Food Science & Technology · Foundations · also Introduction to Food Science and Technology, Fundamentals of Food Technology, Principles of Food Processing and Preservation, Food Preservation
The first map of the whole field: what food is, why it deteriorates, and the families of preservation (heat, cold, drying, chemicals, fermentation, irradiation, packaging and hurdles). Each family is studied in depth later; this subject teaches the logic that connects them.
In the exams: GATE XE-G and XL-U Section 3 (processing principles); ICAR AICE-JRF/SRF 35.a Unit 1 (introductory food technology)
Food science is the study of what food is made of, how it behaves, why it changes, and how it can be kept safe, nutritious and attractive from the field to the plate. Food technology applies that science to processing, preservation, packaging and distribution. This subject is the first map of the whole field. It introduces the food industry and its structure, the composition and quality attributes of foods, the causes of deterioration, and the families of preservation methods: heat, cold, removal of water, chemicals, sugar, salt and smoke, fermentation, irradiation and combinations of hurdles. It ends with fortification, which adds value rather than preventing loss, and with the quality of raw materials, without which no process can make good food.
Every later course in a food technology degree goes deeper into one of these families. Thermal processing works out the mathematics of sterilisation; refrigeration engineering designs freezers; drying technology models dryers; food microbiology explains the organisms each method controls. This subject teaches the logic that links them: every preservation method works by removing, killing, or slowing the agents of spoilage, and the best products combine several mild methods rather than relying on one severe one.
Examinations treat the subject as the foundation of processing principles. Postgraduate entrance examinations in food technology ask about water activity limits, thermal death time concepts, freezing, drying curves, preservatives and their effective pH, irradiation doses, and hurdle technology, with short calculations. Doctoral fellowship examinations ask the same at greater depth. Licensure examinations for food technologists and national food engineer qualifications test the same principles in applied form. The manual covers all of these, with examples from many food traditions and regions.
People have preserved food for as long as they have stored harvests. Sun-drying of fish, fruit and meat, salting, smoking, fermentation of milk, vegetables, cereals and fish, and storage in cool cellars or in snow were developed independently on every continent. These methods worked without any knowledge of microorganisms, by trial and error over generations.
Industrial food preservation began in the early nineteenth century. Nicolas Appert, a French confectioner, showed that food sealed in glass jars and heated in boiling water kept for long periods, and published his method in 1810; tin-plated iron cans followed within a few years in England. Louis Pasteur's work in the 1860s explained why: heat killed the microorganisms that caused spoilage, and sealing kept others out. Mechanical refrigeration spread in the late nineteenth century, quick freezing of packaged foods was commercialised in the 1920s, and spray drying, aseptic processing, freeze drying, controlled-atmosphere storage, extrusion and membrane processes followed in the twentieth century.
The scientific basis of thermal processing was laid in the 1920s, when the heat resistance of Clostridium botulinum spores was measured and methods were developed to calculate the lethality of a canning process. From that point food preservation became an engineering discipline as well as a craft. Today food science draws on chemistry, biochemistry, microbiology, physics, engineering, nutrition, sensory science and economics.
The food industry is the largest manufacturing sector by employment in many countries. It is usually described in stages. Primary processing turns harvested crops, animals and fish into commodities and ingredients: milling grain, crushing oilseeds, slaughtering and dressing animals, grading and packing fruit. Secondary processing turns these into foods: bread from flour, cheese from milk, jam from fruit. Tertiary processing produces ready-to-eat meals and convenience foods. Around these stages sit ingredient suppliers, packaging manufacturers, equipment makers, cold-chain operators, retailers and food service.
The industry ranges from very large multinational firms, which trade across continents and own many brands, to millions of micro and small enterprises: village mills, bakeries, street-food vendors, small dairies and family fermenting businesses. In many low- and middle-income countries most processed food is still made by small and informal enterprises, and a large share of food is sold unprocessed in open markets.
Food loss and waste show why processing matters. FAO has estimated that around 14% of food produced is lost between harvest and retail, and the United Nations Environment Programme has estimated that a further share of around 17% is wasted at retail, food service and household level. Losses are highest for perishable foods such as fruits, vegetables, fish and milk, and are largest after harvest in regions where storage, cooling and processing are scarce. Preservation and processing reduce these losses, extend seasons, allow trade, add value to farm produce and make safe, varied diets available to cities.
