Oilseed processing and edible oil refining

In short

Oilseed processing converts seeds such as groundnut, mustard/rapeseed, soybean, sunflower, sesame, cottonseed, coconut and rice bran into crude oil plus a protein-rich cake, and refining then removes gums, free fatty acids, colour, odour, waxes and contaminants to give a bland, stable, food-grade oil. India is the world's largest importer of edible oil (roughly 55-60% of consumption is imported, chiefly palm, soybean and sunflower), so every step of yield and refining loss carries direct commercial weight. The topic is examined in Food Engineering, Fats and Oils Technology and Unit Operations papers, and in GATE (XE/Food Technology) through extraction, mass balance and refining-loss numericals. Employers who test it include Adani Wilmar, Cargill, Ruchi/Patanjali, Marico, Emami Agrotech, Bunge, ITC, Godrej Agrovet, Mother Dairy (Dhara) and the National Dairy Development Board's oil division, plus FSSAI and Agmark inspection roles.

Explained

Oilseeds, oil composition and quality indices

• Major Indian oilseeds (nine "Agmark" oilseeds): groundnut, mustard/rapeseed, soybean, sunflower, sesame, safflower, niger, linseed, castor (castor is non-edible). Plus the non-conventional sources: rice bran, cottonseed, coconut, palm and palm kernel.
• Typical oil content (dry basis): sesame 45-52%, groundnut kernel 45-50%, mustard 38-42%, sunflower seed 38-45%, palm kernel 45-50%, soybean only 18-20%, rice bran 16-22%, cottonseed kernel 32-36%.
• The choice of process follows oil content: high-oil seeds (>25%) are pressed, then the cake is solvent-extracted; low-oil seeds (<25%) go directly to solvent extraction (direct or "straight" solvent extraction, as for soybean and rice bran).
• Oil chemistry: oils are triacylglycerols (triglycerides) of fatty acids, with minor components - phospholipids (gums), free fatty acids (FFA), mono- and diglycerides, sterols, tocopherols (natural antioxidants), waxes, pigments (carotenoids, chlorophyll), oryzanol (rice bran), gossypol (cottonseed), glucosinolates and erucic acid (mustard), sesamol (sesame), aflatoxin risk (groundnut).
• Quality indices: FFA % (as oleic acid), acid value, peroxide value (PV, meq O₂/kg - primary oxidation), iodine value (IV, g I₂/100 g - unsaturation), saponification value (SV, mg KOH/g - inverse of mean chain length), unsaponifiable matter, moisture and insoluble impurities (M&I), smoke point, colour (Lovibond), p-anisidine value (secondary oxidation), slip melting point, solid fat index/content (SFI/SFC).
• Rancidity: *hydrolytic* (lipase/moisture splits triglycerides → FFA; severe in rice bran, where FFA can rise 1-2% per day if bran is not stabilised) and *oxidative* (autoxidation via free radicals → peroxides → aldehydes/ketones; accelerated by light, heat, metals Cu/Fe, and unsaturation).

Pre-treatment and mechanical expression

1. Cleaning: magnetic separator, sieves/destoner and aspirator remove metal, sand, chaff; dirt causes wear and darkens oil.
2. Drying/conditioning: moisture brought to about 8-10% for decortication, later adjusted for flaking.
3. Dehulling/decortication: groundnut, sunflower, cottonseed, soybean - removing hull raises oil content of the feed, reduces oil retention in cake and raises cake protein. Hulls are abrasive and hold oil.
4. Size reduction: cracking rolls then flaking rolls to 0.2-0.4 mm flakes - rupturing cells shortens the diffusion path, the key to solvent extraction efficiency.
5. Cooking/conditioning (90-110 °C, 20-30 min): coagulates protein, breaks oil cells, reduces oil viscosity, inactivates lipase and myrosinase, and adjusts moisture to about 3% - this is what makes pressing possible.
6. Expression:
   • Ghani/kolhu (traditional): mortar-and-pestle type, low throughput, "kachi ghani" cold-pressed mustard oil sold for flavour and pungency (allyl isothiocyanate).
   • Hydraulic press (batch): cake wrapped in cloth, pressed at 100-200 bar; low capacity, residual oil 6-10%.
   • Screw press / expeller (continuous): a tapered worm in a barrel cage raises pressure along the length; residual oil 4-8% (single press) or 3-5% (double press). Pre-press is a partial press for high-oil seeds leaving 15-20% oil for extraction.
   • Cold pressing: below about 50-60 °C, retains flavour, tocopherols and colour; lower yield, higher cost - the basis of premium virgin coconut, mustard and sesame oils.
7. Clarification: screening, settling and filter press/decanter remove press "foots"; the oil is crude oil.

