Food Science & Technology · Commodity Processing Technologies · also Fruit and Vegetable Technology, Processing of Horticultural Produce, Fruit and Vegetable Preservation
Processed products from fruits and vegetables: juices and concentrates, jams, jellies and marmalades, pickles, sauces and ketchup, dried and frozen products, canned products and minimally processed produce, with pectin chemistry and by-product recovery. Packhouse and storage engineering are in Agricultural Engineering's post-harvest-horticultural-crops.
In the exams: GATE XE-G Section 3 (fruits and vegetables processing; pectin from fruit wastes); ICAR AIEEA-PG Code 18 Unit III; ICAR AICE 35.a Unit 2
Fruit and vegetable processing turns a perishable, seasonal harvest into products that keep, travel and add value: juices and concentrates, squashes and ready-to-serve drinks, jams, jellies and marmalades, candied fruit, pickles and chutneys, tomato paste and ketchup, canned, dried and frozen produce, fresh-cut salads and potato chips. Fresh fruits and vegetables are mostly water, they keep respiring after harvest, and their own enzymes and the microorganisms on their surface start to spoil them within days. Every process in this subject answers the same question in a different way: which of those agents of spoilage is controlled here, by heat, by acid, by sugar, by salt, by drying, by cold or by packaging, and what does that control cost in colour, texture, flavour and nutrients?
The subject sits between food chemistry and food engineering. From chemistry it borrows pectin, the plant acids, pigments and enzymes; from engineering it borrows heat transfer, evaporation, drying and freezing. This manual concentrates on the products and their formulation. The calculation of thermal process lethality is in thermal-processing-canning, the design of evaporators and driers is in evaporation-dehydration, freezing equipment is in chilling-freezing-frozen-foods, and packhouse handling, pre-cooling and controlled-atmosphere storage of fresh produce are in Agricultural Engineering's post-harvest-horticultural-crops. Those topics are referred to here, not repeated.
Fruit and vegetable processing is examined everywhere food technology is taught. In India, GATE XE-G and XL-U Section 3 name fruit and vegetable processing and pectin from fruit waste; ICAR AIEEA-PG Code 18 Unit III and ICAR AICE 35.a Unit 2 ask about juices, jams, jellies, pickles, tomato products and canning. Abroad, the same content appears in food science degree examinations and in professional practice in juice plants, canneries and fresh-cut packhouses. Product standards for these foods differ by country and change with revisions of the regulations; where this manual quotes a classic specification, it says so, and a practitioner must check the current national regulation before labelling a product.
Most fruits and vegetables are 75–95% water. The dry matter is mainly carbohydrate: sugars (glucose, fructose and sucrose in proportions that differ by crop), starch in unripe fruit, potatoes and pulses, and the cell-wall polysaccharides cellulose, hemicellulose and pectin, which together make up dietary fibre. Protein and fat are low in nearly all of them; avocado and olive are the familiar exceptions with much oil. The organic acids give fruit its sourness and set its pH. Citric acid dominates in citrus, berries and tomato, malic acid in apple, pear and stone fruit, and tartaric acid in grape and tamarind. Acidity is reported by titration as the predominant acid, so lime juice is reported as citric acid and grape juice as tartaric acid. Fruits are generally acid (pH below 4.6), while most vegetables are low-acid (pH above 4.6), a distinction that decides how a canned product must be heat-processed.
Vitamin C (ascorbic acid) is the most heat- and oxygen-sensitive nutrient in the group and is often used as an index of processing damage. Carotenoids (beta-carotene, lycopene in tomato and watermelon) are fat-soluble and fairly heat-stable but are oxidised by light and air. Minerals, especially potassium, are high. Phenolic compounds, including tannins, give astringency and are the substrate of enzymatic browning.
Four pigment groups matter. Chlorophyll in green vegetables loses its magnesium under heat in acid conditions and turns into olive-brown pheophytin; short blanching and quick cooling limit the change. Carotenoids give yellow, orange and red colour and survive most processing. Anthocyanins give the red, purple and blue of berries, grapes and red cabbage; they are water-soluble, red in acid and bluish or dull in neutral conditions, and they form dull complexes with tin and iron, which is why coloured fruits are packed in lacquered cans. Betalains give the red of beetroot.
The native enzymes are just as important. Polyphenol oxidase (PPO) oxidises phenolics to quinones that polymerise into brown pigments as soon as cut tissue meets air. Pectin methylesterase (PME) removes methyl groups from pectin and, in juice, causes cloud loss; polygalacturonase (PG) breaks the pectin chain and thins tomato products. Peroxidase and lipoxygenase cause off-flavours in frozen vegetables. Peroxidase is among the most heat-stable of these enzymes, so its inactivation is the usual test that a blanch was sufficient.
A processor buys raw material for the product it will become, not for how it looks on a market stall. Processing quality includes variety, maturity, size and shape uniformity, colour, firmness, total soluble solids (TSS, read on a refractometer as degrees Brix), titratable acidity and their ratio, and freedom from defects and residues. Juice varieties are chosen for yield and Brix; canning peaches for firm flesh that holds its shape; potatoes for chips for high dry matter and low reducing sugar. Maturity indices include TSS, the Brix/acid ratio, firmness by penetrometer, the starch–iodine test in apples, colour charts and days from flowering. Fruit picked too green gives sour, poorly coloured product; fruit picked too ripe gives soft, broken pieces and low pectin. Climacteric fruits (mango, banana, tomato, apple) can ripen after harvest; non-climacteric fruits (citrus, grape, pineapple) do not improve after picking, so they must be harvested at eating quality. Handling between field and factory is covered in post-harvest-horticultural-crops.
