Analysis of dietary fats
Free FATTY ACIDs: markers of lipid hydrolysis
The acid value and acidity account for the hydrolytic alteration of fats.
The acid value (AV) corresponds to the mass of potassium hydroxide (in mg) required to neutralize the free fatty acids present in 1 g of fat. AV is expressed in mg/g.
Acidity corresponds to the content of free fatty acids. It is expressed as a percentage by weight, and is frequently based on the amount of oleic acid.
Free fatty acids: markers of lipid HYDROLysis
PrincipleThe acid value and acidity are determined by dissolving the fat in a solvent (ethanol) and then titrating the acids present in the solution with an ethanolic solution of potassium hydroxide.
R-COOH + KOH → R-COO-K+ + H20
with R-COOH: free fatty acids
Method adapted from the NF EN ISO 660 standard (‘acid value and acidity”)
Free fatty acids : markers of lipid HYDROLysis
Equipment & Reagents
- Standard laboratory equipment, in particular: analytical scale, magnetic stirrer with hot plate, burette, glassware.
- Ethanol at 95%, neutralized just before use by the addition of a few drops of ethanolic potassium hydroxide 0.05 M in the presence of 0.3 mL of a phenolphthalein solution (or equivalent) for 100 mL of solvent.
- Potassium hydroxide 0.05 M in solution with ethanol 95 %. Its titer shall be checked before use with a volumetric solution of hydrochloric acid.
Method adapted from the NF EN ISO 660 standard (“acid value and acidity”)
Free fatty acids: markers of lipid HYDROLysis
Experimental procedure- Accurately weigh 2 g of fat in a 100 mL Erlenmeyer flask (weight adapted for most refined vegetable oils and animal fats).
- Add 50 mL of neutralized ethanol and dissolve the test sample by slightly heating if necessary.
- Add 5 or 6 drops of the colored indicator (phenolphthalein or equivalent).
- Titrate with a standard solution of ethanolic potassium hydroxide 0.05 M, stirring constantly. The end point of the titration is reached once the addition of a single drop of alkali causes a noticeable change in color (even if it is slight) that persists for at least 15 s.
≈ 15 minutes / sampleMethod adapted from the NF EN ISO 3961 standard (“acid value and acidity”)