Food Science & Technology · Food Process Engineering · also Emerging Food Processing Technologies, Non-thermal Processing, Alternative Thermal Technologies, Food Irradiation
The newer ways to make food safe or change its structure with less heat damage: high-pressure processing, pulsed electric fields, ohmic, microwave and radio-frequency heating, irradiation, UV, cold plasma and ultrasound — their mechanisms, equipment, validation and regulatory status.
In the exams: GATE XE-G Section 3 (irradiation, hurdle technology); ICAR AICE 35.a Unit 1 (non-thermal and alternate-thermal technologies); ICAR AIEEA-PG Code 18 Unit III (irradiation, microwave processing)
For a century, heat has been the main tool for making food safe and stable. It works, and its science is mature (thermal-processing-canning), but it also cooks: it softens texture, dulls colour, destroys heat-sensitive vitamins and changes flavour. Consumers increasingly want foods that are safe and long-lasting but taste fresh and minimally processed, and processors want to save energy, water and time. Novel and non-thermal processes answer both demands. Some kill microorganisms with little or no heating — high pressure, pulsed electric fields, irradiation, ultraviolet and pulsed light, cold plasma, ultrasound and chemical agents such as ozone. Others are thermal but deliver heat differently — volumetrically and rapidly through the food by ohmic, microwave and radio-frequency heating, or intensely at the surface by infrared and induction — so that the same lethality is reached with less quality damage than conventional heating from the outside in.
This subject explains how each technology works, what equipment it needs, which microorganisms and enzymes it inactivates (and which it does not — bacterial spores are the common weakness of most non-thermal methods), what it does to food quality, where it is used commercially, and what limits it. It then covers how novel processes are validated, including the use of surrogate organisms and non-log-linear kinetics, and how they are regulated. The principles build on several other subjects: microbial inactivation kinetics and process lethality from thermal-processing-canning; heat and mass transfer from food-transport-phenomena; electrical and dielectric properties from food-physical-properties-rheology; and the chemistry of the constituents being protected or changed from food-chemistry-major-constituents.
The topic is examined in GATE XE-G and XL-U (irradiation and hurdle technology in Section 3), ICAR AIEEA-PG Code 18 Unit III (irradiation, microwave processing) and especially ICAR AICE-JRF/SRF 35.a Unit 1, which names microfiltration, bactofugation, high-voltage and pulsed electric fields, high-pressure processing, irradiation, thermosonication, ohmic, dielectric, infrared and induction heating and biological preservation technologies. Professionally, these processes demand careful validation and regulatory filing, and food safety officers and analysts meet them when checking claims and labels, especially for irradiated foods.
Conventional thermal processes heat food from the outside by conduction and convection. Because foods have low thermal diffusivity, the surface is overheated while the slowest-heating point reaches its target lethality, and quality reactions with high z values continue throughout the long heating and cooling times (thermal-processing-canning). Heat-sensitive nutrients such as vitamin C, thiamin and folate are lost; colours change as chlorophyll degrades and Maillard browning develops; textures soften; fresh flavours are replaced by cooked notes. High-temperature short-time and aseptic processing reduce these effects for pumpable liquids, but for solid foods and particulates the problem remains. Thermal processes also use large amounts of energy and water for heating and cooling. The larger the container and the more viscous or solid the food, the greater the gap between surface and centre, which is why canned solid packs show the most cooked character and why thin pouches and aseptic systems were developed.
Several pressures push the industry towards alternatives. Consumers seek foods perceived as fresh, natural and minimally processed, with fewer additives — 'clean-label' products such as cold-pressed juices, fresh guacamole, ready-to-eat meats without preservatives and chilled soups. At the same time they expect long shelf life and complete safety, especially from Listeria, Salmonella and pathogenic E. coli. Retailers and exporters want longer chilled shelf lives to reach distant markets. Processors want energy and water savings, faster processing and new product possibilities, such as tenderised or texturised foods, improved extraction yields and products impossible to make with heat. Novel processes can also be combined with mild heat, refrigeration, packaging and other hurdles to give stability with better quality — the hurdle concept applied with new tools (shelf-life-stability). Novel processes are not automatically better: each must earn its place by delivering a clear benefit in quality, safety, cost or sustainability over the best conventional option for that product.
Novel technologies are commonly grouped as follows. Non-thermal physical processes inactivate microorganisms mainly without heat: high-pressure processing, pulsed electric fields, ionising irradiation, ultraviolet and pulsed light, cold plasma and power ultrasound. Chemical and biological non-thermal treatments include ozone, chlorine dioxide, electrolysed water, organic acids, bacteriophages, bacteriocins such as nisin and protective cultures. Advanced thermal (or alternative thermal) processes are still thermal but generate heat differently: volumetric heating by ohmic, microwave and radio-frequency energy, and surface heating by infrared and induction. Many of these processes are applied in combination — pressure with heat, ultrasound with heat and pressure (manothermosonication), UV with mild heat — to exploit synergy and overcome individual limitations. A key limitation shared by most non-thermal methods is the high resistance of bacterial spores; for low-acid shelf-stable foods, heat or combined pressure–heat processes remain necessary. Some of these technologies are now mature and widely used, such as HPP, PEF pre-treatment of potatoes, UV for water and irradiation of spices; others, such as cold plasma, remain largely at pilot scale.
