Food Science & Technology · Commodity Processing Technologies · also Edible Oil Technology, Lipid Technology, Oil and Fat Processing
From oilseed to finished fat: mechanical expression and solvent extraction, refining, bleaching and deodorisation, fat modification by hydrogenation, interesterification and fractionation, and the fat products — margarine, shortening, vanaspati and specialty fats — made from them.
In the exams: GATE XE-G Section 3 (oil processing: expelling, solvent extraction, refining, hydrogenation); ICAR AICE 35.a Unit 2 (oilseed processing, vanaspati)
Edible oils and fats supply a large share of dietary energy, carry fat-soluble vitamins and flavours, and give texture to most processed foods. Fats and oils technology covers the whole route from oilseed or fruit to finished product: obtaining crude oil by pressing and solvent extraction, refining it to remove gums, free fatty acids, pigments and odours, modifying it by hydrogenation, interesterification or fractionation to obtain the melting behaviour a product needs, and formulating margarines, shortenings, vanaspati, frying oils and specialty fats such as cocoa butter equivalents. Quality tests follow the oil at every stage, and the by-products of refining are recovered for value.
The subject matters economically and nutritionally. Palm, soybean, rapeseed and sunflower oils dominate world production and trade; India is one of the largest consumers and importers of edible oil, using mustard, groundnut, soybean, sunflower, rice bran, cottonseed and imported palm oil. Health concerns have reshaped the industry: industrially produced trans fatty acids from partial hydrogenation have been restricted or banned in many countries, process contaminants such as 3-MCPD and glycidyl esters are controlled, and saturated fat and frying oil quality are public health issues.
The subject is examined in GATE XE-G Section 3, which names oil processing (expelling, solvent extraction, refining and hydrogenation), and in the ICAR AICE-JRF/SRF 35.a syllabus (oilseed processing, vanaspati). Examiners ask for the steps and purposes of refining, the chemistry of hydrogenation and trans formation, the difference between chemical and enzymatic interesterification, fractionation of palm oil, margarine crystallisation, cocoa butter equivalents, and the meaning and calculation of acid, peroxide, iodine and saponification values. The engineering of expellers and extractors is taught in the Agricultural Engineering subject post-harvest-cereals-pulses-oilseeds, lipid chemistry, oxidation and frying chemistry in food-chemistry-major-constituents, and process contaminants in food-toxicology-contaminants-allergens; this manual builds on them.
Edible oils and fats are mainly triacylglycerols (triglycerides), esters of glycerol with three fatty acids, usually more than 95 per cent of a refined oil. The properties of a fat depend on which fatty acids it contains and how they are arranged on the three positions of glycerol: saturated fatty acids such as palmitic and stearic acids raise the melting point; monounsaturated oleic acid and polyunsaturated linoleic and linolenic acids lower it and increase susceptibility to oxidation. Fats that are solid at room temperature contain enough high-melting triacylglycerols to form a crystal network; oils are liquid. The nomenclature, polymorphism and oxidation chemistry of lipids are covered in food-chemistry-major-constituents.
Crude oils also contain minor components, some valuable and some removed in refining: phospholipids (gums), free fatty acids, mono- and diacylglycerols, sterols (phytosterols in plant oils, cholesterol in animal fats), tocopherols and tocotrienols, which are natural antioxidants and vitamin E, carotenoids (abundant in crude palm oil), chlorophyll, waxes, and characteristic compounds such as sesamol and sesamin in sesame oil, γ-oryzanol in rice bran oil and gossypol in crude cottonseed oil.
Each oil has a characteristic fatty acid profile, which decides its uses; the values below are typical and vary with variety and origin. Palm oil, from the fruit mesocarp, is about half saturated, mainly palmitic acid, with oleic and some linoleic acid; it is semi-solid and is the most produced vegetable oil in the world. Palm kernel oil and coconut oil are lauric oils, rich in lauric acid (about 45 to 50 per cent in coconut), with sharp melting suited to confectionery and coatings. Soybean oil is rich in linoleic acid and contains several per cent of linolenic acid, which makes it prone to flavour reversion; sunflower oil is high in linoleic acid, though high-oleic varieties are now common. Rapeseed oil from canola varieties is high in oleic acid with some linolenic acid and low erucic acid, while traditional mustard and rapeseed oils, preferred in eastern and northern India, contain much erucic acid. Groundnut oil is rich in oleic and linoleic acids; olive oil is predominantly oleic; rice bran oil and cottonseed oil have balanced profiles with notable palmitic acid; sesame oil is stable because of its natural antioxidants. Animal fats include butterfat and ghee, with short-chain fatty acids such as butyric acid, lard, tallow and fish oils, which are rich in the long-chain omega-3 fatty acids EPA and DHA. Cocoa butter is dominated by symmetrical triacylglycerols with oleic acid in the middle position, which give chocolate its sharp melting, discussed in chapter 10.
