you're reading...
Oil Purification Process

Physical Refining – Deodorisation

A Short History

In the early days of the edible oil processing industry, in the first half of the 19th century, there was little or no need for refining. Food fats (e.g. lard, olive oil, milk fat, etc.) were mostly consumed unrefined and their typical flavour was even an attractive characteristic. It was the growth of the margarine industry in Europe at the end of the 19th century that resulted in the development of the edible oil Deodorisation process.

In 1869 Mege Mouries, a Frenchman, invented margarine as a substitute for butter, which was in short supply due to the Franco-German war. The product relied on the careful rendering of animal fats to generate fats that were relatively neutral in flavour and odour. Any residual minimal animal flavour associated with the fat was sufficiently reminiscent of butter that it was not perceived as objectionable.

With the growth of margarine as an economical alternative to butter the demand for odourless and tasteless animal fats exceeded the supply, leading to interest in the use of vegetable oils in margarine. The major obstacle to their use was the strong natural flavour of oils such as soybean oil and sunflower oil, which can be readily used to distinguish between them. This provided the incentive to generate bland vegetable oils.

Also the rapid expansion of cotton acreage at the end of the nineteenth century resulted in large quantities of cottonseed oil, which presented an economic incentive to use this vegetable oil. Caustic refined and bleached cottonseed oil was offered as a cooking or salad oil and blended with tallow or olein stearine as a lard substitute. These products enjoyed a price advantage over lard and olive oil, but the unpleasant flavour was so strong that acceptance was poor. In addition, the hydrogenation process developed to harden vegetable oils imparted a more disagreeable flavour and odour to the oils. Attempts to remove the flavour and odours chemically or to mask them with spices or flavours were unsuccessful.

Experimental studies showed that it was possible to boil the flavour- and odour-causing components out of the essentially non-volatile oil but with a severe risk of damaging or burning the oil due to the high temperature employed. The first successful attempt at removing the disagreeable odours and flavours from a fat and oil consisted of injecting live steam into an oil at high temperatures. It was discovered in England, but this flavour-improvement process was soon adapted by most American fats and oils processors. The advantages of treating oils with steam to remove offensive flavours and odours was recognized in the early 1890s by Henry Eckstein. In the USA, it was Eckstein who developed the first industrial deodorizer. In 1891, he demonstrated that the flavour of alkali refined cottonseed oil could be greatly improved by blowing live steam through the oil at high temperature (160-175°C). The most successful American deodorizing process was that of Wesson, which was introduced in 1900 by the Southern Cotton Oil Company.

David Wesson improved the process by using higher temperatures and maintaining the oil under vacuum while blowing with superheated steam. This allowed further reductions in operating temperatures while minimizing the risks of oxidizing the oil due to air exposure and limiting the level of hydrolysis during the stripping process. The process was not patented and kept secret for a time but it was probably the first vacuum deodorizing process in the US. The quality of Wesson’s deodorized oil was for many decades a standard for edible oils throughout the world. This improved process allowed the development of cottonseed oil, previously considered to be inedible owing to the presence of highly flavoured non-triglyceride components, as a replacement for animal fat in shortenings and liquid oils in the United States and laid the foundation for the current vegetable oil industry.

Purpose of Deodorisation

The final step in refining of fats and oils is deodorisation. It was introduced at the end of the 19th century to improve the taste and smell of refined oils. Today, the process is still commonly named ‘deodorisation’, but the objectives have become much broader than just the removal of off-flavours. In fact, the current deodorisation process has four main objectives:

(1) Stripping of volatile components such as FFA (in the case of physical refining), valuable minor components (tocopherols, sterols etc.) and contaminants (pesticides, light polycyclic aromatic hydrocarbons etc.)

(2) Actual deodorisation by removal of different off-flavours

(3) Thermal destruction of pigments (so-called heat bleaching) and peroxides

(4) Rendering of the oil, by means of some chemical change, as more flavour-stable during its shelf life.

