Deep-fat frying produces desirable or undesirable flavor compounds and changes the flavor stability and quality of the oil by hydrolysis, oxidation, and polymerization. Tocopherols, essential amino acids, and fatty acids in foods are degraded during deep-fat frying. The reactions in deep-fat frying depend on factors such as replenishment of fresh oil, frying conditions, original quality of frying oil, food materials, type of fryer, antioxidants, and oxygen concentration. High frying temperature, the number of fryings, the contents of free fatty acids, polyvalent metals, and unsaturated fatty acids of oil decrease the oxidative stability and flavor quality of oil. Antioxidant decreases the frying oil oxidation, but the effectiveness of antioxidant decreases with high frying temperature. Lignan compounds in sesame oil are effective antioxidants in deep-fat frying.

https://onlinelibrary.wiley.com:443/doi/pdf/10.1111/j.1750-3841.2007.00352.x

Chemistry of deep-fat frying oils

Among vegetable oils, virgin olive oil (VOO) has nutritional and sensory characteristics that to make it unique and a basic component of the Mediterranean diet. The importance of VOO is mainly attributed both to its high content of oleic acid a balanced contribution quantity of polyunsaturated fatty acids and its richness in phenolic compounds, which act as natural antioxidants and may contribute to the prevention of several human diseases. The polar phenolic compounds of VOO belong to different classes: phenolic acids, phenyl ethyl alcohols, hydroxy-isochromans, flavonoids, lignans and secoiridoids. This latter family of compounds is characteristic of Oleaceae plants and secoiridoids are the main compounds of the phenolic fraction. Many agronomical and technological factors can affect the presence of phenols in VOO. Its shelf life is higher than other vegetable oils, mainly due to the presence of phenolic molecules having a catechol group, such as hydroxytyrosol and its secoiridoid derivatives. Several assays have been used to establish the antioxidant activity of these isolated phenolic compounds. Typical sensory gustative properties of VOO, such as bitterness and pungency, have been attributed to secoiridoid molecules. Considering the importance of the phenolic fraction of VOO, high performance analytical methods have been developed to characterize its complex phenolic pattern. The aim of this review is to realize a survey on phenolic compounds of virgin olive oils bearing in mind their chemical-analytical, healthy and sensory aspects. In particular, starting from the basic studies, the results of researches developed in the last ten years will be focused.

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Phenolic molecules in virgin olive oils: a survey of their sensory properties, health effects, antioxidant activity and analytical methods. An overview of the last decade.

Preface to the First Edition. Preface to the Second Edition. Contributors. List of Abbreviations. 1 Production and Trade of Vegetable Oils ( Frank D. Gunstone ). 1.1 Extraction, refining and processing. 1.2 Vegetable oils: Production, consumption and trade. 1.3 Some topical issues. 2 Palm Oil ( Siew Wai Lin ). 2.1 Introduction. 2.2 Composition and properties of palm oil and fractions. 2.3 Physical characteristics of palm oil products. 2.4 Minor components of palm oil products. 2.5 Food applications of palm oil products. 2.5.1 Cooking/frying oil. 2.6 Nutritional aspects of palm oil. 2.7 Sustainable palm oil. 2.8 Conclusions. 3 Soybean Oil ( Tong Wang ). 3.1 Introduction. 3.2 Composition of soybean and soybean oil. 3.3 Recovery and refining of soybean oil. 3.4 Oil composition modification by processing and biotechnology. 3.5 Physical properties of soybean oil. 3.6 Oxidation evaluation of soybean oil. 3.7 Nutritional properties of soybean oil. 3.8 Food uses of soybean oil. 4 Canola/Rapeseed Oil ( Roman Przybylski ). 4.1 Introduction. 4.2 Composition. 4.3 Physical and chemical properties. 4.4 Major food uses. 4.5 Conclusion and outlook. 5 Sunflower Oil ( Maria A. Grompone ). 5.1 Introduction. 5.2 Sunflower oil from different types of seed. 5.3 Physical and chemical properties. 5.4 Melting properties and thermal behaviour. 5.5 Extraction and processing of sunflower oil. 5.6 Modified properties of sunflower oil. 5.7 Oxidative stability of commercial sunflower oils. 5.8 Food uses of different sunflower oil types. 5.9 Frying use of commercial sunflower oil types. 6 The Lauric (Coconut and Palm Kernel) Oils ( Ibrahim Nuzul Amri ). 6.1 Introduction. 6.2 Coconut oil. 6.3 Palm kernel oil. 6.4 Processing. 6.5 Food uses. 6.6 Health aspects. 7 Cottonseed Oil ( Michael K. Dowd ). 7.1 Introduction. 7.2 History. 7.3 Seed composition. 7.4 Oil composition. 7.5 Chemical and physical properties of cottonseed oil. 7.6 Processing. 7.7 Cottonseed oil uses. 7.8 Co-product uses. 8 Groundnut (Peanut) Oil ( Lisa L. Dean, Jack P. Davis, and Timothy H. Sanders ). 8.1 Peanut production, history, and oil extraction. 8.2 Oil uses. 8.3 Composition of groundnut oil. 8.4 Chemical and physical characteristics of groundnut oil. 8.5 Health issues. 9 Olive Oil ( Dimitrios Boskou ). 9.1 Introduction. 9.2 Extraction of olive oil from olives. 9.3 Olive oil composition. 9.4 Effect of processing olives on the composition of virgin olive oils. 9.5 Refining and modification. 9.6 Hardening and interesterification. 9.7 Quality, genuineness and regulations. 9.8 Consumption and culinary applications. 10 Corn Oil ( Robert A. Moreau ). 10.1 Composition of corn oil. 10.2 Properties of corn oil. 10.3 Major food uses of corn oil. 10.4 Conclusions. 11 Minor and Speciality Oils ( S. Prakash Kochhar ). 11.1 Introduction. 11.2 Sesame seed oil. 11.3 Rice bran oil. 11.4 Flaxseed (linseed and linola) oil. 11.5 Safflower oil. 11.6 Argan kernel oil. 11.7 Avocado oil. 11.8 Camelina seed oil. 11.9 Grape seed oil. 11.10 Pumpkin seed oil. 11.11 Sea buckthorn oil. 11.12 Cocoa butter and CBE. 11.13 Oils containing a-linolenic acid (GLA) and stearidonic acid (SDA). 11.14 Tree nut oils. Useful Websites. Index.