Crude oil is obtained from oilseeds by mechanical expression, solvent extraction or both, and from oil-bearing fruits such as palm and olive by pressing or centrifugation after sterilisation or malaxation. The engineering of the machines, from the traditional ghani to the screw press, and the mass balance of pressing are described in post-harvest-cereals-pulses-oilseeds; this chapter summarises what matters for oil quality.
Seed preparation, by cleaning, dehulling, cracking, flaking and cooking or conditioning, ruptures oil cells, coagulates protein, lowers oil viscosity, adjusts moisture and in some seeds inactivates enzymes such as myrosinase in mustard. The screw press or expeller presses seed continuously through a barrel of spaced bars in which pressure rises along the worm shaft; residual oil in the cake depends on seed preparation, pressure, temperature and moisture. Cold-pressed or virgin oils, pressed at low temperature without refining, keep characteristic flavours and minor components, as in extra virgin olive oil and traditional kachi ghani mustard oil, whose pungency comes from allyl isothiocyanate formed from glucosinolates by myrosinase. Oil recovery is calculated by a mass balance on oil-free solids: if 1,000 kg of seed with 40 per cent oil gives a cake with 8 per cent oil, the cake mass is 1,000 × 0.60/0.92, about 652 kg, holding about 52 kg of oil, so about 348 kg of oil, or 87 per cent, is recovered.
Solvent extraction dissolves oil out of prepared flakes or pressed cake, reducing residual oil in the meal to below about 1 per cent. Commercial hexane, a petroleum fraction boiling at about 65 to 70 °C, is the industrial solvent because it dissolves triacylglycerols well, is selective, easily recovered and relatively cheap, but it is flammable and explosive, and residues in oil and meal are regulated. Extractors use counter-current percolation, in which the freshest solvent meets the most extracted flakes. Low-oil seeds such as soybean are extracted directly after flaking; high-oil seeds such as groundnut, sunflower and rapeseed are first pre-pressed to an intermediate oil content and then the cake is extracted.
The oil-solvent solution, called miscella, is distilled in stages, with a final stripping column under vacuum and steam, to recover the oil free of solvent, and the solvent is condensed and recycled. The spent flakes pass through a desolventiser-toaster, where steam drives off solvent and, for soybean meal intended for feed, heat destroys trypsin inhibitors; meal for food uses is desolventised more gently, as noted in pulses-oilseeds-plant-proteins. Alternatives to hexane under investigation or in limited use include ethanol, isopropanol, supercritical carbon dioxide for high-value oils, and aqueous and enzyme-assisted aqueous extraction. Crude solvent-extracted oil generally contains more phospholipids and non-oil components than pressed oil and is always refined before eating.
Crude oils contain phospholipids, called gums, from about 0.1 per cent in some pressed oils to 2 to 3 per cent in crude soybean oil. They cause losses in later refining, darkening and foaming on heating, sediment in storage, and poor performance in physical refining. Degumming removes them as the first refining step. In water degumming, a few per cent of water is mixed with warm oil; hydratable phospholipids such as phosphatidylcholine and phosphatidylinositol absorb water, swell and become insoluble, and the hydrated gums are separated by centrifugation. Water degumming leaves non-hydratable phospholipids, mainly calcium and magnesium salts of phosphatidic acid and phosphatidylethanolamine, which do not hydrate.
Acid degumming adds phosphoric or citric acid, which chelates calcium and magnesium and converts the non-hydratable salts into hydratable forms, followed by water and often partial neutralisation; it achieves low residual phosphorus. Specialised processes combine acid and alkali in carefully controlled steps for oils destined for physical refining, which requires very low phosphorus, commonly a few milligrams per kilogram. Enzymatic degumming uses phospholipases: phospholipase A1 or A2 removes a fatty acid, producing water-soluble lysophospholipids, and phospholipase C removes the phosphate head group, leaving a diacylglycerol that stays in the oil and so increases oil yield. Enzymatic degumming improves yield and reduces waste and chemical use.
