Test For Rancidity Of Fats – Kreis Test

By | January 11, 2018

Fats are smooth, greasy substances that are insoluble in water. Fat is mainly a fuel source for the animal or plant in which it is found, or the animal that eats it. Fats contain more than twice the amount of protein or carbohydrate. Fats also contribute to the texture and give a rich flavour to the food.This article would be revolving around basics of rancidity and different methods of Test For Rancidity Of Fats. So let’s get started.

Test For Rancidity Of Fats

Test For Rancidity Of Fats | Discoverfoodtech.com

What is Rancidity?

Deterioration of fats and oils has been a major problem since ages. This deterioration or degradation of fats is termed as Rancidity. During rancidity, fatty acid esters of oils are converted into free fatty acids, by reaction with air, moisture and/or other materials. These fats degrade to the point of becoming either unpalatable or unhealthy to ingest. Ingestion of rancid fats has been linked to the development of many diseases, including atherosclerosis, cataracts, diarrhoea, kidney disease and heart disease, and can cause cellular membrane damage and carcinogenesis.

What causes Rancidity?

There are two basic types or causes of rancidity that cause and/or contribute to the degradation of stored edible oils: oxidative and hydrolytic. Oxidative rancidity occurs when oxygen is absorbed from the environment. In the presence of oxygen and/or ultraviolet (UV) radiation, most lipids will break down and degrade, forming several other compounds. Oxygen is eight times more soluble in fats than it is in water; it is this exposure that is the main cause of the auto-oxidation process, increasing the saturation of the oil. Hydrolytic rancidity, also called hydrolysis or enzymatic oxidation, occurs in the absence of air, but with moisture present.

Test For Rancidity Of Fats

Quality evaluation of edible oils is tested by different methods. In routine work apart from the free fatty acids determination, the analysis should include the determination of peroxide value, Kries test and ultra-violet absorption at 234 nm and 268 nm to establish rancidity. This article will enlighten you with the Kreis test in detail and the other two i.e. peroxide value and UV absorption in brief.

Among the chemical methods, Kreis test is a promising one for early detection of rancidity, particularly aldehydes with a characteristic odour impact. The colour development in the test is critical and requires optimization.

  1. Kries Test

Principle
Auto-oxidation at fatty acids double bonds occur by reaction with molecular oxygen present in the atmosphere, causing the formation of labile peroxides. The peroxides formed during auto-oxidation are unstable and decompose into free radicals. These initiate chain reactions which eventually lead to the decomposition of the fatty acid into various low-molecular weight aldehydes and ketones.

Aldehydes and ketones react with phloroglucinol developing a pink colour.

Reagents

1) 0.1% Phloroglucinol solution (0.1 gm in100 ml of diethyl ether, freshly prepared)

2) Concentrated Hydrochloric acid

3) Chloroform

4) 30% solution of trichloroacetic acid in glacial acetic acid

5) Ethano

Qualitative | Test For Rancidity Of Fats

  1. Shake 5 ml of the oil vigorously with 5 ml of 0.1% phloroglucinol solution in diethyl ether and add 5 ml of conc. hydrochloric acid.
  2. Allow to stand for 10 minutes.
  3. A pink colour indicates incipient rancidity.

Quantitative | Test For Rancidity Of Fats

First method –

  1. Weigh 0.8 – 1.02 gm of oil or fat into a 100 ml beaker.
  2. Melt sample of fat and add slowly with stirring 20 ml of phloroglucinol until sample dissolved.
  3. Transfer solution to a separating funnel, add 10 ml conc HCl, shake well and allow to separate.
  4. Run off acid layer into a 1inch (2.54mm) Lovibond cell and match the colour using red, yellow and blue glasses. Express result as red Lovibond units.
  5. Up to 3 red units indicates incipient rancidity, between 3 and 8 units indicates the end of induction period, over 8 units indicates definite rancidity.

Second method –

  1. Shake 5 ml of oil and 5 ml chloroform in a stoppered test tube.
  2. Add 10 ml of a 30% solution of trichloroacetic acid in glacial acetic acid and 1 ml of 1 percent solution of phloroglucinol in glacial acetic acid.
  3. Incubate the test tube at 45ºC for 15 min.
  4. After incubation, add 4 ml of ethanol and immediately measure the absorbance at 545 nm. Absorbance values below 0.15 indicate no rancidity.
  5. Absorbance values greater than 0.2 denote incipient rancidity and absorbance values around 1.0 show that the sample is highly rancid.

Peroxide value | Test For Rancidity Of Fats

The peroxide value (PV) determines the concentration of hydroperoxide, the primary oxidation products. The principle involves peroxides liberating iodine from potassium iodide, i.e.

ROOH+KI→ROH+KOH+I2

The amount of ROOH is then determined by measuring the amount of iodine formed, which is done by titration with sodium thiosulfate and using a starch indicator:

I2+starch+2Na2S2O3(blue)→2NaI+starch+Na2S4O6(colourless)

The amount of peroxides is calculated back by the amount of sodium thiosulfate (Na2S4O6) consumed. It is expressed as peroxide value (PV) in units of milli-equivalents (meq) peroxide per 1 kg of fat extracted from the food. A general rule is that PV should not be above 10–20 meq/kg fat to avoid rancidity flavor (Connell, 1975).

Ultra-Violet Absorption | Test For Rancidity Of Fats

Oxidised fatty acids containing conjugated double bonds absorb UV strongly between 230 and 375 nm, dienes absorbing at 234 nm and trienes at 268 nm. Conjugated trienes may be formed by industrial processing, e.g. decolourising with bleaching earths. A secondary absorption by trienes occurs at about 278 nm. In the early stages of oxidation the UV absorption increases somewhat proportionately to the uptake of oxygen and the formation of peroxides. The UV absorption curve forms plateau just before the end of the induction period. The magnitude of UV absorbance is not readily related to the amount of oxidation; so the method is best applicable to detecting relative changes in oxidation of oil in comparison experiments or stability tests.

Test For Rancidity Of Fats Video

References

  1. FSSAI Manual of Methods of Analysis of Foods – ‘Oils and Fats’, 2015
  2. Food Science, 5th edition by Norman Potter and Joseph H. Hotchkiss
  3. http://vlab.amrita.edu/?sub=3&brch=63&sim=688&cnt=1
  4. http://nem.org.uk/Rancidity.htm
  5. http://onlinelibrary.wiley.com/doi/10.1111/j.1745-4522.1999.tb00136.x/abstract
  6. https://www.naturalproductsinsider.com/articles/2009/08/understanding-rancidity-of-nutritional-lipids.aspx
  7. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/peroxide-value

Written By :
Areeb Irshad

Connect to him @ areebirshad95@gmail.com

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