http://health120years.com/cn/pdf/hd_Vegetable.Oils.pdf

Vegetable Oils in Food Technology: Composition, Properties, and Uses

Over the past 5 years growing demands for reducing fat content of fried foods have greatly stimulated the amount of research spent on the issue of fat uptake during deep-fat frying. The results of these efforts are summarized in this review. Most of the increased understanding of the mechanism of fat uptake has been brought about by improved imaging techniques. It turns out that fat uptake is basically determined by two mechanisms: the condensation effect and the capillary effect. Major reductions claimed in literature and patents are found for coating and batter formulations using various types of biopolymers.

Mechanism and reduction of fat uptake in deep-fat fried foods

Integrated approach to deep fat frying: engineering, nutrition, health and consumer aspects

A new method has been developed to estimate the stabilising activity of synthetic and natural food additives at frying. Non-refined and refined vegetable fats and oils were heated at a temperature of 170°C after adding water-conditioned silica gel for two hours. The degraded products were measured to assess the oil stability at frying temperature. The determination of polymeric triglycerides by size exclusion high-pressure liquid chromatography (HPLC) was carried out for the estimation of the oxidative heat stability of vegetable fats and oils.

Tocopherols, various tocopherol esters and phytosterol fractions, phenolic compounds, like quercetin, oryzanol, ferulic acid, squalene, butyl hydroxytoluol (BHT), butyl hydroxyanisol (BHA), and other compounds, like ascorbic acid 6-palmitate and gallates, are added to refined sunflower and rapeseed oil and their efficacy determined.

Both linoleic and oleic rich oils gave comparable results for the activity of the various compounds. α-tocopherol, tocopherol esters, and BHA have low effects at frying temperature. Ascorbic acid 6-palmitate and some phytosterol fractions were found to have the greatest antioxidant activity.

Corn oil was more stable than soybean oil and rapeseed oil better than olive oil. It was also observed that non-refined oils proved to have a better stability at elevated temperature than refined oils. The results show that the stability of the vegetable oils at frying temperature is a function of more than just the fatty acid composition.

There is evidence which supports a co-relationship between the unsaponifiable matter content and oxidative stability.

It is believed that a radical peroxidation mechanism predominates at lower temperatures. When a large volume of oil is heated in a fryer and the oxygen supply is poor, non-radical reactions such as elimination (acid catalysed dehydration) or nucleophilic substitution take place.

