Oxidation Of Fats - BCM 225 Note 2 PDF

**BCM 225 NOTE 2** **Oxidation of Fats** The oxidation of fats, particularly unsaturated fatty acids, is one of the main causes of deterioration of food along with the action of microorganisms, is a crucial biochemical process that affects food quality, nutritional value, and metabolic functions i...

**BCM 225 NOTE 2** **Oxidation of Fats** The oxidation of fats, particularly unsaturated fatty acids, is one of the main causes of deterioration of food along with the action of microorganisms, is a crucial biochemical process that affects food quality, nutritional value, and metabolic functions in living organisms which results in alterations of aroma and flavour, colour, loss of certain nutrients and the formation of potentially harmful substances, which leads to a reduction in the shelf life of the food and has significant implications on the food quality and human health. Fats oxidations is a chemical process in which fats (lipids) react with oxygen, leading to the formation of various byproducts. This process is important in biological systems, food science, and industrial applications. Adequate knowledge on the mechanisms behind lipid oxidation and its effects on metabolism is essential for developing strategies to enhance food preservation and mitigate health risks associated with oxidized lipids. Continued research in this area will help elucidate further insights into lipid chemistry and its biological significance. **Mechanisms of Fat Oxidation** Lipid oxidation is a major contributor to food spoilage. This process involves the gradual oxidation of unsaturated fatty acids in fats when exposed to oxygen, light, or metal ions. It encompasses three primary pathways: autoxidation, photooxidation, and enzymatic oxidation. 1. **Autoxidation:** Autoxidation is a free-radical chain reaction that occurs as a primary interaction between unsaturated fatty acids and oxygen. During the **initiation phase**, oil molecules generate free radicals under the influence of light, heat, or metal catalysts (Figure 1A). This is followed by the **propagation and termination phases** of the free-radical chain reaction (Figures 1B, C). The reaction produces both free-radical and non-free-radical compounds, but the chain reaction terminates when free radicals react with one another to form stable, non-free-radical compounds (Figures 1D--F). As lipid autoxidation progresses, it eventually reaches a point where rancidity develops. This process can also lead to the formation of aromatic compounds, particularly carbonyls, which play a significant role in creating the characteristic flavors and aromas of meat. www.frontiersin.org **Figure 1**. The mechanism of lipid auto-oxidation in food. **(A)** is the initiation period, **(B--C)** are the propagation period and **(D--F)** are the termination period of lipid oxidation. In∙: the free radicals; LH: the unsaturated fatty acid molecules; LOO∙: the lipid peroxyl radical; L∙: the lipid free radicals; O~2~: oxygen; LOOH: the lipid hydroperoxides; LOOL, LL: the lipid polymers. **2. Enzymatic Oxidation** Enzymatic oxidation involves specific enzymes such as lipoxygenases that catalyze the oxidation of fatty acids. This process can lead to the formation of hydroperoxides and other oxidized products that contribute to flavor changes and nutritional degradation. In addition to the non-enzymatic methods stated, an enzyme-mediated mechanism initiates lipid oxidation. The generation of hydroperoxides is the primary distinction between enzyme-catalyzed lipid oxidation and free radical initiation, just as it is in photo-oxidation. The primary enzyme involved in enzymatic oxidation is lipoxygenase. It should be emphasized that the amount of lipoxygenase present influences the progression of oxidation. It is widely known that enzymatic lipid oxidation has an early lag phase that is inversely related to lipoxygenase activity. Thus, enzyme concentration impacts the pace at which lipid oxidation occurs, hence high concentrations encourage oxidative activities. This enzyme has an active site that contains iron, which must be in the ferrous form for the enzyme to function. The active site of an enzyme removes a