All oils, under certain circumstances, will "go rancid" at some point in time. Whether it's flax seed oil or any other kind of oil, with respect to determining its shelf life and the development of rancidity, you need to pay attention to its quality, the processes by which it was manufactured and how it is stored. Since how long any oil will last depends upon a variety of factors, we feel that brush-stroked judgments about the shelf life of particular oils cannot be truly made without taking all of its particular qualities into consideration. Following are the facts about rancidity, as we understand them here at the World's Healthiest Foods.
Rancidity defined
"Going rancid" is a pretty loose term. It generally refers to the production of undesirable odors or flavors by oils, rather than to any specific chemical process. In other words, the definition of rancidity is usually pretty practical: the oil smells and tastes bad. Yet, because there are more strict, chemical definitions of rancidity that are different from the practical definition and not always consistent with it, "going rancid" does not always equal smelling bad and therefore the latter is not always an accurate benchmark for the rancidity process.
These other definitions include "microbial rancidity", "oxidative rancidity" and "hydrolytic rancidity". "Microbial rancidity" refers to a process in which micro-organisms like bacteria use their enzymes (such as lipases) to break down chemical structures in the oil and produce undesirable odors and flavors. "Oxidative rancidity" is a second type of process in which exposure to air (and more specifically, oxygen) does the same thing, usually without involvement of enzymes. "Hydrolytic rancidity" is yet another type of rancidity, in which water accomplishes the same thing. "Oxidative rancidity" is the kind of rancidity that is most likely to damage your flax seed oil.
The chemistry of rancidity - omega 3 and 6 fatty acids
Research in this area of food science has made it very clear that how quickly an oil will go rancid is not just a question of how delicate the oil is in terms of its chemical composition, but how many other factors are present to help protect the oil's delicate chemicals. In the case of flax oil, the delicate part of its chemistry is usually regarded as the part containing its omega-3 and 6 fatty acids. These types of fat are called polyunsaturated. (In terms of their chemistry, polyunsaturated means that there is more than one carbon atom in the fat that is not fully saturated with hydrogen atoms, but instead, is double-bonded together with another carbon atom. The place where the carbon atoms are double-bonded together is a place where oxygen can interact with the fat fairly easily and cause "oxidative rancidity".)
Flax oil contains about 15 grams of alpha-linolenic acid per ounce. Alpha-linolenic acid is a polyunsaturated omega-3 fatty acid not found in a wide variety of foods. Many food scientists look upon the linolenic acid found in flax as the most delicate part of its composition that needs to be protected from oxidative rancidity.
The chemistry of rancidity - saturated fats and other nutrients
There are other types of fats found in flax oil, however, that are much less susceptible to oxidative rancidity. For example, two saturated fats (stearic acid and palmitic acid, each highly stable and containing no double bonds) are found in flax. In fact, flax oil is about 9% saturated in terms of its fat content, making it more saturated than several other cooking oils, including safflower oil and canola oil. This saturated fat component lends stability to the oil.
Flax oil - like most other oils - also contains nutrients that help protect its omega-3 fatty acids. The vitamin E and flavonoids found in this oil are important for keeping it stable.
How manufacturing processes affect rancidity
How your flax oil was produced is important to its quality. There are three ways to get flax oil from flax seeds. Seeds can be soaked in a chemical solvent, but this process damages the oil and leaves toxic residues from the chemical solvent. Air pressure (called supercritical extraction) can be used, although we haven't seen much research on this method and its relationship to flax oil quality. Finally, oil can be pressed out of the seeds using an expeller process.
While expeller pressing is the method we prefer, it does have a potential down side. Pressing the oil from the seed requires a lot of force, and a good amount of heat can be generated during the expelling process. (By "a good amount," we mean that temperatures can reach between 150-200 degrees Fahrenheit, a degree of heat that can definitely damage the omega 3 fats).
The words "cold pressed" on a flax oil label may be very misleading. Since no external source of heat was used in the expelling process, companies are allowed to say "cold pressed" even though high temperatures were reached during the extraction process.
Yet, some companies take steps to prevent this heat build-up. Your best bet would be to contact the manufacturer of the flax seed oil product of interest and inquire about the methods that this manufacturer used in the oil's production.
Practical tips
Even if a manufacturer used great care in the production of the oil, it will not be safe for you to use unless you take the same degree of care with it. Taking care of the product means keeping it refrigerated (and always buying oil that has been refrigerated in the store), opening it only when in use, and keeping it tightly capped when not. It should always be stored in an opaque container so that it is protected from light which can cause rancidity.
Smell and taste the oil before use. If it smells like oil paint or leaves a scratchy sensation in the back of your throat it is rancid and should be discarded.
We like flax seed oil that can be used within approximately one month's period of time, despite the claims of manufacturers that the refrigerated oil will last for much longer. This usually means buying the oil in smaller quantities.
Finally, we should point out that flax capsules, or combination oil products sold in opaque, gel capsules are more stable than flax oil sold as a bulk liquid. When refrigerated, these capsules will usually remain stable for at least 6 months, and it's usually not problematic if the retailer has left them on the shelf at room temperature before you purchased them.
We feel that one of the best ways to benefit from flaxseeds, is to buy organic seeds and grind them in a coffee grinder right before use. Not only will this help to preserve the oil's quality and reduce its ability to go rancid, but by using whole ground flax seeds you will not only benefit from its oil, but from the lignins, fiber and other nutrients contained in the seed coating.
References
Obermeyer, W. R.; Musser, S. M.; Betz, J. M.; Casey, R. E.; Pohland, A. E., and Page, S. W. Chemical studies of phytoestrogens and related compounds in dietary supplements: flax and chaparral. Proc Soc Exp Biol Med. 1995 Jan; 208(1):6-12.
Przybylski, R; Zambiazi. Storage stability of genetically modified canola oils. Proceedings of the 10th International Rapeseed Congress, Canberra, Australia, 1999