Lipids VII Flashcards
What is the major function of true antioxidants?
Have the property of being able to interrupt the PROPAGATION step of the autoxidation process
What other mechanisms can decrease autoxidation?
- Stabilization of hydroperoxides
- Competitive binding of O2
- Retardation of the initiation step
- Removal of oxygen
What is the most important mechanism to slow the autoxidation process?
Interrupting or retarding the propagation step
What are the two properties of an antioxidant?
1) Ability to give a hydrogen more readily than a fatty acid
2) Has a poor tendency to react with molecular oxygen
How do antioxidant free radicals differ from free fatty acid radicals?
Antioxidant free radicals have a poor tendency to react with molecular oxygen
How do antioxidants prefer to end their free radical states?
By combining with other free radicals to form neutral species
What do antioxidants typically give their free radical to?
Generally a peroxy radical
What do antioxidants lengthen?
They lengthen the induction period
Are antioxidants consumed?
Yes, slowly
Compounds with good antioxidant properties are generally _______ compounds.
Phenolic
What is vitamin E present in? What does that cause?
- Seed oils
- They are less susceptible to autoxidation than animal fats
Are seed oils or animal fat oils more susceptible to autoxidation?
Animal fats are more susceptible since they do not have antioxidants
What are the main antioxidants used for food?
1) BHA
2) BHT
3) PG
4) Tert-butyl Hydroquinone (not used in Canada)
In phenolic antioxidants, what donates hydrogens?
Always the OH groups
Which antioxidants does the public prefer?
Complex antioxidants found in natural products (rosemary extract)
What is the disadvantage of using oils with natural antioxidants?
Oils contain natural antioxidants at variable levels; they can be lost during processing
Are antioxidants used alone?
Usually used as combinations because mixtures tend to be more effective
What is the antioxidant legal limit? What is it based on?
- Under 0.02% (200 ppm)
- Based on the fat content of the product
What happens if the amounts of antioxidant surpass 200 ppm?
- Antioxidant can become a pro-oxidant
- Can be toxic
What does an antioxidant free radical form when it accepts hydrogen?
- Forms resonance stabilized antioxidant free radical
- Resonance delocalization (free radical can change spots)
How does gamma-Tocopherol change the quality of an oil?
Prolongs the freshness (less hexanal for longer) through its antioxidants
What happens to antioxidants (like propyl gallate) in frying fats or in systems of high pH?
- Can form a blue complex with ferrous ions
- Discolors a fat, such as lard
How can BHA and BHT (normally stable) be lost? Why?
Through volatilization as a fat is heated
How are the vapor pressures of BHA and BHT?
- Quite low
- But, high enough to allow the migration of the antioxidant impregnated into packaging material to migrate to the food
How does citric acid slow down oxidation? Is it an antioxidant?
- No, it is a preservative
- Natural way to chelate metals, which slows down oxidation
What are powerful catalysts for lipid oxidation? How?
- Metal ions
- Assist in catalyzing the breakdown of hydroperoxides
Metal ions shorten the ____________ and increase the overall __________
induction period, rate of the reaction
What amount of metal ions are required to bring about a significant change in the reaction rate?
Only a small amount (ppm)
Does metal ion catalysis favour the second or third oxidation state reaction? What does it form?
Favours the second oxidation state; forms an alkoxy
What does the third oxidation state of a metal ion form?
A peroxy
Why do metal ions speed up the autoxidation reaction?
Since they catalyze the breakdown of ROOH to form Alkoxy and Peroxy radicals
What does the chelation of metal ions cause?
- Causes steric hindrance
- Eliminates the catalytic role metal ions would otherwise play
Do chelating agents interfere with the primary autoxidation mechanisms? What are they called?
- No
- Not true antioxidants
- Synergistic agents to synergists
Give examples of synergists.
Ascorbic acid, phosphates, EDTA, phospholipids, chelating agents
Why are metal ions largely unavoidable?
Since almost all processing equipment is metallic
How are metal ions contributed naturally?
- By raw materials
- In the form of chlorophyll or myoglobin
Metal ions allow the formation of more reactive ___________, which can attack lipids directly
singlet oxygen
Where is lipoxygenase realtively abundant?
In plant tissues
What does lipoxygenase catalyse?
Catalyses the direct oxidation of polyunsaturated fatty acids which have a cis-cis 1,4 pentadiene group in its molecular structure
What do enzymes catalyze in the lipid of plant tissues?
- Catalyzes the formation of hydroperoxides
- Instigates the free radical mechanism
What happens to frozen/dehydrated vegetables if they are not blanched?
- Undergo rapid autoxidation
- Produce hay like/cardboard aroma
What does the bleaching of pigments (chlorophyll, beta-carotene) occur as a result?
Result of autoxidation initiated by lipoxygenase
Why is heat applied to the oilseed crush?
To reduce the effects of lipoxygenase and lipase
What is responsible for the development of characteristic flavours and aromas in living plant systems?
Lipoxygenase
How can the autoxidation reaction be controlled?
- Antioxidants
- Synergists (chelating agents)
- Protection from light (brown bottles)
- Replacement of air with nitrogen
What are the 11 factors that affect autoxidation?
1) Energy in the form of heat and light
2) Catalysts
3) Double bonds
4) Enzymes
5) Chemical oxidants
6) Oxygen content and types of oxygen
7) Natural antioxidants
8) Phospholipids
9) Fatty acids
10) Mono- and Di-glycerides
11) Polymers
Why can fatty acids chelate?
Because they have a carboxyl group
Which radical is the most reactive?