The wet gums from water degumming of soybean oil are the source of commercial soy lecithin. They are dried under vacuum to low moisture, giving fluid lecithin, a mixture of phospholipids and oil, commonly specified by its acetone-insoluble content, a measure of phospholipids, usually above about 60 per cent. Further processing yields de-oiled powdered lecithin, by acetone extraction of the oil; fractionated lecithins enriched in phosphatidylcholine, by alcohol fractionation; hydroxylated or enzyme-modified lecithins with altered emulsifying properties; and bleached lecithin. Sunflower and rapeseed lecithins are produced as alternatives to soy, partly because soy is a declared allergen and partly to supply non-GM sources.
Lecithin is a widely used food emulsifier, identified as INS 322 (E322). It reduces viscosity of chocolate, allowing less cocoa butter to be used; acts as a wetting agent for instant powders such as milk powder and cocoa drinks; serves as a release agent in baking and as an emulsifier in margarine, where it also reduces spattering in frying; and is used in pharmaceuticals and feed. Gums that are not processed into lecithin are often added back to meal, raising its energy value for feed.
Refining removes free fatty acids, residual phospholipids, pigments, oxidation products, trace metals, odours and contaminants, while keeping desirable components such as tocopherols. In chemical (alkali) refining, the degummed oil is treated with sodium hydroxide solution, which reacts with free fatty acids to form soaps. The soaps, with entrained oil, phospholipids and impurities, are separated by centrifugation as soapstock, and the oil is washed with water to remove residual soap and dried. The amount of alkali is calculated from the free fatty acid content plus a small excess: with free fatty acids expressed as oleic acid (molecular weight 282) and sodium hydroxide molecular weight 40, each tonne of oil with 1 per cent free fatty acids (10 kg) needs about 10 × 40/282 = 1.42 kg of sodium hydroxide before excess. Refining loss, the oil lost with soapstock, rises with free fatty acid content and with emulsified neutral oil, so oils with high free fatty acids are expensive to alkali refine. Alkali refining also removes some pigments, gossypol in cottonseed oil and trace metals.
Bleaching treats the oil with adsorbents, usually acid-activated bleaching earth (montmorillonite clays), sometimes with activated carbon, at about 90 to 110 °C under vacuum, followed by filtration. It adsorbs pigments such as chlorophyll and carotenoids, residual soaps and phospholipids, trace metals and primary and secondary oxidation products; activated carbon is effective against polycyclic aromatic hydrocarbons. Chlorophyll removal matters especially for rapeseed and soybean oils, because chlorophyll is a photosensitiser. Some oils, such as sunflower and rice bran oils, also undergo winterisation or dewaxing, chilling to crystallise and filter out waxes and high-melting components, so that the oil stays clear in the cold.
Deodorisation is steam distillation under high vacuum, commonly about 2 to 6 mbar, at about 220 to 260 °C, which strips volatile odour and flavour compounds, residual free fatty acids, and some contaminants such as light pesticide residues, giving a bland, stable oil. Heat also destroys carotenoids in palm oil (heat bleaching). Side effects include partial loss of tocopherols and sterols into the deodoriser distillate, formation of trans fatty acids from linolenic acid at high temperatures and long times, and formation of 3-MCPD and glycidyl esters, especially in palm oil, as described in food-toxicology-contaminants-allergens; lower temperatures, shorter times and better pre-treatment reduce them.
Physical refining replaces alkali neutralisation by removing free fatty acids during deodorisation itself, by steam distillation at the upper end of the temperature range. It produces less effluent, no soapstock, lower refining loss and a valuable fatty acid distillate, but requires well-degummed and bleached oil with very low phosphorus and metals. It is standard for palm oil and widely used for rice bran oil, where it also preserves γ-oryzanol that alkali refining removes.
Hydrogenation adds hydrogen to the double bonds of unsaturated fatty acids, raising the melting point and oxidative stability of oils, turning liquid oils into semi-solid fats for margarine, shortening and vanaspati. Refined, bleached oil is heated, commonly to about 150 to 220 °C, in a closed reactor with a finely divided nickel catalyst, and hydrogen gas is bubbled through with vigorous stirring. The reaction occurs at the catalyst surface, so the rate depends on temperature, hydrogen pressure, agitation and catalyst concentration and activity. Catalyst poisons, such as sulphur compounds, phospholipids, soaps and oxidation products, reduce activity, which is why oil must be well refined before hydrogenation. After reaction, the catalyst is filtered off and the fat is post-bleached to remove traces of nickel, then deodorised.