All foods are mixtures of a small number of classes of constituents. Water is the largest component of most fresh foods: fruits and vegetables are commonly 75 to 95% water, meat and fish about 60 to 80%, milk about 87%, and cereal grains and pulses about 10 to 14% when dry. Carbohydrates include sugars, starch and dietary fibre and dominate cereals, roots, tubers and many fruits. Proteins are abundant in meat, fish, eggs, milk, pulses and oilseeds. Lipids are concentrated in oilseeds, nuts, animal fats and dairy fat. Minerals (the ash left after incineration) and vitamins are present in small amounts but are essential. Minor constituents such as pigments, flavour compounds, enzymes, organic acids and phenolic compounds decide colour, taste and stability.
Proximate analysis, the classic scheme for describing food composition, measures moisture, protein (as nitrogen multiplied by a conversion factor, commonly 6.25), fat, ash and crude fibre, with carbohydrate often calculated by difference. Food composition tables published by national bodies and by FAO's food composition network give typical values, but composition varies with variety, breed, season, maturity and processing.
Foods are commonly grouped for nutrition and processing purposes as cereals and cereal products; pulses, nuts and oilseeds; fruits and vegetables; roots and tubers; meat, poultry, fish and eggs; milk and milk products; fats and oils; sugars and confectionery; beverages; and spices and condiments. Each group has its own typical deterioration, its own preservation methods and its own industry.
Consumers judge food by attributes they can sense, and by attributes they trust but cannot sense. The sensory attributes are appearance (colour, size, shape, gloss, defects), texture (firmness, crispness, juiciness, viscosity, mouthfeel), and flavour (taste and aroma). The hidden attributes are nutritional value, safety, and authenticity.
Colour is often the first and strongest cue. It comes from natural pigments such as chlorophylls, carotenoids, anthocyanins, betalains and myoglobin, and from browning reactions during processing. Consumers read colour as a sign of ripeness, freshness or doneness, so colour change is one of the main criteria of shelf life. Texture reflects structure: the turgor of plant cells, the state of starch and proteins, the distribution of water and fat. Flavour is the combined perception of the five basic tastes (sweet, sour, salty, bitter and umami) on the tongue and of volatile aroma compounds in the nose; aroma usually carries most of a food's identity.
Nutritional quality depends on composition and on how processing affects it. Heat, light, oxygen, leaching and storage destroy some vitamins, especially vitamin C, thiamin and folate, while processing can also improve digestibility, inactivate anti-nutritional factors and make nutrients more available. Safety means freedom from harmful levels of pathogens, toxins, chemical contaminants and physical hazards. Good food technology protects all these attributes together; a process that makes food safe but inedible has failed.
Microorganisms are the most important cause of food spoilage and the only major cause of foodborne infection. Bacteria, yeasts and moulds grow on foods that give them water, nutrients, a suitable pH and a suitable temperature. Spoilage organisms produce off-odours, slime, gas, softening, acidity or visible growth; pathogens such as Salmonella, Listeria monocytogenes, pathogenic Escherichia coli, Campylobacter and Clostridium botulinum may cause disease without any visible spoilage. Which organisms dominate depends on the food: Gram-negative rods spoil chilled meat, fish and milk; lactic acid bacteria and yeasts spoil acidic and sugary foods; moulds spoil bread, cheese and dry foods.
Enzymes naturally present in foods continue to act after harvest or slaughter. Polyphenol oxidase browns cut fruits and vegetables; lipoxygenase produces off-flavours in frozen vegetables and legumes; lipases release free fatty acids that taste rancid or soapy; pectic enzymes soften fruits and cloud juices; proteases tenderise meat but can over-soften fish. Enzymes from microorganisms add to these. Because enzymes survive mild treatments and act even at low temperatures, many processes, such as blanching before freezing, are designed specifically to inactivate them.
Chemical deterioration proceeds without living cells. Lipid oxidation, a free-radical chain reaction of unsaturated fatty acids with oxygen, produces rancid flavours in oils, nuts, fried snacks, dried milk and fish; it is accelerated by light, heat, metal ions and low water activity near the dry end. Non-enzymatic browning, chiefly the Maillard reaction between reducing sugars and amino groups, darkens and changes the flavour of dried and concentrated foods during storage. Vitamins degrade; pigments fade; proteins cross-link.
Physical deterioration includes moisture gain by dry foods, which makes biscuits soft and powders cake; moisture loss by fresh foods, which wilts vegetables; freezer burn; staling of bread, caused largely by retrogradation of starch; separation of emulsions; and mechanical damage such as bruising, which also opens the way to microbes. Temperature abuse speeds almost all of these.
