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 acidsthink 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 dont 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
- Light (photo oxidation): in the presence of oxygen, light promotes oxidation of unsaturated fatty acids
- Catalysts: trace metal ions, metalloproteins and inorganic salts
Understanding the process
As mentioned above, the main cause of rancidity of lipids is the oxidative deterioration of unsaturated fatty acids via a free-radical chain mechanism, also called lipid peroxidation. It occurs in three stages or phases. The first is the initiation (induction) stage, whereby molecular oxygen combines with unsaturated fatty acids, producing hydroperoxides and peroxyl free radicals, both of which are highly reactive and unstable. The second stage is called propagation. This is when these unstable byproducts of the first stage react with other lipids, starting a continuing free radical lipid peroxidation chain reaction called autoxidation. This results in a continuing and cyclical oxidative degradation process, breaking down the lipid. The final stage, termination, is marked by the slowing or stopping of reactions, completion of making unreactive compounds (e.g. amides, alcohols, aldehyedes, hydrocarbons, ketones, etc.) or when an antioxidant is added or encountered. There are two basic types of oxidative byproducts, primary and secondary.
Primary oxidative products are fatty acids reacting with oxygen-forming peroxide compounds. The primary oxidation is normally measured with a PV test. Primary oxidation byproducts include peroxides (ROO) and hydroperoxides (ROOH)
Secondary oxidation products occur when ROOH degrade further into other substances, primarily carbonyl compounds: volatile and non-volatile aldehydes, amide, carboxylic acids, expoxides, ketones, alcohols and hydrocarbons (alkanes and alkenes). The secondary oxidation is normally measured with an AnV test.
It is worth noting the spontaneous combustion of some oils is also due to an oxidation process, albeit rapid. Many materials can undergo an oxidation process that results in the generation of heat, which eventually reaches auto-ignition temperature. Caution must be taken with materials that come in contact with these oils, such as oil-soaked rags or towels. Many fires in homes or businesses have been caused by this process. Examples of these kinds of oils are: ethyl ester fish oils or drying oils (i.e., castor oil, linseed oil, tung oil, etc.).
Hydrolysis is caused by hydrolysis of the triglycerides in the oil into their component fatty acids and glycerol. The process occurs without oxygen and in the presence of moisture over time, the rate is accelerated by temperature (increase) or a catalyst. These catalysts are usually enzymes (lipase, esterase, lipoxygenase, cyclooxygenase, etc.) or acidic in nature. Lipase in stored oil, for example, usually comes from a bacterial contamination. Lipoxygenase or cyclooxygenase typically come from plant-based oils. A list of catalyst agents is mentioned earlier in this article. These free fatty acids then undergo further secondary autoxidation. Hydrolytic rancidity is more of an issue in animal fats than for vegetable fats.
Microbial Rancidity is caused by micro-organisms (bacteria, mold, and yeast) multiplying in oils after exposure to moisture (water). Bacteria and fungi are everywhere (in water, air, equipment, on people, etc.). These micro-organisms use their enzymes (e.g. lipases) to break down the chemical structures in the oil, resulting in undesirable odors and flavors. The amount of harm done depends upon the type of micro-organism, their numbers, and the physical condition of the oil being stored.
Rancidity is most commonly detected by taste or smell, but chemical tests can also check for rancidity. There are many tests used to determine the quality of an oil; some tests are predictive and others are indicative. The two well-known or commonly performed indicative tests used to measure lipid oxidation are the PV test for primary oxidation products and the p-AnV test for secondary oxidation products.
It is generally accepted that the first compounds formed by oxidation of an oil are hydroperoxides. PV is a measurement of the quantity of peroxide oxygen present in an oil. It is defined as the amount of peroxide oxygen per 1 kg of fat or oil. Traditionally this was expressed in units of milliequivalents (meq), but is normally reported as millimole (mmol) of hydroperoxide per kg of oil (or expressed as milliequivalents of iodine per kilo of oil - 1 millimole = 2 milliequivalents). Standard methods are described by AOCS and IUPAC. PV is a useful measurement for determining actual oxidative status or quality of saturated fats; it is less useful in highly unsaturated fats. Generally speaking, oils with a PV higher than 10 are not considered acceptable.