Hydrogenation is selective to different degrees. Polyunsaturated fatty acids react faster than monounsaturated ones, so linolenic acid is reduced to linoleic, linoleic to oleic and oleic to stearic; selectivity describes how strongly the reaction favours the more unsaturated acids. High temperature, low hydrogen pressure, low agitation and high catalyst concentration increase selectivity and also favour isomerisation. The progress of hydrogenation is followed by the fall in iodine value and the rise in melting point and solid fat content, and the end point is chosen to give the required properties. Full hydrogenation converts all unsaturated acids to saturated ones, giving hard fats with high stearic acid, and produces almost no trans fatty acids.
During partial hydrogenation, adsorbed fatty acids on the catalyst can lose and regain hydrogen at a double bond, which allows the bond to shift position and change from the natural cis configuration to the trans configuration. Partially hydrogenated oils have therefore contained large amounts of trans fatty acids, which have higher melting points than their cis isomers and gave desirable plastic texture. Trans fatty acids from partially hydrogenated oils raise LDL cholesterol and lower HDL cholesterol, and are strongly associated with coronary heart disease. Small amounts of trans fatty acids also occur naturally in ruminant fats such as milk fat, and are formed in small amounts during high-temperature deodorisation.
Regulation has moved from labelling to elimination. Denmark limited industrial trans fat in 2003. The United States determined in 2015 that partially hydrogenated oils are no longer generally recognised as safe, with compliance from 2018. The World Health Organization's REPLACE action package of 2018 called for global elimination of industrially produced trans fat, recommending either a limit of 2 grams per 100 grams of total fat in all foods or a ban on partially hydrogenated oils. India limited trans fatty acids to 2 per cent of total fat in oils and fats from 2022, applied to foods containing them. Industry replaced partially hydrogenated fats with palm and palm fractions, fully hydrogenated fats blended with liquid oils and interesterified, high-oleic oils, and structured fats, at the cost of higher saturated fat in some products.
Interesterification rearranges the fatty acids on and between glycerol molecules without changing the fatty acids themselves, so it creates no trans fatty acids and does not change the overall fatty acid composition, but changes the triacylglycerol composition and therefore melting and crystallisation behaviour. It is used alone on one fat, or on blends of fats with different properties.
Chemical interesterification uses a catalyst, usually sodium methoxide at a fraction of a per cent, with dry, refined oil low in free fatty acids, peroxides and moisture, which would destroy the catalyst, at about 80 to 120 °C. The catalyst randomises the distribution of fatty acids across all three positions of glycerol until a statistical (random) distribution is reached; the reaction is then stopped with water or acid, and the fat is washed, bleached and deodorised. A standard application is to blend a fully hydrogenated hard stock, high in stearic acid, with a liquid oil and randomise the blend, producing a plastic fat with intermediate melting suitable for zero-trans margarines and shortenings. Randomisation of lard improves its crystal habit for shortening, and randomisation of palm oil changes its crystallisation.
Enzymatic interesterification uses lipases, usually immobilised in packed-bed reactors, at about 60 to 70 °C, without the harsh chemicals of the chemical process. Many lipases, for example those from Rhizomucor miehei and Thermomyces lanuginosus, are specific for the outer sn-1 and sn-3 positions, so they exchange fatty acids only at those positions and preserve the fatty acid at the middle (sn-2) position. This allows designed triacylglycerols, called structured lipids, to be made. Examples are cocoa butter equivalents rich in symmetrical POP, POS and SOS triacylglycerols, made by exchanging fatty acids at the outer positions of oleic-rich oils with palmitic or stearic acids; human milk fat substitutes with palmitic acid at the sn-2 position and oleic acid at the outer positions (OPO), which resemble human milk fat and improve fat and calcium absorption in infant formula; and lipids containing medium-chain fatty acids for clinical nutrition.
Enzymatic processes give fewer side products and less colour, need little or no post-treatment, and produce less waste, but enzymes are costly and must be protected from poisons and maintained in activity; they are now widely used for zero-trans margarine and shortening bases. Interesterification has been examined for health effects; current evidence does not show effects different from those expected from the fatty acid composition, but research continues.
