How does foods become rancid?

Foods become rancid due to a process known as lipid oxidation or oxidative rancidity. This process involves the reaction of oxygen from the air with the fatty acids present in foods, particularly unsaturated fats. Here's a breakdown of the steps involved in this process:

1. Initiation: The process begins with the formation of free radicals, which are highly reactive molecules with unpaired electrons. These free radicals can be generated through various factors, including exposure to light, heat, or metal ions present in foods.

2. Propagation: Once free radicals are formed, they react with the fatty acids in foods to form lipid peroxides. Lipid peroxides are unstable and undergo further reactions to form a variety of secondary products, including aldehydes, ketones, and hydrocarbons. These secondary products are responsible for the off-flavors, off-odors, and unpleasant tastes associated with rancidity.

3. Termination: The chain reaction of lipid oxidation continues until the free radicals are neutralized or until antioxidants in the food matrix intervene. Antioxidants are substances that can donate electrons to free radicals, thereby preventing them from reacting with fatty acids.

Factors that accelerate the rate of lipid oxidation and rancidity include:

- Temperature: Higher temperatures increase the rate of chemical reactions, including lipid oxidation. Therefore, foods stored at high temperatures are more prone to rancidity.

- Light: Exposure to light, particularly ultraviolet (UV) light, can generate free radicals and initiate lipid oxidation. This is why foods are often packaged in opaque or light-resistant containers.

- Oxygen: The presence of oxygen is essential for lipid oxidation. Foods with a high surface area exposed to air, such as ground meats and nuts, are more susceptible to rancidity.

- Moisture: Moisture can increase the mobility of oxygen and facilitate the diffusion of reactants, promoting lipid oxidation. Foods with high moisture content are more prone to rancidity.

- Metal ions: Metal ions, such as iron and copper, can act as catalysts for lipid oxidation. Foods that come in contact with metal surfaces or are contaminated with metal ions may experience accelerated rancidity.

To prevent rancidity and extend the shelf life of foods, various methods are employed, such as refrigeration, freezing, vacuum packaging, modified atmosphere packaging, the use of antioxidants, and the addition of preservatives.