Not all of the secondary oxidation products (aldehydes and ketones) can be measured numerically. The AnV specifically measures levels of aldehyde, which does not decompose readily, using them as a marker to determine how much peroxide material has already broken down. The test measures the level of non-volatile aldehydes, primarily the 2-alkene present in the lipid. It is the aldehydes that are mainly responsible for the rancid smell and poor taste of oil. Because PV is non-linear, the AnV is considered by many to be a better indicator of the freshness of an oil, since it shows its oxidative history.
Evaluating a fat should consider both its primary and secondary oxidation test results. This is because just performing the PV test by itself indicates only a part of the story. The peroxides in an oil are really transitory intermediates, which then decompose into secondary compounds. Consider that an oil that has a high PV but is stored for a long time in the absence of oxygen can undergo the secondary oxidative process. Over time, this will result in a reduced PV and an increased AnV. The PV of an oil can be reduced or eliminated by heating the oil, in the absence of oxygen. (This process is actually done as a normal part of the refinement process for some oils.) But, the result of this heating can cause an increase AnV. So, there is a third measurement, TOTOX, which combines both values of PV and AnV, giving a total oxidation measurement or assessment.
Many people consider the TOTOX value to be the most important evaluation in determining an oils freshness, because PV indicates its actual oxidative status and AnV its oxidative history. The combination of both values gives a good indication of the overall rancidity or quality. This value is determined by calculation: TOTOX = AnV + (2 x PV).
There are additional tests that are generally performed on oils to assess quality. The acid value test method determines the acid valuea measure of the acidity or amount of free fatty acidsin an oil and is applicable to all fatty acids and polymerized fatty acids. Low levels of free fatty acids in the oils also suggests low levels of hydrolytic and lipolytic activities in the oils.
The iodine value test measures the degree of unsaturation of an oil or fat, by determining the amount of iodine, in grams, that is taken up by 100 g of the oil. Saturated oils and fats take up no iodine, so their iodine value is zero; unsaturated oils and fats contain molecules with double or triple bonds, which are very reactive toward iodine. The more iodine is attached, the higher is the iodine value. Meaning the more reactive, less stable and more susceptible it is to oxidation and rancidification.
Some other oil/fat tests include acetyl value, AOM, cloud point, color value, ester value, FFA profile, flash point, heavy metals, hexanal value, hydroxyl value, insolubles, moisture, pesticides, refractive index, saponification value, smoke point, TBA test, titer, trans isomers and UV absorption, among others.
Rancidification can be decreased, but not completely eliminated, by storing fats and oils in a place with little or no exposure to oxygen or free radicals, at a low temperature, and away from light. Many oils are packed with nitrogen gas to help limit exposure to oxygen. Antioxidants are often added to fats and oils to retard the development of rancidity due to oxidation and hydrolysis. Antioxidants retard oxidation primarily by either hydrogen donation or electron donation.
Processors can use natural antioxidants such as carotenes, flavonoids, polyphenols, ascorbic acid and tocopherols, or select from a range of synthetic antioxidants (i.e., butylated hydroxyanisole [BHA], butylated hydroxytoluene [BHT], ethoxyquin, phosphoric acid, etc.). Most consumers prefer the use of natural antioxidants. However, these natural antioxidants tend to be shorter-lived, so many bulk producers prefer using synthetic antioxidants when a longer shelf-life is preferred. Vitamin E is among the best antioxidants for oil because it is a powerful antioxidant that is soluble in oils and is considered a natural compound.
Ultimately, rancidity is a natural process; all oils are going rancid all the time. Even the purest vegetable oil will degrade over time, mostly from exposure to air. The shelf life and quality of any oil and its development of rancidity are directly related to how it was manufactured and stored. Working with a trusted supplier and obtaining information on quality control measures, including testing results, can ensure delivering the best possible product to the end consumer.
Robin Koon, senior vice president at Best Formulations , has more than 25 years of pharmaceutical experience in clinical pharmacy as a drug chain executive overseeing operations and managed-care, and in retail mass market.