The quality of fried foods depends not only on the type of foods and frying conditions, but also on the oil used for frying. Thus, the selection of stable frying oils of good quality is of great importance to maintain a low deterioration during frying and consequently a high quality of the fried foods.
Many refined oils and fats are used for frying and the ideal oil composition may be different depending on technical or nutritional considerations. In general, the decision is influenced by many factors amongst which functionality, nutritional properties, cost and availability stand out. Palm olein and partially hydrogenated oils have been considered the most stable oils for frying although, in the last decades, development of genetically modified seeds containing oils with a lower degree of unsaturation than those of the traditional oils has significantly increased the availability of oils of high thermostability in the marketplace [1,2].
However, whatever the oil or fat used, its initial quality may vary significantly and affect the rate of deterioration during frying. Thus, extraction of good-quality seeds and the appropriate development of the different steps in the refining process to fulfill frying oil specifications are necessary. This is the only guarantee for obtaining the best frying performance of the selected oil.
In this article, the main steps of the refining process are discussed briefly with special reference to the changes in the crude oils and their importance in the production of high-quality oils. For complete information on the different conditions and equipments used in the different steps, a wide literature is available [3–5]. Also, specifications for refined frying oils are given and justified. More detailed discussion of the various steps in the refining process is available here..
2. The Refining Process
Refining of crude oil is done to remove unwanted minor components that make oils unappealing to consumers, while trying to cause the least possible damage to the neutral oil as well as minimum refining loss. The components to be removed are all those glyceridic and nonglyceridic compounds that are detrimental to the flavour, colour, stability or safety of the refined oils. They are primarily phosphoacylglycerols, free fatty acids, pigments, volatiles and contaminants.
On the other hand, not all the minor compounds in fats and oils are undesirable. For example, phytosterols are considered of nutritional interest, and tocopherols with vitamin E activity, protecting the oil against oxidation are highly appreciated. Consequently, to reach the maximum oil quality all the steps of the refining process should be carried out with the minimum losses of desirable compounds.
The major steps involved and the main components removed are shown in Table 1. As can be observed, alkali (or chemical) and physical refining are the standard processes used. The main difference between the processes is that alkali refining procedure includes caustic soda treatment to neutralise the oil while, following physical refining, free fatty acid are eliminated by distillation during deodorization. Physical refining reduces the loss of neutral oil, minimises pollution and enables recovery of high-quality free fatty acids. Nevertheless, not all oils can be physically refined.
The purpose of degumming is to remove phospholipids or gums from the crude oil. Two types of phospholipids are present in crude oils according to their level of hydration, i.e. hydratable and nonhydratable ones, the latter mainly present as calcium and/or magnesium salts of phosphatidic acid and phosphatidylethanolamine. After addition of water (1-3%), most of the phospholipids are hydrated and are insoluble in the oil. The hydrated compounds can be efficiently separated by filtration or centrifugation. For the elimination of the nonhydratable fraction, the oil is usually treated with phosphoric acid (0.05 to 1%), which chelates the Ca and Mg converting the phosphatides into the hydratable forms (the acid treatment has the additional function of chelating trace prooxidant metals). Due to the variable content of phospholipids in crude oils, analysis of phosphorus prior to acid treatment is necessary to ensure that the acid dosage is correct, especially when the content of Ca and Mg salts is high.
Depending on the oil composition, the degumming step can be eliminated as the phosphatides are also removed along with the soaps during the next step of neutralization. However, degumming is mandatory for physical refining and the content of phosphorus after degumming should be lower than 10 mg/kg .
In this step, the oil is treated with caustic soda (sodium hydroxide) and free fatty acids are converted into insoluble soaps, which can be easily separated by centrifugation. Thus, the main objective of this step is the removal of free fatty acids, although, as commented above, residual phospholipids in degummed oils or all the phospholipids in the crude oils are also removed as insoluble hydrates. Also, caustic neutralization improves significantly the oil colour partly by reacting with polar compounds (gossypol, sesamol, sterols, hydroxy fatty acids, etc) and partly by solubilization. Alkali refining of oil is compulsory in crude oils of high acidity and pigment contents.
