1.  Introduction

The quality of fried foods depends not only on the type of foods and frying conditions, but also on the oil used for frying. Thus, the selection of stable frying oils of good quality is of great importance to maintain a low deterioration during frying and consequently a high quality of the fried foods.

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Many refined oils and fats are used for frying and the ideal oil composition may be different depending on technical or nutritional considerations. In general, the decision is influenced by many factors amongst which functionality, nutritional properties, cost and availability stand out. Palm olein and partially hydrogenated oils have been considered the most stable oils for frying although, in the last decades, development of genetically modified seeds containing oils with a lower degree of unsaturation than those of the traditional oils has significantly increased the availability of oils of high thermostability in the marketplace [1,2].

However, whatever the oil or fat used, its initial quality may vary significantly and affect the rate of deterioration during frying. Thus, extraction of good-quality seeds and the appropriate development of the different steps in the refining process to fulfill frying oil specifications are necessary. This is the only guarantee for obtaining the best frying performance of the selected oil.

In this article, the main steps of the refining process are discussed briefly with special reference to the changes in the crude oils and their importance in the production of high-quality oils. For complete information on the different conditions and equipments used in the different steps, a wide literature is available [3&#;5]. Also, specifications for refined frying oils are given and justified. More detailed discussion of the various steps in the refining process is available here..

2.  The Refining Process

Refining of crude oil is done to remove unwanted minor components that make oils unappealing to consumers, while trying to cause the least possible damage to the neutral oil as well as minimum refining loss. The components to be removed are all those glyceridic and nonglyceridic compounds that are detrimental to the flavour, colour, stability or safety of the refined oils. They are primarily phosphoacylglycerols, free fatty acids, pigments, volatiles and contaminants.

On the other hand, not all the minor compounds in fats and oils are undesirable. For example, phytosterols are considered of nutritional interest, and tocopherols with vitamin E activity, protecting the oil against oxidation are highly appreciated. Consequently, to reach the maximum oil quality all the steps of the refining process should be carried out with the minimum losses of desirable compounds.

The major steps involved and the main components removed are shown in Table 1. As can be observed, alkali (or chemical) and physical refining are the standard processes used. The main difference between the processes is that alkali refining procedure includes caustic soda treatment to neutralise the oil while, following physical refining, free fatty acid are eliminated by distillation during deodorization. Physical refining reduces the loss of neutral oil, minimises pollution and enables recovery of high-quality free fatty acids. Nevertheless, not all oils can be physically refined.

Table 1. Basic steps of the refining process Alkali or chemical refiningMain groups of compounds removedPhysical Refining Degumming     Phospholipids Degumming Neutralization     Free fatty acids - Bleaching     Pigments/metals/soaps Bleaching Winterization     Waxes/saturated triacylglycerols Winterization Deodorization     Volatiles/free fatty acids Deodorization/ deacidification  

2.1  Degummlng

The purpose of degumming is to remove phospholipids or gums from the crude oil. Two types of phospholipids are present in crude oils according to their level of hydration, i.e. hydratable and nonhydratable ones, the latter mainly present as calcium and/or magnesium salts of phosphatidic acid and phosphatidylethanolamine. After addition of water (1-3%), most of the phospholipids are hydrated and are insoluble in the oil. The hydrated compounds can be efficiently separated by filtration or centrifugation. For the elimination of the nonhydratable fraction, the oil is usually treated with phosphoric acid (0.05 to 1%), which chelates the Ca and Mg converting the phosphatides into the hydratable forms (the acid treatment has the additional function of chelating trace prooxidant metals). Due to the variable content of phospholipids in crude oils, analysis of phosphorus prior to acid treatment is necessary to ensure that the acid dosage is correct, especially when the content of Ca and Mg salts is high.

Depending on the oil composition, the degumming step can be eliminated as the phosphatides are also removed along with the soaps during the next step of neutralization. However, degumming is mandatory for physical refining and the content of phosphorus after degumming should be lower than 10 mg/kg [6].