High-pressure processing (HPP), also called high hydrostatic pressure or pascalisation, subjects packaged foods to very high pressures, commonly about 400 to 600 MPa in commercial practice, for a few minutes, using water as the pressure-transmitting medium. Two principles explain its behaviour. The isostatic principle (Pascal's principle) states that pressure is transmitted instantly and uniformly in all directions through the vessel and the food, regardless of size and shape, so — unlike heat — there is no slow-heating zone and no gradient within the product (provided it contains enough water and no large air pockets). Le Chatelier's principle states that a system under pressure shifts towards states of smaller volume; reactions and changes accompanied by a decrease in volume, such as the disruption of non-covalent hydrophobic and ionic interactions and the formation of some hydrogen bonds, are favoured, while covalent bonds are hardly affected. This is why HPP inactivates microorganisms and denatures proteins while leaving small molecules — vitamins, pigments, flavour compounds — largely intact, preserving fresh-like quality.
Compression also heats the food adiabatically: water-based foods rise by roughly 3 °C per 100 MPa, and fats more, so a product at 5 °C may reach about 20 °C at 500 MPa, returning to its starting temperature on decompression. Pressure units are often confusing: 600 MPa is 6,000 bar or about 87,000 pounds per square inch.
Commercial HPP uses batch vessels — horizontal or vertical thick-walled steel cylinders, often wire-wound to withstand the stresses — of a few hundred litres. Packed products are loaded in carriers, the vessel is closed and filled with water, high-pressure intensifier pumps raise the pressure over a few minutes, the pressure is held (often for one to six minutes), and then released in seconds. Cycle times of several minutes and the capital cost of the equipment make HPP relatively expensive per kilogram, so it is used for high-value products. Bulk systems also treat pumpable beverages in large bags or in the vessel itself before aseptic filling. Packaging must be flexible enough to transmit pressure and withstand compression of around 15 per cent in volume without damage — typically PET or polyolefin bottles and flexible pouches with little headspace; glass and rigid metal containers are unsuitable. Because product, not package, is the cost driver, processors increase vessel fill and choose pack shapes that pack densely in the carriers, and toll-processing centres let small companies use HPP without buying equipment.
Vegetative bacteria, yeasts, moulds, parasites and many viruses are inactivated at commercial pressures by damage to cell membranes, denaturation of proteins and enzymes and disruption of ribosomes. Resistance varies: Gram-positive bacteria are generally more resistant than Gram-negative, cells in stationary phase more than in exponential phase, and low water activity and high sugar or fat protect microorganisms. Bacterial spores are highly resistant to pressure alone at ambient temperature, so HPP products of low acidity must still be refrigerated; combining pressure with temperatures above about 90 °C (pressure-assisted thermal sterilisation, PATS) can inactivate spores, and regulators have accepted filed processes of this type for specific low-acid foods. Enzymes vary widely in pressure resistance: some, such as pectin methylesterase and polyphenol oxidase, can survive commercial pressures, which affects the cloud stability of juices and browning of avocado products, and may need low pH, mild heat or other measures.
HPP is used commercially for avocado products such as guacamole (among its first successes), cold-pressed fruit and vegetable juices, ready-to-eat meats (to control Listeria after packaging), seafood (including shucking oysters, as pressure detaches the adductor muscle), wet salads and dips, dairy products, baby foods and pet foods. Juices processed by HPP in the United States must meet the juice HACCP requirement for a 5-log reduction of the pertinent pathogen. Pressure also modifies texture — tenderising or firming proteins and gelling starches — which is studied for new products.
Pulsed electric field (PEF) processing applies very short pulses — typically microseconds long — of high-voltage electric field across a food placed between two electrodes. The field induces a potential difference across the membranes of cells; when this exceeds a critical value (of the order of 1 volt across the membrane), pores form in the membrane, a process called electroporation. Pores can be reversible, sealing again after the pulse, or irreversible, causing loss of membrane function and cell death. Field strength, pulse width and number, pulse shape (square-wave or exponential decay), total specific energy delivered and treatment temperature determine the effect. Larger cells are porated at lower field strengths than small ones, which is why plant and animal tissues can be permeabilised with much weaker fields than are needed to kill bacteria. Because only membranes are targeted, PEF causes little heating at moderate energies and leaves small molecules such as vitamins and flavour compounds largely intact.
For pasteurisation of pumpable liquids — fruit juices, smoothies, milk and liquid egg — field strengths of roughly 15 to 40 kV/cm are used, with treatment times totalling microseconds to a few hundred microseconds, in continuous treatment chambers (co-field or co-linear designs) through which the liquid flows. Vegetative bacteria and yeasts are inactivated, typically achieving log reductions comparable to mild thermal pasteurisation, especially when combined with moderate temperatures (around 40 to 55 °C) that act synergistically. Bacterial spores and many enzymes are resistant. The product must be free of large air bubbles and particles that could cause arcing, and its electrical conductivity affects the current and the temperature rise; ohmic heating during treatment must be accounted for, and electrode materials are chosen to limit electrochemical reactions and metal release. The specific energy input (kJ/kg) is a key process parameter: it equals the energy per pulse multiplied by the number of pulses, divided by the mass treated. Commercial PEF pasteurisation of juices is established in several countries, giving fresh-like juices with chilled shelf lives of a few weeks.
