Lipid Oxidation and Antioxidants
Lipids are organic products made of carbon, oxygen and hydrogen elements. Apart from these basic components, some lipids may contain elements like nitrogen and phosphorus. A fatty acid is the basic unit of lipid. A fatty acid is organic and it constituents one replaceable hydrogen with a general formula R-COOH, whereby R stands for replaceable hydrogen. The term `oil’ refer to lipids that remain in liquid form at room temperature while fat refers to lipids that remain in a solid state at room temperature Lipids are a key source of energy as well as other vital nutrients to the human body. Lipids contained in vegetable oil are prone to oxidation processes. The process of lipid oxidation takes place in three phases; namely, initiation stage where free radicals are formed, propagation stage where radical chain reaction takes place and termination phase where non-radical products are formed. Deep frying of vegetable oils results in hydrolysis, oxidation as well as polymerization of the oil. The product of lipid oxidation is mainly classified into two; primary product and secondary volatile products.
Lipids are a key source of energy and offer necessary lipid nutrients to the human body. However, over-consumption of lipids of some lipid constituents can be harmful to human health. For instance, over-consumption of saturated fats and cholesterol are the leading cause of cardiovascular diseases. Furthermore,several foodstuffs with lipid content play a key role in determining the physical characteristics, like texture, mouthfeel, appearance and flavour. For this factors,it is challenging to come up with low-lipid alternatives of several foodstuffs,for the simple reason that once the fat content of these foodstuffs is extracted ,these foods will lose the some of the significant features that appeal to consumers. This paper will discuss the process of lipid oxidation, the role of antioxidants in preventing oxidation as well as human health implication as a result of the oxidation process.
Lipids are organic products made of carbon, oxygen and hydrogen elements. Apart from these basic components, some lipids may contain elements like nitrogen and phosphorus. A fatty acid is the basic unit of lipid. A fatty acid is organic and it contains one displaceable hydrogen with a general formula R-COOH whereby R represents a replaceable hydrogen element or carbon group containing hydrogen atom(Hwang). Examples of fatty acids include oleic, palmitic and stearic. Glycerol is water soluble thick liquid which can react with fatty acid to form a complex organic product. In such a chemical reaction, three molecules of the fatty acid combined with one molecule of glycerol, where the hydrogen atom of fatty acid is substituted and a new compound called triglyceride or ester is formed (Schaich). Therefore, all oils and fats are esters of fatty acids. Glycerides are organic compounds present in both animals and plants. And they serve as the storage form of lipids in plants and animals. The term `oil’ refers to lipids that remain in liquid form at room temperature while fat refers to lipids that remain in a solid state at room temperature (Alver, Baştürk, Kılıç, & Karataş).
Lipid oxidation entails different type chemical reactions that take place in the human body with the potential of having both positive as well as a negative implication to human health(Zamora & Hidalgo).In the human body, lipid oxidation is significant for a number of physiological reactions, for example, production of energy from fatty acids via β-oxidation(Purba). Oxidation also takes part in the formation of signalling products called eicosanoids. Lipid oxidation could also lead to unregulated oxidative decomposition of lipids started when free radicals snatch electrons, which is the initial step in the formation of many mutagenic and cytotoxic in the body (C. J,).
Oxidation of vegetable oil is one of the significant processes that take place in the human body and food processing (Hwang). It impacts several interactions among vegetable oil components, resulting in both useful and harmful products. Lipids contained in vegetable oil are prone to oxidation processes. Therefore, lipid oxidation processes are the significant source of deterioration of vegetable oils during manufacturing, distribution, storage and preparation of food. Lipid oxidation components are all always present in vegetable oils, although variation exists with the kind and levels of saturation of vegetable oils. Although levels of oxidation compounds are usually low in vegetable oils, the challenge of lipid oxidation impacts negatively on the quality of food products prepared by vegetable oil and the duration of storage of such foods (Sohrabi, Ross, Martin, & Barker).
