk in storage must be agitated gently to prevent cream from rising and to keep temperature and composition uniform, but not so vigorously that it foams or damages fat globules, which would promote lipolysis. Silos commonly use slow propeller agitators mounted low on the side or bottom, run intermittently. Sampling for composition must follow adequate mixing. Tanks are emptied, cleaned and sanitised after each use or at defined intervals; outdoor silos must be protected from the effects of sudden cooling of hot cleaning solutions, which can create a vacuum and collapse the tank if the vent is blocked. Tank capacity is calculated from geometry, for a vertical cylindrical silo V = π × D² ÷ 4 × H, and the level-to-volume relationship is calibrated so that the plant knows how much milk it holds. Road and rail tankers used to collect and transport milk follow the same principles: insulated stainless steel compartments, hygienic fittings and cleaning after each delivery.
At the start of the chain, bulk milk coolers on farms and at village collection centres hold milk at low temperature until it is collected. They are insulated stainless steel tanks with direct-expansion evaporator plates in the base and lower walls, or ice-bank systems that circulate ice water, together with an agitator, thermostat, outlet valve and often an automatic cleaning system. Their size is matched to the volume of one or two milkings and to the collection frequency, and their refrigeration capacity is chosen so that milk is cooled quickly to the required temperature even when warm milk is added on top of cold milk. Their cleanliness and cooling performance largely determine the bacterial quality of the milk the plant receives.
Dairy separators and clarifiers are disc-stack centrifuges. Milk enters a bowl rotating at high speed, typically several thousand revolutions per minute, and flows into a stack of conical discs separated by narrow gaps. In the gaps, the centrifugal field, thousands of times stronger than gravity, makes fat globules move inwards (they are lighter than skim milk) and dense particles move outwards. Fat collects towards the centre and leaves through the cream outlet; skim milk flows outwards and leaves through the skim outlet; sludge (dirt, cells, bacteria and spores in a bactofuge) collects in the outer sediment space of the bowl. Clarifiers have only one liquid outlet and are designed to remove solids; separators have two. Modern machines are hermetic (completely filled with liquid and closed, so that there is no air contact, which reduces foaming and fat damage) and self-cleaning: the bowl opens intermittently for a fraction of a second to discharge sludge while running. Outlets use paring discs (centripetal pumps) that convert the rotational energy of the liquid into pressure to deliver it from the bowl.
The centrifugal acceleration in the bowl is r ω², where r is the radius and ω the angular velocity (2π × revolutions per second); at a radius of 0.25 m and 6,000 rpm, this is about 99,000 m/s², roughly 10,000 times gravity. Separation follows Stokes' law with g replaced by r ω², so separation improves with speed, radius, particle size, density difference and lower viscosity (warmer milk); the disc stack shortens the settling distance. Separator capacity is often rated using the equivalent clarifying area (sigma value), which allows machines of different design to be compared. Efficiency is judged by the fat left in skim milk (commonly a few hundredths of a per cent with warm separation), and depends on throughput, temperature, bowl condition and air in the feed. Cream fat content is adjusted by throttling the cream outlet or by automatic standardisation systems. Care includes balancing and inspection of the bowl, maintenance of gaskets and the discharge mechanism, and cleaning in place with the bowl rotating and discharging. Because the bowl stores large amounts of kinetic energy, safety interlocks and vibration monitoring are essential.
A homogeniser is a high-pressure positive-displacement pump combined with a homogenising device. The pump usually has three plungers (a triplex pump), driven by a crankshaft and connecting rods, working out of phase so that the flow is reasonably smooth; some large machines have five plungers. Each plunger moves in a cylinder block with suction and discharge valves. The pump forces the product at high pressure through the homogenising valve: a narrow annular gap between a valve seat and a forcer (valve), surrounded by an impact ring. The gap is only a fraction of a millimetre, the product accelerates to a high velocity, and the intense turbulence, shear and cavitation break the fat globules. The homogenising pressure is set by pressing the forcer towards the seat, hydraulically or pneumatically. Most dairy homogenisers have two stages: the first does most of the size reduction and the second, at a lower pressure (often set at about a fifth of the total), breaks up clusters of globules. The theory of homogenisation and its effects on milk are covered in market-milk-processing.
