SOP: Determination of Acid Insoluble Ash (AIA)

GIFSQ — Global Institute for Food Safety & Quality

1. Principle

Acid insoluble ash is a measure of the sandy particles and other siliceous matter present in flour. When the total ash obtained is treated with dilute acid, the inorganic compounds dissolve in the acid, leaving the siliceous and sandy matter as insoluble ash. The warm solution is filtered through ashless filter paper (No. 41), the residue is washed with hot distilled water until free from acid, and the residue is ashed again. The residue so obtained is the acid insoluble ash.

2. Apparatus required

• Muffle furnace (600°C)
• Crucible
• Oven
• Desiccator (with dried silica gel)
• Funnel
• Glass rod
• Wash bottle
• Ashless filter paper (No. 41)

3. Reagents

a. Dilute hydrochloric acid (approximately 5N)

Preparation of HCl: Into a 1000 mL volumetric flask, transfer with care about 400 mL water and 430 mL concentrated hydrochloric acid (sp. gravity 1.18, purity 37%). Allow to cool to room temperature. Make up to the mark with water. Mix well.
b. Distilled water

c. Litmus paper

4. Procedure

Take the crucible containing the total ash

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Add 25 mL of dilute HCl (5N)

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Boil gently for 10 minutes on a steam bath or hot plate inside a fume hood

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Cool and filter through ashless filter paper (Whatman No. 42)

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Wash with hot distilled water until acid free (check with blue litmus)

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Transfer the filter paper with its residue to the original crucible

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Keep it in the electric oven for 3 hours at 105°C

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Ash in the muffle furnace at 550°C until white or grey ash is obtained

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Transfer the crucible to the oven at 100°C for 30 minutes, then transfer to a desiccator for cooling

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Weigh the crucible as soon as it reaches room temperature

5. Calculation

Acid insoluble ash (%) = (Weight of acid insoluble ash ÷ Weight of sample taken) × 100

= [(Z − X) ÷ (Y − X)] × 100 (weight basis)

= [(Z − X) ÷ (Y − X)] × 100 × [100 ÷ (100 − M)] (dry basis)

Where,
– X = weight of dry crucible
– Y = weight of crucible with dried sample
– Z = weight of crucible with acid insoluble ash
– M = moisture percentage

6. Precautions

• Carry out the acid digestion inside a fume hood.
• Cover the crucible with a watch glass to prevent spattering of acid.
• Wash completely until the washings are acid free.

References

IS 1011:2002, Biscuits — Specification; Annex C.