Food Science & Technology · Food Process Engineering · also Refrigeration and Freezing of Foods, Low-Temperature Preservation, Frozen Food Technology
Low-temperature preservation from the food's side: chilling and controlled atmospheres, freezing-point depression and ice crystallisation, freezing-time calculation, freezer types, frozen storage changes and thawing. The refrigeration cycle and cold-store design are in Agricultural Engineering's refrigeration-cold-chain.
In the exams: GATE XE-G Section 3 (chilling, freezing) and Section 4 (freezing and crystallisation); ICAR AICE 35.a Unit 2 (IQF products)
Low temperature slows almost everything that spoils food: microbial growth, enzyme activity, respiration of living produce and chemical reactions such as oxidation. Chilling keeps food just above its freezing point, giving days to weeks of extra life while leaving the food essentially unchanged. Freezing turns most of the water into ice, stopping microbial growth altogether and slowing other changes so much that foods keep for months; but the formation of ice itself can damage texture, and frozen foods continue to change slowly in storage. Together, chilled and frozen foods — fresh meat and fish, dairy products, ready meals, frozen vegetables, ice cream, frozen bakery products — make up a large share of the modern food supply, and they depend on an unbroken cold chain from the plant to the consumer.
This subject looks at low-temperature preservation from the food's side. It explains what happens to foods and microbes when they are chilled; how the cold chain must be managed; why foods freeze over a range of temperatures rather than at 0 °C; how ice crystals form and grow; how long freezing takes and how freezers achieve it; what happens during frozen storage and thawing; and which new freezing methods are being developed. The refrigeration machinery itself — vapour-compression cycles, refrigerants, cold-store construction and refrigeration load — is taught in Agricultural Engineering's refrigeration-cold-chain, and heat-transfer theory (conduction, convection, unsteady heating) in food-transport-phenomena. The physics of the glass transition and of the maximally freeze-concentrated state, Tg′, is introduced in food-chemistry-major-constituents, and the low-temperature microbiology of chilled foods in food-microbiology.
The topic appears in GATE XE-G and XL-U (Food Technology) under chilling and freezing in Section 3 and freezing and crystallisation in Section 4, in ICAR AIEEA-PG Code 18 Unit II (freezing among unit operations) and in ICAR AICE-JRF/SRF 35.a, which names processing methods of frozen fruits and vegetables, IQF products, packaging, storage and thawing. Typical questions ask for freezing-point depression, the heat removed in freezing, Plank's freezing time, half-cooling times and the remaining shelf life of a frozen product with a mixed temperature history, together with recall and reasoning about freezers, ice crystals and storage changes.
Chilling means cooling a food to, and holding it at, a temperature above its freezing point, commonly between about −1 and 8 °C depending on the product and the law that applies. Chilling slows the growth of most spoilage and pathogenic microorganisms, slows enzymatic and chemical reactions, and in living produce slows respiration and ripening. It does not kill microorganisms or inactivate enzymes, so the quality of a chilled food can never be better than the quality with which it entered the chill chain, and its shelf life is limited — days for fresh fish and minced meat, a few weeks for vacuum-packed meats and some ready meals, longer for hard cheeses.
The rate of deterioration falls with temperature. For many reactions and for microbial growth near chill temperatures, a rise of a few degrees can halve shelf life: chilled fish, for example, keeps much longer at 0 °C on ice than at 5 °C. Holding product as close as safely possible to its freezing point therefore extends life, which is the reason for 'superchilling' (holding meat and fish a degree or two below their initial freezing point, with a little ice formed at the surface) and for icing fish at sea.
The microorganisms of concern in chilled foods are the psychrotrophs, able to grow at refrigeration temperatures though their optimum is higher. They include spoilage organisms such as Pseudomonas, Shewanella, lactic acid bacteria and some yeasts and moulds, and pathogens such as Listeria monocytogenes (which grows down to about 0 °C or slightly below), Yersinia enterocolitica and non-proteolytic Clostridium botulinum (which can grow and produce toxin down to about 3 °C). Chilled foods with long shelf lives and reduced oxygen, such as sous-vide meals and vacuum-packed smoked fish, are designed with controls against these last organisms.
Speed of cooling matters as much as the final temperature. Freshly cooked foods pass through the range in which bacteria grow fastest, roughly 10 to 50 °C, and spore-formers such as Clostridium perfringens and Bacillus cereus can germinate and multiply if cooling is slow. Regulations and codes of practice therefore set cooling-time limits. For example, the US Food Code requires cooked foods to be cooled from 57 °C to 21 °C within 2 hours and to 5 °C or below within a total of 6 hours; other countries set different but similar limits for cook-chill catering. Cooling in shallow containers, cutting large joints into smaller pieces, using blast chillers rather than ordinary refrigerators, and stirring liquids all help.