Lower field strengths, roughly 0.5 to 3 kV/cm, with modest energy inputs, are used to permeabilise plant and animal tissues without aiming to kill microorganisms. The largest commercial application is pre-treating potatoes before cutting into French fries and crisps: permeabilisation softens the tuber so that it cuts with less force and less breakage, gives smoother cut surfaces, reduces oil uptake and can lower acrylamide precursors by leaching sugars. PEF also increases juice and oil extraction yields, speeds drying and freezing, aids extraction of pigments, sugars and bioactive compounds, and is used for meat tenderising and in wine-making to increase colour extraction. Because the energy input is low and treatment is continuous, these applications are economically attractive and have spread more widely than PEF pasteurisation. Treatment chambers for solids often use a conveyor belt passing through a water-filled chamber between electrodes, treating several tonnes per hour of whole potatoes.
In ohmic (resistive, Joule or electroconductive) heating, an alternating electric current is passed directly through the food, which acts as an electrical resistor and generates heat internally throughout its volume. The volumetric heat generation rate is q = σ E², where σ is the electrical conductivity of the food (S/m) and E the electric field strength (V/m), as explained in food-physical-properties-rheology. For example, a soup with σ = 1.5 S/m in a field of 1,500 V/m generates 1.5 × 1,500² ≈ 3.4 × 10⁶ W/m³; with density 1,050 kg/m³ and specific heat 3,800 J/kg K, its temperature rises at about 3.4 × 10⁶/(1,050 × 3,800) ≈ 0.85 K per second. Because heat is generated inside the food rather than conducted from a hot surface, there is no need for a temperature gradient to drive heat in, surfaces are not overheated, and fouling of hot surfaces is reduced. Heating is very fast and energy conversion efficient.
The rate of heating depends on electrical conductivity, which rises with ionic content (salt, acids) and moisture, and falls with fat and with non-ionic solids such as sugars and starches. Conductivity of most foods increases with temperature, often by a few per cent per degree, because ion mobility rises as viscosity falls, so regions that heat first become more conductive and may heat faster still. In particulate foods — soups or sauces with vegetable or meat pieces — the relative conductivity of particles and carrier liquid decides which heats faster. If particles are less conductive than the liquid, they may lag behind; if more conductive, they can heat faster than the liquid, the reverse of conventional heating, where particles always lag. Matching conductivities (for example by soaking particles in salt solutions), controlling particle size and orientation, and good mixing are used to achieve uniform heating. Fat particles and fatty tissues, being poor conductors, can form cold spots.
Continuous ohmic heaters pump the product through a series of tubes containing electrodes, with the electric field applied along the flow, followed by a holding tube and cooling. Electrodes are made of materials such as platinised titanium or stainless steel; at low frequencies (50 to 60 Hz mains frequency), electrochemical reactions at the electrodes can cause corrosion, metal migration into the product and gas bubbles, which are reduced by using higher frequencies (kilohertz) and suitable electrode materials. Ohmic heating is used commercially for aseptic processing of fruit preparations with large pieces (for yoghurts and desserts), liquid egg, soups and sauces with particulates, and for blanching, thawing and cooking. Its main advantage in aseptic processing of particulates is that large particles can be sterilised without overcooking the carrier liquid; its main challenges are process validation for particles, control of conductivity variation between batches, and capital cost. Validation follows the principles for particulate aseptic processing in thermal-processing-canning.
Microwave (MW) and radio-frequency (RF) heating use electromagnetic energy that penetrates the food and is converted to heat inside it, mainly by dipole rotation of water molecules following the oscillating field and by ionic conduction of dissolved salts. The dielectric properties of the food — the dielectric constant ε′ and loss factor ε″ — determine how much energy is absorbed and how deeply it penetrates (food-physical-properties-rheology); power absorbed per unit volume is P = 2π f ε0 ε″ E². Frequencies are allocated internationally for industrial, scientific and medical use: microwave heating uses 2,450 MHz (domestic ovens and many industrial systems) and 915 MHz (896 MHz in some regions), which penetrates deeper and is preferred for larger industrial systems; RF heating uses 13.56, 27.12 and 40.68 MHz, with much longer wavelengths and deeper penetration. Microwave systems use magnetrons and waveguides feeding a cavity or tunnel; RF systems apply the field between parallel electrode plates, giving a more predictable field pattern for large blocks.