Fractionation separates a fat into fractions with different melting ranges by partial crystallisation at a controlled temperature followed by separation of the crystals from the liquid. It uses no chemicals in its simplest form, creates no new molecules and is fully reversible in principle, which makes it popular as a natural modification process. The solid fraction is called stearin and the liquid fraction olein.
Dry fractionation cools the molten fat slowly under gentle agitation so that high-melting triacylglycerols crystallise as large, filterable crystals, then separates them by filtration, commonly in membrane filter presses that squeeze the crystal cake to improve separation. It is the most common method because it is cheap and leaves no residues. Detergent fractionation adds an aqueous detergent solution that wets the crystals so they can be separated by centrifugation; it is now rarely used. Solvent fractionation dissolves the fat in hexane or acetone, which gives sharper separation and lower viscosity, and is used for high-value products such as cocoa butter equivalents, at higher cost. Separation efficiency is limited by liquid entrained in the crystal cake; multi-stage fractionation and better filtration improve it. Crystallisation conditions, cooling rate, agitation and holding time decide crystal size and polymorphic form, and therefore yield and quality.
Palm oil is the most fractionated fat. Single-stage dry fractionation gives palm olein, the liquid fraction, widely used as cooking and frying oil in Asia, including India, and palm stearin, the solid fraction, used in margarines, shortenings and vanaspati blends and as hard stock for interesterification. Further fractionation of olein gives super olein, which stays clear at lower temperatures, and palm mid fraction, rich in symmetrical POP triacylglycerols, which is a major component of cocoa butter equivalents. The iodine value of the fractions, higher for olein and lower for stearin, is a quick measure of their composition, and the cloud point indicates the cold stability of olein.
Other applications include fractionation of palm kernel oil to give palm kernel stearin for lauric confectionery fats; fractionation of milk fat to produce fractions for bakery, chocolate and spreadable butter; winterisation of oils (chapter 4), which is a form of fractionation removing small amounts of high-melting material; and fractionation of shea butter to produce shea stearin, rich in SOS, for cocoa butter equivalents. Fractionation, interesterification and blending are often combined in modern fat processing to obtain the solid fat content profile needed for each product without trans fatty acids.
Margarine was invented in France in 1869 by Hippolyte Mège-Mouriès as a cheaper substitute for butter. It is a water-in-oil emulsion: under the Codex standard for fat spreads, margarine contains at least 80 per cent fat, and reduced-fat spreads contain less; national rules on names and fat contents differ. A fat blend, chosen for its solid fat content at refrigerator, room and body temperature, is melted and mixed with emulsifiers (mono- and diglycerides, lecithin), colours (carotenes), flavours, vitamins A and D where fortification is required or permitted, and a water phase containing salt, milk solids or whey and preservatives. The emulsion is pumped through scraped-surface heat exchangers, in which rotating blades scrape crystallising fat from cooled walls (the A units), and pin workers or resting tubes (the B units), which work the crystallising mass to give the required consistency, before packing in tubs or wrappers.
The fat must crystallise in the β′ polymorphic form, which forms small, needle-like crystals that give a smooth, plastic texture and good spreadability. Fats prone to transform to the more stable β form develop large crystals and a grainy, sandy texture over time; palm oil and fats with mixed fatty acid chain lengths tend to stay β′, while rapeseed and sunflower oil-based hard stocks tend to β, so blends are designed accordingly. The solid fat content profile determines spreadability from the refrigerator, stand-up at room temperature and melting in the mouth, measured by pulsed nuclear magnetic resonance.
Shortening is a plastic fat, essentially 100 per cent fat with no water, used in baking and frying. In baking it lubricates gluten, holds air in creamed batters and gives tenderness and flakiness, as described in bakery-pasta-snacks. All-purpose shortenings are plasticised in scraped-surface heat exchangers with a small amount of nitrogen gas to give a white, creamy appearance, and tempered for some days at controlled temperature to stabilise the crystal structure. Specialised shortenings include emulsified (high-ratio) cake shortenings, puff pastry and laminating fats with high plasticity over a wide temperature range, and liquid or pumpable shortenings.
