EXTRACTION AND CHARACTERIZATION OF VEGETABLE OIL USING BREAD FRUIT SEED.
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.
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.
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.
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