Q. My daughter has become something of a health nut in the feeding of her family. One of her latest interests is flaxseed oil. Now I know, as a biochemist, that flaxseed oil is rich in polyunsaturated fatty acids. However, as an amateur woodworker, I am familiar with the wood-finishing product raw linseed oil, also made from flax. Are the flaxseed oil sold in health food stores and the linseed oil sold in paint stores the same? If not, what is different?
A. It’s true that flaxseed oil and linseed oil are the same oil, extracted from the seeds of the flax plant, Linum usitatissimum. But I would not advise your daughter to do her grocery shopping in the hardware store.
If the seeds of the flax plant are simply squeezed to express their oil (“cold pressed”), the oil is edible as flaxseed oil. But cold pressing isn’t 100 percent efficient, so more drastic measures, such as heating and solvent extraction, are used to get the rest of the oil out. That’s the raw linseed oil. Not only is it less pure, but by the time it gets to the hardware store it contains added chemicals to speed drying.
Flaxseed oil is the darling of many “health food” advocates because its fatty acids are about 20 percent monounsaturated and 66 percent polyunsaturated. Most prominent among the latter is alpha-linolenic acid, which in the body can change into an omega-3 fatty acid, and omega-3 fatty acids are believed to protect against heart disease. But according to the American Heart Association, the extent of this change is limited, and further studies are needed before a cause-and-effect relationship can be established between alpha-linolenic acid and heart health.
A good review of flaxseed oil’s health profile can be found at the University of Maryland Medical Center’s Web site,
Q. We recently opened a bottle of extra-virgin olive oil. It was rancid, presumably because of age. There was no expiration date on the bottle. When we called customer service, we were told that according to the coding on the bottle, the oil had passed its “use by” date. They indicated that the shelf life of the oil was 17 to 20 months. What are the chemical reactions that take place in a sealed bottle of olive oil that cause it to go rancid? It seems that this would occur in other oils also.
A. All animal and vegetable fats can turn rancid, a word derived from the Latin rancidus, meaning stinky. So yes, the other oils are also at risk.
As olive oil ages, some fatty acids break off from the fat molecules and its acidity level therefore increases, negatively affecting its bouquet and flavor. Lesser grades than extra-virgin have higher acidity levels to begin with, and hence shorter lifetimes. But your olive oil didn’t die from old age alone.
Fats can go rancid in two ways: by reacting with oxygen (oxidative rancidity) or with water (hydrolytic rancidity). They are slow processes accelerated by heat and light, which generate peroxides and free radicals. These highly reactive chemicals produce compounds called aldehydes and ketones, which are the evil-tasting compounds in rancid fats. Unsaturated vegetable oils are particularly susceptible to oxidation, because oxygen attacks their so-called double bonds.
Thus, the universal recommendation is to keep cooking oils cool and away from bright light. Normal incandescent room light is no problem; it’s the ultraviolet rays in sunlight and fluorescent lamps that do the damage.
Because the oil producer can’t predict how a consumer will store the product, there can be no specification of an expiration date. But if properly stored, olive oil will stay fresh for a couple of years.
Obviously, oxidative rancidity can be inhibited by reducing the oil’s access to air, that is, by keeping the bottle tightly capped or stoppered. Hydrolytic rancidity, on the other hand, isn’t a big problem in a bottle of dry oil.
I keep a small amount of kitchen olive oil in a dark bottle, stoppered with one of those liquor-bottle pouring spouts. I refill the bottle as necessary by tapping my underground oil reserves, a three-liter can in the coolest part of the basement.