Because it is a positive-displacement pump, a homogeniser delivers a fixed flow regardless of downstream resistance; it must never be run against a closed outlet, and it is fitted with a safety (pressure-relief) valve. The power needed is roughly the flow rate times the pressure divided by efficiency: homogenising 10,000 L/h (about 0.00278 m³/s) at 20 MPa needs a hydraulic power of about 56 kW, or about 65 kW at 85 per cent efficiency. Much of this energy becomes heat, raising the product temperature by a few degrees. Homogenisers are placed in the processing line where the product is warm (typically after the regeneration or heating section of a pasteuriser) and, in aseptic UHT lines, either before sterilisation or after it in an aseptic version. Care includes lubrication of the crankcase, cooling and flushing of the plunger seals (water flush prevents product from drying on plungers), inspection and replacement of seals, valves, seats and impact rings (which wear and change performance), checking pressure gauges, and correct start-up and shut-down sequences (pressure released before stopping). Homogenisers also serve as metering pumps in some lines because of their fixed flow.
Plate heat exchangers (PHEs) are the standard heaters and coolers for milk. Thin corrugated stainless steel plates, each with gaskets around its edges and ports at the corners, are clamped together in a frame so that product and heating or cooling medium flow in alternate channels. The corrugations (often chevron patterns) create turbulence at moderate velocities, giving high heat transfer coefficients, and support the thin plates against pressure. Plates are grouped into sections — regeneration, heating, cooling — separated by connector plates, and within each section the flow can be arranged in several passes and parallel channels. PHEs are compact, easy to inspect and extend by adding plates, and have high efficiency, but their narrow channels limit them to low-viscosity products without particles, and gaskets limit their pressure and temperature. The heat duty of a section is Q = m × cp × ΔT, and the area needed is A = Q ÷ (U × LMTD), where the log mean temperature difference LMTD = (ΔT1 − ΔT2) ÷ ln(ΔT1 ÷ ΔT2). For example, heating 10,000 kg/h of milk from 65 to 72 °C with hot water cooling from 80 to 75 °C in counter-current gives ΔT1 = 8 K and ΔT2 = 10 K, LMTD ≈ 8.96 K; with Q ≈ 76 kW and U = 4 kW/m² K, the area is about 2.1 m².
In an HTST pasteuriser, the PHE sections are combined with a balance tank, a timing (metering) pump, a holding tube and a flow diversion valve, with controls and recorders. Regeneration, holding-tube sizing, the flow diversion valve and the pressure relationship that keeps pasteurised milk at a higher pressure than raw milk in the regenerator are described in market-milk-processing. From the equipment side, important points are: gaskets must be inspected and replaced at intervals because a failed gasket or a pinhole in a plate can allow leakage between raw and pasteurised milk; plate integrity is tested periodically by dye penetrant, pressure or conductivity tests; the flow diversion valve must be designed and tested to fail safe (diverting on loss of power or air); and thermometers and recorders must be calibrated. Hot water sets, consisting of a steam-heated water circuit with its own pump and controls, supply the heating section, which gives more uniform and controllable heating than direct steam on plates. PHEs are cleaned in place, and fouling in the heating and holding sections is monitored through pressure drop and temperature approach.
Process lines use hygienic valves that can be cleaned in place: butterfly valves for simple on-off duty, seat valves (single-seat) actuated pneumatically and switched by the control system, and double-seat mix-proof valves that allow product and cleaning solution to flow in adjacent lines while a leakage chamber open to the drain separates them, so that a seal failure shows as visible leakage rather than contamination. Valve clusters (valve matrices) route product and CIP solutions between tanks and lines under automatic control, replacing the swing bends and flow plates of older plants. Instruments measure temperature, pressure, flow, level and conductivity; programmable controllers sequence processing and cleaning; and recorders document critical parameters. Interlocks prevent dangerous or unhygienic states, such as opening a product valve to a line being cleaned with caustic.
