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Abstract
Palm oil is extracted from the flesh of the fruit of the oil palm, Elaeis guineensis, which is a native of West Africa. In Malaysia, the high yielding tenera variety of Elaeis guineensis is the most commonly cultivated oil palm tree. Tenera is a cross between dura and pisifera. There are two types of oil produced from palm fruit: - palm oil from the flesh (the orangish fibrous outer layer of the fruit) and palm kernel oil from the white kernel inside the shell of the oil palm fruit. Out of these two oils, many fractions of palm and palm kernel oil can be produced for variety of food applications. For frying application, only palm oil and palm olein (the liquid fraction of palm oil) are commonly used in industrial frying and food services industry. Palm olein is also used as general household cooking oil in tropical countries. Through industrial experiences and scientific studies, the following facts have been established for palm oil/olein as frying oil: high oxidative stability, extended life span of frying oil, longer shelf life of fried foods, favourable organoleptic quality of fried foods, less maintenance of fryer, the foods cook faster, less oil absorption by fried foods and palm oil/olein is naturally stable that does not need hydrogenation or addition of synthetic anti-oxidant to slow down oxidative degradation.
Keywords: Frying oil, palm oil, palm olein
Introduction

Industrial frying accounts for more than 50% of the total oil used in food applications. Instant noodles, snacks and pre-fried foods, which are industrially produced in bulk need a good frying oil to achieve a good organoleptic quality and long shelf-life of the fried foods. The oil used in frying plays an important role in the eating quality of the fried food and its shelf life. In the past, animal fat, peanut oil, cottonseed oil and hydrogenated fats were the main oils/fats used for frying. These frying fats and oils, due to technical and commercial reasons were gradually replaced with palm oil and palm olein. Numerous studies revealed that palm oil and palm olein have desirable frying performance better than or at least comparable to their alternatives [1]. Today, palm oil and palm olein are the largest oils consumed by the frying industry and frying is the largest single application of palm oil in the world. Palm oil is the world's largest traded oil and it is readily available in most parts of the world.
In recent years, high-oleic oils such as high oleic sunflower, high oleic canola and high oleic soybean oils have also been used for industrial frying but the availability of these oils are limited. The high oleic version of palm oil is produced in Colombia but its use as frying oil is still not popular due to cost and limited supply. Similar products to high oleic palm oil can be obtained by fractionation process where palm olein is fractionated to produce palm olein with higher Iodine Values (IV). These oils are called superoleins. Commonly produced superoleins are the ones with IV60, IV63 and IV65. Higher IV superolein is produced only on demand because of higher cost of production. Superolein is used as household frying and cooking oil mainly in tropical countries. It is also blended with soft oils as cooking oil.
Properties of Palm Oil as Frying Oil
In general, palm oil/palm olein offers the best option as frying oil; it has excellent frying performance and because palm oil is produced all year round, there is ample supply throughout the year for the food industry worldwide. As such, it is a lot easier for the food industry to stick to the same product formulations to keep the taste and quality of the food consistent.
Palm oil is resistant against oxidation even at high frying temperatures due to its low content of linoleic acid and the presence of natural antioxidants, i.e., tocopherols and tocotrienols. Palm oil contains about 50% saturated fatty acids and 50% unsaturated fatty acids. The unsaturated fatty acids consist of 10% linoleic acid, and negligible amount of linolenic acid [2]. Linolenic acid is very susceptible to oxygen attack because of the presence of three double bonds in the fatty acid structure. Thus, palm oil is less susceptible to oxidation because of its low content of linolenic acid compared to polyunsaturated oils, which have excessive amount of linolenic acid. The linolenic acid can undergo auto oxidation process even at ambient temperature. Polyunsaturated oils need to be hydrogenated before it can be used for industrial frying. Unfortunately, hydrogenation process causes the formation of harmful trans fatty acids.
By using the relative oxidation rates of oleic, linoleic, and linolenic acids, which are 1, 12, and 25, respectively, the Inherent Oxidative Stability (IOS) of oils and fats can be calculated. The lower the IOS, the more stable the oil against oxidation. Palm oil, high-oleic sunflower oil, sunflower oil, soybean oil, corn oil, cottonseed oil, canola oil and coconut oil have IOS of 1.5, 1.7, 7.1, 7.1, 6.5, 5.8, 4.5 and 0.2 respectively, indicating that palm oil and high oleic sunflower oil are the most stable oils against oxidation compared to the others [3].


