Measurement of peroxide value is a representative experiment in food chemistry and lipid chemistry used to evaluate the early stages of oxidative deterioration of fats and oils.
During storage, fats and oils are oxidized under the influence of oxygen, light, heat, metal ions, and other factors, producing peroxides such as hydroperoxides.
Peroxide value is an indicator representing the amount of these peroxides and is important when discussing the progress of oxidative deterioration of fats and oils.
In a discussion of peroxide value, it is not sufficient simply to write that “the value was high” or “the oil had deteriorated.”
It is necessary to explain which components of fats and oils are easily oxidized, why unsaturated fatty acids are easily oxidized, and why peroxide value is an indicator of the early stages of oxidation while the value may decrease when oxidation progresses because peroxides decompose.
This article clearly explains, as examples of discussions that can be used in laboratory reports on peroxide-value measurements, oxidative deterioration of fats and oils, formation of hydroperoxides, titration using potassium iodide and sodium thiosulfate, effects of storage conditions, differences from iodine value and acid value, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of peroxide-value measurement results obtained in food chemistry experiments, lipid chemistry experiments, and analytical chemistry experiments at universities and similar institutions.
For the actual sample amount, solvent, potassium iodide solution, sodium thiosulfate standard solution, indicator, blank test, calculation formula, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Peroxide Value?
- Main Items to Include in the Results
- Reference Experimental Values and Calculation Examples for Peroxide-Value Measurement
- Reference Experimental Conditions
- Concept of the Reactions in Peroxide-Value Measurement
- Titration Results Under Different Storage Conditions
- Example Calculation of Blank Correction
- Example Calculation of Peroxide Value
- Relationship Between Storage Conditions and Oxidative Deterioration
- Changes in Peroxide Value With Storage Time
- Example Comparison Among Different Types of Oil
- Example Comparison With Added Antioxidants
- Example Calculation of Inhibition Rate
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- What Is Oxidative Deterioration of Fats and Oils?
- Formation of Hydroperoxides
- Principle of Peroxide-Value Measurement
- Discussion When the Peroxide Value Is High
- Discussion When the Peroxide Value Is Low
- Unsaturated Fatty Acids and Susceptibility to Oxidation
- Effect of Light
- Effect of Contact With Oxygen
- Effect of Temperature
- Effect of Metal Ions
- Relationship With Storage Conditions
- Discussion of Heated Oil and Frying Oil
- Relationship With Iodine Value
- Difference From Acid Value
- Importance of the Blank Test
- Starch Indicator and Endpoint Determination
- Discussion of Sample Amount and Unit Conversion
- Causes of Error in Peroxide-Value Measurement
- When the Results Can Be Considered Good
- Example Discussion When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Peroxide Value
- Summary
What Is Peroxide Value?
Peroxide value is an indicator representing the amount of peroxides contained in fats and oils.
When fats and oils are oxidized, primary oxidation products such as hydroperoxides are first formed.
Peroxide value evaluates the degree of oxidative deterioration of fats and oils by measuring the amount of peroxides produced during this early stage of oxidation.
When the peroxide value is high, oxidation may have progressed in the fat or oil and peroxides may have accumulated.
However, when oxidation progresses further, peroxides decompose and become secondary oxidation products such as aldehydes and ketones.
Therefore, it is important to understand peroxide value as an indicator for evaluating the early stages of deterioration of fats and oils.
Example Discussion:
Peroxide value is a value indicating the amount of peroxides such as hydroperoxides generated in fats and oils.
In the early stages of oxidation, oxidation proceeds near the double bonds of unsaturated fatty acids and peroxides are formed.
Therefore, fats and oils with high peroxide values are considered to have undergone oxidative deterioration and to contain accumulated primary oxidation products.
Main Items to Include in the Results
In peroxide-value measurement results, organize the type of fat or oil sample, sample amount, storage conditions, solvent, addition of potassium iodide, reaction time, concentration of the sodium thiosulfate standard solution, titration volume, blank-test value, endpoint color change, peroxide value, and other information.
Storage conditions and sample history are important because they are closely related to oxidative deterioration.
Main Items to Include in the Results
- Type of fat or oil sample
- Mass of the fat or oil sample
- Storage period of the sample
- Storage temperature
- Exposure to light
- Whether the container was opened or unopened
- Presence or absence of a heating history
- Solvent used
- Amount of potassium iodide added
- Reaction time
- Concentration of the sodium thiosulfate standard solution
- Titration volume in the sample test
- Titration volume in the blank test
- Use of starch indicator
- Color change at the endpoint
- Peroxide value
- Average value from multiple measurements
- Causes of error and points for improvement
Example of How to Write the Results:
Potassium iodide was added to the fat or oil sample, and the iodine liberated by the peroxides in the sample was titrated with a sodium thiosulfate standard solution.
The amount of peroxide in the fat or oil was determined from the difference from the blank test and expressed as the peroxide value per unit mass of sample.
The obtained values were compared with the type of fat or oil, storage conditions, and heating history.
Reference Experimental Values and Calculation Examples for Peroxide-Value Measurement
Here, for peroxide value (PV), an indicator used to evaluate oxidative deterioration of fats and oils, titration volumes under different storage conditions, calculation of peroxide value, progress of oxidation, and factors causing errors are organized using reference experimental values.
Fats and oils are oxidized under the influence of oxygen in the air, light, heat, metal ions, and other factors.