Tubular heat exchangers carry product through tubes with the heating or cooling medium outside them. Types include tube-in-tube (concentric tubes, product in the inner tube or annulus), multi-tube (several product tubes inside a shell) and shell-and-tube units; corrugated or spirally fluted tubes increase turbulence and heat transfer. Compared with plate heat exchangers, tubular units have fewer gaskets, withstand higher pressures and temperatures, tolerate fouling and particles better, and have longer run times with products prone to deposit, which makes them common in UHT plants and for viscous products, concentrated milks, desserts and products with small particles. Their drawbacks are lower heat transfer per unit volume, more floor space, and greater difficulty of inspection. They are cleaned in place at high velocity.
Scraped-surface heat exchangers (SSHEs) handle products too viscous, sticky or particulate for other exchangers, and products that crystallise or freeze on the wall. Product passes through the annular space between a jacketed cylinder and a rotating shaft (dasher) carrying blades that continuously scrape the heat-transfer surface. Scraping removes the thin layer of product at the wall, preventing burn-on or build-up of ice and fat crystals and renewing the surface, so heat transfer stays high even with viscous products. SSHEs are used for heating and cooling puddings, processed cheese, cream cheese, baby foods, sauces and concentrated milks, for continuous khoa and basundi making, for crystallising fat in margarine and spreads, and, most familiarly, as ice cream freezers. Their disadvantages are high capital and maintenance cost (shaft seals, blade wear), energy input from the dasher, and limited capacity per unit. Design parameters include rotor speed, number of blades, annular gap and residence time.
UHT processing is described from the product side in market-milk-processing; this chapter covers the equipment. In direct systems, culinary-quality steam (produced from potable water or filtered to remove boiler-water contaminants) is mixed with the product. In steam injection, steam is injected through a nozzle into a stream of preheated product, heating it almost instantly to the UHT temperature. In steam infusion, product is sprayed or falls as films or droplets into a pressurised vessel filled with steam, where it heats within a fraction of a second; infusion gives gentler heating with less risk of burn-on at the injector. The heated product passes through a holding tube and then into a flash (expansion) vessel under vacuum, where it boils, cools instantly and loses water equal to the condensed steam; the vacuum is controlled so that the outlet temperature matches the inlet temperature before steam addition, which keeps composition unchanged. An aseptic homogeniser after the flash vessel is usual in direct systems because the steam and flash disrupt the fat emulsion. Direct systems need careful control of steam quality, pressure and flash conditions.
Indirect UHT plants heat the product through a wall in plate, tubular or scraped-surface heat exchangers, with extensive regeneration to save energy. Plate units are compact and efficient but foul more quickly and suit low-viscosity products; tubular units are common for longer runs and higher-viscosity products; SSHEs are used for viscous and particulate products. Homogenisers in indirect lines may be placed upstream of sterilisation (non-aseptic) or downstream (aseptic). After sterilisation and cooling, the product passes to an aseptic tank (a sterile buffer tank pressurised with sterile air) or directly to aseptic fillers. The whole downstream system — pipes, valves, tanks, fillers — must be sterilised before production, commonly by circulating pressurised hot water or steam at high temperature for a set time, and must stay sterile: valves have steam barriers, seals are protected, and sterile air filters protect tank headspaces. Plant control records temperatures and holding times and diverts product if conditions fall below the set values. Run length is limited by fouling, after which an intermediate clean or full CIP is needed.