Deodorisation increases the oil’s flavour and oxidative stability by nearly complete removal of FFA and other volatile odour and flavour materials, by partial removal of tocopherols, and by thermal destruction of peroxides. The thermal treatment that is a necessary part of the Deodorisation process also heat bleaches the oil by destruction of the carotenoids that are unstable at Deodorisation temperature.

Materials Removed During Deodorisation

Deodorisation is a vacuum-steam distillation process of an oil at an elevated temperature during which FFA and minute levels of odoriferous materials are removed to obtain a bland and odourless oil.

By virtue of their geographic location and climate, neither North America nor Europe have any indigenous oilseed crops that generate a solid fat like cocoa butter or palm oil that could be used to provide a hard stock for product formulations. As a consequence, hydrogenation has become the key process for modifying the physical characteristics of vegetable oils. Apart from achieving the desired change in the oil’s characteristics, hydrogenation also has a tendency to generate typical off-flavours that are not found in non-hydrogenated oils.  Also bleaching imparts an “earthy” flavour and odour. Fortunately these flavours are also readily removed by Deodorisation.

The materials removed by Deodorisation include free fatty acids (FFA); various flavour and odour compounds classified largely as aldehydes, ketones, alcohols, and hydrocarbons; and other compounds formed by the heat decomposition of peroxides and pigments. As a by-product of the heat treatment received in the deodorizer, many oils emerge from the deodorizer lighter in colour than when they entered owing to the breakdown of pigments, predominantly carotenoids that are unstable at deodorizer operating temperatures. Concentration of individual constituents in the deodorised oil is generally no greater than about 0.1% unless the oil has been abused.

Long-chain aldehydes and ketones are major contributors to the odoriferous and flavour compounds found in vegetable oils together with breakdown products formed during the thermal decomposition of peroxides caused by exposure of the oil to air. Tracking the large number of odoriferous components found in a crude oil is a major undertaking, and fortunately, from a commercial perspective, it is not necessary, because in general these components exhibit vapour pressures similar to those of the free fatty acids found in vegetable oils that provide a ready marker for the success of the Deodorisation process. Normally the level of flavour and odour components in an oil prior to Deodorisation is less than 1000 ppm, and with good handling and refining it can be as low as 200 ppm. Combining this with the human palate’s frequent ability to detect such compounds in the 1–10 ppm concentration range, and in some cases the parts per billion (ppb) range, sets a high target for the Deodorisation process. With normal Deodorisation practice, by the time the peroxide value of the oil is approaching zero and the free fatty acid level of the oil has been reduced to 0.02%, the majority of the flavour and odour components in the oil have been successfully removed. It must, however, be remembered that the achievement of a low free fatty acid level does not guarantee an acceptable oil flavour, because low levels of air leakage into the deodorizer during processing or high levels of prior oxidative abuse can result in flavour component concentrations that exceed the deodorizer’s ability to remove them and generate a bland oil.

Summary of Component Removal During Deodorisation

Physical Removal

FFA: reduced to below 1%*
Odor compounds (aldehydes and ketones): reduced to acceptable levels
Tocopherols: partially removed
Sterols: partially removed
Methyl/ethyl esters: almost completely removed
Antioxidants (BHA, BHT): completely removed
Pesticides: partially removed
Polycyclic aromatic hydrocarbons: partially removed
Monoglycerides: partially removed

Chemical Reaction
Oxidized fatty acids: partially decomposed
Carotene: partially decomposed
Hydroperoxides: partially decomposed
Soaps: completely decomposed
Unsaturated fatty acids: some cis/trans isomerization

Deodorisation Principle

Deodorisation is actually a stripping process in which a given amount of a stripping agent (usually steam) is passed for a given period of time through hot oil at a low pressure. Hence, it is mainly a physical process in which various volatile components are removed. However, since it is usually carried out at high temperature (> 200°C), some chemical, thermal effects may take place as well.

As the temperature of a liquid mixture is raised, those components of the mixture with the lowest boiling points (and hence highest volatility) evaporate first, leaving behind the less volatile compounds with the higher boiling points. By reducing the pressure of the system the temperature at which the evaporation of the more volatile components occurs can be reduced, although the order in which the components of the mixture distill does not change.

