Tags : FSSAI
Method No.: FSSAI 03.012:2023

Scope

Acid Insoluble Ash refers to the ash remaining after the dissolution of total ash in dilute hydrochloric acid. This method is applicable to the determination of acid insoluble ash in most food grains, including cereals and cereal products, pulses and their products, macaroni products, biscuits, bread, and other bakery products.

Caution

• Concentrated hydrochloric acid: Handle with extreme care as it is corrosive. Avoid breathing vapors and contact with skin and eyes. Use only inside a fume hood.
• Muffle furnace: Avoid burns by not touching the exterior or interior surfaces during use or for a period after use. Wear protective insulated gloves and use tongs for loading/unloading the furnace. Stand to one side when opening the door to avoid high temperature exposure.

Principle

The total ash is treated with dilute hydrochloric acid, filtered, and the residue insoluble in acid is incinerated and weighed.

Apparatus/Instrument

1. Dish: Flat-bottomed, with a surface area of at least 15 cm², made of platinum, quartz, porcelain, or another material unaffected by test conditions.
2. Ashless filter paper: Whatman filter paper 42.
3. Water bath.
4. Muffle furnace: Capable of being regulated at 550 ± 10 °C.
5. Electrical hotplate or surface heater.
6. Fume hood or equivalent venting system.
7. Desiccator: Containing desiccant such as orange indicating silica gel.
8. Analytical balance: Accurate up to 0.0001 g.
9. Tongs for crucible: Stainless steel.
10. Thermal protection gloves: Capable of resisting temperatures up to 550-600 °C.
11. Bunsen/Electrical burner.
12. Tripod stand: Iron.
13. Wire gauze.

Preparation of Reagents

• Dilute hydrochloric acid (~ 5.5 N): Into a 1000 mL volumetric flask, transfer about 600 mL water and 170 mL concentrated hydrochloric acid (37%) carefully. Allow cooling to room temperature and make up to the mark with water. Mix well. (Caution: Always add acid to water, not water to acid.)
• Silver nitrate solution (10% m/v): Dissolve 10 g of silver nitrate in distilled water to a total volume of 100 mL.

Method of Analysis

1. Add 15-25 mL of HCl solution to the total ash of the sample and boil for 10 minutes over a boiling water bath, covering the dish with a watch glass to prevent spattering.
2. Filter the contents of the dish through ashless filter paper.
3. Wash the dish and filter paper with hot water until washings are free from hydrochloric acid (about 6 to 8 times). Test for the absence of hydrochloric acid with silver nitrate solution. (Lack of turbidity when a portion of silver nitrate solution is added to the filtrate indicates the absence of hydrochloric acid.)
4. Return the filter paper with the residue to the dish.
5. Evaporate on a water bath and ignite in the muffle furnace at 550 ± 10 °C for 1 hour (or until the ash is carbon-free).
6. When carbon-free ash is obtained, transfer the dish to the desiccator, cool to 25 ± 2 °C, and weigh immediately.
7. Repeat the operations of igniting, cooling, and weighing until the difference between successive weighings does not exceed 0.001 g (W2).

Calculation

Acid insoluble ash on dry mass basis (A) = W2−WW1−W×100\frac{W2 - W}{W1 - W} \times 100W1−WW2−W​×100

Where:

• W = Mass of empty dish in g.
• W1 = Mass of the dish with the dried ash portion taken for test.
• W2 = Mass of the dish and acid insoluble ash in g.

Calculate the mean of two determinations and express the result to one decimal place. Note: Correct the acid insoluble ash weight for the blank of filter paper, if any.

Acid insoluble ash, % by mass (on dry basis) = A×100100−M\frac{A \times 100}{100 - M}100−MA×100​

Where:

• A = Acid insoluble ash, percentage by mass.
• M = Percentage of moisture in the bread.

Reference

1. AOAC, Official Methods of Analysis. Association of Official Analytical Chemistry, Washington DC, 15th Ed. 1990.
2. Pearson, D., Egan, H., Kirk, R.S., and Sawyer, R. (1981) Pearson’s Chemical Analysis of Foods. Churchill Livingstone, Edinburgh, 8th Ed.