International Association for Cereal Chemistry (ICC) - ICC-Standard No. 110/1
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Determination of the Moisture Content of Cereals and Cereal Products (Practical method).
In the case of maize (and whole maize meal) the method of determining moisture content differs in some points from the method for other cereals (and cereal products). In the description of the method in the Standard, the variations are given side by side in two columns: the right-hand column applies to maize and the left-hand to other cereals and cereal products.
This method is applicable specifically to: wheat, rice (hulled paddy), barley, millet, rye and oats as grains, ground grains, semolina and flour. This method gives unsatisfactory results for brewing barley maize grains or flour from whole grains. Because of the very high moisture content which can be found in maize sample (sometimes more than 40%) and because of the size and structure of the grains, problems arise in the predeying and grinding of maize for moisture determination. For this reason both the practical and the basic reference methods can in this case only be carried out by specialist laboratories.
Moisture content is taken to be the loss in weight, expressed as a percentage of the weight of the original sample, which the product undergoes under the conditions specified in the present ICC Standard No. 110.
Determination of the weight loss suffered by The sample when dried at a temperature of 130 to 133°C under precisely fixed conditions so that a result is achieved which corresponds to the result obtained using the basic reference method (ICC Standard No. 109, Determination of the moisture content of cereals and cereal products).
Analytical or semi-analytical balance, minimun accuracy 1 mg, maximum load 100 to 200 9.
Metal dish without a cover; the usable surface must permit a distribution of 100 9 grains in one layer.
Vessel made of corrosion-resistant metal, or glass, with a watertight cover; the usable surface must permit a distribution of the sample of max. 0.3 9 per cm².
Electrically heated, temperature regulated drying cabinet, set at a temperature between 75°C and 85üC and provided with adequate ventilation which is set so that the temperature of the air and the shelves near to the sample is between 130°C and 133°C under normal conditions.
Desiccator with a thick, perforated metal plate, which could alternatively be porcelain. The desiccator contains analytical grade phosphorus pentoxide (P2O5) or anhydrous calcium sulphate, granulated and impregnated with cobaltous chloride indicator, or any other effective desiccant.
According to ICC Method No.101
Weighing is to be carried out with an accuracy of ± 0.001 9.
Preparation of the sample
8.1.1 Products not requiring grinding:
Products with a particle size smaller than or equal to 1.7 mm, with less than 10% by weight larger than 1 mm and more than 50% by weight smaller than 0.5 mm, need not be grounded before the determination.
8.1.2 Products which require grinding:
If the particle size of the sample does not correspond to that given in 8.1.1, it must be ground, with or without preliminary conditioning.
188.8.131.52 Grinding without preliminary conditioning:
This is done when there are no variations in the moisture content of the sample during grinding to be taken into account. This is in general the case with grains having a moisture content between 7 and 17% 9 and 15% (7 and 15% for oats).
The grinding apparatus (6.2) is set so that particle sizes corresponding to 8.1.1 are obtained. A sample quantity of the product is ground and discarded.
A quantity of the sample which is a little larger than the amount intended for the test, which must be at least 5 9, 30 9 of maize, which is quickly homogenised using a spatula, 8 9 of the ground product is then ground rapidly. The ground product is placed in the metal vessel (6.4) which has been dried, cooled and tared; the vessel is quickly closed and weighed (M2). The determination is then carried out.
When using pin-or hammer-mills, the interval between taking the sample and weighing before dehydration must be lest than 2 or 3 min.
184.108.40.206 Grinding with preliminary conditioning:
Products which are too dry (m.c. less than 7%) (m.c. less than 9%) or too moist (m c. above 17%, or 15% for oats) (m.c. above 15%) must be moistened or dried, as appropriate, before grinding.
In the case of products with a moisture content below 7% (below 9% for oats) the sample is moistened by being brought into an atmosphere such that its moisture content is between 7 and 17% or between 9 and 15% in the case of oats. (for other cereals preferably between 9 and 15%).
The most frequent need is for predrying of grain, which is generally undertaken when the moisture content is over 17% (over 15% for oats) in order to bring it within the limits of 7 to 17% (between 7 and 15% 9 to 15% for oats and if possible between 9 and 15% for other cereals).
A sample of whole grains (Mo) somewhat larger than the amount intended for the test is weighted and the process described in 8.3 is continued, except that the time in the drying cabinet (6.5) is only 7 to 10 min. and cooling of the product to laboratory temperature takes place in the open vessel with 100 9 of whole grains is weighed accurately into the metal dish (6.3) (Mo)' which is placed in a drying cabinet at 80°C (6.5) and remains there until the moisture content is between 9 and 15% (ca 1 to 5 h). The dish is removed from the drying cabinet and left to stand for at least 2 h under laboratory conditions, so that the grain is brought to laboratory temperature and a uniform distribution out a desiccator and takes at least 2 h.
