Detection of

peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

gives the initial evidence of

rancidity Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids. When these processes oc ...

in unsaturated fats and oils. Other methods are available, but peroxide value is the most widely used. It gives a measure of the extent to which an oil sample has undergone primary oxidation, extent of secondary oxidation may be determined from

p-anisidine ''p''-Anisidine (or ''para''-anisidine) is an organic compound with the formula CH3OC6H4NH2. A white solid, commercial samples can appear grey-brown owing to air oxidation. It is one of three isomers of anisidine, methoxy-containing anilines. ...

test.Chemistry And Technology Of Oils And Fat
Chemistry And Technology Of Oils And Fats By Dr. M.M. Chakrabarty
The

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

s found in fats and oils play a role in

autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...

. Oils with a high degree of unsaturation are most susceptible to autoxidation. The best test for

(oxidative rancidity) is determination of the peroxide value.

Peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

s are intermediates in the autoxidation reaction. Autoxidation is a

free radical reaction A free-radical reaction is any chemical reaction involving free radicals. This reaction type is abundant in organic reactions. Two pioneering studies into free radical reactions have been the discovery of the triphenylmethyl radical by Moses Gomber ...

involving

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

that leads to deterioration of fats and oils which form off-flavours and off-odours. Peroxide value, concentration of peroxide in an oil or fat, is useful for assessing the extent to which spoilage has advanced.

Definition

The peroxide value is defined as the amount of peroxide oxygen per 1

kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...

of fat or oil. Traditionally this was expressed in units of

milliequivalent An equivalent (symbol: officially equiv; unofficially but often Eq) is the amount of a substance that reacts with (or is ''equivalent'' to) an arbitrary amount (typically one mole) of another substance in a given chemical reaction. It is an arch ...

s, although in

SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

s the appropriate option would be in

millimole The mole, symbol mol, is the unit of amount of substance in the International System of Units (SI). The quantity amount of substance is a measure of how many elementary entities of a given substance are in an object or sample. The mole is define ...

s per

(N.B. 1 milliequivalents = 0.5 millimole; because 1 mEq of O2 =1 mmol/2=0.5 mmol of O2, where 2 is valence). The unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.

Method

The peroxide value is determined by measuring the amount of

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

which is formed by the reaction of peroxides (formed in fat or oil) with

iodide An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine defic ...

ion. :2 I- + H2O + R-OOH -> R-OH + 2 OH- + I2 The base produced in this reaction is taken up by the excess of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

present. The iodine liberated is titrated with

sodium thiosulphate Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate, . The solid is an efflorescent (loses water readily) crystalline substance that dissolves well i ...

. :2S2O3^2- + I2 -> S4O6^2- + 2 I- The acidic conditions (excess acetic acid) prevents formation of hypoiodite (analogous to

hypochlorite In chemistry, hypochlorite is an anion with the chemical formula ClO−. It combines with a number of cations to form hypochlorite salts. Common examples include sodium hypochlorite (household bleach) and calcium hypochlorite (a component of ble ...

), which would interfere with the reaction. The indicator used in this reaction is a starch solution where

amylose Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of starch, making up approximately 20–30%. Because of its tightly packed helical structure, amylose ...

forms a blue to black solution with iodine and is colourless where iodine is

titrated Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

. A precaution that should be observed is to add the starch indicator solution only near the end point (the end point is near when fading of the yellowish iodine colour occurs) because at high iodine concentration starch is decomposed to products whose indicator properties are not entirely reversible.

Taste

Peroxide values of fresh oils are less than 10 milliequivalents/kg; when the peroxide value is between 30* and 40 milliequivalents/kg, a rancid taste is noticeable.

References

{{Reflist Food safety Flavors

Definition

Method

Taste

See also-related test methods

References