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American Journal of Food Science and Nutrition Research 2016; 3(6): 162-171 http://www.openscienceonline.com/journal/fsnr ISSN: 2381-621X (Print); ISSN: 2381-6228 (Online) Lipids: Functions, Applications in Food Industry and Oxidation Abdelmoneim H. Ali1, 2, *, Sherif M. Abed1, 3, Sameh A. Korma1, 2, Hamada M. Hassan2 1 State Key Laboratory of Food Science and Technology, Synergetic Innovation Center of Food Safety and Nutrition, School of Food Science and Technology, Jiangnan University, Wuxi, China 2 Department of Food Sciences, Faculty of Agriculture, Zagazig University, Zagazig, Egypt 3 Food and Dairy Sciences and Technology Department, Faculty of Environmental Agricultural Science, Suez Canal University, El Arish, Egypt Email address [email protected] (A. H. Ali) * Corresponding author To cite this article Abdelmoneim H. Ali, Sherif M. Abed, Sameh A. Korma, Hamada M. Hassan. Lipids: Functions, Applications in Food Industry and Oxidation. American Journal of Food Science and Nutrition Research. Vol. 3, No. 6, 2016, pp. 162-171. Received: August 19, 2016; Accepted: August 29, 2016; Published: September 9, 2016 Abstract Lipids are a group of naturally occurring molecules that includes fats, waxes, monoglycerides, diglycerides, triglycerides, phospholipids, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), and others. The basic biological roles of lipids include energy storage, signaling, and acting as structural components of cell membranes. Lipids have many applications in the cosmetic and food industries as well as in nanotechnology. Lipids are very different in both their individual compositions and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are however soluble in other organic solvents such as ether, acetone, and other lipids. This review is focused on some basic points of lipids such as the difference between docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), the role and function of lipids and their applications in food industry. Moreover, it presents the mechanism of lipids oxidation, and how to measure and prevent the oxidation of lipids. Keywords Lipids, Functions, Applications, Oxidation, Antioxidants 1. Introduction Lipids are defined on the basis of their solubility characteristics, not primarily their chemical composition. The term “lipids” is defined as those organic molecules that are insoluble in water, soluble in organic solvents (e.g., chloroform, methanol, ether), contain hydrocarbon groups as primary parts of the molecule, and are exist in or derived from living organisms [1]. Compound classes covered in this definition or classification include fatty acids (FAs), acylglycerols, fatty acids esters (e.g., waxes), and isoprenoid hydrocarbons. Further compounds also included are regularly considered as belonging to different classes, such as carotenoids, sterols, and the vitamins A, D, E, and K. Lipids tend to be categorized as “simple” or “complex,” referring to the size or structural detail of the molecule. The group of simple lipids includes fatty acids, hydrocarbons, and alcohols, all of which are comparatively “neutral” in terms of charge. While complex lipids, for instance glycolipids and phospholipids, are relatively more charged and are also referred to as “polar.” Fats and oils are fractions of lipids, mainly composed of triglycerides with great importance in food systems, and they are formed through the esterification of fatty acids molecules with one molecule of glycerol [2-5]. Lipid oxidation is one of the major reasons of quality retrogradation in natural and processed food products. Oxidative retrogradation is a large economic issue in the food industry because it affects many quality parameters such as flavor (rancidity), texture, color, and the nutritive value of foods. Furthermore, it results in the production of potentially poisonous compounds [6]. Lipid American Journal of Food Science and Nutrition Research 2016; 3(6): 162-171 oxidation considered very significant to food producers particularly in the case of increasing the content of unsaturated lipids in the final products in order to improve the nutritional profiles. Consequently, lipid oxidation is one of the major factors that limit the shelf life of foods. In addition, the oxidative instability of the polyunsaturated fatty acids frequently limits their applications as nutritionally beneficial lipids in functional foods production. 