hydrogen atom from the methylene group of a polyunsaturated fatty acid, forming a conjugated diene system that reacts with molecular oxygen. The peroxy radical eliminates hydrogen from another unsaturated fatty acid. **3. Photooxidation** Photooxidation occurs when light energy facilitates the oxidation of fats. This process is particularly relevant in food products exposed to light, leading to off-flavors and nutrient loss. To attract customers, meat and meat products are usually exposed to direct light in supermarkets. This fact boosts photooxidation, which is significantly faster than autoxidation. Photo-oxidation is another way for initiating lipid oxidation. Hydroperoxides are produced during this process in the presence of sensitizers (such as myoglobin or hemoglobin) and light. As a result, photo-oxidation is an alternate route for the generation of hydroperoxides to the free radical mechanism described in the autoxidation process. The initial stage in photo-oxidation is the excitation of singlet sensitizers by absorbing light energy, which results in excited triplet sensitizers. The photo-oxidation reactions could be classified into three primary pathways: In the first pathway, stimulated triplet sensitizers (3S\*) combine with molecular oxygen (3O~2~) to form singlet oxygen (1O~2~) via a triplet-triplet annihilation mechanism. This is the most prevalent technique for producing singlet oxygen. The singlet oxygen can then react directly with high electron density double bonds of unsaturated fatty acids to produce a hydroperoxide without forming an alkyl radical. Excited sensitizers can react with triplet oxygen to generate superoxide radical anion (O~2~ −) through electron transfer. This reactive oxygen species has the ability to remove hydrogen from unsaturated fatty acids and induce lipid oxidation. Additionally, superoxide radical anion interacts with hydrogen peroxide to form hydroxyl radical and singlet oxygen (H~2~O~2~ + O~2~ − → HO + OH− + 1O~2~), which can directly react with fatty acids and induce lipid oxidation. This reaction is catalysed in the presence of metals. Finally, the excited triplet sensitizer can extract hydrogen from an unsaturated fatty acid, resulting in the formation of an alkyl radical. The alkyl radical then interacts with molecular oxygen, forming a peroxy radical capable of removing hydrogen from a nearby fatty acid, so beginning the free radical chain reaction mechanism stated earlier in the propagation phase.\ molecule, and eventually a conjugated hydroperoxy diene and alkyl radical are produced. **Implications of Lipid Oxidation** Almost all fat-containing diets have the potential for lipid oxidation, even if the unsaturated fatty acid composition is small. As a result, the risk of consuming lipid oxidation products increases in foods with high amounts of unsaturation (e.g., foods with omega-3 fatty acids), foods subjected to extensive thermal processing (e.g., fried foods), or foods high in pro-oxidants (e.g., meats). Lipid oxidation produces potentially hazardous chemicals that have been linked to inflammatory disorders, cancer, atherosclerosis, diabetes, Alzheimer\'s disease, rheumatoid arthritis, and age-related pathophysiology. These potentially hazardous compounds can enter the body via diet and generate in vivo during the breakdown of lipids. Oxidation products can be absorbed into the bloodstream and, in some situations, delivered to tissues. In all of these cases, macrophages play a critical role in disease development and progression. Lipid oxidation has significant implications for both food science and human health: **Food Quality**: Oxidative reactions can lead to undesirable flavors, aromas, and colors in food products. For instance, the oxidation of omega-3 fatty acids can produce fishy odors, while omega-6 fatty acids may yield grassy aromas. **Nutritional Value**: The degradation of essential fatty acids through oxidation reduces their availability for absorption in the body. Additionally, oxidative products can interact with proteins, potentially altering their structure and functionality. **Health Concerns**: Oxidized lipids have been linked to various health issues, including cardiovascular diseases and