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Lecture 10

FDSC 200 Lecture 10: Chapter 10

8 Pages
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Department
Food Science
Course Code
FDSC 200
Professor
Lawrence Goodridge

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Description
FDSC200 Chapter 10: Lipids: Nature’s Flavor Enhancers Chemical Structure of Lipids • Lipids are organic compounds that are insoluble in water and have a greasy feel - They contain carbon, hydrogen, and oxygen - They differ from carbohydrates in chemical structure and function in food products. They are not polymers, they do not stabilize food structures and they do not dissolve in water. • The 3 types of lipids found in food and in the human body are - Triglycerides, phospholipids, and sterols The Glycerides • Glycerides have 2 basic parts - The base is a glycerol molecule with 3 hydroxyl groups that easily react with other compounds - Fatty acids are carbon chains with a carboxyl group (COOH) at one end. The carboxyl group of a fatty acid will readily react with a hydroxyl group of a glycerol producing a lipid and water. ➢ Common fatty acid chains found in food vary in length from 4 to 24 carbon atoms ➢ There are 10 fatty acids in plants and 20 fatty acids in animal tissue that can combine with glycerol to form lipids ➢ Triglycerides may have 3 identical or different fatty acids, and since there are a variety of fatty acids to choose from, this allows for a wide variety of triglycerides, each with different cooking performances, shell lives and nutritional values. - Glycerol can join with a 1. Monoglyceride: glycerol with one fatty acid 2. Diglyceride: glycerol with 2 fatty acids 3. Triglyceride: glycerol with 3 fatty acids - Glycerol and fatty acids separated are polar molecules. The bodies of the fatty acid are nonpolar (or neutral) by nature, but the end, which is a carboxyl group, is slightly positive (polar). When the positive carboxyl group on a fatty acid joins with the negative hydroxyl group on a glycerol, the polar ends combine, resulting in a nonpolar lipid - Non-polar lipids are insoluble in water (fat-soluble) - Mono- and diglycerides are partially soluble in water because of their hydroxyl group and soluble in fat because of their fatty acid chain(s) ➢ They are added to processed foods to keep mixtures of water and fats stable ➢ Examples: butter and margarine Phospholipids • A phospholipid is a glycerol base with 2 fatty acids and a phosphorus-containing acid attached - Fatty acids dissolve in fat (hydrophobic) and the phosphorus-containing acid dissolves in water (hydrophilic) - This allows for phospholipids to mix with both fat based and water based substances • Phospholipids are important because they - Are part of cell membranes and help transport fats in and out of cells, they make up the cell membrane and their hydrophilic ends stick out into both the interstitial fluid (outside) and the cytoplasm (inside), while their hydrophobic ends make up the interior of the membrane. This helps with allowing fats cross the cell membrane. - Help fats stay mixed in water-based solutions (acts like an emulsifier). Example: there are phospholipids in mayonnaise that keep it from separating. Sterols • Sterols are complicated molecules that are derived from lipids and include - Vitamin D - Steroid hormones including the sex hormones - Cholesterol, a part of every cell in the human body and is a precursor for many steroids Categories of Lipids 1. Molecular Structure • First condensation – how saturated the carbon chains are with hydrogen atoms - Can form zero, one, or many double bonds • Fatty acids that - Have the maximum number of hydrogen atoms are saturated - Do not have the maximum hydrogen atoms possible are unsaturated • Monounsaturated fatty acids: have one double bond - Found in olive and canola oils, almonds and walnuts • Polyunsaturated fatty acids: have 2 or more double bonds - Found in safflower, sunflower and corn oils • Most lipids contain both fatty acids • Fatty acids are grouped based on the amount of double bonds they contain. The less amount of double bonds they have, the more saturated they are with hydrogen atoms. • Saturated fatty acids: have no double bonds and have the maximum amount of hydrogen atoms. Generally, lipids found in animal sources are high in saturated fatty acids. Examples: butyric acid in butter and stearic acid in beef fat. • Monounsaturated fatty acids: have one double bond, examples: olive oil, canola oil, almond or peanut oil • Polyunsaturated fatty acids: have more than one double bond. The double bonds bend the fatty acid chain, making it more difficult for the fatty acids to pack tightly together, so that is why they are found in liquid form at room temperature. Examples: safflower oil, sunflower oil and corn oil. 2. Physical State • Fats are solid at room temperature - Butter, margarine, and shortening • Oils are liquid at room temperature - Vegetable oils • The number of carbon (length of carbon backbone) and hydrogen atoms (how saturated it is) determine the temperature a lipid will liquefy • Double bonds lower the temperature at which a lipid becomes solid • The melting point: the temperature at which a substance changes from solid to liquid. The melting point of a lipid is the temperature at which all the different fats and oils it contains (the fat mixture) are liquid - The more double bonds there are in the fatty acid, the lower the temperature at which it will liquefy (i.e. the lower the melting point). This is why lipids that are liquid at room temperature are normally those that contain one or more double bond in the fatty acid chains. - Polyunsaturated fats become liquid at lower temperatures than monounsaturated fats and saturated fats. - This lower melting point of polyunsaturated fats is due to its bent shape and the decreased amount of hydrogen atoms. - Fats are mostly made up of saturated fatty acids, where as oils are made more of monounsaturated and polyunsaturated fatty acids. This means that fats have more hydrogen atoms, and less double bond than oils, which makes them denser and require more energy to liquefy. This is why fats have a higher melting point and be solid at room temperature. - Since most substances