2.2  Neutralization

In this step, the oil is treated with caustic soda (sodium hydroxide) and free fatty acids are converted into insoluble soaps, which can be easily separated by centrifugation. Thus, the main objective of this step is the removal of free fatty acids, although, as commented above, residual phospholipids in degummed oils or all the phospholipids in the crude oils are also removed as insoluble hydrates. Also, caustic neutralization improves significantly the oil colour partly by reacting with polar compounds (gossypol, sesamol, sterols, hydroxy fatty acids, etc) and partly by solubilization. Alkali refining of oil is compulsory in crude oils of high acidity and pigment contents.

The free fatty acid content of the oil is the main factor that determines the amount and concentration of the caustic soda and also its excess (5 to 20%) for a minimum oil loss. After a reaction time of around 30 minutes at slow stirring and temperature around 80ºC, the water phase is eliminated by centrifugation and the oil washed with water to remove the remaining soap.

2.3  Bleaching

In this step, which is common to both physical and alkali refining, the hot oil (around 100ºC) is slurried with acid-activated bleaching earth (1-2%), normally calcium montmorillonite or natural hydrated aluminium silicate (bentonite). Under these conditions adsorption of colour bodies, trace metals and oxidation products as well as residual soaps and phospholipids remaining after washing neutralized oils takes place. For optimum adsorption of both colour bodies and oxidation products to be achieved, the reaction time has to exceed 15 minutes and no more than 30 minutes at usual bleaching temperatures. The removal of chlorophyllic pigments is very important since they are not eliminated in any other stage of refining, as carotenoid compounds are in deodorization. On the other hand, final filtration must eliminate completely the activated earths as residual traces act as prooxidants during oil storage because of their iron content.

Acid-activated clays are the major adsorbent used, although active carbons and synthetic silicas are also applied industrially with more specific goals. Thus, active carbons are used specifically to eliminate polycyclic aromatic hydrocarbons (PAH) from some oils, especially fish oils and pomace oils [7], while synthetic silicas are quite efficient in adsorbing secondary oxidation products, phospholipids and soaps.

This is a critical step to obtain high-quality oils, because two types of adsorption occur between the compounds to be adsorbed and the absorbent: on one hand, reversible physical adsorption based on intermolecular forces of low strength and, on the other hand, irreversible chemisorption with a strong interaction, which causes chemical reactions.

Chemical changes taking place at this stage have been well studied in olive oil, because of the need to control the presence of refined oils in virgin oils [8]. The two main reactions found extensively in all the vegetable oils are the following:

• Decomposition of hydroperoxides. Previous steps do not modify the peroxide value and it may even increase if air is available in the earlier stages. However, during bleaching, hydroperoxides decompose to form volatiles and oxidized triacylglycerols containing keto and hydroxy functions. After bleaching, peroxide value should be zero or close to zero, but the presence of aldehydes and ketones is clearly detected by the significant increase in the anisidine value.
• Dehydration of alcohols. Hydroxy acids formed from hydroperoxides undergo a partial dehydration by earth catalysis. As the function is at an allylic position, a rapid increase in UV absorption at 232 nm is observed because of the formation of conjugated dienes from oleic acid hydroperoxides and in UV absorption at 268 nm due to formation of conjugated trienes from linoleic acid hydroperoxides. Also, sterols undergo significant dehydration and the formation of the hydrocarbon 3,5-stigmastadiene from the major sterol (β-sitosterol) is considered a proof of the presence of refined oil in virgin olive oil [9].

2.4  Winterization

This step, also called dewaxing, is only applied when the oil is not clear at room temperature because of the presence of waxes or saturated triacylglycerols. It is important to note that these compounds do not affect negatively the oil performance or functionality, but the appearance of the oil is not acceptable to consumers.

Thus, the objective of this step is the removal of high temperature melting components present in small quantities. The crystallization process normally used consists of cooling the oil down gradually to temperatures of 5 to 8ºC in a maturing tank. After increasing the crystal size at this temperature for 24 to 48 h, the solids are separated by centrifugation at 15-16ºC. This treatment ensures excellent clarity of oils when stored at either room or refrigeration temperatures.

