LU3 part 1 rec

The video introduces lipids as chemically diverse compounds soluble in organic solvents, encompassing fats (solid) and oils (liquid) at ambient temperatures. The main learning objectives are to define lipids, classify them, explain fat functions in food, summarize oxidation processes, and detail the chemistry of frying. Fatty acids, the major components of lipids, are defined as aliphatic chains with a carboxylic acid group, mostly ranging from 14 to 24 carbons and classified as saturated or unsaturated.

Lipid molecules feature a hydrophilic head that loves water and hydrophobic tails (up to three fatty acid chains) that love oil. Fatty acid nomenclature can be systematic, common, or abbreviated. Saturated fatty acids are named based on their carbon count (e.g., decanoic acid), while unsaturated fatty acids (those with double bonds) change suffix from '-anoic' to '-enoic'. Numerical abbreviations indicate carbon count and number of double bonds. The Delta system numbers double bonds from the carboxylic end, and the Omega system numbers them from the methyl end, both helping to classify fatty acids like oleic acid.

Saturated fatty acids typically have higher melting points than unsaturated ones, a crucial factor in food technology influencing physical properties of fats and oils. Acylglycerols, or glycerides, are esters of glycerol and fatty acids; over 99% of fatty acids in nature are esterified to glycerol. They exist as mono-, di-, and triglycerides. Triglycerides are the most common in food, while mono- and diglycerides are often used as food additives like emulsifiers.

Beyond triglycerides, which constitute 95% of food lipids, other important types include phospholipids (e.g., lecithin), sphingolipids (e.g., sphingomyelin), sterols (e.g., cholesterol, phytosterols), and waxes. Phospholipids, modified triglycerides with a phosphate group, prevent crystallization and are found in various foods and used as emulsifiers. Sterols, characterized by their ring structure, include animal cholesterol and plant phytosterols, with the latter used to reduce blood cholesterol. Waxes, found on plant and animal tissues, prevent dehydration and are used as fruit coatings to extend shelf life.

Fatty acids vary in chain length and saturation, which affects their melting points. Unsaturated fatty acids, like linoleic and linolenic acid, have lower melting points and are often liquid at room temperature. The video compares the composition of dietary fats, showing how different sources like coconut fat, olive oil, and sunflower oil vary in their mono-saturate, linoleic, linolenic, and saturate content.

Fats serve multiple nutritional and functional roles in food. Nutritionally, they provide essential fatty acids (linoleic and linolenic acid), act as carriers for fat-soluble vitamins (A, D, E, K), and are a concentrated source of energy (9 kcal/gram). Functionally, they influence food appearance (e.g., opacity of whole milk vs. skim milk), texture (tenderness, mouthfeel in baked goods), and flavor (e.g., cheese flavor from short-chain fatty acids, coconut/peach from hydroxy fatty acid hydrolysis). They also contribute to mouthfeel, providing properties from oily to smooth or grainy, and a cooling sensation upon melting.

Auto-oxidation is an undesirable chemical reaction causing the degradation of oils, leading to the formation of primary, secondary, and tertiary oxidation products. This process deteriorates the flavor of fats, leading to rancidity. It's a free radical chain reaction involving unsaturated fatty acids and proceeds in three main stages: initiation, propagation, and termination. Unsaturated fatty acids are the primary reactants, and hydrogen atoms next to double bonds are easily removed.

Initiation begins when a hydrogen atom is removed from an unsaturated fatty acid, forming an alkyl radical. This can be catalyzed by enzymes like lipoxygenases (inactivated by heat) or metal ions (prevented by chelating agents like EDTA or citric acid). Propagation sees the alkyl radical react with oxygen to form a peroxyl radical, which then abstracts hydrogen from another unsaturated molecule, creating a peroxide and a new free radical. This chain reaction can repeat thousands of times. Hydroperoxides are primary oxidation products. They are unstable and decompose into secondary oxidation products like aldehydes, which are primarily responsible for flavor deterioration. Tertiary oxidation products, like oxidized fatty acids, can also form. Termination occurs when free radicals react with each other, forming stable products and ending the chain reaction.

The extent of auto-oxidation is measured by the Totox value, which combines the Peroxide Value (PV) and Anisidine Value (AV). PV measures primary oxidation products (hydroperoxides); a lower PV indicates better oil quality (customers often require <10, sometimes <2). However, PV can decrease as secondary oxidation products appear. AV measures secondary oxidation products, particularly aldehydes, which cause rancid smells; a lower AV also indicates better quality (customers often require <30, sometimes <10). The Totox value provides a more comprehensive assessment of oil quality, especially since PV and AV values change differently over time during oxidation.