2. The Differences Between DHA and EPA 2.1. DHA: Docosahexaenoic Acid (22:6n-3) Fig. 1 shows the structure of docosahexaenoic acid (DHA). It is produced de novo by marine algae and is a primary component of fish oil (approximately 8- 20% by weight). DHA production in animals from linolenic acid occurs through the desaturation/elongation of β-linolenic acid to 24:5n-3. This very long chain unsaturated fatty acid is desaturated by a ∆6 desaturase (possibly a unique ∆6 desaturase enzyme) and the resulting fatty acid undergoes one cycle of β-oxidation to form DHA. Animals seem to have a requirement for DHA for neural function, and they depend on the production of this fatty acid from n-3 precursors by elongation/desaturation cycles or through ingestion of the intact acid. Although the exact role DHA plays in animal physiology is not clear, the great care with which the fatty acid is preserved in certain tissues implies that it may be an essential component of certain cells. Brain and retinal tissues are predominantly enriched in DHA. 163 Fig. 2. Eicosapentaenoic acid. 3. Types and Functions of Lipids Functional lipids for instance, omega-3 and omega-6 fatty acids, conjugated linoleic acids, medium chain triglycerides, and phytosterols have numerous positive influences on human health such as in obesity, blood pressure, cardiovascular diseases, bone health, and in treating and managing depression [7]. There is some misunderstanding between lipids and fats, since not all lipids are fats, but all fats are lipids. There are numerous types of lipids to discover before completely understanding their functions, these types include the following: 3.1. Triglycerides Triglyceride molecules are composed of three of fatty acids and one molecule of glycerol. The fatty acids may be either saturated or unsaturated. Triglycerides have the ability to float in a cell’s cytoplasm since they have a lower density compared to water and are non-soluble, as is the case with all lipids. A triglyceride can be categorized as a fat if it converts to a solid at a temperature of 20°C, otherwise that are classified as oils. They are fundamental in the body for energy storage. 3.2. Steroids Steroids are these organic compounds mainly composed of four rings arranged in a specific configuration (Fig. 3). A few categories of common steroids are cholesterol, vitamin D2, estrogen and testosterone. These fractions of lipids have two main biological roles: certain steroids (such as cholesterol) are substantial constituents of cell membranes which change the fluidity of membranes, and many steroids are signaling molecules which stimulate steroid hormone receptors. Fig. 1. Docosahexaenoic acid. 2.2. EPA: Eicosapentaenoic Acid (20:5n-3) Eicosapentaenoic acid (EPA) is produced de novo by marine algae and in animals by the desaturation/elongation of α-linolenic acid. EPA is the primary fatty acid of fish oil (about 25-20% by weight) although it is not produced de novo by fish. It has also been reported that significant production of EPA can take place in animals through the βoxidation chain shortening of DHA. EPA has been widely investigated for its action as a competitive inhibitor of arachidonic acid metabolism. Although eicosanoids can be produced from EPA, they appear to have either no activity or an activity that opposes arachidonic acid–derived eicosanoids. The chemical structure of EPA is presented in Fig. 2. Fig. 3. Structure of steroids. 164 Abdelmoneim H. Ali et al.: Lipids: Functions, Applications in Food Industry and Oxidation 3.3. Phospholipids materials sciences [8]. Phospholipids can be defined as lipids containing phosphorus. They received their name as their constitution is primarily phosphate groups. They contain molecules that both attract and repel water, playing a key role in the organization of cell membranes structure. There are two major groups of phospholipids; glycerophospholipids and sphingolipids. Fig. 4 shows the basic structure of phospholipids. Purified phospholipids are currently produced commercially and have been applied in nanotechnology and 3.4. Glycolipids Glycolipids are lipids with a carbohydrate attached by a glycosidic bond [9]. Short sugar chains form glycolipids, which can be found in a cellular membrane’s exoplasmic surface. They play an important role in enhancing the body’s immune system, as well as helping to maintain the stability of the membrane and attaching cells to one another to form tissues. Fig. 4. Structure of phospholipids. 3.5. Lipoproteins Lipoproteins are a combination of proteins and lipids found in a cell’s membrane – examples being enzymes and antigens. Different types of lipoproteins are differentiated based on specific proteins attached to the outer layer of phospholipid, called the apolipoprotein. Lipoproteins help fat moving around the body in the bloodstream, and exist in the form of low density lipoprotein (HDL) and high density lipoprotein (LDL). 3.6. Waxes Along with a chain of alcohols, fatty acids are found in waxes. They are chemically stable and insoluble in water and various organic solvents. Because of these properties, waxes are extensively distributed in both animals and plants as protective coverings for tissues. Simple waxes are classified as monoesters of normal fatty acids and normal long-chain alcohols. Complex waxes as well exist, in which either the fatty acids or alcohol components possess complex structures in their own rights such as, vitamin esters or sterol esters [10]. 