2.5  Deodorization/deacidification

Deodorization of fats and oils normally consists of steam distillation at elevated temperature under reduced pressure, although nitrogen has also been used. The purpose of this step is to remove volatile compounds (mainly ketones and aldehydes) contributing to oil taste and odour, total free fatty acids in physical refining and the residual free fatty acids from neutralized bleached oils. The deodorization conditions also contribute to the removal of contaminants (light PAH, pesticides, etc.) and to the reduction of colour of the oil due to the breakdown of the remaining carotenes at high temperature. The efficiency of deodorization is a function of pressure (1 to 5 torr), temperature (200 to 260ºC), residence time (0.5 to 3 h) and volume of stripping gas (1 to 3%). However, differences in the deodorization equipment used also have a major impact on efficiency. After the deodorization, the oil is cooled and addition of citric acid (100 mg/kg of 20% citric acid) is recommended to chelate metal traces and increase its stability during storage.

Apart from the physical changes, chemical reactions taking place in the triacylglycerols due to the drastic conditions of this step have been studied in detail and are summarized as follows:

• Decomposition of oxidation compounds. Even if hydroperoxides were destroyed during bleaching, some new primary and secondary oxidation products formed decompose during heat treatment to form volatile and nonvolatile compounds.
• Dimerization of triacylglycerols. Acyclic dimers of triacylglycerols, i.e. nonpolar dimers (C&#;C bridges) as well as oxygenated dimers (C&#;O&#;C), are detected in significant amounts, which may involve the formation of alkyl and alkoxyl radicals at high temperature even in the absence of oxygen [10].
• Geometrical and positional isomers induced by heat are also formed in this step. Thus, more trans isomers and also more dienoic conjugation are found [11]. However, in oils containing linolenic acid, a decrease in the trienoic conjugation is observed, which is attributed to the formation of cyclic fatty acids and the concurrent elimination of double bonds.
• Finally, an interesterification reaction is detected in vegetable oils deodorized at temperatures above 240ºC by an increase in saturated fatty acids in the 2-position of the triacylglycerols [11].

The importance of these reactions is higher, as expected, as the temperature and the deodorization time increases [12], being dramatic in highly unsaturated oils [13]. It is also remarkable that hydrolytic reactions have not been observed as the content of diacylglycerols remains unchanged, not only in this step but throughout the complete process [14].

Finally, it is important to take into account that long deodorization times and/or too high temperatures can have a devastating effect on the quality of the oil due not only to the chemical changes commented above but also to the distillation of a significant part of the natural tocopherols (20 to 40%), which would decrease the stability of the refined oil [15]. In this respect, the by-product obtained from the deodorization, i.e. deodorizer distillate, contains significant amounts of compounds of high-added value like tocopherols, sterols and hydrocarbons, and a great effort is being made for their recovery [16].

3.  Specifications for Frying Oils

Quality evaluation of refined oil is based mainly on analytical indices giving information on the efficiency of the different steps of the refining process. Table 2 summarizes the specifications of refined oils of good quality. The last three lines include specific recommendations for frying oils. High oxidative stabilities are also required but they are not given because of their dependence on the degree of unsaturation of the refined oil, which in the case of frying oils ranges from polyunsaturated to partially hydrogenated oils.

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Table 2. Specifications for frying oils.  Phosphorus     <1 mg/kg  Free fatty acids     <0.05% oleic acid  Peroxide value     <1 meq O2/kg  Soaps     Traces Metals     Fe < 0.1 mg/kg     Cu < 0.02 mg/kg  Colour     Light yellow  Flavour >8 (scale to 10) Smoke point     <220ºC Linolenic acid (%)     <2 (%) Dimethylpolysiloxane     2 mg/kg

 

Most of the specifications in Table 2 are essential for a good performance of the oil in frying, since they reduce to the minimum the content of detrimental compounds in the process. Thus, a minimum level of phospholipids is necessary to avoid undesirable foaming and a rapid oil darkening with negative consequences for the fried product.

Free fatty acids should be limited because of their prooxidant activity as well as of their contribution to smoke when heating at frying temperatures. In fact, the smoke point and free fatty acid content are interrelated. For example, a smoke point higher than 220ºC is expected for free fatty acid contents lower than 0.05%, while the smoke is clearly observed at a usual frying temperature of around 180ºC in frying oils with free fatty acid levels of 0.3 to 0.4%.