inflammatory conditions due to their role in generating reactive oxygen species (ROS) that can damage cellular components. **Off-flavors and Odors**: Formation of volatile compounds (e.g., aldehydes, ketones) causes rancidity. **Industrial Implications**: Reduced shelf life and quality of food and cosmetic products. **Metabolism of Fatty Acids** The oxidation of fats within the body primarily occurs through a process known as β-oxidation, which takes place in the mitochondria. **Steps in β-Oxidation** **Activation**: Fatty acids are activated by conversion into fatty acyl-CoA by acyl-CoA synthetase. This step consumes one molecule of ATP. The activation is carried out by acyl-CoA synthetase. For each molecule of fatty acid activated, one molecule of coenzyme A and one molecule of adenosine triphosphate (ATP) are used, resulting in a net utilization of the two high-energy bonds in one ATP molecule (which is then converted to adenosine monophosphate \[AMP\] rather than adenosine diphosphate \[ADP\]):  **Transport**: Long-chain acyl-CoA is transported into the mitochondria via carnitine shuttle mechanisms involving carnitine palmitoyltransferase (CPT). The fatty acyl-CoA diffuses to the inner mitochondrial membrane and combines with a carrier molecule called carnitine (an amino acid derivative made from methionine and lysine) in a reaction performed by carnitine acyltransferase. The acyl-carnitine derivative is transported to the mitochondrial matrix and transformed back into fatty acyl-CoA.\ Carnitine.png Chemical structure of the non-proteinogenic amino acid carnitine  **Role of carnitine in the transport of Acyl-CoA from cytosol to the mitochondrial matrix** **Oxidative Reactions**: Within the mitochondrial matrix, β-oxidation proceeds through a series of four reactions: Dehydrogenation: Formation of a double bond between the α and β carbons. Hydration: Addition of water across the double bond. Second Dehydrogenation: Another dehydrogenation step forms a keto group. Cleavage: The β-carbon bond is cleaved to release acetyl-CoA and a shortened acyl-CoA. Each cycle shortens the fatty acid chain by two carbon atoms while producing one molecule each of acetyl-CoA, NADH, and FADH₂ The shortened fatty Acyl-CoA is then degraded by repetitions these four stages, each of which releases one molecule of acetyl CoA. The equation for β-oxidation of palmitoyl-CoA (16 carbons) is as follows: palmitoyl-CoA.jpg **Factors Influencing Fat Oxidation** - **Unsaturation Level**: Polyunsaturated fatty acids (PUFAs) are more prone to oxidation due to their multiple double bonds. - **Oxygen Availability**: Higher oxygen levels accelerate oxidation. - **Temperature**: Heat speeds up the oxidation process. - **Light**: UV light can initiate and accelerate fat oxidation. - **Presence of Pro-oxidants**: Metals like iron and copper can catalyze oxidation. - **Antioxidants**: Natural or synthetic antioxidants (e.g., vitamin E, BHA, BHT) can inhibit oxidation. **Methods to Prevent or Control Fat Oxidation** - **Use of Antioxidants**: Adding natural (e.g., tocopherols) or synthetic antioxidants. - **Proper Storage**: Keeping fats away from heat, light, and oxygen. - **Packaging**: Using vacuum or nitrogen flushing to reduce oxygen exposure. - **Hydrogenation**: Reducing the number of double bonds to make fats more stable (though it may create trans fats). **Unsaturated fatty Acids** unsaturated fat, a fatty acid in which the hydrocarbon molecules have two carbons that share double or triple bond(s) and are therefore not completely saturated with hydrogen atoms. Due to the decreased saturation with hydrogen bonds, the structures are weaker and are, therefore, typically liquid (oil) at room temperature. Unsaturated fats are more likely found in vegetables as well as in fish. Saturated fats, in contrast, are typically found in meat products and are solid at room temperature. Unsaturated fats are an important supply of calories and, therefore, energy to the human body. In general, fats are made of carbon, hydrogen, and oxygen and are the most concentrated source of energy in food. Fats along with protein and carbohydrates are the three main nutrients present in food. Fats are categorized according