Solvent extraction

• Principle: leaching - oil diffuses out of prepared flakes/cake into food-grade hexane (n-hexane, bp 67-69 °C, specific gravity ≈ 0.66), chosen for selectivity for oil, low latent heat, easy recovery and low cost. Hexane loss is typically 1-3 kg per tonne of material.
• Feed preparation: flakes, expanders/collets (porous extrudates that improve percolation and raise bed density) or pre-press cake; bed must percolate, not pack.
• Extractors: immersion type (counter-current screw, for rice bran) and the more common percolation types - Rotocel, Carousel (rotary basket), Stationary-basket, Reflex/De Smet belt (moving-bed) extractors - all counter-current, giving 20-40 wt% oil "full miscella".
• Residual oil in meal: 0.5-1.5%, far below pressing - the main reason solvent extraction dominates.
• Downstream:
  • Miscella distillation: two or three stage evaporators plus a stripping column under vacuum, with steam stripping, recover hexane; final oil must be hexane-free.
  • Desolventiser-toaster (DT): removes solvent from the meal with steam and heat; "toasting" also destroys trypsin inhibitor and urease in soybean. DTDC (desolventiser-toaster-dryer-cooler) finishes the meal.
  • Solvent recovery: condensers, water-hexane separator (decanter), mineral-oil absorption of vent gases.
• Safety: hexane-air mixtures are explosive (LEL ≈ 1.1%); plants are flameproof-wired, earthed, nitrogen/steam purged, with strict hot-work control. Occupational safety is now governed by the Occupational Safety, Health and Working Conditions Code, 2020 (in force 21 November 2025), which replaced the Factories Act, 1948; hexane storage falls under petroleum/explosives licensing (PESO).
• Products: crude solvent-extracted oil (needs refining; it is not fit for direct edible use) and deoiled cake (DOC) - a major Indian export used as protein feed.

Refining: chemical and physical routes

The classic sequence is degumming → neutralisation → bleaching → (winterisation/dewaxing) → deodorisation, abbreviated by the industry as refining-bleaching-deodorising (RBD oil).

1. Degumming - removes phospholipids that would cause foaming, dark colour and settling.
   • *Water degumming:* 1-3% hot water at 60-70 °C, 30 min, hydratable gums swell and are centrifuged out → lecithin (a saleable soybean co-product).
   • *Acid degumming:* 0.05-0.2% phosphoric or citric acid removes non-hydratable (Ca/Mg salts of phosphatidic acid) gums; also chelates trace metals.
   • *Enzymatic degumming:* phospholipase A1 converts phospholipids to water-soluble lysophospholipids (phospholipase C instead splits off diacylglycerol, which stays in the oil), cutting oil loss in the gum phase.
   • Target: residual phosphorus below about 10 ppm for chemical refining, and below about 5 ppm (ideally under 3 ppm) for physical refining, which is the stricter requirement.
2. Neutralisation (alkali refining) - caustic soda (NaOH, 10-24 °Bé) reacts with FFA to form soapstock, removed by centrifuge; followed by hot-water washing and vacuum drying. Soapstock is acidulated to acid oil for soap/oleochemicals.
   • Theoretical caustic = stoichiometric NaOH for the FFA; excess (treat) of 0.05-0.3% covers pigments and non-oil acids.
3. Bleaching (adsorptive, not chemical whitening) - 0.2-2% activated/acid-activated bleaching earth (bentonite/Fuller's earth), sometimes with activated carbon or silica, at 90-110 °C under vacuum for 20-30 min, then filtered on a pressure leaf filter. Removes pigments, soap traces, metals, oxidation products and part of the peroxides. Spent earth retains 20-35% oil and is a fire hazard (spontaneous heating).
4. Winterisation / dewaxing / fractionation - chilling to 5-10 °C and holding, then filtering to remove waxes (sunflower, rice bran) or high-melting glycerides; dry fractionation of palm oil gives palm stearin and liquid palm olein (the cooking oil sold in India).
5. Deodorisation - steam stripping (sparge steam 0.5-3%) at 180-260 °C under 2-5 mbar absolute for 20-120 min in a packed or tray column; removes odour, residual FFA, aldehydes, pesticide residues and some carotenoids (heat bleaching). Tocopherols distilled off are recovered from the "deodoriser distillate" (vitamin E source; oryzanol from rice bran oil).
   • Over-heating (> about 240 °C with long hold) generates trans fatty acids and 3-MCPD esters/glycidyl esters, so modern practice uses lower temperature with longer vacuum-steam contact and good chloride control.
6. Physical (steam) refining - degumming + bleaching followed by combined deacidification-deodorisation in one high-vacuum column; no caustic, so no soapstock and far less effluent. Best for high-FFA, low-gum oils (palm, coconut, rice bran). Chemical refining suits high-gum, low-FFA oils (soybean, mustard).
7. Polishing/filtration, antioxidant addition and nitrogen blanketing before packing; vitamin A and D fortification (the +F logo) under the Food Safety and Standards (Fortification of Foods) Regulations, 2018.