When a food cools in a medium of constant temperature, the temperature difference between food and medium falls roughly exponentially once the initial lag is over. The half-cooling time, Z, is the time for this difference to halve; it stays roughly constant through cooling, so the time to remove seven-eighths of the initial temperature difference — the 'seven-eighths cooling time' widely used for fruit and vegetables — is about three half-cooling times. For example, produce at 30 °C cooled in air at 2 °C with a half-cooling time of 1.5 h reaches 2 + 28/8 = 5.5 °C after about 4.5 h. Half-cooling time depends on the size and shape of the product, its thermal properties, the air velocity and the packaging.
Fresh fruits and vegetables are alive: they respire, producing heat, carbon dioxide and water, and the rate of respiration — and so of ageing — rises steeply with temperature. Removing field heat quickly after harvest by precooling (forced-air, hydro-, vacuum or ice cooling) is the most important single step in keeping them fresh; the engineering of these coolers and of packhouses is covered in Agricultural Engineering's post-harvest-horticultural-crops. Each commodity has a recommended storage temperature and relative humidity: most temperate produce keeps best near 0 °C at high humidity, but many tropical and subtropical produce suffer chilling injury when held below a critical temperature well above freezing — commonly about 10 to 13 °C for bananas, and similar ranges for mangoes, cucumbers, tomatoes and others — showing pitting, discoloration, failure to ripen and decay. Mixed loads must take these differences into account, and ethylene-producing fruits should not be stored with ethylene-sensitive produce.
Carcasses and meat are chilled to remove body heat quickly enough to limit microbial growth on the surface and inside deep muscles, while avoiding cold shortening of pre-rigor muscle, as explained in food-chemistry-major-constituents. Evaporative weight loss from carcass surfaces during chilling is an economic loss, controlled by air conditions and spray chilling. Fish is chilled with ice or refrigerated seawater as soon as it is caught.
The cold chain is the series of refrigerated steps that keep a chilled or frozen food at the right temperature from production to consumption: chilling at the plant, cold storage, refrigerated transport, distribution centres, retail display and the consumer's refrigerator. The chain is only as good as its weakest link, and temperature abuse at any point shortens shelf life or creates a safety hazard that later cooling cannot undo, because microorganisms that have grown do not disappear when the food is cooled again.
Refrigeration equipment in the chain is designed to hold food at temperature, not to cool warm food: transport vehicles and display cabinets have limited spare capacity, so products must be loaded at their correct temperature. International carriage of perishable foods by road and rail in many countries follows the UNECE Agreement on the International Carriage of Perishable Foodstuffs (ATP), which sets standards for insulated and refrigerated equipment and its testing, and specifies maximum temperatures for categories of foods. National food laws set maximum temperatures for chilled foods, commonly in the range of about 5 to 8 °C, with lower limits for some high-risk products; the values differ between countries.
Temperatures in the cold chain are monitored by thermometers and recorders in stores and vehicles, by data loggers that travel with consignments, and increasingly by wireless sensors that send readings in real time. Air temperature and product temperature are not the same: product temperature lags behind changes in air temperature, and the warmest product in a load is the one that matters. Probing product, with proper hygiene, is therefore used at receipt and dispatch. Time–temperature indicators (TTIs) attached to packs integrate the temperature history and change colour irreversibly as the product's estimated remaining life is used up; they communicate abuse to handlers and consumers in a way that a date label cannot.
Good practice includes pre-cooling vehicles before loading, keeping doors open as briefly as possible and using strip curtains, avoiding blocking air circulation in vehicles and cabinets, not overloading display cabinets above their load lines, defrosting evaporators regularly, and rotating stock so that the oldest product is used first — first-expired, first-out (FEFO) rather than simply first-in, first-out.
Surveys in many countries find that a large fraction of domestic refrigerators run warmer than the recommended maximum, commonly cited as 5 °C, and that the time from shop to home can be long in warm weather. Shelf lives set for chilled foods must allow for some abuse in this last link, and labels give storage instructions and use-by dates for perishable foods. For ready-to-eat chilled foods that support the growth of Listeria, regulators in several jurisdictions require manufacturers to show, through challenge tests or predictive modelling, that the organism cannot exceed set limits before the end of the shelf life under reasonably foreseeable storage conditions. Consumer education — keeping the refrigerator cold, cooling leftovers quickly, not overfilling — remains part of cold-chain management. For the manufacturer, the practical consequence is that a shelf life validated only at a perfect 4 °C is not enough: studies at moderately abusive temperatures, or predictive models of growth at the temperatures actually found in homes, are needed to set a date that stays safe in real use.
Pure water freezes at 0 °C, but the water in foods contains dissolved sugars, salts, acids and other solutes that lower its freezing point. For dilute ideal solutions, the freezing-point depression is ΔTf = Kf × m, where m is the molality of solute and Kf, the cryoscopic constant of water, is 1.86 K kg/mol. A solution of 1 mol of sugar per kilogram of water begins to freeze at about −1.86 °C. Foods have initial freezing points typically between about −0.5 and −3 °C: fruits and vegetables, rich in sugars, freeze lower than lean meat or fish.
