Extraction And Characterization Of Vegetable Oil Using Bread Fruit Seed

INTRODUCTION
EXTRACTION AND CHARACTERIZATION OF VEGETABLE OIL USING
BREAD FRUIT SEED.
1.1 Vegetable oil
A vegetable oil is a triglyceride extracted from a plant. Such oils have been part of
human culture for millennia. The term “vegetable oil” can be narrowly defined as
referring only to substances that are liquid at room temperature, or broadly defined
without regard to a substance’s state of matter at a given temperature. For this reason,
vegetable oils that are solid at room temperature are sometimes called vegetable fats.
Vegetable oils are composed of triglycerides, as contrasted with waxes which lack
glycerin in their structure. Although many plant parts may yield oil, in commercial
practice, oil is extracted primarily from seeds.

1.2 Production of Vegetable Oils
To produce vegetable oils, the oil first needs to be removed from the oil-bearing
plant components, typically seeds. This can be done via mechanical extraction using an
oil mill or chemical extraction using a solvent. The extracted oil can then be purified and,
if required, refined or chemically altered.

1.2.1 Mechanical extraction
Oils can also be removed via mechanical extraction, termed “crushing” or
“pressing.” This method is typically used to produce the more traditional oils (e.g., olive,
coconut etc.), and it is preferred by most health food customers in the United States and
in Europe. There are several different types of mechanical extraction: expeller-pressing
extraction is common, though the screw press, ram press, and Ghani (powered mortar and
pestle) are also used. Oil seed presses are commonly used in developing countries, among
people for whom other extraction methods would be prohibitively expensive; the Ghani is
primarily used in India.

1.2.2 Solvent extraction
The processing of vegetable oil in commercial applications is commonly done by
chemical extraction, using solvent extracts, which produces higher yields and is quicker
and less expensive. The most common solvent is petroleum-derived hexane. This
technique is used for most of the “newer” industrial oils such as soybean and corn oils.
Supercritical carbon dioxide can be used as a non-toxic alternative to other solvents.

1.2.3 Sparging
In the processing of edible oils, the oil is heated under vacuum to near the smoke
point, and water is introduced at the bottom of the oil. The water immediately is
converted to steam, which bubbles through the oil, carrying with it any chemicals which
are water-soluble. The steam sparging removes impurities that can impart unwanted
flavors and odors to the oil.

1.2.4 Hydrogenation
Oils may be partially hydrogenated to produce various ingredient oils. Lightly
hydrogenated oils have very similar physical characteristics to regular soya oil, but are
more resistant to becoming rancid. Hardening vegetable oil is done by raising a blend of
vegetable oil and a catalyst in near-vacuum to very high temperatures, and introducing
hydrogen. This causes the carbon atoms of the oil to break double-bonds with other
carbons, each carbon forming a new single-bond with a hydrogen atom. Adding these
hydrogen atoms to the oil makes it more solid, raises the smoke point, and makes the oil
more stable.
Hydrogenated vegetable oils differ in two major ways from other oils which are
equally saturated. During hydrogenation, it is easier for hydrogen to come into contact
with the fatty acids on the end of the triglyceride, and less easy for them to come into
contact with the center fatty acid. This makes the resulting fat more brittle than a tropical
oil; soy margarines are less “spreadable”. The other difference is that trans fatty acids
(often called trans fat) are formed in the hydrogenation reactor, and may amount to as
much as 40 percent by weight of a partially hydrogenated oil. Hydrogenated oils,
especially partially hydrogenated oils with their higher amounts of trans fatty acids are
increasingly thought to be unhealthy.

1.3 Uses of triglyceride vegetable oil
The following are some of the uses of vegetable oils:
1) Culinary uses: Many vegetable oils are consumed directly, or indirectly as ingredients
in food – a role that they share with some animal fats, including butter and ghee;
2) Industrial uses: Vegetable oils are used as an ingredient or component in many
manufactured products. Many vegetable oils are used to make soaps, skin products,
candles, perfumes and other personal care and cosmetic products. Some oils are
particularly suitable as drying oils, and are used in making paints and other wood
treatment products. Dammar oil (a mixture of linseed oil and dammar resin), for example,
is used almost exclusively in treating the hulls of wooden boats. Vegetable oils are
increasingly being used in the electrical industry as insulators .