Raw material is weighed, sampled for quality and sorted before anything else. Washing removes soil, dust, insects, residues and much of the surface microbial load. It is done by soaking, by spray washing on roller conveyors, by flotation and flume washing (where heavy debris sinks and the produce floats), and by brushing. Chlorinated water or other approved sanitisers help limit cross-contamination in the wash water, but they reduce rather than eliminate surface microorganisms, and the wash water itself must be kept clean or it spreads contamination. Sorting removes rotten, damaged and off-grade units; grading separates by size, colour or density so that each later step works on a uniform lot. Optical sorters reject on colour and defects. Uniform size matters because peeling, blanching and heat processing are all timed for a piece size: a mixed lot is over-treated at the small end and under-treated at the large end. Raw material that waits at the factory gate loses quality fast, especially soft fruit in warm weather, so intake is scheduled to the line capacity and produce is held cool and shaded if it must wait. Records of variety, grower, harvest date and intake quality are kept for traceability, which is needed when a defect or residue is found later in the finished product.
Peeling methods are chosen by crop, scale and acceptable loss. Hand peeling is still used for mango and pineapple in small units. Abrasion peeling rubs the skin off on carborundum rollers and suits potatoes and carrots but loses more flesh. Lye peeling dips the produce in hot sodium hydroxide solution, which dissolves the pectin layer under the skin so the skin washes off; concentration, temperature and time are set for each crop, and the produce is rinsed and often dipped in a weak acid to neutralise residual alkali. It is used for peaches, tomatoes, citrus segments and some root crops. Steam peeling exposes the produce to high-pressure steam and then releases the pressure suddenly, so the heated layer under the skin flashes and lifts the skin; it is widely used for tomatoes and potatoes and avoids lye effluent. Flame peeling burns off the outer skin of onions and peppers. Enzymatic peeling with pectinases is used to free citrus segments.
Cutting into halves, slices, dices, strips or wedges follows peeling. Cut surfaces brown quickly, so cut fruit is often held in water, brine or a dilute acid or ascorbic acid solution until the next step. Pitting, coring and destemming machines are designed for particular fruits.
Blanching is a short heat treatment, in hot water or steam, given before freezing, drying or canning. Its main purpose is to inactivate enzymes that would otherwise cause off-flavour, discoloration and loss of vitamins during storage. It also expels air from the tissue, wilts leafy vegetables so they pack easily into containers, fixes the bright green colour of chlorophyll for a short time, and reduces the surface microbial load. Blanching is not a preservation step by itself.
Hot-water blanchers are simple but leach sugars, minerals and water-soluble vitamins and produce large volumes of effluent. Steam blanchers leach less but heat less uniformly when produce is piled deep. Blanch time depends on the size and shape of the piece and the target enzyme; peroxidase is commonly used as the indicator, and some processors use catalase or lipoxygenase. Under-blanching can be worse than none, because it disrupts the tissue and frees enzymes without destroying them; over-blanching softens texture, dulls colour and wastes nutrients. Blanched produce is cooled quickly in cold water or air to stop the cooking.
Juice is the liquid expressed from sound, mature fruit. The extraction method follows the structure of the fruit. Citrus juice is extracted with reamers or with in-line extractors that cut and squeeze each fruit while separating peel oil, so that bitter peel compounds and excess oil stay out of the juice. Apples, pears and grapes are crushed or milled to a mash and then pressed in rack-and-cloth, belt, pneumatic or screw presses. Treating the mash with pectinase enzymes before pressing breaks down pectin, lowers viscosity and raises yield. Soft pulpy fruits such as mango, guava, papaya and banana are not pressed but pulped: the fruit is crushed and passed through a pulper-finisher whose screens separate the pulp from seeds and fibre. Tomato juice is produced the same way. Juice yield is expressed as mass of juice per mass of fruit and differs widely between fruits and presses. Whatever the method, extraction is done quickly and with minimal air contact, because crushed tissue browns and loses vitamin C within minutes, and the extracted juice is screened or centrifuged to the pulp level the product needs.
Some juices are sold clear (apple, grape) and some cloudy (orange, pineapple, cloudy apple). For a clear juice the suspended pectin and particles must be removed. Depectinisation with pectinase enzymes lowers viscosity and lets haze particles settle; fining agents such as gelatin, bentonite and silica sol then flocculate the remaining colloids and polyphenols; and the juice is filtered or, increasingly, ultrafiltered through membranes, which can replace fining and filter aids. A clear juice that later forms a haze has usually suffered from residual pectin, starch or protein–polyphenol complexes.
For a cloudy juice the cloud must be kept. In orange juice the native pectin methylesterase de-esterifies pectin, which then binds calcium and settles out, so the juice separates into a clear serum and a sediment. Heat treatment strong enough to inactivate PME, more severe than that needed to kill vegetative microorganisms in acid juice, stabilises the cloud. Citrus juices can also turn bitter. Naringin causes bitterness in grapefruit, and limonin causes the delayed bitterness that develops in juice from navel oranges some hours after extraction; adsorbent resins or careful extraction reduce it.
