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Vegetable Oils: Oil Production and Processing
Vegetable Oils: Oil Production and Processing AJ Dijkstra and G van Duijn ã 2016 Elsevier Ltd. All rights reserved.
Introduction Oils and fats are mixtures of different organic molecules called lipids. Triglycerides are by far the most common component of oils and fats; these biological molecules are water-insoluble. Oils and fats can be extracted from plant or animal sources. The difference between a fat and an oil is that a fat is solid or semisolid at normal room temperature (20 C in Europe and the United States), while an oil remains liquid. Early vegetable oil production in Europe was mainly concerned with olive oil. For climatic reasons, it was limited to the Mediterranean; more northern countries used lard or butter as the fat source in their diet. This distinction still exists. Countries used to olive oil now use vegetable oil for cooking, the shortage of butter led to the invention of margarine, and lard has been replaced by vegetable shortening in countries that traditionally used lard. Oil production has evolved from the early process comprising a grinding step to open the cells in the oleaginous raw material and an expelling step in which the oil is squeezed out of the ground fruit or seed paste to a process that may also comprise a solvent extraction step. Oils and fats have been used since ancient times for food preparation and in nonfood applications like lamp oil, lubricant, soap manufacturing, and skin care. This article will only deal with the production of oils and fats for use in food preparation or as a food ingredient. Oils and fats in food provide functional and nutritional benefits. They serve as a heat transfer medium at elevated temperatures (frying), provide lubrication (spreads), improve taste (dressings), and give texture to a wide range of foodstuffs. On top of this, they supply a concentrated source of energy, deliver critical building blocks for the body, and act as a carrier for essential minor components like vitamins A and D. A balanced intake of oils and fats is essential for human health.
The Oil and Fat Supply Chains The supply chains of vegetable oils and fats consist of the following:
• • • • •
The growing and harvesting of the oil crop. Oil extraction to recover the oil; the by-product meal is mostly used as animal feed. Purification to reduce the level of unwanted minor components. Modification of the melting characteristics as required for particular products. All transport and storage from grower to end user.
The main origins of supply for the world vegetable oil and fat market fall into three main categories (more details will be given in other articles):
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1. Annually planted oilseed crops like soybean, rapeseed, and sunflower seed 2. Fruit oils like palm and olive oil 3. Kernel oils like palm kernel oil, coconut, and other nut oils Oilseed crops are generally not grown in the regions where the oils are consumed. Oilseeds are transported and stored as seeds, and the separation into oil and meal is mostly performed close to the consumer markets. Under controlled conditions, oilseeds can easily be stored for a long time, at least until the next harvest arrives. Overseas shipping is mainly in the form of seeds. Like other fruits, fruit oils have a short shelf life. After a few days, they start to deteriorate, resulting in a poor oil quality. The extraction of oil from fruits is done close to the crop growing area, and the extracted oil is then stored and transported to the consumer markets. Overseas shipping is only as oil. In contrast, kernels can be stored after drying and transported over long distances, although extraction is generally performed where the fruits are grown. Overseas shipping is mainly as oil.
Vegetable Oil Extraction Olive Oil The annual world production of olive oil varies around 2.8 million tonnes. Spain is the main producer with an average annual production of 1.2 million tonnes, followed by Italy and Greece. After harvesting, the olives are first of all cleaned and washed with water. Then, they are crushed in a hammer mill from where the resulting paste is fed to a malaxer in which the paste is slowly agitated to allow the dispersed oil droplets to coalesce. Talc and/or enzymes can be added to improve the oil yield. In old olive oil mills, the paste is then spread onto circular filter cloths that are stacked inside a hydraulic press. Modern mills use a decanter to separate the oil from the waste. If a twophase decanter is used, this will separate the paste into some 80 wt.% wet pomace and 20 wt.% turbid oil. This oil is then washed with 10–20% water to yield clear oil and not too dirty washing water. The amount of water used affects the residual polyphenol content of the oil and is therefore quite critical. When a three-phase decanter is used, only 50 wt.% dry solids (pomace) are produced, but the addition of further washing water leads to a large effluent stream (black water) that has to be treated before it can be discharged. When the pomace is extracted with, for example, hexane, the oil thus obtained can no longer be called ‘virgin.’ It has to be refined to yield a bland olive oil called ‘pomace oil.’
Oils from Seeds with a High Oil Content Although the production of oil from seeds with a high oil content (around 40%) also leads to the meal by-product, the
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Vegetable Oils: Oil Production and Processing
value of this by-product is low in comparison with the oil. These seeds are therefore grown for their oil. Unlike fruit oils, seeds can be stored so that an oil mill processing oilseeds can operate the whole year round. After harvesting, the seeds are dried to a water content of less than 16% of the dry, oil-free matter to prevent microbial spoilage during storage. At the oil mill, the seeds are first cleaned to remove any stones and other extraneous matter that might damage the flaking rolls. Then, they are conditioned to a temperature of 60–70 C to soften the cell so that the subsequent flaking requires less electrical energy. In the flaking process, two perfectly cylindrical rolls flatten the seeds so that cells within the seed are opened and oil can be squeezed out of the flakes without having to pass through a cell wall. The flakes are passed to a screw press that is fitted with a decreasing-volume Archimedean screw that builds up pressure along its shaft. This shaft rotates inside a slotted barrel that allows the oil to pass through. This oil contains many meal particles that have to be removed. In a first clarification process, the oil is screened or allowed to settle, and then further clarification is realized by filtration or the use of a decanter centrifuge. The solids removed contain a substantial amount of oil and are therefore returned to the screw press. The press cake leaving the screw press is too hot to enter the solvent extraction plant. It therefore has to be cooled and broken into pieces to permit proper percolation during solvent extraction. A very effective way of treating the press cake is by pelletizing, since this treatment also ruptures cells that survived the flaking and pressing processes. When the cake has reached a temperature of no more than 60 C, it is ready for solvent extraction, which will be discussed in the next section. Because the hull of a sunflower seed accounts for up to 30% of its weight, it can be advantageous to dehull the seeds prior to the expelling step. This increases the capacity of the screw presses and the extraction plant; it lowers the wax content of the oil and increases the protein content of the meal. The dehulling process comprises the opening of the seeds by impact and a separation by winnowing. Some hulls have to be included in the feed of the screw press to give it some body. The hulls are usually used on-site as a boiler fuel.
Oils from Seeds with a Low Oil Content The prime example of an oilseed with a low oil content is the soybean that contains around 18–21% oil. Soybeans are primarily grown for the protein content of the meal that is used in the animal feed industry on a large scale; the oil can be regarded as a by-product. Because its use in poultry farming requires a high (>48%) protein content and a low (<3.5%) fiber content, the beans are often dehulled before being extracted. In the classical dehulling process, the beans are dried and held for tempering. This makes the beans shrink away from the hulls so that after cracking, the hulls can be separated from the cracked cotyledons by aspiration. In the so-called ‘hot dehulling’ process, the beans are conditioned to a temperature of 65–70 C in a tower in which tubes, which are heated with low-pressure steam, heat the beans quite quickly. This and the subsequent superficial heating in a fluid bed dryer cause the hull to shrink away from the cotyledon. Cracking leads to a
mixture of separate hull fragments and cracked cotyledons that are separated in cascade aspirators. For the cracking process, two or three sets of superimposed corrugated rolls are used wherein opposing rolls run at different speeds to grip the beans. Cracking should aim at breaking the bean into four to six smaller pieces and avoiding fines and mashed beans. Subsequently, the cracked cotyledons are flattened into flakes of some 0.25–0.30 mm thickness in between flaking rolls that are similar to those used for high oil content seeds. Processing the flakes in an expander is optional. The expander is a kind of extruder in which the flakes are compressed until on leaving the die, the compacted mass expands causing further cell rupture. The resulting collets have the advantage that they have a higher bulk density than the flakes, so expanders are often used to increase the capacity of an existing oil mill. Collets also have the advantage of a reduced liquid holdup, which saves energy in the desolventizer. The extraction solvent used is invariably hexane. Industrial hexane is a mixture of alkanes with a boiling range of 66–69 C comprising n-hexane (62%), methylpentanes (23%), and methylcyclopentane. It is highly inflammable and its explosive range in air is 1–8% by volume. Consequently, extractors are operated at a slightly subatmospheric pressure to prevent the hexane from escaping. Solvent extractors operate countercurrently to minimize the oil content of the extraction residue leaving the extractor and maximize the oil content of the miscella. Two types of extractor are in industrial use: percolation and immersion. In the immersion type, the screen under the material being extracted has so little open area that it constitutes the greatest flow restriction. Immersion extractors provide more contact time between solvent and seed material but require more draining time than percolation-type extractors. Both types ensure a miscella strength of around 30% oil and a residual oil content in the meal of < 1 wt.%. The solvent–wet extraction residue or ‘marc’ leaving the extractor is then treated to remove the solvent, first by indirect heating and then by heating with live steam. This raises the temperature of the meal to 105–100 C in the toasting section where antinutritional enzymes are denatured. Since the live steam causes the moisture content of the meal to increase, it has to be dried. Air is used to dry the meal to a residual water content of 13–14 wt.% and cool it down at the same time. The hexane-laden vapors leaving the desolventizer are at atmospheric pressure. Using their latent heat to evaporate the hexane from the miscella requires the latter to be under a reduced pressure; in industrial practice, this pressure is about 500 hPa absolute. The latent heat content of the vapors leaving the desolventizer suffices to evaporate < 90% of the hexane present in the miscella so that the composition of the liquid leaving the first evaporator is around 80 wt.% oil. It is further concentrated using steam and finally treated in a steam stripper operated at 200–400 hPa absolute. If the oil has to be shipped, it must be degummed with water to prevent a deposit being formed during transport and storage. In this water degumming process, an amount of water that is about the same as the gum content of the oil is mixed with the hot oil, allowed to hydrate the mucilaginous matter. and then separated by a centrifuge. Drying these gums yields lecithin.