4. Applications of Lipids in Food Industry 4.1. Role and Changes of Plant Lipids in Processed Foods Plant lipids have the ability to increase the nutritional values of foods. They also contain tocopherols and tocotrienols, which are the major essential sources of vitamin E. Lipids affect the functional properties of foods; for American Journal of Food Science and Nutrition Research 2016; 3(6): 162-171 example, they help to retain carbon dioxide in dough, thus increasing the final volume of bakery products. The main importance of lipids is their influence on the sensory properties. They affect the texture and increase the viscosity of the morsel after mixing with saliva; high viscosity is appreciated by many consumers. The most desirable influence of lipids is their effects on the odor and flavor of food products. Plant lipids, being more unsaturated than animal lipids, produce different flavor notes as a result of culinary operations. Flavors originating at roasting or frying temperatures are particularly appreciated. 4.2. Frying Oils and Fats The using of oils and fats as a frying medium in both shallow and in deep frying mode is an important component in the overall picture of food applications. Recently, it has been reported that 20 million tonnes of oils and fats is used in this way. This represents a major share of the 90 million tonnes used for dietary purposes. Of course, it should be taken in mind that while some of the frying oil is consumed along with the fried foods, much is thrown away (shallow pan frying) or ultimately finds other uses as spent frying oil. 4.3. Spreads: Butter, Ghee, Margarine Butter. For many centuries, butter from cow’s milk fat has been mainly used as a spread, but also for baking and frying purposes. Butter has become less widespread with the continuous development of good-quality margarine and other spreads. The are some disadvantages associated with butter such as, its comparatively high price, its poor spread-ability (especially from the refrigerator), and its poor health profile resulting from its high fat content, its high content of saturated fatty acids and cholesterol, and the presence of trans unsaturated fatty acids. Butter has the advantages of its completely natural profile and its splendid flavor. Ghee. Milk fat can be consumed partly as butter but also as ghee, however the latter is declining and is now probably below one-quarter of the combined total. Ghee is a concentrate of butterfat with more than 99% of milk fat and less than 0.2% moisture. It has a shelf life of 6- 8 months even at ambient tropical temperatures. Butter or cream is converted to ghee by controlled heating to reduce the content of water to below 0.2%. In other procedures the aqueous fraction is allowed to separate and some of it is run off before residual moisture is removed by heating. Ghee is distinguished by a cooked caramelized flavor varying slightly with the method of preparation. Margarine. Margarine has been produced for more than 10 decades. During the 1860 s, large sections of the European population migrated from country to town and changed from rural to urban occupations. At the same time, there was a rapid increase in population in Europe and a general recession in agriculture leading to a shortage of butter, especially for the growing urban population. So, the price rose beyond the reach of many poor people. So bad was the 165 situation in France that the government offered a prize for the best proposal for a butter substitute that would be cheaper and would also keep better. The production of margarine comprises three basic steps: emulsification of the oil and aqueous phases, crystallization of the fat phase, and plasticification of the crystallized emulsion. Water-in-oil emulsions are cooled in scraped-wall heat exchangers during which time fat crystallization is initiated, a process known as nucleation, and during which the emulsion drop size is reduced. There follows a maturing stage in working units during which crystallization approaches equilibrium, though crystallization may continue even after the product has been packed. The lipid in margarine is part solid (fat) and part liquid (oil), and the proportion of these two varies with temperature. The solid/liquid ratio at different temperatures is of paramount importance in relation to the physical nature of the product. 4.4. Baking Fats, Dough and Shortenings The application of oils and fats in baking processes ranks with frying and spreads as a major food use of these materials. The products range from breads and layered dough to cakes, biscuits (cookies) and biscuit fillings, pie crusts, short pastry, and puff pastry. The fats used to produce this wide range of baked goods vary in their properties and particularly in their melting behavior and plasticity. It is possible to achieve these properties with different blends of oils, and preferred mixtures vary in different areas of the world. Fats used to make dough of various types are almost entirely plastic fats, i.e., mixtures of solid and liquid components that appear solid at certain temperatures and that deform when a pressure is applied. Fats exert their effect by interaction with the flour and (sometimes) sugar, which are the other major constituents of a baked product. 