Hydroxy radical
How many hydroperoxides does oleic acid form?
4
How do the energy levels of triplet and singlet oxygen compare?
Triplet: 0
Singlet: 22.5 kcal
How does the nature of triplet and singlet oxygen compare?
Triplet: diradical
Singlet: no radical
How does the reaction of triplet and singlet oxygen compare?
Triplet: radical
Singlet: electron-rich
How does the degradation of linoleic acid compare if its done with triplet or singlet oxygen?
Triplet: slower
Singlet: much faster
What are dye photosynthesizers?
Methylene blue, crystal violet
What are pigment photosynthesizers?
Chlorophyll, riboflavin
Photosynthesizers absorb light in the ____ - ____ range
380-900
The singlet state is produced first by the _______________, but it has a _____ lifetime, decaying to _______________ to the ground state and by _________________ to the triplet state
light absorbed
short
fluorescence
intersystem crossing
Prior to the development of rancidity, what is the reversion flavor in soybean oil like?
Beany and grassy flavor
What are the detrimental effects of lipoxygenase?
- Destruction of the essential fatty acids
- Free radicals produced damage other compounds
- Development of off-flavor and odor in beans and peans
Lipoxygenase exhibits substrate specificity for what?
cis-cis-penta-1,4-diene unit
What does enzyme-catalyzed oxidation form?
- Abstracts a hydrogen, O2 reacts with the free radical, which produces a hydroperoxide
- Same thing as regular oxidation
What is the role of preventative antioxidants?
Minimizes the formation of initiating radicals
Name some preventative antioxidants.
- Superoxide dismutase
- Catalase
- Singlet-oxygen quencher
What is the role of radical scavenging antioxidants?
Breaks the free radical chain reactions by donating hydrogen to free radicals
Name some radical scavenging antioxidants.
- Vitamin C
- Tocopherol
- Anthocyanin
___________ are opposite to synergists.
Pro-oxidants
The greater the reduction potential in mV, the ______ the species affinity for electrons and the ______ its tendency to be reduced
greater
greater
What are the two ways to minimize lipid oxidation with antioxidants?
1) Preventative antioxidants: inhibit the formation of free alkyl radicals in the initiation step
2) Radical scavenging antioxidants: interrupt the propagation step of the free radical chain
What kind of compounds are antioxidants usually?
Monohydroxy or polyhydroxy phenol compounds
Do antioxidants have a low or high activation energy?
Low activation energy to donate hydrogen
Why don’t antioxidant free radicals initiate another free radical?
Due to the stabilization of the delocalization of the radical electron
Are antioxidant free radicals subject to oxidation?
No, since they are quite stable
What is the function of metal chelators?
Deactivate trace metals that are free of fatty acids by the formation of complex ion or coordination compounds
Give examples of metal chelators.
- EDTA (very efficient)
- Citric acid
- Phosphoric acid
When does synergism occur in lipid oxidation?
When mixtures of antioxidants produce a more pronounced activity than the sum of the activities of the individual antioxidants when used separately
What are primary antioxidants often used with to maximize efficiency?
- Other phenolic antioxidants
- Various metal chelating agents
What are 5 factors affecting the efficiency of antioxidants?
1) Activation energy of antioxidants to donate hydrogen should be low
2) Oxidation potential should be high (should not strip an electron)
3) Reduction potential should be low (can reduce or donate an electron)
4) Stability to pH and processing
5) Solubility in oil should be high
What are the two types of antioxidants?Give examples.
- Natural: tocopherols
- Synthetic: BHA, BHT, PG
What are some factors that affect our choices of antioxidants?
- Safety
- Antioxidant effectiveness
- Off-odor
- Off-color
- Convenience of antioxidant incorporation to foods
- Carry-through effect
- Stability to pH and food processing
- Availability
- Cost
- Non-adsorbable, if possible
What are the 6 tests that can be used to evaluate the quality of the flavour of an oil?
1) Sensory evaluation
2) Peroxide value
3) Benzidine test
4) TBA test
5) GC for volatile compound analysis
6) Active Oxygen Method (AOM)
What does the Benzidine test indicate?
If you have a yellow measurement, you have aldehydes in your oil
What does the TBA test indicate?
If you have an intense red colour, you have aldehydes in your oil (extensive oxidation)
Do decadienals (aldehydes) increase/decrease with light exposure? Does that increase/decrease the flavour score?
- Increase
- Decrease
What can be used to predict oil flavour by GC? Why?
- Linear regression
- To only have to use one taste test panel
What does the AOM method determine? What does it mean?
- Oxygen is bubbled at a controlled rate
- You can determine if the oil is prone to reversion within minutes
- Active Oxygen Method
What happens if an oil is used too long for deep-frying? (2)
1) Aldehydes and ketones will accumulate, which can be toxic
2) Product can be dehydrated to a solid black “rock”
In the lipoxygenase reaction, where is the electron or hydrogen abstracted from? What does that produce?
- Abstracted from the omega-8 position
- Creates a free radical at omega-8
Do efficient antioxidants have low or high activation energies?
Low
Do efficient antioxidants have low or oxidation potentials?
High
Do efficient antioxidants have low or high reduction potentials?
Low
Do efficient antioxidants have low or high solubility in oil?
High
Name 3 factors that affect your choice of antioxidant.
- Safety
- Off-odor
- Stability to pH and food processing
What are the 3 main reactions that occur during deep-frying? What are their consequences?
- Oxidation (alcohols, ketones, aldehydes)
- Dehydration
- Hydrolysis (FFA, diacylglycerols)