Because peroxides such as hydroperoxides are formed during the early stages of oxidation, measuring peroxide value makes it possible to evaluate the progress of oxidative deterioration of fats and oils.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Sample | Edible vegetable oil |
| Sample amount | 5.00 g |
| Measurement method | Iodometric titration |
| Titrant | 0.0100 mol/L sodium thiosulfate standard solution |
| Blank titration volume | 0.05 mL |
| Storage conditions | Cool and dark place, room-temperature dark place, room-temperature light exposure, storage at 40°C, opened and left standing |
| Evaluation items | Titration volume, peroxide value, oxidative deterioration due to storage conditions |
Concept of the Reactions in Peroxide-Value Measurement
In peroxide-value measurement, peroxides in fats and oils oxidize iodide ions and liberate iodine.
The generated iodine is titrated with a sodium thiosulfate standard solution, and the peroxide value is determined from the amount of iodine.
| Step | Reaction or Operation | Meaning |
|---|---|---|
| 1 | Peroxides in the fat or oil oxidize I− | I2 is generated according to the amount of peroxide |
| 2 | I2 is titrated with Na2S2O3 | The amount of iodine generated is determined |
| 3 | Peroxide value is calculated from the titration volume | The degree of oxidative deterioration of the fat or oil is evaluated |
Titration Results Under Different Storage Conditions
The following shows an example in which the same vegetable oil was stored under different conditions for seven days and its peroxide value was measured.
The sample amount is 5.00 g in all cases, and the blank titration volume is assumed to be 0.05 mL.
| Sample | Storage Condition | Storage Period | Sample Mass | Titration Volume | Blank-Corrected Titration Volume | Peroxide Value |
|---|---|---|---|---|---|---|
| A | Fresh oil | 0 days | 5.00 g | 0.18 mL | 0.13 mL | 0.26 meq/kg |
| B | Stored in a cool, dark place | 7 days | 5.00 g | 0.32 mL | 0.27 mL | 0.54 meq/kg |
| C | Stored at room temperature in the dark | 7 days | 5.00 g | 0.62 mL | 0.57 mL | 1.14 meq/kg |
| D | Room temperature with light exposure | 7 days | 5.00 g | 1.48 mL | 1.43 mL | 2.86 meq/kg |
| E | Stored at 40°C | 7 days | 5.00 g | 2.25 mL | 2.20 mL | 4.40 meq/kg |
| F | Opened and left standing | 7 days | 5.00 g | 2.82 mL | 2.77 mL | 5.54 meq/kg |
Example Calculation of Blank Correction
Blank correction is performed to subtract the titration volume originating from the reagents and solvent.
Corrected titration volume = Sample titration volume − Blank titration volume
For sample D, the sample titration volume is 1.48 mL and the blank titration volume is 0.05 mL.
Corrected titration volume = 1.48 − 0.05 = 1.43 mL
This corrected titration volume is used to calculate the peroxide value.
Example Calculation of Peroxide Value
Peroxide value is a value expressing the amount of peroxide contained in 1 kg of fat or oil in equivalents.
In this reference example, it is calculated using the following equation.
Peroxide value (meq/kg) = Corrected titration volume (mL) × Sodium thiosulfate concentration (mol/L) × 1000 ÷ Sample mass (g)
For sample D, the corrected titration volume is 1.43 mL, the sodium thiosulfate concentration is 0.0100 mol/L, and the sample mass is 5.00 g.
PV = 1.43 × 0.0100 × 1000 ÷ 5.00 = 2.86 meq/kg
Therefore, the peroxide value of sample D is calculated to be 2.86 meq/kg.
Relationship Between Storage Conditions and Oxidative Deterioration
Comparing the peroxide values under each storage condition, the increase is small when stored in a cool, dark place, whereas the values become larger under light exposure, heating, and opened-storage conditions.
| Storage Condition | Peroxide Value | Difference From Fresh Oil | Evaluation of Oxidative Deterioration |
|---|---|---|---|
| Fresh oil | 0.26 meq/kg | – | Little oxidation |
| Stored in a cool, dark place | 0.54 meq/kg | +0.28 meq/kg | Slight oxidation |
| Stored at room temperature in the dark | 1.14 meq/kg | +0.88 meq/kg | Oxidation progresses somewhat |
| Room temperature with light exposure | 2.86 meq/kg | +2.60 meq/kg | Oxidation is promoted by light |
| Stored at 40°C | 4.40 meq/kg | +4.14 meq/kg | Oxidation is promoted by heat |
| Opened and left standing | 5.54 meq/kg | +5.28 meq/kg | Oxidation is greatly promoted by contact with air |
In this reference example, the sample that was opened and left standing showed the highest peroxide value.
When the area and duration of contact between the fat or oil and air increase, oxidation by oxygen is considered to proceed more readily.
Changes in Peroxide Value With Storage Time
The following shows an example in which the same oil was stored at room temperature under light exposure and the peroxide value was measured at different storage times.
| Storage Period | Titration Volume | Corrected Titration Volume | Peroxide Value | How to Interpret the Change |
|---|---|---|---|---|
| 0 days | 0.18 mL | 0.13 mL | 0.26 meq/kg | Little oxidation |
| 3 days | 0.72 mL | 0.67 mL | 1.34 meq/kg | Oxidation begins to progress |
| 7 days | 1.48 mL | 1.43 mL | 2.86 meq/kg | Peroxides increase |
| 14 days | 2.95 mL | 2.90 mL | 5.80 meq/kg | Oxidation progresses further |
| 21 days | 3.60 mL | 3.55 mL | 7.10 meq/kg | The increase becomes somewhat more gradual |
The peroxide value increases as the storage period becomes longer.