The free fatty acid content of the oil is the main factor that determines the amount and concentration of the caustic soda and also its excess (5 to 20%) for a minimum oil loss. After a reaction time of around 30 minutes at slow stirring and temperature around 80ºC, the water phase is eliminated by centrifugation and the oil washed with water to remove the remaining soap.
In this step, which is common to both physical and alkali refining, the hot oil (around 100ºC) is slurried with acid-activated bleaching earth (1-2%), normally calcium montmorillonite or natural hydrated aluminium silicate (bentonite). Under these conditions adsorption of colour bodies, trace metals and oxidation products as well as residual soaps and phospholipids remaining after washing neutralized oils takes place. For optimum adsorption of both colour bodies and oxidation products to be achieved, the reaction time has to exceed 15 minutes and no more than 30 minutes at usual bleaching temperatures. The removal of chlorophyllic pigments is very important since they are not eliminated in any other stage of refining, as carotenoid compounds are in deodorization. On the other hand, final filtration must eliminate completely the activated earths as residual traces act as prooxidants during oil storage because of their iron content.
Acid-activated clays are the major adsorbent used, although active carbons and synthetic silicas are also applied industrially with more specific goals. Thus, active carbons are used specifically to eliminate polycyclic aromatic hydrocarbons (PAH) from some oils, especially fish oils and pomace oils , while synthetic silicas are quite efficient in adsorbing secondary oxidation products, phospholipids and soaps.
This is a critical step to obtain high-quality oils, because two types of adsorption occur between the compounds to be adsorbed and the absorbent: on one hand, reversible physical adsorption based on intermolecular forces of low strength and, on the other hand, irreversible chemisorption with a strong interaction, which causes chemical reactions.
Chemical changes taking place at this stage have been well studied in olive oil, because of the need to control the presence of refined oils in virgin oils . The two main reactions found extensively in all the vegetable oils are the following:
- Decomposition of hydroperoxides. Previous steps do not modify the peroxide value and it may even increase if air is available in the earlier stages. However, during bleaching, hydroperoxides decompose to form volatiles and oxidized triacylglycerols containing keto and hydroxy functions. After bleaching, peroxide value should be zero or close to zero, but the presence of aldehydes and ketones is clearly detected by the significant increase in the anisidine value.
- Dehydration of alcohols. Hydroxy acids formed from hydroperoxides undergo a partial dehydration by earth catalysis. As the function is at an allylic position, a rapid increase in UV absorption at 232 nm is observed because of the formation of conjugated dienes from oleic acid hydroperoxides and in UV absorption at 268 nm due to formation of conjugated trienes from linoleic acid hydroperoxides. Also, sterols undergo significant dehydration and the formation of the hydrocarbon 3,5-stigmastadiene from the major sterol (β-sitosterol) is considered a proof of the presence of refined oil in virgin olive oil .
This step, also called dewaxing, is only applied when the oil is not clear at room temperature because of the presence of waxes or saturated triacylglycerols. It is important to note that these compounds do not affect negatively the oil performance or functionality, but the appearance of the oil is not acceptable to consumers.
Thus, the objective of this step is the removal of high temperature melting components present in small quantities. The crystallization process normally used consists of cooling the oil down gradually to temperatures of 5 to 8ºC in a maturing tank. After increasing the crystal size at this temperature for 24 to 48 h, the solids are separated by centrifugation at 15-16ºC. This treatment ensures excellent clarity of oils when stored at either room or refrigeration temperatures.