Metals act as active prooxidants accelerating rapidly the oil degradation, and a minimum peroxide value is a guarantee of either a recent refining or a good stability.

Finally, addition of an antifoaming agent (dimethylpolysiloxane) is strongly recommended in discontinuous frying operations. It is supposed to form a layer at the oil surface impeding the entrance of oxygen when the oil is not protected by the food, and thus, it is especially active to delay oil oxidation [17].

References

1. Hazebroek, J.P. Analysis of genetically modified oils. Progr. Lipid Res. 39, 477-506 ().
2. Marmesat, S., Velasco, L., Ruiz-Méndez, M.V., Fernández-Martínez, J.M. and Dobarganes M.C. Thermostability of genetically modified sunflower oils differing in fatty acid and tocopherol compositions. Eur. J. Lipid Sci.Technol., 110, 776-782 ().
3. Shahidi, F. (ed.). Bailey's Industrial Oil and Fat Products (6th edition). Volume 5. Processing Technologies. (Wiley Inrterscience, Hoboken, New Jersey) ().
4. Dijkstra, A.J. and Seger, J.C. Production and refining of oils and fats. In: The Lipid Handbook, 3rd Edition. pp 143-162 (ed. F.D. Gunstone, J.L. Harwood and A.J. Dijkstra, CRC Press, Boca Raton, Florida) ().
5. Erickson. D.R. (ed.). Edible Fats and Oils Processing: Basic Principles and Modern Practices (AOCS Press, Champaign, Illinois) ().
6. Kovari, K. Recent developments, new trends in seed crushing and oil refining. Oleagineux, Corps gras, Lipides, 11, 381-387 ().
7. León-Camacho, M., Viera-Alcaide, I. and Ruiz-Méndez, M.V. Elimination of polycyclic aromatic hydrocarbons by bleaching of olive pomace oil. Eur. J. Lipid Sci. Technol., 105, 9-16 ().
8. Ruiz-Méndez, M.V. and Dobarganes, M.C. Olive oil and olive pomace oil refining. Oleagineux, Corps gras, Lipides, 6, 56-60 ().
9. Cert, A., Lanzón, A. A., Carelli, A.A., Albi, T. and Amelotti, G. Formation of stigmasta-3,5-diene in vegetable oils. Food Chem., 49, 287-293 ().
10. Ruiz-Méndez, M.V., Márquez-Ruiz, G. and Dobarganes, M.C. Comparative performance of steam and nitrogen as stripping gas in physical refining of edible oils. J. Am. Oil Chem. Soc., 73, - ().
11. León-Camacho, M., Ruiz-Méndez, M.V. and Graciani Constante, E. Changes in olive oil components during deodorization and/or physical refining at the pilot plant scale using nitrogen as stripping gas. Fett/Lipid, 101, 38-43 ().
12. Cmolík, J., Pokorný, J., Dolezal, M. and Svoboda, M. Geometrical isomerization of polyunsaturated fatty acids in physically refined rapeseed oil during plant-scale deodorization. Eur. J. Lipid Sci. Technol., 109, 656-662 ().
13. Fournier, V., Destaillats, F., Juanéda, P., Dionisi, F., Lambelet, P., Sébédio, J.L. and Berdeaux, O. Thermal degradation of long-chain polyunsaturated fatty acids during deodorization of fish oil. Eur. J. Lipid Sci. Technol., 108, 33-42 ().
14. Ruiz-Méndez, M.V., Márquez-Ruiz, G. and Dobarganes, M.C. Relationships between quality of crude and refined edible oils based on quantitation of minor glyceridic compounds. Food Chem., 60, 549-554 ().
15. Mezouari, S., Eichner, K., Kochhar, P., Brühl, L. and Schwarz, K. Effect of the full refining process on rice bran oil composition and its heat stability. Eur. J. Lipid Sci. Technol., 108, 193-199 ().
16. Dumont, M.J. and Narine S.S. Soapstock and deodorizer distillates from North American vegetable oils: Review on their characterization, extraction and utilization. Food Res. Int., 40, 957-974 ().
17. Márquez-Ruiz, G., Velasco, J. and Dobarganes, M.C. Effectiveness of dimethylpolysiloxane during deep frying. Eur. J. Lipid Sci. Technol., 106, 752-758 ().