However, when oxidation of fats and oils progresses further, the generated peroxides decompose into secondary oxidation products such as aldehydes and ketones, so the increase in peroxide value may slow or the value may sometimes decrease.
Example Comparison Among Different Types of Oil
The susceptibility of fats and oils to oxidation also varies depending on fatty-acid composition and the presence or absence of antioxidant components.
Here, reference examples are shown for different oils stored at 40°C for seven days.
| Type of Oil | Storage Condition | Peroxide Value | How to Interpret Oxidation Susceptibility |
|---|---|---|---|
| Olive oil | 40°C, 7 days | 2.10 meq/kg | Relatively resistant to oxidation |
| Canola oil | 40°C, 7 days | 3.25 meq/kg | Moderate |
| Soybean oil | 40°C, 7 days | 4.40 meq/kg | Oxidation tends to progress readily |
| Fish oil | 40°C, 7 days | 8.65 meq/kg | Very susceptible to oxidation |
Oils containing large amounts of highly unsaturated fatty acids tend to be more susceptible to oxidation and show increases in peroxide value more readily.
Therefore, the progress of oxidative deterioration differs among oils even under the same storage conditions.
Example Comparison With Added Antioxidants
Adding antioxidants may suppress oxidation of fats and oils.
Here, an example is shown in which the same oil was stored at 40°C for seven days and compared with and without antioxidants.
| Condition | Peroxide Value | Difference From No Addition | Inhibition Rate | Evaluation |
|---|---|---|---|---|
| No addition | 4.40 meq/kg | – | – | Oxidation progresses |
| Vitamin E added | 2.55 meq/kg | 1.85 meq/kg | 42.0% | Oxidation is suppressed |
| Rosemary extract added | 2.20 meq/kg | 2.20 meq/kg | 50.0% | Oxidation is suppressed further |
Example Calculation of Inhibition Rate
The effect of an antioxidant can be determined by comparing the peroxide values under conditions without and with the antioxidant.
Inhibition rate (%) = (PV without addition − PV with addition) ÷ PV without addition × 100
Under the vitamin E addition condition, the PV without addition is 4.40 meq/kg and the PV after addition is 2.55 meq/kg.
Inhibition rate = (4.40 − 2.55) ÷ 4.40 × 100 = 42.0%
Therefore, the addition of vitamin E was considered to suppress peroxide formation by approximately 42.0%.
Example of How to Write the Results
Edible vegetable oil was stored under different conditions for seven days, and the peroxide value was measured.
The peroxide value of the fresh oil was 0.26 meq/kg, whereas it was 0.54 meq/kg after storage in a cool, dark place and 1.14 meq/kg after storage at room temperature in the dark.
On the other hand, the value was 2.86 meq/kg for the sample exposed to light at room temperature, 4.40 meq/kg after storage at 40°C, and 5.54 meq/kg when opened and left standing, showing differences in the progress of oxidative deterioration depending on the storage conditions.
For sample D, the sample titration volume was 1.48 mL and the blank titration volume was 0.05 mL.
The corrected titration volume was 1.43 mL, and because a 0.0100 mol/L sodium thiosulfate standard solution was used, the peroxide value was calculated to be 2.86 meq/kg.
Points for Connecting the Results to the Discussion
In a discussion of peroxide value, it is important to explain not only the magnitude of the numerical values but also the conditions under which oxidation of the fats and oils was promoted.
- Has the blank value been subtracted from the titration volume in the calculation?
- Can samples with higher peroxide values be explained as containing more early oxidation products?
- Can the reason peroxide value increased because of light, heat, and contact with air be explained?
- Can the reason oxidation was suppressed when stored in a cool, dark place be explained?
- Can differences in oxidation susceptibility among oils be explained in relation to the degree of unsaturation?
- Can the reason peroxide value decreased when antioxidants were added be explained?
- Can it be considered that peroxide value is an indicator of early oxidation and that peroxides may decompose when oxidation progresses too far?
- Can endpoint judgment, volatilization of iodine, reactions caused by light, and delays in titration be considered as sources of error?
Example Discussion
In this experiment, peroxide values were measured for edible oils stored under different conditions, and oxidative deterioration of the oils was compared.
The peroxide value of fresh oil was 0.26 meq/kg, whereas it increased only slightly to 0.54 meq/kg when stored in a cool, dark place.
On the other hand, the peroxide value increased greatly under conditions involving light exposure, storage at 40°C, and opening and leaving the oil standing.
This was considered to result from oxidation reactions in the oil being promoted by contact with light, heat, and oxygen, thereby generating larger amounts of peroxides.
In particular, the sample that was opened and left standing showed a peroxide value of 5.54 meq/kg, the highest value among the conditions used in this experiment.
When the oil is opened, it comes into contact more readily with oxygen in the air, making oxidation reactions more likely to proceed.
In addition, the peroxide value increased under the light-exposure condition, suggesting that light may have promoted radical reactions and accelerated oxidation.
