Oil Refining Process

Fats and oils processing

Oilseeds

Oil content (%)

Soyabean

19.1

Safflower

36.4

Sunflower

38.7

Rapeseed

40.3

Cotton seed

18.2

tallow is derived from mutton or beef lard is derived from pork.

Introduction • Edible fats and oils are separated from animal tissues, oilseeds and oil bearing fruits. • The purpose is to obtain triglycerides in high yield and purity and to produce co-products of maximum value. • The oilseeds are processed by one of three types of processing: -Expeller or screw press extraction - Prepressed solvent extraction - Expander-solvent extraction • Oil bearing fruits are pressed to obtain oil: - after drying as for copra - after sterilising as for palm fruit - cold pressing as for olives to preserve flavour and odour

Oilseed extraction (Cleaning, dehulling and size reduction) • Cleaning of foreign materials: stems, pods, leaves, grains, dirt, small stones, extraneous seeds using high–capacity vibratory dry screeners/ screens and aspirators. • Cottonseed requires an additional delinting process to remove the white or fuzzy linters from the seed. • After cleaning, seeds are dehulled. Hulls have less than 1% oil but will absorb and retain oil in the press cake and reduce yield. Removal of hull also increases the protein content of the meal. Additionally some oilseed hulls contain high melting waxes that extract with the oil.

Thin and fragile hulls are removed by an impact huller in conjunction with multistage aspiration. Thicker hulls are subjected to a cutting action and often need a beating action to aid separation. The hulls are separated by an aspirator which are further processed to separate the oil and meal.

Oil extraction (size reduction) • The next step is to reduce the seeds to smaller particles. The only exception to this are very small seeds, such as rapeseed or canola. The objective is primarily to reduce the particle size sufficiently to ensure uniform cooking without producing an excessive quantity of fine particles. Size reduction also disrupts the oil–bearing cells. • Breaking is achieved either in hammer mills or on roll stands, with two pairs of fluted rolls mounted one above the other. Coarse grinding helps in roller flaking.

Oil extraction (Flaking) • All seeds must be flaked. Seeds are passed between smooth rolls to produce flakes of typically 0.3-0.4 mm thickness. Rolls are hydraulically loaded and are typically 600-700 mm in diameter and 1000-1500 mm long. • Flaking helps to rupture the cell structure and make the oil more readily available. • Production of excessive fines should be avoided as fines create problems in later processing. • Temperature and moisture should be monitored as too cold or too dry seeds will shatter than flake. • In some cases, seeds are conditioned by raising the temperature to 60 oC without significant drying. This helps to make the seed more pliable and give good flakes.

Oilseed extraction (cooking) •

The next step is to heat or cook the ground or flaked oilseed. Cooking temperature varies depending upon seed size; ranges from 85-105 oC. Steam at 6-10 bar gauge pressure is used.

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Cooked seeds are dried to 3-5% moisture for efficient operation of the screw press. Seeds which are solvent extracted without pre pressing such as soyabeans are conditioned to 60-65 o C prior to flaking.

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Cooking temperature in the range from 57-85 oC should be avoided as phospholipase D that makes the phosphatides nonhydratable and more difficult to remove is active in this temperature range.

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Proper cooking results in: – – – – – – – –

Complete breakdown of the oil cells by flashing off intrinsic moisture as steam Coagulation of the proteins to facilitate oil and meal separation Insolubilisation of the phospholipids Increased fluidity of the oil at higher temperatures Destruction of molds and bacteria Inactivation of enzymes Drying to a suitable moisture content Destroys gossipol in cottonseed, a complex polyphenolic compound that is toxic to swine and poultry

• The adverse effect of enzymes upon oils are: – Lipase causes an increase in FFAs – Lipoxygenase causes formation of peroxides and secondary oxidation products – Phospholipase causes increases the nonhydratable phospholipids – Myrosinase causes formation of sulfur compounds, develops meal flavour and digestive problems

Oilseed extraction (cooking in an expander) • The expander is a low shear extruder that heats , homogenises and shapes oilseeds into porous collets or pellets with a high bulk density.

• Steam is injected into the oilseed flakes or cakes in the expander, under pressure, and then this mixture is extruded through dies to the atmosphere. • The collets expand as pressure is released, hence the name expander. • Some expanders have a drainage cage to reduce the oil content of high oilseeds to less than 30%, thus enabling the production of intact collets for direct solvent extraction instead of the prepress extraction process. • The high bulk density of the collets improves the efficiency of solvent extraction process by 15-30% due to higher extraction, low retention time and lower level of nonhydratable phosphatides.