Composition of vegetable oils
Vegetable oil contains majorly 98% of triacylglycerol and other minor ingredients like tocopherols and plant sterols (Sohrabi, Ross, Martin, & Barker). And fatty acid is the basic unit of lipids. Vegetable oils vary their structure of fatty acids. The fatty acid can be saturated like in the case of cooking fat or unsaturated with a single bond commonly referred as monounsaturated or with several double bonds (polyunsaturated).The common unsaturated fatty acids include linoleic acid, linolenic acid and oleic acid(Goddard, McClements, & Decker). Plant sterol or Phytosterols is another ingredient of vegetable oil available in all plants or food products generated from plants. Phytosterols control the permeability and fluidity of cell membranes as well as play a significant role in the adjustment of the membranes to temperature. Cholesterol is the major sterol in the cells of animals but it is only available in small quantities in plant cells (Schaich).
Phytosterols are composed of a tetracyclic phenanthrene ring and side –chain with 17 carbon atoms(C17).They are majorly 28-29 carbon atom steroid. Furthermore, Phytosterols contain a similar basic structure which only differs as additional ethyl or methyls are included to the side chains. The main phytosterol in vegetable oils is campesterol, stigmasterol and sitosterol(Tahir, Kariem, & Ibrahim).Studies indicate that vegetable oils are the prime source of phytosterols,followed by nuts and cereal grains. In the category of vegetable oils, rapeseed and corn contain the highest quantity of phytosterols (C. J).
The process of lipid oxidation takes place in three phases; namely, initiation stage where free radicals are formed, propagation stage where radical chain reaction takes place and termination phase where non-radical products are formed(Runas & Malmstadt). The significant lipids involved in the oxidation process are unsaturated fatty acids oleic, linoleic, and linolenic and moieties. The speed of oxidation of these fatty acids increased with the increase in the degree of unsaturation. For instance, linolenic is 100 times faster; linoleic is 10 times faster while oleic is one time faster. Unsaturated fatty acid contains a labile hydrogen element from an adjacent carbon atom with double bonds.Hydroperoxide (ROOH) is one of the common oxidation products that breaks down to form to form compounds which are in charge of odours and off-flavours.Once hydroperoxide is formed, it may decompose through a number of processes (Hwang). The most common mechanism is called dismutation.In this reaction process hydroperoxide reacts with another radical or molecule to produce two new products(Runas & Malmstadt). Furthermore, the reaction process is in the position of generating aldehydes, alcohols, hydrocarbons and ketones. A number of volatile compounds produced during lipid oxidation process undergo similar dismutations.Since hydroperoxide is not a stable compound, it decomposes as times lapses. The reaction process of free radicals is thermodynamically complex, the formation of the first few radicals required to initiate the propagation process usually takes place under the influence of some form of catalysts such as exposure to light or heat, metal catalyst or hydroperoxide decomposition (Runas & Malmstadt).
Oxidation of Fresh Vegetable Oil
Studies indicate native oxidation of fresh vegetable oils is minimal(Purba). This is widely accepted that the mechanism of lipid oxidation is largely autoxidation. Autoxidation is sudden chemical reactions accelerated by metal, heat and light and entails incorporation of molecular oxygen together with unsaturated fatty acids to form hydroperoxides. Autoxidation process in most cases involves a free radical which undergoes a chain reaction in three phases namely, initiation stage followed by propagation and finally termination phase(Tahir, Kariem, & Ibrahim). In the initiation phase, the production of the first free radical occurs by thermal dissociation, metal catalysis, hydroperoxide decomposition and by photolysis.At the time prior to accelerated oxidation takes place,it is typically noticed in lipid oxidation at the initial stages that when the fresh vegetable is bombarded with oxidative stresses to form the initial few free radicals.
The first few radicals are created,they react with molecular oxygen to produce peroxy radicals. The generated peroxy radicals then abstract a hydrogen atom from unsaturated fatty acids to form a hydroperoxide and free radical is formed in the process (Tahir, Kariem, & Ibrahim). The free radical generated replicates the same reaction resulting in a chain reaction. As more and more hydroperoxides bills up and breaks down to free radicals ,the oxidation reaction takes place at a rapid rate.At the end of propagation process, unsaturated fatty reacts with oxygen to produce hydroperoxides (Sohrabi, Ross, Martin, & Barker).
