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Lipids

Saturated Fatty Acids

8 7 6 5 4 3 2 1 O CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 C OH Oct a noic Acid

Unsaturated Fatty Acids

8 7 6 5 4 3 2 1 O CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 C OH 3 - Oct en oic Acid 8 7 6 5 4 3 2 1 O CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 C OH 3, 6 - Octa dien oic Acid

Fatty Acids Melting Points and Solubility in Water Melting Point z Solubility in H 2 O 2 Fatty acid chain length

Characteristics of Fatty Acids

Fatty Acids M.P.(C) mg/100 ml in H 2 O C4 C6 C8 C10 C12 C14 C16 C18 - 8 - 4 16 31 44 54 63 70 970 75 6 0.55

0.18

0.08

0.04

Where Do We Get Fats and Oils?

    Derived from plant and animal sources Several commercial processes exist to extract food grade oils Most are refined prior to use During oil refining, water, carbohydrates, proteins, pigments, phospholipids, and free fatty acids are removed.

  In general, fat and oil undergo four processing steps:     Extraction Neutralization Bleaching Deodorization Oilseeds, nuts, olives, beef tallow, fish skins, etc.  Rendering, mechanical pressing, and solvent extraction.

Lipid Oxidation

Effects of Lipid Oxidation

 Flavor and Quality Loss  Rancid flavor  Alteration of color and texture  Decreased consumer acceptance  Financial loss  Nutritional Quality Loss  Oxidation of essential fatty acids  Loss of fat-soluble vitamins  Health Risks  Development of potentially toxic compounds  Development of coronary heart disease

  

LIPID OXIDATION and Antioxidants

Fats are susceptible to hydrolyis (heat, acid, or lipase enzymes) as well as oxidation. In each case, the end result can be

RANCIDITY

.

For oxidative rancidity environment a food.

must

to occur, molecular oxygen from the interact with

UNSATURATED

fatty acids in The product is called a peroxide radical, which can combine with H to produce a

hydroperoxide

radical.     The chemical process of oxidative rancidity involves a series of steps, typically referred to as: Initiation Propagation Termination

Simplified scheme of lipoxidation

R H C H H C H H C C H R + Catalyst R H C H H C H C H C * R + Oxygen R H C H H C H C H C O O R

Initiation of Lipid Oxidation

 There must be a catalytic event that causes the initiation of the oxidative process   Enzyme catalyzed “Auto-oxidation”  Excited oxygen states (i.e singlet oxygen): 1 O 2  Triplet oxygen (ground state) has 2 unpaired electrons in the same different orbitals.

spin in  Singlet oxygen (excited state) has 2 unpaired electrons of opposite same orbital.

spin in the  Metal ion induced (iron, copper, etc)  Light  Heat  Free radicals  Pro-oxidants  Chlorophyll  Water activity

Considerations for Lipid Oxidation

 Which hydrogen will be lost from an unsaturated fatty acid?

 The longer the chain and the more double bonds….the lower the energy needed.

Formation of a Peroxyl Radical Oleic acid Radical Damage, Hydrogen Abstraction

Propagation Reactions

Initiation Ground state oxygen Peroxyl radical Hydroperoxide New Radical Hydroperoxide decomposition Start all over again… Hydroxyl radical!!

Secondary Products: Aldehydes

Mechanism of Photooxidation

O 2

or

Singlet Oxygen Oxidation

1 O 2 HOOC OOH HOOC OOH HOOC OOH HOOC OOH HOOC

Autoxidation

+ HOOC 8 9 10 11 H 12 13 14 8 9 10 11 12 H + 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 O 2 8 OO 10 9 11 12 13 14

Singlet Oxygen Oxidation

1 O 2 HOOC HOOC OOH OH O Nonanal

Chemical Tests for Oxidation

Lipid Oxidation Hydrolysis Peroxide Value Oxidation Tests

LIPID OXIDATION

35 30 25 20 15 10 5 0 1

Lipid System Under Oxidizing Conditions

2 3 4 5 Time 6 7 8 9 Oxygen Uptake Peroxides Secondary Products

Free Fatty Acids (FFA’s)

  Degree of hydrolysis (hydrolytic rancidity) High level of FFA means a poorly refined fat or fat breakdown after storage or use.

l

Peroxide Value

Measures peroxides and hydroperoxides in an oil which are the primary oxidation products (usually the first things formed).

l The peroxide value measures the “ present status of the oil ”. Since peroxides are destroyed by heat and other oxidative reactions, a seriously degraded oil could have a low PV.

Peroxide Value

l l l KI + peroxyl radical yields free Iodine (I 2 ) The iodine released from the reaction is measured in the same way as an iodine value.

I 2 in the presence of amylose is blue. I 2 is reduced to KI and the endpoint determined by loss of blue color.

4I + O 2 + 4H 2I 2 + 2H 2 O

Thiobarbituric acid (reactive substances) TBA OR TBARS Tests for end products of oxidation – aldehydes , Malonaldehyde (primary compound), alkenals, and 2,4-dienals l A pink pigment is formed and measured at ~530 nm.

TBARS is firmly entrenched in meat oxidation research and is a method of choice. TBARS measure compounds that are volatile and may react further with proteins or related compounds.

High TBA = High Oxidative Rancidity

l

HEXANAL Determination

Good indictor of the end products of oxidation (if there are any). l l Standard method in many industries. Aldehyde formation from lipid oxidation.

l

Nonenal is also a common end-product

Conjugated Fatty Acids During oxidation, double bond migration occurs and conjugated fatty acids are formed.

They absorb light efficiently and can be monitored in a spectrophotometer.

