5: Rancidity

Question 1

What are the causes of rancidity?

Answer 1

Endogenous enzymes; enzymes produced by invading microbes; chemical transformations

Question 2

How can the causes of rancidity be controlled?

Answer 2

• reducing activity of endogenous enzymes = reduce temp; heat treat food
• minimise microbial growth : reduce temp, heat treat food
• control oxidation = omit air & add antioxidants

Question 3

Give examples of endogenous enzymes and explain how they cause rancidity

Answer 3

Lipases and phospholipases cause hydrolytic rancidity

e.g. lipoprotein lipase (LPL) in freshly produced milk
Triglyceride breaks down into glycerol + 3FFA + hydrolyses fatty acids into diglycerides and monoglycerides
* Pasteurisation inactivates LDL and reduces rancidity

Question 4

Give examples of invading microbes and explain how they cause rancidity

Answer 4

e.g. Actinobacter and Pseudomonas spp
* their lipases release FFA from TG
* are heat stable
* major problem in processed milk products
* improvements in hygiene reduce problem
* heat stable lipases from psychotrophic (‘cold loving’) bacteria can cause problems in UHT milk
Other methods that can activate lipases : temp fluctuations; mechanical agitation; adding raw milk to pasteurised milk

Question 5

Explain hydrolytic rancidity

Answer 5

Changes characteristic of milk, unsalted, crude palm oil and crude fish oil
- butter fat (contains short chain FA which are small and volatile so liberated into the environment leading to goaty odour)
- coconut oil (contains long chain FA so are less volatile, remain in the food but leads to a soapy taste)

Question 6

Explain how lipoxygenase causes rancidity

Answer 6

• Catalyses oxygenation by o2 of fatty acids which contain 2 double bonds (so effectively destroy PUFA)
• Lipoxygenase in soya bean, carrot, maize, cereal grains & soya
• soya bean particularly rich in lipoxygenase
• difficult to inhibit as lacks cofactors - heat treatment effective if over 70 degrees C & antioxidants also effective

Question 7

Explain how ketonic rancidity occurs

Answer 7

• Affects both UFA and SFA
• associated with growth of xerophilic moulds
• TG converted into FFA
• Incomplete B oxidation produces ketones and secondary alcohols
• typically FA with 8-10 carbon atoms are utilised
• e.g. rancid almonds

Question 8

Explain chemical mechanisms of rancidity

Answer 8

1. Chemical hydrolysis
   - occurs when oils are heating (250 degrees C) with water present
   - commonly occurs in frying oils
   - water derived from the food
   - TG + H20 + heat -> glycerol + FFA
   - glycerol is flammable. flash point reduced
   - Reduce risk by using sealed fat fryers & regular change of oil (measure FFA by titration)
2. Auto-oxidation
   - radical mechanism with 3 stages : initiation; propagation or auto catalysis; termination