Clause 11 of FSSAI Manual of Methods of Analysis of Foods – Oils & Fats
11.1 Definition
The acid value is a crucial parameter for assessing the quality and shelf-life of oils and fats. It is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize the free fatty acids present in one gram of fat. This value is a relative measure of rancidity, indicating the degree of decomposition of triglycerides. The acid value can also be expressed as a percentage of free fatty acids, calculated as oleic, lauric, ricinoleic, and palmitic acids.
11.2 Principle
The acid value is determined through a titration process, where the oil or fat sample is dissolved in an alcoholic medium and titrated against a standard potassium hydroxide (KOH) or sodium hydroxide (NaOH) solution.
11.3 Analytical Importance
The acid value measurement is significant for understanding the extent of hydrolysis that has occurred in oils and fats. This hydrolysis is often due to the action of moisture, temperature, and lipolytic enzymes like lipase. The acid value, therefore, provides insight into the freshness and suitability of the oil or fat for consumption or industrial use.
11.4 Apparatus
- 250 mL conical flasks
11.5 Reagents
- Phenolphthalein Indicator Solution: Dissolve one gram of phenolphthalein in 100 mL of ethyl alcohol.
- Alkali Blue 6B Indicator Solution: For rice bran oil or rice bran oil-based blended oils or fats, use Alkali Blue 6B indicator to facilitate the observation of the titration endpoint. Prepare a 2% solution by extracting 2 grams of Alkali Blue 6B with rectified spirit in a Soxhlet apparatus at reflux temperature. Filter if necessary and dilute to 100 mL with rectified spirit. Store in a closed amber-colored bottle to prevent oxidation.
- Ethyl Alcohol:
- 95% alcohol or rectified spirit neutral to phenolphthalein indicator.
- 95% alcohol or rectified spirit neutral to Alkali Blue 6B indicator for rice bran oil or rice bran oil-based blended oils or fats.
- Standard Aqueous Potassium Hydroxide or Sodium Hydroxide Solution: 0.1 or 0.5 N, stored in a brown glass bottle. For refined oils, use a 0.1 N alkali solution.
11.6 Procedure
- Preparation of Sample: Mix the oil or melted fat thoroughly before weighing. The mass of the test sample is determined based on the expected acid value.
- For an expected acid value < 1, use 20 grams of the test sample with an accuracy of 0.05 grams.
- For an expected acid value between 1 and 4, use 10 grams of the test sample with an accuracy of 0.02 grams.
- For an expected acid value between 4 and 15, use 2.5 grams of the test sample with an accuracy of 0.01 grams.
- For an expected acid value between 15 and 75, use 0.5 grams of the test sample with an accuracy of 0.001 grams.
- For an expected acid value > 75, use 0.1 grams of the test sample with an accuracy of 0.0002 grams.
- Titration:
a). Weigh accurately the appropriate amount of the cooled oil sample as mentioned above in a 250 mL conical flask.
b). Add 50 mL of freshly neutralized hot ethyl alcohol and about 1 mL of phenolphthalein indicator solution (or Alkali Blue 6B indicator for rice bran oil-based samples).
c). Heat the mixture for about 15 minutes in a water bath (75-80°C).
d). Titrate the hot mixture against the standard alkali solution, shaking vigorously during the titration.
e). The endpoint using phenolphthalein indicator is the change from colorless to light pink, persisting for 15 seconds.
f). The endpoint using Alkali Blue 6B indicator is the disappearance of the blue color developed during the addition of the indicator.
g). Avoid noting the burette reading after obtaining a dark pink or orangish-red color as it leads to erroneous results. - Calculation:
[
\text{Acid Value} = \frac{56.1 \times V \times N}{W}
]
Acid value = (56.1 x V x N)/W
Where:
- ( V ) = Volume in mL of standard potassium hydroxide or sodium hydroxide used.
- ( N ) = Normality of the potassium hydroxide or sodium hydroxide solution.
- ( W ) = Weight in grams of the sample.
By following this revised method, you can accurately determine the acid value of oils and fats, providing valuable insights into their quality and suitability for various applications.
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