PART VIStandard method of testing for moisture content
1.Principle
The product is dried at a temperature of 130 to 133 oC under normal atmospheric pressure, for a period appropriate to the size of the particles.
2.Scope
This drying method applies to cereals crushed into particles of which at least 50 % pass through a sieve with 0,5 mm mesh and leave not more than 10 % residue on the sieve with a 1,0 mm round mesh. It also applies to flour.
3.Apparatus
Precision balance.
Crusher made of a material which does not absorb moisture, is easy to clean, enables crushing to be effected quickly and evenly without overheating, limits contact with the outside air to the minimum, and meets the requirements mentioned in point 2 (for example a detachable roller mill).
Receptacle made of non-corrodible metal or glass, fitted with a sufficiently tight-fitting lid; working surface allowing distribution of the test sample at 0,3 g per cm2.
Electrically heated isothermic heating chamber, set at a temperature of 130 to 133 oC(1) having adequate ventilation(2).
Dessicator with a metal or, failing metal, porcelain plate (thick, perforated), containing any suitable dessicant.
4.Procedure
Drying
Weigh to the nearest 1 mg a quantity slightly greater than 5 g of the crushed small-grained cereals or 8 g of the crushed maize in the pre-weighed receptacle. Place the receptacle in a heating chamber heated to a temperature of 130 to 133 oC. This should be done as quickly as possible, so as to prevent too great a drop in temperature. Leave small-grained cereals to dry for two hours and maize for four hours after the heating chamber regains a temperature of 130 to 133 oC. Remove the receptacle from the heating chamber, quickly replace the lid, leave to cool for 30 to 45 minutes in a dessicator and weigh (to the nearest 1 mg).
5.Method of calculation and formulae
=
the initial mass, in grams, of the test sample
=
the mass, in grams, of the test sample after preparation
=
the mass, in grams, of the test sample after crushing
=
the mass, in grams, of the dry test sample.
The moisture content as a percentage of the product is equal to:
without previous preparation (E - m) × 100/E,
with previous preparation, ((M’ - m)M/M’ + E - M) × 100/E = 100 (1 - Mm/EM).
Tests to be made in duplicate at least.
6.Repetition
The difference between the values obtained from the two determinations carried out simultaneously or in rapid succession by the same analyst shall not exceed 0,15 g of moisture per 100 g of sample. If it does so, the determinations shall be repeated.
Air temperature inside the heating chamber.
Its heating capacity should be such that, when it has been pre-set to a temperature of 130 to 133 oC, that temperature can be regained in less than 45 minutes after the maximum number of test samples have been placed in the chamber to dry simultaneously. Ventilation should be such that, when small-grained cereals (common wheat, durum wheat, barley and sorghum) are dried for two hours and maize for four hours, the results from all the test samples of semolina or, as the case may be, maize that the heating chamber can hold differ by less than 0,15 % from the results obtained after drying small-grained cereals for three hours and maize for five hours.