3) Pet food additive: Vegetable oil is used in production of some pet foods. In some
poorer grade pet foods though, the oil is listed only as “vegetable oil”, without specifying
the particular oil.
4) Fuel: Vegetable oils are also used to make biodiesel, which can be used like
conventional diesel. Some vegetable oil blends are used in unmodified vehicles but
straight vegetable oil, also known as pure plant oil, needs specially prepared vehicles
which have a method of heating the oil to reduce its viscosity. The vegetable oil economy
is growing and the availability of biodiesel around the world is increasing. It is believed
that the total net greenhouse gas savings when using vegetable oils in place of fossil fuel-
based alternatives for fuel production, range from 18 to 100% [10].

1.4 Negative health effects
Hydrogenated oils have been shown to cause what is commonly termed the
“double deadly effect”, raising the level of low density lipoproteins (LDLs) and
decreasing the level of high density lipoproteins (HDLs) in the blood, increasing the risk
of blood clotting inside blood vessels.
A high consumption of omega-6 polyunsaturated fatty acids (PUFAs), which are
found in most types of vegetable oil (e.g. soyabean oil, corn oil– the most consumed in
USA, sunflower oil, etc.) may increase the likelihood that postmenopausal women will
develop breast cancer. A similar effect was observed on prostate cancer in mice. Plant
based oils high in monounsaturated fatty acids, such as olive oil, peanut oil, and canola
oil are relatively low in omega-6 PUFAs and can be used in place of high-
polyunsaturated oils.

1.5 Uses/Importance of Vegetable oils
1.5.1 Margarine
Margarine originated with the discovery by French chemist Michel Eugene
Chereul in 1813 of margaric acid (itself named after the pearly deposits of the fatty acid
from Greek (margaritēs / márgaron), meaning pearl-oyster or pearl, or (margarís),
meaning palm-tree, hence the relevance to palmitic acid). Scientists at the time regarded
margaric acid, like oleic acid and stearic acid, as one of the three fatty acids which, in
combination, formed most animal fats. In 1853, the German structural chemist Wihelm
Heinrich Heintz analyzed margaric acid as simply a combination of stearic acid and of
the previously unknown palmitic acid.
Emperor Louis Napoleon III of France offered a prize to anyone who could make
a satisfactory substitute for butter, suitable for use by the armed forces and the lower
classes. French chemist Hippolyte Mege-Mouries invented a substance he called
oleomargarine, the name of which became shortened to the trade name “margarine”.
Mège-Mouriès patented the concept in 1869 and expanded his initial manufacturing
operation from France but had little commercial success. In 1871, he sold the patent to
the Dutch company Jurgens, now part of Unilever. In the same year the German
pharmacist Benedict Klein from Cologne founded the first margarine factory “Benedict
Klein Margarinewerke”, producing the brands Overstolz and Botteram.
Margarine is a semi-solid emulsion composed mainly of vegetable fats and water.
While butter is derived from milk fat, margarine is mainly derived from plant oils and
fats and may contain some skimmed milk. In some locales it is colloquially referred to as
oleo, short for oleomargarine. Margarine, like butter, consists of a water-in-fat emulsion,
with tiny droplets of water dispersed uniformly throughout a fat phase which is in a stable
crystalline form. Margarine has a minimum fat content of 80%, the same as butter, but
unlike butter reduced-fat varieties of margarine can also be labelled as margarine.
Margarine can be used both for spreading or for baking and cooking. It is also commonly
used as an ingredient in other food products, such as pastries and cookies, for its wide
range of functionalities.
1.5.1.2 Manufacture of Margarine
The basic method of making margarine today consists of emulsifying a blend of
hydrogenated vegetable oils with skimmed milk, chilling the mixture to solidify it and
working it to improve the texture. Vegetable and animal fats are similar compounds with
different melting points. Those fats that are liquid at room temperature are generally
known as oils. The melting points are related to the presence of carbon-carbon double
bonds in the fatty acids components. Higher number of double bonds give lower melting
points.
