Lecture 9: lipid oxidation Flashcards
Unsaturated fatty acids are susceptible to autoxidation through what mechanism?
Free radical mechanism
What is the primary reason unsaturated fatty acids are more prone to autoxidation than saturated fatty acids?
Unsaturated FA have C=C bonds. The C=C bonds pull electron density towards it, thus electron density at adjacent C-H bonds are lower. Thus, these C-H bonds undergo homolytic cleavage (where bond breaks evenly), causing formation of radicals.
H atom is reactive as electron density in C-H bond are pulled towards C=C , so H atom is cleaved off, leaving behind a radical R*
What are the 3 main stages of free radical mechanism?
- Initiation
- Propogation
- Termination
What happens during initiation? State the chemical reaction
Hydrogen abstraction from a fatty acid (RH) due to heat, UV, or metal ions (Fe²⁺, Cu²⁺).
• Forms a lipid radical (R•).
• Reaction:
RH (fatty acid) —> R• + H•
What happens during propogation? State the chemical reaction of fast and slow steps
During propogation, radicals initiate a series of chain reactions, regenerating lipid hydroperoxides
Reaction:
R* + O2 → ROO(fast) ;; lipid radical
ROO + RH → ROOH + R* (slow) ;; peroxy radical
RO* + RH → ROH + R* ;; alkoxy radical
During propogation, ____ is regenerated and catalyses the free radical mechnism
ROOH, lipid hydroperoxide
What occurs during termination? State the chemical reactions.
R* + R* → non radical product (R-R) ;; lipid radical
ROO* + R* → non radical product (ROOR) ;; peroxy radical
ROO* + ROO* → non-radical product (ROOR)
What is hock cleavage?
An acid-catalysed oxidative reaction where hydroperoxide undergoes rearrangement and leads to cleavage of C-C bond and formation of an alcohol + an aldehyde
The alcohol can isomerise. E.g. if there is a C=C, the enol can become a ketone
Fenton reaction : impt
Fenton reaction involves _____ as catalysts that accelerate lipid oxidation?
In the fenton reaction, lipid hydroperoxides are fragmented into ___ radicals ?
- transition metals
- alkoxy (RO)
Fenton reaction
After alkoxy radicals are formed, the cleavage of C-C bond forms what 2 products? From 1 of the products, what other products can be formed?
Aldehyde + lipid radical
Lipid radical (R) can continue reacting with :
- R + R1H → RH + R1*
- R* + O2 → ROO* → ROH
What are the 3 key aroma compounds from linolenate (C18:3) oxidation?
- Vinyl alcohol
- 1-alken-3-ol (C=C & -OH)
- 1-alken-3-one (C=C & -C=O)
What is the name of another group of volatile aroma compounds generated from aldehydes and ketones, but with additional C=C functional group?
Alkenal, alkenone
(Dialkenal : 2 C=C + CHO)
Lipid-Maillard interaction
How do products from lipid autoxidation interact / fit into Maillard reaction pathway?
Volatiles from FA autoxidation (e.g. dienals) can undergo reaction with nucleophiles NH3 and H2S to form N-containing cyclic compounds (e.g. pyridine) and S-containing cyclic compound (thiopene and thiapyran)
Lipid-Maillard interaction
2,4-decadienal is a volatile with 2 C=C and 1 CHO group, formed from the degradation of linoleic acid (C18:2). What is the difference in the mechanism of action of
- NH3
- H2S
when undergoing reaction with 2,4-decadienal? What are the products formed (no of members in ring and names)
NH3, a stronger nucleophile, attacks the carbonyl carbon on CHO group and undergoes nucleophilic addition.
- Forms 6-membered ring with N atom (2-pentylpyridine)
H2S , a weaker nucleophile, undergoes electrophilic addition at the second C=C.
- Can either form 5 or 6 membered ring; depending on which carbon -SH is added to
(5-member ring : 2-hexylthiopene; 6-member ring : 2-pentyl-2-thiapyran)
Lipid-Maillard interaction
2,4-decadienal is a volatile with 2 C=C and 1 CHO group, formed from the degradation of linoleic acid (C18:2). What is the difference in the mechanism of action of
- NH3
- H2S
when undergoing reaction with 2,4-decadienal?