In the comparison among types of oil, fish oil showed the highest peroxide value, while olive oil showed a relatively low value.
Fats and oils containing large amounts of unsaturated fatty acids contain double bonds and are therefore susceptible to oxidation.
Consequently, even under the same storage conditions, the progress of oxidative deterioration is considered to differ depending on the fatty-acid composition of the oil.
Under conditions where antioxidants were added, the peroxide value was lower than in the condition without additives.
This was considered to result from antioxidant components suppressing radical reactions and delaying peroxide formation.
However, peroxide value is an indicator for evaluating peroxides generated during the early stages of oxidation, and when oxidation progresses further, the peroxides may decompose into secondary oxidation products.
Therefore, to comprehensively evaluate deterioration of fats and oils, it is also important to consider other indicators such as acid value and carbonyl value.
Summary
Peroxide value is an indicator used to evaluate the amount of peroxides generated during the early stages of oxidation of fats and oils.
By correcting the titration volume using the blank value and dividing by the sample mass, the amount of peroxide can be expressed per kilogram of fat or oil.
In this reference example, the increase in peroxide value was small when the oil was stored in a cool, dark place, whereas the peroxide value became large under light exposure, heating, and opened-storage conditions.
In a report, it is useful to discuss storage conditions, oxidative deterioration, type of oil, antioxidants, and sources of error in relation to one another.
What Is Oxidative Deterioration of Fats and Oils?
Oxidative deterioration of fats and oils is a phenomenon in which fatty acids in fats and oils, particularly unsaturated fatty acids, react with oxygen and undergo changes.
As oxidation progresses, hydroperoxides, aldehydes, ketones, acids, and other substances are formed, changing the odor, taste, color, and quality of the fats and oils.
The so-called deteriorated odor or rancid odor of oils is related to oxidative decomposition products.
Oxidation of fats and oils is promoted by oxygen, light, heat, metal ions, enzymes, and other factors.
In particular, fats and oils containing many unsaturated bonds are more susceptible to oxidation.
Peroxide value is an indicator for measuring peroxides generated during this early stage of oxidative deterioration.
Example Discussion:
During oxidative deterioration of fats and oils, unsaturated fatty acids react with oxygen and first form primary oxidation products such as hydroperoxides.
As oxidation progresses, these peroxides are further decomposed to form aldehydes and ketones, leading to deterioration of odor and taste.
Peroxide value is an indicator for evaluating the amount of peroxides formed during this early stage of oxidation.
Formation of Hydroperoxides
In the early stages of oxidation of fats and oils, oxidation proceeds near the double bonds of unsaturated fatty acids and hydroperoxides are formed.
Hydroperoxides are relatively unstable compounds and decompose into secondary oxidation products as oxidation progresses further.
Therefore, peroxide value is treated as a value indicating the progress of the early stages of oxidation.
During the stage in which hydroperoxides increase, the peroxide value rises.
However, when oxidation continues for a long period, hydroperoxides decompose, and the peroxide value does not necessarily remain high.
This point is extremely important when interpreting peroxide value.
Example Discussion:
During the early stages of oxidation of fats and oils, hydroperoxides are formed from unsaturated fatty acids, causing the peroxide value to increase.
However, hydroperoxides are unstable and decompose into aldehydes, ketones, and other compounds as oxidation progresses further.
Therefore, even when the peroxide value is low, the fat or oil is not necessarily fresh, and the possibility that oxidation has progressed too far and the peroxides have already decomposed must also be considered.
Principle of Peroxide-Value Measurement
Peroxide-value measurement uses the reaction in which peroxides in fats and oils oxidize iodide ions, I-, and liberate iodine, I2.
The liberated iodine is titrated with a sodium thiosulfate standard solution to determine the amount of peroxide.
In other words, the larger the amount of peroxide in the fat or oil, the greater the amount of iodine liberated.
In the titration, iodine is reduced to iodide ions by thiosulfate ions.
Near the endpoint, starch indicator is used, and the endpoint is judged as the point at which the blue-violet color caused by the iodine-starch reaction disappears.
It is also important to perform a blank test and correct for values originating from the reagents and procedures.
ROOH + 2I- + 2H+ → ROH + I2 + H2O
I2 + 2S2O32- → 2I- + S4O62-
Example Discussion:
Peroxide-value measurement uses the reaction in which peroxides in fats and oils oxidize iodide ions and liberate iodine.
By titrating the liberated iodine with sodium thiosulfate, the amount of peroxide in the fat or oil can be determined.
Therefore, the greater the sodium thiosulfate titration volume, the greater the amount of peroxide considered to be present in the sample fat or oil.
Discussion When the Peroxide Value Is High
When the peroxide value is high, a large amount of primary oxidation products such as hydroperoxides is considered to have accumulated in the fat or oil.
This may indicate that oxidation progressed during storage under the influence of oxygen, light, and heat.
Oils that have been opened, stored for long periods, or repeatedly heated may show high peroxide values.
However, a high peroxide value indicates that large amounts of peroxides are present during the early to intermediate stages of oxidation.
When deterioration progresses further, the peroxides decompose, and the state of deterioration may not be completely evaluated using peroxide value alone.
Combining peroxide value with other indicators such as acid value and carbonyl value allows a more appropriate evaluation.
Example Discussion:
The high peroxide value suggests that the sample fat or oil contained a large amount of primary oxidation products such as hydroperoxides.
