Detection of
peroxide
In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...
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 ...
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 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
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 ...
(oxidative rancidity) is determination of the peroxide value.
Peroxide
In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...
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 Gombe ...
involving
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
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 spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...
of fat or oil. Traditionally this was expressed in units of
milliequivalents, although in
SI unit
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...
s the appropriate option would be in
millimole
The mole (symbol mol) is a unit of measurement, the base unit in the International System of Units (SI) for ''amount of substance'', an SI base quantity proportional to the number of elementary entities of a substance. One mole is an aggregate ...
s per
kilogram
The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...
(N.B. 1 milliequivalents = 0.5 millimole; because 1 mEq of O
2 =1 mmol/2 of O
2 =0.5 mmol of O
2, 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; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid 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 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 deficiency ...
ion.
:
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 compone ...
present. To measure the iodine, a
redox titration
A redox titration is a type of titration based on a redox reaction between the analyte and titrant. It may involve the use of a redox indicator and/or a potentiometer. A common example of a redox titration is the treatment of a solution of iodine ...
is performed using
sodium thiosulfate
Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent an ...
.
:
The acidic conditions (excess acetic acid) prevents formation of
hypoiodite (analogous to
hypochlorite
In chemistry, hypochlorite, or chloroxide is an oxyanion 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 ...
), which would interfere with the reaction.
A
starch indicator
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
solution is used, for which
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 20–25% of it. Because of its tightly packed Helix, helical struct ...
forms a blue to black solution with iodine and is colorless when the iodine is converted to iodide
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'' ...
.
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.
See also
*
Acid value
In chemistry, acid value (AV, acid number, neutralization number or acidity) is a number used to quantify the acidity of a given chemical substance. It is the quantity of Base (chemistry), base (usually potassium hydroxide (KOH)), expressed as m ...
*
Amine value
In organic chemistry, amine value is a measure of the nitrogen content of an organic molecule. Specifically, it is usually used to measure the amine content of amine functional compounds. It may be defined as the number of milligrams of potassium ...
*
Bromine number
In chemistry, the bromine number is the amount of bromine () in grams absorbed by 100 g of a sample. The bromine number was once used as a measure of aliphatic unsaturation in gasoline and related petroleum samples, but this assay has fallen into ...
*
Epoxy value
Epoxy value derives from the Epoxy equivalent weight (EEW) or Weight Per Epoxide (WPE) and is a measure of the epoxy content of an epoxy resin or epoxy reactive diluent, or glycidyl ether. This is an important parameter as it allows determination ...
*
Hydroxyl value
In analytical chemistry, the hydroxyl value is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize the acetic acid taken up on acetylation of one gram of a chemical substance that contains free hydroxyl groups. ...
*
Iodine value
In chemistry, the iodine value (IV; also iodine absorption value, iodine number or iodine index) is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. Iodine numbers are often used to determine the degree of u ...
*
Saponification value
Saponification value or saponification number (SV or SN) represents the number of milligrams of potassium hydroxide (KOH) or sodium hydroxide (NaOH) required to saponify one gram of fat under the conditions specified. It is a measure of the ...
References
{{Reflist
Food safety
Flavors
Edible oil chemistry
Food analysis