The main technical challenge of microwave heating is non-uniformity. Standing-wave patterns in the cavity create hot and cold spots; fields concentrate at edges and corners of rectangular foods, overheating them; penetration depth at 2,450 MHz is typically only about a centimetre or so in moist foods, so thick items heat from the outside by conduction; frozen regions absorb far less than thawed ones, causing runaway heating during thawing; and salty foods absorb near the surface. Cold spots are a food-safety concern in reheating ready meals and in microwave pasteurisation and sterilisation. Mitigation includes turntables, mode stirrers, rounded and ring-shaped package geometries, controlled pulsing of power, formulation to adjust dielectric properties, metal shielding in packages, and immersion of packs in water during heating to smooth the field and the surface temperature. For safety-critical uses such as pasteurisation and sterilisation, the cold spot is found by mapping with fibre-optic temperature probes or chemical marker methods, and the process is designed for the slowest-heating location, as in conventional thermal processing.
Industrial microwave applications include tempering frozen meat and fish blocks to just below freezing (−2 to −5 °C) in minutes rather than days; finish drying of pasta, snacks and powders; vacuum microwave drying of fruit pieces to give puffed, crisp textures; cooking of bacon and pre-cooked products; and pasteurisation of packaged ready meals. Microwave-assisted thermal sterilisation (MATS), developed at Washington State University, heats packaged low-acid foods immersed in pressurised water in a 915 MHz system, reaching sterilising temperatures much faster than retorting, with better quality; regulators have accepted filed processes using it for specific products. RF heating is used for tempering and thawing large frozen blocks, post-baking drying of biscuits to remove moisture from the centre and prevent checking (cracking), disinfestation of nuts, grains and dried fruit (insects heat preferentially in some conditions), and pasteurisation of low-moisture foods such as flour, nuts and spices against Salmonella, where conventional heating penetrates poorly. Both methods are also used in domestic and catering cooking, where users' reheating instructions — standing times, stirring — address non-uniformity.
Infrared (IR) radiation, with wavelengths between visible light and microwaves (roughly 0.78 to 1,000 µm), is divided into near (about 0.78 to 1.4 µm), medium and far infrared. All bodies emit infrared radiation; hotter emitters radiate more energy at shorter wavelengths, as described by Wien's displacement law, λmax × T ≈ 2,898 µm K — an emitter at 2,000 K has its peak near 1.45 µm, while one at 800 K peaks near 3.6 µm. Infrared energy is absorbed in a thin surface layer of food, typically a few millimetres or less depending on wavelength and the food's composition, so IR heats surfaces quickly while the interior heats by conduction. Water absorbs strongly at certain IR wavelengths, so the spectrum of the emitter can be matched to the food. Emitters include electrically heated quartz tubes and halogen lamps (near IR), ceramic and metal sheath elements (medium to far IR) and gas-fired radiant burners. The emissivity of the food surface affects absorption: dark, moist surfaces absorb well, while shiny or pale surfaces reflect part of the energy.
Infrared heating gives high heat-transfer rates without heating the surrounding air, with fast response and compact equipment. It is used for surface browning and toasting of bakery products, grilling and roasting, drying of thin layers and granular products (often combined with hot air or vacuum), dry peeling of tomatoes and peaches (the skin is loosened by rapid surface heating without water or lye), blanching, surface pasteurisation of ready-to-eat meats and fruits, insect disinfestation of grains and rice, and reheating. Its limitations are shallow penetration, which makes it unsuitable for heating thick products throughout, and sensitivity to surface colour, moisture and geometry, which can cause uneven heating; shadowed areas receive less energy. Combined infrared–hot air and infrared–microwave systems exploit IR for the surface and other methods for the interior. Infrared is also used in catalytic and gas-fired systems for roasting coffee and nuts, where its fast response allows close control of roast colour.
Induction heating uses an alternating magnetic field, produced by a coil carrying alternating current, to induce eddy currents in an electrically conductive (usually ferromagnetic) material, which heats by its own resistance; in ferromagnetic materials, magnetic hysteresis losses add heat. The food itself is not heated directly by the field; instead, a metal vessel, pipe or plate in contact with the food is heated very rapidly and efficiently. Induction cooktops and catering equipment heat pans directly, with high efficiency and precise control; industrial applications include continuous-flow heaters in which a liquid food flows through a metal tube heated by induction, giving fast, controllable heating with little overheating of surfaces, heating of cans and containers, and sealing of induction-sealed foil liners on bottle closures. Induction heating is clean, efficient and fast to respond, but it requires suitable metal components and careful design to avoid hot spots. Because only the metal heats, energy losses to the surroundings are small, and the heating stops almost instantly when the current is switched off, which suits processes needing precise temperature control.
Food irradiation exposes food to ionising radiation: gamma rays from radioactive isotopes, high-energy electron beams, or X-rays generated by striking a metal target with electrons. Gamma sources are mainly cobalt-60 (emitting photons of 1.17 and 1.33 MeV; half-life about 5.3 years) and, less commonly, caesium-137. Electron-beam (e-beam) machines accelerate electrons to energies up to 10 MeV; they switch on and off and need no radioactive source, but electrons penetrate only a few centimetres of food, so they suit thin products and surfaces. X-ray machines (up to 5 MeV, or 7.5 MeV in some jurisdictions) combine machine operation with deep penetration like gamma rays. These energy limits ensure that irradiated food does not become radioactive.
