Research proposal sample

CHAPTER TWO

LITERATURE REVIEW

2.1 Vegetable Oils

Vegetable oils are lipid-based compounds extracted from plant seeds, including sunflower, peanut, sesame, soybean, and cotton seeds (Nzikou et al., 2010). These oils primarily consist of triglycerides (98%) along with minor components (2%) such as free fatty acids, mono- and diglycerides, phosphatides, sterols, tocopherols, and trace metals (O’Brien, 2018). Due to their diverse applications, vegetable oils are widely used in food preparation, cosmetics, pharmaceuticals, margarine production, soap and detergent manufacturing, and paint formulations (Othman & Ngassapa, 2010).

In addition to vegetable oils, plants also produce essential oils, which are volatile aromatic compounds extracted from flowers, leaves, bark, roots, or peels (Bansal, 2016).

2.2 Fatty Acids in Vegetable Oils

Fatty acids are aliphatic monocarboxylic acids composed of a hydrocarbon chain and a carboxyl group. They are classified based on chain length (4–24 carbon atoms) and saturation level (saturated or unsaturated). The most common fatty acids in vegetable oils contain 18 carbon atoms, including stearic, oleic, linoleic, and linolenic acids (Ratnayake & Galli, 2009).

2.2.1 Nomenclature of Fatty Acids

Fatty acids are named systematically by counting carbon atoms from the carboxyl end. However, in biological systems, numbering starts from the terminal methyl group (ω-x or n-x notation), where x indicates the position of the double bond closest to the methyl end. For example, linoleic acid (C18:2n-6) has two double bonds, with the first at the sixth carbon from the methyl group (Mead & Fulco, 1976; IUPAC-IUB, 1978).

2.2.2 Triglyceride Formation

Triglycerides (triacylglycerols) are esters formed when glycerol reacts with three fatty acid molecules, releasing water and forming ester bonds (Boudreaux, 2020). These fatty acids may be identical or structurally different.

2.3 Classification of Fatty Acids

Fatty acids are categorized based on:

1. Chain length: Short (C4–C6), medium (C8–C12), or long (C14–C24).
2. Saturation:
   • Saturated fatty acids (SFAs): No double bonds (e.g., palmitic acid, stearic acid).
   • Unsaturated fatty acids:
     • Monounsaturated (MUFAs): One double bond (e.g., oleic acid).
     • Polyunsaturated (PUFAs): Two or more double bonds (e.g., linoleic acid, α-linolenic acid).

2.3.1 Saturated Fatty Acids

Naturally occurring SFAs typically have even-numbered carbon chains (Table 2.1).

Table 2.1: Major Saturated Fatty Acids in Vegetable Oils (Gunstone, 1996)

2.3.2 Unsaturated Fatty Acids

Unsaturated fatty acids (Table 2.2) may have cis or trans configurations.

Table 2.2: Major Unsaturated Fatty Acids (Gunstone, 1996)

2.3.3 Essential Fatty Acids (EFAs)

EFAs, such as α-linolenic (omega-3) and linoleic acid (omega-6), cannot be synthesized by humans and must be obtained from dietary sources like vegetable oils (Glick & Fischer, 2013). These fatty acids play critical roles in anti-inflammatory processes and hormone synthesis.

• Omega-3 Fatty Acids: Found in flaxseed and fish oils, they exhibit anti-inflammatory and cardiovascular benefits (Weylandt et al., 2012).
• Omega-6 Fatty Acids: Present in sunflower and sesame oils, they help reduce LDL cholesterol and cancer risks (Al-Khudairy et al., 2015).

2.4 Impact of Fatty Acids on Human Health

• SFAs: High intake may elevate LDL cholesterol, increasing cardiovascular disease (CVD) risk (Barbour et al., 2015).
• MUFAs (e.g., oleic acid): Improve lipid profiles by reducing LDL while maintaining HDL (Kris-Etherton, 1999).
• PUFAs: Prone to oxidation, forming harmful trans-fats when heated (Wang et al., 2015).

2.4.1 Nutritional Index (P/S Ratio)

The polyunsaturated-to-saturated (P/S) ratio indicates nutritional quality. Oils with P/S > 1 (e.g., sunflower oil, P/S = 6.76) are considered healthier (Zambiazi et al., 2007).

2.5 Biosynthesis of Fatty Acids

Fatty acid synthesis in plants involves:

1. Carboxylation: Acetyl-CoA → Malonyl-CoA (catalyzed by ACCase).
2. Chain elongation: Via ketoacyl-ACP synthase (KAS I-III).
3. Desaturation: Introduction of double bonds by desaturase enzymes (Buchanan, 2000).

2.6 Factors Affecting Fatty Acid Composition

1. Genetic Factors: Desaturase genes (fad2, fad3) regulate unsaturation levels (Thambugala et al., 2013).
2. Environmental Factors:
   • Temperature: Higher temperatures increase oleic acid content (Byfield & Upchurch, 2007).
   • Soil Nutrients: NPK fertilizers alter fatty acid profiles (Kaptan et al., 2017).
   • Water Stress: Reduces PUFA content (Singh & Sinha, 2005).

2.7 Oil Quality Determinants

• Oxidation Stability: Influenced by fatty acid composition and storage conditions (Kamal, 2006).
• Extraction Methods:
  • Cold-pressing: Preserves antioxidants (Azadmard-Damirchi et al., 2011).
  • Solvent extraction: Higher yield but may leave residues (Takadas & Doker, 2017).

2.8 Analytical Methods

• Fatty Acid Profiling: GC-FID or GC-MS after derivatization to methyl esters (Christie, 1998).
• Heavy Metal Analysis: Atomic Absorption Spectrometry (AAS) (Unak et al., 2007).

2.9 Physicochemical Properties

Key parameters include:

• Viscosity: Higher in saturated oils (Yalcin et al., 2012).
• Acid Value: Indicates hydrolysis (FAO/WHO limit: ≤4 mg KOH/g).
• Iodine Value: Measures unsaturation (Knothe, 2006).

2.10 Heavy Metals in Oils

Sources include soil contamination, refining processes, and packaging. Toxic metals (Pb, Cd) accumulate in the body, causing kidney and neurological damage (WHO, 2003).