Partial hydrogenation of a typical plant oil to a typical component of margarine,
makes most of the C=C double bonds be removed in this process, which elevates the
melting point of the product. Commonly, the natural oils are hydrogenated by passing
hydrogen through the oil in the presence of a nickel catalyst, under controlled conditions.
The addition of hydrogen to the unsaturated bonds (alkenic double C=C bonds) results in
saturated C-C bonds, effectively increasing the melting point of the oil and thus
“hardening” it. This is due to the increase in van der Waals’ forces between the saturated
molecules compared with the unsaturated molecules. However, as there are possible
health benefits in limiting the amount of saturated fats in the human diet, the process is
controlled so that only enough of the bonds are hydrogenated to give the required texture.
Margarines manufactured in this way are said to contain hydrogenated fat. This method is
used today for some margarines although the process has been developed and sometimes
other metal catalysts are used such as palladium. If hydrogenation is incomplete (partial
hardening), the relatively high temperatures used in the hydrogenation process tend to
flip some of the carbon-carbon double bonds into the “trans” form. If these particular
bonds aren’t hydrogenated during the process, they will still be present in the final
margarine in molecules of trans fats, the consumption of which has been shown to be a
risk factor for cardiovascular disease. For this reason, partially hardened fats are used less
and less in the margarine industry. Some tropical oils, such as palm oil and coconut oil,
are naturally semi solid and do not require hydrogenation.
Three types of margarine are common:
 Soft vegetable fat spreads, high in mono- or polyunsaturated fats, which are made
from safflower, sunflower, soybean, cottonseed, rapeseed or olive oil.
 Margarines in bottle to cook or top dishes
 Hard, generally uncolored margarine for cooking or baking.
1.5.2 Soap
In chemistry, soap is a salt of a fatty acid. Soaps are mainly used as surfactants for
washing, bathing, cleaning, in textile spinning and are important components of
lubricants. Soaps for cleansing are obtained by treating vegetable or animal oils and fats
with a strongly alkaline solution. Fats and oils are composed of triglycerides; three
molecules of fatty acids are attached to a single molecule of glycerol. The alkaline
solution, which is often called lye, (although the term “lye soap” refers almost
exclusively to soaps made with sodium hydroxide) brings about a chemical reaction
known as saponification. In saponification, the fats are first hydrolyzed into free fatty
acids, which then combine with the alkali to form crude soap. Glycerol (glycerin) is
Figure 1: Hydrogenation of vegetable oils
Partial hydrogenation of a typical plant oil to a typical component of margarine,
makes most of the C=C double bonds be removed in this process, which elevates the
melting point of the product. Commonly, the natural oils are hydrogenated by passing
hydrogen through the oil in the presence of a nickel catalyst, under controlled conditions.
The addition of hydrogen to the unsaturated bonds (alkenic double C=C bonds) results in
saturated C-C bonds, effectively increasing the melting point of the oil and thus
“hardening” it. This is due to the increase in van der Waals’ forces between the saturated
molecules compared with the unsaturated molecules. However, as there are possible
health benefits in limiting the amount of saturated fats in the human diet, the process is
controlled so that only enough of the bonds are hydrogenated to give the required texture.
Margarines manufactured in this way are said to contain hydrogenated fat. This method is
used today for some margarines although the process has been developed and sometimes
other metal catalysts are used such as palladium. If hydrogenation is incomplete (partial
hardening), the relatively high temperatures used in the hydrogenation process tend to
flip some of the carbon-carbon double bonds into the “trans” form. If these particular
bonds aren’t hydrogenated during the process, they will still be present in the final
margarine in molecules of trans fats, the consumption of which has been shown to be a
risk factor for cardiovascular disease. For this reason, partially hardened fats are used less
and less in the margarine industry. Some tropical oils, such as palm oil and coconut oil,
are naturally semi solid and do not require hydrogenation.