Juice is concentrated to cut the cost of storage and transport and to make it more stable. Multiple-effect and thermally accelerated short-time evaporators remove water at reduced pressure; volatile aroma compounds are stripped early and recovered as an essence that is added back on reconstitution. Freeze concentration and membrane processes (reverse osmosis) give better aroma but reach lower final Brix or cost more. Orange juice concentrate is commonly concentrated to around 65 °Brix. Concentration is a mass balance on soluble solids: the mass of solids is unchanged, so the mass of concentrate equals the mass of juice multiplied by the ratio of feed Brix to product Brix. Equipment design is in evaporation-dehydration.
Juices are preserved by pasteurisation (acid juices need only a mild heat treatment because spore-formers cannot grow below pH 4.6), by chemical preservatives where permitted, by chilling, by freezing (frozen concentrate) and by aseptic processing and packaging. Deaeration before heating protects vitamin C and colour. Hot filling, cartons, glass, cans and PET bottles are all used; the oxygen and light barrier of the pack sets the shelf life. Spoilage of acid juices is mainly by yeasts, moulds and acid-tolerant bacteria; Alicyclobacillus, a spore-forming acidophile, causes a medicinal off-flavour in some juices. Thermal process calculation is in thermal-processing-canning.
Fruit beverages are classified by how much juice or pulp they contain, their total soluble solids and whether they are drunk as they are or diluted. A ready-to-serve (RTS) beverage is drunk straight from the bottle and has low juice content and low TSS. A nectar has more pulp or juice and more body; it is also drunk without dilution and is the usual form for pulpy fruits like mango, guava and apricot. A squash is a concentrated sweetened fruit drink, often cloudy and containing pulp, that is diluted with water before drinking. A cordial is a sparkling-clear squash from which all pulp and suspended matter have been removed. A crush is similar to a squash but sweeter, and a fruit syrup is sweeter still.
Indian textbooks teach the specifications of the old Fruit Products Order (FPO), which became the basis of the present FSSAI standards: an RTS beverage with at least 10% juice and at least 10% TSS; a nectar with at least 20% juice and at least 15% TSS; a squash and a cordial each with at least 25% juice, with at least 40% and 30% TSS respectively; a crush with at least 25% juice and 55% TSS; and a syrup with at least 25% juice and 65% TSS. These are useful as teaching values. Current legal limits, including permitted preservatives and their maximum levels, are set in the national food standards regulations and differ between countries, so they must be checked before a product is labelled.
A beverage is formulated by balancing juice, sugar, acid and water to hit the specified juice content, TSS and acidity. Take a batch of 100 kg of RTS beverage required to contain 10% juice, 10% TSS and 0.3% acidity as citric acid, made from juice of 10 °Brix and 1.0% acidity. The 10 kg of juice brings 1.0 kg of soluble solids and 0.1 kg of acid. The beverage needs 10 kg of soluble solids, so 9 kg of sugar is added, and 0.3 kg of acid, so 0.2 kg of citric acid is added. Water makes up the rest: 100 − 10 − 9 − 0.2 = 80.8 kg. (The added citric acid also contributes slightly to TSS; textbook calculations usually ignore it.) The same logic, applied with the juice and TSS of a squash, gives the squash recipe. When two syrups of different strengths must be mixed to a target strength, the Pearson square or the two-component mass balance gives the proportions.
Sugar is dissolved, usually by heating with part of the water and acid, strained, cooled, and mixed with the juice. Heating the juice itself is kept to a minimum to save flavour and colour.
RTS beverages and nectars are usually pasteurised in the bottle or hot-filled, because their low sugar content gives no protection. Squashes and cordials are diluted at home and opened many times, so they have traditionally carried a chemical preservative: sulphur dioxide (from potassium metabisulphite) or benzoic acid (as sodium benzoate). Sulphur dioxide bleaches anthocyanins, so benzoate is preferred for coloured fruits such as jamun, phalsa and pomegranate, while sulphur dioxide is common in light-coloured products such as mango and lemon squash. Both preservatives are more effective at low pH, because the undissociated acid form is the active one.
Typical defects include sedimentation and separation of pulp in squashes and nectars, controlled by fine pulping, homogenisation and stabilisers where permitted; browning during storage from Maillard reactions and ascorbic acid degradation, worse at higher storage temperature; fermentation and gas production by yeasts when preservation is inadequate; and loss of colour. Storage in cool, dark conditions extends shelf life. Carbonated fruit drinks and other soft drinks are covered in beverage-technology.
Pectin is a family of cell-wall polysaccharides found mainly in the middle lamella, where it cements neighbouring cells together. Its backbone is a chain of α-(1→4)-linked D-galacturonic acid units. Some of the carboxyl groups are esterified with methanol; the fraction of galacturonic units that carry a methyl ester is the degree of esterification (DE), also called degree of methylation. Stretches of the backbone are interrupted by rhamnose units that carry side chains of neutral sugars. In unripe fruit much of the pectin is insoluble protopectin bound into the wall; during ripening it is converted to soluble pectin, and in over-ripe fruit enzymes de-esterify and break it down to pectic acid, which does not form a sugar–acid gel. This is why slightly under-ripe fruit is preferred for jelly making.
