Advanced Food Chemistry Lipids PDF

Advanced Food Chemistry Lipids Outline Definition Functions of lipids Classifications Lipids classes Chemistry of frying oil Lipids oxidation Definition Definition No exact definition of lipids exists. ‘‘a wide variety of natural products including fatty acids and thei...

Advanced Food Chemistry Lipids Outline Definition Functions of lipids Classifications Lipids classes Chemistry of frying oil Lipids oxidation Definition Definition No exact definition of lipids exists. ‘‘a wide variety of natural products including fatty acids and their derivatives, steroids, terpenes, carotenoids, and bile acids, which have in common a ready solubility in organic solvents such as diethyl ether, hexane, benzene, chloroform, or methanol.’’ Definition ‘‘those substances which are (a) insoluble in water; (b) soluble in organic solvents such as chloroform, ether or benzene; (c) contain long-chain hydrocarbon groups in their molecules; and (d) are present in or derived from living organisms.’’ Definition ‘‘a chemically heterogeneous group of substances, having in common the property of insolubility in water, but solubility in nonpolar solvents such as chloroform, hydrocarbons or alcohols.’’ The traditional definition of total fat of foods used by the U.S. Food and Drug Administration (FDA) has been the ‘‘sum of the components with lipid characteristics that are extracted by Association of Official Analytical Chemists (AOAC) methods or by reliable and appropriate procedures.’’ Functions of lipids Functions of lipids Act as building blocks in the formation of biological membranes which surround cells and subcellular particles. Such lipids occur in all foods, but their content is often less than 2%. Have important properties that affect food processing such as their melting behavior and the pleasant creamy or oily taste.  Functions of lipids Fats also serve as solvents for certain taste substances and numerous odor substances. In addition, foods can be prepared by deep frying, i. e. by dipping the food into fat or oil heated to a relatively high temperature. The lipid class of compounds also includes some important food aroma substances or precursors which are degraded to aroma compounds. Some lipid compounds are food emulsifiers, while others are important as fat- or oil soluble pigments or food colorants. Classification Lipids definition Classification of lipid structures is possible based on: – physical properties at room temperature (oils are liquid and fats are solid) – their polarity (polar and neutral lipids) Neutral lipids include fatty acids, alcohols, glycerides, and sterols polar lipids include glycerophospholipids and glyceroglycolipids. – their essentiality for humans (essential and nonessential fatty acids) – their structure (simple or complex). Neutral lipids include Lipids classification Based on structure, lipids can be classified as: – Derived – Simple – Complex Derived lipids The derived lipids include fatty acids and alcohols, which are the building blocks for the simple and complex lipids. Simple lipids Simple lipids, composed of fatty acids and alcohol components, include acylglycerols, ether acylglycerols, sterols, and their esters and wax esters. In general terms, simple lipids can be hydrolyzed to two different components, usually an alcohol and an acid. Complex lipids Complex lipids include glycerophospholipids (phospholipids), glyceroglycolipids (glycolipids), and sphingolipids. These structures yield three or more different compounds on hydrolysis. Lipid classes Lipids classes Fatty acids Acylglycerol Serols and sterol esters Waxes Phosphoglycerides (PHOSPHOLIPIDS) Glyceroglycolipids (glycosylglycolipids) Sphingolipids Fat soluble vitamins Hydrocarbons Fatty acids Saturated Unsaturated Acetylenic Fatty Acids Trans fatty acids Branched fatty acids Cyclic fatty acids Hydroxy and Epoxy Fatty Acids Furanoid fatty acids Fatty acids Fatty acids are usually denoted in the literature by a “shorthand description”, e. g. 18:2. for linoleic acid. – Such an abbreviation shows the number of carbon atoms in the acid chain and the number, positions and configurations of the double bonds. – All bonds are considered to be cis; whenever trans-bonds are present, an additional “tr” is shown. Saturated fatty acids Unbranched, straight-chain molecules with an even number of carbon atoms are dominant among the saturated fatty acids (Table 3.6). Saturated fatty acids: Nomenclature Name of Name of No. of carbon saturated fatty saturated atoms acids hydrocarbons Alkanoic Alkane Methanoic Methane 1 Ethanoic Ethane 2 Propanoic Propane 3 Butanoic Butane 4 Pentanoic Pentane 5 Hexanoic Hexane 6 Heptanoic Heptane 7 Octanoic Octane 8 Nonanoic Nonane 9 Decanoic Decane 10 Saturated fatty acids: Nomenclature Name of Name of No. of carbon saturated fatty saturated atoms acids hydrocarbons Undecananoic Undecane 11 Dodecanoic Dodecane 12 Tridecanoic Tridecane 13 Tetradecanoic Tetradecane 14 Pentadecanoic Pentadecane 15 Hexadecanoic Hexadecane 16 Heptadecanoic Heptadecane 17 