Australian Journal of Basic and Applied Sciences, 5(7): 582-587, 2011

ISSN 1991-8178

Uncertainty of Moisture Measurements Methods for Grains

N.I. ElSayed M.M. Mekawy and F.M. Megahed

Thermometry Department National Institute for Standards Tersa St.

P.O. Box: 136 Giza, Code. No. 12211, El-Haram, Egypt.

Abstract: It is important for food scientists to be able to reliably measure moisture content as it is
one of the most commonly measured properties of food materials especially grains. In addition, the
moisture monitoring techniques will provide for better control of processing for value added
applications and improved end product quality. They will provide more precise moisture information
that can be used in optimizing grain drying with resulting higher quality products and significant
energy savings in grain drying operations. A number of techniques and electronic meters have been
developed for this purpose, which vary in their accuracy, cost, speed, sensitivity, specificity, ease of
operation, etc…The uncertainty in the moisture content measured by drying method, which is the
highest standard method in grain moisture measurements, was evaluated. The uncertainties for other
electronics meters to be used as secondary standard for determination of moisture in grain, which are
very fast and easy to be used, were calculated and its traceability to Inter-National System of units(SI)
was established.

Key words: Electronic moisture meters uncertainty, Calibration of moisture meters, Moisture
measurements

INTRODUCTION

Foods are heterogeneous materials that contain different proportions of chemically bound, physically bound,
capillary trapped or bulk water. In addition, foods may contain water that is present in different physical states:
gas; liquid or solid which can be problematic for the food analyst trying to accurately determine the moisture
content of foods.
Moisture is generally refers to the presence of water, often in trace amount. It is important to food
scientists for a number of different reasons. First, it is the single most important quality characteristic that
determines the safe storage potential for cereal grains and oilseeds (Wikipedia, 2003; Grain Moisture, 1995).
It is also important for harvesting, storage transport, shelf lives, package for storage, energy savings in grain
drying and processing.. There are legalities to the maximum or minimum amount of water that must be present
in (FAQS, 1969) certain type of food. The coast of many foods depends on the amount of water they contain
as the manufacturers often try to incorporate water as much as possible in a food without exceeding some
maximum legal requirements because water is an inexpensive ingredient.
There are many techniques used for the determination of moisture in grains the most important techniques
used are discussed below (NIST, 2006; Moisture, 2007; German, 2006; Develops, 2008; On-Line 2003; Adam
Moisture Balances, 2003).

1-evaporation Method (Oven Dry Method or the Standard Method):

Its principle relies on measuring the mass of water in known mass of sample. The moisture content is
determined by measuring the mass of a grains before and after the water is removed by evaporation. The basic
principle of this technique is that water has a lower boiling point than the other major components within
grains, e.g., proteins, carbohydrates and minerals.
To obtain an accurate measurement of the moisture content of a grain using evaporation methods it is
necessary to remove all of the water molecules that ware originally present, without changing the mass of the
grain matrix. This is often extremely difficult to achieve in practice because the high temperatures or long
times required to remove all of the water molecules would lead to changes in the mass of the grain matrix,
e.g., due to volatilization or chemical changes of some components. For this reason, the drying conditions used
in evaporation methods are usually standardized in terms of temperature and time so as to obtain results that
are as accurate and reproducible as possible. Using standard method of sample preparation and analysis helps

Corresponding Author: N.I. ElSayed, Thermometry Department National Institute for Standards Tersa St. P.O. Box: 136
Giza, Code. No. 12211, El-Haram, Egypt.
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to minimize sample-to- sample variations within and between laboratories.

Evaporation Devices:
Thermal energy used to evaporate the water from a grain sample can be provided directly (e.g., transfer
of heat from an oven to grain) or indirectly (e.g., conversion of electromagnetic radiation incident upon a grain
into heat due to absorption of energy by the water molecules using micro wave oven or infra red lamb). The
three techniques used in this study are:

Vacuum oven:
Weighed samples are placed under reduced pressure (typically 25-100 mm Hg) in vacuum oven for a
specified time and temperatures untell constant weight and their dried mass is determined. The thermal energy
used to evaporate the water is supplied directly to the sample via the metallic shelf that it sits upon. There
is an air inlet and outlet to carry moisture lost from the sample out of the vacuum oven, which prevent the
accumulation of moisture within the oven. The boiling point of water is reduced when it is placed in vacuum.
Drying grains in vacuum oven therefore has a number of advantages over drying techniques. If the sample is
heated at the same temperature, drying can be carried much quicker. Alternatively, lower temperature can be
used to remove the moisture (e.g., 70oC instead of 100oC), and so problems associated with degradation of heat
labile substances can be reduced.

