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Lipids

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Lipids (generally soluble in organic solvents) Plant sources (olive, palm) Animal sources (butter, lard, tallow) Oils and Fats Sterols Waxes (monoesters) Triacylglycerol Food 98 %

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Lipids Definition of Lipids Naturally occurring biological substances made primarily of C, H, and O of pronounced hydrophobicity that are soluble in organic solvents but have limited solubility in water Petroleum distillates (e.g. hexane) Chloroform Ethers (e.g. diethyl ether) Alcohols Term Fat vs. Oil, chemically identical (both lipid) but Fat = solid at room temp. Oil = liquid at RT Special case the term fat can be used to refer to oils from food sources to avoid confusion with other oils such as petroleum oils Lipids have more C and H than carbohydrates, which is why they are said to be energy dense, when utilized 2.25 times more = 9 kcal/g vs. 4 kcal/g.

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Lipids Biological roles A. Structural - found in membranes - protective barriers B. Regulatory - steroids/prostaglandins (hormones) - phospholipids C. Storage - triglyceride is an energy storage molecule D. Vitamins - fat-soluble - precursor molecules

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Lipids Role in foods A. Calories (kcal) American Heart Association recommends energy from fat < 25- 35 % of all calories ideal - satiety B. Essential fatty acids (cant synth.) - linoleic acid, linolenic acid C. Flavor - lipid soluble compounds or off- flavors D. Texture mouth feel & appearance E. Color - carotenoids F. Heat transfer medium (frying) v.s.

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Fat Content of Some Selected Foods Food% FatFood% Fat Brazil nuts67Hamburger20 Walnuts61Avocado16 Almonds54Ice Cream13 Peanuts50Tuna, canned8 Sunflower seeds 47Poultry, dark meat 7 Pork roast30Salmon6 Cheese30Whole milk4 Beef roast25Poultry, light meat 4 Ham, cured22Shredded wheat cereal 2 6

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Lipids Classification of lipids (structure) 1) Simple lipidsSimple lipids Mono, Di, and Triacylglycerols Account for 98 % lipids in foods Waxes 2) Compound lipids - some polarityCompound lipids Phospholipids Glycolipids 3) Derived lipids often hydrolyzed 1&2Derived lipids Free fatty acids Sterol esters Tocopherol (Vit-E) -carotene Triglyceride Glycerol backbone Fatty acids

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Lipids 1) Non-polar lipids (neutral lipids) Fatty acids Mono-, di-, & triacylglycerols Waxes Sterols Carotenoids Tocopherols Classifications of lipids (polarity) 2) Polar lipidsPolar lipids Glycerophospholipid Glyceroglycolipid Sphingophospholipid Sphingoglycolipid contains sphingoid base (nitrogen) and sugar

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Lipids Structure & properties of fatty acids Fatty acid are composed of a hydrocarbon chain with methyl group (CH 3 ) on one end and a carboxyl group (COOH) on the other. Basic properties common to most fatty acids 1. Most are even carbon # 2. Most are monocarboxylic acids 3. Most are part of triacylglycerides (triglycerides)

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Lipids NOMENCLATURE 3 ways to name 1) Short - # of Carbons, # double bonds 2) Common reflective of source, little if any structural info 3) Systematic follows IUPAC rules 1. Number of carbonsNumber of carbons C4-C24 most common E.g. C8 = octa C12 = dodec, C18 = ? 2. Saturation = saturated with H bonds (no double bonds) Unsaturated (double bonds) Mono (1 = bond) Poly (>1 = bond)

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Lipids 2. Saturation Systematic Naming No double bond = Anoic E.g. C18:0 One double bond = Enoic E.g. C18:1 Two double bonds = Dienoic E.g. C18:2 Three double bonds = Trienoic E.g. C18:3 3. Geometric configuration of double bonds Cis vs. Trans Has an influence on the fatty acid backbone structure

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Lipids 4. Position of double bonds Delta ( ) system - count # of carbons to the = bond from the COOH end E.g. 9-octadecenoic acid Means: a) C18 = octadecenoic b) 1 double bond = octadecenoic c) double bond is 9 carbons from the COOH end Omega ( ) system - count # of carbons to the = bond from the CH 3 end used for abbreviations of fatty acids E.g. 9-octadecenoic acid would be C18:1 9 What about all-cis-9,12,15-octadecatrienoic? Delta = same as systematic = 9,12,15-ocatadecatrienoic acid Omega = C18:3 3 (1 st double bond is at C16, 3 carbons from the methyl end) -3, -6 and -9 the most common Methyl (CH 3 ) end dictates biological activity (more commonly used in nutrition and food science) -3 essential b/c our bodies are not able to synthesize fatty acids that have double bonds between an existing double bond and the methyl end