Oil and Oilseed Processing III

Crude Oil Refining and Preparation for Biodiesel Production

Crude oil obtained by both solvent extraction and mechanical pressing contains desirable and undesirable compounds. Desirable compounds include triacylglycerides (TAGs) (neutral lipids) and health beneficial compounds such as tocopherols and phytosterols. Free fatty acids (FFAs), phospholipids (PLs), also referred to as gums, and lipid oxidation products are the major impurities removed during oil refining. There are several unit operations in a crude oil refining operation. Degumming, deacidification/refining, bleaching, deodorization and winterization are commonly used for edible oil production. Vegetable oils to be used for biodiesel production must be at least degummed and deacidified.

Degumming

PLs are natural components of oils and oilseeds. They are not desirable because they settle out of the oil during shipping and storage. PLs have adverse effects on the color and flavor of oil. They are surface-active compounds that reduce interfacial tension between immiscible liquids, i.e. water/oil. The presence of PLs creates problems during oil processing and some food applications, i.e. frying. PLs are removed from oil during the degumming process.

There are two types of PLs: hydratable and nonhydratable. In general, crude vegetable oils contain a small amount of nonhydratable PLs. However, the amount may vary significantly depending on quality of the seed, type of seed and conditions during the oil milling operation. Oil degumming is usually carried out at the crushing or extraction plant. Hydratable PLs can be removed from the oil by water-degumming. Hot water (at 160-176°F) or steam is injected into the warm oil. The amount of water/steam added depends on the amount of hydratable PLs present in the oil. As a rule of thumb about 2 percent water is added to oil and mixed for one hour during a batch operation. Continuous degumming processes utilize an on-line mixer for mixing oil and water (2 percent based on oil amount) and the residence time is usually 10-15 minutes. During this process, PLs absorb water and lose their lipophilic (affinity to lipids) characteristics, become oil insoluble and agglomerate into a gum phase. Gums are separated by centrifugation and added back to meal. Gums can be further processed to produce lecithin, which is used as an emulsifier in food and feed applications. The residual phosphorous level in degummed oil is about 100 parts per million after water degumming. PL content of the oil can be further decreased to about 30-50 parts per million by adding - parts per million organic acid into the oil at 104-131°F, a process called super-degumming. The oil from the degumming centrifuge is cooled to 90-100°F before entering a feed tank for the refining operation.

There are also enzymatic degumming processes, which are already competing with traditional processes. Enzymatic degumming increases oil yields by converting hydratable PLs to diacylglycerols that remain in neutral oil and are not lost during the centrifugation process.

Deacidification/Refining

Good quality oil contains more than 95 percent neutral lipids (TAGs). Commercial crude oils usually contain about 1-3 percent FFAs. High quality oils contain 0.5 percent or less FFA. However, palm, olive, fish and some specialty oils such as wheat germ and rice bran oils may contain 20 percent or more FFAs. As an industry rule, the FFA content of refined oils should be less than 0.1 percent. Although most of the long-chain FFAs do not significantly impair the taste of the oil, the short-chain FFAs may have a soapy and rancid flavor. Furthermore, FFAs accelerate oxidation reactions, consequently, reducing the oxidative stability of the oils. Crude oils are traditionally deacidified or refined by chemical methods. During chemical refining, a heavy soapstock (sodium or potassium salts of fatty acids) is formed. Soapstock is separated from refined oil by gravity settling, filtration or centrifugation. Sodium hydroxide, also referred to as caustic or lye, is widely used for chemical oil refining. The proper strength and amount of lye is critical for achieving high FFA removal with minimal neutral oil loss and degradation, and needs to be determined by trials for different oil types and quality. Not only the FFA content, but also the presence of color and surface-active compounds in oil make reaction of FFAs with lye highly variable. The amount of lye needed for refining soybean oil can be calculated from the following equation:

[(% FFA x 0.142 + %  excess) x 100 ] / (% NaOH in caustic) 
(E.G. Latondress, Journal of the American Oil Chemists Society, vol. 61, no. 8, pp: -, August ).