Microwave oven (Rapid , 2005; Microwaves, 2004; 2002; Nelson and Trabelsi, 2004; 2005; Trabelsi, S., S.O.
Nelson, 2005, 2005; 2004; 2005; ):
Moisture content was determined by this method with accuracies of better than one half of one percent.
It offer good potential for saving considerable time and labor in moisture testing required when drying grains
and testing them for safe storing and marketing. Weighed samples are placed in a microwave oven for
specified time and power-level and their dried mass is weighed. Alternatively, weighed samples may be dried
until they reached constant final mass. The water molecules in the food evaporate because they absorb
microwave energy, which causes them to become thermally excited. The major advantage of microwave
methods over other drying methods is that they are simple to use and rapid to carry out.

Infrared lamp drying (Determination of moisture content, 2007; Development, 2006:

The sample to be analyzed is placed under an infrared lamp and its mass is recorded as function of time.
The water molecules in the food evaporate because they absorb infrared energy, which causes them to become
thermally excited. One of the major advantages of Infrared lamp drying methods is that moisture contents can
be determined rapidly using inexpensive equipments, e.g., 10-25 minutes. This is because the IR energy
penetrates into the sample, rather than having to be conducted and converted inwards from the surface of the
sample.

The Following Practical Considerations Were Taken into Account in Our Study:
The first is sample dimension as the rate and extent of moisture removal depends on the size and shape
of the sample. The second is the decomposition of other grain component because if the temperature of drying
is too high, or the drying is carried out for too long, there may be decomposition of some of the heat-sensitive
components in the grains. This will cause a change in the mass of the sample matrix and lead to error in the
moisture content determination. It is therefore normally necessary to use a compromise time and temperature,
which are sufficient to remove most of the moisture, but not too long to cause significant thermal
decomposition of the grain matrix. Besides most evaporation methods stipulate a definite temperature or power
level to dry the sample so as to standardize the procedure and obtain reproducible results. Finally it is
important to use appropriate pans to contain samples, and to handle them correctly, when carrying out a
moisture content analysis. Typically aluminum pans are used because they are relatively cheap and have a high
thermal conductivity. Pans were handled with tongs because fingerprints can contribute to the mass of a
sample. Also pans were dried in an oven and stored in desecrator prior to use to ensure that no residual
moisture is attached to them.

Advantages and Disadvantages of Evaporation Method:

These methods are Precise, relatively cheap, easy to use, and officially sanctioned for many applications.
Many samples can be analyzed simultaneously. But they are destructive and time consuming.

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Uncertainty of Oven Dry Method (Standard Method):

The Two Ways to Evaluate Individual Uncertainty Contributions Are:
1- Type (A) (UA) uncertainty.:
After ~ ten measurement results the mean was obtained from the relation:
in
qi
q (1)
i 1 n
Where
q is the mean
n is the number of readings .10
qi is the i value and
i is the number of the reading
Then the variance is equal to
1 in
S q2  
n  1 i 1
(qi  q ) 2 (2)

The standard deviation (Sq) (Type A of uncertainty) is equal to

UA or  REP  S q  S q2 (3)

2-Calculation of type (B) (UB) uncertainty:

Type (B) uncertainty was calculating from the following table.

Table 1: Calculation of type (B) uncertainty UB.

Symbol Source
UT Uncertainty caused by the temperature distribution in the dryer oven.
Um0 Uncertainty of the combined mass of the grain sample and the weighing can before drying.
Um1 Uncertainty of the combined mass of the grain sample and the weighing can after drying.
Us Uncertainty in the calibration of the weighing balance.

Then the total uncertainty U total  UB 2 UA2

2- Physical Methods (USDA, 2002; Trabelsi, S., Nelson, S.O. 2005; 2005; Nelson, 2005).
According to the previous disadvantages of the evaporation method physical methods are used. A number
of physical methods have been developed to determine the moisture content of grains that are based on the
fact that water has appreciably different bulk physical characteristics than the grain matrix, e.g.; density,
electrical conductivity or refractive index. One of these methods is using the electronic meters. These meters
are calibrated to give the moisture content values directly. They is done calibrated using samples which were
dried using the oven standard method. The data of uncertainty of the dry samples were used in the
determination of moisture content and uncertainty budget of the electronic meters. Then the total uncertainty
of the meter is calculated.