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TABLE 1: FATTY ACID COMPOSITIONS OF SELECTED OILS AND FATS AND THEIR OSI AT 1100C
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SBO CO GNO RSO SFO PO POL CNO
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Laurie 0.2 48.0
Myristic 0.1 0.1 0.1 0.1 1.0 1.0 18.2
Palmitic 11.0 11.0 11.5 4.3 7.0 46.0 40.6 9.0
Stearic 3.6 2.0 2.0 1.7 4.2 4.0 3.8 2.0
Oleic 24.7 27.0 48.0 59.0 19.5 37.8 42.0 7.0
Linoleic 53.5 58.5 31.0 27.8 68.9 10.0 11.5 2.0
Linolenic 6.4 0.5 1.0 8.3 0.5 0.4
OSI (hr,1lOC) 5 7 8 7 5 25 24 75
=====================================================================================

Source: Ismail, R,[5]
Note: SBO = Soybean Oil, CO = Corn Oil, GNO = Groundnut Oil, RSO = Rapeseed Oil, SFO = Sunflower Seed Oil, PO = Palm Oil, POL = Palm Olein, CNO = Coconut Oil.

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TABLE 2: OSI VALUES OF OILS AND FATS
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Fats/Oils OSI (hr/llOC) Fats/Oils OSI (hr,110°C)
Sunflower Oil 3.0-6.0 High-Oleic Canola Oil 12.0-18.0
Soybean Oil 5.0-6.7 Palm Oil 20.0-30.0
Canola Oil 6.0-8.0 High-Oleic Soybean Oil 25.0-65.0
Corn oil 7.0-11.0 Partially Hydrogenated Soybean Oil 20.0-85.0
=====================================================================================

The presence of tocopherols and tocotrienols in palm oil/palm olein provides greater oxidative stability to the oil. For Refined, Bleached and Deodorized (RBD) palm oil, the range of antioxidants presence in the oil is from 280 - 890 ppm while for palm olein, the levels are between 560 - 900 ppm [4], All this contributes to high Oil Stability Index (OSI) of palm oil and palm olein. The OSI is a parameter that measures the relative oxidative stability of edible oils against oxidation and is done at elevated temperatures to accelerate oxidation process. The higher the OSI value, the better the oil against oxidation.
Coconut oil has the highest OSI due to high content of lauric and myristic acids. However, lauric fatty acid will cause hydrolysis of oil and ultimately rancidity to the food. Thus, coconut oil is not suitable for industrial frying in which the fried products need a long shelf life of between one to two years depending on the type of fried foods.
Palm olein when blended with soft oils, will increase the OSI of the blend and at the same time the soft oils increase the resistance to crystallization of the blend by reducing its Cloud Point (CP). Based on IOS and OSI above, and by classifying the oils with highest IOS and lowest OSI as having poor oxidative stability, the oxidative stability of oils and fats can be summarized as per Table 3.

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TABLE 3: OXIDATIVE STABILITY OF OILS AND FATS
===============================================================
Fats/Oils Oxidative Stability
Coconut oil Excellent
Hydrogenated soybean oil Good/Excellent
Palm oil Good
Palm olein Good
High-oleic sunflower oil Good
High-oleic canola oil Fair/Good
Peanut oil Fair
Cottonseed oil Fair
Rapeseed/Canola oil Fair
Corn oil Poor/Fair
Sunflower oil Poor
Soybean oil Poor
===============================================================

For quality evaluation of frying oil, OSI should be used in combination with other quality parameters such as Free Fatty Acids (FFA), Peroxide Value (PV), p-Anisidine Value (AV), Total Polar Materials (TPM), total polymeric compounds and sensory evaluation. Quick tests are available to monitor the quality of frying oil during frying. If antioxidants and anti-foaming agents are added to palm oil/palm olein, the oxidative stability of the oil will be excellent. For soft oils, it is necessary to add antioxidants and anti-foaming agents to increase their oxidative stability. Otherwise, the oils should only be used for a very limited number of repeated fryings [6] and should not be used in heavy duty frying. Consumption of heated polyunsaturated oils could cause cellular damage to liver and kidneys.[7]
Soft oils can also be blended with palm olein/superolein to improve the oxidative stability. For example, blending of palm olein with cottonseed oil significantly improved the frying performance of the latter.[81
Palm-based frying oils are available in many forms; palm shortening, pourable palm oil, palm olein, special quality palm oil/palm olein (low colour with better oxidative stability) and various grades of superoleins. The various forms of palm based frying oil provide the food industry the flexibility to select the most suitable frying oil for a specific application. For example, palm olein is used by major snack food manufacturers in European Union countries [9]. This is also true in many other countries in Asia, Africa and America. In the USA, palm shortening is the popular frying medium for donuts. In Malaysia, pourable palm oil is used by the fast food chains. The instant noodle industry usually uses palm oil or palm olein as the frying medium. In Vietnam, palm stearin is also used for instant noodle frying. In short, palm oil offers the solution for every type of frying operation; be itshallow or deep frying, continuous or batch frying. At least one form or more of palm frying oils will fit into the required frying operation.
Specifications for Frying Oil
Several chemical reactions take place at once at accelerated rates during frying. These reactions include hydrolysis, oxidation, ring formation and polymerization. The consequence of these chemical reactions is degradation of the frying oil and the quality of food fried in it. The higher the temperature, the faster the degradation process takes place. Soft oils degrade faster than palm oil/palm olein due to higher content of polyunsaturated fatty acids. Generally, a good frying oil should comply with the specifications as given in Table 4.