Commercial pectin is classified by DE. High-methoxyl (HM) pectin has a DE above 50%; low-methoxyl (LM) pectin has a DE below 50%. Amidated LM pectin is made by treating pectin with ammonia, which replaces some ester groups with amide groups and makes the gel less sensitive to calcium level. Pectin is sold standardised to a grade, the number of parts of sugar that one part of the pectin will set into a jelly of standard firmness under standard conditions; 150-grade pectin sets 150 parts of sugar.
HM pectin forms a gel only when three conditions are met together: enough pectin, high soluble solids and low pH. The sugar competes for water and dehydrates the pectin chains, and the acid suppresses the ionisation of the remaining carboxyl groups, so that the chains no longer repel each other and can associate through hydrogen bonds and hydrophobic interactions between methyl ester groups. In practice an HM pectin gel needs soluble solids above about 55% (traditional jams and jellies are made at 65–68% or more) and a pH of roughly 2.8–3.5, with an optimum near 3.0–3.2; pectin content of the gel is usually well under 1%. Above about pH 3.5 the gel does not set; much below about pH 2.8 it sets too fast and tends to weep. Rapid-set HM pectins have high DE and set at higher temperature and pH; slow-set HM pectins have lower DE and suit jellies that must be poured without trapping air.
LM pectin gels by a different mechanism. Its many free carboxyl groups bind divalent calcium ions, which bridge neighbouring chains in junction zones described by the egg-box model. LM gels therefore need calcium but not high sugar, and they set over a wider pH range. They are used for low-sugar and reduced-calorie jams and for fruit preparations in yoghurt. Amidated LM pectin needs less calcium and gives thermoreversible gels.
Before making a jelly, the processor needs to know whether the fruit extract has enough pectin. The alcohol test is the classic quick check: a small volume of extract is mixed with about three times its volume of alcohol. A single firm clot means the extract is rich in pectin and needs about an equal weight of sugar; several smaller clots mean a moderate pectin content and less sugar; a flocculent or no precipitate means too little pectin, so the extract must be concentrated or pectin added. The jelmeter, a calibrated viscosity pipette, estimates pectin strength from how fast the extract flows. Laboratory methods include precipitation as calcium pectate and measurement of galacturonic acid. Good pectin sources among fruits include guava, apple, citrus (especially the albedo of the peel), sour plum, quince, gooseberry and karonda; strawberry, cherry and pineapple are low in pectin and need added pectin or a pectin-rich partner fruit.
A jam is made by boiling fruit pulp or pieces with sugar, and pectin and acid where needed, to a consistency firm enough to hold the fruit in place. A jelly is a clear, sparkling gel made from strained fruit extract, sugar and acid; it should hold its shape when turned out, quiver but not flow, and cut cleanly. A marmalade is a citrus jelly in which thin shreds of peel are suspended; the Indian distinction between jelly marmalade and jam marmalade follows the base used. A preserve, known in South Asia as murabba, is whole fruit or large pieces impregnated with heavy syrup until the fruit is translucent. Fruit cheese and fruit leather are firmer, more concentrated pulp products.
In jam making, the prepared pulp is heated with sugar, acid is adjusted to bring the pH into the gelling range, and boiling continues until the end point. During boiling, part of the sucrose is hydrolysed to glucose and fructose (invert sugar). Some inversion is desirable, because a mixture of sucrose and invert sugar is more soluble than sucrose alone and does not crystallise; too much gives a sticky, syrupy product. Boiling should be quick, because long boiling darkens the jam, degrades pectin and destroys flavour. Vacuum boiling allows lower temperatures for better colour and flavour. Jams are filled hot into sterilised jars, sealed, and in many plants inverted or given a short pasteurisation to treat the headspace.
The end point is the moment when soluble solids have reached the target and the product will set on cooling. It is judged by a refractometer reading of the hot product (commonly about 68–70 °Brix for traditional jams and jellies), by the boiling temperature, which at sea level is about 105–106 °C for a jam at the right concentration and is adjusted for altitude because water boils at a lower temperature in the hills, by the sheet or flake test, where the product falls from a ladle in a sheet rather than drops, and by the drop test in a glass of water. Classic Indian textbook specifications have required finished jams and jellies to have at least 65–68.5% TSS, depending on the edition of the standard; the current figure is in the national regulation.
The defects follow from the gel conditions. Crystallisation of sucrose occurs when too little inversion took place, for example when acid was too low or boiling too short; crystallisation of glucose occurs when inversion went too far. A stiff, tough jelly comes from too much pectin; a runny one from too little pectin, too little acid or too low a TSS. Syneresis, or weeping, the separation of liquid from the gel, is caused by excess acid, too little pectin, or too low a sugar concentration. Mould growth on the surface points to low TSS, poor sealing or condensation in the headspace. Cloudiness in jelly comes from using immature fruit, squeezing the extract, or pouring too slowly.
A candied fruit is fruit or peel impregnated with sugar syrup to a high soluble solids content, then drained and dried. Sugar cannot be forced in quickly: a strong syrup applied at once draws water out faster than sugar can diffuse in, and the fruit shrivels and toughens. So the fruit is treated with syrup whose strength is raised gradually over several days, often starting near 30–40 °Brix and ending at about 70–75 °Brix, with the syrup reboiled or topped up at each step. Pre-treatments include blanching or pricking to make the tissue permeable, and in some products holding in brine or a calcium salt for firmness. Small amounts of glucose syrup or invert sugar in the final syrup help prevent crystallisation. Candied products include amla, papaya tutti-frutti, ginger, citrus peel and cherries.