Retorts are pressure vessels in which sealed containers are heat-treated to commercial sterility, as described in market-milk-processing for in-container sterilised milk and in concentrated-dried-milks for evaporated milk. Batch retorts may be vertical or horizontal and use saturated steam, steam-air mixtures, water spray or full water immersion as the heating medium. Steam retorts must be vented to remove air before timing begins, because air pockets reduce heat transfer and create cold spots. Water spray and water immersion retorts with overpressure (air pressure above the saturation pressure) are used for flexible pouches, plastic bottles and trays, which would burst or deform without external pressure balancing the internal pressure. Rotary retorts (end-over-end or axial rotation) agitate the contents to increase heat penetration and shorten processes, reducing quality damage. Each retort process is established by heat penetration studies with thermocouples at the slowest-heating point of the container, by a competent process authority, and must achieve the required lethality (F0); retorts carry calibrated temperature indicators and recorders, and records are kept for every batch.
Continuous sterilisers handle large volumes of cans and bottles. Hydrostatic sterilisers are tall towers in which containers carried on chains pass down a water leg (preheating), through a steam chamber at sterilising temperature and pressure, and up a second water leg (cooling); the height of the water legs balances the steam pressure, so no pressure locks are needed. Continuous rotary cookers move cans along a spiral reel inside a horizontal pressure shell, with rotation improving heat transfer, and transfer them through rotary valves between pressure zones. Both systems need careful control of steam pressure, chain or reel speed (which sets process time) and cooling water quality (chlorinated cooling water prevents post-process contamination through can seams). As F0 = t × 10^((T − 121.1) ÷ 10) with t in minutes, a process of 15 minutes at 118 °C, for example, has an F0 of about 7.3 minutes. Container integrity (double seams for cans, seals for pouches and bottles) is checked routinely, since leaking containers are a main cause of spoilage after sterilisation.
Mixing is needed throughout dairy processing: keeping milk uniform in silos, dissolving powders, blending ice cream and yoghurt mixes, suspending cocoa, dispersing stabilisers, standardising, and heating or cooling in jacketed vessels. Agitators are chosen by liquid viscosity and the duty. Propeller agitators (axial flow, high speed) suit low-viscosity liquids such as milk; turbine agitators (radial flow) give stronger shear and are used for dispersing and heat transfer; paddle agitators work at low speed in moderate viscosities; anchor and gate agitators, which sweep close to the vessel wall, often with scrapers, suit viscous products in jacketed kettles. Baffles prevent swirling and vortex formation in low-viscosity liquids. The power drawn by an agitator in turbulent flow is P = Np × ρ × N³ × D⁵, where Np is the power number (a property of the impeller type), ρ the density, N the speed in revolutions per second and D the impeller diameter; for example, a propeller with Np = 0.35 and D = 0.5 m at 2 rev/s in milk of density 1,030 kg/m³ draws about 90 W. Because power rises with the fifth power of diameter and cube of speed, scale-up must be done carefully.
Dissolving powders such as skim milk powder, sugar, stabilisers and cocoa into liquids is difficult because powders form lumps with dry centres and incorporate air. Powder mixers use a high-shear rotor-stator head and a vacuum or venturi powder inlet: liquid is recirculated through the mixer while powder is drawn in under vacuum and immediately wetted and dispersed under high shear, with minimal air incorporation. Batch high-shear mixers in tanks, inline high-shear mixers, and specialised recombination systems for milk powder and fat are used. Static mixers — fixed elements inside pipes that repeatedly divide and recombine the flow — blend streams continuously with no moving parts, for example in standardisation and in-line dosing of flavours. After powder dissolution, hydration time and deaeration may be needed to avoid defects in the finished product. Mixing equipment must be hygienic, with seals and shafts designed for cleaning in place, and with attention to foam, air incorporation and shear damage to fat globules.
Butter-making processes are described in fat-rich-dairy-products; here the focus is on the machines. Batch churns are rotating stainless steel vessels, cylindrical, conical, cubical or double-conical in shape, filled to about half with aged cream and rotated at a speed that throws the cream about and incorporates air. They have sight glasses, drain valves for buttermilk, spray devices for washing butter grains and for CIP, and internal working devices (rollers or ribs) that work the butter as the churn rotates slowly after salting. Churn speed, fill level and temperature are controlled to give suitable churning time and grain size. Wooden churns, once common, have been replaced by stainless steel for hygiene. Small farm and village butter is still made in manual churns and with wooden beaters (as in desi butter from dahi in South Asia).
