6.3 Lipid oxidation and antioxidants

Lipid oxidation can turn your favorite foods rancid, ruining their taste and smell. It's a chain reaction triggered by free radicals, causing unsaturated fats to break down into stinky compounds. But don't worry, there's hope!

Enter antioxidants, the superheroes of food preservation. These compounds, both natural and synthetic, fight off free radicals and stop oxidation in its tracks. They keep your food fresh and tasty for longer, making them essential in food science and production.

Lipid Oxidation

Rancidity and Free Radicals

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• Rancidity occurs when lipids undergo oxidative deterioration resulting in off-flavors and odors
• Caused by the formation of free radicals, highly reactive molecules with an unpaired electron
• Free radicals are generated through exposure to light, heat, or metal ions (iron, copper)
• React with unsaturated fatty acids in lipids to initiate a chain reaction of lipid oxidation
• Leads to the formation of various aldehydes, ketones, and other compounds responsible for rancid flavors and odors (hexanal, pentanal)

Autooxidation Process

• Autooxidation is a spontaneous reaction between atmospheric oxygen and unsaturated fatty acids
• Occurs in three stages: initiation, propagation, and termination
• Initiation involves the formation of lipid free radicals through the abstraction of a hydrogen atom from an unsaturated fatty acid
• Propagation involves the reaction of lipid free radicals with oxygen to form peroxy radicals, which then abstract hydrogen from another unsaturated fatty acid, creating a new lipid free radical and a lipid hydroperoxide
• Termination occurs when two radicals react with each other to form non-radical products or when antioxidants donate hydrogen atoms to neutralize the radicals

Hydroperoxides and Decomposition Products

• Lipid hydroperoxides are the primary products of autooxidation
• Unstable and readily decompose into various secondary oxidation products (aldehydes, ketones, alcohols, hydrocarbons)
• Decomposition is accelerated by heat, light, and metal ions
• Secondary oxidation products contribute to the off-flavors and odors associated with rancidity
• Measuring the concentration of hydroperoxides and secondary oxidation products is used to assess the extent of lipid oxidation in foods (peroxide value, thiobarbituric acid reactive substances)

Antioxidants

Natural Antioxidants

• Compounds naturally present in foods that can delay or prevent lipid oxidation
• Act by donating hydrogen atoms to neutralize free radicals or by chelating metal ions that promote oxidation
• Examples of natural antioxidants include tocopherols (vitamin E), ascorbic acid (vitamin C), carotenoids (beta-carotene, lycopene), and phenolic compounds (flavonoids, phenolic acids)
• Tocopherols are the most widely used natural antioxidants in foods, particularly in vegetable oils and nuts
• Ascorbic acid is commonly used in meat products to prevent oxidation and maintain color stability

Synthetic Antioxidants

• Chemically synthesized compounds added to foods to prevent or delay lipid oxidation
• Commonly used synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiary butylhydroquinone (TBHQ), and propyl gallate (PG)
• These antioxidants are effective at low concentrations and are often used in combination to achieve synergistic effects
• BHA and BHT are widely used in cereals, baked goods, and processed meats, while TBHQ is commonly used in vegetable oils and fried foods
• The use of synthetic antioxidants is regulated by food safety authorities, with maximum permitted levels established for each antioxidant

Chelating Agents

• Compounds that can bind metal ions (iron, copper) and prevent them from promoting lipid oxidation
• Work by forming stable complexes with metal ions, rendering them inactive and unable to catalyze oxidation reactions
• Common chelating agents used in foods include citric acid, ethylenediaminetetraacetic acid (EDTA), and phosphates
• Citric acid is widely used in beverages, jams, and canned fruits and vegetables to prevent oxidation and maintain color stability
• EDTA is often used in salad dressings, mayonnaise, and other emulsified products to prevent oxidation and extend shelf life