Edible Oil

Production process of edible oil

The production process of edible oil typically involves the removal of oil from plant components, such as for example corn, sunflower or rape seeds. This can be done via mechanical extraction using an oil mill or chemical extraction using a solvent. The extracted oil is then purified and, if required, refined or chemically altered. It is during the refinery process that Jongia mixers are in operation. 

In large oil storage tanks side entry mixers, type JDRW/JRWM or RWM are used to keep the contents of the vessel homogeneous and to prevent sedimentation.  In case of a high solids contents and an advanced degree of impurity of the oils top entry mixers may be necessary. 

In particular during the degumming stage  when solvents such as sodiumhydroxide (NaOH) solution and hydrochloric acid are used to remove soapstock and gums from the oil gentle treatment of the product is required. The Jongia L-type mixer is the mixer of choice especially in combination with pitch blade elements or hydrofoils.

Typically the solvents are added in-line either by means of a static mixer or an IM type high shear mixer. After the in-line mixers come the resident tanks where gentle mixing action….

The oil mill route

This option starts with the separation of the soybeans into two fractions: oil and meal. There are, basically,two process alternatives to achieve this purpose: pressing and solvent extraction. Each one of the fractions is then further processed to yield a multitude of products and by-products, with practically no waste. Since oil meal operations are often the starting point in the preparation of soybean protein products, they will be reviewed in some detail in Chapter 3. The processes and products associated with the oil fraction will be described here in some detail. Soybean protein products which branch-off from the meal fraction will be just mentioned here for the sake of completeness and taken up in detail in specific chapters to follow.

Utilization of the oil fraction:

a- Oil refining: The preparation of marketable soybean oil for human consumption from crude soybean oil requires a series of operations known as ” refining “. Several alternative technologies are available for each one of these operations. Each one can be carried out in batchwise, continuous or semi-continuous fashion.

The first step in refining crude soybean oil is the removal of the phospholipids, or “degumming“. Degumming is necessary in order to prevent the separation and settling of gums (sticky, viscous oil-water emulsions stabilized by the phospholipids) during transportation and storage of crude oil, to reduce oil losses in the subsequent phases of refining and to avoid excessive darkening of the oil in the course of high-temperature deodorization. Crude oil is mixed thoroughly with a small amount of water and an acid ( usually phosphoric acid ). “Gums” are formed and precipitated, carrying in the emulsion a certain amount of oil. They are separated by centrifugation, dried under vacuum and bleached. The resulting product consists approximately of 50% phospholipids and 50% oil and has the consistency of honey.

The phospholipid fraction may be separated from practically all the oil by a series of solvent extraction and precipitation processes. Oil free soybean phospholipids are solid. All these by-products of the degumming process are known as “soybean lecithin” and sold under different trade-names and in a variety of quality grades. The principal quality parameters for commercial lecithins are: phospholipid content ( measured as percent acetone insolubles ), free acidity, non-lipid impurities ( measured as hexane insolubles ), viscosity and colour. For certain applications requiring an extremely bland lecithin, the phospholipids are separated from the crude soybean oil fraction, purified and then redissolved in any desired type of refined oil. Lecithins are mainly used for their activity at the interface between fats and hydrophillic phases. They act as emulsifiers in sauces and salad dressings, as viscosity reducers and stabilizers in chocolate, as anti-spattering agents in margarine, as pan release agents in bakery and confectionery , as dough improvers and staling retardants in bread, as wetting agents in instant food powders etc. They also have some antioxidant property.

Degumming is usually carried out at the extraction plant, even if the subsequent steps of refining are performed elsewhere. Whenever further processing of the crude gums is not economically feasible, due to insufficient plant scale or insufficient market demand, the crude gums can be added-back to the meal, increasing the bulk and caloric value of the latter.

There are two major types of processes for refining degummed oil. They differ in the way the free fatty acids are removed. In the “chemical ” or “caustic” refining process,the most common process applied to soybean oil, the fatty acids are neutralized with alkali (sodium hydroxide and sodium carbonate ) to form salts ( soaps ) soluble in water. Treatment with caustic solutions also removes residues of phospholipids not removed by degumming and results in some degree of bleaching due to the destruction of some of the pigments or their adsorption by the heavy phase.

The resulting aqueous soap solution, known as “soap stock” is removed from the neutralized oil by centrifugation. The amount of alkali to be added is calculated according to the free fatty acid content of the oil plus a slight excess (about 0.1%).

Crude soybean oil contains typically 0.3 to 0.7% free fatty acids. After neutralization, the oil is thoroughly mixed with hot soft water to remove traces of soap (washing ), then centrifuged again and dried by heating under vacuum,in preparation to the next step, bleaching. Soap stock can be used for making soap or it can be converted back to fatty acids by treating with a strong mineral acid. The crude mixture of fatty acids obtained, known as “acidified soap stock” can be used as a caloric component in animal feed or for the manufacture of distilled fatty acids. In the “physical refining” process, less commonly applied to soybean oil, fatty acids are removed by steam distillation under high vacuum, simultaneously achieving deodorization. Oil for physical refining must be degummed more thoroughly than in the case of alkali refining process.

