Reactions of Oils and Fats

Reactions of Oils and Fats Hydrolysis Oxidation Hydrogenation Esterification

Hydrolysis Chemical (Autocatalytic) Enzymatical (Lipase) +3H20 O C triacylglycerol = - R2 - R3 HC H2C OH glycerol HC H2C HO - C O - R1 3 fatty acids + - R3 - R2 +3H20

Acid Value Number of mgs of KOH required to neutralize the Free Fatty Acids in 1 g of fat.

Oxidation of Oils and Fats The reaction of molecular oxygen with organic molecules has for long been a process of considerable interest. Although a wide variety of organic molecules are susceptible to chemical attack by oxygen, a great deal of attention has recently been focused on lipids because of the remarkable implications of their oxidative damage.

Oxidation of Oils and Fats The results of the oxidation of fats and oils is the development of objectionable flavors and odors characteristic of the condition known as “oxidative rancidity.” Loss of shelf-life, functionality and nutritional value. Adverse health effects (carcinogenic)

Oxidation of Lipids Autoxidation of Lipids is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical chain mechanism. The chain of reaction includes Initiation Propagation Termination

What is Free radical? A free radical is a group with an odd number of unpaired electrons. They are extremely unstable and immediately react with another molecule to form stable substances.

Initiation The initiation of lipid oxidation starts with the removal of an hydrogen atom from unsaturated TGs or FFAs (RH) to form a free radical (R•) (Eq.1). C O - O - H C O - O - H • + H• Represent as RH R• + H• (Eq.1)

Initiation The removal of hydrogen takes place at the carbon atom next to the double bond. C O - O - H C O - O - H • + H• Represent as RH R• + H• (Eq.1)

Formation of Lipid Radical Hydrogens on carbons next to double bonds most easily removed (-carbon) Energy for H removal (kcal/mole) H - CH2 - CH2 - CH3 H - CH = CH2 H - CH2 - CH = CH2 CH2 = CH - CH - CH = CH2 H 100 103 85 65 H on carbon next to double bond easier to remove

Initiation mechanisms Photosynthesized Oxidation (Photooxdation) Metal Catalysis Thermal Oxidation Enzymatic Oxidation

Initiation mechanisms-PO Light, in the presence of oxygen, promotes oxidation of unsaturated fatty acids. Photooxidation energy from light is captured aided by sensitizer molecules (pigments: chlorophile) Light excites these sensitizers to the triplet state that promotes oxidation by type I and type II mechanisms.

Initiation mechanisms-PO In type I photosensitized oxidation, the triplet state sensitizer abstracts a hydrogen or electron from the unsaturated oil, producing radicals that initiate chain propagation In type II photooxidation, the energy of the triplet sensitizer is transferred to molecular oxygen, converting it to its excited singlet state. light sens sens* sens* + RH R• + H• light sens* + 3O2 sens + 1O2

Initiation mechanisms-PO Singlet oxygen more reactive than triplet oxygen RH + 1O2 ROOH RO• + •OH RO• provides free radical to start propagation Initiated by singlet oxygen (1O2) metastable, excited energy state of O2 two unpaired electrons in same orbital triplet oxygen ground state 2 electrons w/ same spin in 2 orbitals singlet oxygen excited state 2 electrons w/ different spin in 1 orbital

Initiation mechanisms-Metal Catalysis Metal ions (e.g. Fe, Co, Cu) can also initiate reaction found naturally in foods, from metal equipment RH + M+2 R• + H+ + M+

Initiation mechanisms-Thermal Oxidation The energy requirements for the abstraction of H to form a lipid radikal can be supplied in the form of thermal energy. High temperatures (like frying) facilitate the all stages of the chain reaction Initiation mechanisms-Enzymatic Oxidation Enzyme-catalysed oxidation is initiated even in the absence of hydroperoxides. This means the enzyme alone is able to overcome the energy barrier of this reaction

Propagation R• + O2 ROO• (Eq.2) ROO• + R1H ROOH+ R1• (Eq.3) This highly reactive lipid (alkyl) radical (R•) can then react with oxygen to form a peroxy radical (ROO•) in a propagation reaction (Eq.2) During propagation, peroxy radicals can react with lipids (others R1H or same RH) to form Hydoperoxide (ROOH) and a new unstable lipid radical (Eq.3) R• + O2 ROO• (Eq.2) ROO• + R1H ROOH+ R1• (Eq.3)