Modification of fats and oils

• Hydrogenation: H₂ added across double bonds over a nickel catalyst at 120-200 °C, 1-4 bar, with agitation; raises melting point and oxidative stability → vanaspati. Partial hydrogenation isomerises cis to trans fatty acids (elaidic acid).
  • Selectivity: linolenic → linoleic → oleic → stearic; high selectivity, low H₂ pressure, high temperature and low agitation favour trans formation.
  • India's FSSAI limit on industrial trans fat is not more than 2% by mass of total oils/fats in edible oils, fats and all foods containing them (fully in force from 1 January 2022).
• Interesterification and enzymatic interesterification: rearranging fatty acids among glycerol backbones (chemical catalyst sodium methoxide, or an immobilised sn-1,3-specific lipase) to build hard stock without trans fat - the main modern replacement for partial hydrogenation.
• Blending: mixing a liquid oil with a harder fat, or two oils for fatty-acid balance. FSSAI permits blended edible vegetable oil with a minimum 20% by weight of each of only two oils, labelled accordingly; multi-source edible oil declarations must follow the labelling regulations.
• Fractionation: dry (crystallisation + filtration), solvent (acetone/hexane) and detergent fractionation to separate stearin and olein.
• Texturisation: margarine and bakery shortening production via votator (scraped-surface heat exchanger) crystallisation and pin-working.

Standards, regulators and by-product utilisation

• Food Safety and Standards Act, 2006, with the Food Products Standards and Food Additives Regulations, 2011, set identity standards for each oil - acid value/FFA, iodine value, saponification value, refractive index (butyro-refractometer reading), Bellier turbidity temperature, unsaponifiable matter, and tests for argemone oil (mustard), mineral oil and castor oil. Limits on PV, M&I, antioxidants (TBHQ, BHA, BHT) and metals are in the FSS (Contaminants, Toxins and Residues) Regulations, 2011.
• Refined oil typically requires acid value not more than 0.5 mg KOH/g (about 0.25% FFA); raw/expeller "kachi ghani" mustard oil is allowed a much higher acid value - the exact figure for each oil must be read from the current FSSAI standard, which is amended frequently.
• Loose/unpacked edible oil sale is prohibited; oil must be sold packed and labelled, and AGMARK grading under the Agricultural Produce (Grading and Marking) Act, 1937 is voluntary (except where a specific order requires it) and is used for mustard, groundnut and blended oils.
• Bureau of Indian Standards publishes methods and specifications for oilseeds, oils and their analysis; quote a specific IS number only after checking its current edition, since several older oil specifications have been withdrawn in favour of FSSAI standards.
• By-products: oil cake and DOC (soybean, groundnut, mustard, rice bran) as cattle/poultry feed and export; lecithin; soapstock/acid oil; deodoriser distillate (tocopherols, sterols, oryzanol); spent earth; hulls as fuel/fibre; glycerol from splitting; biodiesel from non-edible oils (castor, jatropha, used cooking oil - FSSAI's RUCO initiative diverts used cooking oil to biodiesel).
• Missions and economics: the National Mission on Edible Oils (Oilseeds, and the Oil Palm sub-mission) targets import substitution; refining margins turn on refining loss, usually benchmarked against the Wesson loss or the 1.5-3× FFA rule of thumb. Edible oils attract the 5% GST slab (slabs since 22 September 2025 are 5%/18%/40%); confirm the current rate for a specific oil before quoting it commercially.

