As a firm believer in Walnut oil I thought I should share this. from http://www.artpurveyors.com/rancidity.html
IMPORTANT INFORMATION ON RANCIDITY AND VEGETABLE OILS.
Some authors of books regarding art materials, i.e. Mayer and Kay, have expressed opinions about rancidity and walnut oil but there is no reliable published data or analytical testing which supports their statements. As a result we believe there is a high degree of misinformation within the art community about rancidity and vegetable oils.
The actual truth of the matter is that all vegetable drying oils go rancid in the process of drying and that linseed does so much faster than walnut oil. The shelf life of most vegetable oils is about a year with that of linseed even less. If the rancidity of vegetable drying oils, used as an artists’ medium was an issue, then no one would be able to paint with linseed oil. Since both oils turn rancid and walnut oil exhibits superior stability in this regard, it is erroneous to conclude that walnut oil products are inferior to linseed oil products based on rancidity.
Every time something is written about walnut oil and rancidity without including all oils, especially linseed, in the discussion it does disservice to the art community. This only helps to continue the promotion of misleading information lacking a factual basis and prevents artists from achieving a fuller understanding of the materials they use.
For far too long artist’s have been mislead about the dangers of rancidity with walnut oil and as a result, have been deprived of its benefits of vibrancy, fluidity and resistance to cracking and yellowing.
Rancidity is a natural oxidative process common to all vegetable oils. In artists’ color it is part of the drying process by which the color turns into a tough, leathery film that makes oil painting possible.
While all vegetable oils turn rancid over time and develop a strong aroma, some do so to a greater degree and more rapidly than others. Both degree and speed of rancidity are associated with oxidation and the speed of drying. In artists’ colors, linseed oil is chief among those oils with the greatest degree of rancidity achieved over the shortest period of time.
Rancidity is determined by the fatty acid composition of the oil in question. The amount of unstable compounds such as linolenic acid and the total percentage of saturated vs unsaturated acids determine the time and degree of rancidity. Oils such as linseed containing higher amounts of linolenic acid are prone to turn rancid more rapidly than oils such as walnut.
Oil chemists, for all major producers such as Archer Daniels Midland and Cargill, etc. test vegetable oils at time of manufacture for their resistance to rancidity and utilize quantitative measures to determine stability and shelf life. These tests include Iodine Value, Peroxide Value, Free Fatty Acid Content and Oxidative Stability Index. Linseed oil consistently shows poor resistance to rancidity in these tests. Linseed oil is also known on an empirical basis to turn rancid more rapidly than Walnut or Sunflower oils.
Manufacturers of linseed oil typically include a shelf life restriction of one year after which they recommend the oil be discarded. In fact linseed oil is so unstable that by the time the artist purchases it in an art supply shop, it has already begun to turn rancid. (The characteristic aroma artists associate with linseed oil is it’s rancidity.)
To further prove the point commercially available samples obtained from an art material retailer of walnut oil and two brands of linseed oil were sent to an independent analytical research laboratory for testing. This laboratory concluded that both oils turn rancid but that walnut oil does so less rapidly than linseed oil.
Not withstanding, all these oils perform quite well for painting purposes, can be used almost indefinitely, have equivalent shelf lives when stored in collapsible metal tubes and should be evaluated by artists for meaningful attributes such as yellowing, flow and drying rate rather than rancidity which has no practical meaning for artists’ color other than to tell us the oil has begun to dry.
Quotes on Rancidity
“Rancidity is the natural oxidation chemical degradation of oils. This process converts fatty acid esters of oils into free fatty acids by reaction with air and other materials. It is the free fatty acids that have the peculiar tainted smell of spoilage such as is seen in butter and most any vegetable oil over time. Some oils are more prone to rancidity than others, but all vegetable oils easily degrade through this oxidation mechanism.”
The Nutrition Farm http://www.nutritionfarm.com/message%20board/Message_Board/flaxseed2.htm
“LNA (Alpha Linolenic Acid) is 5 times more unstable than LA (Linoleic Acid) and quickly goes rancid if exposed to light or oxygen. It is so unstable, in fact, that when it is pressed from the seeds that possess it, the pressing must be done in the total absence of light and oxygen. It must be handled this way right through the packaging stage, then quickly refrigerated or frozen”
“Oxygen is eight times more soluble in fats than in water and it is the oxidation resulting from this exposure that is the primary cause of rancidity. The more polyunsaturated a fat is, the faster it will go rancid. This may not, at first, be readily apparent because vegetable oils have to become several times more rancid than animal fats before our noses can detect it. An extreme example of rancidity is the linseed oil (flaxseed) that we use as a wood finish and a base for oil paints. In just a matter of hours the oil oxidizes into a solid polymer. This is very desirable for wood and paint, very undesirable for food.”
“Unless they have been specially treated, *unopened* cooking oils have a shelf life of about a year, depending upon the above conditions. Some specialty oils such as sesame and flax seed have even shorter usable lives. If you don’t use a great deal of it, try not to buy your fats in large containers. This way you won’t be exposing a large quantity to the air after you’ve opened it, to grow old and possibly rancid, before you can use it all up.”
Food Storage FAQ http://www.survival-center.com/foodfaq/ff10-fat.htm
“Flaxseed oil is not suitable for cooking and should be stored in an opaque, airtight container in the refrigerator or freezer. If the oil has a noticeable odor it is probably rancid and should be discarded.”
Vitamin Guide http://www.gnc.com/health_notes/Supp/Flaxseed.htm
“Description.-The U.S.P. describes linseed oil as a “yellowish, or yellow oily liquid having a slight, peculiar odor, and a bland taste. When exposed to the air, it gradually thickens, and acquires a strong odor and taste; and if spread, in a thin layer, on a glass plate, and allowed to stand in a warm place, it is gradually converted into a hard, transparent, resin-like mass (absence of non-drying oils). Specific gravity 0.930 to 0.940 at 15o C. (59o F.). It does not congeal above 20o C. (-4o F.). Soluble in about 10 parts of absolute alcohol and, in all proportions, in ether, chloroform, benzin, carbon disulphide, or oil of turpentine:-(U.S.P.). When cooled to 27o C. (-16.6o F.) linseed oil congeals to a yellowish mass. Upon exposure to the air, old oil is liable to become rancid. On account of its drying properties, facilitated by warmth, linseed oil is a most important article, being used in the making of paints and varnishes, of printer’s ink, oil cloth, etc. Its affinity for the oxygen of the air is so great that it is liable to inflame cotton waste and other fibrous materials soaked with it.”
King’s American Dispensatory http://www.ibiblio.org/herbmed/eclectic/kings/linum_oleu.html
“All oils are subject to the onset of rancidity, but the rate rancidity occurs differs from oil to oil. Of the more common vegetable oils flaxseed oil is the most susceptible. Walnut oil is also fairly susceptible but not nearly as much as flaxseed, although probably more so than other vegetable oils such as soya or sunflower oils.
In a true sense, all oils are “rancid” to some extent as the process starts as soon as the oil is extracted, but it can be minimized by limiting exposure to air, light and higher temperatures and we would recommend storage in a cool dark place.”