R C C C C C C C C R

TECHNIQUES OF MEASURING OXIDATIVE STABILITY Induction Period: is defined as the length of time before detectable rancidity or time before rapid acceleration of lipid oxidation

Iodine Value

 Measure of the degree of unsaturation in an oil or the number of double bonds in relation to the amount of lipid present  Defined as the grams of iodine

absorbed

100-g of sample.

per  The higher the amount of unsaturation, the more iodine is absorbed.

 Therefore the higher the iodine value, the greater the degree of unsaturation.

Iodine Value

 A known solution of K I is used to

reduce

excess I Cl (or IBr) to free iodine R-C-C = C-C-R + I Cl  R-C-C I - C Cl -C-R + [Excess] (remaining) I Cl  Reaction scheme: I Cl + 2K I  K Cl + K I + I 2  The liberated iodine is then titrated with a standardized solution of sodium thiosulfate using a starch indicator  I 2 + Starch + thiosulfate = colorless endpoint (Blue colored)

Iodine Value

Used to characterize oils:  Following hydrogenation  During oil refining (edible oils)  Degree of oxidation (unsaturation

decreases

during oxidation)  Comparison of oils  Quality control

Iodine value: g absorbed I 2 / 100 g fat

Iodine Value of some oils (Table 14-2) Source

Beef Tallow Olive, Palm, Peanut Corn, Cottonseed Linseed, Soybean, safflower, conola Fish

I 2 Value

<50 < 100 100 - 130 > 130 >150 Highly saturated High in 18:1 High in 18:1 and 18:2) 18:1, 18:2, 18:3 18:1, 18:2, 18:3 (longer chains)

Chemical Tests

Saponification Value

Saponification Value

Saponification is the process of breaking down or degrading a neutral fat into glycerol and fatty acids by treating the sample with alkali.

Heat Triacylglyceride ---> Fatty acids + Glycerol KOH Definition: mg KOH required to titrate 1g fat (amount of alkali needed to saponify a given amount of fat) Typical values: Peanut = 190, Butterfat = 220

Lipid Oxidation

Primary Drivers

 Temperature basic rxn kinetics  Water Activity  Both high and low Aw  At low Aw, peroxides decompose faster and metal ions are better catalysts in a dry environment  Metal Ions -catalysts  Light -energy source  Singlet Oxygen - ROS, highly electrophilic  Reacts 1,500 times faster at C=C than ground state O 2  Enzymes ie. Lipoxygenase (LOX)

Implications to food products

 A major cause of quality deterioration  Develop rancidity in raw or fatty tissues  Produces WOF in cooked meats  Oxidized flavors in oils  Loss of functional properties  Loss of nutritional values  Formation of toxic compounds  Forms colored products

Production of toxic compounds

 Many secondary by-products of lipid oxidation are potential carcinogens  Hydroperoxides are known to damage DNA  Carbonyl compounds may affect cellular signal transduction  Aldehydes: 4-OH-nonenal and malondialdehyde  Epoxides and hydrogen peroxide by-products are known carcinogens

Lipid Oxidation

Factors affecting the development of lipid oxidation in foods

 Fatty acid compositions  Oxygen, free radicals  Pro-oxidants  Antioxidants and additives  Processing conditions of food  Irradiation  Cooking  Grinding, cutting, mixing, restructuring etc.

 Packaging  Storage: time and conditions

Bond Energy and Lipid Oxidation

Types of Fatty Acids and Oxidation Rates

 As # of double bonds increases # and reactivity of radicals increases

Type of Fatty Acid

18:0 18:1Δ9 18:2Δ9,12 18:3Δ9,12,15

Rate of Reaction Relative to Stearic Acid

1 100 1200 2500

Lipid Modifications

Hydrogenation  Method Oil is heated with catalyst (Ni), heated to the desired temperature (140-225°C), then exposed to hydrogen at pressures of up to 60 psig and agitated.

Hydrogenation - Conditions  Starting oil must be:  Refined  Bleached  Low in soap  Dry  The catalysts must be:  Dry  Free of CO 2 and NH 4

Hydrogenation  Hydrogenation Limitations  Selectivity is never absolute  Little preference for C18:3 over C18:2 

trans

-fatty acids acids may be formed

Altering Fats for Oxidative Stability

 Blending solid and liquid fats/oils  Hydrogenation (full or partial)  Random inter-esterification to change sn positions  Natural re-arrangement  Addition of desired fatty acids  Targeted inter-esterifications  1,3 lipases for a 1, 3 inter-esterification

Interesterification

 Exchanging positions from one glyceride to another to alter chemical composition and physical properties sn-1 sn-2 sn-3 O P S S O P P O S P S O

Cocoa butter

 Palmitic, stearic, and oleic acids  95% of the fat  Predominant sn positions:  sn-2 oleic acid  sn-1 or 3 palmitic or stearic acid  POS: 40%  POP: 15%  SOS: 27.5% sn-1 P sn2 O sn-3 S

Enzymatic Interesterification



Lipase catalyzed



Lipozyme (immobilized)



Selective FA-interchange on sn-1, 3 positions

The Polar Paradox Theory

 In bulk oils, with water and phospholipids…  Polar antioxidants are more effective in non-polar or less polar systems  Non-polar antioxidants are more effective in polar systems  Due to an “interfacial phenomenon”?

Polar Antioxidants

 Most effective in nonpolar or less polar environment  Bulk oils  Located at the oil-air interface or in reverse micelles  High amount of oxidants present here Yellow Oil Blue water Phospholipids

Non-polar Antioxidants

  Most effective in polar environment  Oil-in-water emulsions Located at the water-oil interface   Dissolved in oil droplets of the emulsion Allows access to oxidizing agents located in the water phase   Peroxides Oxidizing metals