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TABLE 4: SPECIFICATIONS FOR DEEP FRYING OILS/FATS
===========================================================
1. FFA < 0.1%
2. Odour & Flavour Bland
3. Colour Light
4. Moisture < 0.1%
5. Smoke point > 2000C
6. PV < 1 meq/kg
7. Fatty Acid Composition
8. Linolenic < 2% (French Legislation) or < 3%
===========================================================

Source: Stevenson, S.T. et al. [10] and Binkman, B. [11]
Conclusion
The type of food, nutritional aspects, type of frying operation - continuous, batch, maintenance cost, oil prices etc. are factors usually considered in selecting a frying oil. For frying purpose, palm oil/olein offers several techno-economic advantages over most other oils and fats in terms of price, availability, frying properties, nutritional aspects etc. Experiences of the frying industry have shown that when palm oil/palm olein is used for frying, productivity is higher, production breakdown and maintenance costs are reduced, slower increase in free fatty acid, polar and polymers formation and the fried foods have longer shelf-life.[1]
References
1. Berger, K.G.(2005). The use of palm oil in frying.Malaysian Palm Oil Council.pg 72-106.
2. MS 814:2007, AMD. 1 :(2018). Palm oil - Specification (Second revision)
3. Min Hu, (2018). Oxidative stability of oils and fats. Inform. http://www.informmagazinedigital.org/informmagazine/february_2018/
MobilePagedArticle.action?articleld=1330640
4. Gapor M.T. (1989). Effect of refining and fractionation on vitamin E in palm oil, PORIM International Development Conference Proceedings, Kuala Lumpur.
5. Ismail, R, (2019). Personal Communication.Unpublished data.
6. FEDIOL Nutrition Factsheet.
http://www.fediol.be/data/FEDIOL%20Factsheet%20on%20Frying%20oils%20-final.pdf
7. Hageman G1, Verhagen H, Schutte B, Kleinjans J.(1991). Biological effects of short-term feeding to rats of repeatedly used deep-frying fats in relation to fat mutagen content. Food ChemToxicol. Oct;29(10):689-98.
8. FatmaNurArslan, F.,Sappi, A.N., Duru, F. and Kara, H. (2017). A study on monitoring of frying performance and oxidative stability of cottonseed and palm oil blends in comparison with original oils. Journal. International Journal of Food Properties, 20:704-717.
9. Kochhar, P. (1999) Stable and healthful frying oil for the 21st century. Inform, 11:642-647.
https://www.researchgate.net/publication/284100043_Stable_and_healthful_frying_oil_for_the_ 21st-Century
10. Stevenson, S.T, Vaisey-Genser. M. &Eskin, NA M. (1984). Quality control in the use of deep frying oils, JAOCS, 61:6,1102-1108.
11. Binkman, B. (2000). Quality criteria of industrial frying oils and fats. European Journal of Lipid Science and Technology 102(8-9):539-541 http://www.fediol.be/data/FEDIOL%20Factsheet%20on%20Frying%20oil s%20-final.pdf
7. Hageman G1, Verhagen H, Schutte B, Kleinjans J.(1991). Biological effects of short-term feeding to rats of repeatedly used deep-frying fats in relation to fat mutagen content. Food ChemToxicol. Oct;29(10):689-98.8. FatmaNurArslan, F., Sapgi, A.N., Duru, F. and Kara, H. (2017). A study on monitoring of frying performance and oxidative stability of cottonseed and palm oil blends in comparison with original oils. Journal. International Journal of Food Properties, 20:704-717.9. Kochhar, P. (1999) Stable and healthful frying oil for the 21st century.Inform, 11:642-647.
https://www.researchgate.net/publication/284100043_Stable_and_healthful_frying_oil_for_the_21st_century10.Stevenson, S.T, Vaisey-Genser.M. &Eskin, NA M. (1984). Quality control in the use of deep frying oils, JAOCS, 61:6,1102-1108.11. Binkman, B. (2000). Quality criteria of industrial frying oils and fats. European Journal of Lipid Science and Technology 102(8-9):539-54.
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