4.5. Salad Oils and Mayonnaise Salad oils, used in the preparation of mayonnaise and salad cream, should be oxidatively stable and free of solids even when stored in refrigerator. Numerous vegetable oils may be applied. Those containing linolenic acid (soybean oil and canola oil) are frequently lightly hydrogenated to improve the oxidative stability. All oils are mostly winterized in order to remove high-melting glycerides that would crystallize, as well also waxes present in solvent-extracted oil. The latter lead to a haze in the oil when it is been cooled. Salad oils must pass a cold test, which needs that the oil remains clear for 5.5 hours at the refrigeration temperature. After appropriate treatment, soybean, canola, corn, and sunflower oils can be used for mayonnaise production. 4.6. Incorporation of Vegetable Oils into Dairy Products Vegetable oils could be incorporated into dairy products as a substitute of milk fat. This occurs when local supplies are insufficient as in several tropical countries where the climate 166 Abdelmoneim H. Ali et al.: Lipids: Functions, Applications in Food Industry and Oxidation is not appropriate for large-scale dairy farming and also for consumers concerned about the saturated acids and cholesterol existing in milk fat. Furthermore, it is possible to produce milk fat substitutes in a more convenient form as, for example, in long-life cream. This reaction scheme is able to generate aldehydes, ketones, alcohols and hydrocarbons. Many of the volatile compounds produced during lipids oxidation originate through similar dismutations. 5. Lipids Oxidation The mechanism of lipids oxidation is shown in Fig. 5. Lipid oxidation or rancidity is clearly the major challenge for stabilizing specialty oils, particularly since oils with special nutraceutical properties have predominantly polyunsaturated fatty acids. While there is relatively little data yet available regarding oxidation of specialty oils per se, all oils follow the same fundamental processes, modified by endogenous proand antioxidants and innate variances in fatty acid composition. What has been learned about lipids oxidation in conventional food oils can be applied to predict the stability of specialty oils and to explain the observed behaviors. Hydroperoxides are not stable compounds and given time, they will break down. A typical mechanism, as presented below, results in the formation of two radicals from a single hydroperoxide molecule. Both of these new radicals can initiate further oxidation, and some metals can speed up this reaction. Both ions and free radicals were formed, and copper was the catalyst. Copper did not initiate the reaction, but once the hydroperoxides were formed, it speeds up their breakdown. Fig. 5. Mechanism of lipids oxidation 5.1. Measurements of Lipid Oxidation 5.1.1. Peroxide Value One of the most widely used parameters for oxidative rancidity. The detection of peroxide gives an initial indication of rancidity in the unsaturated fats and oils [11]. Peroxides are the basic initial products of autoxidation, they can be measured by using procedures based on the principle of their ability to release iodine from potassium iodide, or to oxidize ferrous to ferric ions. Their content is frequently expressed in terms of milliequivalents of oxygen/kg of fat. Although the peroxide value is applicable for following peroxide formation at the early stages of oxidation, it is, however, highly experimental. The precision is questionable, the results vary American Journal of Food Science and Nutrition Research 2016; 3(6): 162-171 with details of the technique used, and the test is very sensitive to temperature variations. Throughout the course of oxidation, peroxide values reach a peak and then decline [12]. 5.1.2. Thiobarbituric Acid (TBA) TBA is the most extensively used examination for measuring the extent of lipid peroxidation in foods because of its simplicity and because the obtained results are extremely associated with the sensory evaluation scores. The principle of this method depends on the reaction of one molecule of malonaldehyde with two molecules of TBA to generate a red malonaldehyde-TBA complex (Fig. 6), which can be spectrophotometrically quantitated at 530 nm. Nevertheless, this technique has been evaluated as being nonspecific and insensitive for the detection of low concentrations of malonaldehyde. Other TBA-reactive substances (TBARS) including sugars and other aldehydes could interfere with the malonaldehyde-TBA reaction. If some of the malonaldehyde reacts with proteins in an oxidizing system, abnormally low values may result. In many cases, however, the TBA test is appropriate tool to compare samples of a single material at different stages of oxidation. Fig. 6. Proposed TBA reaction [12]. 