Vegetable Oils: Oil Production and Processing
Vegetable Oil Refining Process Sequence The vegetable oil refining process was introduced at the start of the twentieth century to produce good-quality oils and fats for use in margarine production and as frying oil. Before 1900, oils were cleaned by removing water and solids by decanting. The refining process has been optimized to reduce the natural taste and color and to remove most of the free fatty acids present in the extracted oil. Later, it was discovered that the process also removes many of the contaminants present in the extracted oils, like solvent residues, metals, polyaromatic hydrocarbons, and pesticide residues, bringing these contaminants below the legal limits. The refining process is a combination of the following process steps; more details are given in the following sections:
• •
•
Degumming: A pretreatment process mainly applied to seed oils to reduce their phospholipid contents. Neutralization: The purpose of neutralization is to reduce the concentration of free fatty acids by reaction with a diluted alkali solution, typically sodium hydroxide. The reaction product (soap) is removed by decanting or by centrifugation. Bleaching: The main purpose is to remove residual soap, color pigments, and oxidized components. In this process,
• •
bleaching earth (natural or activated clay and/or silica) is added to the oil as an adsorbent. The earth and adsorbed impurities are subsequently removed by filtration. Dewaxing: Removal of natural waxes present in some vegetable oils by cooling and filtration. Deodorization: Under high vacuum, the oil is heated to 180–270 C, while steam is blown through the oil. The objective is to remove volatile components and to create an odorless oil with a bland taste and increased storage stability.
The process sequence of combined degumming/neutralization followed by bleaching and deodorization is called chemical refining, referring to the reduction of free fatty acids by a chemical reaction. The process sequence of degumming followed by bleaching and deodorization is called physical refining, referring to the reduction of free fatty acids by the physical process of vacuum stripping. An overview of the possible process sequences is given in Figure 1.
Degumming Processes As indicated in the refining process flow sheet (Figure 1), many different routes lead from crude oil to fully refined oil, and the number of steps can differ.
CRUDE OIL
water degumming or enzymatic degumming
oil with NHP
acid degumming
acid refining
oil with < 30 ppm P
enzymatic PLA gum
dry degumming alkali neutralization
oil with < 5 ppm P
bleaching
bleaching
deodorizing
FULLY REFINED OIL
deodorizing/ stripping
Figure 1 A block diagram of possible vegetable oil refining process routes.
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Vegetable Oils: Oil Production and Processing
The route with only two steps consists of the dry degumming process, followed by physical refining. In the dry degumming process, some phosphoric acid is mixed into the oil and allowed to react with the phosphatides. Then, bleaching earth is added, and this adsorbs the phosphatides, the excess phosphoric acid, and also the coloring matter present in the oil. This bleaching earth is removed by filtration, and the bleached oil is ready for physical or steam refining. Since the bleaching earth requirement in the dry degumming process is proportional to the phosphatide content of the crude oil, this two-step route is only economical for oils and fats with a low phosphatide content. Examples are palm oil, lauric oils, lard, and tallow. In the acid degumming step, the crude or water-degummed oil is treated with a strong acid that is subsequently diluted with water to form a gum phase that is removed by centrifuge. However, this dilution raises the pH insufficiently to pull all the gums into the gum phase. Consequently, the acid-degummed oil requires a further degumming treatment before the oil can be physically refined. In the acid refining process, the crude or water-degummed oil is also treated with a strong acid, but instead of diluting the acid with water, it is neutralized with caustic soda. This raises the pH to a value where still no soaps are formed but where all the phosphatidic acid that has been liberated by the acid treatment is dissociated. It therefore moves completely into the aqueous phase and can be separated from the oil as sodium phosphatidate. The acid refining process produces an oil that can be physically refined after it has been bleached. The water degumming process and the acid refining process produce a gum phase as by-product. This gum phase also contains oil, and if the gums are disposed of by mixing with the meal, the oil is sold at meal value that is less than if it were to be sold as oil. To recover at least some of this oil, enzymatic gum treatment processes using a phospholipase have been developed. As shown in Figure 1, phospholipase C (PLC) can be used in the water degumming step or on the gums obtained by water degumming. This enzyme catalyzes the hydrolysis of the bond between the phosphate moiety and the glycerol moiety in phosphatides as indicated in Figure 2. This hydrolysis generates diglycerides that move into the oil phase and
A1 H2C O R2
C O
O
CH H2C
Another phospholipase that can be used to treat the gums and thereby increase the oil yield is phospholipase A (PLA). Figure 2 shows that there are two enzymes that catalyze the hydrolysis of a fatty acid ester bond: PLA1 and PLA2. Both enzymes are commercially available. They form lysophosphatides that retain less triglyceride oil and free fatty acids that are removed during a subsequent neutralization process. They can be added to oil that has already been acid-refined or to the gums resulting from the acid refining treatment. Because the oil savings resulting from the various enzyme treatments are proportional to the amount of gums, these treatments only make sense when applied to oils with a high phosphatide content.
Neutralization Processes In the neutralization process, oil containing FFAs is treated with lye, a solution of sodium hydroxide or caustic soda in water. This reacts with the FFA to form sodium soaps that are insoluble in the oil and therefore form a separate soap phase. This soap phase also contains phosphatides, when present in the crude oil, and triglyceride oil. After this soap phase has been separated from the oil, it is called soap stock. Formerly, it was used in a soap works adjacent to the refinery. It can be used as an animal feed ingredient, but in Europe, it is more common to react the soap stock with sulfuric acid at elevated temperatures and recuperate its fatty matter as ‘acid oils,’ a mixture of fatty acids and triglyceride oil. This process also generates an acid-aqueous effluent stream that contains phosphates and sulfates and has a high BOD. It has to be neutralized, its phosphate may have to be removed by a lime treatment, the resulting precipitate will then have to be separated, and then what is left has to be treated in an effluent plant to lower the BOD. All this makes soap stock splitting a cumbersome process that is preferably avoided. It has been a powerful driving force for the shift towards physical refining. In alkali refining, there are two routes. The oldest route that has been developed and is still in use in the United States is the
O C
A2
thus contribute to oil yield. It also produces phosphate esters of choline, which are water-soluble and do not retain any oil.
R1
H2 C
HO
D choline
O O
P
O
H2 C
X HO
C
CH3 H2 C N CH3 CH3
H2 C
NH2
O
X = choline (phosphatidylcholine or PC)
ethanolamine OH OH
X = ethanolamine (phosphatidylethanolamine, PE) X = inositol (phosphatidylinositol or PI) X = hydrogen (phosphatidic acid or PA)
OH O
OH OH
inositol, link in 1-position Figure 2 Chemical structure of phosphatides and indications which phospholipase enzyme catalyzes the hydrolysis of which bond.
Vegetable Oils: Oil Production and Processing so-called ‘long mix’ process. Dilute lye (14–18 Be´ or 130–180 g NaOH/L of water) is added and mixed into cold oil, and then the oil is heated until a clear separation between oil and soap stock is obtained. The soap stock is removed by centrifugal separator, and to lower the residual soap content, the oil is either washed with water or treated with silica hydrogel. In Europe, the ‘short mix’ process is almost universally used. In this process, a degumming acid (phosphoric acid) is finely dispersed in the hot (90–100 C) oil, it is allowed to react in a holding vessel, and then less dilute lye (20–28 Be´ or 145–215 g NaOH per liter of water) is dispersed in the oil. Separation between oil and soap stock is almost instantaneous, allowing the mixture to be sent to the centrifuge without much holding time. Again, the oil phase can be washed or treated with silica hydrogel, but in Europe, water washing is more common.