After conditioning, the sample is weighed again (M') and immediately ground in the grinding apparatus which has been set up in advance (6.2). The ground product is placed in the test vessel (6.4) and weighed again (M2), the interval between the two weightings (M' and M2) not exceeding 2 to 3 min.
The determination is then carried out. of moisture occurs. The sample is then weighed again accurately (M,). Some 30 9 of the conditioned grains is removed and immediately ground in the grinding apparatus (6.2) the powder is quickly homogenised with the spatula. Exactly 8 9 (M2) is placed in the test vessel (6.4). The interval between the two weighings (M, and M2) must be less than 2 to 3 min.
Sample for analysis
8.2.1 For products which do not require grinding (see 8;1.1), the following are quickly performed:
220.127.116.11 At least 5 9 of the substance 8 9 of the substance is placed in the metal vessel (6.4) which, after standing in the drying cabinet (6.5) has been tared and cooled to laboratory temperature in the desiccator (6.6)
18.104.22.168 The vessel (6.4) is closed and weighed (Ma).
8.2.2 For products which require grinding (see 8.1.2) the ground material in the closed and weighed vessel (6.4) is used as the sample for analysis (see 22.214.171.124 and 126.96.36.199).
The open vessel (6.4) with the sample is placed in the drying cabinet (6.5) for 2 h (90 min for flour).
The time is calculated from the point when the drying cabinet has regained a temperature of 130°C. The vessel is then quickly removed from the drying cabinet, covered and placed in the desiccator (6.6); the vessel must never be placed one on top of another in the desiccator. As soon as the vessel has cooled to laboratory temperature (usually 30 to 45 min after being put into the desiccator), it is weighed (M3)
The determination must be performed at least twice.
9. PRESENTATION OF RESULTS
Method of calculation and formulae
The percentage moisture content of the product is
9.1.1 without preconditioning 100
without grinding (M0 - M3) x 100/M0
with grinding (M2 - M3) x 100/M2
M0 = original weight in g of the original sample (whole grains before conditioning in the case
of a determination with preliminary drying or damping),
M1 = weight in g of the original sample after condition,
M2 = weight in g of the sample after girding,
M3 = weight in g of the sample after complete drying.
The determination must be carried out at least twice. The arithmetic mean of the determinations, when the conditions for accuracy of determination (9.2) are fulfilled, is taken as the result. The result is rounded to the nearest 0.05 9 moisture per 100 9 sample.
Accuracy of the determination
The difference between the results of two determinations is calculated to an accuracy of 0.01%. The difference between the results of these determinations, which are carried out simultaneously or in quick succession by the same person, must not exceed 0.15 9 moisture per 100 9 sample. If this is not the case, a further determination must be carried out in duplicate.
In comparsion with the results of the basic reference method, these results vary in general by less than 0.15 g moisture per 100 9 sample.
10. EXPERIMENTAL REPORT
The experimental report should give the method used and the results obtained. In addition, all details of the procedure which are not provided for in the present ICC Standard or are optional must be mentioned, as well as any incidents which might have influenced the result.
The experimental repros must contain all the data necessary for complete identification of the sample.
The moisture content range given for the conditioning of cereal grains prior to grinding is suitable for a temperature of approximately 20°C and relative humidity of 40 to 70% in the laboratory. Under different atmospheric conditions the moisture content range would have to be altered.
Wet products and dry samples should never be put in the drying cabinet together, since this results in a partial rehydration of the dry samples.
Study Group: Moisture determination
Chairman: Dr. J.L. Multon, Nantes, France
Choice of method for temperature measurement
For measurement of the temperature of grain in bags, a bimetallic thermometer can be used but it is slow and measures an average of temperature over a large proportion of the stem. A thermocouple or thermistor system incorporated in a metallic spear is preferable for bags or for taking spot measurements in bulk grain at depths up to five metres.
Monitoring of the temperature within a grain bulk over a long period is best carried out with either a thermistor or thermocouple system. In both cases the leads should be placed in situ before the silo is filled with grain. If monitoring at a small number of points is required, then the thermistor system is most suitable, and thermistors are essential if long-term monitoring by automatic recording is required. Thermocouples are cheap and convenient to use where large numbers of points are to be monitored.
Monitoring systems requiring many sensors also need a switching device, which should be located, if possible in an air-conditioned room. Otherwise, in hot humid climates, they may fail.
Temperature gradients near the surfaces of a bulk of grain can be severe in tropical and subtropical climates. Such gradients are best measured with thermocouple or thermistor systems, because their sensing heads can be made small enough to take meaningful measurements of temperatures at points in close proximity to each other.
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