Vanaspati is the Indian name for hydrogenated vegetable fat used as a cheaper substitute for ghee, introduced in the 1930s and long one of the most widely used cooking fats in India. Traditionally it was made by partial hydrogenation of oils such as groundnut, cottonseed and later imported soybean and palm oils, and therefore contained high levels of trans fatty acids. Standards under the old Prevention of Food Adulteration rules required the addition of a small proportion of sesame oil so that adulteration of ghee with vanaspati could be detected by the Baudouin colour test for sesame oil, and required fortification with vitamin A. With FSSAI's limit on trans fatty acids, manufacturers have moved to formulations based on palm stearin, interesterified and fully hydrogenated fats, and blends. The current FSSAI standard for vanaspati, including melting range, permitted oils, vitamin fortification and trans limits, should be consulted, because these requirements have been revised.
Cocoa butter is solid at room temperature and melts sharply just below body temperature, giving chocolate its snap, gloss and melt-in-the-mouth quality. This behaviour comes from its narrow composition: most of its triacylglycerols are symmetrical, with saturated palmitic or stearic acid at the outer positions and oleic acid in the middle, as POP, POS and SOS. Cocoa butter is polymorphic, with six crystal forms, and chocolate must be tempered, by controlled cooling, reheating and seeding, so that it crystallises in form V (β2), which gives gloss, snap and resistance to fat bloom; uncontrolled crystallisation or transformation to form VI causes grey bloom.
Because cocoa butter is expensive and variable in supply, alternative fats are used. Cocoa butter equivalents (CBEs) are non-lauric fats with the same type of symmetrical triacylglycerols, made from palm mid fraction, shea stearin, illipe, sal, kokum and mango kernel fats, often by fractionation and blending, or by enzymatic interesterification. They are fully compatible with cocoa butter and can be mixed with it in any proportion and tempered like it. European Union law allows chocolate to contain up to 5 per cent of vegetable fats other than cocoa butter from six named sources (palm, illipe, sal, shea, kokum gurgi and mango kernel), with labelling; other countries have their own rules. Cocoa butter substitutes (CBS) are lauric fats from palm kernel and coconut oil, which melt sharply and need no tempering but are incompatible with cocoa butter and cause soft texture and bloom if mixed with more than a little of it; they are used in compound coatings with cocoa powder. Cocoa butter replacers (CBR) are non-lauric fats, traditionally partially hydrogenated, that tolerate moderate amounts of cocoa butter.
Many other specialty fats are designed for particular products. Confectionery coating fats, filling fats and ice cream coating fats are formulated for melting behaviour, gloss and resistance to bloom and migration. Bakery fats for laminated pastry need plasticity over a wide temperature range. Frying fats for industrial use are formulated for stability, with high-oleic oils and antioxidants. Dairy fat replacers and vegetable fats for non-dairy creamers and whipped toppings, often based on hydrogenated or fractionated lauric fats, give whipping and stability. Medium-chain triacylglycerols (MCT oils), made from caprylic and capric acids from coconut or palm kernel oil by esterification with glycerol, are absorbed directly into the portal circulation and used in clinical nutrition. Omega-3 oils from fish and algae are refined gently and microencapsulated to protect them from oxidation in foods. Oleogels, liquid oils structured into solid-like materials by waxes, ethylcellulose or other gelators, are being developed to reduce saturated fat in solid fat applications.
Formulating a specialty fat means matching a solid fat content profile, crystallisation rate and polymorphic stability to the product and process, while meeting nutritional targets, cost, origin and sustainability requirements. Sustainability of palm oil, including certification of deforestation-free supply, is an important commercial issue for fat processors.
The acid value is the number of milligrams of potassium hydroxide required to neutralise the free fatty acids in 1 g of fat, determined by titration in neutral solvent. Free fatty acid content is often expressed as a percentage as oleic acid; because the molecular weight of oleic acid is 282 and that of KOH 56.1, FFA (% as oleic) = acid value / 1.99, so an acid value of 2 corresponds to about 1.0 per cent FFA. It measures hydrolysis by lipases or by heat and moisture, and is a key criterion for crude oil price and refining loss and for refined oil quality.
The peroxide value measures hydroperoxides, the primary products of oxidation, usually in milliequivalents of active oxygen per kilogram, by iodometric titration: peroxides liberate iodine from potassium iodide, which is titrated with sodium thiosulphate. Fresh refined oils have very low values, and values rise during storage; because hydroperoxides break down, a low peroxide value in an old or heated oil does not prove good quality. The anisidine value measures secondary oxidation products, mainly aldehydes, and the Totox value, twice the peroxide value plus the anisidine value, combines both: an oil with a peroxide value of 5 and an anisidine value of 10 has a Totox of 20.