Possible causes include oxidation of unsaturated fatty acids progressing during storage under the influence of oxygen, light, and heat.
In particular, fats and oils stored for long periods after opening tend to undergo oxidation because of contact with air and therefore tend to show increased peroxide values.
Discussion When the Peroxide Value Is Low
When the peroxide value is low, oxidation of the fat or oil may not have progressed greatly and only a small amount of peroxide may have been formed.
Fresh fats and oils, fats and oils stored under dark, low-temperature, sealed conditions, and fats and oils with a low degree of unsaturation tend to show lower peroxide values.
However, a low peroxide value does not necessarily mean that the fat or oil is fresh.
In fats and oils in which oxidation has progressed considerably, the generated peroxides may decompose into secondary oxidation products and the peroxide value may decrease.
Therefore, information such as odor, color, acid value, and carbonyl value must also be considered.
Example Discussion:
When the peroxide value is low, peroxide formation in the fat or oil may be small and oxidative deterioration may not have progressed greatly.
However, in fats and oils in which oxidation has progressed too far, hydroperoxides may decompose and the peroxide value may become low.
Therefore, freshness should not be judged from peroxide value alone, and the result must be interpreted together with storage conditions, odor, and other deterioration indicators.
Unsaturated Fatty Acids and Susceptibility to Oxidation
The susceptibility of fats and oils to oxidation is related to the amount of unsaturated fatty acids.
Unsaturated fatty acids contain carbon-carbon double bonds, and regions near these double bonds are susceptible to oxidation.
Fatty acids with multiple double bonds, such as linoleic acid and linolenic acid, tend to be particularly susceptible to oxidation.
Fats and oils with high iodine values have high degrees of unsaturation and may be more susceptible to oxidation.
Therefore, fats and oils with high iodine values may show more rapid increases in peroxide value during storage.
However, the oxidation rate changes depending on the presence or absence of antioxidant components and the storage conditions.
Example Discussion:
Unsaturated fatty acids contain double bonds and are therefore susceptible to oxidation by oxygen.
In particular, fats and oils containing large amounts of polyunsaturated fatty acids are likely to form hydroperoxides during the early stages of oxidation and show increases in peroxide value.
Therefore, when discussing peroxide value, the relationship with the degree of unsaturation and iodine value of the fat or oil must also be considered.
Effect of Light
Light is one of the factors that promotes oxidation of fats and oils.
Particularly when fats and oils are stored in transparent containers in bright locations, light may promote oxidation reactions and increase the peroxide value.
Light promotes radical reactions and photooxidation and accelerates oxidation of unsaturated fatty acids.
Storing fats and oils in light-blocking containers or in dark places suppresses oxidation caused by light.
In experiments comparing storage conditions, comparing the peroxide values of samples exposed to light and samples protected from light makes it easier to discuss the effect of light.
Example Discussion:
If the peroxide value was high in fat or oil stored under light exposure, light was considered to have promoted oxidation of unsaturated fatty acids and caused hydroperoxide formation.
On the other hand, under light-blocking conditions, photooxidation is suppressed, so the increase in peroxide value may be smaller.
This suggests that storage away from light is effective for preventing oxidative deterioration of fats and oils.
Effect of Contact With Oxygen
Oxygen is necessary for oxidation of fats and oils.
Fats and oils that have been opened or stored with a large amount of air in the container have increased contact with oxygen and are more likely to oxidize.
Leaving the container open or repeatedly opening and closing it may cause the peroxide value to increase more readily.
Conversely, oxidation can be suppressed by storing fats and oils in sealed containers and reducing contact with air.
Methods such as nitrogen replacement or oxygen absorbers are also available, but they are used according to the purpose of the experiment or food storage.
When discussing peroxide value, whether the container was opened and the amount of air inside the container are also important.
Example Discussion:
If the peroxide value was high in fat or oil after opening, the increased opportunity for contact with oxygen in the air was considered to have promoted oxidation of unsaturated fatty acids.
When stored in a container containing a large amount of air, oxygen continues to be supplied and hydroperoxide formation becomes more likely.
Therefore, to prevent oxidative deterioration of fats and oils, it is important to reduce contact with oxygen by sealed storage.
Effect of Temperature
The higher the temperature, the more readily oxidation reactions in fats and oils proceed.
High-temperature storage, cooking by heating, and repeated use of frying oil may promote oxidation reactions and increase the peroxide value.
Heat increases the rate of oxidation reactions and promotes both hydroperoxide formation and decomposition.
However, in fats and oils heated at high temperatures for long periods, the generated peroxides may decompose further, causing the peroxide value to become low.
Therefore, when evaluating deterioration of heated fats and oils, it is useful to consider not only peroxide value but also acid value, carbonyl value, and other indicators.
Example Discussion:
In fats and oils stored or heated at high temperatures, oxidation reactions are promoted, so the peroxide value is considered likely to increase.
However, when heating continues for a long period, the generated hydroperoxides decompose into secondary oxidation products, so peroxide value alone may not reveal the overall state of deterioration.
Therefore, when evaluating heated fats and oils, the temperature and heating time must be recorded and other deterioration indicators must also be considered.
Effect of Metal Ions
Metal ions such as iron and copper may promote oxidation of fats and oils.
When trace amounts of metal ions are present, radical reactions become more likely to proceed and hydroperoxide formation and decomposition may be promoted.