Bleaching of Vegetable Oils Crude and degummed/neutralized vegetable oils differ widely in color. Palm oil is deep red, rapeseed oil is greenish, and sunflower seed oil has not much color. These colors are caused by two groups of compounds. The red color stems from carotenes, with b-carotene being the most common, and the green color originates from chlorophyll. Carotenes are thermally unstable so they break down when the oil is deodorized or physically refined. Because palm oil also contains the thermally more stable chlorophyll, which is removed by bleaching earth, palm oil has to be treated with this adsorbent to yield an almost colorless fully refined oil. The bleaching process itself is quite simple. Depending on the color of the oil, an amount of 0.5–4.0 wt.% of bleaching earth is mixed into hot (80–110 C) oil, the suspension is given some time to approach the adsorption equilibrium, and the earth is then removed by filtration. The filters used for this step are invariably pressure leaf filters that are precoated by circulating oil with bleaching earth over the filters. Filter aids like diatomaceous earth can also be used. Bleaching earth is a pretty universal adsorbent, but it insufficiently adsorbs certain compounds that are considered to be undesirable. Among these compounds are the polyaromatic hydrocarbons (PAHs). Low-molecular-weight polyaromatic hydrocarbons are sufficiently volatile to be removed by vacuum stripping, but high-molecular-weight polyaromatic hydrocarbons require another means of removal. This means is provided by treating the oil with activated carbon, a highly effective adsorbent for this kind of compounds. Bleaching clay and activated carbon can be used simultaneously.
Dewaxing of Vegetable Oils Some vegetable oils like sunflower seed oil, rice bran oil, and corn germ oil contain waxes. These waxes are esters of fatty acids and fatty alcohols. When both of them are saturated, the wax has a melting point > 70 C, which causes the waxes to crystallize at refrigeration temperatures and form an unsightly deposit. To prevent this from happening, the oil has to be dewaxed. In the dewaxing process, the oil is cooled down to cause the waxes to crystallize. They can then be removed by filtration with a pressure leaf filter and filter aid. Since the
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amount of filter aid is proportional to the wax content and oil is also retained in the filter cake, filtration becomes quite expensive when the oil has a high wax content. Then, it is more economical to remove the bulk of the waxes in a wet dewaxing process using a centrifugal separator and remove residual waxes by filtration. This wet dewaxing process employs water with a detergent, which can be soap, to concentrate the wax crystals in the aqueous phase. It can therefore be combined with the neutralization step. The ideal cooling profile comprises a slow cooling from above the cloud point of the oil to below this cloud point to limit the number of crystal nuclei. Cooling from below this cloud point can be fast and maturation is not necessary.
Deodorization The deodorization process is carried out at a high temperature (180–270 C). Because of this high temperature, residual peroxides and carotenes will decompose, and when the temperature is too high, trans isomers can be formed. The more double bonds a fatty acid contains, the more prone it is to this kind of isomerization. This is the reason why refined oil specifications contain an upper limit of 1.5 wt.% for oils containing linolenic acid (soybean oil and canola oil) and an upper limit of 1.0 wt. % for sunflower seed oil and corn oil, which do not contain this fatty acid. Apart from these chemical reactions, vacuum stripping is a physical process that can be described by the laws of Raoult and Dalton. The reduction of volatiles in the oil depends on the stripping steam flow, the vacuum pressure, the oil temperature, the oil volume, the vapor pressures of the volatiles, and the equipment design. The deodorization process can be carried out in batch, continuous, or semicontinuous installations. Batch deodorizers have one single compartment in which the process steps of deaeration, heating, stripping, and cooling are sequential in time. In continuous deodorizers, the process steps are simultaneous in different compartments (trays) of the deodorizer, and both input and output are constant over time. In semicontinuous deodorizers, the process steps are simultaneous in the different trays, but tray filling and emptying is discontinuous. Filling stops after a certain level has been reached in the first tray. After allowing sufficient time to perform the process step for that tray, the bottom valve is opened and the content drops into the next tray for the next process step. Figure 3 shows a basic layout of a semicontinuous deodorizer. Oil can also be fed to the top of a packed column and trickled down through this column, while the stripping flows countercurrently. This type of column has a lower stripping steam usage than the crossflow system to reach the same final concentration of volatiles. Obtaining stable oil is more than just getting rid of the volatiles present in the oil to be deodorized or physically refined. When triglyceride oils are heated for prolonged periods, strong off-flavors are developed. Deep fat frying in coconut oil smells different from frying in sunflower oil, let alone tallow. The chemical nature of these smells is not known, and since they are not released by freshly deodorized oil, they must be formed during deep fat frying from flavor precursors. Accordingly, the deodorization process should
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Vegetable Oils: Oil Production and Processing
Degasser Vacuum system
Heat exchange
Final heating
HP steam boiler Stripping steam
Cooling water Guard filters Citric Acid
Pre-treatment Cooler
Storage
Figure 3 The layout of a semicontinuous deodorizer. From G. van Duijn, lecture material.
provide some holding time at a high temperature to enable the thermal breakdown of these flavor precursors. There are two reasons why the deodorization process is the most energy-intensive step of the refining process. Firstly, it is carried out at temperatures of up to 270 C, and secondly, it is carried out under a vacuum of 2–10 hPa, which has traditionally been maintained by steam ejectors. Accordingly, heat exchange between ingoing and outgoing material either directly in a plate heat exchanger or indirectly by using one or even two thermosiphons has led to energy savings. The use of a mechanical vacuum pump in combination with a surface condenser for the distillate has limited the steam usage to the stripping steam but increased the consumption of electrical energy.
Vegetable Oil Modification Processes The Purpose of Oil Modification When Me`ge-Mourie`s invented margarine in 1869, he used tallow olein as the fat phase, but when more and more margarine was produced, a shortage of solid fat emerged. Accordingly, the invention of the hydrogenation process in 1903 led to an almost immediate industrialization despite the fact that this also involved building plants for elemental hydrogen. Me`geMourie`s also described a dry fractionation process to obtain his tallow olein, but that process only began to be applied on a large scale with the increased production of palm oil in the last quarter of the twentieth century. Networks of solid fat crystals play an important role in many food products. For example, margarines consist of an
emulsion of small droplets of an aqueous phase dispersed in liquid oil stabilized by a structure of solid fat crystals. The fat crystals supply structure and texture to the product, they prevent the oil from leaking out of the product, and they avoid coalescence of the dispersed aqueous phase droplets. Solid fat crystal networks have similar functions in whipped cream and ice cream, where they stabilize a foam of air bubbles dispersed in an emulsion of fat droplets in water; they also play an important role in bakery products. Most of the seed oils are too low in solids at the temperature at which the product is used (generally ambient temperature) to fulfill this role. Tropical oils, such as palm oil, palm kernel oil, and coconut oil, have a higher solid fat content, but for many applications, they are still too soft. Modification of the melting performance of naturally occurring oils and fats is required to produce fats with optimal properties for the aforementioned applications. The modification technologies that can be applied are the following: 1. Hydrogenation 2. Interesterification 3. Fractionation
Hydrogenation Hydrogenation of oils and fats has been applied on a large scale since the start of the past century to produce hard fats from a wide variety of oils and fats (mainly liquid oils and fish oil). Hydrogenation involves the addition of two hydrogen atoms to unsaturated double bonds (C]C) in the fatty acid moieties of the triglycerides. The more saturated fat resulting
Vegetable Oils: Oil Production and Processing
from this reaction has a higher melting point than the starting material. In the hydrogenation process, hydrogen gas reacts with liquid oil at elevated temperatures and pressures in the presence of a solid catalyst. The hydrogenation of edible oils is always carried out as a batch process. Oil is preheated by the previous batch, the catalyst (nickel on an inert support) is added, and hydrogen gas is dispersed in the oil. Hydrogenation is an exothermic reaction so the temperature of the batch starts to rise and has to be controlled by cooling. The reaction is continued until the product properties aimed for have been reached (partial hydrogenation). The catalyst is removed by filtration and residual nickel is removed in a post-bleaching step. As well as saturation, the isomerization of double bonds also takes place during the hydrogenation reaction. Two types of isomerization reaction will occur: geometric isomerization (cis–trans) and positional isomerization (shift of double bond along the fatty acid chain). The complete reaction mechanism is quite complicated because of the many steps involved, and it is still not yet fully understood and kinetically quantified. In response to the findings of adverse effects of trans fatty acids on blood lipids, the food industry has practically eliminated components containing trans fatty acids from their fat phase compositions. This has drastically reduced the application of partial hydrogenation as a method of producing solid fats. Full hydrogenation of almost all double bonds reduces the trans isomer content to an acceptable level but results in a product with a very high melting temperature. Such a product can only be used after interesterification with a liquid oil.