For vegetable oil which contains unsaturated fatty acids, the vulnerability to oxidation relies on their tendency to donate a hydrogen atom to react with peroxy radicals. Free radicals are normally formed at the joining points of double bonds because at these joints bonding energy is less and hydrogen atoms at these joints can be easily taken away by peroxy radicals.
Another significant way vegetable oils can undergo oxidation is through exposure to light in the presence of a sensitizer and oxygen. The activation of oxygen in triplet state triggers electronic excitation to produce singlet oxygen which reacts with unsaturated fatty acids. Singlet oxygen can be produced in a number of ways, but the most common way is that of exposing fatty acids to light energy in the presence of a photosensitizer. This method of exposing fatty acids to light energy in order for oxidation to take place is called Photooxidation.
Lipid Oxidation in Frayed Vegetable Oils
Frying of food is one of the most popular methods of preparing food. In deep frying, food to be prepared is completely immersed in vegetable cooking oil, which serves as a medium of transferring heat (C. J). Fundamentally, frying is a dehydration mechanism at high temperatures. The aim of frying food using vegetable oils is to form a unique crust, texture and flavour. Frying is important in the preparation of all kinds of food such as vegetables, fish and meat. Potato is probably one of the food stuff that is mostly subjected to frying because potatoes are a key ingredient in the production of French fries. French fries have 8% to 15% of fats. On the other hand, potato crisps have a high content of lipids, which can be as high as 35%(Alver, Baştürk, Kılıç, & Karataş).
During frying, vegetable oil is subjected to a number of deterioration processes like hydrolysis, thermal alteration and oxidation, which produces a variety of compounds such as free fatty acids, diacylglycerols, cyclic compounds, monoacylglycerols, geometric isomers containing unsaturated fatty acids and volatile compounds(C. J). Oxidised dimeric,monomeric and oligomeric triacylglycerols produced from native triacylglycerols may have adverse effects on human health.
Diverse vegetable oils used during the frying process are characterised by diverse fatty acids profiles (Hwang). With solid vegetable fats having a higher content of fatty acids as compared to liquid vegetable oils. Vegetable oils with a high concentration of fatty acids are more stable in the frying process as compared to vegetable oils with a low concentration of fatty acids. However, due to health concerns associated with saturated vegetable oils, the use of monounsaturated oils in food frying has been on the rise in the recent past (C. J).
Minor constituents of vegetable oils other than acyl-lipids, like analogous sterols and cholesterol, can also be subjected to oxidation to produce several products commonly referred to as cholesterol oxidation products (Purba). Phytosterols also referred to as plant sterols are chemically similar to cholesterol and therefore they can undergo oxidation process just like the common cholesterol. Cholesterol oxidation products attracted much attention in the recent past due to the increasing cases of obesity and cardiovascular diseases which are closely associated with consumption of vegetable oils. Cholesterol oxidation products (COPs) negatively impact human health due to biological effects like atherogenicity, cytotoxicity, mutagenicity, sterol metabolism interruptions and the subsequent absorption of these components into the bloodstream of an individual. The correction between long-run consumption of vegetable oil and individual health is not explicitly clear, but definitely abused and over-use of vegetable oil poses health risks (Zamora & Hidalgo).
A variety of vegetable fats and oils are available for the purpose of frying foods. These comprise vegetable oils such as rapeseed oil, cottonseed oil, palm oil, soybean oil and olive oil as we animals fats such as tallow and lard. For many people, the choice of frying oils depends on a number of factors such as stability availability, affordability and flavour. Resistance to oxidation in the course of exposure to high temperature is a key property that frying oil used for the industrial process of food must possess (Hwang).
Palm oil is the most common vegetable oil which is used for preparing foods. Palm oil is popular because it is the highly saturated content of fatty acids and an excellent lipid oxidation. Palm oil is used in many countries to prepare food.However,it has negative nutritional impacts due to its highly concentrated content of fatty acids and high content of cholesterol which is a leading cause of cardiovascular diseases and obesity (Afshari & Sayyed-Alangi).
Vegetable oils used for the frying process can always be altered to boost their stability. The common modification processes include hydrogenation, fractionation as well as genetic modification of the composition of fatty acids in plant seeds(Hwang). Oil fractionation is used to minimise the content of saturated fatty acids of palm oil. Palm olein has a higher concentration of monounsaturated fatty acid and lower concentration of saturated fatty acids as compared to palm oil. Thus palm olein is widely used to prepare snacks due to its higher oxidation ability when compared with palm.