Contact with metal containers or cooking utensils may also have an effect depending on the conditions.
Avoiding long-term contact between fats and oils and metals may help prevent oxidation during storage.
In experiments, contamination of equipment and introduction of metal ions can also be considered as sources of error.
Because even trace components may promote oxidation reactions, cleanliness of equipment is important.
Example Discussion:
Metal ions may promote oxidation of fats and oils, and contamination with iron or copper in particular may promote radical reactions.
If metal components contaminated the sample or equipment, hydroperoxide formation may have progressed and the peroxide value may have become high.
Therefore, in peroxide-value measurements, it is important to use clean equipment and avoid metal contamination.
Relationship With Storage Conditions
Oxidative deterioration of fats and oils changes greatly depending on storage conditions.
Oxidation progresses more readily in places exposed to light, at high temperatures, in containers that allow easy contact with air, and during long-term storage.
On the other hand, oxidation is more readily suppressed by dark, low-temperature, sealed, and short-term storage.
When discussing peroxide-value measurement results, it is essential to confirm how the sample was stored.
Even for the same fat or oil, the peroxide value may differ between unopened and opened states, refrigerated and room-temperature storage, and transparent and light-blocking containers.
Example Discussion:
Fats and oils stored at room temperature, in bright locations, and in an opened state are more readily affected by oxygen, light, and heat and are therefore considered likely to show increased peroxide values.
On the other hand, under low-temperature, light-blocking, and sealed conditions, oxidation reactions are suppressed and the increase in peroxide value may be smaller.
Therefore, peroxide-value results must be discussed in relation to storage temperature, light exposure, opened or unopened state, and storage period.
Discussion of Heated Oil and Frying Oil
In frying oils and fats and oils that have been repeatedly heated, oxidation, hydrolysis, and thermal decomposition may proceed simultaneously.
While heating produces peroxides, the peroxides may also be further decomposed into secondary oxidation products such as aldehydes and ketones.
Therefore, care is required when interpreting peroxide values in heated oils.
Peroxide value may increase during the early stages of heating, but during prolonged heating, the peroxides may decompose and the value may decrease.
Therefore, when evaluating deterioration of frying oil, not only peroxide value but also acid value, carbonyl value, color, odor, viscosity, and other factors are considered.
Example Discussion:
In repeatedly heated fats and oils, hydroperoxides are generated by oxidation, but under high-temperature conditions they also decompose and form secondary oxidation products.
Therefore, the peroxide value of heated oil does not necessarily continue to increase in proportion to the progress of deterioration.
When discussing deterioration of frying oil, the peroxide value should be evaluated together with acid value and changes in odor and color.
Relationship With Iodine Value
Iodine value is an indicator representing the amount of unsaturated bonds in fats and oils.
Fats and oils containing more unsaturated bonds tend to be more susceptible to oxidation and more likely to form peroxides.
Therefore, fats and oils with high iodine values may show greater increases in peroxide value under poor storage conditions.
However, iodine value indicates the degree of unsaturation of fats and oils and does not directly indicate how far oxidation has actually progressed.
Peroxide value indicates the amount of early oxidation products.
Therefore, iodine value should be distinguished as an indicator of susceptibility to oxidation, while peroxide value indicates the state of oxidation progress.
Example Discussion:
Fats and oils with high iodine values contain many unsaturated bonds and therefore tend to be susceptible to oxidation.
Consequently, when exposed to oxygen, light, or heat, they may readily form hydroperoxides and show increases in peroxide value.
However, iodine value indicates susceptibility to oxidation, while the actual progress of oxidation must be evaluated using peroxide value.
Difference From Acid Value
Acid value is an indicator representing the amount of free fatty acids contained in fats and oils.
When fats and oils undergo hydrolysis, free fatty acids increase and the acid value rises.
In contrast, peroxide value represents the amount of peroxides generated during the early stages of oxidation of fats and oils.
In other words, acid value and peroxide value evaluate different aspects of deterioration of fats and oils.
During storage, oxidation and hydrolysis may proceed simultaneously in fats and oils.
Therefore, measuring both peroxide value and acid value allows the state of deterioration to be evaluated from multiple perspectives.
In a report, it is important not to confuse the meanings of acid value and peroxide value.
Example Discussion:
Peroxide value is a value indicating the amount of peroxides that are early oxidation products of fats and oils, whereas acid value indicates the amount of free fatty acids.
Therefore, peroxide value can be distinguished as an indicator of oxidative deterioration, while acid value is an indicator used to consider hydrolytic deterioration.
Because multiple reactions are involved in deterioration of fats and oils, considering both peroxide value and acid value allows the deterioration state to be evaluated in greater detail.
Importance of the Blank Test
In peroxide-value measurements, it is important to perform the same procedure without adding the fat or oil sample as a blank test.
The blank test makes it possible to correct for iodine generation and sodium thiosulfate consumption originating from reagents, solvents, and operational procedures.
Without a blank test, values not originating from the sample fat or oil may be included in the calculated peroxide amount.
Particularly for fats and oils with low peroxide values, the titration volume in the sample test is small, so the effect of the blank test becomes relatively large.
If the blank value is large, deterioration of reagents, contamination of the solvent, insufficient cleaning of equipment, and other causes must be suspected.
Example Discussion:
The blank test is necessary to correct for sodium thiosulfate consumption originating from the reagents, solvent, and operation.