Key points

• Pre-press + solvent extraction - the standard route for high-oil seeds; direct solvent extraction for soybean and rice bran.
• Flaking - reduces diffusion path; the single biggest determinant of extraction rate and residual oil.
• Cooking - coagulates protein, ruptures cells, lowers viscosity, inactivates lipase; without it pressing yields poorly.
• Residual oil - 4-8% in expeller cake, 0.5-1.5% in solvent-extracted meal.
• Hexane - food-grade n-hexane, bp 67-69 °C, LEL ≈ 1.1%; losses 1-3 kg/tonne; flameproof plant mandatory.
• DT/DTDC - desolventiser-toaster(-dryer-cooler): strips hexane from meal and destroys soybean trypsin inhibitor.
• Degumming - removes phospholipids; water for hydratable, acid or enzymatic for non-hydratable gums; by-product lecithin.
• Neutralisation - NaOH converts FFA to soapstock; theoretical caustic plus 0.05-0.3% excess "treat".
• Bleaching - adsorption on activated earth at 90-110 °C under vacuum; removes pigments, soap, metals, peroxides.
• Deodorisation - steam stripping at 180-260 °C and 2-5 mbar; removes odorants and residual FFA; yields tocopherol-rich distillate.
• Physical refining - degum, bleach, then steam deacidify-deodorise; no soapstock, minimal effluent; suits palm, coconut, rice bran.
• Winterisation/dewaxing - chill and filter to remove waxes (sunflower, rice bran) and keep oil clear in cold weather.
• Fractionation of palm oil - gives palm stearin (hard) and palm olein (liquid cooking oil).
• Hydrogenation - Ni catalyst, 120-200 °C; makes vanaspati but creates trans fat; FSSAI caps industrial trans fat at 2% of total fat.
• Interesterification - trans-free route to hard stock, chemically or with sn-1,3-specific lipase.
• Rice bran - must be stabilised (extrusion/steam) within hours because lipase raises FFA by 1-2% per day.
• Key quality indices - FFA/acid value (hydrolysis), PV and p-anisidine (oxidation), IV (unsaturation), SV (chain length).
• Refining loss - typically 1.5-3 times the FFA content for chemically refined oil; the plant's profit lever.

Formulas and examples

Example 1 - Overall mass balance on a pre-press/solvent plant. 1,000 kg of groundnut kernel with 48% oil is pre-pressed, leaving a cake with 18% oil; the cake is then extracted to 1% residual oil in the meal.

• Oil in feed = 480 kg; non-oil (meal solids) = 520 kg.
• Pre-press cake: solids 520 kg are 82% of the cake → cake = 520/0.82 = 634.1 kg, containing 0.18 × 634.1 = 114.1 kg oil.
• Press oil = 480 − 114.1 = 365.9 kg.
• Final meal: solids 520 kg are 99% → meal = 520/0.99 = 525.3 kg, oil in meal = 5.3 kg.
• Extracted oil = 114.1 − 5.3 = 108.8 kg. Total crude oil = 365.9 + 108.8 = 474.7 kg → overall oil recovery = 474.7/480 = 98.9%; yield on seed = 47.5%.
• Extraction-stage efficiency = (114.1 − 5.3)/114.1 = 95.4%.

Example 2 - FFA, acid value and caustic requirement. A 5.0 g sample of crude soybean oil needs 4.5 mL of 0.1 N KOH.

