Mechanism of rancidity
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Mechanism of Rancidity
Introduction to Rancidity
Rancidity is a significant concern in food science, leading to undesirable flavors, odors, and reduced nutritional value. It primarily occurs through the oxidative deterioration of lipids, which are essential components in many foods. Understanding the mechanisms behind rancidity is crucial for developing strategies to prevent it and extend the shelf life of food products.
Autoxidative Rancidity: Free Radical Reactions
Autoxidative rancidity is a complex process involving numerous reaction products such as aldehydes, alcohols, alkyl formates, and hydrocarbons. This type of rancidity is initiated by the formation of lipid hydroperoxides, particularly from unsaturated fatty acids like oleic acid. These hydroperoxides decompose into free radicals, which further react to produce a variety of compounds responsible for the rancid taste and odor. The free radical chain reactions propagate through successive steps, leading to the formation of secondary oxidation products that contribute to the off-flavors in rancid foods.
Photochemical Rancidity: Role of Photosensitizers
Photochemical rancidity occurs when light exposure disrupts the stability of lipids. In vegetable oils, chlorophyll acts as a photosensitizer, while in animal fats, pigments like hemoglobin can initiate rancidity. These photosensitizers generate nascent hydrogen, which reacts with molecular oxygen to form unstable hydrogen peroxide. This peroxide then interacts with unsaturated triglycerides, forming glyceride peroxides that decompose into aldehydes, leading to rancid compounds.
Oxidative Rancidity: Atmospheric Oxygen
Oxidative rancidity is driven by the reaction of lipids with atmospheric oxygen. This process involves a series of complex reactions that produce both volatile and non-volatile compounds, even at very low concentrations. These compounds are responsible for the off-flavors and odors associated with rancid foods. Factors such as the presence of prooxidative transition metals, lipid hydroperoxides, and the physical properties of the lipid-water or lipid-air interface significantly influence the rate of lipid oxidation .
Hydrolytic Rancidity: Enzymatic Activity
Hydrolytic rancidity is caused by the enzymatic breakdown of lipids into free fatty acids and other compounds. In pearl millet, for example, the activity of triacylglycerol lipases leads to the accumulation of free fatty acids and volatile aldehydes, which are characteristic of rancidity. Mutations in specific lipase genes can reduce this enzymatic activity, thereby decreasing the rate of rancidity. Similarly, in dairy products like condensed milk, the presence of raw milk introduces lipolytic enzymes that accelerate rancidity unless inactivated by heat treatment.
Influence of Salts and Antioxidants
Certain salts, such as sodium chloride, can accelerate rancidity in muscle foods by altering the protein environment and promoting lipid oxidation. However, the addition of antioxidants and chelating agents can mitigate this effect by stabilizing the lipids and preventing oxidative reactions. The balance between pro-oxidant and antioxidant compounds is crucial in managing the oxidative stability of foods.
Conclusion
Rancidity is a multifaceted phenomenon influenced by various chemical and physical factors. Understanding the mechanisms of autoxidative, photochemical, oxidative, and hydrolytic rancidity is essential for developing effective strategies to prevent it. By controlling factors such as light exposure, enzymatic activity, and the presence of prooxidants, the food industry can enhance the shelf life and quality of food products.
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