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Lipids Commonly Encountered Major fatty acids in foods Saturated Palmitic (16:0) Stearic (18:0) Monoenoic Oleic (18:1 9) Dienoic Linoleic (18:2 6) 9, 12 - common in plants; some in animal Trienoic Linolenic (18:3 3) 9, 12, 15 Tetraenoic Arachidonic (20:4 6) - 5, 8, 11, 14 - part of membrane phospholipids

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Lipids Structure/function properties of fatty acids 1.Length of fatty acids Longer chain length leads to increase in melting point and gives more stable fat crystals Classes: C4 C8 - liquid @ room temperature (20-25C) These are water soluble good emulsifiers C10 - C14 - viscous @ room temperature C16 - C26 - solid @ room temperature For example: C6:0 MP = -2C C10:0 MP = 31.5C C16:0 MP = 63C

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Lipids Factors affecting the properties of fatty acids 2. Double bondsDouble bonds An increase in # of double bonds decreases the melting point Example: 18:0 = 71.2C 18:1 = 16.3C 18:2 = -5C 18:3 = -11C

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Lipids Factors affecting the properties of fatty acids 3. Cis vs. TransCis vs. Trans Cis has lower melting point than Trans Cis produces a kink in the fatty acid chain which creates a more open fatty acid crystal structure Kink Melting Point 18:1c15C 18:1t44 C 18:2c-5 C 18:2t29 C 18:3c-11 C 18:3t71 C

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Lipids >98% of fatty acids in food products are found as triacylglycerols (also the largest group of neutral lipids) Structure: Fatty acid esters of glycerol (three carbon alcohol) Most triglycerides are mixed (i.e. contain different fatty acids) Triacylglycerol Glycerol backbone Fatty acids The structure and properties of triacylglycerols

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Lipids We use stereochemical numbering system (sn) to indicate the position of the fatty acids on the glycerol backbone If you have 20 fatty acids to chose from then you have 20 3 (i.e. 8000) possible numbers of different triacylglycerols Complete randomization at positions seldom observed

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Lipids Arrangement of fatty acids on triacylglycerides 1. Not random (usually) 2. Specificity controlled 3. General pattern The arrangement can significantly affect physical properties of fat LC : Long chain; SC: Short chain PositionPlantMammalMilkBirdFish 1SSSSS-LC 2UUSUU 3ULCU or SCS or ULC

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Lipids Solid fat index (SFI) Reflects percentage of oil that is solid Therefore, the rest is liquid Curve shows that tallow has a broader melting point while cocoa butter has a uniform sharp melting point (desirable)

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Lipids One can produce triacylglycerols with specific properties Examples: 1. Medium chain triglycerides (MCTs)Medium chain triglycerides (MCTs) C8:0 and C10:0 fatty acids (from palms, coconuts, milk) Hydrolyze, fractionate, re-esterify with glycerol Metabolized in the liver (not through the gut) and thus used more for energy than for deposition as fat and thus have fewer kcal (8.3 kcal/g) Used as flavor, color and vitamin carriers in foods and pharmaceuticals; also in reduced fat applications MCTs are bland

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Lipids 2. Salatrim (short and long chain triacylglyceride molecule), Benefat (Danisco) Triacylglycerol made to contain a short chain (C2:0, C4:0 or C6:0) and a long chain (C18:0) fatty acid esterified to glycerol backbone Get only 5 kcal/g because: Get less energy from short chain fatty acids Stearic acid (C18:0) is incompletely absorbed Can be custom made to suit a variety of applications but it is not suitable for frying 3. Caprenin (Proctor and Gamble Co.) Has C8:0, C10:0 and C22:0 Only 5 kcal/g due to partial absorbance of behenic acid (C22:0) Confectionary applications

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Lipids Important Compound Lipids 1. PhospholipidsPhospholipids Make up cellular membranes Lipid molecules that contain a phosphate group attached to a functional group Have both hydrophobic (fatty acids) and hydrophilic (phosphate and functional group) portions Good emulsifiers May have a protective effect against ulcers (milk PL)