• AV = 56.1 × 0.1 × 4.5 / 5.0 = 5.05 mg KOH/g.
• FFA = 28.2 × 0.1 × 4.5 / 5.0 = 2.54% as oleic acid (check: 5.05/1.99 = 2.54 ✓).
• For a 10,000 kg batch: FFA mass = 0.0254 × 10,000 = 254 kg.
• Theoretical NaOH = 254 × 40/282 = 36.0 kg; with a 0.15% excess treat on oil = 15 kg, total ≈ 51 kg of NaOH (as 100%); as a 12 °Bé lye (≈ 8% NaOH) this is about 640 kg of solution.
• Expected refining loss at a factor of 2 = 2 × 2.54 = 5.1%, i.e. about 508 kg lost as soapstock and entrained oil.

Example 3 - Peroxide value and shelf-life judgement. A 10.0 g sunflower oil sample consumes 1.6 mL of 0.01 N sodium thiosulphate.

• PV = 0.01 × 1.6 × 1000 / 10.0 = 1.6 meq O₂/kg - fresh refined oil (typical limit for refined oil is around 10 meq O₂/kg; read the current FSSAI figure for the specific oil).
• If after 6 months the same oil titrates 12.0 mL, PV = 0.01 × 12.0 × 1000/10 = 12 meq O₂/kg - beyond the usual refined-oil limit, indicating oxidative rancidity.

Example 4 - Hydrogenation: hydrogen demand and trans check. A soybean oil of IV 130 is hydrogenated to IV 70 to make a vanaspati base; batch size 5,000 kg.

• ΔIV = 60 units. H₂ per kg oil = 60 × 0.0794 = 4.76 g/kg.
• Total H₂ = 4.76 × 5,000 = 23,800 g ≈ 23.8 kg, i.e. 23.8/2.016 = 11.8 kmol → about 264 m³ at STP (11.8 × 22.4).
• If the product analyses 6% trans fat, it fails the FSSAI 2% industrial trans-fat limit; the plant must switch to interesterified/fully hydrogenated hard stock blended with liquid oil.

Example 5 - Rice bran stabilisation economics. Fresh rice bran (20% oil) has 4% FFA; unstabilised bran gains 1.5% FFA per day and is extracted after 5 days.

• FFA at extraction = 4 + 5 × 1.5 = 11.5%.
• Refining loss at a factor of 2 = 23% of the crude oil, against 2 × 4 = 8% if extracted fresh - a 15 percentage-point loss.
• On 100 tonnes of bran → 20 tonnes crude oil; extra loss = 0.15 × 20 = 3 tonnes of oil. At ₹110 per kg of refined rice bran oil that is about ₹3.3 lakh lost per 100 tonnes of bran - which is why extrusion stabilisation pays for itself.

Where it is used

• Large refineries and port-based plants: Adani Wilmar (Fortune), Cargill, Bunge, Emami Agrotech, Patanjali/Ruchi Soya, Marico (Saffola), Godrej Agrovet - crude palm/soy/sunflower imported and refined at Kandla, Mundra, Krishnapatnam, Kakinada, Haldia.
• Oilseed belts and co-operatives: groundnut in Gujarat and Rajasthan, mustard in Rajasthan, UP and Haryana, soybean in Madhya Pradesh and Maharashtra, coconut in Kerala and Tamil Nadu; NDDB/Dhara and state federations run processing and blending units.
• Rice bran oil industry: solvent extraction units attached to rice mills in Punjab, Haryana, AP and Telangana; oryzanol and wax as high-value co-products.
• Job roles: production/shift engineer (extraction, refinery), QC chemist (titrations, Lovibond, GC for fatty-acid profile and trans fat), process/R&D engineer (yield and loss optimisation), FSSAI-trained food safety supervisor, Agmark/BIS grader, hexane plant safety officer.
• Downstream users: vanaspati, bakery shortening and margarine plants, snack and namkeen frying lines, soap and oleochemical units (acid oil, glycerol), feed mills buying DOC, biodiesel units using RUCO used cooking oil.
• Regulatory and testing labs: FSSAI-notified laboratories and the Solvent Extractors' Association's testing network, checking argemone and mineral oil adulteration, PV, trans fat and 3-MCPD esters.