Pests, chiefly insects, mites and rodents, damage stored grains, pulses, dried fruits, spices and nuts. They eat food, contaminate it with excreta, hair and fragments, raise its moisture and temperature, and spread moulds and pathogens. Storage losses from pests can be large where storage is poor.
The rates of most deteriorative reactions rise with temperature. A useful rule is the temperature quotient Q10, the factor by which a rate increases for a 10 °C rise; for many food reactions it lies between about 2 and 3. If Q10 is 2, a product that keeps 6 months at 30 °C would keep roughly 12 months at 20 °C.
The water content of a food does not tell how available that water is to microorganisms and chemical reactions. Water bound to solutes, proteins and polysaccharides is less free to act. Water activity (aw) expresses this availability. It is defined as the ratio of the vapour pressure of water over the food to the vapour pressure of pure water at the same temperature: aw = p/p0. It ranges from 0 to 1. At equilibrium, water activity equals the equilibrium relative humidity (ERH) of the air around the food divided by 100; a food with aw 0.80 neither gains nor loses water in air at 80% relative humidity.
Water activity is measured with instruments that equilibrate a small sample in a sealed chamber and measure the humidity of the headspace, most commonly by chilled-mirror dew point or by capacitance sensors. For dilute ideal solutions, Raoult's law gives aw as the mole fraction of water: one mole of sucrose dissolved in 1 kg of water (55.5 mol) gives aw = 55.5/56.5 ≈ 0.982. Salts dissociate, so one mole of sodium chloride gives about twice the depression. Real concentrated solutions deviate from Raoult's law.
The relationship between water content and water activity at constant temperature is the moisture sorption isotherm. It is usually sigmoid for foods. The lower region corresponds to tightly bound monolayer water, which can be estimated by the Brunauer-Emmett-Teller (BET) or Guggenheim-Anderson-de Boer (GAB) models; the middle region to multilayer water; and the upper region to water in capillaries and solution. Adsorption and desorption isotherms differ, a phenomenon called hysteresis.
Microorganisms need available water, and each has a minimum aw for growth. Typical textbook minima are about 0.91 for most spoilage bacteria, 0.88 for most yeasts and 0.80 for most moulds. Some organisms tolerate much lower values: Staphylococcus aureus can grow down to about 0.86 aerobically, halophilic bacteria to about 0.75, xerophilic moulds to about 0.61 and osmophilic yeasts to about 0.60. Clostridium botulinum type A does not grow below about 0.94. No microorganism grows below about 0.6, which is why dried foods are microbiologically stable as long as they stay dry, although they can still carry surviving pathogens.
Chemical reactions also depend on aw. Lipid oxidation is fastest at very low aw, slowest near the monolayer value (about 0.2 to 0.3), and rises again at intermediate aw. Non-enzymatic browning peaks at intermediate aw, about 0.6 to 0.8, where reactants are concentrated but still mobile. Enzyme activity generally falls as aw falls. These patterns, often drawn as a 'food stability map', explain why each dried food has an optimum moisture: too dry and the fat oxidises, too moist and it browns, cakes or grows moulds.
Water activity is the basis of intermediate-moisture foods and of hurdle technology, and is used in food law: for example, many regulations treat foods with aw at or below 0.85 as not requiring refrigeration for control of pathogens, and use combinations of aw and pH to decide when a food needs temperature control.
Heat kills microorganisms by denaturing proteins, including enzymes, and damaging membranes and nucleic acids. At a constant lethal temperature, the number of survivors falls logarithmically with time. The decimal reduction time, D, is the time at a given temperature to reduce the population by 90%, that is by one log cycle. The z value is the temperature change needed to change D tenfold; for Clostridium botulinum spores it is taken as 10 °C. The lethality of a process is expressed as an F value, the equivalent time at a reference temperature; F0 refers to 121.1 °C with z = 10 °C.
Bacterial spores are far more heat resistant than vegetative cells, yeasts or moulds. The design target for low-acid canned foods is the 'botulinum cook' or 12D process, which would reduce C. botulinum spores by twelve log cycles. With a D121 of about 0.21 minutes, 12D is about 2.5 minutes at 121.1 °C, and in practice processes are often designed for F0 of 3 minutes or more for safety, and higher for spoilage spores. Because a 10 °C fall in temperature multiplies D by ten, the same 12D process at 111.1 °C would take about 25 minutes.
pH divides foods into groups. C. botulinum does not grow or form toxin below pH 4.6, so acid and acidified foods (pH 4.6 or below) need only a process that destroys vegetative cells, yeasts, moulds and acid-tolerant spoilage spores, usually at or below 100 °C, while low-acid foods (pH above 4.6 and aw above 0.85) need a full sterilisation process under pressure.