Deodorization of fats and oils normally consists of steam distillation at elevated temperature under reduced pressure, although nitrogen has also been used. The purpose of this step is to remove volatile compounds (mainly ketones and aldehydes) contributing to oil taste and odour, total free fatty acids in physical refining and the residual free fatty acids from neutralized bleached oils. The deodorization conditions also contribute to the removal of contaminants (light PAH, pesticides, etc.) and to the reduction of colour of the oil due to the breakdown of the remaining carotenes at high temperature. The efficiency of deodorization is a function of pressure (1 to 5 torr), temperature (200 to 260ºC), residence time (0.5 to 3 h) and volume of stripping gas (1 to 3%). However, differences in the deodorization equipment used also have a major impact on efficiency. After the deodorization, the oil is cooled and addition of citric acid (100 mg/kg of 20% citric acid) is recommended to chelate metal traces and increase its stability during storage.
Apart from the physical changes, chemical reactions taking place in the triacylglycerols due to the drastic conditions of this step have been studied in detail and are summarized as follows:
- Decomposition of oxidation compounds. Even if hydroperoxides were destroyed during bleaching, some new primary and secondary oxidation products formed decompose during heat treatment to form volatile and nonvolatile compounds.
- Dimerization of triacylglycerols. Acyclic dimers of triacylglycerols, i.e. nonpolar dimers (C–C bridges) as well as oxygenated dimers (C–O–C), are detected in significant amounts, which may involve the formation of alkyl and alkoxyl radicals at high temperature even in the absence of oxygen .
- Geometrical and positional isomers induced by heat are also formed in this step. Thus, more trans isomers and also more dienoic conjugation are found . However, in oils containing linolenic acid, a decrease in the trienoic conjugation is observed, which is attributed to the formation of cyclic fatty acids and the concurrent elimination of double bonds.
- Finally, an interesterification reaction is detected in vegetable oils deodorized at temperatures above 240ºC by an increase in saturated fatty acids in the 2-position of the triacylglycerols .
The importance of these reactions is higher, as expected, as the temperature and the deodorization time increases , being dramatic in highly unsaturated oils . It is also remarkable that hydrolytic reactions have not been observed as the content of diacylglycerols remains unchanged, not only in this step but throughout the complete process .
Finally, it is important to take into account that long deodorization times and/or too high temperatures can have a devastating effect on the quality of the oil due not only to the chemical changes commented above but also to the distillation of a significant part of the natural tocopherols (20 to 40%), which would decrease the stability of the refined oil . In this respect, the by-product obtained from the deodorization, i.e. deodorizer distillate, contains significant amounts of compounds of high-added value like tocopherols, sterols and hydrocarbons, and a great effort is being made for their recovery .
3. Specifications for Frying Oils
Quality evaluation of refined oil is based mainly on analytical indices giving information on the efficiency of the different steps of the refining process. Table 2 summarizes the specifications of refined oils of good quality. The last three lines include specific recommendations for frying oils. High oxidative stabilities are also required but they are not given because of their dependence on the degree of unsaturation of the refined oil, which in the case of frying oils ranges from polyunsaturated to partially hydrogenated oils.
Most of the specifications in Table 2 are essential for a good performance of the oil in frying, since they reduce to the minimum the content of detrimental compounds in the process. Thus, a minimum level of phospholipids is necessary to avoid undesirable foaming and a rapid oil darkening with negative consequences for the fried product.
Free fatty acids should be limited because of their prooxidant activity as well as of their contribution to smoke when heating at frying temperatures. In fact, the smoke point and free fatty acid content are interrelated. For example, a smoke point higher than 220ºC is expected for free fatty acid contents lower than 0.05%, while the smoke is clearly observed at a usual frying temperature of around 180ºC in frying oils with free fatty acid levels of 0.3 to 0.4%.
Metals act as active prooxidants accelerating rapidly the oil degradation, and a minimum peroxide value is a guarantee of either a recent refining or a good stability.
Finally, addition of an antifoaming agent (dimethylpolysiloxane) is strongly recommended in discontinuous frying operations. It is supposed to form a layer at the oil surface impeding the entrance of oxygen when the oil is not protected by the food, and thus, it is especially active to delay oil oxidation .
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