In oil refineries lye strength is measured by its specific gravity and expressed in degrees Baumé. The percentage of excess lye for degummed soybean oil is usually 0.10-0.12 percent and the lye used for refining oil is 14-18°Bé (9.5-12.7 percent NaOH in water). Details for the calculation of lye requirement for refining can be found in Bailey&#;s Industrial Oil and Fat Products (3rd edition, editor, D. Swern, John Wiley & Sons, Inc., N.Y., , pp.735-740). The degummed oil at 90-100°F is mixed with the required amount of lye and pumped through a high shear mixer. The mixing time is 5-10 minutes. Then, oil is heated to 165°F and centrifuged to remove soapstock (sodium salts of FFAs). Soda ash or sodium carbonate also can be used to remove FFAs from crude oil. However, carbon dioxide released during refining causes foaming. In addition, entrainment of gas in the soapstock prevents proper settling.

In cottonseed, gossypol, a complex polyphenolic compound, contributes to oil toxicity and dark color and is regarded as an undesirable component. However, recent studies have shown that gossypol possesses antitumor and contraceptive activities in males. Today, gossypol is considered a value-added natural product from cottonseed with health beneficial properties. Nevertheless, during cottonseed processing, gossypol must be removed to produce edible oil and animal feed. Gossypol in crude cottonseed oil is typically removed in the miscella (mixture of oil + hexane) before hexane removal from the oil at the hexane extraction plants. In this process, the crude oil-hexane mixture (45-65 percent oil:35-55 percent hexane) is filtered to remove any meal, scale or insoluble impurities that may be carried from the extraction process. Next, the crude miscella is pumped to a reaction vessel, where lye is added and mixed thoroughly until the impurities in the crude oil precipitate in the soap phase. Then, the light-colored refined miscella is separated from the dark, gummy, fluid soapstock by using a specially designed centrifuge. The light yellow miscella is pumped to a stripper to recover hexane. Leaving the stripper at 220°F, the refined oil passes to a pressure leaf-type filter to remove the last traces of soap and any impurities before cooling and entering the storage tank. During miscella refining, FFAs and PLs also are removed along with gossypol from hexane miscella.

Although it is not widely used, selective solvent extraction is practiced by small operations to neutralize oils with very high FFA content, e.g. cocoa butter from rinds and olive oil from the oil cake. Isopropanol is the choice of solvent for selective extraction of FFAs. Water soluble silicates such as sodium silicate also are effective in neutralizing FFAs. This process allows soapstock removal by filtration or decanting. Silicate concentrations between 10-50 percent in aqueous solutions have been used to neutralize FFAs. At high silicate concentrations, the soapstock tends to agglomerate into a firm solid phase. Refined oil, with less than 0.02 percent FFAs, can be obtained with minimal oil loss. The soluble silicate refining increases oil yield, eliminates centrifugation for separating soapstock and water washing of the oil.

Physical refining, also known as deacidification by steam distillation, is a process where FFAs and other volatile compounds are distilled off the oil. Physical refining, a viable alternative for the caustic/chemical refining process, is based on the higher volatility of FFAs than TAGs at high temperatures and low pressures. During the process, volatile compounds, including FFAs, are volatilized and neutral oil droplets are entrained within the stripping steam. The final FFA content in the refined oil can be reduced to 0.005 percent when physical refining is used.

Adsorption processes also have been examined to remove FFAs from oils. A process, which utilizes magnesium oxide as adsorbent to remove FFAs from oils, has been patented. Aluminum hydroxide gel also is effective for removing FFAs.

Bleaching

Oils are usually bleached after deacidification/refining and before deodorization. Originally bleaching was used to remove color compounds such as carotenoids and chlorophyll. Today, bleaching is designed to remove undesirable oil components including peroxides, aldehydes, ketones, phosphatides, oxidative trace metals, soaps and other contaminants such as pesticides and polycyclic aromatic hydrocarbons.