Blanching is a short heat treatment of vegetables and some fruits, in hot water or steam, mainly before freezing, drying or canning. Its purposes are to inactivate enzymes that would cause off-flavours, browning and vitamin loss during storage, to expel air from tissues, to wilt leafy material for packing, and to reduce surface microorganisms. Peroxidase and catalase are commonly used as indicator enzymes because they are relatively heat resistant: if peroxidase is inactivated, most other enzymes will be. Over-blanching softens texture and leaches water-soluble vitamins and minerals.
Pasteurisation is a mild heat treatment that destroys vegetative pathogens and reduces spoilage organisms, giving a product that is safe but not sterile and usually needs refrigeration. Milk pasteurisation standards include 63 °C for 30 minutes (batch) and 72 °C for 15 seconds (high temperature short time); fruit juices, liquid eggs, beer and acidic foods are also pasteurised under conditions set for their key organisms or enzymes. Alkaline phosphatase, an enzyme slightly more heat resistant than the target pathogens in milk, is used to verify milk pasteurisation.
Sterilisation, often called commercial sterility, renders a food free of microorganisms capable of growing under normal storage conditions. It is achieved by heating sealed containers in retorts (canning) or by heating product continuously at ultra-high temperature and filling it aseptically into sterile packages. High-temperature short-time processes favour quality because microbial destruction has a smaller z value than most quality losses, so raising temperature kills microbes faster than it degrades colour, flavour and nutrients.
Chilling lowers food temperature to just above freezing, typically between about 0 and 5 °C, to slow microbial growth, enzymatic activity and chemical reactions. It does not kill microorganisms, and psychrotrophic organisms such as Pseudomonas, Listeria monocytogenes and some Clostridium and Bacillus species grow slowly at chill temperatures. Chilled foods therefore have limited shelf lives that depend on initial load, temperature control and packaging.
Fresh produce is alive and respires after harvest, consuming sugars and producing heat, carbon dioxide and water. Cooling soon after harvest, called precooling, by forced air, hydrocooling, vacuum cooling or icing, removes field heat and slows respiration. Not all produce tolerates low temperatures: many tropical and subtropical fruits and vegetables, such as bananas, mangoes, cucumbers and some tomatoes, suffer chilling injury below about 10 to 13 °C, showing pitting, discoloration and failure to ripen. Controlled and modified atmospheres, with low oxygen and raised carbon dioxide, extend the storage of apples and other produce further.
Meat, fish and dairy products are chilled immediately after slaughter, catch or milking. Fish is often stored in ice, which holds it near 0 °C. For many perishable foods each degree matters: a product stored at 8 °C instead of 2 °C may lose half or more of its shelf life. The cold chain, from producer to consumer, is only as strong as its weakest link.
Freezing preserves food by lowering temperature and by turning water into ice, which reduces the water available to microbes and reactions. Most foods begin to freeze a little below 0 °C because of dissolved solutes, and much of the water freezes between about −1 and −5 °C, the zone of maximum ice crystal formation. Fast passage through this zone gives many small ice crystals; slow freezing gives fewer, larger crystals, often outside the cells, which draw water out of cells and damage structure, causing drip loss and soft texture on thawing.
Freezing methods include air-blast freezers, plate (contact) freezers, fluidised-bed freezers for individually quick frozen small items, and cryogenic freezing with liquid nitrogen or carbon dioxide, which is very fast. Freezing time can be estimated with Plank's equation, which shows that it rises with product thickness, with latent heat and with lower heat-transfer coefficients.
Frozen foods are commonly stored at −18 °C or below, the temperature widely used in standards and codes of practice for quick frozen foods. Even at this temperature, slow changes continue: lipid oxidation, protein denaturation in fish, recrystallisation of ice (made worse by temperature fluctuation), and freezer burn, the dehydration of surface tissue when ice sublimes from poorly packed food. Vegetables are blanched before freezing to stop enzyme activity. Freezing does not reliably kill bacteria, so thawed food should be handled like fresh food, although freezing does kill some parasites, which is why some regulations require freezing of fish to be eaten raw.