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Lipids 2. GlycolipidsGlycolipids Contain at a minimum one sugar Some may also have a phosphate amino group (glycosphingolipids) Found in all tissues of animals Have same solubility characteristics as regular lipids 3. Sterols Made of four fused hydrophobic rings with a hydrophilic OH group Not so important as a food ingredient but important for dietary reasons Cholesterol mostly in animal foods Can contribute to coronary heart disease (arteriosclerosis) 300 mg/day the recommended intake limit

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Lipids 4. Fat substitutes Sucrose fatty acid polyesters Olestra, brand name Olean (Procter and Gamble Co.) FDA approved for use in frying oils (snacks) in 1996 6-8 fatty acids (>C12) esterified to sucrose Caloric free due to its bulky structure and because lipases cannot hydrolyze it May lead to loss of fat soluble vitamins and can give diarrhea FDA now requires warning labels Sucrose and polyol fatty acid esters 1-3 fatty acids esterified to sucrose or a polyol (e.g. sorbitol) Have caloric value (polyol fatty acid esters only about 1.5 kcal/g) Used as emulsifiers and stabilizers

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Lipids Functional Properties

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Lipids Functional Properties Crystallization Solid fats are most all in a crystal form Cooling liquid fat (oil) results in it loosing heat and molecular motion is decreased Fat molecules come in close contact and the non-polar nature of the fatty acids align via strong hydrophobic interactions (attractive forces) and a crystal is formed Simple triacylglycerides (identical FA on the glycerol) strong bond and tightly packed crystals Mixed triacylglycerides (the FA are different) Weaker bonds and packing is less = weaker and more numerous crystals Most fats fall into this category The actual nature and thus the functionality of the crystals formed is highly influenced by the type of fatty acids that are on the glycerol backbone

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Lipids Functional Properties -crystals Formed on rapid cooling Randomly associated triglycerides Size < 1m Very delicate, needle like, smooth, shiny and fine-grained texture Unstable due to their disorder Heating transforms this form to the two other higher stability forms ( or )

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Lipids Functional Properties -crystal Close packing Crystal axes alternate Intermediate of and -crystals Many food fats are processed in this manner to produce a fine-grained texture (more grainy than but less than ) The desired form for many processed fats Shortenings & Margarine Ideal texture Good at incorporating air

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Lipids Functional Properties -crystals Have more order and greater packing than the -crystals Most stable Very large coarse crystals with grainy texture Size = 25-45 m This crystal is the ideal form for some fats Chocolate fat (cocoa butter ) Mp = 35-36 C Brittle/firm until eaten Develops a glossy sheen fat bloom Improper storage leads to chocolate bloom (change in crystal structure)chocolate bloom

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Lipids Functional Properties Most of the crystal forms are interconvertible Polymorphism Determined by: Fatty acid type (length, unsaturation and cis vs. trans) Distribution of fatty acid on glycerol backbone Rate of cooling Agitation Storage conditions LIQ. '

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Lipids Functional Properties Crystal structure and melting point relationship With longer fatty acids there is more packing and stronger crystals result MP Fatty acid heterogeneity (nonuniformity) = more packing and MP Fatty acid heterogeneity = less packing and MP double bonds (cis) kink in the fatty acid = less packing (bulkier crystal) and MP (trans form = MP) For the same fatty acid the melting point follows this order: > >

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Lipid crystal forms summary 33 random weakest association between chains = some order most association between chains = most order some association between chains =

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Lipids Functional Properties Hydrogenation Very important chemical modification method The objective is to chemically reduce unsaturated fatty acids into saturated fatty acids (i.e. remove double bonds) The result Functional properties of the oil/fat is modified to ones desired Liquid oil solid (vegetable oil margarine) Higher melting point (due to less double bonds and trans fatty acids) By proper control of the reaction one can get a range of textures (SFI profiles) Main reasons to perform hydrogenation 1. Impart specific physical or chemical property 2. A cheaper oil/fat source can be converted to mimic a more expensive oil/fatcheaper oil/fat source (e.g. using cottonseed oil and hydrogenate to a product similar to cocoa butter) 3. Give product unique functional characteristics 4. Increased stability of oil/fat = shelf life Fewer double bonds and more trans fatty acids lead to less oxidation problems Reduced nutritional value More saturation and more trans fatty acids Most hydrogenated oils/fats are partially hydrogenated (trans, cis, removed double bonds) Canola and Soybean oil blend

