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
explain the relevance of autooxidation of PUFAs
- faster reaction than FAs with a single double bond (monoenoics)
- initial segregation products are unsaturated
- smaller and more volatile fragments are produced
increased likelihood of diradicals
what is the dietary significance of lipid peroxidation? what is the main source of lipid per oxidation products in the diet?
fried food- main source
lipid peroxidation products have adverse effects on the diet (decreased vitamin A levels in lipid rich diets) and a link to arterial disease
what provides protection against auto-oxidation
- protective packaging (away from light)
- suitable storage instructions (dry, dark place)
carotenoids are present in many crude oils, they produce a yellow/ orange colour that is unacceptable to the consumer (oils are refined and bleached to remove these)
what is an antioxidant
a substance that inhibits oxidation, especially one used to counteract the deterioration of stored food products.
what are some examples of radical scavengers
vitamin E
- good scavenger of of peroxyl radicals
- important inhibitor of the radical chain reaction
- tocopherol radical produced is non-reactive
- lipid soluble
vitamin C
- water soluble scavenger
- donates hydrogen
- produced ascorbic radical which is non-reactive
- ascorbic radical is the lipophilic variant of ascorbic acid
what are oxygen scavengers and displacers
asc
what is an example of protective packaging
foil-like packaging
how is rancidity measured
sensory - odor - taste - colour Odor is very sensitive and is a method that can be used to directly measure consumer acceptability. Typical smells include acetaldehyde (caused by bacterial spoilage and oxidation) and phenolic (caused by bacterial spoilage by coliforms)
chemically
Hydroperoxides
- primary product
- concentration is determined by titration
Aldehydes
- secondary product
- determined by TBA reaction (pink= reaction) and nucleophile and electrophile (red = reaction)
Volatiles
- tertiary product
- measured by gas chromatography
It is best to use all 3 methods as each measures something different
Totox value takes into account more than one type of product
what methods are used to predict susceptibility to rancidity
Iodine value
- iodine monochloride binds to unsaturated FA, more double bonds= more iodine mono chloride absorption, more uptake= more susceptibility to rancidity
- does not take antioxidants into account
Reactive index
- very rapid
- indicates the proportion of unsatisfyingly FAs present
- does not take antioxidants into account
Response to abuse
- heat oil, bubble through O2, measure Peroxide value over time
- more unsatisfyingly FA and PUFA = more oxidation
- can assess different antioxidants which is useful
- hard to related to actual food products
what are oxygen scavengers and displacers
An oxygen scavenger is a material in which one or more reactive compounds can combine with oxygen to reduce or completely remove oxygen in fluids and enclosed packaging
- ascorbic acid reacts with oxygen
- flushing packaging with nitrogen displaces oxygen