The iodine value is the number of grams of iodine absorbed by 100 g of fat, measured by the Wijs method with iodine monochloride, and measures overall unsaturation: pure triolein has an iodine value of about 86, coconut oil less than 10, and linseed oil above 170. It is used to identify oils, follow hydrogenation and characterise fractions. The saponification value is the number of milligrams of KOH required to saponify 1 g of fat, and is inversely related to the average molecular weight of the fatty acids: mean triacylglycerol molecular weight ≈ 3 × 56,100/SV, so a saponification value of 190 corresponds to about 886, typical of C18-rich oils, while lauric fats such as coconut oil, with shorter chains, have saponification values around 250. Unsaponifiable matter is the fraction that does not form soaps, mainly sterols, tocopherols, hydrocarbons and pigments.
Physical tests include colour, by the Lovibond tintometer in red and yellow units; refractive index or butyro-refractometer reading, which rises with unsaturation; specific gravity; melting point and slip point; cloud point and cold test for liquid oils; smoke point; and moisture and impurities. Solid fat content is measured by pulsed nuclear magnetic resonance and is the main specification for fats used in margarine, shortening and confectionery. Fatty acid composition is determined by gas chromatography of fatty acid methyl esters, and triacylglycerol composition by liquid chromatography. Oxidative stability is tested by accelerated methods such as the Rancimat or oxidative stability index, which record the time until volatile acids form at high temperature with air flow.
Adulteration of oils is a long-standing problem in India and elsewhere, and classical colour tests remain in food law and examinations: the Baudouin test detects sesame oil (a red colour with furfural and hydrochloric acid, from sesamolin), the Halphen test detects cottonseed oil (a red colour from cyclopropenoid fatty acids), the Bellier turbidity temperature test detects groundnut oil (from arachidic acid), and tests for argemone oil (nitric acid test, a toxic adulterant of mustard oil that caused epidemic dropsy outbreaks) and mineral oil are standard. The Reichert-Meissl and Polenske values, measuring volatile water-soluble and insoluble fatty acids, detect adulteration of butter and ghee with other fats, because butterfat contains butyric acid. Modern authentication uses fatty acid and sterol profiles, triacylglycerol patterns and spectroscopy.
During deep-fat frying, oil is held at about 150 to 190 °C in contact with air, water and food, conditions that drive three types of degradation, whose chemistry is described in food-chemistry-major-constituents. Hydrolysis by steam from the food releases free fatty acids, mono- and diacylglycerols, lowering the smoke point. Thermal oxidation forms hydroperoxides that rapidly decompose at frying temperatures into volatile aldehydes, ketones and acids, which give fried flavour and, in excess, rancid odours, and non-volatile oxidised triacylglycerols. Polymerisation forms dimers and polymers that increase viscosity, cause foaming, gum deposits on fryers, and darkening. Food components such as sugars, proteins and pigments, and metals such as copper and iron, accelerate degradation, while antioxidants and minor components slow it.
The products of degradation are collectively measured as total polar compounds (TPC), which include free fatty acids, oxidised triacylglycerols, polymers and mono- and diacylglycerols. Fresh oil typically has a few per cent of polar compounds; values rise with use. Heavily used oil has poorer flavour, gives higher oil uptake in food, and contains compounds of health concern; it is also associated with acrylamide and other contaminants in fried foods, as noted in food-toxicology-contaminants-allergens.
Frying oil quality is monitored by total polar compounds, measured by column chromatography in the laboratory or by capacitance-based handheld sensors in kitchens and plants; by free fatty acids; by colour, foaming, smoke and odour; and by rapid test kits. Many countries set discard limits for total polar compounds, typically in the range of about 24 to 27 per cent; FSSAI sets 25 per cent, and requires food businesses using large quantities of oil for frying to keep records and dispose of used oil to authorised collectors. FSSAI's RUCO initiative (Repurpose Used Cooking Oil), launched in 2018, promotes collection of used oil for biodiesel and other non-food uses, to prevent it from being reused in food or sold back into the food chain.
Good frying practice extends oil life and product quality. Choose stable oils, such as high-oleic varieties, palm olein or blends with antioxidants. Keep temperatures no higher than needed, commonly below about 180 °C, and lower or switch off fryers when idle. Filter oil regularly to remove food particles, which burn and catalyse degradation. Top up with fresh oil to replace oil absorbed by food; a high turnover rate (fresh oil added relative to fryer capacity) keeps oil quality steady in industrial fryers. Avoid contact with copper and brass, salt and seasonings in the fryer, and water. Clean fryers thoroughly to remove polymer deposits. Industrial continuous fryers use external heat exchangers, continuous filtration and oil management systems to keep total polar compounds stable. Adsorbent filter powders can remove some free fatty acids and polar compounds, but cannot restore badly degraded oil.