The absorbed dose is the energy absorbed per unit mass, measured in grays (1 Gy = 1 J/kg); food doses are usually given in kilograys (kGy). The dose required depends on purpose: low doses (below about 1 kGy) inhibit sprouting of potatoes and onions, delay ripening of some fruits and disinfest grains, pulses and fruit of insects (phytosanitary treatment); medium doses (about 1 to 10 kGy) reduce spoilage organisms (radurisation) and pathogens such as Salmonella, E. coli O157:H7 and Listeria (radicidation) in meat, poultry, seafood and spices; high doses (above 10 kGy, radappertisation) sterilise foods for special purposes such as space missions and some hospital diets. Because absorbed dose varies through a package, dosimetry measures the minimum and maximum doses, and the dose uniformity ratio (Dmax/Dmin) must lie within limits that achieve the minimum effective dose without exceeding the maximum permitted.
Ionising radiation damages microorganisms mainly by breaking DNA, directly and through free radicals formed by radiolysis of water. Sensitivity is expressed by the D10 value, the dose for a one-log reduction. Larger and more complex organisms are more sensitive: insects and parasites are killed or sterilised at low doses, vegetative bacteria at moderate doses, and bacterial spores, viruses and prions need much higher doses; low water activity and freezing increase resistance because fewer free radicals form or diffuse. For example, if a pathogen has D10 = 0.5 kGy, a 5-log reduction requires 2.5 kGy.
At the doses used, irradiation causes only small chemical changes: some vitamin losses (thiamin and vitamin E are among the most sensitive), lipid oxidation and off-flavours in fatty foods at higher doses, softening of some fruits, and formation of small amounts of radiolytic products, most of which also form in cooking. Extensive toxicological and nutritional studies, reviewed by expert bodies of FAO, IAEA and WHO, concluded that food irradiated to appropriate doses is safe and nutritionally adequate. Irradiation does not inactivate preformed toxins and is not a substitute for good hygiene; it is used as a final step for packaged foods because it treats products in their packs without significant heating.
The Codex General Standard for Irradiated Foods sets principles: irradiation must serve a technological need, be done in licensed facilities using good practice and records, and the overall average dose should generally not exceed 10 kGy except where higher doses are needed for a legitimate technological purpose. National laws list permitted foods, purposes and dose limits, which vary considerably: some countries permit a wide range of foods, others very few. Irradiated foods must be labelled — Codex and many countries require a statement such as 'irradiated' or 'treated with ionising radiation', and many use the international Radura symbol (a stylised plant in a broken circle); ingredients that have been irradiated must also be declared in many systems. Analytical methods detect irradiation to enforce labelling: thermoluminescence of silicate minerals adhering to herbs, spices and fresh produce; electron spin resonance of radicals trapped in bone, shells and crystalline sugars; and gas chromatography–mass spectrometry of 2-alkylcyclobutanones, compounds formed only when fat is irradiated. Consumer acceptance varies by country and has limited irradiation's use, although it is widely applied to spices, herbs and phytosanitary treatment of traded fruit.
Ultraviolet radiation is divided into UV-A (315 to 400 nm), UV-B (280 to 315 nm) and UV-C (200 to 280 nm). UV-C is germicidal: DNA absorbs strongly around 260 nm, and absorbed UV forms pyrimidine (mainly thymine) dimers that block replication and transcription. Low-pressure mercury lamps emitting mostly at 253.7 nm are the traditional source; amalgam lamps, medium-pressure lamps and UV-C light-emitting diodes are also used. The UV dose (fluence) is the product of irradiance (mW/cm²) and exposure time (s), expressed in mJ/cm²; for example, an irradiance of 2.0 mW/cm² for 15 s gives 30 mJ/cm². Microorganisms differ in sensitivity, with spores, moulds and some viruses needing higher doses than vegetative bacteria, and repair mechanisms (photoreactivation in light) can reverse some damage.
UV is used extensively to disinfect water, air and surfaces in food plants — conveyor belts, packaging materials, cutting equipment — and for surface treatment of fruits, eggs and cheese. Its great limitation is penetration: UV is absorbed strongly by most food components, so it treats only surfaces and thin layers, and shadows, dirt and surface roughness protect organisms.
In liquids such as juices, cider and milk, UV absorbance can be very high — UV penetrates only a fraction of a millimetre in many juices — so treatment requires thin films and turbulent mixing to bring all of the liquid close to the lamps. Reactor designs include thin-film reactors, annular reactors with lamps in quartz sleeves, coiled-tube reactors generating Dean vortices, and Taylor–Couette reactors with a rotating inner cylinder. The delivered dose depends on the liquid's absorbance, turbidity and flow pattern, so validation uses biodosimetry with test organisms. In the United States, UV treatment of juice is a permitted process and must achieve the 5-log reduction required by juice HACCP rules; UV-treated apple cider is a commercial example. UV exposure can also enrich foods with vitamin D: UV-B treatment converts ergosterol in mushrooms to vitamin D2, and UV-treated mushrooms and baker's yeast have been approved as novel foods in some jurisdictions.