Drying, or dehydration, removes water to lower water activity below the levels that support microbial growth, and reduces weight and volume for transport. Methods range from sun and solar drying of grains, fruits, fish and spices to hot-air cabinet, tunnel and belt dryers, drum dryers for pastes, spray dryers for liquids such as milk and coffee extract, fluidised-bed dryers for particles, and freeze drying for high-value products.
In hot-air drying of a wet solid, a constant-rate period usually comes first: the surface stays wet, evaporation proceeds as from free water, and the product surface sits near the wet-bulb temperature of the air. When the surface can no longer be kept wet by internal movement of water, the falling-rate period begins; drying then depends on diffusion inside the product, and the product temperature rises towards the air temperature. The moisture content at the change is the critical moisture content. Drying too fast at high temperature can form a hard, dry surface layer that slows further drying, called case hardening.
Moisture is expressed on a wet basis (kg water per kg of total product) or a dry basis (kg water per kg of dry solids). A product with 80% moisture wet basis contains 4 kg of water per kg of solids. In mass balances the dry solids are conserved: 100 kg of a vegetable at 80% moisture holds 20 kg of solids, so drying it to 10% moisture leaves 20/0.9 = 22.2 kg of product, and 77.8 kg of water is removed.
Concentration removes part of the water from liquid foods to make products such as tomato paste, fruit juice concentrates, evaporated and sweetened condensed milk, and sugar syrups. It reduces storage and transport costs and lowers aw, but rarely enough to preserve the product alone; concentrates are usually also heated, acidified, sweetened, chilled or frozen. Methods include evaporation, often under vacuum in multiple-effect or mechanical-vapour-recompression evaporators to lower boiling temperature and save energy; freeze concentration, which removes water as ice and preserves delicate aromas; and membrane processes such as reverse osmosis.
Intermediate-moisture foods have aw roughly between 0.6 and 0.85 or 0.9, enough water to be soft and ready to eat but too little for most bacteria. Examples include jams and fruit pastes, dried fruits, some cured and dried meats, honey, soft confectionery, certain semi-moist pet foods and traditional products such as dried sweet milk confections. They are made by partial drying, by adding humectants such as sugars, salt, glycerol or sorbitol, or both. Because moulds and yeasts can still grow, intermediate-moisture foods usually also rely on low pH, preservatives such as sorbate, and packaging. They are a clear example of combining hurdles.
Chemical preservatives are substances added to inhibit microorganisms or chemical deterioration. Their use is controlled by law: each must be approved for specific foods at maximum levels, and Codex sets these in its General Standard for Food Additives, with acceptable daily intakes evaluated by the Joint FAO/WHO Expert Committee on Food Additives.
Weak organic acid preservatives such as benzoic acid, sorbic acid and propionic acid act mainly in their undissociated form, which crosses the microbial cell membrane and then releases protons and anions inside the cell. The fraction undissociated depends on pH and on the acid's pKa: by the Henderson-Hasselbalch relation it is 1/(1 + 10^(pH − pKa)). Benzoic acid (pKa about 4.2) is therefore effective in acidic foods such as soft drinks, pickles and fruit products; at pH 4.2 half of it is undissociated, but at pH 6.2 only about 1%. Sorbic acid (pKa about 4.8) works at somewhat higher pH and is widely used against moulds and yeasts in cheese, bakery and intermediate-moisture foods. Propionates inhibit moulds and rope-forming Bacillus in bread.
Sulphur dioxide and sulphites inhibit microbes, enzymatic browning and non-enzymatic browning in dried fruits, wines and juices, but destroy thiamin and can provoke reactions in sensitive asthmatics, so they must be labelled. Nitrite in cured meats inhibits Clostridium botulinum and forms the pink nitrosomyoglobin colour; its use is limited because nitrite can form nitrosamines. Antioxidants such as tocopherols, ascorbic acid and approved synthetic phenolic antioxidants slow lipid oxidation.
Sugar and salt preserve mainly by lowering water activity. High-sugar products such as jams, jellies, candied fruits and sweetened condensed milk keep because their soluble solids are high: jams are typically made to about 65% or more total soluble solids, which with the natural acidity and the heat of boiling makes them stable once sealed. Pectin forms a gel only within a range of sugar concentration and pH, typically pH about 3.0 to 3.5 for high-methoxyl pectin.
Salt is used dry or as brine for fish, meat, vegetables, cheese and butter. Besides lowering aw, it draws water from tissues, makes the environment selective for salt-tolerant lactic acid bacteria in fermented vegetables, and contributes flavour. Heavily salted fish can still spoil through halophilic bacteria that cause reddening. Concentration of brine is expressed as percentage salt by mass of the solution: to make a 3% brine from 50 kg of water needs 50 × 0.03/0.97 ≈ 1.55 kg of salt.