Three types of margarine are common:
 Soft vegetable fat spreads, high in mono- or polyunsaturated fats, which are made
from safflower, sunflower, soybean, cottonseed, rapeseed or olive oil.
 Margarines in bottle to cook or top dishes
 Hard, generally uncolored margarine for cooking or baking.
1.5.2 Soap
In chemistry, soap is a salt of a fatty acid. Soaps are mainly used as surfactants for
washing, bathing, cleaning, in textile spinning and are important components of
lubricants. Soaps for cleansing are obtained by treating vegetable or animal oils and fats
with a strongly alkaline solution. Fats and oils are composed of triglycerides; three
molecules of fatty acids are attached to a single molecule of glycerol. The alkaline
solution, which is often called lye, (although the term “lye soap” refers almost
exclusively to soaps made with sodium hydroxide) brings about a chemical reaction
known as saponification. In saponification, the fats are first hydrolyzed into free fatty
acids, which then combine with the alkali to form crude soap. Glycerol (glycerin) is
liberated and is either left in or washed out and recovered as a useful byproduct,
depending on the process employed.
When used for cleaning, soap allows otherwise insoluble particles to become
soluble in water and then be rinsed away. For example: oil/fat is insoluble in water, but
when a couple drops of dish soap are added to the mixture the oil/fat apparently
disappears. The insoluble oil/fat molecules become associated inside micelles, tiny
spheres formed from soap molecules with polar hydrophilic (water-loving) groups on the
outside and encasing a lipophilic (fat-loving) pocket, which shielded the oil/fat molecules
from the water making it soluble. Anything that is soluble will be washed away with the
water. Synthetic detergents operate by similar mechanisms to soap.
The type of alkali metal used determines the kind of soap produced. Sodium
soaps, prepared from sodium hydroxide, are firm, whereas potassium soaps, derived from
potassium hydroxide, are softer or often liquid. Historically, potassium hydroxide was
extracted from the ashes of bracken or other plants. Lithium soaps also tend to be hard
these are used exclusively in greases.
Typical vegetable oils used in soap making are palm oil, coconut oil, olive oil,
and laurel oil. Each species offers quite different fatty acid content and, hence, results in
soaps of distinct feel. The seed oils give softer but milder soaps. Soap made from pure
olive oil is sometimes called Castile/Marseille soap, and is reputed for being extra mild.
The term “Castile” is also sometimes applied to soaps from a mixture of oils, but a high
percentage of olive oil.
1.5.2.1 Purification and finishing
Figure 2: A generic bar of soap, after purification and finishing
In the fully boiled process on factory scale, the soap is further purified to remove
any excess sodium hydroxide, glycerol, and other impurities, colour compounds, etc.
These components are removed by boiling the crude soap curds in water and then
precipitating the soap with salt. At this stage, the soap still contains too much water,
which has to be removed. This was traditionally done on chill rolls, which produced the
soap flakes commonly used in the 1940s and 1950s. This process was superseded by
spray dryers and then by vacuum dryers. The dry soap (about 6–12% moisture) is then
compacted into small pellets or noodles. These pellets or noodles are then ready for soap
finishing, the process of converting raw soap pellets into a saleable product, usually bars.
Soap pellets are combined with fragrances and other materials and blended to
homogeneity in an amalgamator (mixer). The mass is then discharged from the mixer into
a refiner, which, by means of an auger, forces the soap through a fine wire screen. From
the refiner, the soap passes over a roller mill (French milling or hard milling) in a manner
similar to calendering paper or plastic or to making chocolate liquor. The soap is then
passed through one or more additional refiners to further plasticize the soap mass.
Immediately before extrusion, the mass is passed through a vacuum chamber to remove
any trapped air. It is then extruded into a long log or blank, cut to convenient lengths,
passed through a metal detector, and then stamped into shape in refrigerated tools. The
pressed bars are packaged in many ways.
Sand or pumice may be added to produce a scouring soap. The scouring agents serve to
remove dead cells from the skin surface being cleaned. This process is called exfoliation.
Many newer materials that are effective, yet do not have the sharp edges and poor particle
size distribution of pumice, are used for exfoliating soaps.
