Rancidity

Question 1

What is rancidity?

Answer 1

Rank, unpleasant, stale taste produce by the decomposition of fats and oils

Question 2

How are fats protected against rancidity?

Answer 2

Antioxidants
Biochemical processes which eliminate free radicals
Enzymes

Question 3

What products contain desirable deterioration of fats?

Answer 3

Sour cream
Blue cheese
Greek cheese

Question 4

What are the six main causes of rancidity?

Answer 4

1. Endogenous enzymes
2. Enzymes produced by invading microbes
3. Chemical transformations
4. Hydrolytic rancidity
5. Enzyme action
6. Ketonic rancidity

Question 5

How do endogenous enzymes causes rancidity?

Answer 5

Lipases and phospholipases cause hydrolytic rancidity = lipolysis
- Fatty acids are removed from the glycerol backbone by the addition of water (OH attaches to fatty acid and H attaches to glycerol)

Question 6

What inhibits the action of endogenous enzymes in milk?

Answer 6

Pasteurising

Question 7

What activates enzymes produced by invading microbes?

Answer 7

Temperature fluctuations
Mechanical agitation
Adding raw milk to pasteurised milk

Question 8

What can be done in order to minimise rancidity by enzymes produced by invading microbes?

Answer 8

Improvement of farm hygiene

Question 9

What chemical transformations cause rancidity?

Answer 9

Oxidation 
Chemical hydrolysis (when oils are heated)

Question 10

What occurs as a result of hydrolytic rancidity?

Answer 10

Changes in characteristics
Free fatty acids and glycerol are produced
- Length of fatty acid chains changes taste

Question 11

What enzymes causes rancidity?

Answer 11

Catalyse = oxidises fatty acids

Lipoxygenase

Question 12

What happens during ketonic rancidity?

Answer 12

Free fatty acids and glycerol are produced

Fatty acids are oxidised producing ketones and secondary alcohols

Question 13

What is ketonic rancidity associated with?

Answer 13

Xerophilic moulds

Question 14

How can rancidity be controlled?

Answer 14

Reduce enzyme action
Minimise microbial growth
Control oxidation (omit air)

Question 15

What developments have reduced the problem of rancidity?

Answer 15

Fridges
Freezers
Heat processing
Packaging

Question 16

What does auto oxidation cause?

Answer 16

Deterioration of fats

Question 17

What are the three stages of auto oxidation?

Answer 17

1. Initiation
2. Propagation
3. Termination

Question 18

What happens during the initiation stage of auto oxidation?

Answer 18

Reaction between fatty acid double bond and excited singlet oxygen to produce (hydro)peroxide = by sensitization

Question 19

What do sensitizers do?

Answer 19

Absorb light and transfer energy to triplet oxygen to produce singlet oxygen

Question 20

Name three sensitizers.

Answer 20

Chlorophyll
Myoglobin
Riboflavin

Question 21

How can lipids be protected against initiation?

Answer 21

Remove carotenoids via refining
Store in dark
Light absorbing packaging

Question 22

What happens during the propagation stage of auto oxidation?

Answer 22

Hydroperoxide forms alkoxyl and peroxyl radicals which abstract H+ from vulnerable sites on unsaturated fatty acids

Question 23

What is the most vulnerable site on an unsaturated fatty acid?

Answer 23

Alpha methylene group (CH2)

Question 24

What do alkoxyl radicals form?

Answer 24

Aldehyde
Secondary alcohols
Ketone termination

Question 25

What is the main food source of lipid peroxide?

Answer 25

Fried foods

Question 26

What are the two ways in which rancidity can be measured?

Answer 26

Using senses
- Odour 
- Taste 
- Colour change 
Chemical methods

Question 27

What chemical methods can be used to measure rancidity?

Answer 27

• Lipid hydroperoxide concentration = levels increase as rancidity occurs
• Aldehyde concentration = secondary product
• Volatile concentration = tertiary product (ethane, pentane)
• Toxo value = takes into account the presence of more than one product

Question 28

What does hydrolytic rancidity do?

Answer 28

Releases free fatty acids from triglycerides

Question 29

How can hydrolytic rancidity be measured?

Answer 29

By acid-base titration = the amount of potassium hydroxide required to neutralise acidity of 1g of sample

Question 30

What type of triglyceride is most susceptible to oxidation?

Answer 30

Those containing polyunsaturated fatty acids

Question 31

How can susceptibility to rancidity be predicted?

Answer 31

• Knowledge of the degree to which fatty acids are unsaturated
• Iodine monochloride = the more unsaturation the more iodine absorbed (extent to which iodine is absorbed is determined by titrations)
• Refractive index indicates the proportion of unsaturation
• Response to abuse = heat and bubble through oxygen as unsaturated fatty acids are oxidised more readily

Question 32

What are antioxidants?

Answer 32

Compounds that retard or delay rancidity

Question 33

Name a natural antioxidant.

Answer 33

L ascorbic acid

Question 34

Name a synthetic antioxidant.

Answer 34

Butylated hydroxytoluene

Question 35

How do antioxidants prevent rancidity?

Answer 35

Scavenge free radicals faster than radicals attack fatty acids

Question 36

Name two antioxidants that are important radical scavengers.

Answer 36

Vitamin C and E

Question 37

What are chelating agents and how do they work?

Answer 37

Retard or delay rancidity by collating metal ions = breakdown of hydroperoxide is not promoted

Question 38

Why does hydrogenation reduce rancidity?

Answer 38

Reduces the number of unsaturations and therefore reduces oxidation

Question 39

How does flush packaging with nitrogen reduce rancidity?

Answer 39

Displaces oxygen and therefore oxidation is reduced