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The reaction factors: Oil source type of FA and their position has a dramatic effect on the final oil/fat properties High temperature H 2 gas bubbled into oil under pressure Agitation Catalyst usually Ni on an inert silica support removed by centrifugation or filtration Possible reactions (example with linolenic acid) 18:3 18:2 18:1 18:0 Lipids Functional Properties These will have a great impact on which of the double bonds in a fatty acid are hydrogenated and which will migrate in the fatty acid

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The hydrogenation process: A.Oleic acid (18:1) (letters correspond to figure) B.Nickel catalyst adsorbs onto the double bond C.Hydrogen atom binds to a carbon leaving one Ni bond At this point the reaction can go in several possible directions: D.1. Another hydrogen atom can bind to the second carbon and give a saturated fatty acid (1 in figure) 2. A H-atom may be lost from the carbon giving 2 unsaturated trans (or cis) isomers (2 & 4 in figure) 3. The original H-atom may be lost and a new H-atom can come in to form a trans double bond (3 in figure) 4. The fatty acid may detach from the catalyst and thus lead to no change in its structure (A in figure) 12341234

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Lipids Functional Properties Hydrolytic rancidity Happens faster with extracted fats/oils General mechanism involves the cleavage of the fatty acids from the glycerol backbone (is a hydrolysis = cleavage & addition of water) Usually only the fatty acids at position 1 and 3 (outside fatty acids) Indication of quality loss in foods and fats/oils Free fatty acids are volatile (especially short chain) and can exert an unfavorable odor and flavor Free fatty acids are more prone to lipid oxidation reactions (i.e. to become rancid) Free fatty acids may react with other food components, e.g. can make proteins lose their functionality

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Lipids Functional Properties A. Chemical hydrolysis Catalyzed primarily by heat (225-280 C) Deep fat frying Viscosity increases Foaming increases Fat degradation products polymerize Color darkens Off odors/flavors form Smoke point decreases % FFA SMOKE PT ACID VALUE 0.01450F 0.02 1 320F 1.9 10260F 19 100200F 190 EXAMPLE

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Lipids Functional Properties B. Enzymatic hydrolysis Caused by enzymes that are naturally present in the foods native Lipases Lipases can also be introduced via contaminating microorganisms (can be intentional ex. cheese making) Thermal processing can be used to inactivate Lipases (generally above 60 C but time & temp factor in) Grains, flour a w FFA loaf volume Fish frozen storage FFA (due to phospholipases) toughening water holding ($$) flavor and color problems (rancidity) Dairy Lipases specific for sn-3 Agitation, pumping, etc favors the reaction Inactivated by high heat Problems 1. Off-flavors 2. Poor churning (mono and diglycerides - emulsifiers) 3. Poor cheese (FFA inhibit the enzyme Rennin)

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Lipids Functional Properties Oxidative rancidity Oxidative rancidity is the result of chemical reactions usually involving O 2 and lipid Referred to as autoxidation since it is a autocatalytic process which reaction rate increases as reaction proceeds Leads to major quality problems in foods 1. Off-flavors 2. Color change (browning, loss of pigments) 3. Degradation of nutrients Essential fatty acids Essential amino acids Vitamins 4. Toxicity?

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Lipids Functional Properties Rate of oxidation reaction affected by Fatty acid composition (saturated vs. unsaturated) Degree of unsaturation Rxn rate increases with degree of unsaturation (18:0, 18:1, 18:2, 18:3) except for conjugated series Presence of pro- and antioxidants Pro-oxidants catalyze oxidation Metals, light Antioxidants delay oxidation Synthetic BHA, BHT, polyphenols Partial pressure O 2 Low pressure minimizes oxidation, less O 2 ! (Vacuum packaging, flushing) Storage conditions Temperature Light Water activity pH

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Lipids Functional Properties The three steps of autoxidation A Hydrogen atom is abstracted from the fatty acid (R) by an initiator and a fatty acid free radical (missing an electron) is formed O2O2 A peroxyl free radical (ROO) is formed in the presence of O 2 Hydroperoxide (ROOH) is formed in the presence of another FA. Rxn repeats rapidly! The propagation step is terminated by the reaction between two radicals

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Lipids Functional Properties Initiation Induced by: Light (Visible, UV-rays, -radiation) Chlorophyll (sensitizer) 1 O 2 = singlet oxygen Heme compounds (hemoglobin and myoglobin in muscle foods) Metal compounds Only low concentration needed (0.1 ppm) From soil (plants), animal (needed nutrient), metallic processing & storage equipment M + n+ RHM [n-1] + R H+H+ ++ Cu 2+ Fe 3+ Cu + Fe 2+ Direct rxn of a metal with substrate (RH)