Nanoscopic metals are commonly added to certain soaps specifically for both colouration
and antibacterial properties. Titanium dioxide powder is commonly used in extreme
“white” soaps for these purposes; nickel, aluminium and silver compounds are less
commonly used. These metals exhibit an electron-robbing behaviour when in contact
with bacteria, stripping electrons from the organism’s surface, thereby disrupting their
functioning and killing them. Since some of the metal is left behind on the skin and in the
pores, the benefit can also extend beyond the actual time of washing, helping reduce
bacterial contamination and reducing potential odours from bacteria on the skin surface.
1.5.3 Biodiesel production
Biodiesel production is the process of producing the biofuel/biodiesel, through the
chemical reactions: transesterification and esterification. This involves vegetable or
animal fats and oils being reacted with short-chain alcohols (typically methanol or
ethanol). The major steps required to synthesize biodiesel are as follows:
1. Feedstock pretreatment: Common feedstock used in biodiesel production
include yellow grease (recycled vegetable oil), “virgin” vegetable oil, and tallow.
Recycled oil is processed to remove impurities from cooking, storage, and
handling, such as dirt, charred food, and water. Virgin oils are refined, but not to a
food-grade level. De-gumming to remove phospholipids and other plant matter is
common, though refinement processes vary. Regardless of the feedstock, water is
removed as its presence during base-catalyzed transesterification causes the
triglycerides to hydrolyse, giving salts of the fatty acids (soaps) instead of
producing biodiesel.
2. Determination and treatment of free fatty acids: A sample of the cleaned
feedstock oil is titrated with a standardized base solution in order to determine the
concentration of free fatty acids (carboxylic acids) present in the vegetable oil
sample. These acids are then either esterified into biodiesel, esterified into
glycerides, or removed, typically through neutralization.
3. Reactions: Base-catalyzed transesterification reacts lipids (fats and oils) with
alcohol (typically methanol or ethanol) to produce biodiesel and an impure co-
product, glycerol. If the feedstock oil is used or has a high acid content, acid-
catalyzed esterification can be used to react fatty acids with alcohol to produce
biodiesel. Other methods, such as fixed-bed reactors, supercritical reactors, and
ultrasonic reactors, forgo or decrease the use of chemical catalysts.
4. Product purification: Products of the reaction include not only biodiesel, but
also byproducts, soap, glycerol, excess alcohol, and trace amounts of water. All of
these byproducts must be removed to meet the standards, but the order of removal
is process-dependent. The density of glycerol is greater than that of biodiesel, and
this property difference is exploited to separate the bulk of the glycerol co-
product. Residual methanol is typically recovered by distillation and reused.
Soaps can be removed or converted into acids. Residual water is also removed
from the fuel.
1.5.3.1 Reactions
Animal and plant fats and oils are composed of triglycerides, which are esters
containing three free fatty acids and the trihydric alcohol, glycerol. In the
transesterification process, the alcohol is de-protonated with a base to make it a stronger
nucleophile. Commonly, ethanol or methanol are used. As can be seen, the reaction has
no other inputs than the triglyceride and the alcohol. Under normal conditions, this
reaction will proceed either exceedingly slowly or not at all, so heat, as well as catalysts
(acid and/or base) are used to speed up the reaction. It is important to note that the acid or
base are not consumed by the transesterification reaction, thus they are not reactants, but
catalysts. Common catalysts for transesterification include sodium hydroxide, potassium
hydroxide, and sodium methoxide.
Almost all biodiesel is produced from virgin vegetable oils using the base-
catalyzed technique as it is the most economical process for treating virgin vegetable oils,
requiring only low temperatures and pressures and producing over 98% conversion yield
(provided the starting oil is low in moisture and free fatty acids). However, biodiesel
produced from other sources or by other methods may require acid catalysis, which is
much slower.
The transesterification reaction is base catalyzed. Any strong base capable of de-
protonating the alcohol will do (e.g. NaOH, KOH, sodium methoxide, etc.), but the
sodium and potassium hydroxides are often chosen for their cost. The presence of water
causes undesirable base hydrolysis, so the reaction must be kept dry. In the
transesterification mechanism, the carbonyl carbon of the starting ester (RCOOR1)
undergoes nucleophilic attack by the incoming alkoxide (R2O−) to give a tetrahedral
intermediate, which either reverts to the starting material, or proceeds to the
transesterified product (RCOOR2). The various species exist in equilibrium, and the
product distribution depends on the relative energies of the reactant and product.
GENERAL PROPERTIES OF VEGETABLE OILS

1.6 Vegetable oils – General properties
Vegetable oils are obtained from oil containing seeds, fruits, or nuts by different pressing
methods, solvent extraction or a combination of these (Bennion, 1995). Crude oils
obtained are subjected to a number of refining processes, both physical and chemical.
These are detailed in various texts and articles (Bennion, 1995), (Fennema, 1985). There
are numerous vegetable oils derived from various sources. These include the popular
vegetable oils: the foremost oilseed oils – soybean, cottonseed, peanuts and sunflower
oils; and others such as palm oil, palm kernel oil, coconut oil, castor oil, rapeseed oil and
others. They also include the less commonly known oils such as rice bran oil, tiger nut
oil, patua oil, ko_me oil, niger seed oil, piririma oil and numerous others. Their yields,
different compositions and by extension their physical and chemical properties determine
their usefulness in various applications aside edible uses.
Cottonseed oil was developed over a century ago as a byproduct of the cotton industry
(Bennion, 1995). Its processing includes the use of hydraulic pressing, screw pressing
and solvent extraction (Wolf, 1978). It is classified as a polyunsaturated oil, with palmitic
acid (C16H32O2) consisting 20 – 25%, stearic acid (C18H36O2) 2 – 7%, oleic acid
(C18H34O2) 18 – 30%, and linoleic acid (C18H32O2)40 – 55% (Fennema, 1985). Its
primary uses are food related – as salad oil, for frying, for margarine manufacture, and
for manufacturing shortenings used in cakes and biscuits.
Palm oil, olive oil, cottonseed oil, peanut oil, and sunflower oil amongst others are
classed as Oleic – Linoleic acid oils seeing that they contain a relatively high proportion
of unsaturated fatty acids, such as the monounsaturated oleic acid and the
polyunsaturated linoleic acid (Dunn, 2005; Gertz et al., 2000). They are characterized by
a high ratio of polyunsaturated fatty acids to saturated fatty acids. As a consequence of
this, they have relatively low melting points and are liquid at room temperature. Iodine
values, saponification values, specific compositions and melting points in addition to
other physical properties have been determined and are widely available in the literature
(Williams, 1966), (Oyedeji et al., 2006).
Other oils fall under various classes such as the erucic acid oils which are like the oleic
linoleic acid oils except that their predominant unsaturated fatty acid is erucic acid (C22).
Rapeseed and mustard seed oil are important oils in this class. Canola oil is a type of
rapeseed oil with reduced erucic acid content (Applewhite, 1978). It is a stable oil used in
salad dressings, margarine and shortenings. Soybean oil is an important oil with
numerous increasing applications in the modern day world. It is classed as a linolenic
acid oil since it contains the more highly unsaturated linolenic acid. Other oils include
castor oil (a hydroxy-acid oil) which contains glycerides of ricinoleic acid (Erhan et al.,
2006). Also worthy of note is that coconut oil, which unlike most vegetable oils, is solid
at room temperature due to its high proportion of saturated fatty acids (92%) particularly
lauric acid. Due to its almost homogenous composition, coconut oil has a fairly sharp
melting point (Bennion, 1995).

1.7 Auto oxidation and oxidative stability in vegetable oils
By definition, the oxidative stability of oil is a measure of the length of time taken for
oxidative deterioration to commence. On a general level, “the rates of reactions in auto-
oxidation schemes are dependent on the hydrocarbon structure, heteroatom concentration,
heteroatom speciation, oxygen concentration, and temperature (Ferrari et al., 2004).