An efficient removal of these volatile substances depends mainly upon their vapour pressure and their concentration in the oil. The vapour pressure for a given constituent is a function of the temperature and increases with increasing temperature (See Fig below).The lower the vapour pressure, the lower the volatility, and the more difficult it is to remove the constituent from the oil. The use of an inert (non-reacting) stripping agent effectively enhances the vaporization of the volatile components.

vapour pressure

The four process variables that determine the effectiveness of deodorisation are as discussed in detail below:

Pressure

The low absolute pressure necessary for low-temperature distillation of the odoriferous substances is affected by the vacuum system. The boiling point of the fatty acids and the vapour pressure of the odoriferous materials decrease as the absolute pressures decreases. The required low absolute pressure, usually between 2 and 4 mbar, is commonly generated by vacuum systems consisting of a combination of steam jet ejectors, vapour condensers, and mechanical vacuum pumps. Special vacuum systems have been developed to reach lower pressures and operating costs and, at the same time, reduce emissions by a more efficient condensing of the volatiles.

Temperature

Deodorisation temperatures must be high enough to make the vapour pressure of the volatile impurities in the oil conveniently high. The vapour pressure of the odoriferous materials increases rapidly as the temperature of the fat is increased. For example, the vapour pressure of palmitic fatty acid is 1.8 mm Hg (2.4mbar)  at 350°F (176.7°C), 7.4 mm Hg (9.86mbar) at 400°F (204.4°C), 25.0 mm Hg (33.3mbar) at 450°F (232.2°C), and 72 mm Hg (95.9mbar) at 500° F (260°C). Assuming that the vapour pressure–temperature relationship for all the odoriferous materials is similar to that of palmitic fatty acid, each 50°F (27.8°C) deodorizer temperature increase would triple the odoriferous material removal rate. Or, stated another way, it would take nine times as long to deodorize an oil at 350°F (177°C) than at 450°F (232°C). Higher deodorizer temperatures definitely provide shorter Deodorisation times; however, excessive temperature results in the development of the polymerization, isomerization to produce trans fatty acids, thermal cracking with formation of odoriferous and low boiling products, colour reversion, and distillation of tocopherols. Generally, trans fatty acid formation during Deodorisation is negligible below 428°F (220°C), becomes significant between 428 and 464°F (220 and 240°C), and is nearly exponential above 464°F (240°C).

Thermal degradation of the tocopherols becomes significant at Deodorisation temperatures above 500°F (260°C).113 It has been determined that twice as many tocopherols and sterols are stripped out at 525°F (275°C) as at 465°F (240°C), and that pressure variations of 2 to 6 mbar had only a slight effect on tocopherol/sterol stripping. Deodorizer operation at elevated temperatures can also promote thermal decomposition of some constituents naturally present in oils, such as pigments and some trace metal–pro oxidant complexes. The carotenoid pigments can be decomposed and removed by Deodorisation beginning at 446°F (230°C), therefore, a compromise must be determined between time and temperature for deodorizing particular fats and oils.

Optimum deodorizer operating temperatures vary from product to product. In general, animal fats require less stringent conditions than the vegetable oil products. Chemically refined oils are easier to deodorize than physically refined oils due to lower FFA levels and more effective removal of polar components, oxidation products, and pigments. Among the vegetable oils, those containing relatively short-chain fatty acids, such as coconut and palm kernel oils, require lower Deodorisation temperatures than the domestic oils composed of longer chain fatty acids. Hydrogenated oils are usually more difficult to deodorize because of higher FFA contents and the distinctive odour imparted by the hydrogenation reaction.

Stripping Medium

Steam deodorization is feasible because the flavour and odour compounds that are to be removed have appreciably greater volatility than do triglycerides. Operation at high temperature increases the volatility of these odoriferous compounds; furthermore, introduction of an inert gas, such as stripping steam, into the deodorizer greatly increases the rate at which the odoriferous compounds are volatilized. Steam is sparged to carry away the volatiles and to provide agitation.