Hydrogenation of vegetable oil is a chemical process in which hydrogen gas is used to minimise the level of saturation of vegetable oil with the help of a catalyst (Tahir, Kariem, & Ibrahim). However,partial or full hydrogenation of vegetable oils,does not only increase the risk of atherogenic lipoprotein and low-density lipoprotein (LDL) but also increases, but also reduces the levels of high-density lipoprotein (HDL) due to the formation of trans fatty acids in the process(Goddard, McClements, & Decker).
Genetic alteration of the fatty acid content of oilseeds normally entails improving the relative concentration of oleic acid and reducing the concentration of linolenic and linoleic acid in oil seeds (Sohrabi, Ross, Martin, & Barker).Several studies have assessed oil stability in altered vegetable oil like in high oleic sunflower oil, oleic rapeseed oil and oleic soybean (Niu; Khazaei, Hwang).Olive oil can as well be used for food frying purposes. Olive oil is extracted from virgin olive trees and it is refined in such a manner that does not lead to any modification in the original glyceride structure. Because of the low content of saturated fatty acids, olive oil is recommended for the purpose of frying food from the medical perspective.
During frying,fats and vegetable oils are subjected to high temperatures where a number of chemical reactions take place. When oxygen is available, temperatures are high and food moisture, the vegetable oils experience three kinds of chemical reactions namely, thermal modification, hydrolysis and oxidation. Hydrolysis is one of the main chemical reactions that take place as one prepares food due to the availability of moisture originating from the food been cooked and fairly high temperature in use. Hydrolysis of ester bonds available in lipids leads to the production of free fatty acids, glycerols,mono- acylglycerols as well as diacylglycerols (Khazaei).
Lipid oxidation is a significant source of food spoilage.Oxidation, which gains momentum due to high temperature used during the frying process, generates rancid flavour and decreases the organoleptic features of the fried food. (Zamora & Hidalgo). Hydroperoxides are the main original reaction outcome of lipid oxidation.Unfortunately,they are unstable and breaks down spontaneously leading to the formation of other compounds like ketones, acids, hydrocarbons, aldehydes among other products (Goddard, McClements, & Decker).
At high temperatures during the process of frying,results to triacylglycerols in vegetable oils to take part in chemical decomposition reactions like polymerization ,isomerization and cyclisation (Runas & Malmstadt).Polymeric as well as dimeric glycerides acids by oxidative and the thermal combination of free radicals (Alver, Baştürk, Kılıç, & Karataş).The proportion of saturated fatty acids to polyunsaturated is perceived to be the main factor influencing oxidation of the vegetable oil.The availability of the high quantity of unsaturated vegetable oils accelerates the susceptibility of vegetable oils to oxidation(Schaich).Vegetable oil resistance to chemical oxidation depends on solely antioxidant content as well as the composition of fatty acids of the vegetable oil.
Deep frying of vegetable oils results in hydrolysis, oxidation as well as polymerization of the oil. Hydrolysis increases the quantity of free fatty acids, glycerols, mono- and diacylglycerols in vegetable oils. Oxidation occurs at a faster rate as compared to hydrolysis during deep frying. Oxidation yields hydroperoxides as well as less volatile compounds such as short-chain alkenes and alkanes, carboxylic acids, ketones and aldehydes.
Prevention of Vegetable Oil from Deterioration
The oxidation process can be restricted by antioxidants naturally available in vegetable oils or added so that they can enhance the stability of the vegetable oils(Hwang). Tocotrienols and tocopherols are the main antioxidant available in vegetable oils. Some non-natural are also made available to increase the stability of vegetable oils. Studies show that in addition to tocotrienols or tocopherols to improve oil stability in the thermal environment have demonstrated that antioxidant effect of these compounds, of tocotrienols and tocopherols, depends solely on fatty acids structures and the composition of tocopherols in vegetable oil.
