Rancidity and Lipid Peroxidation
The shelf life of a product refers to the length of time a product may be stored without becoming unsuitable for use or consumption.
After the shelf life of a product has reached, the food undergoes processes which results in rancidity.
Rancidity, is the natural process of decomposition of fats or oils by either hydrolysis or oxidation, or both.
The process of degradation converts fatty acid esters of oils into free fatty acids.
This gives rise to an unpleasant odour and
taste in food.
These lipids degrade to the point of becoming either unpalatable or unhealthy to ingest.
There are 3 types/pathways of rancidity:
1. Oxidative Rancidity
Known as autoxidation
It is due to the auto-oxidation of PUFA present in triacylglycerols by the atmospheric O2 on free radicals.
The end product is the formation of aldehyde epoxide and peroxide.
Known as hydrolysis/enzymatic oxidation.
It is due to the contamination of fat by lipase leading to the formation of diacyl & triacylglycerols with free fatty acids.
The end product is the formation of aldehyde epoxide and peroxide.
The main cause of rancidity of lipids is the oxidative deterioration of lipids via a free- radical chain mechanism, which is also called lipid peroxidation.
This process occurs only within our bodies.
It occurs in three stages or phases:
Initiation
Propagation
Termination
Rancidity is most commonly detected by taste or smell, but it is also accompanied by a marked increase in the acid value of the fat, which is tested by using two basic laboratory tests:
Peroxide Value for primary oxidation products
Anisidine Value for secondary oxidation products.
Solution A
Acetic acid + chloroform 3:2 ratio
90 acetic acid + 60 mL chloroform in black bottle
Solution B: 1% Starch solution
1g starch dissolved up to 100 mL distilled water on hot plate by stirring with glass rod and then filtered with filter paper.
Solution C: 0.01N sodium thiosulphate
Take 0.25g of sodium thiosulphate in 80 mL distilled water + add 0.02g of sodium carbonate + mix well + heat to dissolve sodium thiosulphate + cool+ make final volume up to 100 mL with Distilled water
Solution D: KI saturated solution in 2 mL of distilled water
Take a flask+ add 10g of Oil + 30mL of solution A + shake well + add 1mL of KI solution + shake well + 30 mL of distilled water + few drops of 1% starch solution + solution becomes blue
titrate with 0.01N Sodium thiosulphate in burette add drop wise with vigorously shaking flak + wait to disappear blue color.
Peroxide value:V x N x 1000/weight of sample
4.5 x 0.01 x 1000/10= 4.5 meq/Kg
Normal Value: 1 to 3 mmol O2 / kg
Solution A: 30% trichloro-acetic acid (15g trichloro-acetic acid dissolve up to 50 mL acetic acid)
Quantitative Solution B: 1% Phloroglucinol in acetic acid (1 g phloroglucinol dissolve up to 100 mL acetic acid)
Qualitative Solution C: 0.1% Phloroglucinol solution in diethyl ether (10mg dissolve up to 10mL of diethyl ether)
Qualitative method 5mL of oil + 5mL of 0.1% phloroglucinol + shake well + 5mL of concentrated HCl + shake pink color shows rancidity
If There is no color, then there is no rancidity
Quantitative: 5mL of oil in 50 mL falcon tube + 5 mL chloroform + close tube + shake well + 10 mL of 30% trichloroacetic acid + 1 mL of 1% Phloroglucinol solution + close tube + shake well + incubate at 45 °C for 15 minutes Take absorbance at 545nm
Blank cuvette Absorbance adjust at zero
Fresh oil sample: A=0.126 no rancidity if absorbance is less than 0.15
Rancid oil sample A=0.423