Testing and comparing oxidative stability of vegetable oils and fats at frying temperature

The deep-frying process, normally carried out at 140-200 °C, is a very complex system due to the combination of heat and mass transfer between food and frying medium. The system becomes more complicated as the frying operation proceeds, because the composition of the food being fried and the frying medium is changing continuously due to the progressive deterioration of the frying medium. Apart from a variety of chemical reactions occurring, several changes take place in the frying food, such as gelatinisation of starch, denaturation of protein, and decrease of moisture. These changes bring about swelling of the product, formation of a crusty layer, and the appearance of a golden colour, good texture and taste. The precise control of the fryer enables these physical and chemical changes in the frying of food to convert it into a desirable finished product. This article discusses various types of reactions occurring in the food frying operation, possible mechanisms, a new realistic method - OSET index for measuring heat stability of frying oils - and the protective behaviour of substances that enhance the frying stability of oils.

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New theoretical and practical aspects of the frying process

Deep-fat frying is a complex, thermal chemical process that produces fried foods with desirable colour, appearance, flavour, and texture. Normally, less stable liquid oils are hydrogenated to enhance their oxidative stability for deep-fat frying purposes. However, considerable amounts of trans and positional isomer fatty acids are formed during hydrogenation, which are nutritionally undesirable. The stability of frying oils is sometimes increased by careful blending of polyunsaturated oils with more saturated oils. The natural way of improving oxidative and flavour stability of frying oils and fats is by adding natural antioxidative components and precursors present in the plant kingdom, such as 'virgin' olive oil, sesame seed oil (SSO) and rice bran oil (RBO). A variety of natural antioxidative components, present in these oils, comprise tocopherols and tocotrienols, special sterols e.g. Δ5-avenasterol and sterol esters, squalene, sesamolin, sesamol, sesaminol and related compounds, polyphenols, etc. Various antioxidative components present in SSO and RBO are largely retained in Good-Fry® Constituents (GFC), manufactured according to European patent as well as USA and worldwide patent applications pending (Silkeberg and Kochhar, 2000). Generally, palm olein, palm oil, partly hydrogenated rapeseed oil/soybean oil and/or their blends are mainly used by the frying industry for the production of a variety of snack products and pre-fried convenience foods. Several new frying oils with good oxidative stability, which do not require hydrogenation, are now commercially available on the European market, for example high-oleic sunflower seed oil stabilised with GFC. The results showed that the addition of 6% GFC to unhydrogenated rapeseed provided crisps, produced on industrial scale, with stability similar to those fried in palm olein without GFC. Shelf life of crisps fried in soybean oil, iodine value 130, was substantially increased by addition of 5% GFC. The Good-Fry® Constituents can also be added, with advantages of flavour stability of fried snacks, to oils such as palm oil or palm olein at lower levels of 2%. It is forecasted, to meet an ever-growing consumer demand of healthier' snack products, the usage of natural antioxidative components in stabilising frying oils rich in monounsaturated fatty acids (MUFAs) will grow tremendously.

Stabilisation of frying oils with natural antioxidative components

The formation of acrylamide during food frying is generally influenced by food type, thermal treatment and equipment. The acrylamide concentration is increased when frying oils containing a higher level of polar materials or silicone or larger amounts of diglycerides. This effect may be caused by moisture escaping from food that has an enhancing effect on the heat transfer. It was noticed that if the moisture in the frying operation was bound by special adsorbents, the acrylamide content could be reduced by more than 50%. The effects of several additives like citric acid on the formation of acrylamide during frying of chips were also investigated. The mechanism of acrylamide formation in fried foods is discussed to explain these findings.

https://www.ocl-journal.org/fr/articles/ocl/pdf/2003/04/ocl2003104p297.pdf

Deep frying: the role of water from food being fried and acrylamide formation

The effects of the chemical refining process on the minor compounds of rice bran oil and its heat stability were investigated. After 8 h of heating, about 50% and 30% of total tocopherols remained in crude and refined rice bran oil, respectively. The individual tocopherols were differently affected by the refining process. The order of heat stability of tocopherols and tocotrienols in crude oil was found to be different from that in fully refined oil. A similar tendency was observed for sterols. After 8 h of heating, 65% and 72% of total sterols, and 14% and 46% of sterol esters, of crude or fully refined rice bran oil, respectively, disappeared. The heating process led to a 4% and 10.3% increase in polymer contents in crude and refined rice bran oil, respectively. Although refined rice bran oil showed good heat stability, when compared to crude oil its heat stability was decreased to some extent.

S. Parkash Kochhar

Papers

Testing and comparing oxidative stability of vegetable oils and fats at frying temperature

New theoretical and practical aspects of the frying process

Stabilisation of frying oils with natural antioxidative components

Deep frying: the role of water from food being fried and acrylamide formation

Effect of the full refining process on rice bran oil composition and its heat stability