GB/T 24304-2024 Animal and vegetable fats and oils- Determination of anisidine value
1 Scope
This standard specifies a method for the determination of the anisidine value in animal and vegetable fats and oils.
This is a measure of the amount of aldehydes present (principally α, β-unsaturated aldehydes).
Milk and milk products (or fat coming from milk and milk products) are excluded from the scope of this standard.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 661 Animal and vegetable fats and oils - Preparation of test sample
Note: GB/T 15687-2008 Animal and vegetable fats and oils - Preparation of test sample ( ISO 661: 2003, IDT)
ISO 3696 Water for analytical laboratory use - Specification and test methods
Note: GB/T 6682-2008 Water for analytical laboratory use - Specification and test methods (ISO 3696: 1987, MOD)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
anisidine value
one hundred times the increase in absorbance, measured at a wavelength of 350 nm in a 10 mm cell, of a test solution when reacted with p-anisidine under the test conditions specified in this standard
Note: The anisidine value has no dimensions, and is calculated and quoted on the basis of 1g of the test sample in 100ml of a mixture of solvent and reagent.
4 Principle
A test solution is prepared in isooctane (2, 2, 4-trimethylpentane). It is reacted with anacetic acid solution of p-anisidine. The increase in absorbance at 350 nm is measured. The anisidine value is calculated.
5 Reagents
Use only reagents of analytical grade, and water complying with grade 3 of ISO 3696, unless otherwise indicated.
5.1 Sodium sulfate (Na₂SO₄), anhydrous.
5.2 Isooctane (2, 2, 4-trimethylpentane), having an absorbance not exceeding 0.01 against water in the wavelength range 300 nm to 380 nm.
5.3 p-anisidine (4-Methoxyaniline), anhydrous cream-coloured crystals.
WARNING - p-anisidine is toxic and care shall be taken to avoid contact with the skin.
Store the p-anisidine in a dark bottle at 0℃ to 4℃ in the dark. No coloration (grey or pink) shall be observed. If this is present, purify the p-anisidine as follows.
Dissolve 4 g of p-anisidine in 100 ml of water at 75 ℃. Add 0.5 g of sodium sulfite (Na₂SO₃) and 2 g of charcoal. Stir for 5 min and filter through a medium retention filter paper to give a clear solution. Cool the filtrate to 0℃ and leave at this temperature for at least 4 h. Filter off the crystals, preferably under vacuum, and wash with a small volume of water at about 0℃. Dry in a vacuum desiccator containing an efficient desiccant.
5.4 Glacial acetic acid, of water content not greater than 0.1% (mass fraction).
5.5 Anisidine reagent
On the day of use, prepare the minimum quantity of reagent required for the analysis, in view of its toxicity and limited life. Prepare, for example, 50 ml of reagent as follows. Dissolve 0.125 g of the p-anisidine in the glacial acetic acid in a 50 ml volumetric flask and dilute to the mark with the same solvent, avoiding exposure to strong light.
Check the absorbance against isooctane before use and discard the reagent when the difference is larger than 0.2. In any case, discard any reagent left over on the day of use.
6 Apparatus
6.1 Spectrometer, double-or single-beam, suitable for use at a wavelength of 350 nm, with cells of optical path length 10 mm.
When a double-beam spectrometer is used, it is recommended that a pair of matched 10 mm cells be used.
6.2 Volumetric flasks, of 25 ml capacity.
6.3 Test tubes, of 10 ml capacity, fitted with ground glass stoppers.
6.4 Pipettes, of 1 ml and 5 ml capacities.
7 Sampling
A recommended sampling method is given in ISO 5555.
A representative sample should have been sent to the laboratory. It should not have been damaged or changed during transport or storage.
8 Preparation of test sample
Prepare the test sample in accordance with ISO 661.
If the moisture content of the sample is greater than 0.10% (mass fraction), it should be dried using the following procedure.
Add sodium sulfate (5.1) in the proportion of 1g to 2 g per 10 g of the thoroughly mixed sample, at a temperature of not more than 10℃ above the melting point in the case of a solid fat. Stir thoroughly and filter, maintaining the temperature to prevent solidification.
9 Procedure
9.1 Preparation of test solution
Weigh, to the nearest 1 mg, a sufficient mass of the prepared test sample (Clause 8) directly into a 25ml volumetric flask. Preheat solid samples to 10℃ above their melting point. Dissolve the sample in 5 ml to 10 ml of the isooctane (5.2) and make up to the mark with the same solvent.
The size of the test portion depends on the quality of the sample and the characteristics of the spectrometer used, and should be chosen to avoid readings near the upper and lower ends of the scale. In general, 0.4 g to 4.0 g is used.
9.2 Unreacted test solution
By means of a pipette (6.4), transfer 5 ml of the test solution (9.1) to a test tube (6.3). Add 1 ml of glacial acetic acid (5.4), stopper the tube and shake well. Keep the test tube in the dark at (23±3)℃ for 8 min.
Within a further 2 min, transfer the solutions to a clean, dry spectrometer cell. After a total reaction time of 10 min±1 min, follow the procedure specified in 9.5.
9.3 Reacted test solution
Transfer, by means of a pipette (6.4), 5ml of the test solution (9.1) to a test tube (6.3). Add, by means of a pipette (6.4), 1ml of the anisidine reagent (5.5). Stopper the tube and shake well. Keep the test tube in the dark at (23±3)℃ for 8 min.
Within a further 2 min, transfer the solutions to a clean, dry spectrometer cell. After a total reaction time of 10 min±1 min from the addition of the anisidine reagent, follow the procedure specified in 9.5.
9.4 Blank
Transfer, by means of a pipette (6.4), 5 ml of isooctane (5.2) to a test tube (6.3). Add, by means of a pipette (6.4), 1 ml of the anisidine reagent (5.5). Stopper the tube and shake well. Keep the test tube in the dark at (23±3)℃ for 8 min.
Within a further 2 min, transfer the solutions to a clean, dry spectrometer cell. After a total reaction time of 10 min±1 min from the addition of the anisidine reagent, follow the procedure specified in 9.5.
9.5 Spectrometric measurement
Adjust the zero absorption of the spectrometer with isooctane (5.2) at 350 nm. Measure the following absorbances against isooctane:
- A₁ of the reacted solution (9.3),
- A0 of the unreacted test solution (9.2), and
- A2 of the blank (9.4).
9.6 Absorbance range
If the measured absorbance A₁ of the reacted solution (9.3) is not in the range 0.2 to 0.8, repeat the determination (9.2 to 9.4) with an adjusted amount of test sample.
If the measured absorbance A2 of the blank exceeds 0.2, purify the anisidine reagent, and prepare fresh anisidine reagent. Repeat this test with the fresh anisidine reagent.