Interesterification Interesterification involves a rearrangement or reshuffling of the fatty acids on the glycerol backbone of the triglyceride molecule. Interesterification is catalyzed by an alkaline catalyst or by a lipase enzyme. The most commonly used alkaline catalysts are sodium methanolate and sodium ethanolate. The early interesterification process was used to extend the plastic range of lard used as shortening. Later blends of different raw materials were used to change their physical properties so that they could be used as hard fat in margarine and as shortening. For a long time, the interesterification reaction has been assumed to proceed with a glycerolate anion as the catalytic intermediate. This anion would then attack the slightly positive carbon atom of a carboxyl group. If this group left as a hydroxyl anion, ester interchange would have been the overall result. An alternative reaction mechanism has been proposed recently that assumes that the initial catalyst (sodium methanolate) abstracts an a-hydrogen from a fatty acid moiety, thereby forming an enolate anion. This enolate anion then acts as the catalytic intermediate. When it reacts with water, it will form a free fatty acid, the formation of which could not be explained by the glycerolate mechanism. The alkali (sodium methanolate or ethanolate)-catalyzed reaction takes place in prerefined oil (low in water and in free fatty acids) at elevated temperatures (100–110 C). The reaction is very fast; full randomization results within a few minutes even in factory-scale vessels (10–40 t oil content). After the reaction, the catalyst is inactivated by the addition of water. This addition of water leads to the formation of free
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fatty acids and sodium hydroxide that will combine to form soaps. The reaction of sodium methanolate with a fatty acid moiety leads to the formation of methanol, and when this reacts with an enolate anion, methyl esters will be formed. The soap is removed by water washing and decanting. Methyl esters are more volatile than triglyceride molecules and are removed during high-temperature deodorization. Both soap and esters constitute a yield loss. In addition to the basic catalysts used, lipase enzymes have also been introduced for the interesterification process. Interest in the use of lipase originates from the attempts to produce cocoa butter equivalents (CBEs), symmetrical monounsaturated triglycerides that are compatible with cocoa butter. This isomeric requirement ties in with the sn-1,3 specificity of the lipase enzyme. Interesterifying a triglyceride that consists almost entirely of trioleate (high-oleic sunflower seed oil, for instance) with stearic acid or methyl stearate should generate a CBE. Lipase enzymes are also used in a nonstereospecific interesterification process. To overcome the fundamental problem that enzymes need water to be active but use this water to hydrolyze fats, this process uses a large amount of rather dry, immobilized enzymes. It is a continuous process, using four reactors that are filled with enzymes. The first of these, which is filled with an enzyme that has lost most of its activity, acts as a kind of guard for downstream reactors. It is emptied when the product leaving the last reactor is insufficiently randomized, filled with fresh enzymes, and relocated as the last reactor. What impurities in the oil cause the enzyme to lose its activity is still not quite clear, so as a precaution, the oil fed to the first reactor has been neutralized, bleached, and deodorized. The enzymatically interesterified oil needs to be deodorized before it can be used in food products.
Fractionation Fractionation is the controlled crystallization of the more saturated and/or longer-chain triglycerides, followed by separation of the solid phase (stearin) and liquid phase (olein). By far, the most important oil, fractionated worldwide, is palm oil, the main reason being the demand for clear liquid oil (palm olein). More recently, there has been a growing interest in the solid product of palm oil fractionation (palm stearin) for the production of cocoa butter equivalents, cocoa butter replacers, and margarine hard stocks. Besides palm oil, palm kernel oil, partly hydrogenated liquid oils, cottonseed oil, and milk fat are also fractionated.
Processes for Producing Virtually Trans-Free Hard Stocks The combination of full hydrogenation, interesterification, and fractionation and the availability of a variety of different oils and fats can be used to produce virtually trans-free hard stocks with various melting ranges. Liquid seed oils, low in solid fats, are first fully hydrogenated to generate solids combined with a very low trans level (<1.25%). These fully hydrogenated oils may subsequently be interesterified with nonhydrogenated liquid oil to reduce the solid fat content at high temperature (>40 C). This solid fat content can be further reduced by fractionation (see Figure 4).
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Vegetable Oils: Oil Production and Processing
Full hydrogenation
Liquid oils
Interesterification
TFA-free fat-phase component
Tropical oils
Fractionation Figure 4 Process routes to produce trans-free hard fat components.
The presence of relatively high solid contents in tropical oils creates more flexibility in oil modification routes. Fractionation alone will produce a relatively soft stearin, which is not optimal for structuring margarine. Fractionation followed by interesterification with other (fractionated) components is used to produce ‘nonhydrogenated’ hard fats. Full hydrogenation followed by interesterification is an alternative way to obtain a hard fat high in solids with a steep melting line without fractionation (see Figure 4).
See also: Carotenoids: Occurrence, Properties and Determination; Drying: Principles and Types; Enzymes: Analysis and Food Processing; Fatty Acids: Fatty Acids; Fatty Acids: Trans Fatty Acids; Heat Treatment: Principles and Techniques; Oxidation of Food Components; Phospholipids: Properties and Occurrence; Quality Control in Food Processing; Rapeseed Oil/Canola; Sunflower Oil; Tocopherols: Properties and Determination; Triacylglycerols: Characterization and Determination; Triacylglycerols: Structures and Properties.
Dijkstra AJ (2007) Degumming of oils and fats. In: Gunstone FD, Harwood JL, and Dijkstra AJ (eds.) The lipid handbook, 3rd ed., pp. 177–191. Boca Raton, FL: Taylor & Francis Group LLC. Dijkstra AJ (2010) Enzymatic degumming. European Journal of Lipid Science and Technology 112: 1178–1188. Dijkstra AJ (2012) Kinetics and mechanism of the hydrogenation process - The state of the art. European Journal of Lipid Science and Technology 114: 985–998. Dijkstra AJ (2014) Oil refining. In: Dunford NT, Martinez Force E, and Salas Lin˜a´n JJ (eds.) Sunflower, Chemistry and Technology. Urbana: AOCS Press. Dijkstra AJ, To˜ke ER, Kolonits P, Recseg K, Ko˜va´ri K, and Poppe L (2005) The basecatalyzed, low temperature interesterification mechanism revisited. European Journal of Lipid Science and Technology 107: 912–921. Johnson LA (2008) Oil recovery from soybeans. In: Johnson LA, White PJ, and Galloway R (eds.) Soybeans, production, processing, and utilization, pp. 331–376. Urbana: AOCS Press. Kemper TG (2013) Solvent extraction. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 97–125. Chichester: John Wiley & Sons. Van Doosselaere P (2013) Production of oils. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 55–96. Chichester: John Wiley & Sons. Van Duijn G (2000) Technical aspects of trans reduction in margarines. Oleagineux, Corps Gras, Lipides 7(1): 95–98. Van Duijn G and Den Dekker G (2013) Oil processing design basics. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 267–310. Chichester: John Wiley & Sons. Van Duijn G (2014) Safety of fats and beverages: oils and fats. In: Motarjemi Y (ed.) Encyclopedia of food safety, pp. 315–323. Amsterdam: Elsevier Inc.
Further Reading Relevant Websites Blake Hendrix WM (1990) Neutralization I. Theory and practice of conventional caustic (NaOH) refining. In: Erickson DR (ed.) Edible fats and oils processing: basic principles and modern practices. Champaign, IL: American Oil Chemists’ Society. Dijkstra AJ (1999) Stripping medium requirements in continuous countercurrent deodorization. Journal of the American Oil Chemists Society 76: 989–993. Dijkstra AJ (2007) Bleaching of oils and fats. In: Gunstone FD, Harwood JL, and Dijkstra AJ (eds.) The lipid handbook, 3rd ed., pp. 212–231. Boca Raton, FL: Taylor & Francis Group LLC.
www.aocs.org. www.eurofedlipid.org. www.fediol.be. http://lipidlibrary.aocs.org/processing. http://apps.fas.usda.gov/psdonline/circulars/oilseeds.pdf. http://www.dr-baumann-international.co.uk/science/E-book%20Lipid% 20Glossary.pdf.