In addition high concentration of tocopherols not only does it act as pro-oxidant but also reduces the stability of fats or vegetable oils. Preventive antioxidants slow down the speed of chain initiation. Important suppressors are mainly metal deactivators that catalyse chain initiation. Metal deactivators can be used to stabilise vegetable oils and lipids containing foodstuffs like tartaric acid, phospholipids, phosphoric and citric (Alver, Baştürk, Kılıç, & Karataş).
An oxidant is a stable molecule that is in a position of donating to an electron uncontrolled free radical as well as neutralises it, thus mitigating the extent of damage (Afshari & Sayyed-Alangi). Antioxidants inhibitor delays cellular destruction by their free radical scavenging characteristics. The low-molecular-weight property of antioxidants enables them to interact safely with free radicals as well as stop chain reaction prior to the damage of vital molecules. Examples of antioxidants include uric acid, ubiquinol and glutathione, which are formed during the usual metabolism in the body (Hwang). Other minor antioxidants are contained in the diet. Although there are a number of enzymes systems within the human body that search for free radicals, the main micronutrients antioxidants are vitamins such as vitamin C , B-carotene and vitamin E. The human body is incapable of producing these vital micronutrients, hence the must be provided in the diet.
Antioxidant serves as free radical searchers, electron donor, a hydrogen donor, oxygen quencher, synergist, enzyme inhibitor as well as metal-chelating agents. At the same time, enzymatic and non-enzymatic antioxidants occur in both extracellular and intracellular environment for detoxification purpose (Sohrabi, Ross, Martin, & Barker).Antioxidants work through two main mechanisms (Purba).Firstly, a chain-breaking mechanism in which primary antioxidants supply an electron to a free radical available in the system. The second defence mechanism entails removal of explosive nitrogen initiators or secondary antioxidants (Alver, Baştürk, Kılıç, & Karataş). Antioxidants may subject their impact on biological systems through diverse mechanisms such as metal ion chelation, gene expression control, co-antioxidants or by co-antioxidants.
Furthermore, the level of antioxidant action varies. Antioxidants in the defence act at diverse levels such as radical scavenging, preventative, repair or adaption(Hwang). The preventive antioxidants are the first line defence and they work by suppressing the production of free radicals. To suppress reactions, antioxidants minimises the production of hydrogen peroxide and hydroperoxides beforehand to water and alcohol respectively, without forming any free radical and some sequester metal ions. (Niu). Peroxidase, glutathione-s-transferase and glutathione peroxidase break down lipid hydroperoxides to their respective constituents.
Radical scavenging is the second line of defence. They scavenge active radicals to repress chain initiation or decompose chain propagation activities. Examples of radical-scavenging antioxidants include uric acid, vitamin C, albumin, thiols, bilirubin and vitamin E which are considered as the most effective lipophilic antioxidant. The third line of defence contains repair antioxidants (Purba). Proteolytic enzymes, peptidases, proteases and proteinases,available in the mitochondria and cytosol of mammalian cells, detects, degrades and ejects oxidatively altered protein and avoids the accumulation of oxidised materials.
The most effective method of preventing lipid oxidation in food substances is through the inclusion of antioxidants(Zamora & Hidalgo). They function through several mechanisms such as mitigating oxidation ingredients such as oxygen and lipids, inactivation of free radicals as well as controlling of prooxidants.Recently; interest has been drawn to naturally existing antioxidants which are assumed to be safe as they are plant and animal materials. Antioxidants are substances with the potential to retard or inhibit oxidation. Antioxidants react by interrupting the chain reaction processes.
However, if the number of free radicals is low, lipid oxidation will not occur. For a molecule to function well as an antioxidant, it must react with free radicals rather than lipids. The resultant products of the reaction process are soluble lipids and not pro-oxidants. Moreover, free radicals produced by conjugated molecules exist in many resonance structures. Alternatives to antioxidants include the elimination of oxygen, removal of sensitive substances such as substitution of polyunsaturated oils with less unsaturated oil like palm or olive oil and reducing the rate of oxidation by storing lipids in low temperatures, in dark areas or use of substances that are rich in antioxidants(Zamora & Hidalgo). They occur in vegetable oil as native additives or constituents. Antioxidants are classified into two according to the mode of their action. Primary antioxidants are chain-breaking in nature and acceptors of free radicals(Zamora & Hidalgo). Primary antioxidants represent hydrogen donors as they are in a position of scavenging lipid radicals in vegetable oil.
