Food Chem- lecture 6
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What is the rate of autoxidation a function of?
The rate of autoxidation is a function of the degree of unsaturation of the fatty acids making up the lipid system
Why are molecules with double bonds more susceptible to abstraction of a hydrogen?
Because it takes less energy to release them ad the lifetime of the free radical formed is extended significantly
Which hydrogens are preferentially abstracted?
Hydrogen is preferentially abstracted from the alpha position, next to the double bond as they are more liable (easily abstracted)
What happens during resonance of a fatty acid?
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
What is an important consequence of resonance?
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
What are the 2 major difference of faty acids with double bonds
- Both more susceptible to hydrogen abstraction
- The resulting free radical has a longer lifetime due to resonance, providing more time for oxygen to attack
How does molecular rotation happen?
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
Dexscribe hydroperoxide formation in linoleic acid
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
How does the number of double bonds affect the stability?
As the number of double bonds increases, the stability of the resonane forms increases allowing oxygen more opportunity to attack
Why is linolenic acid a special case in hydroperoxide formation?
Linolenic acid is a special case- found naturally in the unconjugated form, which is readily converted into a mixed bonding system
What does the bixed bonding configuration produce?
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
What happens when the free radical has reacted with molecular oxygen in linolenic acid?
Once the free radical has reacted with molecular oxygen, the linolenic acid hydroperoxides are very unstable and will undergo almost immediate decomposition
Why are TBA or AV tests useful even in relatively fresh oils?
Because they rapidly form low molecular weight aldehydes, ketones, alcohols and carboxylic acids- even before detectable peroxide levels develop
What is reversion?
Reversion is an accelerated oxidation reaction, which does not allow hydroperoxides to accumlate significantly before rancidity becomes apparent
Describe how free radicals are resonance stabilized
Increasing in stability with increased double bonds
What do mono- and bi- molecular decomposition reactions lead to?
The mono- and bi- molecular decomposition reactions lead to a very diverse array of breakdown products-positionally oxygenated, variable short chain organic compounds
Where do mono- and bi- moleculr decomposition reactions usually take place?
This reaction takes place in triclygerides
Is the lipid system increasing or decreasing in entropy?
The lipid system is basically increasing its entropy (disorder) incorporating oxygen and becoming more polar
How much oxidation is required in lipids to produce an off flavor in foods?
The oxidation of only small amounts of lipids is required to cause significant off flavor development in foods
What is the most common approach to slow down autoxidaiton?
How do antioxidants prevent oxidation?
True antioxidants have the property of being able to interrupt the propagation step of the autoxidation process (reducing its statistical dominance)
What other methods (other than antioxidants) can be used to slow down lipid oxidation?
- Stabilization of hydroperoxides
- Competitive binding of O2
- Removal of oxygen
- Use of chelating agents
What is the propagation step of oxidation?
- 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
What property does a true antioxidant have the helps stop the propagation step?
Has the property of donating a hydrogen more readily than a fatty acid
How does an antioxidant prefer to terminate its free radical state?
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
Why do antioxidants not stop the oxidation completely?
Antioxidants to not last forever, but are slowly consumed, and eventually the autoxidation reaction predominates and destroys the lipid
Where is vitamin E or alpha-tocopherol naturally found?
- Vegetable oils
- Present in many seed oils- hence these are less susceptible to autoxidation than animal fats where vitamin E is not naturally present
What is Gossypol?
A natural antioxidant found in crude cotton seed oil, however, it is toxicologically problematic
Why can only selected antioxidants be used in foods?
Because the toxicity of the additive has to be considered
What are the main synthetic antioxidants used in food?
- Butylated hydroxy anisole (BHA)
- Butylated hydroxy toluene (BHT)
- Propyl gallate (PG)
- Tert-butyl hydroquinone
Why are antioxidants usually used as mixtures?
Because a mixture of antioxidants tend to be more effective than a single antioxidant by itself
What is a typical commercial antioxidant mixture?
- Tenox BHA:
- 20% BHA
- 6% Propyl gallate
- 4% Citric acid
- propylene glycol as a carrier
What is the antioxidant legal limit?
<0.02%, based on the fat content of the product
What happens if too much of an antioxidant is used?
It cab ne come a pro-oxidant
What is the general mechanism of BHT?
- 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
What are the cons of propyl gallate?
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
How are BHT and BHA lost?
- Although relatively stable, they are slowly lost via volatilization as a fat is heated
- Vapor pressures are both quite low, and can sublime
How are BHA and BHT used in dry cereals?
Often incorporated into packaging materials and allowed to migrate into the food
Why is citric acid used to slow down oxidation?
- 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
Why are chelating agents not considered true antioxidants?
Chelating agents do not interfere with the priary autoxidation mechanism itself- commonly termed synergistic agents or synergists
What are some other examples of synergists?
- Ascorbic acid
Why are metal ions unaviodable?
- Almost all processing equipment is metallic, so Cu++ and Fe++ ions are largely unavoidable
- Metal ions are aso contributed naturally by the raw materials
What do metallo-porphyrin compounds do?
Metalo-porphyrin compounds allow the formation of very reactive singlet oxygens, which can attack lipids directly- promoting autoxidation
what does loipoxygenase do?
Lipoxygenase catalyzes the direct oxidation of polyunsaturated fatty acids which have a cis-cis 1,3 pentadiene group in its molecular structure
Why are most lipids susceptible to the action of lipoxygenase?
Because the natural unconjugated bonding system (cis-cis 1,4 pentadiene group) is found in most of the poluynsaturated lipids
what does lipoxygenase catalyze the formation of?
Catalyzes the formation of hydroperoxides and instigates the free radical mechanism in the lipid of plant tissues
Why must frozen/dehydrated vegetables blanched first?
If not blanched, they undergo very rapid autoxidation catalyzed by lipoxygenase- producing hay llike/cardboard aroma
How does lipoxygenase affect pigments?
Pigments such as chlorophyll and beta carotene, which are highly unsaturated, can be bleached by lipoxygenase action
What is done to oilseed to denature enzymes such as lipoxygenase and lipase?
Head is generally aplied to the oilseed crush to denature enzymes and thereby reducing the detrimental effects of both lipoxygenase and lipase
How does a hydraulic press work?
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
What is hydraulic pressing usually used for and why?
Preferred method for producing virgin olive oil- here we want the flavor and free fatty acids- as is
What is the downside to hydraulic pressing?
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
What is the first step in moden commercial processes?
Typically, the first step is to heat and crush the seed continuously using an expeller or extruder
How does an expeller work?
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
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