Smoking combines several effects: drying of the surface, heat, and deposition of smoke components. Wood smoke contains phenols, which are antioxidant and antimicrobial, organic acids, carbonyls that colour the surface, and formaldehyde. It also contains polycyclic aromatic hydrocarbons such as benzo[a]pyrene, some of which are carcinogenic, so regulations limit them and the industry uses controlled smoke generation, filtered smoke and liquid smoke flavourings. Traditional smoked fish and meat in many regions owe their keeping quality more to drying and salting than to smoke itself.
Fermentation, in the food sense, is the controlled growth of selected microorganisms or the action of their enzymes to transform a food, usually making it safer, longer-keeping and more flavourful. It is one of the oldest and most widespread preservation methods: fermented milks, cheeses, breads, beers and wines, soy sauce and miso, fermented fish sauces, sauerkraut and kimchi, fermented cassava, sourdoughs, injera, idli and dosa batters, fermented sausages and cocoa and coffee processing all depend on it.
Fermentation preserves through the products of microbial metabolism. Lactic acid bacteria convert sugars to lactic acid, lowering pH to about 4 or below; homofermentative species produce mainly lactic acid, while heterofermentative species also produce carbon dioxide, ethanol or acetic acid. Yeasts produce ethanol and carbon dioxide. Acetic acid bacteria oxidise ethanol to acetic acid in vinegar. Propionibacteria produce propionic acid and the eyes of Swiss-type cheese. Besides acid and alcohol, fermenting organisms produce hydrogen peroxide, diacetyl and bacteriocins, and they compete with spoilage and pathogenic organisms for nutrients. The combined effect is a set of hurdles.
Spontaneous fermentations rely on organisms naturally present on raw materials, selected by conditions such as salt concentration, temperature and absence of oxygen. In sauerkraut, shredded cabbage with about 2 to 3% salt ferments through a succession: heterofermentative Leuconostoc mesenteroides starts the fermentation, producing acid and carbon dioxide that exclude air, and is followed by more acid-tolerant lactobacilli such as Lactiplantibacillus plantarum, which bring the pH lower. Too little salt allows softening and spoilage; too much slows the desired bacteria.
Industrial fermentations increasingly use defined starter cultures, which give faster, more predictable acidification, consistent flavour and protection against failure. Back-slopping, adding part of a previous batch to the new one, is a traditional intermediate that maintains a house culture. Cleanliness, temperature control and correct raw materials remain essential.
Fermentation also improves foods nutritionally: it can break down anti-nutritional factors such as phytates and some toxic compounds (as in the fermentation and processing of bitter cassava, which helps remove cyanogenic glycosides), increase some B vitamins, and pre-digest proteins and carbohydrates. Risks exist too: some fermentations can produce biogenic amines, ethyl carbamate or mycotoxins if poorly controlled, and acid fermentations that fail to reach a safe pH can allow pathogens to survive. Nisin, a bacteriocin produced by Lactococcus lactis, is approved as a preservative in many countries, for example in processed cheese and canned foods.
Food irradiation exposes food to ionising radiation to kill insects, parasites and microorganisms, inhibit sprouting or delay ripening. Three types of radiation are permitted under the Codex General Standard for Irradiated Foods: gamma rays from cobalt-60 or caesium-137, X-rays from machine sources up to a set energy limit, and electrons from machine sources up to 10 MeV. These energies are too low to induce radioactivity in food, so irradiated food is not radioactive.
The absorbed dose is measured in grays (Gy); one gray is one joule absorbed per kilogram. Low doses up to about 1 kGy inhibit sprouting of potatoes and onions (around 0.05 to 0.15 kGy), disinfest grains, fruits and spices of insects, and delay ripening. Medium doses of about 1 to 10 kGy reduce spoilage organisms and pathogens such as Salmonella and Escherichia coli O157:H7 in meat, poultry and seafood (radurisation and radicidation) and decontaminate spices. High doses above 10 kGy can sterilise food (radappertisation), used for some special diets and space foods. Even 10 kGy deposits only 10 kJ/kg, enough to raise the temperature of water by only about 2.4 °C, which is why irradiation is called a cold process.