A glazed fruit is a candied fruit coated with a thin transparent film of syrup that dries to a glossy finish. A crystallised fruit is a candied fruit coated with fine sugar crystals. The low water activity of these products, not sterility, gives them their long shelf life; packaging must keep moisture out, or the surface becomes sticky and moulds.
Pickles are vegetables or fruits preserved by salt, acid, oil, spices, or a combination of these. Two broad routes exist. In fermented pickles, salt draws juice and sugars out of the vegetable and selects for lactic acid bacteria, which ferment the sugars to lactic acid and lower the pH until spoilage organisms cannot grow. In non-fermented pickles the acid is added directly as vinegar or is supplied by the fruit itself (lime, raw mango), and salt, oil and spices complete the barrier. Indian pickles are mostly oil pickles: the cut vegetable or fruit is first salted and partly dried, then mixed with spices and mustard, sesame or another oil, which forms a layer that excludes air from the surface. Vinegar pickles rely on enough acetic acid in the finished product to stop spoilage organisms.
Common salt concentrations differ with the method. Dry salting at a few per cent by weight (as in sauerkraut, about 2–2.5% salt) allows fermentation, while brining at higher salt slows fermentation and lets vegetables be stored before processing. Brine strength is often expressed in degrees salinometer, where 100° corresponds to a saturated salt solution.
In sauerkraut and fermented cucumbers, the fermentation follows a succession. Leuconostoc mesenteroides, a heterofermentative lactic acid bacterium, starts it, producing lactic acid, acetic acid, carbon dioxide and flavour compounds and lowering the pH. As the acid rises, more acid-tolerant homofermentative species such as Lactobacillus plantarum and Pediococcus take over and finish the fermentation, giving a final pH usually below about 3.8 in sauerkraut. Temperature and salt control this succession: too much salt or too high a temperature lets the wrong organisms dominate. The same microbiology underlies kimchi and many fermented vegetables; the wider subject is in fermentation-food-biotechnology.
Defects in pickles are well catalogued. Softness comes from pectinolytic enzymes of moulds or of the plant itself, encouraged by low salt and air contact. Hollow cucumbers arise from gas formed during fermentation or from the raw material. Shrivelling results from placing vegetables directly in very strong salt, sugar or vinegar solutions. Darkening may come from iron in water or utensils reacting with tannins, or from hydrogen sulphide produced by bacteria. A white film on the surface of brine is usually film yeast, which consumes acid and lets other spoilage follow. Clean equipment, correct salt, adequate acidity and exclusion of air prevent most of these.
A chutney is a sweet, spiced, thick preparation of fruit or vegetable pieces cooked with sugar, vinegar or another acid, salt and spices to a jam-like consistency; mango, tomato, apple and tamarind chutneys are common. Its preservation depends on sugar, acid, spices and heat processing together, not on any one of them alone. Sauces are thinner, smoother and usually strained; tomato sauce and ketchup are the familiar examples and are treated in the next chapter. Chilli sauce and soy-based sauces belong to the same group. Whole spices are often tied in a muslin bag and boiled with the product so their flavour is extracted while the particles are removed; ground spices darken the product. Onion and garlic are added for flavour.
Sauces and chutneys are filled hot, sealed and cooled. Where the acid and sugar level are low, a preservative such as sodium benzoate may be permitted, subject to the national regulation. Because the product is acid, its thermal process is mild; the rules for acid and acidified foods are in thermal-processing-canning.
Tomato is the most heavily processed vegetable in the world. Ripe tomatoes are washed, sorted, chopped or crushed, and heated at once, because chopping releases pectin methylesterase and polygalacturonase, which start to break down pectin within seconds. In the hot break process the crushed tomatoes are heated rapidly to a high temperature, commonly above about 85–90 °C, which inactivates these enzymes before they can act. The pectin is preserved and the product is thick, so hot break is used for paste, ketchup and sauces where consistency is valued. In the cold break process the tomatoes are heated more gently, commonly to about 60–70 °C, so the enzymes act for a time; viscosity is lower but colour and fresh flavour are better, and the product is used for juice and some purees. The heated pulp is then passed through a pulper and finisher, which remove skin and seeds.
The red colour of tomato comes from lycopene, a carotenoid that is fairly stable to heat and becomes more available after processing. Good colour, high soluble solids, and a high viscosity after break are the processing qualities a tomato variety is bred for.
Tomato juice is the unconcentrated pulp, seasoned with salt and sometimes sugar, homogenised to keep the pulp suspended, and heat processed. Puree and paste are concentrated pulp, distinguished by their soluble solids. Indian textbooks teach puree as concentrated pulp of roughly 9–12% TSS and paste as at least 25% TSS, with products described as single, double or triple concentrates; definitions and minimum values differ between national standards, so the current regulation must be checked. Concentration is done in vacuum evaporators to protect colour and flavour; forced-circulation and multiple-effect evaporators are used because the product becomes very viscous.
