bk-2003-0836.ch015

Chapter 15

Downloaded by UNIV LAVAL on May 17, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch015

Oxidative Stability of Edible Oils as Affected by Their Fatty Acid Composition and Minor Constituents Fereidoon Shahidi Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland A 1 B 3X9, Canada Edible oils are composed of triacylyglycerols and minor constituents. The nutritional quality and stability of the oils is affected primarily by the chemical nature of the fatty acids involved, as reflected in their degree and type of unsaturation. However, minor components present in the oils also exert a major influence on their stability characteristics. The minor constituents of the oils include phospholipids, tocols, ubiquinones and other phenolics, phytosterols, carotenoids, chlorophylls, hydrocarbons, waxes and wax esters. During processing, the content of minor constituents is generally reduced by 35-95%. Thus, the Rancimat induction period of red palm oil was reduced by over 15 h at 100°C upon removal of its minor components, mainly tocols and carotenoids. Stability of other oils was also affected by the chemical nature of their minor constituents to different degrees. Under photooxidative conditions, the positive effects of minor constituents may be overwhelmed by the photosensitizing effect of chlorophylls, i f present. Thus, proper storage and packaging conditions are deemed important for quality preservation and oxidative stability of edible oils. Edible oils originate from plant, animal and algal sources. They provide a concentrated source of energy and essential fatty acids through daily dietary intake. Lipids also serve as an important constituent of cell walls and carrier of fat-soluble vitamins. In addition, lipids provide flavor, texture and mouthfeel to the food. Edible vegetable oils are also used for frying purposes and in this way serve as a © 2003 American Chemical Society

Cadwallader and Weenen; Freshness and Shelf Life of Foods ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

201


202 heating medium which results in the generation of aroma, some of which are a direct consequence of the interaction of lipids and/or their degradation products with the constituents of food that is being fried. Oilseeds and tropical fruits are a major source of food lipids although speciality fats and oils may also be obtained from a variety of unconventional source materials. The edible oils from oilseeds may be produced by pressing, solvent extraction or their combination. The seeds may first be subjected to a pretreatment heating to deactivate enzymes present. The oil after extraction is subsequently subjected to further processing of degumming, refining, bleaching and deodorization. Edible oils from source materials are composed primarily of triacylyglycerols (TAG). In addition, phospholipids, glycolipids, waxes, wax esters, hydrocarbons, tocopherols and tocotrienols, other phenolics, carotenoids, sterols and chlorophylls, and hydrocarbons among others, may be present as minor constituents and these are collectively referred to as unsaponifiable matter ( 1 ). During processing, storage and use, edible oils undergo chemical and physical changes. Often, process-induced changes of lipids are necessary to manifest specific characters of food, however, such changes should not exceed a desirable limit. Both T A G and minor constituents of the oil exert a profound influence on the desirability of an oil source for nutrition and in health promotion and disease prevention. In addition, in oilseeds and other oil sources, following oil extraction, the left over meal may serve as a source of fat-insoluble phytochemicals. Obviously, hulls, might be included in the meal i f the seeds are not dehulled prior to oil extraction. The importance of bioactivities in processing by-products of oilseeds may thus require attention. The quality characteristics of edible oils generally depend on the composition of their fatty acids, positional distribution of fatty acids, non-triacylglycerol components, presence/absence of antioxidants, the system in which the oil is present such as bulk oil versus emulsion and low-moisture foods, as well as the storage conditions and packaging material. A n overview of the relevant topics is provided in this chapter.

Triacylyglycerols Triacylyglycerols are generally the main fraction of neutral lipids and usually account for over 95% of edible oils. The fatty acids present are either saturated or unsaturated. Although saturated lipids were generally condemned because of their perceived negative effect on cardiovascular disease, more recent studies have shown that C18:0 is fairly benign while C14:0 may possess adverse health effects. Furthermore, the appreciation about detrimental effects of trans fats has served as