Introduction Oils and fats are mixtures of different organic molecules called lipids. Triglycerides are by far the most common component of oils and fats; these biological molecules are water-insoluble. Oils and fats can be extracted from plant or animal sources. The difference between a fat and an oil is that a fat is solid or semisolid at normal room temperature (20 C in Europe and the United States), while an oil remains liquid. Early vegetable oil production in Europe was mainly concerned with olive oil. For climatic reasons, it was limited to the Mediterranean; more northern countries used lard or butter as the fat source in their diet. This distinction still exists. Countries used to olive oil now use vegetable oil for cooking, the shortage of butter led to the invention of margarine, and lard has been replaced by vegetable shortening in countries that traditionally used lard. Oil production has evolved from the early process comprising a grinding step to open the cells in the oleaginous raw material and an expelling step in which the oil is squeezed out of the ground fruit or seed paste to a process that may also comprise a solvent extraction step. Oils and fats have been used since ancient times for food preparation and in nonfood applications like lamp oil, lubricant, soap manufacturing, and skin care. This article will only deal with the production of oils and fats for use in food preparation or as a food ingredient. Oils and fats in food provide functional and nutritional benefits. They serve as a heat transfer medium at elevated temperatures (frying), provide lubrication (spreads), improve taste (dressings), and give texture to a wide range of foodstuffs. On top of this, they supply a concentrated source of energy, deliver critical building blocks for the body, and act as a carrier for essential minor components like vitamins A and D. A balanced intake of oils and fats is essential for human health.
The Oil and Fat Supply Chains The supply chains of vegetable oils and fats consist of the following:
• • • • •
The growing and harvesting of the oil crop. Oil extraction to recover the oil; the by-product meal is mostly used as animal feed. Purification to reduce the level of unwanted minor components. Modification of the melting characteristics as required for particular products. All transport and storage from grower to end user.
The main origins of supply for the world vegetable oil and fat market fall into three main categories (more details will be given in other articles):
Encyclopedia of Food and Health
1. Annually planted oilseed crops like soybean, rapeseed, and sunflower seed 2. Fruit oils like palm and olive oil 3. Kernel oils like palm kernel oil, coconut, and other nut oils Oilseed crops are generally not grown in the regions where the oils are consumed. Oilseeds are transported and stored as seeds, and the separation into oil and meal is mostly performed close to the consumer markets. Under controlled conditions, oilseeds can easily be stored for a long time, at least until the next harvest arrives. Overseas shipping is mainly in the form of seeds. Like other fruits, fruit oils have a short shelf life. After a few days, they start to deteriorate, resulting in a poor oil quality. The extraction of oil from fruits is done close to the crop growing area, and the extracted oil is then stored and transported to the consumer markets. Overseas shipping is only as oil. In contrast, kernels can be stored after drying and transported over long distances, although extraction is generally performed where the fruits are grown. Overseas shipping is mainly as oil.
Vegetable Oil Extraction Olive Oil The annual world production of olive oil varies around 2.8 million tonnes. Spain is the main producer with an average annual production of 1.2 million tonnes, followed by Italy and Greece. After harvesting, the olives are first of all cleaned and washed with water. Then, they are crushed in a hammer mill from where the resulting paste is fed to a malaxer in which the paste is slowly agitated to allow the dispersed oil droplets to coalesce. Talc and/or enzymes can be added to improve the oil yield. In old olive oil mills, the paste is then spread onto circular filter cloths that are stacked inside a hydraulic press. Modern mills use a decanter to separate the oil from the waste. If a twophase decanter is used, this will separate the paste into some 80 wt.% wet pomace and 20 wt.% turbid oil. This oil is then washed with 10–20% water to yield clear oil and not too dirty washing water. The amount of water used affects the residual polyphenol content of the oil and is therefore quite critical. When a three-phase decanter is used, only 50 wt.% dry solids (pomace) are produced, but the addition of further washing water leads to a large effluent stream (black water) that has to be treated before it can be discharged. When the pomace is extracted with, for example, hexane, the oil thus obtained can no longer be called ‘virgin.’ It has to be refined to yield a bland olive oil called ‘pomace oil.’
Oils from Seeds with a High Oil Content Although the production of oil from seeds with a high oil content (around 40%) also leads to the meal by-product, the
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value of this by-product is low in comparison with the oil. These seeds are therefore grown for their oil. Unlike fruit oils, seeds can be stored so that an oil mill processing oilseeds can operate the whole year round. After harvesting, the seeds are dried to a water content of less than 16% of the dry, oil-free matter to prevent microbial spoilage during storage. At the oil mill, the seeds are first cleaned to remove any stones and other extraneous matter that might damage the flaking rolls. Then, they are conditioned to a temperature of 60–70 C to soften the cell so that the subsequent flaking requires less electrical energy. In the flaking process, two perfectly cylindrical rolls flatten the seeds so that cells within the seed are opened and oil can be squeezed out of the flakes without having to pass through a cell wall. The flakes are passed to a screw press that is fitted with a decreasing-volume Archimedean screw that builds up pressure along its shaft. This shaft rotates inside a slotted barrel that allows the oil to pass through. This oil contains many meal particles that have to be removed. In a first clarification process, the oil is screened or allowed to settle, and then further clarification is realized by filtration or the use of a decanter centrifuge. The solids removed contain a substantial amount of oil and are therefore returned to the screw press. The press cake leaving the screw press is too hot to enter the solvent extraction plant. It therefore has to be cooled and broken into pieces to permit proper percolation during solvent extraction. A very effective way of treating the press cake is by pelletizing, since this treatment also ruptures cells that survived the flaking and pressing processes. When the cake has reached a temperature of no more than 60 C, it is ready for solvent extraction, which will be discussed in the next section. Because the hull of a sunflower seed accounts for up to 30% of its weight, it can be advantageous to dehull the seeds prior to the expelling step. This increases the capacity of the screw presses and the extraction plant; it lowers the wax content of the oil and increases the protein content of the meal. The dehulling process comprises the opening of the seeds by impact and a separation by winnowing. Some hulls have to be included in the feed of the screw press to give it some body. The hulls are usually used on-site as a boiler fuel.
Oils from Seeds with a Low Oil Content The prime example of an oilseed with a low oil content is the soybean that contains around 18–21% oil. Soybeans are primarily grown for the protein content of the meal that is used in the animal feed industry on a large scale; the oil can be regarded as a by-product. Because its use in poultry farming requires a high (>48%) protein content and a low (<3.5%) fiber content, the beans are often dehulled before being extracted. In the classical dehulling process, the beans are dried and held for tempering. This makes the beans shrink away from the hulls so that after cracking, the hulls can be separated from the cracked cotyledons by aspiration. In the so-called ‘hot dehulling’ process, the beans are conditioned to a temperature of 65–70 C in a tower in which tubes, which are heated with low-pressure steam, heat the beans quite quickly. This and the subsequent superficial heating in a fluid bed dryer cause the hull to shrink away from the cotyledon. Cracking leads to a
mixture of separate hull fragments and cracked cotyledons that are separated in cascade aspirators. For the cracking process, two or three sets of superimposed corrugated rolls are used wherein opposing rolls run at different speeds to grip the beans. Cracking should aim at breaking the bean into four to six smaller pieces and avoiding fines and mashed beans. Subsequently, the cracked cotyledons are flattened into flakes of some 0.25–0.30 mm thickness in between flaking rolls that are similar to those used for high oil content seeds. Processing the flakes in an expander is optional. The expander is a kind of extruder in which the flakes are compressed until on leaving the die, the compacted mass expands causing further cell rupture. The resulting collets have the advantage that they have a higher bulk density than the flakes, so expanders are often used to increase the capacity of an existing oil mill. Collets also have the advantage of a reduced liquid holdup, which saves energy in the desolventizer. The extraction solvent used is invariably hexane. Industrial hexane is a mixture of alkanes with a boiling range of 66–69 C comprising n-hexane (62%), methylpentanes (23%), and methylcyclopentane. It is highly inflammable and its explosive range in air is 1–8% by volume. Consequently, extractors are operated at a slightly subatmospheric pressure to prevent the hexane from escaping. Solvent extractors operate countercurrently to minimize the oil content of the extraction residue leaving the extractor and maximize the oil content of the miscella. Two types of extractor are in industrial use: percolation and immersion. In the immersion type, the screen under the material being extracted has so little open area that it constitutes the greatest flow restriction. Immersion extractors provide more contact time between solvent and seed material but require more draining time than percolation-type extractors. Both types ensure a miscella strength of around 30% oil and a residual oil content in the meal of < 1 wt.%. The solvent–wet extraction residue or ‘marc’ leaving the extractor is then treated to remove the solvent, first by indirect heating and then by heating with live steam. This raises the temperature of the meal to 105–100 C in the toasting section where antinutritional enzymes are denatured. Since the live steam causes the moisture content of the meal to increase, it has to be dried. Air is used to dry the meal to a residual water content of 13–14 wt.% and cool it down at the same time. The hexane-laden vapors leaving the desolventizer are at atmospheric pressure. Using their latent heat to evaporate the hexane from the miscella requires the latter to be under a reduced pressure; in industrial practice, this pressure is about 500 hPa absolute. The latent heat content of the vapors leaving the desolventizer suffices to evaporate < 90% of the hexane present in the miscella so that the composition of the liquid leaving the first evaporator is around 80 wt.% oil. It is further concentrated using steam and finally treated in a steam stripper operated at 200–400 hPa absolute. If the oil has to be shipped, it must be degummed with water to prevent a deposit being formed during transport and storage. In this water degumming process, an amount of water that is about the same as the gum content of the oil is mixed with the hot oil, allowed to hydrate the mucilaginous matter. and then separated by a centrifuge. Drying these gums yields lecithin.
