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Food lipids. Lipids are organic substances that are relatively nonpolar. Structures Fats and oils are chemically known as triacylglycerols, or triglycerides (TGs). TGs are triesters of glycerol and fatty acids (see fig). Glycerol: simple organic molecule (sometimes called glycerine) is a 3-carbon Molecule containing three alcohol groups. Fatty acids: organic molecules that contain chains of carbon bound to hydrogen, Plus an acid (COOH) at one end and a methyl group (CH3) at the other. Ester bonds hold fatty acids to glycerol, joining the OH groups of glycerol to The COOH group of fatty acids (with the loss of water)

Chemical structure of TG. Note the ester bonds.

Saturated and unsaturated fats Fats and oils are mixtures of fatty acids (FA) differing in chain length and degree of unsaturation. Saturated: FA that does not contain any carbon to carbon double bonds. Unsaturated: FA contains carbon to carbon double bonds (see fig). This can be monounsaturated (having only 1 double bond in the carbon chain) or polyunsaturated (having 2 or more double bonds in the chain). (see table). HOT: Omega 3 fatty acids: ? (a) Saturated or unsaturated?

Melting point Definition: A temperature at which a solid is converted into liquid. Fat molecules exist in crystalline forms, and the strength of the bonding forces between fatty acids in a fat crystals determine its melting point. Fatty acids exhibit unique melting points. Thus TGs with their array of 3 fatty acids exhibit melting points based on which fatty acids are Present. Thus types of fatty acids determine whether the TGs will be a solid, liquid or even plastic (soft and pliable due the mixture of solid and liquid TGs) at room temperature.

Melting point is determined by many factors such as fatty acid chain length And degree of unsaturation. All else equal? Short chain show lower melting point than long chain Saturated have higher melting points than unsaturated. When a mixed fat is heated e.g. stearic-oleic-palmitic, the melting is often gradual and over a range of temperature (rather than one distinct measureable value), reflecting impure fatty acid composition. Cis configuration has lower melting point than trans configuration (see table). Another fat property: it carries flavor compounds, plus the fat molecules themselves actually contribute to flavor.

Cis and trans fats Unsaturated fatty acids come in two configurations, defined by their structure At the double bonds. Cis: the hydrogen atom bonding to the C=C are located on the same side of the Double bond (see fig) Trans: the hydrogen atoms attached to the carbon atoms of the double bond are Opposite each other. Most cases; fatty acids existing in food in cis rather than trans configuration. These structural differences affect properties such as melting point.

Cis and trans fatty acids and melting point (MP). The MP of saturated fat is higher than for unsaturated fat. The trans configuration results in a higher melting point than the cis. Fatty acid name Fatty acid type Notation MP (C) Stearic saturated C18:0 70 Oleic Cis-unsaturated C18:1 19 Elaidic Trans-unsaturated 43 Thus, Oleic is liquid at room temperature whilst elaidic is solid.

Polar lipids These lipids have polar groups i.e. potentially charged hydrophilic groups containing oxygen or nitrogen in their structure, and are thus have a degree water solubility. These lipids are both lipophilic and hydrophilic. Example: Lecithin or phosphatidylcholine. Thus in food, Lecithin can function as emulsifier (see fig): choline is the hydrophilic part. Other lipids: Pigments and waxes.

General structure of a phospholipid (lecithin): it is amphipilic, hydrophobic hydrophilic General structure of a phospholipid (lecithin): it is amphipilic, Thus can function as emulsifier!

Chemical reactions of lipids Most important are: Fractionation, hydrogenation, hydrolysis, inter-esterification, oxidation. (a) Fractionation Splitting of oil into its higher melting point components (e.g. stearic acid) And lower melting point components (e.g. oleic acid), thus allowing Crystal portions to be separated from the liquid portion. Example: An oil can be fractionated to produce a good oil to be used for Frying. The same oil can be fractionated to produce a good plastic fat For shortening or improved spreadability right out of a fridge..

Hydrogenation Forced addition of hydrogen atoms to the unsaturated bonds in an unsaturated fat. The process raises the fat’s melting point, thus harden liquid oil into semi-solid fats. Fas with most double bonds hydrogenate more quickly than the less saturated ones. Process can be controlled; e.g. vegetable oil into butter, while retaining high linoleic and oleic acids. Side effect of hydrogenation: percentage of cis-unsaturated is changed to trans unsaturated FAs. In trans form, linoleic acid (a dietary essential FA) becomes biologically inactive! So what? Since unsaturation is reduced, the tendency of fat to oxidize is also reduced, making the hydrogenated frying oil more stable (longer frying life), and higher smoke points. (Industry loves this)!

Hydrolysis To separate FAs from the glycerol portions of a TG molecule. Requiring heat and water molecules. TG + 3H2O + heat, to yield 3 FAs + glycerol. Continouos heating will yield acrolein due to glycerol breakdown. Acrolein is odorous and irritating fumes in the smoke of an overheated TG. Another term: hydrolytic rancidity. Stored fat become rancid by the hydrolysis reaction with water. Thus Free FA is produced through hydrolysis. For every molecule of water That combines with TG, one free FA is liberated.

Rancidity is governed by the size of liberated FAs. Short chain FA: objectionable flavors and odors associated with hydrolytic rancidity. Long chain FA? They do not contribute to off flavor and odors. If butter is left at room temperature for too long, hydrolytic rancidity causes the butter to smell and taste bad because butter TG (butterfat) has high content of butyric (C4) and caproic (C6) acids. Interesterification Removal of FAs from glycerol and their subsequent rearrangement or recombination into numerous configurations, most of which differ from the original TG molecule.

Thus It is possible to modify FA distribution within a TG, that yield TG with Desired properties. E.g. Industrially, hard fat is interesterified to produce mixed glycerides that offer improved creaming. Recall: ester bonds that attach FA to glycerol, thus the term interesterification. It is this bonds that are broken and then reformed during the reaction. Interesterification creates new FA distribution of glycerides molecule with Different melting point and crystallization behavior. Coarse-crystal, grainy-textured fat like lard can be changed into a lower Melting point, finer crystal size, more plastic, and smoother texture and Mouthfeel. It is even possible to produce TG with lower calories! A B FA A and FA B Showing random Distribution within A single TG molecule.

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Oxidation Oxygen reacts with the double bonds of unsaturated FAs, yielding small organic compounds, which in turn generate undesirable “rancid” odors in foods that contain oxidized fats or oils. Rancid foods are not edible. The original FAs along with their nutritional value are lost.