As soon as ice begins to form, it removes pure water from the solution, so the remaining solution becomes more concentrated and its freezing point falls further. Freezing therefore proceeds over a wide range of temperatures: most of the water freezes in the first few degrees below the initial freezing point, but some remains liquid at −18 °C and below. Eventually the remaining solution becomes so concentrated and viscous that it no longer freezes; it forms a glass at Tg′ with an unfrozen water content Wg′, or, for some solutes, crystallises with ice at a eutectic temperature. A further fraction of water, bound to polar groups of proteins and carbohydrates, never freezes.
A simple model of freezing-point depression predicts the fraction of freezable water that has turned to ice at any temperature T below the initial freezing point Tf (both in °C): ice fraction ≈ 1 − Tf/T. For a food with initial freezing point −1.5 °C, about 1 − 1.5/3 = 50 per cent of the freezable water is frozen at −3 °C, about 83 per cent at −9 °C and about 92 per cent at −18 °C. More exact methods use Raoult's law with the effective molecular weight of solutes, or empirical equations, and take account of bound water that never freezes.
The ice fraction explains much of frozen-food behaviour. Because so much water freezes just below the initial freezing point, most of the latent heat is removed there, which is why temperature–time curves during freezing show a plateau, the thermal arrest period. It explains why foods are much harder and more brittle at −18 °C than at −3 °C, why reactions continue in the concentrated unfrozen phase even in the frozen state, and why lower storage temperatures reduce deterioration: they freeze more water and raise the viscosity of what remains.
Freezing changes thermal properties. Ice has a thermal conductivity about four times that of liquid water and a much higher thermal diffusivity, so frozen foods conduct heat better than unfrozen ones; this is one reason freezing (heat flowing out through a frozen layer) is faster than thawing (heat flowing in through a thawed layer). The apparent specific heat of a food rises to a sharp peak just below the initial freezing point, where latent heat is released, and falls to that of the frozen food below.
Simple estimates use Siebel's equations, in which w is the water mass fraction: specific heat above freezing ≈ 3.35w + 0.84 kJ/kg K, specific heat below freezing ≈ 1.26w + 0.84 kJ/kg K, and latent heat of the food ≈ 334w kJ/kg. The heat removed in freezing a food from an initial temperature to a final storage temperature is then approximately the sum of three parts: sensible heat above freezing, latent heat, and sensible heat below freezing. For lean meat with 75 per cent water cooled from 5 °C to −18 °C with Tf = −2 °C, this gives roughly 3.35 × 0.75 + 0.84 = 3.35 kJ/kg K above freezing (× 7 K ≈ 23 kJ/kg), 250 kJ/kg latent heat and 1.79 kJ/kg K below (× 16 K ≈ 29 kJ/kg), about 300 kJ/kg in all — dominated by the latent heat. These estimates treat all water as freezing at Tf and so overstate the latent heat slightly; enthalpy tables and more detailed models are used for design.
Ice formation begins with nucleation: the formation of tiny ordered clusters of water molecules large enough to grow rather than dissolve again. In very pure water, homogeneous nucleation requires deep supercooling, to about −40 °C; in foods, heterogeneous nucleation on surfaces, particles, cell walls and impurities starts at much smaller supercooling, usually a few degrees below the initial freezing point. A food is said to be supercooled when it is below its freezing point without ice; once nucleation occurs, the latent heat released raises the temperature back towards the freezing point and freezing proceeds. The extent of supercooling and the rate of nucleation determine how many crystals form: deep supercooling and fast heat removal produce many nuclei and hence many small crystals, while little supercooling and slow cooling produce few nuclei and large crystals. On a freezing curve recorded at the thermal centre, supercooling shows as a dip below the freezing point followed by a sudden rise when nucleation releases latent heat, after which the temperature stays near the freezing point during the thermal arrest period while most of the water turns to ice.
After nucleation, crystals grow by adding water molecules from the surrounding solution, at a rate limited by removal of the latent heat and by diffusion of water to the crystal surface. The freezing rate — often expressed as the rate at which the ice front advances, in centimetres per hour, or as the time to pass through the zone of maximum ice crystal formation (roughly from the initial freezing point to about −5 °C) — strongly affects the result in cellular foods.
In slow freezing, ice nucleates first in the extracellular spaces, where the solution is more dilute. Water then moves out of the cells by osmosis towards the growing extracellular crystals, the cells shrink and are dehydrated, and large extracellular crystals form that distort and rupture cell walls and membranes. On thawing, the cells cannot reabsorb all the water and it leaks out as drip, carrying soluble nutrients and flavour, and the tissue becomes soft. In fast freezing, nucleation occurs both inside and outside cells, many small crystals form, water has little time to migrate, and the tissue structure is better preserved. This is the main reason quick freezing gives better quality in fruits, vegetables, meat and fish, though the advantage is partly lost if the product is later stored at fluctuating temperatures. Very fast freezing of large products can also cause cracking, because the frozen outer shell cannot accommodate the expansion of the interior as it freezes (water expands by about 9 per cent on freezing).