While deodorisation removes FFA from the oil, the FFA content cannot be reduced below about 0.005% because hydrolysis of the oil by the stripping steam is continually producing more FFA. The rate of hydrolysis decreases as the oil’s FFA content decreases. Eventually the rate of FFA formation by hydrolysis equals the rate of removal by the stripping steam, and the oil’s FFA cannot be reduced any further. A reduction in absolute pressure in the deodorizer also reduces the hydrolysis rate by a small but definite amount.

 Due to this, nitrogen was proposed as a replacement for steam. However using nitrogen give rise to the following disadvantages:

  1. Cost of nitrogen is higher than steam
  2. Larger capacity vacuum system required

 Any inert gas can be used, but steam has the advantage of being readily available and is readily condensed; thus cost of the vacuum-producing equipment is minimized

The amount of stripping steam required is a function of both the absolute operating pressure and the mixing efficiency of the equipment design. Agitation of the oil, necessary to constantly expose new oil surfaces to the low absolute pressure, is accomplished by the use of carefully distributed stripping steam. Therefore, oil depth is a primary factor for establishing both the stripping steam requirement and the deodorizing or holding time. The quantity of fatty acids distilled with each kg of steam is directly proportional to the vapour pressure of the fatty acids. It must be noted that when removing volatiles, it is not the mass but the volume of live steam which determines the results. For example, 1-mbar operation will require a lower weight percentage of stripping steam than will 6-mbar operation. Therefore higher temperature and low pressure are beneficial for increasing the steam volume.

Excessive live steam as mentioned above may cause hydrolysis and increased energy consumption for the vacuum system. Typical stripping steam deodorization conditions for chemically refined oils are 5 to 15 wt% of oil for batch systems and 0.5 to 2% for continuous and semi-continuous deodorizer systems.

Retention Time

Deodorizer holding time is the period during which the fat or oil is at deodorizing temperature and subjected to stripping steam. Stripping time for efficient deodorization has to be long enough to reduce the odoriferous components of the fats and oils products to the required level. This time will vary with the equipment design. For example, a batch deodorizer with an 8- to 10-foot depth of oil above the sparging steam distributor will require a longer deodorization time than will a continuous or semi-continuous system that treats shallow layers of oil. Typically, the holding time at elevated temperatures for batch deodorizer systems is three to eight hours, whereas the holding time for continuous and semi-continuous systems vary from 15 to 120 minutes. Additionally, certain reactions with the oils deodorized are not related to removal of FFA, but instead help provide a stable oil after deodorization.

These reactions and the heat bleaching are time and temperature dependent, thus deodorizing systems provide a retention period at deodorizing temperatures to allow these reactions and the heat bleaching to occur.

Properties of some RBD oils

Properties of Some Crude and Refined, Bleached, Deodorized (RBD) Oils

Quality parameter

Effect of process variables on deodorized oil quality

Typical conditions for deodorisationTypical Process Conditions for Deodorisation

End.

Reference:

1. Food Technology Fact Sheet: Oil and Oilseed Processing III, the Oklahoma Cooperative Extension Service
2. Bailey’s Industrial Oils & Fats Products (6th Edition), Wiley-Interscience (2005)
3. Palm Oil: Production, Processing, Characterization and Uses, AOCS Press (2012)
4. Fats and Oils Handbook, AOCS Press (1998)
5.Fats and Oils: Formulating and Processing for Application (3rd Edition), CRC Press (2009)

6. Introduction to Oil & Fats Technology, AOCS Press (2000)

7. Green Vegetable Oil Processing, AOCS Press (2012)

8. Edible Oil Processing (2nd Edition), AOCS Press (2013)

9. Physical Properties of Lipid, Marcel Dekker (2002)

10. Food Lipids (2nd Edition, Marcel Dekker (2002)

11. The Lipid Handbook (3rd Edition), CRC Press (2007)

12. The AOCS Lipid Library: Deodorisation

Advertisements

Discussion

No comments yet.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: