CHAPTER ONE

INTRODUCTION

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.

2

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.

3

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;

4

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.

5

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

6

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.

7

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

8

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

9

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.

10

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

11

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:

12

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

13

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.

14

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 (RCOOR

1

)

undergoes nucleophilic attack by the incoming alkoxide (R

2

O

−

) to give a tetrahedral

intermediate which either reverts to the starting material or proceeds to the

transesterified product (RCOOR

2

). 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.

15

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.

16

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).

17

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

18

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 110

o

C 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 (Frega et al.

1999). Hence refining practices are important seeing Aluyor and Ori-Jesu 4839 that

improper handling and raw material abuse can result in the stimulation of enzymatic

activity which could produce free fatty acids (Applewhite 1978). Further investigations

on manufacturing practices also reveal research which indicates the importance of the

solvent used in the extraction of vegetable oils. Traditional solvents utilized such as

19

hexane or petroleum ether have the characteristic of extracting only non-polar species.

Isopropanol however as documented by Oyedeji et al. (2006) would extract some polar

and high molecular weight compounds. Among these compounds are the natural

antioxidants and pigments in oilseeds which presence lead to extended shelf life and

hence better oxidative stability (Oyedeji et al. 2006).

1.8 Antioxidants and stability of vegetable oils

Numerous experimental works have established the positive effect of anti-oxidants on the

oxidative stability of vegetable oils for both edible uses and industrial uses. An important

class of anti-oxidants consists of the phenolic compounds butylhydroxyanisole (BHA)

butylhydroxytoluene (BHT) propyl gallate and tert-butyl

hydroquinone (TBHQ). Their use in vegetable oils meant for domestic and industrial

processes is widespread.

Vegetable oils in their natural form possess constituents that function as natural

antioxidants. Amongst them are ascorbic acids _-tocopherole _-carotene chlorogenic

acids and flavanols (Ullah et al. 2003). Tests conducted to investigate the effectiveness

of natural anti-oxidants contained in red pepper oil added to soybean and sunflower oils

indicate that they provide variable protection against light induced auto-oxidation.

In the above mentioned study on the inhibitive effect of natural antioxidants contained in

red pepper oil it was additionally observed that the phenolic anti-oxidant

butylated hydroxytoluene (BHT) shows more effectiveness generally than natural anti-

oxidants (Ullah et al. 2003). In the work done by Robert (2005) the common phenolic

anti-oxidants were tested for their effectivenessin improving the oxidative stability of

biodiesel obtained from soybean oil. Dunn monitored the oxidative stability by means of

20

pressurized differential scanning calorimetry (P-DSC). For both static and dynamic

conditions improvements in oxidative stability are observed with the application of anti-

oxidants which included BHA BHT TBHQ propyl gallate (PrG) and α-tocopherol. The

work of (Dunn 2005) further showed that the relative effectiveness of the different anti-

oxidants differed for static and dynamic conditions although all showed superior

performance when compared with α-tocopherol.

A recent area of interest in antioxidant research is concerned with finding effective

replacements for the conventional synthetic antioxidants from among various natural

extracts from plant species which are seen to possess antioxidant properties. Such

research is in the main prompted by the reported possibility of synthetic antioxidants

having adverse health effects on humans exposed to them. Specifically they are known

to contribute to liver enlargement and an increase in microsomal activity (Khanahmadi et

al. 2006; Morteza- Semnani et al. 2006). Maduka et al. (2003) investigated the

effectiveness of a Nigerian alcoholic beverage additive Sacoglottis gabonensis stem bark

extract as an antioxidant for common stored vegetable oils. Inhibition of lipid peroxi-

dation was found to be comparable to inhibitions obtained with treatment with vitamins C

and E (Maduka et al. 2003). The Ferulago angulata plant indigenous to the west of

Iran also has proven antioxidant properties. Experimental studies documented indicate

that these plants’ essential oils and extract begins to show preservative properties on

vegetable oils at a minimum concentration of 0.02%. In fact it even shows more

effectiveness that TBHQ at concentrations of 0.5% (Khanahmadi et al. 2006). When

evaluated by measuring reducing power ability to inhibit linoleic acid peroxidation and

22-diphenyl picrylhydrazyl radical scavenging activities the alkaloid extracts of

Fumaria capreolata and Fumaria bastardii demonstrated strong total antioxidant

21

activity with effectiveness marginally less than that of the common synthetic antioxidant

butylated hydroxyanisole and better than quercetine and caffeine. These species have

wide distribution in the Mediterranean region and have a reputation for effectiveness in

treating hepatobiliary disfunction and gastrointestinal disorders via local therapies (Maiza

et al. 2007). Methanolic extracts of Phlomis bruguieri P. herbaventi P. olivieri Stachys

byzantine S. inflata S. lavandulifolia and S. laxa were tested in sunflower oil stored at

70

o

C for antioxidant effectiveness using peroxide values as a measure. Comparisons

included samples containing BHA. Highest effectiveness in stabilizing sunflower was

obtained from methanolic extracts of P. bruguieri and S. laxa. These tests and their

findings suggest strongly the possibility of having in these plants a viable source of

natural antioxidants of high performance (Morteza- Semnani et al. 2006).

1.9 Vegetable oils as lubricants bio-fuels and transformer coolants

The application of vegetable oils and animal fats for industrial purposes and specifically

lubrication has been in practice for many years. Inherent disadvantages and the

availability of inexpensive options have however brought about low utilization of

vegetable oils for industrial lubrication (Honary 2004). When applied in the science of

tribology vegetable oils fall under the class known as fixed oils (Gunther 1971). They

are so named because they do not volatilize without decomposing. Prior to recent

developments vegetable and animal oils in tribology have functioned mainly as additives

to mineral lubricating oil formulations although in some cases they are applied

exclusively or in blends. For instance tallow (acidless) has been used as an emulsifying

agent for steam cylinder oils while castor peanut and rapeseed oils have been used in

blends with mineral oils to improve lubrication performance. Palm oil has been used in

22

isolation as a fluxing dip in the tin plating of steel while olive oil has applications as a

yarn lubricant (Gunther 1971).

Reasons for the use of vegetable oils in the science of lubrication abound. Their superior

lubricity and emulsifying characteristics increase their desirability as additives to the

cheaper but less effective mineral oil aced lubricants. Their superior lubricity in industrial

and machinery lubrication sometimes even necessitates the addition of friction materials

in tractor transmissions in order to reduce clutch slippage (Honary 2004).

Other advantages that encourage the use of vegetable oils include their relatively low

viscosity-temperature variation; that is their high viscosity indices which are about twice

those of mineral oils (Honary 2004). Additionally they have low volatilities as

manifested by their high flash points (Honary 2004). Significantly they are

environmentally friendly: renewable non toxic and biodegradable (Howell 2007). In

summary engine lubricants formulated from vegetable oils have the following

advantages deriving from their base stock

chemistry: higher Lubricity resulting in lower friction losses and hence more power and

better fuel economy; lower volatility resulting in decreased exhaust emissions; higher

viscosity indices; higher shear stability; higher detergency eliminating the need for

detergent additives; higher dispersancy; rapid biodegradation hence decreased

environmental / toxicological hazards (Erhan et al. 2002).

In a comparison of palm oil and mineral based lubricants palm oil based lubricants were

found to be more effective in reducing the hydrocarbon and carbon monoxide emission

levels among other things (Masjuki et al. 1999).

23

Vegetable oils have also been identified as having a lot of potential as alternative diesel

engine fuels (Kayisoglu et al. 2006). This is supported by an interest in a cleaner

environment as well as the increasing cost of mineral deposit based energy (Howell

2007). Based on the potential availability to meet demand soybean peanut and

sunflower oils have been identified as the most promising fuel sources (Kayisoglu et al.

2006). When used as a fuel the term “biodiesel” is applicable.

Biodiesel is defined strictly as “...the mono alkyl ester (usually methyl ester) of

renewable fats and oils...” (Howell 2007). It consists primarily of long chain fatty acid

esters produced by the transesterification reaction of vegetable oils with short chain

alcohols. Distinct advantages of biodiesel include a high flash point of over 100

o

C

excellent lubricity a BTU content comparable to that of petrol diesel and virtually no

sulfur or aromatic content. Above all biodiesel is non-toxic and biodegradable (Howell

2007). Results from investigating performance of vegetable oils in blends with diesel

indicate that blending up to 25 percent biodiesel (sunflower) with mineral diesel has no

adverse effect on performance (Kayisoglu et al. 2006).

Vegetable oils have also been applied as transformer coolant oils and have been found to

conform to all industry standards with performances and cost profiles comparable to the

conventional mineral oils applied in transformer cooling (ABB Inc. 2002). Transformer

oil products have been produced from soybean oils as well as castor oils (Honary 2004).

Whether applied for lubrication purposes or as biodiesel or as transformer cooling fluid

one of the major challenges in the utilization of the more environmentally friendly

vegetable oils is their poor oxidative stability (Honary 2004) (Howell 2007).

Combating the issue of oxidative instability in vegetable oils for industrial use is a

24

continuing research area. In the United States for instance three avenues are being

pursued. These are (Howell 2007):

 Genetic modification of oils to give higher mono unsaturated compounds;

 Chemical modification

 The use of various additives and property enhancers

Genetic modification has been made possible by recent advances in biotechnology.

DuPont Technology has developed a soybean seed that presents 83% oleic acid as against

having the more unsaturated linolenic acid as the major constituent. This new seed

provides oils that show about 30 times the oxidative stability and viscosity stability of the

conventional oil. High oleic varieties of rapeseed canola and sunflower seed oils are

increasingly being used as base stocks for lubricant formations (Honary 2004).

Chemical modifications involve the partial hydrogenation of the vegetable oil and a

shifting of its fatty acids (Honary 2004). In one study epoxidized soybean oil was

chemically modified with various alcohols in the presence of sulfuric acid as a catalyst.

Better performance was recorded (Hwang et al. 2001).

The use of additives known as antioxidants to control the development of oxidative

rancidity has been applied in the US since 1947 (Bennion 1995). They still remain one of

the most efficient and cost effective ways to improve the oxidative stability of oils in both

domestic and industrial conditions.

25