Pulsed light (PL) uses very short, intense flashes — typically lasting from microseconds to a millisecond — of broad-spectrum light from xenon flash lamps, covering ultraviolet, visible and near-infrared wavelengths (roughly 200 to 1,100 nm), with high peak power. Its antimicrobial effect comes mainly from the UV component, possibly enhanced by brief local heating at the surface and by photophysical damage from the high peak power. A few flashes can achieve several log reductions on smooth surfaces in a fraction of a second, making PL attractive for in-line decontamination of packaging materials (caps, lids, preforms), conveyor surfaces and smooth food surfaces. Like UV, pulsed light treats only surfaces it reaches and is less effective on rough, porous or shadowed surfaces and in opaque liquids; repeated or intense flashes can heat and discolour sensitive foods. In the United States, pulsed light is a permitted treatment of food with a limit on the cumulative fluence, and other jurisdictions assess it case by case.
Plasma is an ionised gas containing electrons, ions, free radicals, excited atoms and molecules, and ultraviolet photons; it is sometimes called the fourth state of matter. In thermal plasmas, as in arcs, all species are hot; in non-thermal or cold plasmas, the electrons are highly energetic but the heavy particles — ions and neutral gas — remain near room temperature, so the plasma can treat heat-sensitive materials. Cold plasma for food is generated at atmospheric pressure (or under low pressure) by applying high voltage to a gas such as air, nitrogen, oxygen, argon or helium. Common devices include dielectric barrier discharges (DBD), in which an insulating layer between electrodes prevents arcing and the discharge forms in the gap, and plasma jets, which blow the activated gas onto a target. In-package plasma generates the discharge inside a sealed package placed between electrodes, so the reactive species act on the food and then decay back to stable gases within the closed pack, avoiding recontamination.
The antimicrobial action of cold plasma comes from reactive oxygen and nitrogen species — ozone, atomic oxygen, hydroxyl radicals, singlet oxygen, nitric oxide and others — together with UV photons and charged particles, which oxidise and etch cell membranes, damage proteins and DNA and inactivate microorganisms on surfaces. Vegetative bacteria, yeasts, moulds and some spores, as well as viruses and biofilms, can be reduced, depending on the gas composition, power, treatment time, distance and humidity. Plasma also degrades pesticide residues and mycotoxins on surfaces, modifies the surface properties of packaging and seeds (improving wettability and germination), and can modify proteins and starches. Plasma-activated water — water exposed to plasma, which becomes acidic and rich in reactive species — is studied as a washing and disinfecting agent for produce. The relative importance of each species depends on the gas used: air plasmas produce many reactive nitrogen species, oxygen-containing plasmas more ozone and atomic oxygen, and noble-gas plasmas more UV.
Cold plasma is being developed for decontamination of fresh produce, sprouted seeds, nuts, spices, dried foods, eggshells and meat surfaces, for sterilising packaging materials and for treating food-contact surfaces. Its advantages are low temperature, short treatment times, no residues once reactive species decay, and the possibility of in-package treatment. Its limitations are that it acts mainly on surfaces, with effectiveness reduced on rough, porous or shadowed surfaces and in thick or opaque products; it can cause lipid oxidation, colour changes and loss of some nutrients through the same reactive species that kill microorganisms; scale-up to industrial throughput is still developing; and its regulatory status is not settled in many jurisdictions, so applications must be assessed and validated case by case. Worker safety measures are needed for ozone and other gases produced by open systems. As with other surface treatments, combining plasma with other hurdles such as refrigeration and modified-atmosphere packaging is the most realistic route to commercial benefit.
Ultrasound is sound above the range of human hearing, above about 20 kHz. Two ranges serve very different purposes. High-frequency, low-intensity ultrasound (usually in the megahertz range, below about 1 W/cm²) does not change the material and is used for measurement: ultrasonic velocity and attenuation reveal composition (fat and solids content), texture, droplet size and the progress of processes such as crystallisation and gelation, and ultrasound is used for non-destructive inspection and level and flow measurement. Low-frequency, high-intensity (power) ultrasound, typically 20 to 100 kHz at intensities of roughly 10 to 1,000 W/cm², changes the material. Its main mechanism in liquids is acoustic cavitation: the pressure waves create microbubbles that grow and collapse violently, producing local hot spots, intense shear, microjets and shock waves, and free radicals from water sonolysis. The intensity delivered depends strongly on the equipment and declines rapidly with distance from the transducer, so reported effects must be linked to measured acoustic power or energy density.
Ultrasound alone inactivates microorganisms only slowly at practical energy inputs, because cavitation is weakened by viscosity and solids and many organisms are resistant, but it acts synergistically with heat and pressure. Thermosonication combines ultrasound with mild heat; manosonication combines it with moderate pressure, which intensifies cavitation; manothermosonication combines all three. These combinations can achieve useful log reductions of vegetative cells, and some inactivation of spores and heat-resistant enzymes, at lower temperatures than heat alone. Ultrasound also cleans surfaces and helps remove biofilms in washing systems. Energy input, temperature rise and product damage — such as off-flavours from radicals in fatty foods — must be controlled. In practice, ultrasound for preservation is mostly studied for liquids such as juices and milk, where it can be applied in flow cells, rather than for solid foods.
