Understanding Rancidity of Nutritional Lipids

by Robin Koon

Since ancient times, deterioration of lipids has been a major problem in the storage of oils and fats. Often referred to as rancidity, it is the natural process of decomposition (degradation) of fats or oils by either hydrolysis or oxidation, or both. The development of rancidity is accompanied by a marked increase in the acid value of the fat, which is tested by using two basic laboratory tests: Peroxide Value (PV) and Anisidine Value (AnV).

The process of degradation converts fatty acid esters of oils into free fatty acids, by reaction with air, moisture and/or other materials. This includes triglycerides (95 percent of all dietary fats), which are naturally occurring esters of three fatty acids and glycerol. However, there are some oils that are naturally high in free fatty acids—think conjugated linoleic acid (CLA) or saw palmetto. These lipids degrade to the point of becoming either unpalatable or unhealthy to ingest. Ingestion of rancid lipids has been linked to the development or exacerbation of many diseases, including atherosclerosis, cataracts, diarrhea, kidney disease and heart disease, and can cause cellular membrane damage, nausea, neurodegeneration and carcinogenesis.

Vegetable oils tend to be less stable and turn rancid more quickly than do animal fats. They can also become several times more rancid than animal fats, even before the human sense of smell can detect it. Unsaturated fats are more susceptible to oxidation than are saturated fats, meaning the more polyunsaturated a fat is, the faster it will go rancid. This is due to the more unstable double bonds, which allow more oxygen to react at those points. Oils generally don’t suddenly go rancid; they tend to slowly become more oxidized over time.




Causes of Rancidity
There are two basic types or causes of rancidity that cause and/or contribute to the degradation of stored edible oils: oxidative and hydrolytic. Oxidative rancidity, known as autoxidation, occurs when oxygen is absorbed from the environment. In the presence of oxygen and/or ultraviolet (UV) radiation, most lipids will break down and degrade, forming several other compounds. Oxygen is eight times more soluble in fats than it is in water; it is this exposure that is the main cause of the autoxidation process, increasing the saturation of the oil.

Hydrolytic rancidity, also called hydrolysis or enzymatic oxidation, occurs in the absence of air, but with moisture present. This normally is accomplished through enzymatic peroxidation, where enzymes found naturally in plant oils (i.e., lipoxygenase, cyclooxygenase) and animal fats (i.e., lipase) can catalyze reactions between water and oil.

Another degradation process is microbial rancidity, in which micro-organisms such as bacteria, molds and yeast use their enzymes to break down chemical structures in the oil, producing unwanted odors and flavors. Water needs to be present for microbial growth to occur.

There are other contributing factors that can significantly speed up the rancidity process, including:

Temperature: decomposition rate increases as temperature rises
Time
Light (photo oxidation): in the presence of oxygen, light promotes oxidation of unsaturated fatty acids
Water
Catalysts: trace metal ions, metalloproteins and inorganic salts