to their percentage of hydrogen bonds as saturated (all hydrogen bonds) or unsaturated (not all hydrogen bonds). Eating more unsaturated fats and less saturated fat (such as butter) can help lower cholesterol and heart-related health risks by decreasing the bad cholesterol (low-density lipoprotein \[LDL\]) even though the average person makes about 75 percent of cholesterol in his or her liver and only about 25 percent is obtained from the diet. The most important influence on the blood cholesterol level is the actual mix of different types of cholesterol in the diet. A diet with more unsaturated versus saturated fats is important because unsaturated fats are necessary for the body, and they also protect against illness. The American Heart Association recommends a moderate intake of all types of fats. There are two main types of the unsaturated fats: monounsaturated and polyunsaturated. Monounsaturated fats---which include olive, peanut, and canola oils---have one double bond present per molecule. They are considered the healthiest types of fats because they lower total cholesterol, bad cholesterol, and triglycerides (the amount of fat circulating in the blood). Polyunsaturated fats have more than one double bond and are more likely to be found in fish, especially salmon; soy beans; mayonnaise; soft margarine; and fish oil. They provide essential fatty acids for healthy skin and the development of body cells. Features of Unsaturated Fatty Acids 1. **Double Bonds**: - Single double bond = **Monounsaturated fatty acids (MUFA)** (e.g., oleic acid in olive oil). - Two or more double bonds = **Polyunsaturated fatty acids (PUFA)** (e.g., linoleic acid, alpha-linolenic acid). 2. **Liquid at Room Temperature**: - Unsaturated fatty acids are typically liquid at room temperature, unlike saturated fatty acids, which are solid. 3. **Cis vs. Trans Configuration**: - **Cis**: Most naturally occurring unsaturated fats have a cis configuration, causing a bend in the molecule. - **Trans**: Industrial processes like hydrogenation can create trans fats, which are more linear and mimic saturated fats in behavior. ### **Types of Unsaturated Fatty Acids** #### A. Monounsaturated Fatty Acids (MUFA): - Contain one double bond. - Examples: - Oleic acid (found in olive oil, avocados). - Palmitoleic acid (found in fish and macadamia oil). - Health Benefits: - May reduce bad cholesterol (LDL) and support heart health. #### B. Polyunsaturated Fatty Acids (PUFA): - Contain two or more double bonds. - Examples: - **Omega-3 Fatty Acids**: - Alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA). - Found in flaxseeds, walnuts, and fatty fish. - Known for anti-inflammatory properties and cardiovascular benefits. - **Omega-6 Fatty Acids**: - Linoleic acid, arachidonic acid. - Found in vegetable oils and seeds. - Important for brain function and growth but should be balanced with omega-3 intake. ### **Biological Roles** - **Cell Membrane Structure**: Unsaturated fatty acids are integral to the phospholipid bilayer, contributing to membrane fluidity. - **Energy Source**: Provide calories and are stored in adipose tissue. - **Precursor to Bioactive Compounds**: - Omega-3 and omega-6 fatty acids are precursors to eicosanoids, which regulate inflammation and immune responses. - **Brain and Nervous System**: Essential for brain development and function. ### **Sources of Unsaturated Fatty Acids** - **Monounsaturated Fats**: Olive oil, avocados, nuts (almonds, hazelnuts). - **Polyunsaturated Fats**: Fatty fish (salmon, mackerel), seeds (chia, flaxseed), vegetable oils (sunflower, soybean). ### **Health Benefits** ### **Incorporating unsaturated fatty acids into the diet has been linked to numerous health benefits:** ### **Cholesterol Regulation: Unsaturated fats can help lower levels of low-density lipoprotein (LDL) cholesterol while increasing high-density lipoprotein (HDL) cholesterol, contributing to cardiovascular health.** ### **Anti-inflammatory Properties: These fats may reduce inflammation, benefiting individuals with chronic conditions such as arthritis or autoimmune diseases.** ### **Nutrient Absorption: Unsaturated fats enhance the absorption of fat-soluble vitamins (A, D, E, and K).**