Oil extraction (Expeller or screw press) • Expeller pressing mechanically squeezes the oil from the seed. Used for seeds with relatively high oil content. • In the screw press, the cooked flakes are separated into crude oil and press cake. The press cake has 3-10% oil and used as animal feed. The oil is allowed to settle and then filtered and transferred to refiners. Continuous screw presses are also available. • Solids in this oil can be simply removed screening the oil over either a static or a vibratory screen. Final clarification is done in a hermetically-sealed filter with stainless steel leaves in which the material itself is used to generate a pre coat through which the oil is filtered. • In large plants, horizontal decanters are used in place of filters. The solids that settle down are again returned to the press feed stream for reprocessing.

Oil extraction (prepress solvent extraction) • In prepress solvent extraction, a portion of the oil is removed with expellers and the remainder of the oil is extracted with an organic solvent. • During prepressing, the expeller is choked so that less pressure develops and less oil is recovered. The oil content of the prepress cake is 15-18%. The remaining oil in this partially deoiled cake is then solvent extracted using the same procedure as for direct solvent extraction. • Both the oil fractions are mixed before refining. • The advantages are that the capacity of screw press is increased and a small solvent extraction plant is required to recover the oil from the deoiled press cake. • The cake produces from a high-pressure press can be quite large pieces, which becomes very hard on cooling. Breaking rolls are used.

Oil extraction (direct solvent extraction) •

This process removes the oil directly from conditioned oilseeds with an organic solvent.

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Used for soybeans; it is problematic for oilseeds with high oil content , such as cottonseed, sunflower, rapeseed, safflower, and peanut. The oil content of the oilseed flakes caused them to disintegrate into fines during the extraction process.

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For these high oil seeds, a low–shear extrusion method is followed before solvent extraction.

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Process steps: – Leach the oil out of the cake, flakes or collets with a solvent, usually hexane. It is done at 50-55 oC due to the vapour pressure of hexane. – Oil is separated from solvent from the miscella by conventional distillation method. – The recovered solvent is separated from the accumulated moisture in a gravity separation tank and reused in a solvent extraction operation. – The hexane free oil is cooled and filtered.

Oil extraction from oil bearing plants • Oil bearing plants of commercial importance are olive and palm. • Their oil recovery process is different from those used for oilseeds and animal tissues. Olives 1. Olives soon after harvest are processed to prevent rise in acidity. The foreign material is removed and olives are washed. 2. The olives are ground or milled to a coarse paste to release the oil from oil bearing cells and formation of large oil droplets from the smaller droplets. 3. The paste is subjected to any one of the three methods: hydraulic press, continuous centrifuge or adhesion filtering. 4. The adhesion filtering unit has series of steel blades that are dipped into the olive paste and then withdrawn, after which the oil is allowed to drip off the blades. 5. The husks or residue are dried and oil is further extracted using solvent. 6. Two oil types are obtained from olives: 1. 2.

Olive oil which is pressed without further processing (other than washing, decantation, centrifugation, and filtration) and contains less than 3.5 % FFA. Pomace oil that is obtained by solvent extraction and does not qualify as olive oil.

Oil extraction for palm oil • Palm oil is extracted by cooking and pressing. • The palm is sterilised with steam at 130-145o C for about an hour to: – Inactivate hydrolytic enzymes – Loosen the individual fruits for subsequent processing

• The sterilised fruits are removed from the stalk by vigorous shaking and reheated to 95-100 oC for 20-30 min in a digester to loosen the pericarp from the nuts and to break the oil cells. • The liquid is separated from the semi solid phase in a screw press followed by centrifugation and vacuum drying to remove the moisture. • Overripe fruit bruises easily, accelerates FFA rise through enzymatic hydrolysis and adversely affects bleachability of the extracted oil.

Animal fat recovery • Animal fat is recovered from fatty tissues by cooking process known as rendering, either wet or dry rendering. Wet rendering process is followed for obtaining edible animal fat. • The fatty tissues are cut into small pieces of 2-5 cm with a crusher. The pieces are cooked in batch or continuous cooker with agitation to evaporate the moisture, breakdown the fat cells and release the fat. • The cooked fatty tissues are subjected to wet rendering. Wet rendering can be done below boiling point of water or using steam at 40-60 psi (2.7-4 bar) and a corresponding high temperature. Under these conditions, the fats separates from the solids and rises to the top of the vessel. The fat is drawn off and purified by settling or with a centrifuge. There is generation of ~o.35% FFA during wet rendering.