The mass balance is the same as for juice concentrate. If 1000 kg of pulp at 5% TSS is concentrated to a paste of 28% TSS, the paste mass is 1000 × 5 ÷ 28 = 178.6 kg and the water evaporated is 821.4 kg. A common error is to subtract the percentages instead of using the ratio. Paste is hot-filled or aseptically packed in drums and bags and is the bulk intermediate from which sauces, soups and ketchup are made worldwide.
Ketchup is a strained tomato product cooked with sugar, vinegar, salt, onion, garlic and spices to a smooth, thick, pourable consistency; its total soluble solids are typically much higher than those of puree, commonly about 25–30% or more depending on the standard. Spices are added in a muslin bag or as extracts, and vinegar is added late in cooking to limit loss of acetic acid. The finished product is acid enough for a mild heat process and is hot-filled.
Quality control of tomato products uses colour (by colorimeter or comparison with standards), consistency (by Bostwick consistometer, which measures how far the product flows in a channel in a set time), soluble solids by refractometer, acidity and pH, and the Howard mould count, a microscopic test that counts the proportion of fields containing mould filaments and so shows whether rotten fruit was used. Acceptable limits for these tests are set by each standard. A dark ring at the neck of ketchup bottles, called black neck, has been attributed to iron reacting with tannins and to oxidation in the headspace, and is prevented by avoiding iron contamination and minimising headspace air. Separation of serum (syneresis) indicates low pectin, often after a cold break or poor homogenisation.
Canning preserves food by sealing it in a container and heating it enough to destroy or inactivate the microorganisms that could grow in it. The steps for fruits and vegetables are selection and grading, washing, peeling, coring or cutting, blanching (mainly for vegetables), filling into cans or jars, adding a covering liquid, exhausting, sealing, heat processing, cooling, and storage. The covering liquid is syrup for fruits and brine (often about 2% salt) for vegetables; it improves heat transfer, flavour and appearance and fills the spaces between pieces.
Exhausting removes air from the container before sealing, either by heating the filled can in an exhaust box, by filling hot, by mechanical vacuum, or by steam injection into the headspace during closing. A good exhaust gives a vacuum in the cooled can, reduces internal pressure during processing so seams are not strained, and limits oxidation, corrosion and loss of vitamin C. A proper headspace, the space left between the product and the lid, is needed for the same reasons.
Fruit syrups are described by their Brix; the syrup and fruit come to equilibrium during processing and storage, and the soluble solids of the liquid when the can is opened is the cut-out Brix. To reach a target cut-out Brix, the syrup strength is calculated from the mass and Brix of the fruit and the mass of syrup. Suppose a can holds 300 g of fruit at 10 °Brix and 200 g of syrup, and the cut-out target is 20 °Brix. Total soluble solids needed = 500 × 0.20 = 100 g; the fruit brings 30 g; so the syrup must carry 70 g in 200 g, a strength of 35 °Brix. Students often average the two Brix figures without weighting by mass, or forget the fruit's own sugar.
Different fruits are packed in syrups of different strengths, from light to heavy, depending on the fruit's own sugar and the market. Light syrups and fruit packed in juice are increasingly common. Acid such as citric acid is sometimes added to bring the pH of low-acid fruit (such as some varieties of pear, fig or banana) clearly below 4.6, a practice called acidification, so that a boiling-water process becomes adequate.
Acid fruits (pH below 4.6) are processed at or near 100 °C, in boiling water or atmospheric steam, because Clostridium botulinum cannot grow in them. Low-acid vegetables such as peas, beans, corn and okra must be processed under pressure in a retort, typically at about 115–121 °C, for a time calculated for each product and can size to give commercial sterility. Process calculation, the 12D concept and F0 are in thermal-processing-canning. Cans are cooled quickly in water to about 35–40 °C, warm enough that the surface water evaporates and does not rust the can.
Tinplate cans are lacquered inside to protect the product and the can. Acid-resistant lacquer (often called R-enamel) is used for coloured fruits, whose anthocyanins would otherwise react with tin and lose colour; sulphur-resistant lacquer (C-enamel) containing zinc oxide is used for sulphur-rich vegetables such as peas and corn, to prevent black iron or tin sulphide staining. Spoilage of canned fruits and vegetables includes swells caused by gas from surviving or leaking microorganisms or by hydrogen from corrosion (hydrogen swells, common in acid fruits in poorly lacquered cans), flat sour spoilage by thermophilic spore-formers in low-acid vegetables, and sulphide stinker spoilage. Any swollen can must be treated as potentially dangerous.
Drying removes water until the water activity is too low for microorganisms and most enzymes. Sun drying remains common for grapes (raisins), apricots, figs, dates and chillies, but it is slow, weather-dependent and exposed to dust, insects and birds. Solar driers protect the product and dry faster. Industrial methods include cabinet and tunnel driers with heated air, belt driers, drum driers for purees, spray driers for juices and pulps (often with carriers such as maltodextrin because sugary juices are sticky), and freeze driers for high-value products. Osmotic dehydration in sugar or salt solution removes part of the water before air drying and gives a softer, sweeter product. Drier design and drying kinetics are in evaporation-dehydration.
