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<pre>This may be very boring! You've been warned.


Responding to Sadoun's request, supported by Phil, Hanna, Daniela...(and
following on Gareth and Antonio's response), I'll try to throw a little
light on some of the terms used in the chemical specification of vegetable
oils. Stan's no doubt the resident edible oil specialist on this group, and
hopefully he'll assist.

Herewith my contribution on FATTY ACIDS AND ACIDITY:

Olive oil is composed mainly of triacylglycerols (triglycerides). Chemically
speaking, these are molecules derived from the natural esterification of
three fatty acid molecules with a glycerol molecule. The glycerol molecule
can simplistically be seen as an "E-shaped" molecule, with the fatty acids
in turn resembling longish hydrocarbon chains, varying (in the case of olive
oil) from about 14 to 24 carbons atoms in length. Thus the triacylglycerols
can, for our purpose, be visualised as elongated E-shaped molecules, each
with three long extensions, being the three fatty acid chains "attached to
each horizontal bar of the E".

Please note that we are dealing here with fatty acids forming part of the
triacylglycerol molecule. They are distinct from FREE FATTY ACIDS, which
we'll talk about later!

Various fatty acids are found in nature. They differ in length (number of
carbon atoms in the chain) as well as in the type of chemical bonds found
within the chain. Mostly these carbon-carbon bonds in the chain are "single"
bonds, comprising 2 electrons shared between adjacent carbon atoms. However,
in certain of the fatty acids, some of the bonds are "double bonds", where 4
electrons are shared between adjacent carbon atoms. The fatty acids that
have no double bonds in their chains are called "saturated" fatty acids (all
the carbons in their carbon chain are "saturated" by hydrogen atoms).
Examples of saturated fatty acids are Palmitic Acid (16 carbons long),
Stearic Acid (18 carbons long) and Arachidic Acid (20 carbons long). The
fatty acids that have one carbon-carbon double bond somewhere along their
length are called monounsaturated fatty acids (one carbon-carbon bond which
is not fully saturated with hydrogens), i.e. one of the bonds available at
each of 2 adjacent carbons is now used to form a double bond between
themselves instead of being used to bond externally to hydrogen atoms.
Examples of monounsaturated fatty acids are Palmitoleic Acid (16 carbons
long) and our famous Oleic Acid (18 carbons long). Oleic acid is the most
abundant fatty acid found in nature. The double bond in Oleic acid occurs in
the mid position of the molecule, between carbon 9 and carbon 10.

I don't want to make this sound too complicated, but as soon as one brings a
double bond into the picture, one must bear in mind that, unlike the single
bonds (wherein the molecular chain has complete rotational freedom of
movement in the bond axis), the double bond is a rigid bond insofar as it
does not allow rotation around its longitudinal axis. Thus, with each double
bond, one introduces what are called isomers. These have the same chemical
structure, but different stereochemistry. In other words, the shape of the
molecule differs, and so does its chemical reactivity (and effect on
health). This leads to things like "trans" fatty acids (TFA's) and "cis"
fatty acids (CFA's). Trans fatty acids are normally produced when oils are
artificially and chemically converted into margarines. They are said to
raise LDL's (the "bad" cholesterol) and lower HDL's (the "good"
cholesterol), and are thus to be avoided. Oleic acid is a cis fatty acid,
and more specifically a cis monounsaturated C18 acid. Cis means the rest of
the chain is "on the same side" of the bond axis as the carbon chain prior
to the double bond, and trans means "on opposite sides" of the bond axis.
Wow, is there anyone still reading this!?

Well, we're not yet finished. Now for polyunsaturated fatty acids. If you've
understood the above, they're a simple extension - just more than one double
(unsaturated) bond along the length of the fatty acid carbon chain. In olive
oils the maximum number of double bonds per fatty acid is three, wheras one
can get up to six unsaturated double bonds in certain fatty acids derived
from
fish. Generally, however, the greater the number of double bonds in the
fatty acid, the more unstable, and more easily broken down by heat, light,
etc. That's why olive oil, made up predominantly of monounsaturated oleic
acid, is so much more heat-stable than the highly polyunsaturated seed oils.
Olive oil can, for example, be re-used substantially more often in frying
than other seed oils (including canola, which has about three times the
amount of polyunsaturation than olive oil). An example of a polyunsaturated
fatty acid with two unsaturated double bonds along its carbon chain is
Linoleic Acid (18 carbons long). Linolenic Acid has three double bonds in
its carbon chain, and is also 18 carbons long.

Triacylglycerols are normally composed of a mixture of three of the some of
the above-mentioned fatty acids. Most prevalent in olive oil is the
oleic-oleic-oleic (OOO) triacylglycerol, followed, in order of incidence, by
palmitic-oleic-oleic (POO), then oleic-oleic-linoleic (OOL), then
palmitic-oleic-linoleic (POL), then stearic-oleic-oleic (SOO), etc.

Now lets look ACIDITY, which is probably the most fundamental quality
measurement of an edible oil.

As we know, freshly pressed oil, made from sound, healthy, freshly picked
olives, normally has a pretty low "acidity", in the order of well under
0,5%. This "acidity" is the result of a degree of breakdown of the
triacylglycerols due to a chemical reaction called hydrolysis, in which
free fatty acids are formed. (In exceptional circumstances, even oils made
from fresh, healthy olives can have significant amount of acidity, caused by
anomalies during the actual biosynthesis of the oil in the olive fruit).
Once the oil has been extracted, however, carelessness can lead to a very
significant further breakdown of the triacylglycerides into fatty acids -
these "broken off" fatty acids being called FREE FATTY ACIDS. Sometimes just
one of the three fatty acids breaks off, leaving a diacylglycerol. If two
fatty acids break off a certain triacylglycerol, we're left with a
monoacylglycerol. If all three break off, we're left with glycerol.

Factors which lead to a high free fatty acidity in an oil are: fruit fly
infestation of fruit, delays between harvesting and extraction, especially
if the fruit has been bruised or damaged during harvesting, fungal diseases
in the fruit (gloesporium, macrophoma, etc.), prolonged contact between oil
and vegetation water (after extraction), etc. Thus we see that the
traditional
way in which olives are/were stored in heaps/silos to encourage enzymatic
breakdown of cell structure so as to facilitate oil release (as practiced in
Portugal, etc.) is certainly not conduucive to producing a high quality, low
acid oil.

The free fatty acidity is thus a direct measure of the quality of the oil,
and reflects the care taken right from blossoming and fruit set to the
eventual sale and consumption of the oil.

Measurement of FFA (free fatty acidity) is a very simple procedure. The
principle is based on dissolving the free fatty acids present in a carefully
weighed sample of oil into a mixture of solvents (usually
alcohol/peroxide-free ether), and then titrating, with constant stirring,
against a standard alkali solution (usually standardised Potassium
Hydroxide) in the presence of an acid/base indicator (usually
phenolpthalein). The results are presented as grams oleic acid per 100 grams
oil, commonly known as the free fatty acidity (ffa or acidity) of the oil
(in %).

Have I left anything out?!!

Cheers

Guido
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