Vegetable Oils: Oil Production and Processing
Vegetable Oil Refining Process Sequence The vegetable oil refining process was introduced at the start of the twentieth century to produce good-quality oils and fats for use in margarine production and as frying oil. Before 1900, oils were cleaned by removing water and solids by decanting. The refining process has been optimized to reduce the natural taste and color and to remove most of the free fatty acids present in the extracted oil. Later, it was discovered that the process also removes many of the contaminants present in the extracted oils, like solvent residues, metals, polyaromatic hydrocarbons, and pesticide residues, bringing these contaminants below the legal limits. The refining process is a combination of the following process steps; more details are given in the following sections:
• •
•
Degumming: A pretreatment process mainly applied to seed oils to reduce their phospholipid contents. Neutralization: The purpose of neutralization is to reduce the concentration of free fatty acids by reaction with a diluted alkali solution, typically sodium hydroxide. The reaction product (soap) is removed by decanting or by centrifugation. Bleaching: The main purpose is to remove residual soap, color pigments, and oxidized components. In this process,
• •
bleaching earth (natural or activated clay and/or silica) is added to the oil as an adsorbent. The earth and adsorbed impurities are subsequently removed by filtration. Dewaxing: Removal of natural waxes present in some vegetable oils by cooling and filtration. Deodorization: Under high vacuum, the oil is heated to 180–270 C, while steam is blown through the oil. The objective is to remove volatile components and to create an odorless oil with a bland taste and increased storage stability.
The process sequence of combined degumming/neutralization followed by bleaching and deodorization is called chemical refining, referring to the reduction of free fatty acids by a chemical reaction. The process sequence of degumming followed by bleaching and deodorization is called physical refining, referring to the reduction of free fatty acids by the physical process of vacuum stripping. An overview of the possible process sequences is given in Figure 1.
Degumming Processes As indicated in the refining process flow sheet (Figure 1), many different routes lead from crude oil to fully refined oil, and the number of steps can differ.
CRUDE OIL
water degumming or enzymatic degumming
oil with NHP
acid degumming
acid refining
oil with < 30 ppm P
enzymatic PLA gum
dry degumming alkali neutralization
oil with < 5 ppm P
bleaching
bleaching
deodorizing
FULLY REFINED OIL
deodorizing/ stripping
Figure 1 A block diagram of possible vegetable oil refining process routes.
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The route with only two steps consists of the dry degumming process, followed by physical refining. In the dry degumming process, some phosphoric acid is mixed into the oil and allowed to react with the phosphatides. Then, bleaching earth is added, and this adsorbs the phosphatides, the excess phosphoric acid, and also the coloring matter present in the oil. This bleaching earth is removed by filtration, and the bleached oil is ready for physical or steam refining. Since the bleaching earth requirement in the dry degumming process is proportional to the phosphatide content of the crude oil, this two-step route is only economical for oils and fats with a low phosphatide content. Examples are palm oil, lauric oils, lard, and tallow. In the acid degumming step, the crude or water-degummed oil is treated with a strong acid that is subsequently diluted with water to form a gum phase that is removed by centrifuge. However, this dilution raises the pH insufficiently to pull all the gums into the gum phase. Consequently, the acid-degummed oil requires a further degumming treatment before the oil can be physically refined. In the acid refining process, the crude or water-degummed oil is also treated with a strong acid, but instead of diluting the acid with water, it is neutralized with caustic soda. This raises the pH to a value where still no soaps are formed but where all the phosphatidic acid that has been liberated by the acid treatment is dissociated. It therefore moves completely into the aqueous phase and can be separated from the oil as sodium phosphatidate. The acid refining process produces an oil that can be physically refined after it has been bleached. The water degumming process and the acid refining process produce a gum phase as by-product. This gum phase also contains oil, and if the gums are disposed of by mixing with the meal, the oil is sold at meal value that is less than if it were to be sold as oil. To recover at least some of this oil, enzymatic gum treatment processes using a phospholipase have been developed. As shown in Figure 1, phospholipase C (PLC) can be used in the water degumming step or on the gums obtained by water degumming. This enzyme catalyzes the hydrolysis of the bond between the phosphate moiety and the glycerol moiety in phosphatides as indicated in Figure 2. This hydrolysis generates diglycerides that move into the oil phase and
A1 H2C O R2
C O
O
CH H2C
Another phospholipase that can be used to treat the gums and thereby increase the oil yield is phospholipase A (PLA). Figure 2 shows that there are two enzymes that catalyze the hydrolysis of a fatty acid ester bond: PLA1 and PLA2. Both enzymes are commercially available. They form lysophosphatides that retain less triglyceride oil and free fatty acids that are removed during a subsequent neutralization process. They can be added to oil that has already been acid-refined or to the gums resulting from the acid refining treatment. Because the oil savings resulting from the various enzyme treatments are proportional to the amount of gums, these treatments only make sense when applied to oils with a high phosphatide content.
Neutralization Processes In the neutralization process, oil containing FFAs is treated with lye, a solution of sodium hydroxide or caustic soda in water. This reacts with the FFA to form sodium soaps that are insoluble in the oil and therefore form a separate soap phase. This soap phase also contains phosphatides, when present in the crude oil, and triglyceride oil. After this soap phase has been separated from the oil, it is called soap stock. Formerly, it was used in a soap works adjacent to the refinery. It can be used as an animal feed ingredient, but in Europe, it is more common to react the soap stock with sulfuric acid at elevated temperatures and recuperate its fatty matter as ‘acid oils,’ a mixture of fatty acids and triglyceride oil. This process also generates an acid-aqueous effluent stream that contains phosphates and sulfates and has a high BOD. It has to be neutralized, its phosphate may have to be removed by a lime treatment, the resulting precipitate will then have to be separated, and then what is left has to be treated in an effluent plant to lower the BOD. All this makes soap stock splitting a cumbersome process that is preferably avoided. It has been a powerful driving force for the shift towards physical refining. In alkali refining, there are two routes. The oldest route that has been developed and is still in use in the United States is the
O C
A2
thus contribute to oil yield. It also produces phosphate esters of choline, which are water-soluble and do not retain any oil.
R1
H2 C
HO
D choline
O O
P
O
H2 C
X HO
C
CH3 H2 C N CH3 CH3
H2 C
NH2
O
X = choline (phosphatidylcholine or PC)
ethanolamine OH OH
X = ethanolamine (phosphatidylethanolamine, PE) X = inositol (phosphatidylinositol or PI) X = hydrogen (phosphatidic acid or PA)
OH O
OH OH
inositol, link in 1-position Figure 2 Chemical structure of phosphatides and indications which phospholipase enzyme catalyzes the hydrolysis of which bond.
Vegetable Oils: Oil Production and Processing so-called ‘long mix’ process. Dilute lye (14–18 Be´ or 130–180 g NaOH/L of water) is added and mixed into cold oil, and then the oil is heated until a clear separation between oil and soap stock is obtained. The soap stock is removed by centrifugal separator, and to lower the residual soap content, the oil is either washed with water or treated with silica hydrogel. In Europe, the ‘short mix’ process is almost universally used. In this process, a degumming acid (phosphoric acid) is finely dispersed in the hot (90–100 C) oil, it is allowed to react in a holding vessel, and then less dilute lye (20–28 Be´ or 145–215 g NaOH per liter of water) is dispersed in the oil. Separation between oil and soap stock is almost instantaneous, allowing the mixture to be sent to the centrifuge without much holding time. Again, the oil phase can be washed or treated with silica hydrogel, but in Europe, water washing is more common.