Uncertainty in Calibrations of Electronic Meter (Working Standard):

Many national standards laboratories have chosen to maintain a secondary standard as their primary
reference for moisture measurements. The traceability to a primary standard is realized by calibrating the
secondary standard(s) with the national standard of the laboratory.

Experimental Procedure:
The procedure used to evaluate the uncertainty of measurement of electronic meters used in our laboratory
is as follows:
Saples of grains were prepared with moisture content covering the range of moisture meter scale. The
preparation were discribed in a privous paper (N.I.El-Sayed,Mostafa M. Makawy, 2010). Four types of grains
were used which are; Adas, Rice, Lobya and Beans; Each sample was divided into five portions. Three were
used for the deternination of moisture content using ovrn dry method,microwave oven; and the third was used
with the IR appratous. The two other samples were used for moisture measurement by two electronic meters
with resolution of 0.1 and 0.2 respectivly. A comparison was done between the results of each sample using

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the five measuring methodes. This pross were repeated for four kind of grains and the results are shown in
table(3).

Sources of Uncertainty:
The uncertainty of a calibration is contributed by the uncertainty of the reference value (i.e. uncertainty
sources related to the measurement standard and calibration equipment) and uncertainty sources related to the
electronic meters.
Uncertaint of moisture meter considered here includes:
- Repeatability type (A) uncerttainty
- Resolution
- Non-linearity/interpolation error is taken into account if calibration curve or function is reported (usually
taken from the manufactrar catalouge).
- Ambient temperature changes affecting the display unit
- Long-term stability is usually added by the user of the meter after a peroid of using time.

The repeatability type A uncertainty (UA) was calculating using the previous equation (3)
Type (B) uncertainty was calculating using table(2)

Table 2: Uncertainty of electric meter type (B).

Symbol Source of Uncertainty Probabilit distribution Divisor
Rs Calibration of moisture meter Normal 2

RD drift of moisture meter Rectangular 3

Ss Ambient temperature changes affect Rectangular 3
TT Resolution of moisture meter Rectangular 3

 UB UA
2 2
Then the total uncertainty U total

Exprimental results:
Table 3: The moisture content with its uncertainty for four grain samples.
Moisture Value Moisture using oven Moisture using oven Moisture using oven
St. method 7.0 ± 0.1% St. method @ 11.0 ± 0.1% St. method @ 15.0 ± 0.1%
--------------------------------------------------------------- ------------------------------------------------------------------- ---------------------------------------------------------------------
Moisture Microwave IR ± 0.6 Meter Meter Microwave IR ± 0.6 Meter Meter Microwave IR ±0.6 Meter Meter
byp Sample ± 0.5 (1) ± 0.8 (2 ) ± 1.2 ± 0.5 (1) ± 0.8 (2) ± 1.2 ± 0.5 (1) ± 0.8 (2) ± 1.2
Adas 7.6 7.9 7.7 7.9 11.2 11.5 11.9 11.3 15.1 15.3 15.9 15.2
Rice 7.4 7.6 7.1 6.9 11.8 10.7 11.4 10.7 15.5 14.9 14.7 14.7
Lobya 6.8 6.7 6.5 7.1 11.1 11.7 10.3 10.8 14.9 15.6 15.1 14.9
Beans 7.5 7.8 7.3 6.6 11.6 11.2 10.8 10.9 15.0 15.0 14.6 15.9

Conclousion
In this study:, the uncertainties in the grain moisture content measured by the five appratous; oven dry
method (st) ,microwave oven and the IR appratous and the other two electronic meters were evaluated and
the data were referd to the SI units. The uncertainty of the oven dry method was . ±0.1 %. The uncertainties
of the Microwave oven was (± 0.5) while for thr IR is (± 0.6 ) the electronic meters were found to be (± 0.8)
and (± 1.2). It is clear that its values are a bit bigger because it includes the draft of the meter, its resolution
and type A uncertainty beside the uncertainty of the st. methode(± 0.1).
It is also clear from a general analysis of propagation of uncertainty in moisture measurement that the
uncertainty data measured by different meters cotain the uncertainty of the standard.

It is clear that the domenating componant affecting the uncertainty is the resolution, temperature effect
and the drift. For meters has low resolution may be mainely affected by the drift of the instrument and
temperature effect.

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Although the uncertainty of the electric meters are high but it is widely used according to its easyer in
use nondistractive, gives fast results and some meters has long pins which enables the measurements in a deep
lot of grains especially during export and import of grains.