Clays used for bleaching are commonly called &#;Bentonites.&#; Activated carbon, alumina, silicic acid, aluminium- and magnesium-silicate, silica gel and synthetic silicates also are used to adsorb impurities from refined oil. The bleaching is normally carried out under vacuum (20-30 mm Hg) to minimize oxidation reactions and control moisture levels. Preheated oil (194°F) is pumped into a slurry tank and adsorbent is added to the tank simultaneously. After mixing, the clay/oil system is fed into a vacuum bleacher. The bleaching process takes 15-30 minutes in a temperature range of 176-248°F. Although high temperature increases the adsorption efficiency, bleaching at very high temperatures is not recommended because it promotes undesirable reactions. The temperature should be high enough to maintain a low oil viscosity, which improves diffusion and mass transfer rates. Wet bleaching is practiced when processing oils containing PLs, because water will act as a carrier for the PLs into the bleaching clay particle. The optimal amount of water used for wet bleaching is about 50-100 percent of the adsorbent used for the process. Initially oil (about 0.5 percent moisture) is treated with water and adsorbent (8-15 percent moisture) at 158-194°F for 20 minutes under atmospheric conditions. Then, bleaching is carried out under a vacuum for 15-30 minutes. The amount of adsorbent required for bleaching depends on the types of adsorbent and the oil and its pre-treatment. The adsorbent dosage range is quite wide, usually 0.1-2.0 percent (of oil processed), but in some cases it can be as high as 5 percent. Physically refined oils require a higher amount of adsorbent than chemically refined oils. After bleaching, oil is filtered and separated from the adsorbent.

Deodorization

Deodorization is a steam-distillation process in which volatile and odoriferous compounds are stripped off with steam. The objective is to produce a bland and stable product. Deodorization removes FFAs, aldehydes, ketones and peroxides from bleached oil. Temperature plays a critical role during deodorization. If the temperature is increased from 350°F to 400°F, the rate at which odor compounds are removed is expected to triple. If the temperature is further raised to 450°F, that rate can be expected to triple again. This means higher deodorization temperature reduces processing time. However, high temperatures cause development of undesirable polymers. Hence, optimization of time and temperature is necessary for a given process. High vacuum is desirable for deodorization because it inhibits oil hydrolysis. The volume of stripping steam needed in the deodorizer also is affected by vacuum. For example a deodorizer operating at 12 mm Hg pressure would require twice the stripping steam of a unit operated at 6 mm Hg. Currently, 6 mm Hg vacuum is commonly used for vegetable oil deodorizers. Batch, continuous and semi-batch deodorizers are available for vegetable oil processing.

Winterization

Winterization is a separation process by which higher melting point acylglycerides and waxes that are responsible for the turbidity of some edible oils in the winter or after refrigeration are crystallized and removed. Composition of the oil, rate of cooling, temperature of crystallization and mobility of TAG molecules in the oil are critical factors affecting efficiency of winterization. These factors play a significant role both in separating the solid phase and then separation of the solids from the liquid portion. The edible oil industry utilizes the liquid fraction to make high-quality salad oils, whereas the solid fraction is used in shortening or margarine formulations. During the winterization process, the oil is cooled from room temperature to a predetermined temperature of crystallization. The cooled oil is kept at this temperature for a certain period of time prior to the separation of solid phase from the liquid oil by filtration of the oil-solid fat slurry. In a winterization process, cooling rate and temperature of crystallization are extremely important. Too low a temperature and high cooling rates will result in high viscosity and reduce crystal growth rate. A mild agitation is recommended to provide a gentle motion to the crystals to enhance their growth rate and keep the temperature and composition uniform in the bulk oil. The agitator design should be such that no shear to break the crystals is generated. In commercial winterization operations crystal modifiers or an appropriate solvent are used to facilitate filtration of solid phase from the liquid oil. In certain applications, scrape surface heat exchangers are preferred.

Tips for Preparing Crude Oil for Biodiesel Production

In general, crude oil preparation for biodiesel production includes at least degumming, neutralization and drying. Oil to be converted to biodiesel should have the following specifications:

Phosphorous content: 2-10 ppm

Water content: 500- ppm

Acid value: 0.05-0.25 percent FFA, max

 

Nurhan Dunford
FAPC Oil/Oilseed Specialist

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