Rancidity

having a rank, unpleasant, stale flavour or taste due to the decomposition of fats and oils

protection by antioxidants and enzymes biochemical processes eliminate free radicals

protection is lost and lipids become more susceptible to hydrolytic and oxidative changes

changes in colour, odor, flavour that are usually undesirable although they can sometimes be desirable e.g. sour cream

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)

• 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

• 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

an enzyme that catalyses the oxygenation of FA • widespread in plants (soybean, carrot, pea) • produces derivatives with characteristic tastes and flavours • difficult to inhibit

• 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)

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

Xerophilic means “dry loving.” Thus, xerophilic molds are those that can or prefer to grow in “dry” environments.

spontaneous oxidation of a substance at ambient temperatures in the presence of oxygen

initiation propagation termination

• 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

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

• 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

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

• 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)

a substance that inhibits oxidation, especially one used to counteract the deterioration of stored food products.

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

asc

foil-like packaging

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

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