A joint FAO/IAEA/WHO expert committee concluded in 1980 that irradiation of any food up to an overall average dose of 10 kGy presents no toxicological hazard. Codex states that the maximum absorbed dose should not exceed 10 kGy except where necessary for a legitimate technological purpose. Irradiated foods must be labelled, often with the international Radura symbol. Acceptance by consumers and permitted uses vary widely between countries.
Hurdle technology, developed by Lothar Leistner and colleagues, preserves food by combining several moderate preservative factors, or hurdles, instead of one severe treatment. The main hurdles are high temperature (F), low temperature (t), reduced water activity (aw), acidity (pH), low redox potential (Eh), preservatives, competitive microflora and packaging. Each hurdle alone would be insufficient, but microorganisms cannot overcome them together. The result can be a food that is safe and stable with milder heat, less salt or sugar, and better quality.
The approach works because microorganisms must spend energy to maintain their internal environment, their homeostasis, against each stress. When several stresses act at once, the organisms exhaust their energy and die or cannot multiply, a principle called metabolic exhaustion; and hurdles that hit different targets in the cell (membrane, enzymes, pH balance, DNA) act synergistically, the multitarget principle.
Many traditional foods are hurdle foods without the name: fermented sausages combine nitrite, salt, low aw, low pH, competitive lactic acid bacteria and smoke; cheeses combine low pH, salt, low aw and competitive flora; dried, salted and smoked fish combine low aw with phenols. Modern applications include shelf-stable bakery products and minimally processed fruit kept with mild heat, low pH, humectants and sorbate. The approach needs care: hurdles must be measured and controlled, and a product designed this way can fail if one hurdle slips, for example if pH drifts upward.
Adding nutrients to food is used to prevent or correct deficiencies, to restore what processing removes, and to make a substitute food nutritionally equivalent to the food it replaces. Terms are used as follows in Codex and the FAO/WHO guidelines on food fortification. Fortification, or enrichment in some usage, is the addition of one or more essential nutrients to a food, whether or not normally contained in it, to prevent or correct a demonstrated deficiency in the population or specific groups. Restoration is the addition of nutrients to replace those lost during processing, storage or handling. Nutritional equivalence means adding nutrients to a substitute food, such as a plant-based beverage, so it matches the food it replaces. Enrichment is often used, especially for cereal products, to describe adding back B vitamins and iron lost in milling.
FAO and WHO distinguish mass fortification of foods eaten widely by the whole population, such as salt, flour, oil, sugar or rice; targeted fortification of foods for specific groups, such as complementary foods for young children; and market-driven fortification decided by manufacturers within regulatory limits. Biofortification, raising nutrient content by plant breeding or agronomy, is a related approach, for example vitamin A-rich orange-fleshed sweet potato and high-iron beans and millets.
A good fortification vehicle is eaten regularly in fairly constant amounts by the target population, is centrally processed so the nutrient can be added under control, does not change in taste, colour or shelf life when fortified, and allows monitoring.
Several fortification programmes have had large public-health effects. Universal salt iodisation, adding potassium iodate or potassium iodide to salt, has sharply reduced iodine deficiency disorders such as goitre and cretinism worldwide. Folic acid fortification of wheat flour has lowered the incidence of neural tube defects where it is mandatory. Vitamin A is added to edible oils, sugar and margarine in parts of Africa, Asia and Latin America; vitamin D to milk and margarine in many temperate countries; and iron, zinc and B vitamins to wheat and maize flour and to rice.
The technical choices matter. The form of a nutrient must be stable and bioavailable without harming the food: potassium iodate is preferred to iodide in humid tropical climates because it is more stable. Iron is the hardest nutrient: soluble forms such as ferrous sulphate are well absorbed but can cause rancidity and colour changes, while less reactive forms such as elemental iron powders are poorly absorbed; sodium iron EDTA is used in high-phytate flours. Double fortified salt carries both iodine and iron. Rice can be fortified by coating kernels or by extruding fortified kernels mixed with ordinary rice.
Calculations follow from composition. Potassium iodate is about 59.3% iodine by mass, so to supply 30 mg of iodine per kg of salt about 30/0.593 ≈ 50.6 mg of potassium iodate is needed. Fortification levels are set from dietary intake surveys, with overage added to cover losses during storage, and are monitored at the factory, in markets and in households.
No process can make a better product than its raw material allows. Processing can preserve quality, but rarely improve it; defects, contamination and unsuitable composition pass through to the finished food or are removed only at a cost in yield. Raw-material quality for processing is judged by suitability for a particular process, not only by appearance for the fresh market.