NH3, a stronger nucleophile, attacks the carbonyl carbon on CHO group and undergoes nucleophilic addition.
Frying of potato chips
During deep frying of potato chips with oil, what do volatile and non-volatile compounds affect / contribute to?
- Volatile compounds affects / contribute to food flavour (volatiles from autooxidation of fatty acids)
- Non-volatile compounds (polymerisation products) affect the shelf-life of potato chips
Frying of potato chips
Phase 1 : What occurs when potato chips just start frying?
- Little oxidation of oil
- Little flavour development
Frying of potato chips
What are the key reactions occuring during frying of potato chips, and their reaction products? [4]
1) Hydrolysis
- TAG / DAG (in frying oil) → FA ;;
- Starch (in potato chips) → glucose / simple sugar
2) Dehydration / polymerisation : forms dimers, trimers, alcohols, hydrocarbon (C=C)
3) Autoxidation of FA (presence of O2 and high temp)
- Formation of hydroperoxides / conjugated dienes (e.g. 2,4-decadienal) → alcohols, ketones, aldehydes → acods, hydrocarbons
4) Maillard reaction
- Reducing sugar + asparagine (AA in potato) → acrylamide
- lipid-maillard interactions (conjugated dienes like 2,4-decadienal + nucleophiles from strecker degradation, NH3 and H2S)
Frying of potato chips
Second phase : oil is at its optimum, food develops desriable golden brown colour
What main reactions occur during this phase? [3].
Describe the stability / quality of oil and if it can be reused
- Oxidation of oil, in the presence of heat, produces hydroperoxides → secondary oxidation products (aldehydes / ketones), giving rise to flavour
- Some hydrolysis of starch to give rise to reducing sugars
- Maillard reaction, leading to non-enzymatic browning and development of golden colour
Oil quality is good, can still be re-used
Check : during deep frying, starch is gelatinised rather than hydrolysed ;; so maillard rxn occur between AA + free sugars
Frying of potato chips
Third phase : deterioriation of oil, food has darker brown color and slight-off flavours
What main reactions occur during this phase? [4].
Describe the stability / quality of oil and if it can be reused
1) Oxidation of oil continues
2) Hydrolysis of TAG / DAG into FA (may lead to rancidity), and continued hydrolysis of starch
3) Higher extent of Maillard reaction, leading to more non-enzymatic browning
- Volatiles
- Non-volatiles : asparaging (AA in potato) + glucose/fructose (reducing sugar) → acrylamide, a harmful / carcinogen
4) Dehydration, leading to polymerisation
- Polymerisation of sugars
- Polymerisation of FA (FA undergo intermolecular cyclisation and form dimer, trimer….)
Oil quality decreases but still considered acceptable
Frying of potato chips
Fourth phase : Further detioriation of oil
What is the main problem that occurs at this stage, leading to poor quality of oil and thus cannot be reused?
Foaming of the oil (oil bubble too much due to aeration [O2]), leading to uneven frying
Frying of potato chips
Fifth phase : Severe oil degradation
What happens to the flavour of fried food at this stage?
What occurs during this stage and can the oil still be reused?
Food develops unacceptable flavours (burnt, acrid flavour).
More foaming of oil occurs, leading to uneven frying due to limited contact between oil and food. Oil has to be discarded.
Acrid = sharp, harsh
Frying of potato chips
Describe the odour characteristic of these 3 fatty acids:
1. Palmitic acid (C16:0)
2. Linoleic acid (C18:2)
3. Linolenic acid (C18:3)
- Palmitic acid : fruity, waxy (undesirable)
- Linoleic acid (C18:2) : deep fried fat flavour (desirable)
- Linolenic acid : fishy odour (undesirable)
Frying of potato chips
What are the 2 main considerations when frying potato chips with oil?
1) Development of desirable flavours
- Oils with higher content of linoleic acid (C18:2) gives rise to desirable deep fried, fatty flavour (but oil is more unstable and degrade faster, cannot reuse many times)
2) Stability of oil
- Oil higher in unsaturated FA such as palmitic acid (C16:0) are less likely to undergo autoxidation, thus is more stable and can be re-used more times (but can lead to waxy, plastic, fruity off-odour)
Thus, must have a balance between different FA composition in oil to give rise to desriable flavour + higher stability (so can reuse frying oil more times, more cost-effective)