Octadecanoic Octadecane 18 Nanodecanoic Nanodecane 19 Icosanoic Icosane 20 Unsaturated fatty acids By far, the most common monounsaturated fatty acid is oleic acid (18:1n-9). More than 100 monounsaturated fatty acids have been identified in nature. Unsaturated fatty acids: Nomenclature (general rules) 1 double bond Alkenoic Alkanoic 2 double bond Alkadienoic Alkanoic 3 double bond Alkatrienoic 4 double bond Alkatetraenoic 5 double bond Alkapentaenoic 6 double bond Alkahexaenoic Common unsaturated fatty acids Acetylenic Fatty Acids A number of different fatty acids have been identified having triple bonds. The nomenclature is similar to double bonds, except that the - ane ending of the parent alkane is replaced with –ynoic acid, - diynoic acid, etc. Acetylenic Fatty Acids Shorthand nomenclature uses a lowercase ‘‘a’’ to represent the acetylenic bond; 9c,12a-18:2 is an octadecynoic acid with a double bond in position 9 and the triple bond in position 12. Trans fatty acids Trans fatty acids include any unsaturated fatty acid that contains double-bond geometry in the (trans) configuration. Nomenclature differs from normal cis fatty acids only in the configuration of the double bonds. Trans fatty acids The three main origins of trans fatty acids in our diet are bacteria, deodorized oils, and partially hydrogenated oils. The preponderance of trans fatty acids in our diets is derived from the hydrogenation process. Trans fatty acids The action of bacteria in the anaerobic rumen results in biohydrogenation of fatty acids and results in trans fatty acid formation in dairy fats (2%–6% of total fatty acids). Branched fatty acids The fatty acids can be named according to rules for branching in hydrocarbons. Besides standard nomenclature, several common terms have been retained, including iso-, with a methyl branch on the penultimate (ῳ2) carbon, and anteiso, with a methyl branch on the antepenultimate (ῳ 3) carbon. Cyclic fatty acids Many fatty acids that exist in nature contain cyclic carbon rings. Ring structures contain either three (cyclopropyl and cyclopropenyl), five (cyclopentenyl), or six (cyclohexenyl) carbon atoms and may be saturated or unsaturated. Cyclic fatty acid structures resulting from heating the vegetable oils have been identified Hydroxy and epoxy fatty acids Furanoid fatty acids Some fatty acids contain an unsaturated oxolane heterocyclic group. There are more commonly called furanoid fatty acids because a furan structure (diunsaturated oxolane) is present in the molecule. 1  Acylglycerol Acylglycerols are the predominant constituent in oils and fats of commercial importance. Glycerol can be esterified with one, two, or three fatty acids, and the individual fatty acids can be located on different carbons of glycerol. The terms monoacylglycerol, diacylglycerol, and TAG are preferred for these compounds over the older and confusing names mono-, di-, and triglycerides Glyceride structure + + 3 H2O Ester bond Structure Common name Scientific name Glycerol + 1 fatty acid = Monoglyceride = Monacylglycerol Acylglycerols sn1, sn2, and sn3 designations are used for the top (C1), middle (C2), and bottom (C3) OH groups. Sterols and sterol esters ‘‘Any hydroxylated steroid that retains some or all of the carbon atoms of squalene in its side chain and partitions nearly completely into the ether layer when it is shaken with equal volumes of ether and water’’ Steroid Squalene Sterols and sterol esters The sterols may be derived from plant (phytosterols) or animal (zoosterols) sources. The predominant zoosterol is cholesterol. Although a few phytosterols predominate, the sterol composition of plants can be very complex. For example, as many as 65 different sterols have been identified in corn Waxes Waxes (commonly called wax esters) are esters of fatty acids and long-chain alcohols. Waxes are found in animal, insect, and plant secretions as protective coatings. Waxes of importance in foods as additives include beeswax, carnauba wax, and candelilla wax. Phosphoglycerides (phospholipids) Phosphoglycerides (PLs) are composed of glycerol, fatty acids, phosphate, and (usually) an organic base or polyhydroxy compound. The phosphate is almost always linked to the sn-3 position of glycerol molecule. Phosphoglycerides (phospholipids) Sphingolipids The glycosphingolipids are a class of lipids containing a long-chain base, fatty acids, and various other compounds, such as phosphate and monosaccharides. The base is commonly sphingosine, although more than 50 bases have been identified. Fat soluble vitamins Vit. A Vit. D Vit. E Vit. K Hydrocarbons The hydrocarbons include normal, branched, saturated, and unsaturated compounds of varying chain lengths. Chemistry of frying oil Oil frying chemistry Deep-fat frying imparts desired sensory characteristics of fried food flavor, golden brown color, and crisp texture in foods. During frying, at ~190 C, as oils thermally and oxidatively decompose, volatile and nonvolatile products are formed that alter functional, sensory, and nutritional qualities of oils. Oil frying