Interview questions

• Q: Why is soybean extracted directly with solvent while groundnut is pre-pressed? A: Soybean has only 18-20% oil, too little for economic pressing, and flakes percolate well; groundnut at 45-50% would flood the extractor, so most oil is pressed out first, leaving 15-20% for extraction.
• Q: What does cooking before pressing achieve? A: It coagulates protein and ruptures oil cells, lowers oil viscosity, adjusts moisture to about 3% and inactivates lipase and enzymes, all of which raise press yield and keep FFA low.
• Q: Why is n-hexane preferred over other solvents? A: It dissolves oil selectively, has a low boiling point (67-69 °C) and low latent heat for easy recovery, is cheap and does not react with the oil - its drawback is flammability and toxicity, so recovery and flameproof design are essential.
• Q: What is the purpose of the desolventiser-toaster? A: It strips residual hexane from the meal with steam and heat, and the toasting step destroys anti-nutritional factors such as trypsin inhibitor and urease in soybean meal.
• Q: Difference between hydratable and non-hydratable gums? A: Hydratable phospholipids (phosphatidylcholine, phosphatidylinositol) swell with water and centrifuge out; non-hydratable ones, mainly Ca/Mg salts of phosphatidic acid, need acid (phosphoric/citric) or enzymatic degumming.
• Q: Why is deodorisation done under high vacuum? A: Vacuum lowers the stripping temperature needed to volatilise odorants and FFA, cuts steam consumption, and prevents oxidation and trans-fat formation that high temperature in air would cause.
• Q: When would you choose physical refining over chemical refining? A: For high-FFA, low-phosphatide oils such as palm, coconut and rice bran, because deacidification by steam stripping avoids caustic, soapstock and the heavy effluent load, and gives better oil recovery.
• Q: What causes the FFA of rice bran oil to be so high? A: Milling ruptures the bran and brings active lipase into contact with the oil, so FFA rises 1-2% per day unless the bran is stabilised by extrusion or steam within a few hours of milling.
• Q: How is trans fat formed and what does Indian law allow? A: Partial hydrogenation isomerises cis double bonds to trans; severe deodorisation adds a little. FSSAI limits industrial trans fat to not more than 2% by mass of total oils and fats in edible oils, fats and foods containing them.
• Q: What is winterisation and which oils need it? A: Chilling the oil so that waxes and high-melting glycerides crystallise and are filtered out, so the bottled oil stays clear in winter - needed mainly for sunflower, rice bran and some cottonseed oil.
• Q: How do you test mustard oil for argemone adulteration? A: The nitric acid test - a red-brown colour in the acid layer indicates argemone oil - confirmed by paper chromatography/HPLC for sanguinarine; argemone oil causes epidemic dropsy and is prohibited.
• Q: Name the economically useful by-products of a refinery. A: Lecithin from degumming, soapstock/acid oil from neutralisation, spent earth, wax and stearin from winterisation/fractionation, and deodoriser distillate rich in tocopherols, sterols and oryzanol.

Common mistakes

• Treating "oil content of seed" and "oil yield" as the same number - yield is always less, by the residual oil in cake or meal and the refining loss.
• Writing mass balances on total mass instead of oil-free solids, which are what remain constant through pressing and extraction.
• Confusing the FFA conversion factors: acid value uses 56.1 (KOH), FFA as oleic uses 28.2, and AV ÷ 1.99 = FFA%.
• Saying bleaching "whitens" oil chemically - it is physical adsorption on activated earth, and over-dosing strips tocopherols and costs oil in the spent cake.
• Claiming deodorisation removes FFA completely or that it is the deacidification step in chemical refining - caustic neutralisation does that; deodorisation only strips the residue.
• Quoting an FSSAI or IS limit from memory without noting that the standards are amended often, and mixing up raw "kachi ghani" limits with refined-oil limits.
• Forgetting hexane's explosion hazard and the solvent-free requirement for both oil and meal when asked about plant design or safety.

Related topics

Fats and oils chemistry: rancidity and antioxidants · Unit operations: leaching, distillation and evaporation · Frying and bakery fats · Food analysis and quality control of fats · Food safety law: FSSAI standards and labelling · Food plant safety and effluent treatment