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Oxidative Rancidity Food Chemistry

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Sola Park

on 20 March 2014

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Transcript of Oxidative Rancidity Food Chemistry

Food Chemistry F.7
F.7.1 Describe the steps in the free radical chain mechanism occurring during oxidative rancidity
Once the radicals have been formed, the alkyl radical (R∙ ) produced in the initiation step can propagate in the presence of oxygen to react to form a peroxyl radical (ROO∙):
R∙ O2 →ROO∙
Super Lipid Brothers
The journey to decomposition with oxidative rancidity...
The condition reached in certain foods as the lipid undergoes oxidation reactions producing aldehydes, hydroxyl acids and other compounds
Relates to our perception of flavor of fats and oils (lipids ) and unpleasant smells
Hydrolytic Rancidity
A process by which fats and oils are broken down into their constituent fatty acids and glycerol.
Oxidative Rancidity
Known as auto-oxidation
A process due to the oxidation of the fatty acid chains by the addition of oxygen across the carbon to carbon double bond in unsaturated fatty acids.
The accumulation of aldehydes and ketones is responsible for the off-flavor and odor of foods containing oxidized lipids.
Ex) Oily fish (mackerel and herring) are susceptible to oxidative rancidification.
The greater the degree of unsaturation present, the more predisposed a lipid will be to oxidative rancidification.

I can't wait to learn about the chain mechanism!!
Free Radical Mechanism- atoms or group of atoms with an unpaired number of electrons and can be formed when oxygen interacts with certain molecules; homolytic
Unsaturated fatty acid containing a C-H bond can be broken homolytically
Like other free-radical chain reactions, it involves three stages: initiation, propagation, and termination
The free radical mechanism catalyzed by light in the presence of enzymes or metal ions which leads to the formation of hydro peroxides (ROOH) which degrade to volatile aldehydes and ketones with strong off flavors.
1. Initiation
RH-> R. + H.
RH represents any unsaturated fatty acid with hydrogen atoms that can be lost to form free radicals
R and H with two dots represent the free radicals.
The carbon-to-hydrogen bonds are strong with an average bond enthalpy of 412 KJ mol-1.
Thus, the hemolytic fission of the C-H bond requires a significant amount of energy input due to the strength of the bond.

Since the activation energy for the formation of the first free radicals is high, the strong carbon to hydrogen bond R—H of an unsaturated fatty acid is first broken homolytically by presence of a metal catalyst or exposure to light.
2. Propagation
This peroxyl radical is able to remove hydrogen from an intact unsaturated fatty acid molecule to form a hydroperoxide and regenerate the alkyl radical.
ROO∙ + RH→ R ∙ + ROOH
When the hydroperoxide and alkyl radical is generated, the hydroperoxide is colorless and odorless but it quickly changes into a mix of aldehydes, ketones, or carboxylic acids. So, these NOT hydroperoxide give the rancid smell and taste.
3. Termination
R∙ + R∙ →R--R
R∙ + ROO∙ → ROOR
ROO ∙ + ROO∙ → ROOR +O2

Two free radicals combine to form non-radical products and eliminate the unpaired electrons. This stage produces a mixture of compounds
Further Reactions of Organic Hydroxides
The hydro peroxides formed in the propagation step are unstable as the O—O bond is very weak.
They undergo further free-radical reactions to produce aldehydes and ketones which are responsible for the unpleasant smells and taste of rancid food.
The decomposition of the hydroperoxide is dependent on photolysis of the O—O bond or on catalysis with transition metals.

Mission Accomplished
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