Power ultrasound is used more widely to assist processes than to preserve foods. It enhances extraction of compounds such as polyphenols, oils, pigments and proteins by disrupting cell walls and improving mass transfer; it forms fine, stable emulsions; it promotes nucleation and controls crystal size in crystallisation of fats, sugars and ice (ultrasound-assisted freezing, chilling-freezing-frozen-foods); it degasses liquids before bottling and defoams; it speeds drying, brining, marinating and filtration by improving mass transfer; it tenderises meat; and ultrasonic knives, vibrating blades, cut sticky and fragile products such as cakes, cheese and frozen desserts cleanly with less waste. Equipment includes ultrasonic baths (low intensity, uneven), probe (horn) systems (high intensity near the tip, for small volumes and laboratory work) and continuous flow cells and plate transducers for industrial scale. Scale-up is limited by the decay of intensity with distance from the transducer and by energy cost. In many of these uses, the benefit is a shorter process time or lower temperature in an existing operation, which can improve quality and reduce energy use without changing the product category.
Ozone (O3) is a powerful oxidising agent, more reactive than chlorine, that inactivates bacteria, viruses, fungi and protozoa by oxidising cell membranes, enzymes and nucleic acids. It is unstable and decomposes to oxygen, leaving no residue, so it must be generated on site, usually by corona discharge through dry air or oxygen or by UV lamps. In water its half-life is short (minutes to tens of minutes, depending on temperature, pH and organic load), and it reacts rapidly with organic matter, which lowers its effectiveness in dirty water. In the United States, ozone was approved in 2001 as an antimicrobial agent for treating, storing and processing foods in gas and aqueous phases, and it is widely used to wash fruits and vegetables, treat process water, sanitise equipment, control moulds and ethylene in cold stores and fumigate stored grains. Ozone is toxic to humans; occupational exposure limits are low (of the order of 0.1 ppm over a working day), so enclosed systems, monitoring and destruction of off-gas are required. High concentrations can damage the surface of produce and oxidise lipids and pigments.
Chlorine compounds — sodium hypochlorite and chlorine gas — remain the most common sanitisers for wash water and surfaces, but their effectiveness falls rapidly with organic load and rising pH, and they can form by-products such as trihalomethanes. Chlorine dioxide, a gas or solution generated on site, is more effective over a wider pH range and less reactive with organic matter. Electrolysed water, produced by electrolysis of dilute salt solution, gives an acidic anode product rich in hypochlorous acid (a strong sanitiser) and an alkaline cathode product used for cleaning. Peracetic acid, a mixture of acetic acid and hydrogen peroxide, is widely used for washing produce and poultry carcasses and for sanitising equipment and aseptic packaging; it breaks down to harmless products. Organic acids (lactic, acetic, citric) are sprayed on carcasses and used as dips. Acidified sodium chlorite and other antimicrobials are permitted for specific uses in some countries. Use of these chemicals on food is regulated as food additives or processing aids, with permitted applications, concentrations and residue limits that differ between jurisdictions.
Biological methods use microorganisms or their products to inhibit spoilage and pathogenic organisms. Bacteriocins are antimicrobial peptides produced by bacteria; nisin, produced by Lactococcus lactis, is permitted as a preservative in many countries for processed cheese, dairy desserts and canned foods, where it inhibits spore-forming bacteria, and other bacteriocins are studied. Bacteriophages — viruses that infect specific bacteria — have been approved as processing aids in some countries to reduce Listeria, Salmonella or E. coli on foods and surfaces; their high specificity is both an advantage and a limitation. Protective cultures of lactic acid bacteria, added to meats, fish and dairy products, compete with pathogens and spoilage organisms and produce acids and bacteriocins. Antibacterial enzymes such as lysozyme (from egg white), used to prevent late blowing in some cheeses, and lactoperoxidase are further examples. These approaches fit the hurdle concept and consumer demand for 'natural' preservation, but they act on particular organisms and must be validated in each product.
A novel process used for food safety must be validated: shown, with scientific evidence, to deliver the required reduction of the target pathogen under the worst-case conditions of production. Validation begins by identifying the pertinent microorganism (the most resistant pathogen likely to be present and able to cause harm in the product), the performance criterion (for example a 5-log reduction for juices in US rules, or the reduction needed to meet a food safety objective), and the critical process parameters — pressure and hold time for HPP, field strength and specific energy for PEF, dose and dose uniformity for irradiation, UV dose and absorbance for UV, temperature and residence time for ohmic and microwave systems.