If a blank test is not performed, iodine not originating from peroxides in the fat or oil may also be included in the measured value, causing the peroxide value to be overestimated.
Particular care is required for samples with low peroxide values because the effect of the blank value becomes larger.
Starch Indicator and Endpoint Determination
In sodium thiosulfate titration, starch indicator may be added near the endpoint.
Starch reacts with iodine to produce a blue-violet color.
When iodine is reduced by sodium thiosulfate, the blue-violet color disappears, and this disappearance is used to determine the endpoint.
If too much sodium thiosulfate is added beyond the endpoint, the titration volume becomes larger and the peroxide value may be overestimated.
Conversely, if the titration is stopped while the blue-violet color remains, the peroxide value may be underestimated.
Near the endpoint, it is important to add the titrant one drop at a time and mix thoroughly.
Example Discussion:
Starch indicator can be used to determine the endpoint of sodium thiosulfate titration because it produces a blue-violet color with iodine.
At the endpoint, iodine is reduced and the blue-violet color disappears.
If titration continues beyond the endpoint, the sodium thiosulfate titration volume becomes too large and the peroxide value may be overestimated, so careful operation is necessary near the endpoint.
Discussion of Sample Amount and Unit Conversion
Because peroxide value is calculated as a value per unit mass of fat or oil sample, errors in weighing the sample greatly affect the result.
If the sample amount is recorded as smaller than the actual amount, the peroxide value is calculated as higher.
Conversely, if the sample amount is recorded as larger, the peroxide value is calculated as lower.
In addition, the concentration of the sodium thiosulfate standard solution, titration volume, blank correction, and unit conversions must be handled correctly.
Errors in conversions between mL and L, g and kg, milliequivalents, and other units can cause a large deviation in the peroxide value.
It is important to confirm not only the calculation formula but also the meaning of the units.
Example Discussion:
Because peroxide value is expressed per unit mass of the fat or oil sample, errors in sample mass directly affect the result.
In addition, incorrect handling of the concentration of the sodium thiosulfate standard solution, titration volume, blank value, or unit conversion can cause the peroxide value to deviate greatly.
Therefore, accurate weighing and calculation procedures are important.
Causes of Error in Peroxide-Value Measurement
Causes of error in peroxide-value measurement include errors in weighing the sample, errors in the concentration of the sodium thiosulfate standard solution, insufficient blank correction, endpoint-determination errors, deterioration of reagents, contamination of the solvent, oxidation of potassium iodide, reactions caused by light, oxidation of the fat or oil during measurement, and variation in the storage condition of the sample.
Because peroxides are unstable, handling of the sample is also important.
Causes that may increase the value include insufficient blank correction, titration beyond the endpoint, progress of oxidation during measurement, and iodine generation originating from reagents.
Causes that may decrease the value include stopping titration before the endpoint, decomposition of peroxides, nonuniform sample collection, and errors in sodium thiosulfate concentration.
Organizing the causes of error into overestimation and underestimation makes the discussion easier.
Example Discussion:
Possible causes of error in peroxide value include deviations in endpoint determination, insufficient blank correction, concentration errors in the sodium thiosulfate standard solution, and oxidation of the fat or oil during measurement.
If titration continues beyond the endpoint or blank correction is insufficient, the peroxide value may be overestimated.
On the other hand, if the peroxides had decomposed before measurement, a low peroxide value may be obtained even when actual deterioration has progressed.
When the Results Can Be Considered Good
Peroxide-value measurements can be considered to have produced good results when the blank value is small and stable, variation among repeated titration values is small, and the measured values do not contradict the trends expected from the storage conditions.
For example, if low values are obtained for unopened fats and oils stored at low temperature and protected from light, while high values are obtained for opened fats and oils stored at high temperature and in bright conditions, the results are consistent with the storage conditions.
However, old and deteriorated fats and oils may show low values because the peroxides have decomposed, so information such as odor, color, acid value, and carbonyl value should also be used as references.
The results should be judged with awareness that peroxide value is an indicator of the early stages of oxidation.
Example Discussion:
In this experiment, there was no large variation among repeated titration values and the blank value was also small.
In addition, fats and oils stored in an opened state in a bright place showed high peroxide values, whereas fats and oils stored under light-blocking and low-temperature conditions showed low values.
This trend agrees with the idea that light, oxygen, and temperature promote oxidation of fats and oils, and the measurement results were considered to generally reflect differences in oxidative deterioration caused by the storage conditions.
Example Discussion When the Experiment Did Not Go Well
When peroxide-value measurement does not go well, possible causes are considered from results such as variation in titration values, a large blank value, disagreement between the storage conditions and the trend in values, difficulty identifying the endpoint, or a low value despite the oil being old.
Organizing the causes according to sample storage, reagents, blank testing, titration operation, endpoint determination, and calculation makes the discussion easier.
Example Discussion:
In this experiment, the peroxide value of the fat or oil stored for a long period was lower than expected.
One possible cause is that hydroperoxides generated during the early stages of oxidation had further decomposed and changed into secondary oxidation products such as aldehydes and ketones.
In addition, stopping the titration before the endpoint or nonuniform sample collection may also cause the peroxide value to be underestimated.
How to Write Points for Improvement
In a discussion of peroxide-value measurement, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sample storage, reagent management, titration operation, blank testing, and calculation and analysis.