Continuous butter-making machines, developed from the Fritz process, produce butter in a continuous stream. Aged cream is pumped into a horizontal churning cylinder in which a beater rotates at high speed, forming butter grains within seconds. Grains and buttermilk pass into a separation section where buttermilk drains and grains may be washed with chilled buttermilk or water. The grains are then forced by twin screws through a series of perforated plates and working sections (texturisers), often with a vacuum chamber that removes air for a denser butter, and with injection points for salt slurry, water and cultures (as in the NIZO method). In-line measurement of moisture and salt controls dosing so that the butter meets composition targets, and the butter leaves through a nozzle to a packing machine or bulk packing. Continuous machines require cream of consistent fat content and temperature; beater speed, cylinder temperature and screw speed are adjusted for season. They are cleaned in place, though some parts need manual cleaning. Butter is packed into wrapped blocks or tubs by forming-and-wrapping machines or into bulk cartons for storage.
Traditional open cheese vats are rectangular, jacketed stainless steel vessels in which milk is renneted, cut with wire harps (knives) and stirred by hand or by travelling mechanical agitators; they are still used for small and artisan production. Modern plants use enclosed vats, which are cleaned in place and give better hygiene and less fat loss: horizontal cylindrical vats with a rotating shaft carrying combined cutting and stirring frames (the blades cut in one direction of rotation and stir in the other), and 'double-O' vats with two overlapping cylinders and two shafts. Vertical vats are used for some varieties. Cutting speed and stirring intensity are programmed to give uniform curd particles and minimum fines. For Cheddar-type cheese, curd and whey are pumped to draining and matting belts or continuous cheddaring machines, followed by mills, salting systems and block-forming towers in which curd fuses under its own weight and vacuum into uniform blocks. For Dutch-type cheese, pre-pressing columns or buffer tanks feed curd under whey to moulding machines.
Curd is formed into cheeses in moulds of plastic or stainless steel, often with perforated liners that drain whey and leave a rind pattern. Moulds are filled by dosing machines, and pressing is done in vertical presses, pneumatic tunnel presses (in which conveyors of moulds pass under pneumatic pressing heads) or by the weight of curd in towers. After pressing, cheeses are demoulded, and moulds are washed in mould-washing machines. Brining systems include deep brine tanks with racks or cages, shallow brining with floating cheeses turned automatically, and brine circulation systems with filters, pasteurisers or UV units to control brine quality. Pasta filata cheeses are made in cooker-stretchers (augers working curd in hot water or steam) and moulders that shape and cool the cheese. Ripening rooms have racking, turning and coating machines, and humidity and temperature control. The process science behind these machines is in cheese-science-technology.
Ice cream freezing is described from the product side in frozen-dairy-desserts. The continuous ice cream freezer is a scraped-surface heat exchanger: a horizontal cylinder surrounded by a jacket in which refrigerant (ammonia or a halocarbon) evaporates at a low temperature, with a dasher carrying scraper blades rotating inside. Mix is pumped in by a positive-displacement mix pump, and air is metered in by a separate air pump or by an air-injection system, both at a controlled ratio that sets overrun. Ice forms on the cold wall and is scraped off; the dasher whips air into fine cells and destabilises fat. A back-pressure valve at the outlet keeps the cylinder under pressure, compressing air during freezing so that it expands as the product leaves. Controls regulate refrigerant evaporating pressure (and hence temperature), dasher speed, mix flow, air flow and back-pressure to give the target draw temperature (commonly around −5 to −6 °C) and overrun. Output equals mix flow multiplied by (1 + overrun): 1,000 L/h of mix at 100 per cent overrun gives 2,000 L/h of ice cream.