Ice crystals are not stable once formed. During frozen storage, small crystals tend to disappear and large ones grow, because small crystals have higher surface energy and slightly lower melting points. This recrystallisation takes several forms: isomass recrystallisation, in which irregular crystals become rounder; migratory recrystallisation (Ostwald ripening), in which large crystals grow at the expense of small ones; and accretive recrystallisation, in which neighbouring crystals fuse. The rate increases with storage temperature and is greatly increased by temperature fluctuations, during which ice partly melts and then refreezes onto the larger crystals.
Recrystallisation gradually undoes the benefit of quick freezing, coarsening texture in fruits, vegetables and fish and causing the icy, coarse texture of ice cream that has been 'heat-shocked' by cycles of warming and cooling in retail cabinets and home freezers. It is controlled by low and stable storage temperatures, by storage below Tg′ where molecular mobility is very low, and by ingredients: stabilisers such as hydrocolloids in ice cream slow recrystallisation, and ice-structuring (antifreeze) proteins, found in some fish, plants and insects, bind to ice surfaces and inhibit crystal growth.
Freezing time determines freezer capacity, so predicting it is a basic task of food refrigeration engineering. The classical method is Plank's equation, derived in 1913 for the freezing of a material initially at its freezing point: t = (ρ λ / (Tf − Ta)) × (P a / h + R a² / k), where t is freezing time, ρ the density of the food, λ its latent heat per unit mass, Tf its freezing temperature, Ta the temperature of the cooling medium, a the thickness of a slab or the diameter of a cylinder or sphere, h the surface heat transfer coefficient and k the thermal conductivity of the frozen food. The shape factors are P = 1/2 and R = 1/8 for an infinite slab, P = 1/4 and R = 1/16 for an infinite cylinder, and P = 1/6 and R = 1/24 for a sphere.
The two terms in the bracket represent the two resistances to heat removal: external (surface convection, a/h) and internal (conduction through the frozen layer, a²/k). When h is small, as in still air, the first term dominates and freezing time is roughly proportional to thickness; when h is very large, as in good plate or cryogenic freezers, the second term dominates and freezing time increases with the square of thickness. The ratio of the two, related to the Biot number ha/k, shows which resistance controls.
A slab of lean fish 5 cm thick (a = 0.05 m), density 1,000 kg/m³, latent heat 250 kJ/kg, initial freezing point −1.5 °C and frozen conductivity 1.5 W/m K is frozen in an air-blast freezer at −30 °C with h = 20 W/m² K. Then ρλ/(Tf − Ta) = 1,000 × 250,000/28.5 ≈ 8.77 × 10⁶ J/m³ K; Pa/h = 0.5 × 0.05/20 = 1.25 × 10⁻³; Ra²/k = 0.125 × 0.0025/1.5 = 2.08 × 10⁻⁴. The freezing time is 8.77 × 10⁶ × 1.458 × 10⁻³ ≈ 12,800 s, about 3.6 h. In a plate freezer with h = 200 W/m² K the first term falls to 1.25 × 10⁻⁴ and the time to about 2.9 × 10³ s, less than an hour, showing why contact freezing is so much faster than air for flat products. Common slips in such problems are using the unfrozen rather than frozen conductivity, forgetting to convert latent heat from kJ to J, using the half-thickness when the equation uses the full thickness, and using the wrong shape factors.
Plank's equation makes several simplifying assumptions: the food is initially at its freezing point, so no sensible heat must be removed before freezing; freezing occurs at a single temperature rather than over a range; thermal conductivity of the frozen layer is constant; and the sensible heat removed from the frozen layer as it cools below Tf is ignored. Because real foods start above their freezing point and finish well below it, Plank's equation usually underestimates freezing time. Its definition of freezing time is also specific: the time for the thermal centre to freeze, not to reach storage temperature.
Many refinements have been proposed. Nagaoka and colleagues added a correction factor for the sensible heat above freezing; Cleland and Earle developed empirical modifications to P and R based on Stefan and Biot numbers that fit experimental data for a wide range of foods; Pham's method divides freezing into precooling, phase change and subcooling stages with separate calculations, and is widely used because it is simple and accurate; and numerical methods (finite differences or finite elements) solve the heat conduction equation with temperature-dependent properties for any shape. Equivalent heat transfer dimensionality factors extend slab results to irregular shapes. In examinations, Plank's equation is the standard method, and the skill is in substituting consistent units and choosing the right shape factors.
Air-blast freezers remove heat by blowing cold air, typically at about −30 to −40 °C, over the food at velocities of a few metres per second. They are the most versatile freezers and handle products of almost any shape and size, packaged or unpackaged. Batch blast rooms and cabinets hold trolleys or pallets; continuous tunnel freezers carry products on trolleys or belts through a cold tunnel; spiral freezers wind a long belt around a rotating drum in a compact, insulated enclosure, giving long residence times in a small floor area and suiting bakery products, meat patties and ready meals. Surface heat transfer coefficients in blast freezers are moderate, so freezing is slower than in contact or cryogenic freezers, and unpackaged products lose weight by evaporation and sublimation, while ice collects on the evaporator coils and must be removed by regular defrosting. Impingement freezers direct high-velocity jets of cold air onto the product surface, breaking up the boundary layer and greatly increasing the heat transfer coefficient, which suits thin products such as fish fillets and patties.
