Q. Determine the acid insoluble ash in the sample of flour?

After undertaking this activity, you will be able to:

• assess the mineral content in the given sample of flour,

• check the given sample for conformance to the standard for ash content, and

• determine the acid insoluble mineral content for dry samples.

Principle

Ash content of a foodstuff represents inorganic residue remaining after destruction of organic matter. High ash content or a lower alkalinity of ash may be suggestive of the presence of adulterants. The acid-insoluble ash is a measure of sand and other silicious matter present. Overheating of ash results in the fusion of some of the inorganic salts which affects the acid-insoluble ash. During the process of overheating, the ashed material changes from the molecular to the polymerized form and boiling with 1:9 hydrochloric acid (HCl) does not remove the acid soluble constituents of the total ash as the acid is not able to act on the entire periphery of each molecule. Thus, many components remain unreacted and insoluble in acid and results in the higher value of acid-insoluble ash.

Materials Required

Collect the following material for conducting the activity:

• Sample of flour

• Dilute hydrochloric acid (HCl)

• Flat-bottom dish - of stainless steel, porcelain, silica or platinum.

• Muffle Furnace-maintained at 550 ± 10°C.

• Desiccator

• Weighing balance

• Whatman number 42 filter paper

Procedure

Now carry out the practical step-by-step as enumerated herewith:

1)  Weigh accurately about 3-5 g of the sample in the  dish, previously dried in the air oven and weighed.

2)  Heat the dish gently on a flame at first and then strongly in a muffle furnace at 550 ± 10°C till grey ash results.

3)  Cool in the dessicator and weigh.

4)  Repeat the process of heating in muffle furnace, cooling and weighing at 30 minutes intervals until the difference between two consecutive weighings is less than one milligram.

5)  Record the lowest weight.

6)  To the ash contained in the dish, add 25 ml of dilute hydrochloric acid.

7)  Cover with watch-glass and heat on a water bath for 10 minutes.

8)  Allow to cool and filter the contents of the dish through a whatman filter paper no. 42.

9)  Wash the filter paper with water until the washings are free from the acid and return them to the dish.

10) Keep it in an oven maintained at 100 ± 2°C for about 3 hours.

11) Ignite in a muffle furnace at 550 ± 10°C for one hour.

12) Cool the dish in a desiccator and weigh.

13) Heat the dish again at 550 ± 10°C for 30 minutes, cool in the desiccator  and weigh.

14) Repeat this process of heating for 30 minutes, cooling and weighings until the difference between two successive weighings is less than one milligram. Record the lowest weight.