203 a catalyst for the return from using hydrogenated fats in place of naturaUy-occurring edible oils with a high degree of saturation. O f the unsaturated lipids, monounsaturated fatty acids, similar to saturated fatty acids, are non-essential as they could be synthesized in the body de novo. However, polyunsaturated fatty acids (PUFA; containing 2 or more double bonds) could not be made in the body and must be acquired through dietary sources. The parent compounds in this group are linoleic acid (LA, C18:2ω6) and linolenic acid ( L N A , C18:3G)3). The symbols G)6 (or n-6) and ω3 (or n-3) refer to the position of the first double bond from the methyl end group as this dictates the biological activity of the fatty acid molecules involved (2). Over 80% of our dietary lipids are composed of CI8 fatty acids and it is recommended that the ratio of ω6 to ω3 fatty acids be at least 5:1 to 10:1, but the western diet has a ratio of 20:1 or less. Enzymes in our body convert both groups of P U F A through a series of desaturation and elongation steps to C20 and C22 products, some of which are quite important for health and general well-being. The C20 compounds may subsequently produce a series of hormone-like molecules known as eicosanoids which are essential for maintenance of health. Obviously, elongation of L A and alpha-linolenic acid ( A L A ) to other fatty acids may be restricted by rate determining steps and lack of the required enzymes in the body (See Figure 1). Thus, pre-term infants and the elderly may not be able to effectively make these transformations and that production of docosahexaenoic acid (DHA) from eicosapentaenoic acid (EPA) is rather inefficient. Furthermore, supplementation with gamma-linolenic acid (GLA) as a precursor to arachidonic acid (AA) might also be necessary. Lipids are generally highly stable in their natural environment; even the most unsaturated lipids from oilseeds are resistant to oxidative deterioration prior to extraction and processing. Nature appears to be able to protect itself as higher level of antioxidants are generally found in highly unsaturated oils. It is believed that unsaturated oils generally co-exist with antioxidants in order to protect themselves from oxidation; of course the natural capsule or seed coat provides a barrier to light and oxygen as well as compartmentalization of oil cells and inactivity of enzymes prior to crushing. However, upon crashing and oil extraction, the stability of edible oils is compromised and this is dictated by several factors, including the degree of unsaturation of fatty acid constituents. This topic will be discussed in further detail in a later section. In addition, the position of fatty acids in the triacylglycerol molecule (Sn-1, Sn-2 and Sn-3) would have a considerable effect on their assimilation into the body. Generally, fatty acids in the Sn-1 or Sn-3 position are hydrolyzed by pancreatic lipase and absorbed while those in the Sn-2 position are used for synthesis of new T A G and these might be deposited in the body.


204 Omega-6

Omega-3

18:2 (LA)

18:3 ( A L A )

i 18:3 (GLA) i 20:3 ( D G L A )


1 20:4 (AA) i 22:4 I 22:5

I

18:4 1 20:4 1 20:5 (EPA) I 22:5 (DPA) - 24:5 i

i

22:6 (DHA) «- 24:6

Figure 1. Essential fatty acids (EFA) of the ω-6 and ω-3 families.

Non-triacylyglycerols Constituents The non-triacylyglycerol or unsaponifiables matter content varies from one oil to another. Different classes of compounds belonging to this group are summarized in Table I. In most oils these constitute approximately 1%, but in others they may be present at 2-8%. Many of the unsaponifiable matter are recovered from oil during processing steps of degumming, refining, bleaching and deodorization. Thus, loss of sterols, tocopherols, carotenoids and related compounds during oil processing may range from 35 to 95%. These material may be collected as distillates during the deodorization process. Distillates that are rich in certain components may be separated and marketed for use in nutraceutical applications. Thus, mixed tocopherols, tocotrienols, carotenoids, lecithin and other constituents may be separated from soybean, palm oil, rice bran oil and barley oil, depending on their prevalence.


205 Table L Non-tricylyglycerol Classes of Compounds i n Edible Oils

Class of Compound


Hydrocarbons Sterols Tocols Ubiquinones Phenolic Compounds Carotenoids Polar Compounds

Example Squalene Phytosterols Tocopherols/Tocotrienols Ubiquinone 9/Ubiquinone 10 Phenolic acids, Phenylpropanoids, Flavonoids/Isoflavonoids, Tannins Carotenes/Xanthophylls Phospholipids/Glycolipids

Stability of edible oils is affected by their constituents and this is primarily dictated by the degree of unsaturation of fatty acids involved, the minor constituents of the oil and storage conditions. Thus as the degree of unsaturation of an oil increases, its susceptibility to oxidation, under similar conditions, increases. Furthermore, the condition in which the oil is present, e.g. bulk versus emulsion, has a profound effect on the stability of the oil. In addition, presence of antioxidants as well as metal ions, light, chlorophylls and other pigments might influence the stability of the oil. A cursory evaluation of the factors involved is given in the subsequent sections. Degree of Unsaturation As the number of double bonds in a fatty acid increases, its rate of oxidation increases. Thus, stability of fatty acid in edible oil sources follows the trend given below. Docosahexaenoic acid > Eicosapentaenoic acid > (C22:6(03) (C22:5G>3) Linolenic acid > Linoleic acid > Oleic acid > (C18:3o>3) (C18:2o>6) (C18:lG)9)

Arachidonic acid > (C20:46)6) Stearic acid (C18:0)

This trend is also reflected i n the triacylyglycerols of different oils. Thus, as the iodine value (IV) of oils increases, their oxidation potential is also increased. Therefore, coconut oil with I V = 9 is far more stable than illipe oil with I V = 3 3 which is in turn more stable than canola oil with I V = 107 and black currant oil with I V =143.