Plate freezers press the food, in packages or as blocks, between hollow metal plates through which refrigerant or cold brine circulates. Direct contact gives high heat transfer and freezing from both faces, so flat packages freeze quickly and uniformly, with little dehydration. Horizontal plate freezers are used for retail packs of uniform thickness — fish fillets, meat, vegetables in cartons, ready meals — and vertical plate freezers for freezing unpacked fish, meat or offal into blocks, especially on fishing vessels. Their limitations are that products must be flat and of regular thickness, and the plates must be pressed firmly in contact; air gaps or bulging packages slow freezing. Batch loading and unloading also limit throughput, although automatic horizontal plate freezers that load and unload packs continuously are used in large plants. Cooled-surface freezers such as drum freezers and belt freezers with cooled steel belts freeze liquids, purées and thin products by contact.
Cryogenic freezers use liquefied gases that boil at very low temperatures — liquid nitrogen at −196 °C and carbon dioxide, which sublimes as dry ice at −78.5 °C at atmospheric pressure. The product is sprayed with liquid nitrogen in a tunnel, or immersed in it, and the cold gas is used to precool incoming product. The large temperature difference and high surface heat transfer give very fast freezing, small ice crystals, low weight loss and low capital cost and floor space, making cryogenic freezing suitable for high-value and delicate products such as shrimps, berries and some ready meals, and for flexible or seasonal capacity. The cost of the consumed gas is the main disadvantage, and very rapid freezing of large items can crack them.
Immersion freezing in cold brine, sugar solutions or other liquids gives fast heat transfer and is used for fish on board vessels and for packaged products. Individual quick freezing (IQF) freezes small particles separately so that they flow freely after freezing: fluidised-bed freezers blow cold air upwards through a bed of peas, sweetcorn, diced vegetables, berries or shrimps, suspending and separating the pieces while freezing them in minutes. IQF products can be portioned by the consumer and are the basis of the frozen-vegetable trade. Selection of a freezer depends on product size, shape and value, required throughput, quality, weight loss, capital and running costs and available floor space.
Most vegetables are blanched before freezing, by brief exposure to hot water or steam, to inactivate enzymes such as peroxidase, catalase and lipoxygenase that would otherwise cause off-flavours, discoloration and loss of nutrients during frozen storage; blanching also removes air from tissues, brightens green colour, reduces microbial load and wilts leafy vegetables for easier packing. Peroxidase, being among the most heat-stable of these enzymes, is commonly used as the indicator of adequate blanching, though some workers argue that lipoxygenase relates better to quality. Over-blanching softens texture and leaches nutrients, so the time–temperature is set for each vegetable and size. After blanching, vegetables are cooled quickly, drained and frozen, often by IQF.
Fruits are generally not blanched, because heat damages their texture and flavour; instead enzymatic browning is controlled by adding ascorbic acid or citric acid, by packing in sugar or syrup, which also excludes oxygen and protects texture, or by sulphite treatment where permitted. Soft fruits such as strawberries are frozen individually as quickly as possible to limit drip and softening, and fruit purées are frozen in bulk for later manufacture.
Meat should normally be frozen after rigor mortis is complete; muscle frozen before rigor contracts violently when thawed (thaw rigor), losing much drip and becoming very tough. Carcasses and cuts are frozen in blast freezers, while boneless meat is frozen in blocks in plate freezers for manufacturing. Packaging that fits closely prevents freezer burn. Frozen meat keeps for many months, with lipid oxidation the main limit for fatty meats and pork.
Fish deteriorates quickly after catch, so it is frozen as soon as possible, often at sea in plate, blast or brine freezers. Whole fish and fillets are commonly glazed — dipped in or sprayed with cold water to form a thin layer of ice — which protects the surface against dehydration and oxidation during storage; the glaze weight must be declared or excluded from the net weight under labelling rules. Fatty fish such as mackerel and herring are limited by rancidity and need low storage temperatures and good protection from air. Lean gadoid fish such as cod and hake suffer toughening during frozen storage because an enzyme converts trimethylamine oxide into dimethylamine and formaldehyde, and formaldehyde cross-links the muscle proteins; lower and stable temperatures slow this. Surimi, the washed, minced fish protein used for imitation crab and other products, is frozen with cryoprotectants — commonly sugar and sorbitol with a little polyphosphate — that protect the myofibrillar proteins from freeze denaturation.
Frozen ready meals combine components with different freezing behaviours. Sauces thickened with ordinary starch may separate and weep after freezing and thawing because the starch retrogrades, so freeze–thaw-stable modified starches and hydrocolloids are used. Meals are usually frozen in their trays in blast or spiral freezers, and must be cooled quickly after cooking to limit microbial growth before freezing.