Pre-treatments protect colour and texture. Light-coloured fruits such as apricot, apple and peach are sulphured, by exposing cut fruit to the fumes of burning sulphur or dipping in a sulphite solution; sulphur dioxide inhibits enzymatic and non-enzymatic browning and also protects vitamin C. Grapes are dipped in an alkaline emulsion of oils to crack the waxy bloom and speed drying. Vegetables are blanched to inactivate enzymes. The yield of dried product follows from a solids balance: 100 kg of fruit at 85% moisture (wet basis) dried to 5% moisture gives 100 × 0.15 ÷ 0.95 = 15.8 kg, a drying ratio of about 6.3 kg of fresh fruit per kg of dried product. Rehydration ratio, the mass of rehydrated product per mass of dry product, is a common quality index.
Freezing stops microbial growth and slows chemical change, but it does not stop enzymes completely, so vegetables are blanched before freezing to prevent off-flavours, colour loss and vitamin loss during frozen storage. Fruits are usually not blanched because blanching would cook them; instead they are packed in sugar or syrup, which excludes air and limits browning, often with ascorbic acid as an antioxidant. Individually quick frozen (IQF) products, frozen in fluidised-bed or belt freezers, stay free-flowing so the consumer can use part of a pack. Fast freezing forms small ice crystals and damages tissue less than slow freezing, which forms large crystals that rupture cells and cause drip loss on thawing.
Frozen storage at a constant low temperature, commonly −18 °C or below, is needed for quality; temperature fluctuation causes ice recrystallisation and freezer burn, the dry, discoloured patches from sublimation at the surface. Freezing equipment and the calculation of freezing time are in chilling-freezing-frozen-foods.
Minimally processed or fresh-cut produce, such as bagged salads, cut fruit, baby carrots and spiralised vegetables, is washed, trimmed, cut and packed but remains living tissue. Cutting is a wound: respiration and ethylene production rise, cell contents spill onto the surface, enzymatic browning starts at the cut, and microorganisms gain access to nutrients. Shelf life is therefore short and depends on a chain of hurdles rather than on any one treatment.
The chain is: high-quality raw material; cutting with sharp blades, because blunt blades crush more cells; washing and sanitising in clean, sanitised water to reduce the microbial load and prevent cross-contamination (chlorine is widely used, though approved sanitisers and their levels differ by country); removing surface water by centrifuge or air; anti-browning dips such as ascorbic acid, calcium ascorbate or citric acid, and calcium salts for firmness; modified atmosphere packaging (MAP) in films whose permeability is matched to the product's respiration, so that oxygen falls and carbon dioxide rises to levels that slow respiration without causing anaerobic fermentation; and an unbroken cold chain, commonly at about 0–5 °C. Listeria monocytogenes, pathogenic Escherichia coli and Salmonella are the main safety concerns, and an atmosphere that becomes anaerobic raises concern about Clostridium botulinum in some products. Packhouse handling and controlled-atmosphere storage of whole produce are covered in post-harvest-horticultural-crops.
Potato quality for processing is defined mainly by dry matter and reducing sugars. Dry matter, closely related to specific gravity, sets yield and texture: high-dry-matter tubers give crisp chips and mealy fries that absorb less oil. Reducing sugars (glucose and fructose) react with amino acids, especially asparagine, during frying through the Maillard reaction, producing brown colour and acrylamide, a probable human carcinogen. Processors therefore set low limits on reducing sugars for chips and fries; the exact limits are company and market specifications.
Storage temperature matters. Potatoes stored cold, below roughly 8–10 °C, accumulate reducing sugars, a change called cold-induced sweetening, and give dark chips. Holding them for some days or weeks at a warmer temperature, about 15–20 °C, reconditions them by letting respiration use up part of the sugar, though not always fully. Processing potatoes are therefore stored warmer than table potatoes, with sprout suppression. Greening on exposure to light is linked with the formation of glycoalkaloids (solanine), which are bitter and toxic in excess, so tubers are stored in the dark.
Potato chips (crisps) are thin slices, washed to remove surface starch, fried in oil at a temperature commonly around 170–190 °C until the moisture is very low and the slice is crisp, then salted and seasoned and packed in nitrogen-flushed, light-barrier packs to delay oil oxidation. Blanching the slices in hot water leaches reducing sugars and lowers browning and acrylamide. French fries are cut into strips, blanched to gelatinise the surface starch and even out sugars, dried, par-fried, frozen and finished by the consumer or caterer. Dehydrated potato products include flakes, made by cooking, mashing and drum drying, and granules, made by gentle cooking, mashing with recycled dry product (add-back) and air drying so that most cells stay intact and the reconstituted mash is not gluey. Free starch from broken cells makes mash pasty, so processes are designed to keep cells whole. Other products include canned small potatoes, potato starch and frozen formed products. Frying is treated in more depth in the fats and oils and snack subjects.
Fruit and vegetable processing leaves large amounts of peel, seeds, cores, pomace (the pressed residue from juice extraction) and trimmings; for citrus, mango and pineapple these can be a third to a half or more of the raw fruit. Disposal costs money and, if dumped, the waste ferments and pollutes. Recovering useful products turns a cost into income. The main by-products are pectin from citrus peel and apple pomace; essential oils from citrus peel; flavonoids such as hesperidin from orange and naringin from grapefruit; dietary fibre powders; seed oils and fats such as mango kernel fat; enzymes such as bromelain from pineapple stems and papain from papaya latex; vinegar and alcohol from fermentable waste; and animal feed, compost and biogas from what remains. The order of preference usually follows value: recover the highest-value compound first (an oil, enzyme or pectin), then use the residue for fibre or feed, and send only the final remainder to biogas or compost. Whether recovery pays depends on the volume of waste at one site, how quickly it can be processed before it ferments, the cost of drying, and the local market for each product. Small seasonal plants often sell wet peel or pomace to a central processor rather than recover products themselves.