5.1.3. Iodine Value Iodine value is used to measure the unsaturated double bonds in fat, and is expressed in terms of percentage of the iodine absorbed. The decline in the iodine value is occasionally used to observe the reduction of dienoic acids during the course of the autoxidation. The higher the iodine number, the more double bonds are presented in the fat [13]. 5.1.4. Active Oxygen Method (AOM) This technique determines the stability of a fat by bubbling air through a solution of the fat using specific conditions of flow rate, temperatures, and concentration. At intervals, peroxides and hydroperoxides resulted by this treatment are determined by titration with iodine. The AOM value is defined as the number of hours needed for the peroxide concentration to reach 100 meq/kg of fat. The more stable the fat, the longer time it will take to reach that level. 5.2. Applied Methods to Evaluate Lipids Oxidation The analysis and examination of lipid oxidation in different food samples is a significant issue since the compounds produced in the process are associated with the undesirable sensory and biological influences. Suitable measurement of lipid oxidation considered a challenging 167 mission since the process of oxidation is complicated and depends on the category of lipid substrate, the oxidation causes and the environmental influences. A great number of procedures have been developed and applied so far, in order to determine both primary and secondary products of lipids oxidation. The most common techniques and classical methods include, peroxide value, TBARS analysis and chromatographic tests. Particular additional procedures such as chemiluminescence, fluorescence emission, Raman spectroscopy, infrared spectroscopy or magnetic resonance deliver interesting and promising outcomes. Consequently, consideration should be paid to these additional different methods in the field of food lipid oxidation analysis [14]. Estimating the status of lipid oxidation is a challenging mission because of a number of evidences such as the diverse compounds which are produced depending on the time, the extent of oxidation and the involved mechanism. Consequently, selecting only one parameter to analyze the oxidative status is rather tough and it is often more appropriate to combine different procedures. In addition, as indicated by Eymard, Baron [15], not only nature and composition of lipid as the substrate of the reaction have an influence on lipid oxidation process, but also proteins kind and the concentration, antioxidants and prooxidants existing in the food matrix, as well as its physicochemical properties are parameters worthwhile to take in account. Lipid oxidation varies depending on the source of food. For example, in meat samples, studies suggested that the rates of lipid oxidation may depend on the comparative ability of haemoglobins from different animal classes to promote it [16], and also suggested that colloidal structures designed by phospholipids in vegetable oils could have an influence on the oxidative stability of food oils [17]; similarly lipid oxidation was expected to be delayed in fish sausages after the addition of numerous antioxidants [18], and in milk samples, the role of catechins and ascorbic acid in oxidation has been recently studied [19]. Additionally, each method permits a number of different experimental environments, and combined with the lack of uniformity among laboratories, it leads to different results that are currently unavoidable. Most of the oxidation compounds are disposed to be moreover degraded, which provides an added source of divergence. Therefore, accurate control of the experimental technique should be kept. 5.2.1. Volumetric Methods Amongst the different suggested methods for peroxides analysis, iodometry has been the most conventional and common technique mostly because of the simplicity of the experimental process. Though the procedure involves prior extraction of lipids, rapid and clear results are provided. In acidic medium, hydroperoxides and other peroxides react with the iodide ion to produce iodine, which is titred with a sodium thiosulfate solution, in the existence of starch. The AOAC provides an approved manner since 1965 [20]. According to this technique, peroxide value is considered to represent the quantity of active oxygen (in meq) contained in 1 kg of lipid and which could oxidize the potassium iodide. It 168 Abdelmoneim H. Ali et al.: Lipids: Functions, Applications in Food Industry and Oxidation shows, however, some drawbacks, mainly derived from the iodide high susceptibility to oxidation in the presence of molecular oxygen and accelerated by light exposure. As well, spontaneous hydroperoxide formation can occur, which would result in overestimation and absorption of iodine by unsaturated fatty acids leading to underestimation [21]. Furthermore, it involves anhydrous systems in order to avoid interference complications. Consequently, lipids have to be extracted, and this procedure stage increases the contact with oxygen. Furthermore, the peroxide value determination does not provide actual measure of the oxidative degradation; since peroxides are frequently further degraded; therefore simultaneous measurement of the secondary products is suitable. 