Bakery products — bread, part-baked bread, doughs, pastries and cakes — are frozen to extend their life and to allow baking at the point of sale. Freezing yeast doughs damages some yeast cells and gluten structure, so formulations are adjusted with more yeast and dough improvers. Bread stales fastest at refrigeration temperatures but very slowly when frozen, so freezing preserves freshness provided the product passes rapidly through the chill range.
Ice cream is frozen in two stages: a continuous scraped-surface freezer partly freezes the mix while whipping in air (overrun), producing many small ice crystals, and the packed product is then hardened in a blast or plate hardening tunnel to about −20 °C or below. Its smooth texture depends on small ice crystals, fine air cells and partially coalesced fat, and is easily lost by recrystallisation in storage. The detailed technology of ice cream is part of dairy technology.
Frozen foods lose moisture by sublimation whenever the vapour pressure of ice at their surface is higher than that of water vapour in the surrounding air, which happens because evaporator coils in a cold store are colder than the product and continually condense moisture from the air. Unpackaged or loosely packed foods therefore dry out at the surface during storage. Where drying is severe, it produces freezer burn: greyish-white or brownish, dry, spongy patches on meat, poultry and fish, in which the porous dried tissue is exposed to oxygen and oxidises, giving off-flavours and a tough texture that do not recover on cooking. Moisture lost from the product often appears as ice crystals inside the package, especially when temperatures fluctuate. Freezer burn is prevented by tight, moisture-proof packaging with little headspace, vacuum packing, glazing of fish, and stable storage temperatures. Large cold stores also limit weight loss of unpacked products by keeping the difference between evaporator and room temperature small, by controlling air velocity over stacks, and by covering pallets.
Drip is the fluid released when a frozen food is thawed. It comes from damage to cells by ice crystals, from the inability of denatured proteins to rebind the water released by melting ice, and from recrystallisation during storage. Drip carries soluble proteins, vitamins, minerals and flavour compounds with it, reduces weight and juiciness and spoils appearance. Its amount depends on the freezing rate, the storage temperature and duration, temperature fluctuations, the state of the meat at freezing (pre- or post-rigor, pH) and the thawing method.
Proteins also change during frozen storage. In the unfrozen phase, salts and other solutes become concentrated, pH can shift, and proteins near ice surfaces can denature and aggregate. Fish myofibrillar proteins are particularly sensitive, which is why fish becomes dry, tough and spongy after long or warm frozen storage, and why surimi needs cryoprotectants. Egg yolk forms an irreversible gel if frozen without added sugar or salt, and some emulsions and sauces break.
Lipid oxidation continues slowly in the frozen state and is often the factor that limits storage life of fatty fish, pork, poultry and nuts. Freezing does not stop it; the concentration of solutes and pro-oxidant metals in the unfrozen phase, and surface drying that exposes lipids to air, can even accelerate it. Low storage temperatures, oxygen-excluding packaging, glazing, antioxidants and avoidance of light are the controls. Colour changes include the browning of meat surfaces as myoglobin oxidises to metmyoglobin, loss of green colour in vegetables as chlorophyll converts to pheophytin, and enzymatic browning in unblanched or poorly protected fruits. Vitamin C is the most sensitive vitamin in frozen fruits and vegetables; losses are small at −18 °C or below for well-blanched, well-packed products, but increase markedly at warmer and fluctuating temperatures. In ice cream and frozen desserts, recrystallisation and lactose crystallisation (sandiness) are the main storage defects.
The general rule is that lower and more stable storage temperatures slow all of these changes, which is why −18 °C, the temperature used in many legal standards and in Codex texts for quick-frozen foods, is regarded as a maximum rather than an ideal for long storage of sensitive products, and why many cold stores for fish and fatty meat operate at −25 °C or below.
Studies at the US Department of Agriculture's Western Regional Research Laboratory in the 1950s and 1960s established the time–temperature tolerance (TTT) concept for frozen foods. For each product there is a relationship between storage temperature and the time it can be stored before quality falls below a defined level; the quality loss is cumulative and, to a first approximation, independent of the order in which different temperatures are experienced. This allows the effect of a mixed temperature history — cold store, transport, retail cabinet, home freezer — to be estimated by adding up the fractions of shelf life used at each stage.
Two definitions of storage life are common. High quality life (HQL) is the time from freezing until a trained panel can first detect a statistically significant difference from a control stored at a very low reference temperature, often defined by triangle tests in which a set proportion of panellists (commonly 70 per cent) correctly identify the odd sample; it is the time to a 'just noticeable difference'. Practical storage life (PSL) is the longer period during which the product remains acceptable for consumption or for its intended use. PSL is the more useful figure for commerce. Both shorten as storage temperature rises, usually more steeply for fatty products than for lean ones, and both depend on the raw material, processing and packaging.