The stability of several vegetable oils is credited to the presence of natural antioxidants and native tocols. Tocols include four tocotrienol isomers and four tocopherol each designed chromanol ring. They are one type of phenolic compound which is widely spread in plants and plays a significant role in regulating oxidative processes in both plants as well as extracted oils (Purba). They prevent lipid oxidation in chemical systems and in food by stabilizing free radicals and hydroperoxy.
Measurement of the Quality of Frying Oil
Several methods such as p-anisidine, value free fatty acids (FFA), total polar compounds (TPC), smoke point, peroxide value (PV), colour, viscosity and sensory analysis have been employed to test the quality of vegetable oils(Purba). Peroxide value (PV) is a method employed to determine the level of hydroperoxides as the inceptive product of lipid oxidation. PV is shown as milliequivalent oxygen per kilogramme of oil or fats.Since hydroperoxide does not accumulate due to its volatility at the frying thermal conditions, therefore Peroxide value(PV) cannot be used to measure the assessment of frying vegetable oil. p-Anisidine value (p-AV) measures secondary chemical decomposition substances like aldehydes.
Free fatty acids (FFA),measures the quantity of fatty acids chemically hydrolysed against triacylglycerol backbone. Free fatty acid (FFA) uses a chemical marker to monitor the amount of frying operations. This parameter is commonly used to assess the suitability of fats and vegetable oils available for human consumption, where the value of 2% indicates the point of rejection. Total polar compounds (TPC) is a significant measure of vegetable oil and fats deterioration in the process of frying.During frying process,vegetable oils decompose,and products such as monoacylglycerol, peroxides, acids, cyclic compounds and diacylglycerol are formed.Further oxidised produced such as oligomeric triacylglycerol, monomeric and dimeric are formed(Zamora & Hidalgo). For several countries in Europe, the highest permitted Total polar compounds (TPC) is between 24-27% (Sohrabi, Ross, Martin, & Barker).
Products of Lipid Oxidation and Their Significance
Mono Hydroperoxides are the basic products of lipid oxidation. A number of hydroperoxides with structural and geometrical isomers are produced depending on the number of double bonds and their structural position. The advantage of producing hydroperoxides is that they are odourless. However, hydro peroxides are unstable and they are significant precursors for a number of both volatile as well as nonvolatile secondary products that are significant to flavour stability, chemical and physical characteristics of vegetable oils in food application, nutrition and toxicology concerns to consumers.
The second category of products as result of lipid oxidation is called secondary volatile products. The secondary products are volatile in nature. The impact of the volatility of secondary products on flavour can be positive as well as negative. For instance, large quantities of lactones available in coconut oil are assumed to be positively contributing to its unique aroma and flavour. However, nutritionists have never agreed in terms of the odour coconut produces.Crude soybean oxidation is characterised “green-beany” flavour which during bleaching, refining as well as deodorization is removed to form light coloured oil.
In conclusion, lipid oxidation takes place in three stages; namely, initiation stage where free radical are formed, propagation stage where radical chain reaction takes place and termination phase where non-radical products are formed. The speed of lipid oxidation increases with the increase in the degree of lipid unsaturation. Different vegetable oils used during the frying process are characterised by diverse fatty acids profiles. Oxidation of vegetable oils is affected by energy sources available such as heat or light,types of oxygen, a combination of fatty acids and minor components like metals, phospholipids, pigments, free fatty acids, heat oxidised components, diacylglycerols and antioxidants. Frying of vegetable oils, present serious challenges in determining to what extent vegetable oils should be cooked in order to turn phytosterols into potentially harmful phytosterol oxidation products,in order to reduce negative health effects associated with these products. Oxidation of phytosterols is influenced by many factors like light, oxygen,high temperatures, free radical initiators and metal ions. The common method of preventing lipid oxidation in food substances is through the inclusion of antioxidants. Antioxidants work via several mechanisms such as mitigating oxidation ingredients such as oxygen and lipids, inactivation of free radicals as well as controlling prooxidants.