Inactivation by many non-thermal processes does not follow simple first-order (log-linear) kinetics: survivor curves often show shoulders and tails, especially for pressure, UV and PEF. Models such as the Weibull model, log(N/N0) = −(t/δ)^p, where δ is the time for the first decimal reduction and p a shape parameter (p < 1 for tailing, p > 1 for shoulders), describe such data better than a single D value. Because pathogens cannot be used in production plants, validation in plant conditions uses surrogate organisms — non-pathogenic organisms with equal or greater resistance to the process, demonstrated in laboratory comparisons. Examples include Listeria innocua for Listeria monocytogenes, non-pathogenic E. coli strains for E. coli O157:H7, Enterococcus faecium for Salmonella in low-moisture foods treated by heat or RF, and Clostridium sporogenes for C. botulinum. Chemical and physical indicators, such as dosimeters for irradiation and time–temperature integrators, support surrogate studies. Validation must cover the range of product compositions, package sizes and equipment conditions expected, and the process is then monitored continuously and verified periodically.
Regulators treat novel processes in different ways. Some processes are specifically regulated: irradiation is permitted only for listed foods, purposes and doses, with labelling (chapter 7); UV and pulsed light treatments are permitted under specific conditions in some countries; and chemical treatments such as ozone, chlorine dioxide, peracetic acid and bacteriophages are approved as food additives or processing aids. Other processes, such as HPP and PEF, are not generally subject to a separate approval in many countries but must meet the same safety requirements as any process, and if they are used for low-acid shelf-stable foods, the scheduled process must be filed and supported by a process authority, as for thermal processes (thermal-processing-canning); pressure-assisted and microwave-assisted thermal sterilisation processes have been accepted on this basis in the United States for specific products. In the European Union, a food produced by a process not used before 15 May 1997 that gives rise to significant changes in composition, nutritional value, metabolism or undesirable substances may be a novel food requiring authorisation under the Novel Food Regulation (EU) 2015/2283; UV-treated mushrooms, for example, were authorised as a novel food because of their increased vitamin D, whereas HPP-treated juices have generally not been considered novel. Codex provides general principles, and countries such as India regulate irradiation and permitted processing aids through their food regulators.
Labelling rules affect commercial success. Irradiated foods must be labelled as such; HPP products are often marketed with claims such as 'cold-pressed' or 'high-pressure processed', which must not mislead — for example, HPP juices are not 'unpasteurised' in a safety sense if they achieve a pasteurisation-equivalent reduction, and claims of 'raw' or 'fresh' are regulated in some jurisdictions. Shelf-life claims for novel-processed products must be supported by validation and storage studies (shelf-life-stability). Food business operators, inspectors and analysts need to know which processes require labelling, how to verify them (for example detection of irradiation), and what evidence supports the safety of products sold with extended shelf lives. Clear communication about the science of novel processes helps build consumer trust, which has historically been a greater barrier to adoption than the technology itself. Businesses adopting a novel process therefore plan labelling, marketing claims and regulatory checks alongside technical validation, before launch rather than after.
A student uses this subject to explain how each novel technology works and why most fail against spores, to compare HPP, PEF, ohmic, microwave, irradiation, UV, plasma and ultrasound on mechanism, equipment, uses and limits, and to calculate irradiation doses from D10 values, UV doses from irradiance and time, ohmic heating rates and adiabatic heating in HPP. In the laboratory, students may compare UV inactivation in clear and turbid liquids or measure microwave heating patterns with thermal imaging. They also learn to read a process validation report.
A process or product development technologist five years in evaluates whether a novel process suits a new product: arranges trials with HPP toll processors, designs validation studies with surrogate organisms, sets critical limits for pressure, dose or field strength, plans packaging compatible with the process, checks labelling and claims for the target markets, and works with quality teams on monitoring and records. In regulatory roles, the same knowledge is used to check irradiation labelling and detection results and to assess shelf-life claims.
A technical director decides whether to invest in novel processing — buying an HPP line or using a toll processor, adopting PEF pre-treatment for fries, installing UV for water and surfaces, or using MATS or PATS for shelf-stable products — weighing capital and running costs, product value, market acceptance and regulatory status in each country. They lead regulatory submissions, coordinate process authorities and laboratories, and manage consumer communication. They also plan how a new technology fits with existing lines, staff skills and maintenance, and they review the long-term reliability record of suppliers' equipment before committing.
A research leader studies mechanisms of non-thermal inactivation, especially of spores and enzymes; develops combined processes and kinetic models for non-log-linear inactivation; designs equipment for scale-up of plasma, UV and ultrasound; advises regulators on validation standards, surrogates and novel food assessment; supervises doctoral research; and sets examination questions linking physics, microbiology and food quality. They also assess life-cycle energy and cost comparisons between novel and conventional processes, and help translate laboratory results into industrial guidance on equipment and process control.
This manual does not give product-specific process conditions, D10 values or permitted irradiation doses by food and country, because these vary and must be taken from validated studies and current law; questions supply the values they need. Equipment design details (vessel engineering, pulse generators, magnetron and applicator design) are described only in outline. The regulatory picture changes as new processes are approved, so references must be checked for current status. Microfiltration and bactofugation, named in the AICE syllabus with non-thermal methods, are covered in mechanical-unit-operations; heat and mass transfer in food-transport-phenomena; dielectric properties in food-physical-properties-rheology.
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. Items tagged to food safety officer, analyst, licensing or recruitment exams whose syllabi Laisha has not yet verified are applied questions written to the expected scope. Regulatory statements reflect the frameworks when written and must be checked for amendments.
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