If a product's practical storage life at each temperature is known, the fraction of its life consumed during a period of t days at temperature T is t/PSL(T). Summing these fractions over the product's history gives the total fraction used; one minus that sum is the fraction remaining, which can be converted into days at any planned storage temperature. For example, suppose a frozen vegetable has PSL values of 360 days at −24 °C, 270 days at −18 °C and 90 days at −12 °C. After 120 days in a cold store at −24 °C (120/360 = 0.333), 30 days in distribution at −18 °C (30/270 = 0.111) and 15 days in a retail cabinet at −12 °C (15/90 = 0.167), 61 per cent of the life has been used; the remaining 39 per cent corresponds to about 0.389 × 270 ≈ 105 days at −18 °C in the consumer's freezer.
The additivity assumption works well for many products but not all. Some foods show 'product–process–packaging' interactions, and temperature fluctuations can do more damage than their average temperature suggests, because they drive recrystallisation and moisture migration inside the pack. The TTT approach is therefore an estimate, best used together with direct quality measurements.
Codex texts on quick-frozen foods and many national laws define quick-frozen foods as foods that have passed rapidly through the zone of maximum crystallisation and are kept at −18 °C or colder at the thermal centre after thermal stabilisation, with only brief upward fluctuations of a few degrees permitted during transport and in retail display; exact tolerances are set by each law. Frozen foods carry best-before dates rather than use-by dates, because quality, not safety, limits their life when the cold chain is maintained; labels give storage times for different domestic freezer star ratings and instructions not to refreeze after thawing. Time–temperature indicators, discussed under the cold chain, are used on some frozen products to show whether the product has been held at acceptable temperatures. Retail display cabinets are the warmest regular link in many frozen chains, so products at the top and front of open cabinets, nearest the load line and the warm store air, use up their life fastest.
Thawing is not freezing in reverse. During freezing, heat leaves through the already frozen outer layer, whose thermal conductivity is high (ice conducts about four times better than water); during thawing, heat must enter through the already thawed outer layer, whose conductivity and thermal diffusivity are much lower. In addition, the temperature difference available is limited: the surface of the food cannot be heated much above ambient without cooking it or encouraging microbial growth, whereas freezing media can be tens of degrees below the freezing point. Thawing therefore takes considerably longer than freezing under comparable conditions, and the food spends a long time close to its freezing point, where the thawed outer layers may be warm enough for microorganisms to grow while the centre is still frozen. Plank-type equations can be written for thawing with the thawed-layer conductivity, and they confirm the longer times; in practice thawing schedules are set by trials that record both surface and centre temperatures.
Thawing methods include still or moving air, at ambient or controlled temperatures and humidity; water, by immersion or spraying, which gives faster heat transfer but can leach solutes and needs hygienic control; vacuum (condensing-steam) thawing, in which water vapour condenses on the product under reduced pressure at low temperature; and contact thawing between heated plates for flat blocks. Electrical methods — microwave, radio-frequency and ohmic heating — generate heat inside the food and can thaw much faster. Microwave thawing, however, tends to be uneven: liquid water absorbs microwave energy far more strongly than ice, so any region that thaws first heats rapidly while neighbouring frozen regions stay cold — the 'runaway' heating that causes cooked edges and frozen centres. Domestic microwaves use pulsed defrost cycles to allow heat to spread; industrial radio-frequency systems, with longer wavelengths and deeper penetration, give more uniform results in large blocks.
For safety, consumers and caterers are advised to thaw foods in the refrigerator, in cold water changed regularly, or by microwave immediately before cooking, rather than at room temperature, and to cook large items such as poultry only when fully thawed unless the cooking method allows for frozen product. Thawed foods should not be refrozen unless cooked first, because of quality loss and microbial growth during thawing.
Tempering means raising the temperature of frozen food from storage temperature to just below its freezing point, commonly to about −2 to −5 °C, without thawing it. Tempered meat and fish blocks are firm enough to slice, dice, mince or form cleanly but soft enough not to damage machinery, and they process with less drip than fully thawed material. Tempering is done slowly in controlled air rooms, which takes days for large blocks, or rapidly by microwave or radio-frequency tempering systems, which can temper a block in minutes; because little ice melts, runaway heating is limited in tempering. Tempering is widely used in meat and fish processing and in preparing frozen raw materials for further manufacture. Because tempered blocks stay below the initial freezing point, microbial growth is negligible and drip is low, and the product can be held tempered for a limited time before processing. Uniform tempering matters: a block whose edges have thawed while its core is at −15 °C slices badly and loses quality at the edges.
Supercooling preservation holds a food below its freezing point without forming ice, gaining much of the shelf-life benefit of sub-zero temperature without the damage caused by ice crystals. Because the supercooled state is metastable, it can collapse suddenly into freezing when disturbed by vibration, temperature fluctuation or a nucleating surface; research focuses on stabilising it with precise temperature control, careful cooling profiles, and the use of electric or magnetic fields, whose effectiveness is still debated. Controlled-nucleation methods take the opposite approach: they trigger nucleation at a chosen temperature, uniformly throughout the product, to give uniform small crystals. Ultrasound-assisted freezing uses acoustic cavitation to induce nucleation and break growing crystals, and has shown improved crystal structure in research studies. Industrial adoption of these methods is still limited, and most published work is at laboratory scale, so claims of commercial benefit should be checked against trials on the product in question.