contain a mixture of both solid fats and oils, the melting point of a lipid is the temperature at which all the different fats and oils (with different melting points) liquefy. It’s the point at which the entire lipid (fat mixture) liquefies. - Each type of lipid has a different melting point. For example, oleic acid is a monounsaturated fat. Much margarine has high oleic acid content, meaning they have high monounsaturated fat content. They will have a lower melting point and be much softer at room temperature than butter, which have a high content of saturated fatty acids. • Fats and oils come from different sources. Fats normally come from animal sources like butterfat in milk, pig lard, and tallow from sheep and cow. Where as, oils usually come from plant sources like soybean, peanut, canola, and olive... • Hydrogenation: is the process of adding hydrogen atoms to an unsaturated lipid to increase its saturation level - Hydrogenation is achieved through bubbling hydrogen through the liquid oil in the presence of a nickel catalyst. The double bonds in the oil with break, and the carbon atoms will then be free to pick up hydrogen atoms, and become saturated. - The process can be stopped at any time, and it is important to avoid making the oil completely saturated, as they become too brittle for use as solid fats. - Hydrogenation makes oils solid at room temperature, the result is a substance with a melting point higher than oils - Most common hydrogenated oil: soybean oil (sometimes palm oil and cottonseed oil are added as well) - Advantages of hydrogenation include: i. Longer shelf-life than oil or lard ii. Greater stability and lower production costs than lard iii. Faster dissolving and setting properties in chocolate production 3. Dietary Sources: triglycerides come from seven main groups of dietary sources, each group of triglycerides have similar molecular characteristics and physical properties. i. Milkfats contain short chain fatty acids of 4 to 12 carbons and come from cows, goats, and sheep. They are high in palmitic, oleic (MUFA) and stearic acids (PUFA). ii. Lauric acids: are the main component of a group of lipids found in the oils of fruits and seeds from tropical palm trees like coconut palm (make up 40-50% of their fatty acids). Lauric acid itself has a low melting point, but all the lipids in this group contain the most saturated fatty acids in plants. iii. Vegetable butters: come from seeds of tropical plants. Each molecule of lipid in vegetable butters contains at least one unsaturated fatty acid and one saturated fatty acid. All vegetable butters have similar molecular characteristics, and therefore have a very narrow melting range. One example is cocoa butter (used to make candies). iv. Oleic-linoleic acids: come from corn, peanuts, sunflowers, olives, cottonseeds and sesame seeds. This is the largest group of triglycerides. Each is made up of 20% saturated fatty acids. v. Linolenic acid (omega-3): is found in soybeans and wheat germ and requires refrigeration right after opening. This is because omega-3 is made of polyunsaturated fatty acids made of 3 double bonds per fatty acid chain, so they are at high risk of being attacked by oxygen (oxidation) if left out, which will make the food rancid (smell and taste bad). vi. Animal fats: are found in meats and poultry as seen by the marbling of meat. They contain LARGE amounts of fully saturated fatty acids, which make them have a very high melting point. vii. Marine oils: are highly polyunsaturated oils found in fish that contain 6 double bonds or more. They must be frozen or eat right away, because if left out, due to the high amount of double bonds they can go bad and develop an off-flavor very quickly (rancid). Marbling • The specks or streaks of fat in muscle tissue are called marbling – the fatter, the more marbling. • Marbling is an indication of high flavor and tenderness. • Steak that has a lot of fat or is highly marbled, generally is considered to be a higher quality than a steak that doesn’t • The white parts on steak is fat, the degree of fat determines its amount of marble • Higher amount of fat, higher marble quantity, tastes very good, higher quality steak • In moderation, a highly marbled steak is okay but not in big quantities Physical Characteristics: 1. Melting and Solidifying • Lipids melt and solidify over a temperature range because of the mixture of fatty acids • Unlike water, lipids do not have a specific melting point. This is because most lipids are a mixture of different kinds of fatty acids, and each fatty acid has a different melting point. Because of this, the lipids in a mixture will melt and also become solid at different temperatures. • The solidification point: is the temperature at which all lipids in a mixture are in a solid state • Lipids containing more saturated fatty acids than unsaturated fatty acids will have a higher melting point, this is because the number of hydrogen atoms and carbon atoms affect the melting point. The more saturated the fatty acid and the longer the length of the carbon chain, the higher the melting point • The solidification point is lower than the melting point for most lipids 2. Nonpolar Molecules • Equal or balanced sharing of electrons means lipid molecules are nonpolars • Substances that are nonpolar will readily mix with other nonpolar substances, and will also only dissolve in nonpolar substances • Water, a polar molecule, will not mix with oil because polar and nonpolar molecules are not attracted to each other • Lipid molecules are large molecules, and the variety and shape of fatty acids do not allow them to be tightly packed together, creating huge spaces between fatty acids. But, since fatty acids are nonpolar, and water molecules are polar, water molecules cannot slip through and fill up the spaces between fatty acids. This is why lipids have a low density compared to water, because of the huge spaces between fatty acids with nothing to fill it in, so there is little mass per volume (density). This is why oil floats on water. LIPIDS ARE LESS DENSE THAN WATER. 3. Tendency to Deteriorate a) Auto-oxidation: is a complex chain reaction that starts when lipids are exposed to oxygen
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