Foods high in fat develop an unpleasant odor when stored for long periods of time due to oxidation. This can be prevented by storing the food at a low temperature and using antioxidants.
Foods high in fat, such as nuts, can develop an unpleasant odor when stored for a long time. This is usually due to rancidity. Rancidity is a phenomenon in which foods develop an abnormal taste and odor during storage. When fat is exposed to air for a long time and is affected by heat, light, etc., oxidation occurs, resulting in rancidity. This rancidity is related to the structure of the components that make up the fat.
In general, fat forms a chain, and one molecule of fat is combined with one molecule of glycerin and three molecules of fatty acids. Fatty acids are composed of a chain structure that includes carbon-hydrogen and carbon-oxygen bonds, centered on carbon-carbon bonds, and all but the carbon bonded to glycerol is bonded to hydrogen. Most carbon-carbon bonds in fatty acids are single bonds, but some are double bonds. Fatty acids without double bonds are called saturated fatty acids, and those with one or more double bonds are called unsaturated fatty acids. Omega-3 and omega-6 fatty acids are examples of unsaturated fatty acids. Oxidative rancidity occurs in lipids with unsaturated fatty acids, and the more double bonds there are, the more likely it is to occur. Glycerin has no significant effect on the rancidity of lipids.
For example, let’s say there is a fat called A that is composed of only omega-6 fatty acids. Oxidation occurs on the carbon atoms in the omega-6 fatty acid chain of fat A, leading to rancidity. Radical molecules play an important role in this process. Most molecules have an even number of electrons, but they can sometimes change to molecules with an odd number of electrons due to the influence of external energy. This changed molecule is called a radical molecule. In general, radical molecules are high-energy and unstable, so they easily react with surrounding molecules. Through this reaction process, they change into non-radical molecules that are low in energy and stable.
When the carbon next to the double bond of A-fat is affected by heat or light, the A-fat molecule changes into an allyl radical, which is energetic and unstable. An allyl radical combines with oxygen to form a peroxy radical. The peroxy radical reacts with another nearby omega-6 fatty acid chain to form a new allyl radical, while the peroxy radical itself becomes a hydroperoxide, a non-radical molecule. The newly formed allyl radical combines with oxygen again to form a peroxy radical, and the above chain reaction is repeated. The hydroperoxides produced are broken down and transformed into compounds such as alcohols and aldehydes. These compounds are the primary cause of off-flavors.
Rancidity has a significant impact on food quality and safety. Food that has gone rancid is no longer edible and, in severe cases, can be harmful to health. Therefore, it is important to use methods that prevent food from going rancid.
One method of delaying the onset of rancidity in fats is to add antioxidants. Antioxidants give electrons to radical molecules so that they have an even number of electrons and are less likely to react with other molecules. For example, vitamin E, a natural antioxidant found in plants, stabilizes peroxy radicals and interferes with the process by which omega-6 fatty acid chains are converted to allyl radicals. There are many other ways to inhibit the action of factors that influence the progression of rancidity. For example, storing foods at low temperatures and minimizing exposure to light and air are effective. In addition, the use of special packaging techniques such as nitrogen packaging can slow down rancidity.
Thus, fat rancidity is an important issue in food preservation and quality maintenance, and efforts should be made to prevent it through various methods. For foods stored for long periods of time, more attention should be paid to preventing rancidity in order to maintain freshness and provide safe products for consumers.