High pressure lowers the freezing point of water: at about 200 MPa, water remains liquid down to about −21 °C. In pressure-shift freezing, a product is cooled under high pressure to well below 0 °C without freezing and the pressure is then released rapidly; the sudden supercooling throughout the whole volume causes instantaneous, uniform nucleation of small ice crystals, even in large items, followed by conventional completion of freezing at atmospheric pressure. Pressure-assisted thawing uses the reverse effect. These methods give excellent microstructure but need expensive high-pressure equipment, and the high pressure itself can denature proteins and change the colour of meat.
Isochoric freezing holds the product, usually immersed in a liquid, in a rigid sealed chamber of constant volume. As ice forms it expands, raising the pressure inside the chamber, and the rising pressure lowers the freezing point; the system reaches an equilibrium at a sub-zero temperature in which part of the chamber is ice and the product, positioned in the unfrozen liquid region, remains unfrozen. Isochoric storage thus keeps foods at sub-zero temperatures without ice in the food itself, and is being studied for high-value foods and for biological materials.
Other emerging or specialised methods include dehydrofreezing, in which fruits and vegetables are partially dried before freezing, reducing the amount of water to freeze, the freezing load and drip, and producing a denser product for manufacturing use; impingement and hydrofluidisation freezing, which raise surface heat transfer with high-velocity jets of air or refrigerated liquid; the use of ice-structuring (antifreeze) proteins and ice-nucleating proteins to control crystal formation; and cryoprotective formulations based on sugars, polyols and hydrocolloids. Electrostatic and magnetic-field-assisted freezing are marketed for some applications, but published results are mixed and claims should be judged on independent evidence.
When evaluating a new freezing method, the food technologist should ask: does it improve crystal size or distribution measurably, and does that translate into lower drip, better texture or longer storage life in sensory and instrumental tests? What does it cost in capital, energy and throughput compared with a well-run conventional freezer? Is it scalable and safe, and does it raise any regulatory questions? Many quality problems blamed on freezing are in fact caused by poor storage and fluctuating temperatures, which no freezing method can undo.
A student uses this subject to solve the numerical questions that recur in food technology exams — freezing-point depression, heat loads in freezing, Plank's freezing time with the right shape factors and units, half-cooling and seven-eighths cooling times, and remaining shelf life from TTT data — and to explain in words why quick freezing gives less drip, why thawing is slower than freezing, why vegetables are blanched and why fish is glazed. In the laboratory the same student records freezing curves with thermocouples, compares drip loss after slow and fast freezing, and tests blanching adequacy with the peroxidase test.
Five years into a frozen-food or chilled-food plant, a technologist sets blanching times and checks peroxidase results, chooses freezer settings and belt speeds for each product, monitors freezer outlet temperatures and weight loss, manages cold-store temperatures, defrost cycles and stock rotation, investigates complaints of freezer burn, ice crystals in packs, drip or rancidity, and verifies cooling records for cooked products against legal limits. In a chilled-food business, the same person runs shelf-life and challenge studies for Listeria and checks the temperature records of every link in the distribution chain.
A senior manager or engineer decides on freezer technology for new lines — spiral or tunnel, plate or IQF, mechanical or cryogenic — weighing quality, throughput, energy, refrigerant regulations and capital cost; sets corporate cold-chain standards and audits logistics partners; negotiates storage temperatures and shelf lives with retailers in different countries; and leads energy-reduction programmes in large cold stores. They also answer for the plant in incidents involving temperature abuse, deciding whether product can be released, downgraded or must be destroyed.
A research leader works on the science of ice in foods — nucleation control, recrystallisation inhibition by ice-structuring proteins and hydrocolloids, glass-transition-based stability models — and on emerging technologies such as pressure-shift, isochoric, supercooled and ultrasound-assisted freezing, together with low-carbon refrigeration and cold-chain digitalisation with sensors and predictive shelf-life models. They advise regulators on temperature standards, review the evidence behind commercial claims for new freezing methods, supervise doctoral students and design examinations. They also help set the scientific basis for storage-temperature rules, such as whether −18 °C remains the right reference as energy costs and climate targets change.
This manual does not cover refrigeration machinery, refrigerants or cold-store design and load calculations (see Agricultural Engineering's refrigeration-cold-chain), or modified- and controlled-atmosphere storage of produce (post-harvest engineering). Product-by-product recommended storage temperatures, practical storage lives and legal temperature limits are not tabulated, because they vary by commodity, source and country; questions supply the values they need. Ice-cream manufacture is treated only in outline, as part of dairy technology. Refined freezing-time methods (Pham, Cleland and Earle) are described but their formulas are not given in full.
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 and storage lives are given in each question because published values vary by product and source.
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