Bleaching of Vegetable Oils Crude and degummed/neutralized vegetable oils differ widely in color. Palm oil is deep red, rapeseed oil is greenish, and sunflower seed oil has not much color. These colors are caused by two groups of compounds. The red color stems from carotenes, with b-carotene being the most common, and the green color originates from chlorophyll. Carotenes are thermally unstable so they break down when the oil is deodorized or physically refined. Because palm oil also contains the thermally more stable chlorophyll, which is removed by bleaching earth, palm oil has to be treated with this adsorbent to yield an almost colorless fully refined oil. The bleaching process itself is quite simple. Depending on the color of the oil, an amount of 0.5–4.0 wt.% of bleaching earth is mixed into hot (80–110 C) oil, the suspension is given some time to approach the adsorption equilibrium, and the earth is then removed by filtration. The filters used for this step are invariably pressure leaf filters that are precoated by circulating oil with bleaching earth over the filters. Filter aids like diatomaceous earth can also be used. Bleaching earth is a pretty universal adsorbent, but it insufficiently adsorbs certain compounds that are considered to be undesirable. Among these compounds are the polyaromatic hydrocarbons (PAHs). Low-molecular-weight polyaromatic hydrocarbons are sufficiently volatile to be removed by vacuum stripping, but high-molecular-weight polyaromatic hydrocarbons require another means of removal. This means is provided by treating the oil with activated carbon, a highly effective adsorbent for this kind of compounds. Bleaching clay and activated carbon can be used simultaneously.
Dewaxing of Vegetable Oils Some vegetable oils like sunflower seed oil, rice bran oil, and corn germ oil contain waxes. These waxes are esters of fatty acids and fatty alcohols. When both of them are saturated, the wax has a melting point > 70 C, which causes the waxes to crystallize at refrigeration temperatures and form an unsightly deposit. To prevent this from happening, the oil has to be dewaxed. In the dewaxing process, the oil is cooled down to cause the waxes to crystallize. They can then be removed by filtration with a pressure leaf filter and filter aid. Since the
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amount of filter aid is proportional to the wax content and oil is also retained in the filter cake, filtration becomes quite expensive when the oil has a high wax content. Then, it is more economical to remove the bulk of the waxes in a wet dewaxing process using a centrifugal separator and remove residual waxes by filtration. This wet dewaxing process employs water with a detergent, which can be soap, to concentrate the wax crystals in the aqueous phase. It can therefore be combined with the neutralization step. The ideal cooling profile comprises a slow cooling from above the cloud point of the oil to below this cloud point to limit the number of crystal nuclei. Cooling from below this cloud point can be fast and maturation is not necessary.
Deodorization The deodorization process is carried out at a high temperature (180–270 C). Because of this high temperature, residual peroxides and carotenes will decompose, and when the temperature is too high, trans isomers can be formed. The more double bonds a fatty acid contains, the more prone it is to this kind of isomerization. This is the reason why refined oil specifications contain an upper limit of 1.5 wt.% for oils containing linolenic acid (soybean oil and canola oil) and an upper limit of 1.0 wt. % for sunflower seed oil and corn oil, which do not contain this fatty acid. Apart from these chemical reactions, vacuum stripping is a physical process that can be described by the laws of Raoult and Dalton. The reduction of volatiles in the oil depends on the stripping steam flow, the vacuum pressure, the oil temperature, the oil volume, the vapor pressures of the volatiles, and the equipment design. The deodorization process can be carried out in batch, continuous, or semicontinuous installations. Batch deodorizers have one single compartment in which the process steps of deaeration, heating, stripping, and cooling are sequential in time. In continuous deodorizers, the process steps are simultaneous in different compartments (trays) of the deodorizer, and both input and output are constant over time. In semicontinuous deodorizers, the process steps are simultaneous in the different trays, but tray filling and emptying is discontinuous. Filling stops after a certain level has been reached in the first tray. After allowing sufficient time to perform the process step for that tray, the bottom valve is opened and the content drops into the next tray for the next process step. Figure 3 shows a basic layout of a semicontinuous deodorizer. Oil can also be fed to the top of a packed column and trickled down through this column, while the stripping flows countercurrently. This type of column has a lower stripping steam usage than the crossflow system to reach the same final concentration of volatiles. Obtaining stable oil is more than just getting rid of the volatiles present in the oil to be deodorized or physically refined. When triglyceride oils are heated for prolonged periods, strong off-flavors are developed. Deep fat frying in coconut oil smells different from frying in sunflower oil, let alone tallow. The chemical nature of these smells is not known, and since they are not released by freshly deodorized oil, they must be formed during deep fat frying from flavor precursors. Accordingly, the deodorization process should
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Vegetable Oils: Oil Production and Processing
Degasser Vacuum system
Heat exchange
Final heating
HP steam boiler Stripping steam
Cooling water Guard filters Citric Acid
Pre-treatment Cooler
Storage
Figure 3 The layout of a semicontinuous deodorizer. From G. van Duijn, lecture material.
provide some holding time at a high temperature to enable the thermal breakdown of these flavor precursors. There are two reasons why the deodorization process is the most energy-intensive step of the refining process. Firstly, it is carried out at temperatures of up to 270 C, and secondly, it is carried out under a vacuum of 2–10 hPa, which has traditionally been maintained by steam ejectors. Accordingly, heat exchange between ingoing and outgoing material either directly in a plate heat exchanger or indirectly by using one or even two thermosiphons has led to energy savings. The use of a mechanical vacuum pump in combination with a surface condenser for the distillate has limited the steam usage to the stripping steam but increased the consumption of electrical energy.
Vegetable Oil Modification Processes The Purpose of Oil Modification When Me`ge-Mourie`s invented margarine in 1869, he used tallow olein as the fat phase, but when more and more margarine was produced, a shortage of solid fat emerged. Accordingly, the invention of the hydrogenation process in 1903 led to an almost immediate industrialization despite the fact that this also involved building plants for elemental hydrogen. Me`geMourie`s also described a dry fractionation process to obtain his tallow olein, but that process only began to be applied on a large scale with the increased production of palm oil in the last quarter of the twentieth century. Networks of solid fat crystals play an important role in many food products. For example, margarines consist of an
emulsion of small droplets of an aqueous phase dispersed in liquid oil stabilized by a structure of solid fat crystals. The fat crystals supply structure and texture to the product, they prevent the oil from leaking out of the product, and they avoid coalescence of the dispersed aqueous phase droplets. Solid fat crystal networks have similar functions in whipped cream and ice cream, where they stabilize a foam of air bubbles dispersed in an emulsion of fat droplets in water; they also play an important role in bakery products. Most of the seed oils are too low in solids at the temperature at which the product is used (generally ambient temperature) to fulfill this role. Tropical oils, such as palm oil, palm kernel oil, and coconut oil, have a higher solid fat content, but for many applications, they are still too soft. Modification of the melting performance of naturally occurring oils and fats is required to produce fats with optimal properties for the aforementioned applications. The modification technologies that can be applied are the following: 1. Hydrogenation 2. Interesterification 3. Fractionation
Hydrogenation Hydrogenation of oils and fats has been applied on a large scale since the start of the past century to produce hard fats from a wide variety of oils and fats (mainly liquid oils and fish oil). Hydrogenation involves the addition of two hydrogen atoms to unsaturated double bonds (C]C) in the fatty acid moieties of the triglycerides. The more saturated fat resulting
Vegetable Oils: Oil Production and Processing
from this reaction has a higher melting point than the starting material. In the hydrogenation process, hydrogen gas reacts with liquid oil at elevated temperatures and pressures in the presence of a solid catalyst. The hydrogenation of edible oils is always carried out as a batch process. Oil is preheated by the previous batch, the catalyst (nickel on an inert support) is added, and hydrogen gas is dispersed in the oil. Hydrogenation is an exothermic reaction so the temperature of the batch starts to rise and has to be controlled by cooling. The reaction is continued until the product properties aimed for have been reached (partial hydrogenation). The catalyst is removed by filtration and residual nickel is removed in a post-bleaching step. As well as saturation, the isomerization of double bonds also takes place during the hydrogenation reaction. Two types of isomerization reaction will occur: geometric isomerization (cis–trans) and positional isomerization (shift of double bond along the fatty acid chain). The complete reaction mechanism is quite complicated because of the many steps involved, and it is still not yet fully understood and kinetically quantified. In response to the findings of adverse effects of trans fatty acids on blood lipids, the food industry has practically eliminated components containing trans fatty acids from their fat phase compositions. This has drastically reduced the application of partial hydrogenation as a method of producing solid fats. Full hydrogenation of almost all double bonds reduces the trans isomer content to an acceptable level but results in a product with a very high melting temperature. Such a product can only be used after interesterification with a liquid oil.