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Molecular mechanism of initiation 4 isomers are formed C H 2 C H 2 C H C H C H 2 C H 2 Attack is adjacent to the double bond CHC H 2 C H C H CHC H 2.. Radical can potentially form at either site

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Initiation of linoleic acid (18:2) (1) (2)

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Propagation of linoleic acid (18:2) peroxyl radical formation stability (1) (2)

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ROOH (primary products) can be very unstable and decompose to form secondary oxidation products: Acids Alcohols Aldehydes (acetaldehyde) Carbonyls Ketones These are responsible for the rancid odor/flavor of oxidized fat Propagation of linoleic acid (18:2) hydroperoxide formation (OOH)

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You can follow the progress of lipid oxidation chemically and sensorially Time Sensory detection Hydroperoxides Aldehydes

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Lipids Functional Properties For the exam your should be able to predict how many fatty acid radicals can form for oleic, linoleic and linolenic acid and know where they would be located Where would the first attack be located? Where else could attack occur? How many radicals can form per lipid? Which radical would be the most important? Stability?

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Lipids Functional Properties Prevention/retardation of autoxidation Remove oxygen Vacuum or modified atmosphere packing Reduce light E.g. use opaque packaging, filters, cans Remove catalysts (e.g. metals) Chelators (EDTA; citric acid; phosphoric acid) Avoid high temperatures Use less unsaturated fatty acids or use saturated fatty acids No double bonds far less to none oxidation Hydrogenation fewer double bonds Use antioxidants EDTA Cu 2+ EDTA in soda http://en.wikipedia.org/ wiki/Vault_%28soft_drin k%29 http://www.squirtsoda.c om/

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Lipids Functional Properties Antioxidants Can be very effective in slowing down lipid oxidation They function by inhibiting/delaying the propagation chain reaction by scavenging the free radical intermediates Some foods have natural antioxidants but to stabilize them even further we add both synthetic and natural antioxidants to them Propagation rxn Antioxidants Versus Propagation rxn

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Lipids Functional Properties Common synthetic antioxidants Vitamin E (tocopherol) - A natural antioxidant - Common antioxidants Vitamin C (ascorbic acid) - A natural antioxidant -

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Lipids Functional Properties "Quench free radicals" - therefore prolong induction period - will eventually oxidize Induction period

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Lipids Functional Properties Emulsions Consist of 2 immiscible phases Dispersed phase (also called discontinuous phase) Continuous phase Oil and water These phases do not like each other and strive to separate Oil in water (o/w) - milk, salad dressing Water in oil (w/o) - butter, margarine

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Lipids Functional Properties Macro-emulsion Particle size 0.5 - 100 m Milky due to light scattering Micro emulsion Particle size 0.01 - 0.5 m Clear Can have high viscosity - higher than equivalent volume of o/w or w/o

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Lipids Functional Properties Emulsion formation To form an emulsion the dispersed phase needs to be divided into small particles and then needs to be stabilized You need input of energy to form the emulsion Without stabilizers (emulsifiers) the emulsion will rapidly break down since the small droplets will coalesce and form larger droplets High shear (more work) Large droplets (salad dressing) Small droplets (milk) http://www.youtube.co m/watch?v=oBbWJX EZoRQ - put sound off

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Lipids Functional Properties Emulsion stabilization To provide the emulsion with long term stability one needs to employ emulsifiers along with high energy input Emulsifiers decrease the tension (surface tension) between the two phases They can do this by having a hydrophobic and hydrophilic character at the same time

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Lipids Functional Properties Common emulsifiers Mono and diacylglycerides Detergents (Tween) Phospholipids (lecithin) Proteins All have polar (e.g. OH) groups and non polar (e.g. fatty acid or hydrophobic amino acids) groups OH Oil droplet Monoglyceride H 2 O phase

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Lipids Functional Properties The effectiveness and function of an emulsifier is defined by its HYDROPHILIC - LIPOPHILIC balance (HLB) HLB = % weight of hydrophilic portion of emulsifier 5 Scale goes from 1-20 1 = entirely non-polar (hydrophobic) 20 = entirely polar (hydrophilic)

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Lipids Functional Properties Emulsifier classification HLB 1 - 8= hydrophobic HLB 8 - 11= intermediate HLB 11 - 20= hydrophilic HLB values for industrial/food applications 3 - 6 HLB= w/o emulsifiers 8 - 18HLB= o/w emulsifier