The main factors are variety or breed, maturity, composition, size and shape, freedom from defects and disease, microbial load, chemical residues and contaminants, and the time and conditions between harvest and processing. Processing varieties are often bred for different traits from fresh-market varieties: tomatoes for paste need high soluble solids, firm flesh and concentrated ripening for mechanical harvest; potatoes for chips and crisps need high dry matter (high specific gravity) and low reducing sugars, because reducing sugars cause dark colour and acrylamide formation on frying; wheat for bread needs strong gluten-forming proteins, while soft wheat suits biscuits and cakes.
Maturity is judged by indices suited to each crop: soluble solids, acidity and their ratio for fruits; firmness by penetrometer; tenderness of peas and sweet corn by tenderometer or similar instruments, because their sugars turn rapidly to starch; colour; starch-iodine staining for apples; and days from flowering. Climacteric fruits such as bananas, mangoes, tomatoes and apples can ripen after harvest, while non-climacteric fruits such as citrus, grapes and pineapples do not improve after picking, so they must be harvested ripe.
Raw materials deteriorate from the moment of harvest, slaughter or catch. Respiration, enzymatic changes, water loss and microbial growth continue, and mechanical damage accelerates all of them. Sweet corn and peas can lose a large fraction of their sugar within a day at warm temperatures; fish quality declines quickly unless iced; milk must be cooled within hours. Processors therefore schedule harvest to match plant capacity, cool produce quickly, minimise holding time and handle gently.
Storage conditions can change suitability. Potatoes stored at low temperature accumulate reducing sugars through cold-induced sweetening, which darkens fried products and increases acrylamide; processing potatoes are therefore stored at moderate temperatures with sprout inhibition, and may be reconditioned at warmer temperatures before use.
At intake the processor inspects, samples and grades raw material against a specification: size and shape grading by screens, rollers or optical sorters; colour and defect sorting; tests for soluble solids, moisture, oil, protein or fat; microbial and residue tests; and checks for foreign matter. Payment is often linked to quality, for example by soluble solids for tomatoes and sugar content for sugar beet and cane. Contracts with growers may specify variety, agronomic practice, pesticide use and harvest date. Good agricultural practice and supplier approval form the first prerequisite programmes of a food safety management system.
A student uses this subject to build a mental map before the specialised courses. When a later course introduces thermal process calculation, freezing time or sorption isotherms, the student already knows why they matter and which spoilage agent each controls. In examinations the subject gives reliable marks on water activity limits, D and z values, preservative pH, irradiation doses and hurdles. In laboratory practicals the student learns to measure moisture and water activity, to check blanching with the peroxidase test and to calculate brines and syrups.
Five years into a career, a production or quality technologist uses these principles to solve daily problems: a jam batch that grows mould because soluble solids were low, frozen vegetables with hay-like off-flavours because blanching was too short, crisps that fry dark because potatoes were stored too cold, a sauce whose pH has crept above the level the preservative needs. They set specifications for raw materials, design shelf-life studies with Q10 estimates, and explain to managers why a cheaper process step would compromise safety.
A senior technical manager with twenty-five years' experience uses the subject to make strategic choices: which preservation route to adopt for a new product line, whether to invest in a freezer, a dryer or an aseptic line, how to reformulate products to cut salt and sugar without losing stability, and how to answer regulators and customers on irradiation, preservatives and fortification. They lead hazard analysis teams, judge supplier programmes and plan fortification projects with public-health partners. When a product recall or an outbreak occurs, they use the same principles to find which hurdle failed and to decide what must change in process, specification and training so that it does not happen again.
A senior researcher uses these principles as the framework for new work: modelling microbial inactivation and growth across combined hurdles, mapping reaction rates against water activity and glass transition, designing milder processes that keep nutrients and flavour, and evaluating new technologies against established ones. They advise standard-setting bodies on preservation, irradiation and fortification, design fortification trials and teach the next generation to reason from mechanism rather than recipe. They also review the evidence behind accepted limits, such as minimum water activity values or blanching indicators, and test whether those limits still hold for new formulations and new organisms.
This is an overview subject: it does not give full thermal process calculations (general and formula methods), freezing-time derivations, dryer design, packaging science or the microbiology of individual pathogens, which are covered in later subjects. Minimum water activity and pH values are representative textbook figures that differ with strain, solute and other conditions. Permitted preservatives, maximum levels, irradiation approvals and mandatory fortification levels differ by country and change over time; the manual describes the principles and the Codex framework and does not list national limits. Food loss and waste figures are the FAO and UNEP estimates current when the sources were written.
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.
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