REFERENCE

Adam Moisture, 2003. Balances. (Processing).(Adam Equipment) Food Engineering & Ingredients.
Develops Moisture 2008. Content Determination Method US Fed News Service, Including US State News;
pp: 339.
Determination of moisture content and density of fresh-sawn red oak lumber by near infrared spectroscopy.
Forest Products Journal; May 1,2007; Defo, Maurice Taylor, Adam M. Bond, Brian.
Development 2006. of a Near-Infrared Imaging System to Determination of Rice Moisture Cereal
Chemistry; Sep. Lin, Lian-Hsiung; Lu, Fu- Ming; Chang. Yung-Chiung.
El-Sayed, N.I., M. Mostafa, 2010. Makawy Comparison of methods for Determination of moisture in food
In research Journal of Agriculture and Biological Sciences, 6(6): 906-911.
German, 2006. Inventors Develop Material Moisture Content Determination Apparatus US Fed News
Service. Including US State News; Sep 29: 289.
Grain Moisture content effects and Management 3-1995 Dr. Kenneth J. Hellevang, PE, Extension
Agricultural Engineer NDSU www.ag.ndsu.edu.
Microwaves 2004. sense grain moisture, impacting harvest, storage and price .Emerging Food R&D Report;
Feb, pp: 572.
Microwaves, 2002. sense grain moisture. Emerging Food R&D Report; Feb.
Misc. survivalism FAQS maintained by Alan T.Hagan, athagan@sprintmail.com copyright © 1969,97,98,99.
Alan T. Hagan. Why moisture is Important.
Moisture content determination of different polymers by Karl Fischer titration. (Tech Service)
Nelson, S.O., S. Trabelsi, 2004. Microwave dielectric properties of shelled and unshelled peanuts.
Transactions of the ASAE, 47(4): 1215-1222.
Nelson, S.O., Trabelsi, S. 2005. Universal microwave moisture sensor. In: Proceedings of ISEMA 2005
6th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, May 29-
June 1, Weimar, Germany, pp: 231-235.
Nelson, S.O., 2005. Dielectric properties measurement for agricultural applications. In: Proceedings of the
American Society of Agricultural Engineers, St. Joseph, MI. Paper No. 053134.
NIST, 2006. Handbook 44-2007 Edition Specifications, Tolerances, and other Technical Requirements for
weighing and measuring Devices as adopted by the 91th National conference on weights and easures.
On-Line, 2003. moisture determination of one concentrates; A review of traditional methods and the
introduction of a novel solution Minerals & Metallurgical Processing; Cancilla, P; Roy, D; Rosenblum, F.
Rapid determination of moisture/solids and fat in dairy products by microwave and nuclear Magnetic
resonance analysis. (FOOD COMPOSITION AND ADDiTIVESM Report) Journal of AOAC International; Jan
1,2005; Cartwright, Gary McManus, Bobbie H. Leffler, Timothy P. Moser, Cindy R.
Rubber World, Jul 1, 2007. Chakraborty, S. Debnath, M. Bandyopadhyay, S. Mukhopadhyay, R. Deuri,
A.S.
Trabelsi, S., S.O. Nelson, 2005. Dielectric study of temperature-dependent behavior of bound water in
grain. In: Proceedings of ISEMA 2005 6TH International Conference on Electromagnetic Wave Interaction with
Water and Moist Substances, May 29-June 1, Weimar, Germany., pp: 41-46.
Trabelsi, S., S.O. Nelson, 2005. Dielectric study of temperature-dependent behavior of bound water in
grain. In: Proceedings of ISEMA 2005 6TH International Conference on Electromagnetic Wave Interaction with
Water and Moist Substances, May29-June 1, Weimar, Germany, pp: 41-46.
Trabelsi, S., S.O. Nelson, 2005. Microwave dielectric methods for rapid, nondestructive moisture sensing
in unshelled and shelled peanuts. In: Proceedings of the American Society of Agricultural Engineers, St.
Joseph, MI. Paper no.056162.
Trabelsi, S., S.O. Nelson, 2004. Calibration methods for nondestructive microwave sensing of moisture
content and bulk density of granular materials. Transactions of the ASAE, 47(6): 1999-2008.
Trabelsi, S., Nelson, S.O. 2005. Effect of nonequilibrated moisture on microwave dielectric properties of
wheat. In: Proceedings of the 22nd IEEE Instrumentation and Measurement Technology Conference. Ottawa,
Ontario, Canada, pp: 369-371.

586
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USDA, 2002. (United States Department of Agriculture) Richard B. Russell RSCh. Ctr. 5-5-2008.
productsby dielectric properties. pp: 29-7.
Wikipedia the free encyclopedia 23 April 2009 Moisture content.

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