SOP: Ash Analysis of Vinegar

GIFSQ — Global Institute for Food Safety & Quality

Principle

The ash content and the alkalinity of the ash reveal whether a vinegar is brewed or artificial: brewed vinegars leave a substantial alkaline ash, while artificial vinegars leave almost none. The total ash is found by ignition; the alkalinity number of the ash is the number of millilitres of normal acid required to neutralize 1 g of the ash, found by back titration; and the ash is further divided into water-soluble and water-insoluble fractions, the soluble filtrate being kept for the determination of sodium chloride and the alkalinity of the water-soluble ash.

Apparatus required

• Muffle furnace
• Silica dishes
• Desiccator
• Watch glass
• Ashless filter paper

Reagents

a. 0.1 N HCl.

b. 0.1 N NaOH.

c. Methyl orange indicator.

d. Water.

Procedure

Total ash: after the determination of total solids, ignite the residue in a muffle furnace at 450° C to get the ash; cool in a desiccator and weigh

↓

Alkalinity of ash: to the ash in one of the silica dishes, add an excess of 0.1 N HCl and warm the mixture; cool and back titrate with 0.1 N NaOH solution using methyl orange as the indicator; carry out a blank titration using a similar volume of acid as added to the ash

↓

Water-soluble and insoluble ash: to the ash in the second silica dish, add 25 ml of water; cover with a watch glass to avoid loss by spattering, and heat nearly to boiling; filter through an ashless filter paper and wash with an equal volume of water; place the filter paper and the residue again in the dish, ignite, and weigh — this gives the insoluble ash; the water-soluble ash is the total ash minus the insoluble ash; make up the filtrate which contains the water-soluble ash to 100 ml with distilled water and keep it for determining the alkalinity of the water-soluble ash and the sodium chloride

↓

Alkalinity of water-soluble ash: pipette 25 ml of the filtrate from the water-soluble ash solution and titrate with 0.1 N HCl using methyl orange as indicator; report the result as the number of ml of 0.1 N acid required to neutralize the ash from 1 g of the sample

Calculation

Alkalinity number of ash = [(ml of blank titre − ml of sample titre) × Normality of NaOH] ÷ (Wt of ash × 1 N)

Where,

• ml of blank titre = ml of 0.1 N NaOH used in the blank titration
• ml of sample titre = ml of 0.1 N NaOH used in the sample titration
• Normality of NaOH = normality of the standard NaOH solution (the print reads “Normality of alkali”; the acid and the back-titrating alkali are both 0.1 N)
• Wt of ash = weight of the ash taken (g)

Precautions

• Determination of the alkalinity of ash is not useful if the sample contains added mineral acid.
• Avoid loss by spattering when heating the ash with water; keep the dish covered with the watch glass.

Interpretation

• Genuine malt vinegar contains not less than 0.2 g of ash in 100 ml.
• The water-soluble ash from 100 ml of malt vinegar requires not less than 4 ml of 0.1 N acid to neutralize its alkalinity.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.