chemistry During the past 30 years, scientists have reported extensively on the physical and chemical changes that occur during frying and on the wide variety of decomposition products formed in frying oils. A small amount of oxidation in frying oils is important to develop the delicious deep- fried flavor characteristic of fried foods. However, as oils breakdown further because of the processes of oxidation, hydrolysis, and polymerization, compounds are formed that can cause off flavors and may even be toxic if formed in high concentrations. Oil frying chemistry Hydrogenated oils have been commonly used for commercial deep-fat frying in the United States since the 1950s, but concerns about trans fatty acids in hydrogenated oils have encouraged the use of alternative oils. Sometimes, these alternatives are less oxidatively stable oils and subsequent problems developed because of the use of unstable oils for frying. Changes in oil during frying Changes in oil during frying Changes in oil during frying Physical changes Chemical changes Physical changes Deep-fat frying is a process of cooking and drying in hot oil with simultaneous heat and mass transfer. As heat is transferred from the oil to the food, water is evaporated from the food and oil is absorbed by the food. Physical changes Physical changes in the oil include increases in color, foaming, and viscosity. Methods exist to measure these changes; however, qualitative changes can also be determined subjectively by visual inspection. Although these practices are not recommended, many small-scale oil users, such as restaurants, discard frying oils when frying causes excessive foaming of oil or when the oil color darkens. Chemical changes During deep-fat frying the deteriorative chemical processes of hydrolysis, oxidation, and polymerization occur. Hydrolysis As food is placed in oil at frying temperatures, air and water initiate a series of interrelated reactions. Water and steam hydrolyze triglycerides, which produce mono- and diglycerides, and eventually free fatty acids and glycerol. Hydrolysis Glycerol partially evaporates because it volatilizes above 1508C, and the reaction equilibrium is shifted in favor of other hydrolysis products. The extent of hydrolysis depends on factors such as oil temperature, interface area between the oil and the aqueous phases, and amount of water and steam because water hydrolyzes oil more quickly than steam. Free fatty acids and low molecular weight acidic products arising from fat oxidation enhance the hydrolysis in the presence of steam during frying. Hydrolysis products, like all oil degradation products, decrease the stability of frying oils and can be used to measure oil fry life, for example, free fatty acids. Oxidation Oxygen is present in the fresh frying oil and more oxygen is added into the frying oil at the oil surface and by addition of food. Oxygen activates a series of reactions involving formation of many compounds including free radicals, hydroperoxides, aldehydes, ketones, and conjugated dienoic acids. Oxidation The chemical reactions that occur during the oxidation process contribute to the formation of both volatile and nonvolatile decomposition products. The oxidation mechanism in frying oils is similar to autoxidation at 25 C; however, the unstable primary oxidation products—hydroperoxides— decompose rapidly at 190 C into secondary oxidation products such as aldehydes and ketones. Oxidation Secondary oxidation products, such as aldehydes, that are volatile, significantly contribute to the odor of the oil and flavor of the fried food (desirable and undesirable) Oils that are unstable to oxidation, such as polyunstaturated ones, form the highest amounts of unacceptable degradation products. On the other hand, oxidatively more stable oils, such as high oleic sunflower, that have little linoleic or linolenic acids, form only low amounts of oxidation products. Polymerization Polymerization of frying oil results in the formation of compounds with high molecular weight and polarity. Polymers can form from free radicals or triglycerides. Cyclic fatty acids can form within one fatty acid. Dimeric fatty acids can form between two fatty acids, either within or between triglycerides. Polymers with high molecular weight are obtained as these molecules continue to cross- link. As polymerized products increase in the frying oil, viscosity of the oil also increases, the color of the oil darkens and oil absorption by food also increases. Factors affecting oil decomposition Lipid oxidation Lipid oxidation Lipid oxidation causes: – nutritional losses – produces undesirable flavor, color, and toxic compounds – makes foods less acceptable or unacceptable to consumers. Lipid oxidation can occur by either diradical triplet oxygen or nonradical singlet oxygen. Chemistry of triplet and singlet oxygen The most abundant and stable oxygen is triplet oxygen which we breathe now. The differences in the chemical properties of triplet and singlet oxygen are best explained by their molecular orbitals (Figures 