Commercial pectin is made mainly from dried citrus peel and apple pomace; citrus peel generally yields more pectin than apple pomace on a dry basis. The raw material is washed to remove sugars and acids, and may be blanched to inactivate pectin-degrading enzymes, then dried for storage if it is not extracted at once. Extraction is done with hot dilute acid, commonly at a pH of about 1.5–3 and a temperature of about 60–100 °C for a set time; mineral acids or citric acid are used. The extract is separated from the solids by pressing and filtration and clarified. Pectin is then precipitated, usually with alcohol (ethanol or isopropanol), and the alcohol is recovered by distillation; precipitation with aluminium salts is an older alternative. The precipitate is washed, dried, ground and standardised with sugar to a fixed grade.
Extraction conditions determine the degree of esterification: longer or harsher acid treatment de-esterifies the pectin. LM pectin is made by controlled de-esterification of HM pectin with acid, alkali or enzymes, and amidated LM pectin by treatment with ammonia. Yield, DE, galacturonic acid content and jelly grade are the main quality tests.
Citrus peel oil, mainly d-limonene, is recovered by cold pressing during or before juice extraction; it is a flavouring and a solvent. Peel can also be candied or made into marmalade, or dried, limed and pressed to make citrus pulp pellets for cattle feed, with the press liquor concentrated into citrus molasses. Apple pomace is used for pectin, fibre and feed. Grape pomace from wine making is a source of tartrates, grape-seed oil and anthocyanins. Mango peel contains pectin and dietary fibre, and mango kernels yield a fat used in confectionery fat blends. Tomato pomace of skins and seeds contains lycopene and seed oil. Pineapple waste can be fermented to vinegar or used for bromelain. Vegetable trimmings are used for feed and composting, and wet wastes for biogas.
Any by-product intended for food must be handled as food from the start: fast processing to prevent fermentation and mould, and control of pesticide residues and contaminants that concentrate in peel.
A student uses this subject to answer the product questions that appear in every food technology paper: the difference between a squash and a cordial, why a jelly weeps, how HM and LM pectins set, what hot break does to tomato paste, and why light-coloured fruit is sulphured before drying. In the pilot plant the same student makes a batch of RTS beverage, jam and pickle, reads the end point on a refractometer, runs the alcohol test for pectin, measures a Bostwick reading on ketchup, and works the mass balances for concentration, drying yield and syrup strength. Knowing the chemistry behind each step turns a list of recipes into a subject that can be reasoned through.
Five years into a juice, preserves or tomato plant, a technologist sets and defends specifications: raw-material Brix, acid and maturity at intake; break temperature and finisher screen size; evaporator end point; jam end point and fill temperature; preservative levels within the national regulation. When a customer complains of weeping jam, separated nectar, dark chips or cloudy apple juice, the technologist traces the defect to its cause (acid, pectin, enzyme, sugar, temperature, storage) and corrects the process. The same person qualifies new suppliers, signs off label claims against the current standard, and runs the hold-and-release checks before product leaves the plant.
After twenty-five years a senior manager decides which products a plant makes and how: whether to invest in aseptic bulk packing of pulp, membrane clarification, aroma recovery or IQF lines; how to source fruit across seasons and countries; how to turn peel and pomace from a waste cost into pectin, oil, fibre or feed income; and how to meet changing rules on preservatives, acrylamide, residues and labelling in several export markets. The senior manager also leads incident response when a can swells in the market or a fresh-cut product is linked to an outbreak, deciding on recall, root cause and changes to the hurdle design.
A researcher with long industry experience works on the questions processors cannot solve by trial: the molecular basis of pectin gelation and its control through DE, amidation and calcium; enzyme engineering for juice yield and cloud; non-thermal and minimal processes that keep fresh flavour while assuring safety; acrylamide mitigation through raw material, enzymes and process; the physiology of wounded tissue and active packaging for fresh-cut produce; and biorefinery routes that recover several products from one waste stream. Such work builds models and data that regulators and companies use to set specifications and evaluate new processes.
This manual gives the classic Indian FPO teaching values for fruit beverages and jams but does not reproduce current legal limits for each country, including preservative maximum levels; these change with revisions of the regulations and must be read from the current national standard. It does not give equipment design calculations for evaporators, driers, freezers or retorts, which are in evaporation-dehydration, chilling-freezing-frozen-foods and thermal-processing-canning, or packhouse and controlled-atmosphere storage practice, which is in post-harvest-horticultural-crops. Crop-by-crop recipes for every Indian pickle, murabba and chutney, and acceptance limits for Howard mould count, Bostwick consistency and reducing sugars in potatoes, are not listed because they are set by each standard or buyer. Carbonated fruit drinks and fruit wines are in beverage-technology.
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. Product specifications quoted are the classic teaching values from Indian textbooks; current legal limits are set by each country's food standards regulation.
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