Propagation R1• + O2 R1OO• (Eq.4) ROOH RO• + OH• (Eq.5) This lipid radical (R1•) will then react with oxygen to produce another peroxy radical (R1OO•) resulting in a cyclical, self-catalyzing oxidative mechanism (Eq.4) Hydroperoxides (Eq.3) are unstable and can degrade to produce radicals that further accelerate propagation reactions (Eq.5) and (Eq.6) R1• + O2 R1OO• (Eq.4) ROOH RO• + OH• (Eq.5) 2ROOH ROO• + RO• + H2O (Eq.6)

Propagation Hydroperoxides are readily decomposed by high-energy radiation, thermal energy, metal catalysis, or enzyme activity. Transion metals such as Fe and Cu ROOH + M+ RO• + OH• + M+ (Eq.7) ROOH + M2+ ROO• + H+ + M+ (Eq.8) 2ROOH ROO• + RO• + H2O (Eq.6)

Termination The propagation can be followed by termination if the free radicals react with themselves to yield non-reactive (stable) products, as shown here: Carbonyl compounds (aldehydes and ketones)and hydrocarbons R• + R• RR RO• + R• ROR ROO• + R• ROOR ROO• + ROO• ROO•R + O2

Pentane Formation from Linolenic Acid + _ . - CH 3 (CH 2 ) CH CH CH CH CH CH COOH CH CH CH H O Initiation (metal) Propagation OH Hydroperoxide Decomposition H C CH CH CH CH CH Termination Pentane 14 13 12 11 10 9 n

Oxidation Product Primary Oxidation Products Hydroperoxides Secondary Oxidation Products Aldehydes and ketones

Factors Affecting Autoxidation 1. Energy in the form of heat and light 2. Catalysts (Metal) 3. Double bonds 4. Enzymes 5. Chemical oxidants 6. Oxygen content and types of oxygen 7. Natural antioxidants 8. Phospholipids 9. Free Fatty acids

Oxidation Rates: Types of Fatty Acids As # of double bonds increases # and stability of radicals increases Rate increases Rate of Reaction Relative to Stearic Acid 1 100 1200 2500 Type of Fatty Acid 18:0 18:1D9 18:2D9,12 18:3D9,12,15

Kinetics of Autoxidation

ANALYSIS OF OIL OXIDATION 1. Peroxide Value Peroxide Value = ml of Na2S2O3  N  1000 (milliequivalent peroxide/kg of sample) Grams of Oil

Aldehyde + p-AnV Yellowish Products (Under acidic conditions) p-Anisidine Value. p-AnV is defined as 100 times the optical density measured at 350 nm in a 1.0 cm cell of a solution containing 1.0 g oil in 100 ml of a mixture of solvent and reagent. This method determines the amount of aldehyde (principally 2-alkenals and 2,4-alkadienals ) in animal fats and vegetable oils. Aldehyde + p-AnV Yellowish Products (Under acidic conditions) 3. Totox Value = 2* PV + p-AnV

K232 and K270 Oxidation of PUF is accompanied by an increase in the UV absorption of the products. Lipids containing methylene-interrupted dienes and trienes show a shift in their double-bond position during oxidation due to isomerization and conjugate formation. The resulting conjugated dienes exhibite an intense absorption at 232 nm; similarly conjugated trienes absorb 268 nm. K232 and P.V correlate well in the early stages of oxidation.

Oxidative Stability of Oils and Fats Active Oxygen Method (AOM) Determined the time required to obtain certain peroxide value under specific experimental conditions. The larger the AOM value, the better the flavor stability of the oil. Oil Stability Index / Rancimat Methods OSI and Rancimat measure the change in conductivity caused by ionic volatile organic acids, mainly formic acid, automatically and continuously.

Antioxidants Primary Antioxidants Secondary Antioxidants Chain-breaking antioxidants are free radical acceptors that delay or inhibite the initiation step or interrupt the propagation step of autoxidation. Secondary Antioxidants Act through numerous possible mechanisms, but they do not convert free radicals to more stable products.

Primary Antioxidants R• + AH RH + A• RO• + A• ROA ROO• + AH ROOH + RH

Natural and Synthetic Antioxidants

Secondary Antioxidants Chelators: citric acid, EDTA Oxygen Scavengers and Reducing Agents: Ascorbic acid, ascorbyl palmitate, Singlet Oxygen Quenchers: Caretenoids (beta-carotene, lycopene, lutein) Deplete singlet oxygen’s excess energy and dissipate the the energy in the form of heat.