11.1 and 11.2). Chemistry of triplet and singlet oxygen The spin multiplicity used to define spin states of molecules is defined as 2S +1, where S is the total spin quantum number. One spin is designated as +1/2 or -1/2. For triplet oxygen – The total spin quantum number (S) of triplet oxygen is ½ + ½= 1. – Triplet state oxygen has a spin multiplicity of 3, which can show three distinctive energy levels under magnetic field. – Triplet oxygen is paramagnetic with diradical properties (Figure 11.1). – Triplet oxygen reacts readily with other radical compounds in foods. – However, most food compounds are nonradical and in the singlet state Chemistry of triplet and singlet oxygen For singlet oxygen – The molecular orbital of singlet oxygen differs from that of triplet oxygen where electrons in the bi-antibonding orbital are paired (Figure 11.2). – The total spin quantum number (S) is -1/2 +1/2 =0 and the multiplicity of the state is 1 under magnetic field. – Singlet oxygen is a highly energetic molecule. – Singlet oxygen is not a radical compound and reacts with nonradical, singlet state, and electron- rich compounds containing double bonds. Chemistry of triplet and singlet oxygen The oxidation rate of lipid is dependent on: Temperature the presence of inhibitors or catalysts the nature of the reaction environment the nature of the compounds Temperature has little effect on singlet oxygen oxidation but has a significant effect on triplet oxygen oxidation, which requires high activation energy. Polyunsaturated fatty acids are more susceptible to radical-initiated triplet oxygen oxidation than monounsaturated fatty acids. Reaction of triplet oxygen with fatty acids The mechanism of triplet oxygen oxidation with linoleic acid is shown in Figure 11.3. The triplet oxygen oxidation has three steps: initiation, propagation, and termination. Initiation is a step for the formation of free alkyl radicals. Heat, light, metals, and reactive oxygen species facilitate the radical formation of food components. Reaction of triplet oxygen with fatty acids Decomposition of hydroperoxide The primary oxidation products are lipid hydroperoxides, which are relatively stable at room temperature and in the absence of metals. At high temperature or in the presence of metals, hydroperoxides are readily decomposed to alkoxy radicals and then form aldehydes, ketones, acids esters, alcohols, and short-chain hydrocarbons. Decomposition of hydroperoxide Singlet oxygen oxidation Formation of singlet oxygen Singlet oxygen can be formed: –Chemically –Enzymatically –photochemically as shown in Figure 11.6. The important mechanism for the formation of singlet oxygen is by photosensitization. Photosensitization Photosensitizers such as chlorophyll, pheophytins, porphyrins, riboflavin, myoglobin, and synthetic colorants in foods can absorb energy from light and transfer it to triplet oxygen to form singlet oxygen. Photosensitization The chemical mechanism for the formation of singlet oxygen in the presence of sensitizer, light, and triplet oxygen in foods is shown in Figure 11.7. Photosensitization Photosensitization The photosensitizer absorbs light energy rapidly and becomes an unstable, excited, and singlet state molecule (1Sen*). The excited singlet sensitizer loses its energy by: – internal conversion – light emission – Intersystem crossing Internal conversion Internal conversion involves the transformation from high-energy to low energy state by releasing energy as heat. Light emission Emission of fluorescence converts the excited singlet sensitizer (1Sen*) to ground state singlet sensitizer (1Sen). Intersystem crossing The excited singlet sensitizer may also undergo an intersystem crossing to become an excited triplet state molecule (3Sen*). – Intersystem crossing (ISC) is a radiation less process involving a transition between the two electronic states with different states spin multiplicity. Intersystem crossing Light emission The emission of phosphorescence converts 3Sen* to 1Sen. Photosensitization The lifetime of the 3Sen* is greater than 1Sen*. The 3Sen* reacts with 3O2 to form 1O2 and 1Sen. The sensitizer returns to ground state (1Sen) and may begin the cycle again to generate singlet oxygen. Sensitizers may generate 103–105 molecules of singlet oxygen before becoming inactive Type I and type II pathways Once 3Sen* is formed, there are two major pathways: Type I and Type II. Type I pathways The excited triplet sensitizer (3Sen*) may react directly with a compound (RH) such as linoleic acid or phenol compounds by donating and accepting hydrogen or electron and producing free radicals or free radical ions. The 3Sen* can also react with 3O2 to form superoxide anion by electron transfer to triplet oxygen. – Less than 1% of the reaction of triplet sensitizer and triplet oxygen produces superoxide anion. The rate of the Type I pathway is mostly dependent on the type and concentration of sensitizers and substrate compound. Type II pathways The excited triplet sensitizer (3Sen*) may react with triplet oxygen to