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Hi Mike,
Every once in a while we get asked about the safety of using
walnut oil
because it may go rancid. I am attaching the answer. We are on
a quest to
get this information out to the artists. I know this is really
lengthy so
maybe it should be pared down a bit. It has been submitted to
the magazine
that last published the rancidity remark so some of this may
show up in
print. Once you have read through it, let me know if you have
any ideas
about making this subject more visible.
Diana
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 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”
CureZone www.curezone.org/foods/flaxseed_oil.asp
“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 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 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 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.”
Norman Harris
For Anglia Oils www.angliaoils.co.uk
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