5.2.2. VIS–UV Spectroscopic Techniques Besides the volumetric technique, spectroscopic ones are relatively simple and are reliable, moderately sensitive and reproducible when they are achieved under standardized conditions. On the other hand, they are highly experimental in the view of the point that they measure complex mixtures of oxidized molecules. Furthermore, they are normally workintensive and consume large volumes of solvents and reagents that might be dangerous [22]. 5.2.3. Ferrous Oxidation Method The ferrous oxidation technique for the quantification of peroxide content is easy and simple to use compared to iodometry. The chief reason is the lower sensitivity of ferrous ion towards the spontaneous oxidation by the oxygen existing in the air, when compared to the high susceptibility to oxidation of the iodide solutions. It involves the oxidation of Fe (II) to Fe (III), mediated by hydroperoxide reduction in acidic conditions and in the presence of either thiocyanate or xylenol orange. 5.2.4. Iodide Oxidation Method A spectrophotometric iodidedependant technique has similarly been set for the determination of hydroperoxide content. In this not so regularly applied methodology [23], the lipid sample is placed in an acidic solution, which is then combined with iodide. The lipid hydroperoxide oxidizes iodide to iodine. Then, the produced iodine and iodide in excess react to give triiodide anion, which is identified spectrophotometrically at 350 nm. Bloomfield [24] used Fe (II) as a catalyst in this reaction. The closed conditions prevent interference from atmospheric oxygen, and the short reaction time reduces the interference from side reactions. 5.2.5. Chromatographic Methods All the methodologies described above are generally rather simple regarding the theory base, the application of the technique and the ulterior interpretation of the data, presenting low-to-moderate selectivity and sensitivity, though. In this case, chromatographic methods are far more precise, sensible and specific for the compounds of concern, allowing improved identification of the individual products. In fact, their application for hydroperoxides determination rather than that of volumetric and spectroscopic measurements displays a growing trend over the last few years. As an unavoidable consequence, chromatographic approaches frequently need long or particular experimental work, precise control of the experimental conditions and the data processing is relatively complex [14]. Liquid chromatography. High-performance liquid chromatography (HPLC) has been recently applied for hydroperoxides identification. This technique is highly sensitive and pretty multipurpose considering both column and detector properties, allowing to identify compounds with diverse properties of volatility, molecular weight or polarity. However, the preparation of samples is often tedious and regularly needs lipid extraction. Zeb and Murkovic [25] found that the isocratic HPLC–ESI–MS was a convenient technique for the identification and classification of oxidized classes of triacylglycerols (TAGs), that is, mono- and bishydroperoxides. Gas chromatography. Gas chromatography coupled with mass spectrometry (GC–MS) can also be applied for the quantification of lipid hydroperoxides, nevertheless because of their thermolability, previous reduction of the hydroperoxides is required. This point, along with the previous lipid extraction and successive derivatization step, makes it an unwieldy and time-consuming technique [26]. 5.3. Reasons for Lipids Oxidation As shown in Fig. 7, the overall reaction mechanism of lipid oxidation consists of three phases: (1) Initiation, the formation of free radicals. (2) Propagation, the free radical chain reactions. (3) Termination, the formation of non-radical products. The important lipids involved in oxidation are the unsaturated fatty acid moieties, oleic, linoleic, and linolenic. The rate of oxidation of these fatty acids increases with the degree of unsaturation, as oleic acid has 1 times rate, linoleic acid has 10 times and linolenic acid has 100 times. Fig. 7. Reaction mechanism of phenolic antioxidant and hydroperoxide. 5.4. Free Radicals Free radicals are atoms or groups of atoms with an odd number of electrons and can be produced when oxygen interacts with certain molecules. As soon as these highly reactive radicals are formed, they can start a chain reaction, like dominoes. American Journal of Food Science and Nutrition Research 2016; 3(6): 162-171 Types of free radicals: The most common types of free radicals include; superoxide (O2-), hydrogen peroxide (H2O2), hydroxyl radical (OH-), singlet oxygen (1O2), hydroperoxy radical (HOO-), lipid peroxide radical (ROO-), nitric oxide (NO-), and peroxynitrite, (ONOO-) Properties of free radicals: Highly reactive - very short half-life - generate new radicals by chain reaction - cause damage to biomolecules, cells and tissues in various disease conditions such as diabetes mellitus, neurodegenerative diseases, cancer, and cardiovascular diseases. 