SOP: Polarimetric Examination of Vinegar
GIFSQ — Global Institute for Food Safety & Quality
Principle
The optical rotation of a vinegar, measured in a polarimeter tube, distinguishes the types: malt and molasses vinegars are dextro-rotatory, cider and grape vinegars are laevo-rotatory, and spirit vinegar has no optical activity. Turbid or dark samples are clarified before reading, and a right-handed reading on a cider vinegar indicates adulteration.
Apparatus required
- Polarimeter with 100-mm, 200-mm and 400-mm tubes
- Filter paper
Reagents
a. 10% basic lead acetate solution.
b. Lead subacetate and alumina cream, in about equal quantities.
Procedure
Light-coloured vinegars, free from turbidity: measure directly in a 100-mm tube
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Slight turbidity: remove by filtering twice through a filter paper; it is advisable to add 10% basic lead acetate solution to precipitate malic or tartaric acid and to filter before polarization
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Turbid or dark-coloured vinegar: add 5 ml of about equal quantities of lead subacetate and alumina cream to 50 ml of the sample, shake, filter, and measure in a 200-mm tube; add 10% to the reading to account for dilution
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Express the polarization value of the vinegar as the actual direct reading obtained by the undiluted sample in a 200-mm or 400-mm tube
Precautions
- Filter turbid samples before polarization; suspended matter disturbs the reading.
- Correct the reading for dilution (add 10%) when the clarified sample was measured.
Interpretation
- Malt vinegars are dextro-rotatory.
- Cider vinegar is laevo-rotatory; its polarization expressed for 200 mm of the undiluted sample should be between 0.1 and 4.0 Ventzke — a right-handed polarization of 0.5° may safely be assumed as evidence of adulteration.
- Grape vinegar is slightly laevo-rotatory.
- Molasses vinegar is dextro-rotatory before and laevo-rotatory after inversion.
- Spirit vinegar has no optical activity in polarized light.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.