Food Chem- lecture 6

The rate of autoxidation is a function of the degree of unsaturation of the fatty acids making up the lipid system

Because it takes less energy to release them ad the lifetime of the free radical formed is extended significantly

Hydrogen is preferentially abstracted from the alpha position, next to the double bond as they are more liable (easily abstracted)

During resonance the absolute double bond character is partly lost as the radical electron tries to resonate back and forth to stabilize the high energy free radical structure

Another important consequence of resonance is that a variety of isomeric hydroperoxides can be formed, depending on which free radical form exists at the time oxygen attacks

• Both more susceptible to hydrogen abstraction
• The resulting free radical has a longer lifetime due to resonance, providing more time for oxygen to attack

Molecular rotation is possible due to the fact that the double bond caracter is partly lostwhile in the resonance state, allowing molecular rotation around the bond

Unconjugated double bonds hae an even more labile methylene group between the double bonds, and once resonance is initiated, six possible positional free radical forms can exist and their correspoding cis/trans isomers

As the number of double bonds increases, the stability of the resonane forms increases allowing oxygen more opportunity to attack

Linolenic acid is a special case- found naturally in the unconjugated form, which is readily converted into a mixed bonding system

The mixed bonding configuration produces an activated methylene group, which is very readily abstrated to form a relatively stable a free radical with many resonance forms

Once the free radical has reacted with molecular oxygen, the linolenic acid hydroperoxides are very unstable and will undergo almost immediate decomposition

Because they rapidly form low molecular weight aldehydes, ketones, alcohols and carboxylic acids- even before detectable peroxide levels develop

Reversion is an accelerated oxidation reaction, which does not allow hydroperoxides to accumlate significantly before rancidity becomes apparent

Increasing in stability with increased double bonds

The mono- and bi- molecular decomposition reactions lead to a very diverse array of breakdown products-positionally oxygenated, variable short chain organic compounds

This reaction takes place in triclygerides

The lipid system is basically increasing its entropy (disorder) incorporating oxygen and becoming more polar

The oxidation of only small amounts of lipids is required to cause significant off flavor development in foods

Antioxidants

True antioxidants have the property of being able to interrupt the propagation step of the autoxidation process (reducing its statistical dominance)

• Stabilization of hydroperoxides
• Competitive binding of O2
• Removal of oxygen
• Use of chelating agents

• R-O-O∙+R-H --> R-O-O-H+R∙
• The peroxy radical has the penhcant for ending its free radical state by abstracting a hydrogen from another fatty acid- propagating the reaction

Has the property of donating a hydrogen more readily than a fatty acid

An antioxidant prefers to terminate its free radical state by combining with other free radicals to form neutral species, rather than to react with molecular oxygen

Antioxidants to not last forever, but are slowly consumed, and eventually the autoxidation reaction predominates and destroys the lipid

• Vegetable oils
• Present in many seed oils- hence these are less susceptible to autoxidation than animal fats where vitamin E is not naturally present

A natural antioxidant found in crude cotton seed oil, however, it is toxicologically problematic

Because the toxicity of the additive has to be considered

• Butylated hydroxy anisole (BHA)
• Butylated hydroxy toluene (BHT)
• Propyl gallate (PG)
• Tert-butyl hydroquinone

Because a mixture of antioxidants tend to be more effective than a single antioxidant by itself

• Tenox BHA:
• 20% BHA
• 6% Propyl gallate
• 4% Citric acid
• propylene glycol as a carrier

<0.02%, based on the fat content of the product

It cab ne come a pro-oxidant

• 1. Free radical accepts hydrogen and forms resonance stabilized antioxidant free radical
• 2. Resonance stabilized antioxidant radical can regenrate antioxidant free radical by reacting with R

Propyl gallate breaks down in frying fats or in systems with a high pH- can form a blue complex with ferrous ions- discolor a fat such as lard

• Although relatively stable, they are slowly lost via volatilization as a fat is heated
• Vapor pressures are both quite low, and can sublime

Often incorporated into packaging materials and allowed to migrate into the food

• Citric acid is a good chelator
• Metal ions are powerful catalysis for lipid oxidation (catalyze the breakdown of hydroperoxides)
• Metal ions also shorten the induction period and increase the overall rate of the oxidation reaction

Chelating agents do not interfere with the priary autoxidation mechanism itself- commonly termed synergistic agents or synergists

• Ascorbic acid
• Phosphates
• EDTA
• Phospholipids

• Almost all processing equipment is metallic, so Cu⁺⁺ and Fe⁺⁺ ions are largely unavoidable
• Metal ions are aso contributed naturally by the raw materials

Metalo-porphyrin compounds allow the formation of very reactive singlet oxygens, which can attack lipids directly- promoting autoxidation

Lipoxygenase catalyzes the direct oxidation of polyunsaturated fatty acids which have a cis-cis 1,3 pentadiene group in its molecular structure

Because the natural unconjugated bonding system (cis-cis 1,4 pentadiene group) is found in most of the poluynsaturated lipids

Catalyzes the formation of hydroperoxides and instigates the free radical mechanism in the lipid of plant tissues

If not blanched, they undergo very rapid autoxidation catalyzed by lipoxygenase- producing hay llike/cardboard aroma

Pigments such as chlorophyll and beta carotene, which are highly unsaturated, can be bleached by lipoxygenase action

Head is generally aplied to the oilseed crush to denature enzymes and thereby reducing the detrimental effects of both lipoxygenase and lipase

Rarely used in North America- simply involves the use of a screw press to apply pressure, crush the seed/tissue and squeeze the oil out

Preferred method for producing virgin olive oil- here we want the flavor and free fatty acids- as is

For oils other than olive oil, this process is generally detrimental due to the time factor, which allows enzymes (lipase and lipoxygenase) to act and release significant levels of free fatty acids and develop off flavors

Typically, the first step is to heat and crush the seed continuously using an expeller or extruder

An expeller is a simplified version of an extruder- Acrhimedes screw in a barrel which tapers towrd the end- pressure is increased and the oil is squeezed out continuously