Rancidity Flashcards
define rancid
having a rank, unpleasant, stale flavour or taste due to the decomposition of fats and oils
in vivo, fats and oils are stable- what does stable mean in this context
protection by antioxidants and enzymes
biochemical processes eliminate free radicals
once a fat/ oil has been harvested what is lost
protection is lost and lipids become more susceptible to hydrolytic and oxidative changes
what are the physical effects of rancidity
changes in colour, odor, flavour that are usually undesirable
although they can sometimes be desirable e.g. sour cream
what are the causes of rancidity
endogenous enzymes
- lipases and phospholipases (lipoprotein lipase) cause hydrolytic rancidity
- present in freshly produced milk
- can reduce rancidity through heat treatment
enzymes produced by invading microbes
- e.g. actinobacter
- their lipases release FFAs from TGs
- heat stable lipases
- can be an issue in processed milk products
chemical transformations
- antioxidants start the auto-oxidation process (initiation, propagation, termination)
how can rancidity be controlled
- reduce the activity of endogenous enzymes by reducing temperatures/ heat treating food
- minimise bacterial growth by heat treating food
- control oxidation by omitting air
- controlling rancidity has allowed the development pf fridges and freezers
what is hydrolytic rancidity
- caused by TG hydrolysis producing FFAs
- measured by an acid-base titration
- hydrolytic rancidity changes the characteristics of milk, unsalted butter, crude palm oil and fish oil.
- short chains= volatile e.g. butter
- long chains= less volatile e.g. butter
what is lipoxygenase
an enzyme that catalyses the oxygenation of FA
- widespread in plants (soybean, carrot, pea)
- produces derivatives with characteristic tastes and flavours
- difficult to inhibit
what is ketonic rancidity
- attack fats containing short‐chain fatty acids and produce ketones with a characteristic odour and taste.
- butter, coconut, and palm kernel oils are most susceptible
- associated with the growth of xerophilic moulds e.g bread
- caused by the incomplete beta oxidation of TG to FFA, resulting in the production of ketones and secondary alcohols
- affects both saturated and unsaturated FAs (8-10 carbons)
what are the chemical mechanisms that cause rancidity
chemical hydrolysis
- water must be present
- converts TG to FFA and glycerol
- glycerol is flammable (due to a reduce flash point) so can be dangerous the frying oils
- risk can be reduced by using sealed fat friers and changing oil regularly
what is a xerophilic mould
Xerophilic means “dry loving.” Thus, xerophilic molds are those that can or prefer to grow in “dry” environments.
define auto-oxidation
spontaneous oxidation of a substance at ambient temperatures in the presence of oxygen
what are the stages of auto-oxidation
initiation
propagation
termination
outline what occurs during the initiation phase of auto-oxidation
- formation of the first radical by a reaction with an excited O2 molecule
- reaction between unsat FAs and O2 is thermodynamically difficult and slow
- production of the first few radicals occur due to photosensitisation- this involves an interaction between FA double bond and an excited O2 molecule
- oxygen in the triplet state is activated by light in the presence of a sensitiser
- 18:3 FAs are most susceptible
outline what occurs during the propagation phase of auto-oxidation
free radicals produced in the initiation phase react freely with other unsaturated FAs to propagate the reaction
most vulnerable to oxidation is the CH2 group next to a double bond
18:3 FAs are most susceptible to propagation due to higher number of double bonds