169 5.5. Analyzing the Type and Path of Oxidation Chemical analysis monitors the formation of primary and secondary oxidative by-products (Fig. 8). Primary oxidative by-product measurements are often used to determine how close a material or matrix is to the point of failure (oxidation). Often, only the peroxide value is used in order to evaluate the level of lipids oxidation in different matrixes. In fact, without secondary oxidative by-products, it is not easy to truly judge the status of a product by only peroxide values. Consequently, even if a product has a low peroxide value, it does not necessarily always refer that this product is fresh. Fig. 8. The volatiles and non-volatiles contain the secondary oxidative by products [27]. Primary oxidative by-product measurements are only one aspect to ensure the quality of food products. In order to get a full clear picture of where a product is on the oxidation path, secondary oxidative by-products, such as hexanals or 2,4decadienals should be measured as well. These secondary byproducts are associated with the flavor components connected to lipids oxidation. Chemical analysis gives an insight into the timing of the oxidation pathway that sensory analysis might not show until it is too late. There are numerous ingredient options to choose from that can help to control lipids oxidation, but remember when analyzing these options, confirm the two commonly asked questions have been answered. It is difficult to meet all consumer requirements and expectations when delivering quality food products that look good, taste fresh and are consistently the same each time purchased. 5.6. Prevention of Lipids Oxidation Lipid oxidation in foods considered a serious dilemma, difficult to overcome often and leads to loss of shelf life, palatability, functionality, and nutritional quality. Loss of palatability is due to the generation of off-flavors that arise primarily from the breakdown of unsaturated fatty acids during autoxidation. The high reactivity of the carbon double bonds in unsaturated fatty acids makes these substances primary targets for free radical reactions. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical chain mechanism. 5.7. Antioxidants In foods containing lipids, antioxidants can delay the beginning of oxidation or slow the rate at which it proceeds. These substances can occur as natural components of foods, but also they can be deliberately added to products or formed during processing. Their role is not to enhance or improve the quality of foods, but they do maintain food quality and extend their shelf-life. Antioxidants used in food processing should be distinguished by their low-cost, nontoxic, influential at low concentrations, stable, and capable of surviving processing (carry-through effect); color, flavor, and odor must be negligible. The choice of the type of antioxidant which will be used depends mostly on the compatibility of the product and regulatory guidelines. Antioxidants can slow lipids oxidation through inactivating or scavenging free radicals, consequently preventing initiation and propagation reactions. Free radical scavengers (FRS) or chain-breaking antioxidants are able to 170 Abdelmoneim H. Ali et al.: Lipids: Functions, Applications in Food Industry and Oxidation accept a radical from oxidizing lipids species such as peroxyl (LOO•) and alkoxyl (LO•) radicals by the following reaction: Antioxidants not only extend the shelf life of the product, but also reduce raw material waste, reduce nutritional losses, and widen the range of fats that can be used in specific products. By extending keeping quality and increasing the number of oils that can be used in food products, antioxidants allow food producers to use more available and/or less costly oils for product formulation. 5.7.1. Synthetic Antioxidants (1) Butylated hydroxyanisole. (2) Butylated hydroxytoluene. (3) Tertiary butylhydroxyquinone. (4) 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline (Ethoxyquin). (5) Gallates. (6) Tocopherols. (7) Erythorbic acid and ascorbyl palmitate. Fig. 9. Chemical composition of synthetic antioxidants. 5.7.2. (1) (2) (3) (4) (5) (6) (7) (8) Natural Antioxidants Tocopherols and tocotrienols. Ascorbic acid and ascorbate salts. Carotenoids. Enzymatic antioxidants: glucose oxidase, superoxide dismutase, catalase, and glutathione peroxidase. Proteins and related substances. Maillard reaction products. Phospholipids and sterols. Sulfur dioxide and other sulfites. 6. Conclusion Lipids have a functional and significant role in foods because of their contribution to palatability, satiety, and nutrition. Consequently, lipid quality is a very important issue to consumers and may show a relation to numerous health problems. 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