206


Tocols and Ubiquinones Tocols include both tocopherol (T) and tocotrienol (T3) family of compounds which are present in edible oils in different compositions and proportions. Eight different compounds exist; each series of Τ and T3 includes four components designated as α, β, γ and δ, depending on the number and position of methyl groups on a chromane ring. The α-isomer is 5, 7, 8-trimethyl; β-isomer, 5, 8dimethyl; γ-isomer, 7,8-dimethyl; and δ is the 8-methyl isomer. The occurrence of tocopherols in vegetable oils is diverse, but animal fats generally contain only α-tocopherol. However, absence of α-tocopherol in blood plasma and other body organs may not negate the importance of other tocopherols such as γ-tocopherol. In oilseed lipids, there appears to be a direct relationship between the degree of unsaturation as reflected in the IV, and the total content of tocopherols. Most vegetable oils contain α-, γ- and ô-tocopherols, while βtocopherol is less prevalent, except for wheat germ oil. Meanwhile, tocotrienols are present mainly in palm and rice bran oils. The antioxidant activity of tocotrienols generally exceeds that of their corresponding tocopherols. Meanwhile, the antioxidant activity of tocopherols is generally in the order of δ ) γ ) β ) α. The content of tocopherol/tocotrienol in selected oils is summarized in Table Π. With respect to ubiquinones, also known as coenzyme Q, they occur as 6 to 10 isoprene unit compounds; that is Q (UQ-6) to Q (UQ-10). Coenzymes Q (UQ-10) to a lesser extent Q (UQ-9) are found in vegetable oils. Ubiquinone provides efficient protection in-vivo for mitochondria against oxidation, similar to vitamin Ε in the lipids and lipoproteins. 6

1 0

1 0

9

Phospholipids Phospholipids possess fatty acids which are generally more unsaturated than their associated tricylglycerols. Therefore, phospholipids are more prone to oxidation than their associated triacylglycerols. However, this situation does not necessarily hold for marine oils such as seal blubber oil whose phospholipids are less unsaturated than its triacylglycerols. The role of phospholipids as pro- or antioxidants is, however, complex because in addition to their lipid moiety, they contain phosphorous - and nitrogen-containing groups that dictate their overall effect in food systems. Extensive studies have demonstrated that phospholipids may exert an antioxidant effect in vegetable oils and animal fats. The exact mechanism of action of phospholipids in stabilizing fats and oils remains speculative; however, evidence points out to the possibility of their synergisms with tocopherols, chelation of prooxidant metal ions as well as their role in the formation of MaiUard-type reaction products. King et al. (3) found a positive relationship between the presence and type of phospholipids and stability of salmon oil in the order given below.


207 Sphingomyelin, lysophosphatidyleholine « phosphatidyl choline » phosphatidylethanolamine > phosphatidylserine > phosphatidylinositol > phosphatidylglycerol. Phytosterols Edible oils generally contain a variety of sterols which exist in the free form, as sterol ester of fatty acids and sterol glycosides or esters of sterol glycosides. Sterols are heat-stable molecules with no flavor of their own and exhibit antipolymerization activity during frying. Sterols serve as a means for fingerprinting of vegetable oils and lend themselves for detection of adultration of oils. Among sterols, A -avenasterol, fucosterol and citrostadienol have been shown to exhibit antioxidant properties. Donation of a hydrogen atom from the allylic methyl group in the side chain is contemplated. Most oils contain 100-800 mg/100g sterols (See Table Π). Brassicasterol is specifically found in canola, rapeseed and mustard oils.


5

Carotenoids Carotenoids are widespread in oilseeds, but are found in the highest amount in palm oil at 500-700 ppm levels. Both hydrocarbon-type carotenoids, namely α and β-carotene, as well as xanthophylls are present. Carotenoids act as scavengers of singlet oxygen and hence are important in stability of oils exposed to light. While β-carotene, and α-carotene, are usually the dominant components,

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