ISO 6540:2021 pdf – Maize — Determination of moisture content (on milled grains and on whole grains).
1 Scope This document specifies two methods: — a reference method for the determination of the moisture content of maize grains and ground whole maize, groats, grits and maize flour, see Clause 4 ; — a routine method for the evaluation of the moisture content of maize in whole grains, see Clause 5 . The latter is not suitable for use for experts ’ reports, or for calibration or checking of humidity meters, because of its significant bias to the reference method (see Table B.3). 2 Normative references There are no normative references in this document. 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses: — ISO Online browsing platform: available at https://www.iso .org/obp — IEC Electropedia: available at http://www.electropedia .org/ 3.1 moisture content of maize loss in mass undergone by a product under specified conditions Note 1 to entry: It is expressed as a percentage. 4 Reference method 4.1 Principle If necessary, grinding of a sample, after pre-conditioning, if required. Drying of a test portion at a temperature between 130 °C and 133 °C, under conditions that enable a result to be obtained in agreement with that obtained by the absolute method (see Annex A). 4.2 Apparatus 4.2.1 Analytical balance, able to weight with an accuracy of ±0,001 g and therefore having a display accuracy of 0,000 1 g. 4.2.2 Analytical balance, able to weight with an accuracy of ±0,1 g and therefore having a display accuracy of 0,01 g.
4.2.3 Grinding mill, having the following characteristics: a) made of material that does not absorb moisture; b) easy to clean and having as little dead space as possible; c) enabling grinding of 30 g of maize grains to be carried out rapidly and uniformly, without appreciable development of heat and, as far as possible, without contact with the outside air; d) adjustable so as to obtain particles of the dimensions indicated in 4.4.1. 4.2.4 Metal boat, without lid, with an effective surface area enabling 100 g of maize grains to be distributed in a single layer. 4.2.5 Metal capsule, of suitable dimensions, non-corrodible under the test conditions, or, failing this, a glass dish, with a sufficiently tight-fitting lid, and having an effective surface area such as to allow distribution of the test portion with no more than 0,3 g per square centimetre. 4.2.6 Constant-temperature oven, electrically heated, adjustable between 60 °C and 80 °C, and with adequate ventilation. 4.2.7 Constant-temperature oven, electrically heated, capable of being controlled in such a way that the temperature of the air and of the shelves carrying the test portions is within the range of 130 °C to 133 °C in the neighbourhood of the test portions, in normal working condition. The oven shall have a heat capacity such that, when initially adjusted to a temperature of 131 °C, it can again reach this temperature in less than 45 min (preferably in less than 30 min) after insertion of the maximum number of test portions that can be dried simultaneously.
The effectiveness of the ventilation shall be determined using durum wheat semolina, with a maximum particle size of 1 mm, as the test material. The ventilation shall be such that, after inserting all the test portions that the oven can hold and drying at a temperature of 130 °C to 133 °C, the results after a heating period of 2 h and then a further 1 h will not differ by more than 0,15 g of moisture per 100 g of sample. 4.2.8 Desiccator, containing an efficient desiccant. 4.3 Sampling Sampling is not part of the method specified in this document. A recommended sampling method is given in ISO 24333. The laboratory should be provided with a truly representative sample, in a sealed package, that is undamaged and unmodified during transport and storage. 4.4 Preparation of the test sample 4.4.1 Products not requiring to be ground Products that have particles of sizes less than or equal to 1,7 mm, less than a mass fraction of 10 % being over 1 mm and more than a mass fraction of 50 % being less than 0,5 mm, do not need to be ground before the determination. Mix the laboratory sample thoroughly before taking the test portion (see 4.5.3 ).