Interesterification Interesterification involves a rearrangement or reshuffling of the fatty acids on the glycerol backbone of the triglyceride molecule. Interesterification is catalyzed by an alkaline catalyst or by a lipase enzyme. The most commonly used alkaline catalysts are sodium methanolate and sodium ethanolate. The early interesterification process was used to extend the plastic range of lard used as shortening. Later blends of different raw materials were used to change their physical properties so that they could be used as hard fat in margarine and as shortening. For a long time, the interesterification reaction has been assumed to proceed with a glycerolate anion as the catalytic intermediate. This anion would then attack the slightly positive carbon atom of a carboxyl group. If this group left as a hydroxyl anion, ester interchange would have been the overall result. An alternative reaction mechanism has been proposed recently that assumes that the initial catalyst (sodium methanolate) abstracts an a-hydrogen from a fatty acid moiety, thereby forming an enolate anion. This enolate anion then acts as the catalytic intermediate. When it reacts with water, it will form a free fatty acid, the formation of which could not be explained by the glycerolate mechanism. The alkali (sodium methanolate or ethanolate)-catalyzed reaction takes place in prerefined oil (low in water and in free fatty acids) at elevated temperatures (100–110 C). The reaction is very fast; full randomization results within a few minutes even in factory-scale vessels (10–40 t oil content). After the reaction, the catalyst is inactivated by the addition of water. This addition of water leads to the formation of free
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fatty acids and sodium hydroxide that will combine to form soaps. The reaction of sodium methanolate with a fatty acid moiety leads to the formation of methanol, and when this reacts with an enolate anion, methyl esters will be formed. The soap is removed by water washing and decanting. Methyl esters are more volatile than triglyceride molecules and are removed during high-temperature deodorization. Both soap and esters constitute a yield loss. In addition to the basic catalysts used, lipase enzymes have also been introduced for the interesterification process. Interest in the use of lipase originates from the attempts to produce cocoa butter equivalents (CBEs), symmetrical monounsaturated triglycerides that are compatible with cocoa butter. This isomeric requirement ties in with the sn-1,3 specificity of the lipase enzyme. Interesterifying a triglyceride that consists almost entirely of trioleate (high-oleic sunflower seed oil, for instance) with stearic acid or methyl stearate should generate a CBE. Lipase enzymes are also used in a nonstereospecific interesterification process. To overcome the fundamental problem that enzymes need water to be active but use this water to hydrolyze fats, this process uses a large amount of rather dry, immobilized enzymes. It is a continuous process, using four reactors that are filled with enzymes. The first of these, which is filled with an enzyme that has lost most of its activity, acts as a kind of guard for downstream reactors. It is emptied when the product leaving the last reactor is insufficiently randomized, filled with fresh enzymes, and relocated as the last reactor. What impurities in the oil cause the enzyme to lose its activity is still not quite clear, so as a precaution, the oil fed to the first reactor has been neutralized, bleached, and deodorized. The enzymatically interesterified oil needs to be deodorized before it can be used in food products.
Fractionation Fractionation is the controlled crystallization of the more saturated and/or longer-chain triglycerides, followed by separation of the solid phase (stearin) and liquid phase (olein). By far, the most important oil, fractionated worldwide, is palm oil, the main reason being the demand for clear liquid oil (palm olein). More recently, there has been a growing interest in the solid product of palm oil fractionation (palm stearin) for the production of cocoa butter equivalents, cocoa butter replacers, and margarine hard stocks. Besides palm oil, palm kernel oil, partly hydrogenated liquid oils, cottonseed oil, and milk fat are also fractionated.
Processes for Producing Virtually Trans-Free Hard Stocks The combination of full hydrogenation, interesterification, and fractionation and the availability of a variety of different oils and fats can be used to produce virtually trans-free hard stocks with various melting ranges. Liquid seed oils, low in solid fats, are first fully hydrogenated to generate solids combined with a very low trans level (<1.25%). These fully hydrogenated oils may subsequently be interesterified with nonhydrogenated liquid oil to reduce the solid fat content at high temperature (>40 C). This solid fat content can be further reduced by fractionation (see Figure 4).
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Vegetable Oils: Oil Production and Processing
Full hydrogenation
Liquid oils
Interesterification
TFA-free fat-phase component
Tropical oils
Fractionation Figure 4 Process routes to produce trans-free hard fat components.
The presence of relatively high solid contents in tropical oils creates more flexibility in oil modification routes. Fractionation alone will produce a relatively soft stearin, which is not optimal for structuring margarine. Fractionation followed by interesterification with other (fractionated) components is used to produce ‘nonhydrogenated’ hard fats. Full hydrogenation followed by interesterification is an alternative way to obtain a hard fat high in solids with a steep melting line without fractionation (see Figure 4).
See also: Carotenoids: Occurrence, Properties and Determination; Drying: Principles and Types; Enzymes: Analysis and Food Processing; Fatty Acids: Fatty Acids; Fatty Acids: Trans Fatty Acids; Heat Treatment: Principles and Techniques; Oxidation of Food Components; Phospholipids: Properties and Occurrence; Quality Control in Food Processing; Rapeseed Oil/Canola; Sunflower Oil; Tocopherols: Properties and Determination; Triacylglycerols: Characterization and Determination; Triacylglycerols: Structures and Properties.
Dijkstra AJ (2007) Degumming of oils and fats. In: Gunstone FD, Harwood JL, and Dijkstra AJ (eds.) The lipid handbook, 3rd ed., pp. 177–191. Boca Raton, FL: Taylor & Francis Group LLC. Dijkstra AJ (2010) Enzymatic degumming. European Journal of Lipid Science and Technology 112: 1178–1188. Dijkstra AJ (2012) Kinetics and mechanism of the hydrogenation process - The state of the art. European Journal of Lipid Science and Technology 114: 985–998. Dijkstra AJ (2014) Oil refining. In: Dunford NT, Martinez Force E, and Salas Lin˜a´n JJ (eds.) Sunflower, Chemistry and Technology. Urbana: AOCS Press. Dijkstra AJ, To˜ke ER, Kolonits P, Recseg K, Ko˜va´ri K, and Poppe L (2005) The basecatalyzed, low temperature interesterification mechanism revisited. European Journal of Lipid Science and Technology 107: 912–921. Johnson LA (2008) Oil recovery from soybeans. In: Johnson LA, White PJ, and Galloway R (eds.) Soybeans, production, processing, and utilization, pp. 331–376. Urbana: AOCS Press. Kemper TG (2013) Solvent extraction. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 97–125. Chichester: John Wiley & Sons. Van Doosselaere P (2013) Production of oils. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 55–96. Chichester: John Wiley & Sons. Van Duijn G (2000) Technical aspects of trans reduction in margarines. Oleagineux, Corps Gras, Lipides 7(1): 95–98. Van Duijn G and Den Dekker G (2013) Oil processing design basics. In: Hamm W, Hamilton RJ, and Calliauw GH (eds.) Edible oil processing, pp. 267–310. Chichester: John Wiley & Sons. Van Duijn G (2014) Safety of fats and beverages: oils and fats. In: Motarjemi Y (ed.) Encyclopedia of food safety, pp. 315–323. Amsterdam: Elsevier Inc.
Further Reading Relevant Websites Blake Hendrix WM (1990) Neutralization I. Theory and practice of conventional caustic (NaOH) refining. In: Erickson DR (ed.) Edible fats and oils processing: basic principles and modern practices. Champaign, IL: American Oil Chemists’ Society. Dijkstra AJ (1999) Stripping medium requirements in continuous countercurrent deodorization. Journal of the American Oil Chemists Society 76: 989–993. Dijkstra AJ (2007) Bleaching of oils and fats. In: Gunstone FD, Harwood JL, and Dijkstra AJ (eds.) The lipid handbook, 3rd ed., pp. 212–231. Boca Raton, FL: Taylor & Francis Group LLC.
www.aocs.org. www.eurofedlipid.org. www.fediol.be. http://lipidlibrary.aocs.org/processing. http://apps.fas.usda.gov/psdonline/circulars/oilseeds.pdf. http://www.dr-baumann-international.co.uk/science/E-book%20Lipid% 20Glossary.pdf.