form singlet oxygen and singlet sensitizer. More than 99% of the reaction between triplet sensitizer and triplet oxygen produces singlet oxygen. The rate of Type II pathway is mostly dependent on the solubility and concentration of oxygen in the food system. Antioxidants Antioxidants In foods containing lipids, antioxidants delay the onset of oxidation or slow the rate at which it proceeds. These substances can occur as natural constituents of foods, but they can also be intentionally 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 shelf life. Antioxidants Antioxidants for use in food processing must be: – Inexpensive – Nontoxic – effective at low concentrations – Stable – capable of surviving processing (carry-through effect) – color, flavor, and odor must be minimal. The choice of which antioxidant to use depends on product compatibility and regulatory guidelines. Classification Antioxidants can be classified by mechanism of action as: – primary antioxidants – secondary antioxidants – multiple-function antioxidants  Primary antioxidants Primary, type 1, or chain-breaking antioxidants are free radical acceptors that delay or inhibit the initiation step or interrupt the propagation step of autoxidation. Primary antioxidants Primary antioxidants react with lipid and peroxy radicals and convert them to more stable, nonradical products. Primary antioxidants donate hydrogen atoms to the lipid radicals and produce lipid derivatives and antioxidant radicals (A.) that are more stable and less readily available to further promote autoxidation. Primary antioxidants The antioxidant radical produced by hydrogen donation has a very low reactivity with lipids. The antioxidant radical is stabilized by: – delocalization of the unpaired electron around a phenol ring to form stable resonance hybrids. – Antioxidant radicals are capable of participating in termination reactions with peroxy (Equation 15.10), oxy (Equation 15.11), and other antioxidant radicals (Equation 15.12). Primary antioxidants Primary antioxidants are most effective if they are added during the induction and initiation stages of oxidation when the cyclical propagation steps have not occurred. Addition of antioxidants to fats that already contain substantial amounts of peroxides will quickly result in loss of antioxidant function. Primary antioxidants are mono- or polyhydroxy phenols with various ring substitutions. Primary antioxidants The most commonly used primary antioxidants in foods are synthetic compounds. Examples of important primary phenolic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tertiary butylhydroquinone (TBHQ). Secondary antioxidants Secondary, preventive, or type 2, antioxidants act through numerous possible mechanisms. These antioxidants slow the rate of oxidation by several different actions, but they do not convert free radicals to more stable products. Secondary antioxidants These antioxidants are often referred to as synergists because they promote the antioxidant activity of type 1 antioxidants. Types – Chelators – Oxygens scavengers and antioxidants – Singlet oxygen quenchers Chelators Several heavy metals with two or more valence states (Fe, Cu, Mn, Cr, Ni, V, Zn, A1) promote oxidation by acting as catalysts of free radical reactions. Citric acid, phosphoric acid, and ethylenediaminetetraacetic acid (EDTA) can chelate metals are considered as chelating agents Oxygens scavengers and antioxidants Ascorbic acid, ascorbyl palmitate, erythorbic acid, sodium erythorbate, and sulfites prevent oxidation by scavenging oxygen and acting as reductants. Oxygen scavenging is useful in products with headspace or dissolved oxygen. Singlet oxygen quenchers Singlet oxygen quenchers deplete singlet oxygen of its excess energy and dissipate the energy in the form of heat. Carotenoids, including b-carotene, lycopene, and lutein, are active singlet oxygen quenchers at low oxygen partial pressure. Synthetic antioxidants Synthetic antioxidants are intentionally added to foods to inhibit lipid oxidation. Synthetic antioxidants approved for use in food include BHA, BHT, PG and TBHQ. The synthesis of novel antioxidants for food use is limited by: – Rising costs of research and development, costs associated with safety assessment. – the time required to obtain regulatory approval of additives. – growing consumer preference for natural food additives, have led industry to emphasize natural materials as a source Synthetic antioxidants The characteristics of a product ultimately determine the selection of the phenolic antioxidant. – BHA and BHT are fairly heat stable and are used in heat processed foods. – PG decomposes at 148 C and is inappropriate for high temperature processing. – Therefore, heat-stable TBHQ is useful in frying applications. – BHA and BHT are strongly lipophilic and are used extensively in oil-in-water emulsions. – BHA and BHT are also typically used together in mixtures, acting synergistically.  Synthetic antioxidants Phenolic antioxidants are effective at low concentration and are often used at levels