The next step of refining is “bleaching‘. Its purpose is to remove the yellow-orange carotenoid pigments and the green chlorophyll of the oil. The extent of bleaching depends on market requirements. The market in the U.S.A. requires almost water-clear appearance while somewhat darker colour may be perfectly acceptable or even preferred in other markets. Bleaching is carried out by treating the oil with solid adsorbents such as Fuller’s earth or activated carbon or both. The pigments and some other impurities are adsorbed on the solid surface and removed by filtration. In order to prevent oxidation, the process is carried out under vacuum. Continuous “in-flow” bleaching processes are available.

The last refining operation is “deodorization“. It consists in the removal of odorous substances by steam distillation under high vacuum and at temperatures in the range of 2500 C. Typically,the deodorizer is a vertical cylindrical vessel with internal baffles and other devices to ensure exposure of a large surface area of oil and intimate contact between the oil and steam. At the end of the stripping process, the oil must be cooled while still under vacuum to prevent oxidation. Citric acid is usually added in order to chelate any metal ions which may catalyze peroxide formation. In modern deodorizers, all the parts in contact with oil are made of stainless steel to prevent such metal contamination. While the main objective of deodorization is the removal of odour-bearing compounds such as aldehydes, ketones and hydrocarbons, other substances such as sterols and tocopherols are also distilled off. In physical refining, this operation is responsible for the removal of free fatty acids. All these substances may be recovered from the deodorizer condensate stream, if necessary.

b- Further processing and utilization of refined soybean oil: Freshly refined soybean oil is practically odourless and bland. However, objectionable off-flavour described as “green, grassy,fishy” is known to develop quickly if the oil is heated (as in cooking and frying), or stored under conditions which expose it to light and oxygen or permits contamination with certain metals such as copper and iron.

This type of flavour deterioration has been called “flavour reversion”, expressing the thought that it brings back the off-flavours of crude oil. Although this has been shown to be false, the term of “flavour reversion” is still used sometimes, when referring to the flavour deterioration of refined soybean oil. The process is apparently triggered by the oxidation of the unsaturated fatty acids and most particularly that of linolenic acid. Unlike oxidative rancidity, flavour reversion occurs at very low levels of oxidation and is not retarded appreciably by antioxidants. It can be retarded by minimizing exposure to oxygen ( bottling under nitrogen ) and to light ( opaque containers, dark glass bottles).

Another method of flavour stabilization is the reduction of the linolenic acid content by selective hydrogenation, followed by chilling ( winterization ) to remove the high melting point saturated fatty acids formed. The partially hydrogenated- winterized soybean oil is perfectly suitable as an all-purpose (salad and cooking) oil. The crystalline fraction separated after chilling is known as “soybean stearin” and used in different solidified fats.

More complete hydrogenation of soybean oil is the basis for the manufacture of shortenings, margarines and tailor-made fats used by various food industries.

Utilization of the meal fraction:

a- Soybean meal as animal feedstuff: By far the largest portion of the soybean oil meal and cake production is used as a protein source in animal feed. Although the terms “meal” and “cake” are often used interchangeably, meal refers to the product of solvent extraction, while cake is the product resulting from expeller pressing of soybeans. The different types of soybean meals are characterized mainly by their protein content and the extent of heat treatment applied in their production to inactivate anti-nutritional factors. If the soybeans are extracted without dehulling,or if the hulls are added back after extraction, the meal will contain about 44% protein. Meals produced from dehulled beans contains approximately 50% protein.

The extent of heat treatment or toasting is measured in terms of residual urease activity or as the solubility of the protein under 

Oils can also be removed via mechanical extraction, termed “crushing” or “pressing”. 

solvent extraction

The processing vegetable oil in commercial applications is commonly done by chemical extraction, using solvent extracts, which produces higher yields and is quicker and less expensive. The most common solvent is petroleum-derived hexane. This technique is used for most of the “newer” industrial oils such as soybean and corn oils.

Supercritical carbon dioxide can be used as a non-toxic alternative to other solvents.

Hydrogenation

Oils may be partially hydrogenated to produce various ingredient oils. Lightly hydrogenated oils have very similar physical characteristics to regular soy oil, but are more resistant to becoming rancid. Margarine oils need to be mostly solid at 32 °C (90 °F) so that the margarine does not melt in warm rooms, yet it needs to be completely liquid at 37 °C (98 °F), so that it doesn’t leave a “lardy” taste in the mouth.

Hardening vegetable oil is done by raising a blend of vegetable oil and a catalyst in near-vacuum to very high temperatures, and introducing hydrogen. This causes the carbon atoms of the oil to break double-bonds with other carbons, each carbon forming a new single-bond with a hydrogen atom. Adding these hydrogen atoms to the oil makes it more solid, raises the smoke point, and makes the oil more stable.

Hydrogenated vegetable oils differ in two major ways from other oils which are equally saturated. During hydrogenation, it is easier for hydrogen to come into contact with the fatty acids on the end of the triglyceride, and less easy for them to come into contact with the center fatty acid. This makes the resulting fat more brittle than a tropical oil; soy margarines are less “spreadable”[compared to?]. The other difference is that trans fatty acids (often called trans fat) are formed in the hydrogenation reactor, and may amount to as much as 40 percent by weight of a partially hydrogenated oil. Hydrogenated oils, especially partially hydrogenated oils with their higher amounts of trans fatty acids are increasingly thought to be unhealthy.

References: Cargill/Bunge/Azersun ADM/IOI Loders Croklaan