Refining produces several by-products, and recovering their value is part of the economics of an oil refinery. Soapstock from alkali refining contains soaps, neutral oil, phospholipids and impurities. It is acidulated with sulphuric acid to split the soaps, giving acid oil, a mixture of free fatty acids and glycerides used in soap making, animal feed and the oleochemical and biodiesel industries; the acidic wastewater must be treated. Rice bran soapstock is a source of γ-oryzanol. Fatty acid distillate from physical refining, especially palm fatty acid distillate, is a cleaner source of free fatty acids, used for soaps, feed fats and oleochemicals, and is often more valuable than soapstock.
Deodoriser distillate, collected from the vapours of the deodoriser, is rich in tocopherols and phytosterols as well as free fatty acids. It is the main commercial source of natural vitamin E and of plant sterols, which are added to spreads and other foods to lower blood cholesterol. Gums from degumming are processed into lecithin, as described in chapter 3, or added to meal. Waxes recovered by dewaxing, especially rice bran wax, are used in cosmetics, polishes and food coatings.
Spent bleaching earth retains a substantial amount of oil, commonly about 20 to 40 per cent of its weight, together with adsorbed pigments and oxidation products. It is liable to self-heating and spontaneous combustion, so it must be handled carefully, and its oil can be recovered by solvent extraction or pressing for non-food uses; the residue is used in cement kilns, as fuel or in soil applications where permitted, or disposed of in landfill. Spent nickel catalyst from hydrogenation is recovered for its nickel. Hulls removed before extraction are used as fuel or fibre; oilseed meals, the largest by-product of oilseed processing, are used as feed and as food ingredients, as discussed in pulses-oilseeds-plant-proteins.
Effluents from refineries, including soapstock wash water, acid water from acidulation and condensates from deodorisers, carry fats, soaps and acids, and need treatment by fat traps, dissolved air flotation and biological treatment. Using by-products well reduces waste and improves profitability, and is increasingly part of sustainability reporting for refineries. For a technologist, by-product decisions also involve food law: tocopherols and phytosterols for food use must meet purity specifications, and acid oils and fatty acid distillates must not re-enter the food chain where not permitted.
A student uses this subject to follow an oil from seed to bottle and to master the quality constants: what each value measures, how it is determined and what it reveals about an oil's identity, freshness or adulteration. The short calculations recur: oil recovery from a mass balance, alkali for neutralisation, FFA from acid value, mean molecular weight from saponification value, Totox. Running acid and peroxide values and the classical adulteration tests in the laboratory fixes the chemistry.
Five years into an oil mill, refinery or fat-processing plant, a technologist controls degumming and refining losses, sets bleaching earth dose and deodoriser conditions to meet colour, FFA and contaminant targets, runs hydrogenation or interesterification to a solid fat profile, crystallises margarine and shortening, tests incoming oils for quality and adulteration, manages by-products, and keeps products compliant with trans fat, contaminant and labelling rules.
A senior technical or business leader with twenty-five years' experience plans oil sourcing across volatile markets, decides on investments in physical refining, enzymatic degumming or interesterification, designs zero-trans product ranges for bakery, confectionery and food service, manages process contaminant mitigation and sustainability certification, and represents the industry on standards and regulations.
A researcher at this level works on lipid crystallisation and structuring, enzymatic processes and structured lipids, mitigation of 3-MCPD, glycidyl esters and trans isomers, alternative extraction solvents, oxidation and antioxidant mechanisms, frying chemistry, and nutritional effects of fat modification. Such work combines physical chemistry, enzymology, engineering and nutrition.
This manual gives typical ranges for fatty acid compositions, process conditions and quality limits; exact values vary with variety, process and standard, and national standards for oils, vanaspati and trans fats change and must be checked. Expeller and extractor engineering is in post-harvest-cereals-pulses-oilseeds, lipid chemistry and oxidation in food-chemistry-major-constituents, process contaminants in food-toxicology-contaminants-allergens and frying practice in bakery-pasta-snacks. Biodiesel, oleochemicals and detailed reactor design are beyond its scope.
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