If untreated, oils from vegetable origin oxidize during use and polymerize to a plastic like
consistency (Honary, 2004). Even when they are not subjected to the intense conditions
of industrial applications, fats and oils are liable to rancidity (Eastman Chemical
Company, 2001; Morteza- Semnani et al., 2006). This happens more so in fats that
contain unsaturated fatty acid radicals (Charley,
1970). Indeed the oxidisability of a vegetable oil is dependent on the level of unsaturation
of their olefinic compounds. In general terms, oxidative rancidity in oils occurs when
heat, metals or other catalysts cause unsaturated oil molecules to convert to free radicals.
These free radicals are easily oxidized to yield hydroperoxides and organic compounds,
such as aldehydes, ketones, or acids which give rise to the undesirable odors and flavors
characteristic of rancid fats (Eastman Chemical Company, 2001). The role of peroxides is
exploited in monitoring oxidative deterioration by measuring peroxide values (POV)
(Mochida et al., 2006) Lipid oxidation occurs via auto oxidation or lipoxygenase catalysis. Auto oxidation refers
to a complex set of reactions which result in the incorporation of oxygen in lipid
structures. Auto oxidation reactions are seen to progress more rapidly in oils that contain
predominantly unsaturated fat molecules; other relevant factors include the presence of
light, transition metal ions, oxygen pressure, the presence or absence of antioxidants and
pro oxidants, temperature and moisture content. Auto oxidation reactions occur at an
increasing rate after the initial induction period. This behavior can be explained by
assuming that oxidation proceeds by a sequential free radical chain reaction mechanism.
Relatively stable radicals that can abstract hydrogen atoms from the allylic methylene
groups in olefinic compounds are formed. Hence auto oxidation is a radical induced chain
reaction which proceeds through the traditional stages of initiation, propagation and
termination. Detailed proposed mechanisms for these free radical chain reactions are
available in literature (Fennema, 1985).
Lipoxygenases are metal proteins with an iron atom as the active center. They catalyze
the oxidation of unsaturated fatty acids to hydroperoxides as with auto oxidation. Enzyme
activation usually occurs in the presence of hydroperoxides, even though enzyme
catalyzed oxidation can occur even in the absence of hydroperoxides (Fennema, 1985).
As earlier stated, the more unsaturated the fatty acid involved is, the greater its
susceptibility to oxidative rancidity. For instance, the linolenic acid esters present in
soybean oil (with twice the unsaturation as monounsaturated esters) is particularly
sensitive to even oxidation of the slightest kind, commonly referred to as flavor
reversion, resulting in beany, grassy or painty flavors (Wolf, 1978). A highly saturated
fatty acid level is confirmed to be of benefit in terms of storage ability when compared to
more unsaturated vegetable oils (Ferrari et al., 2004). Indeed, the tendency of an oil to
combine with oxygen of the air and become gummy (known as drying) is measured with
the iodine number, which in fact is merely a measure of the level of unsaturation of the
oil in question (a higher iodine number will indicate higher unsaturation seeing that
iodine is absorbed primarily by the mechanism of addition to the double bonds
characteristic of unsaturation) (Gunther, 1971).
Based on studies by Toshiyuki. (1999), the oxidative stability of refined vegetable oils is
found to be determined considerably by the fatty acid composition, the tocopherols
content and the carbonyl value (Toshiyuki, 1999). When observed at frying temperatures,
it is seen that in general, non-refined oils prove to have a better stability than refined oils
(Gertz et al., 2000). This could be attributed to the fact that refining steps, in particular
deodorization, remove a percentage of the tocopherols, which act as natural anti-oxidants
in vegetable oils (Applewhite, 1978). Corn oil has a better stability than soybean oil,
while rapeseed oil is seen to give a better performance than olive oil. This can be
explained in terms of their compositions (Isbell et al., 1999). When investigated at a
temperature of 110oC, vegetable oils still show the trend of increased stability in the
unrefined state than when refined. Meadow foam oil is reported as the most stable oil in
the study conducted by Isbell et al. (1999). High oleic sunflower oil and crude jojoba oil
also had good values of oxidative stability (Isbell et al., 1999). Other studies indicate that
the presence of free fatty acids has a pro-oxidant effect on vegetable oils