Peroxide value
Encyclopedia
The peroxide value of an oil or fat
is used as a measurement of the extent to which rancidity reactions have occurred during storage. Other methods are available but peroxide value is the most widely used.
The double bond
s found in fats and oils play a role in autoxidation
. Oils with a high degree of unsaturation
are most susceptible to autoxidation. The best test for autoxidation (oxidative rancidity) is determination of the peroxide value. Peroxide
s are intermediates in the autoxidation reaction.
Autoxidation is a free radical reaction
involving oxygen
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.
The peroxide value is defined as the amount of peroxide oxygen per 1 kilogram
of fat or oil. Traditionally this was expressed in units of milliequivalents, although if we are using SI units then the appropriate option would be in millimoles per kilogram (N.B. 1 millimole = 2 milliequivalents). Note also that the unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.
which is formed by the reaction of peroxides (formed in fat or oil) with iodide
ion.
2 I- + H2O + ROOH -> ROH + 20H- + I2
Note that the base
produced in this reaction is taken up by the excess of acetic acid
present. The iodine liberated is titrated with sodium thiosulphate.
2S2O32- + I2 -> S4O62- + 2 I-
The acidic conditions (excess acetic acid) prevents formation of hypoiodite (analogous to hypochlorite
), which would interfere with the reaction.
The indicator used in this reaction is a starch solution
where amylose
forms a blue to black solution with iodine and is colourless where iodine is titrated
.
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.
and peroxide value depends on the type of oil and is best tested with taste panels. The odours and flavours associated with typical oxidative rancidity are mostly due to carbonyl
-type compounds. The shorter-chain aldehyde
s and ketone
s isolated from rancid fats are due to oxidative fission and are associated with advanced stages of oxidation. The carbonyl-type compounds develop in low concentrations early in the oxidative process.
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.
Vegetable fats and oils
Vegetable fats and oils are lipid materials derived from plants. Physically, oils are liquid at room temperature, and fats are solid. Chemically, both fats and oils are composed of triglycerides, as contrasted with waxes which lack glycerin in their structure...
is used as a measurement of the extent to which rancidity reactions have occurred during storage. Other methods are available but peroxide value is the most widely used.
The double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s found in fats and oils play a role in autoxidation
Autoxidation
Autoxidation is any oxidation that occurs in open air or in presence of oxygen and/or UV radiation and forms peroxides and hydroperoxides. A classic example of autoxidation is that of simple ethers like diethyl ether, whose peroxides can be dangerously explosive. It can be considered to be a slow,...
. Oils with a high degree of unsaturation
Unsaturated fat
An unsaturated fat is a fat or fatty acid in which there is at least one double bond within the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond. Where double bonds are formed, hydrogen atoms are...
are most susceptible to autoxidation. The best test for autoxidation (oxidative rancidity) is determination of the peroxide value. Peroxide
Peroxide
A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion .The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.The simplest stable peroxide is hydrogen peroxide...
s are intermediates in the autoxidation reaction.
Autoxidation is a free radical reaction
Free radical addition
Free radical addition is an addition reaction in organic chemistry involving free radicals. The addition may occur between a radical and a non-radical, or between two radicals....
involving oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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.
The peroxide value is defined as the amount of peroxide oxygen per 1 kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
of fat or oil. Traditionally this was expressed in units of milliequivalents, although if we are using SI units then the appropriate option would be in millimoles per kilogram (N.B. 1 millimole = 2 milliequivalents). Note also that 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 iodineIodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
which is formed by the reaction of peroxides (formed in fat or oil) with iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
ion.
2 I- + H2O + ROOH -> ROH + 20H- + I2
Note that the base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
produced in this reaction is taken up by the excess of acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
present. The iodine liberated is titrated with sodium thiosulphate.
2S2O32- + I2 -> S4O62- + 2 I-
The acidic conditions (excess acetic acid) prevents formation of hypoiodite (analogous to hypochlorite
Hypochlorite
The hypochlorite ion, also known as chlorate anion is ClO−. A hypochlorite compound is a chemical compound containing this group, with chlorine in oxidation state +1.Hypochlorites are the salts of hypochlorous acid...
), which would interfere with the reaction.
The indicator used in this reaction is a starch solution
Starch indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide . However, the complex is not formed if only iodine or only iodide is present...
where amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...
forms a blue to black solution with iodine and is colourless where iodine is titrated
Redox titration
Redox titration is a type of titration based on a redox reaction between the analyte and titrant.Redox titration may involve the use of a redox indicator and/or a potentiometer.-Example:...
.
A precaution that should be observed is to add the starch indicator solution only near the end point
Equivalence point
The equivalence point, or stoichiometric point, of a chemical reaction when a titrant is added and is stoichiometrically equal to the amount of moles of substance present in the sample: the smallest amount of titrant that is sufficient to fully neutralize or react with the analyte...
(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
Correlation of rancid tasteTaste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....
and peroxide value depends on the type of oil and is best tested with taste panels. The odours and flavours associated with typical oxidative rancidity are mostly due to carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
-type compounds. The shorter-chain aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s isolated from rancid fats are due to oxidative fission and are associated with advanced stages of oxidation. The carbonyl-type compounds develop in low concentrations early in the oxidative process.
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.