Batch freezers, used in shops and small plants, have a horizontal or vertical refrigerated barrel with a dasher; a fixed charge of mix is frozen and whipped for a set time and discharged, giving lower overrun and often warmer draw temperatures. Soft-serve machines are small batch or continuous freezers that hold mix refrigerated and freeze on demand. Low-temperature extruders are cooled screw units attached after the freezer that further freeze the product under shear to around −12 to −15 °C, giving smaller ice crystals and reducing hardening load. Downstream equipment includes ingredient feeders and variegating pumps, fillers for cups, cones and tubs, extrusion and cutting lines for stick bars, moulding machines that freeze mix in moulds immersed in cold brine or with direct expansion, coating and enrobing stations for chocolate, and hardening tunnels or spiral freezers with air at around −30 to −40 °C. The freezers are cleaned in place, with dashers and blades removed and inspected regularly, because worn blades reduce scraping and heat transfer.
Filling machines deliver an accurate quantity of product into a package and seal it hygienically. Pouch machines for milk, common in South Asia and elsewhere, are vertical form-fill-seal machines: a reel of polyethylene film, often decontaminated with ultraviolet light, is formed into a tube, sealed longitudinally, filled with a measured volume, sealed across and cut, at rates of thousands of pouches per hour. Bottle fillers (gravity, vacuum, volumetric or flow-meter based) fill glass or plastic bottles, followed by cappers. Gable-top carton fillers form, fill and seal paperboard cartons, with clean or ultra-clean filling zones for ESL milk. Cup fillers for yoghurt and desserts either fill preformed cups or thermoform cups from plastic reels (form-fill-seal), dose product (and fruit), heat-seal foil lids and cut the cups apart. Output is set by the number of lanes and cycles per minute: for example, 60 pouches of 0.5 L per minute is 1,800 L per hour.
Aseptic fillers for UHT products sterilise the packaging material, commonly with hydrogen peroxide (by bath or spray) followed by hot sterile air to evaporate the residue, and form, fill and seal the package in a sterile zone maintained under sterile air overpressure; the filler itself is sterilised before production. Package types include brick-shaped laminated cartons, bottles and pouches. Residual hydrogen peroxide in packages is limited by regulations and monitored. All fillers need control of fill volume or weight (with checkweighers rejecting under- and overfilled packs to meet legal metrology rules), seal integrity (tested by destructive and non-destructive methods), coding with date marks and batch numbers, and hygiene: filling zones are classed as high-hygiene areas, with HEPA-filtered air, controlled access and cleaning and sanitation programmes. After filling, packages are collated, crated or cased, palletised and sent to cold stores or warehouses. Packaging materials and their properties are covered in dairy-packaging.
A student uses this subject to identify and describe machines in the plant, explain why stainless steel grades and hygienic features are chosen, calculate heat exchanger areas, homogeniser and agitator power, separator g-force and freezer output, and learn care routines. It supports AIEEA-PG group 17 Unit II and plant training.
A maintenance or production engineer five years in keeps separators, homogenisers, PHEs, UHT plants, freezers and fillers running: scheduling gasket, seal and valve replacement, checking plate integrity, adjusting homogeniser pressure and separator discharges, troubleshooting fill weights and seal failures, and supporting CIP. This subject gives the equipment knowledge behind those tasks.
A plant head or engineering director selects and specifies equipment for new lines, balancing capacity, hygiene, energy, flexibility and cost; manages maintenance strategy and spare parts; and ensures equipment meets hygienic design and safety standards. This subject frames those investment and reliability decisions.
A professor or research director works on equipment design and modelling — heat exchanger fouling, homogenising valve flow, separator hydrodynamics, aseptic system validation — and teaches and examines AICE Dairy Engineering on these topics.
This manual does not yet give: detailed mechanical design calculations for pressure vessels, shells and supports; manufacturer-specific operating limits and maintenance intervals; detailed aseptic system validation protocols; retort process establishment procedures in full; standards text from 3-A or EHEDG; and packaging machine engineering in depth. Speeds, pressures, temperatures and capacities are representative values.
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