Improvements to Sample Storage and Pretreatment
- Store the fat or oil sample protected from light
- Store at low temperature
- Seal the container to reduce contact with oxygen
- Record the storage period after opening
- Record the heating history
- Mix the sample thoroughly before collecting it
- Keep the standing time before and after measurement short
Improvements to Reagents and Titration Operation
- Accurately standardize the sodium thiosulfate standard solution
- Use fresh potassium iodide solution
- Perform the blank test under the same conditions
- Add starch indicator at the appropriate timing
- Add the titrant one drop at a time near the endpoint
- Carefully confirm disappearance of the blue-violet color
- Perform the operation while avoiding the effects of light
Improvements to Calculation and Analysis
- Correctly apply the blank correction
- Accurately reflect the sample mass in the calculation
- Check the titrant concentration and unit conversions
- Perform multiple measurements and calculate the average value
- Compare the results among storage conditions
- Organize the differences from iodine value and acid value
- Consider peroxide decomposition when oxidation has progressed too far
Example of How to Write Points for Improvement:
To improve the accuracy of peroxide-value measurement, the fat or oil sample must be stored under light-blocking, low-temperature, and sealed conditions to suppress oxidation as much as possible before measurement.
In addition, it is important to perform the blank test under the same conditions and confirm the concentration of the sodium thiosulfate standard solution.
During titration, starch indicator should be added near the endpoint and the point at which the blue-violet color disappears should be carefully determined to reduce endpoint errors.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of peroxide value, simply writing that “the value was high” or “the oil had deteriorated” results in a superficial discussion.
A good discussion relates early oxidation products, storage conditions, peroxide decomposition, the titration principle, blank testing, and differences from other indicators for fats and oils.
| Superficial Discussion | Good Discussion |
|---|---|
| The peroxide value was high. | The high peroxide value suggests that primary oxidation products such as hydroperoxides accumulated through oxidation of unsaturated fatty acids. |
| The value was low even though the oil was old. | In fats and oils in which oxidation has progressed, the generated hydroperoxides may decompose into secondary oxidation products, causing the peroxide value to become low. |
| There were differences depending on storage conditions. | Oxidation is promoted by oxygen, light, and heat under bright, high-temperature, and opened conditions, whereas oxidation is suppressed under light-blocking, low-temperature, and sealed conditions, causing differences in peroxide value. |
| The titration values varied. | The variation in titration values may have been caused by deviations in endpoint determination, insufficient blank correction, errors in the standard-solution concentration, nonuniformity of the sample, and progress of oxidation during measurement. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of peroxide-value measurements.
Adjust the necessary parts according to your own experimental results.
- Peroxide value is an indicator representing the amount of peroxides contained in fats and oils.
- During the early stages of oxidation of fats and oils, primary oxidation products such as hydroperoxides are formed.
- When the peroxide value is high, oxidative deterioration of the fat or oil may have progressed.
- When oxidation progresses further, peroxides may decompose and the peroxide value may decrease.
- Fats and oils containing large amounts of unsaturated fatty acids are susceptible to oxidation.
- Light, oxygen, heat, and metal ions are factors that promote oxidation of fats and oils.
- Light-blocking, low-temperature, and sealed storage are methods for suppressing oxidative deterioration of fats and oils.
- Peroxide value is an indicator of the early stages of oxidation and represents a different aspect of deterioration from acid value.
- The blank test is important for correcting for effects originating from reagents and procedures.
- Deviation in endpoint determination can lead to overestimation or underestimation of the peroxide value.
Points to Check When Discussing Peroxide Value
Checking the following points before writing the report makes the discussion easier to write.
- Is the definition of peroxide value explained?
- Are the early oxidation products of fats and oils explained?
- Are the formation and decomposition of hydroperoxides considered?
- Is the principle of titration using potassium iodide and sodium thiosulfate described?
- Is the meaning of a high peroxide value explained?
- Is the possibility of deterioration considered even when the peroxide value is low?
- Are the effects of light, oxygen, temperature, and metal ions considered?
- Are the storage conditions related to the measured values?
- Are the differences from iodine value and acid value distinguished?
- Is the meaning of the blank test explained?
- Are errors in endpoint determination and standard-solution concentration considered?
- Do the points for improvement correspond to the causes of error?
Summary
Peroxide value is a value representing the amount of peroxides such as hydroperoxides contained in fats and oils and is an indicator for evaluating the early stages of oxidative deterioration of fats and oils.
Unsaturated fatty acids in fats and oils readily react with oxygen and form peroxides during the early stages of oxidation.
Therefore, fats and oils with high peroxide values are considered to have undergone oxidation and to contain accumulated primary oxidation products.
However, peroxides are unstable and decompose into secondary oxidation products such as aldehydes and ketones as oxidation progresses further.
Therefore, even if the peroxide value is low, the fat or oil is not necessarily fresh.
When evaluating deterioration of fats and oils, it is important to consider storage conditions, odor, color, acid value, iodine value, and other information together.
In a report, rather than simply writing that “the peroxide value was high or low,” organize and discuss oxidative deterioration of fats and oils, formation and decomposition of hydroperoxides, storage conditions, titration using potassium iodide and sodium thiosulfate, blank testing, endpoint determination, causes of error, and points for improvement.
Peroxide value is an important analytical value for chemically understanding the storage stability and oxidative deterioration of fats and oils.
