How to assess grain quality
January 01, 1996
by Teresa Acklin
International consultant Robin Wilkin discusses techniques to analyze the physical characteristics of grain.
Moisture in grain is very important as it affects both the value and storage properties of the grain. Unfortunately, measuring moisture content is not a simple process, and different approaches to measurement are likely to give different results. This is because moisture is present in at least three forms: bound water, adsorbed water and absorbed water. Each form presents its own problem for measurement.
Almost every contract for grain will specify an upper limit for moisture. These limits will vary, ranging in the European Union from 14.5% for intervention grain to 16% for some feed grain contracts. Exceeding the limit may result in rejection of a load or the imposition of drying charges or some other financial penalty.
Sampling can have a fundamental influence on the results of moisture determination. Delivering grain from a bulk store or large bin and relying on results from a few surface samples taken from the store is a formula for disaster because variation both within and between loads is probable.
Moisture in grain controls the growth of mold and the development of mites. However, the availability of water in grain to mites and mold also is affected by temperature. Therefore, when calculating “safe” moisture content for storage, account must also be taken of grain temperatures. Table 1 provides some indication as to the effect of different moisture contents and temperatures on mites and mold.
The most widely used method of measuring moisture under laboratory conditions is to heat a weighed sample of ground grain to drive off water and then measure the weight loss. This is a direct method and sounds simple and straightforward.
In practice, problems arise because several different methods are in use worldwide and even within individual countries. Differences between methods include the temperature and time of heating, and these lead to differences in results. Also, grain contains non-water volatiles, and some of these may be driven off by heating, leading to overestimation of moisture.
In the United Kingdom, two main oven heating methods are used. One, the ISO R712 method, dries at 130° to 133°C for two hours; the other method dries at 99° to 101°C for at least four hours. The difference in results between these two methods can be as much as 0.5% at 15% moisture, with the ISO method giving the higher value.
Since oven moisture determinations are carried out on small samples, these must be homogeneous and representative. It should also be noted that the ISO method allows 0.2% variation between replicates, which could allow 0.2% error in the results.
A wide range of electrical moisture meters also are on the market. None of these meters measures moisture directly, other than the electrical heating type. In general, a characteristic of grain that is related to moisture, such as resistance or capacitance, is measured. Unfortunately, the relationship is often not linear so that calibration is not straightforward.
Electrical meters are extremely valuable tools, but the results obtained must be taken in context. In general, resistance meters measure the moisture of a small sample of grain which may or may not be ground first. Capacitance meters generally accept larger samples of whole grain.
Most types of meter will measure the moisture content of wheat, barley and oilseeds. More complex meters may have calibrations for other cereals and even different varieties of the same cereal.
The accuracy of electrical meters tends to be related to their calibration and the level of moisture in the sample. Meters tend to be more effective within the range of 12% to 18% moisture and may be less accurate with very wet or very dry grain. Temperature will also affect the results, and best results are obtained when the meter and samples are at about the same temperature.
Most manufacturers of moisture meters will accept that their instruments are subject to a small variation; 0.5% is commonly quoted. This is not important during general testing, but could lead to problems when selling grain. The seller must either allow a margin to cover the possible error of his instrument or confirm his results using an oven test or the buyer's meter.
Regular calibration of electrical meters must be part of any quality assurance system. It is not uncommon for the calibration of some meters to change between seasons.
The chart on page 26 shows the range of variation found with a group of moisture meters tested at random by the Flour Milling and Baking Research Association at Chorleywood, U.K., in 1994. These results show that over the range covered by the six samples tested, more than half the meters had an error of more than plus or minus 0.5%, with the worst variation reading plus or minus 2%.
This variation illustrates the value of regular calibration, and results obtained with a meter should be checked against oven tests whenever possible. Many laboratories supply this service, and the meter manufacturer also may provide the same service.
Just as with oven tests, the quality of information given by moisture meters will be governed by the quality of the samples tested. However, with electrical meters, it is more practical to test a range of samples.
Another approach to the electrical measurement of moisture is to use the reflectance or absorption of near infra red radiation (NIR systems). The water content of grain strongly influences the absorption or reflection of NIR radiation so that it offers good potential for moisture measurement. Such analyzers also will measure a range of other quality parameters of grain during a single determination.
However, the calibration of NIR analyzers is a complex process and may require regular updating. It is normal to calibrate such machines using oven determinations at the start of each season. A good approach to quality management is to carry out regular comparisons between an NIR analyzer and oven determinations.
The operator plays an important role in providing accurate and consistent results from all types of electrical moisture meters. Training of the operator to ensure consistent operating procedures is important, as is a high level of cleanliness.
Records of equipment calibration should be kept for reference in case of dispute and so that trends can reveal when equipment replacement may be required.
Although no exact definition of screenings exists, they are considered to be the material that is removed from grain when it is sieved over a specific mesh. Screenings also are considered to be material that does not pass a mesh that allows whole grains through. Alternatively, the term admixture may be used to describe all material which is not whole grain (or seeds in the case of oilseeds), usually removed from the sample manually.
Screenings or admixture may consist of earth, small stones, weed seeds, broken grain, straw and many other contaminants. With screenings determined by sieving, the percentage removed from a sample will be affected by the size of screen used, which will, in turn, be controlled by the quality of sieve.
Standards exist for the quality of sieves, both mesh and slotted varieties, and it may be necessary to use standard sieves when measuring screenings for commercial purposes. However, some debate exists about the validity of the sieve standardization.
Most contracts or specific quality standards set levels for screenings and/or admixture. Sometimes, but not always, contracts or standards will define methods of measurement. Examples of some of these E.U. standards are given in Table 2 on page 27.Detecting Pests.
Almost all grain is at risk from infestation. Insect and mite pests are widely distributed in both farm and commercial grain stores. Data from the Ministry of Agriculture, Fisheries and Food in the U.K. show that, at any one time, 10% of farms stores and 30% of commercial stores will have some insect pests. Mites are much more widely distributed.
When receiving grain from several suppliers, a high risk exists that some loads will be infested. All the pests are relatively small (less than 4 millimeters), shun light and are extremely difficult to detect. Once a few pests become established in a grain store they can increase in numbers and also can persist in the store between seasons.
If insects are obvious during casual inspection, they usually are present at rates of more than 10 or 20 per kilogram. At this density, the infestation will soon damage the grain and also will trigger secondary problems such as hot spots and moisture migration.
The most common method of looking for insects in grain is to collect samples, sieve the grain and check the sievings for insects. This is used both for stored grain and when grain is in transit.
The major limitation of this approach is that relatively small amounts of grain are examined in relation to the total quantity in a store or consignment. Therefore, finding insects using this method involves a large element of chance.
Sellers of grain should note that most buyers will reject loads of grain or penalize the seller if a single insect is found in a load of grain.
When grain is in storage, sampling must be targeted toward the areas of highest risk: the warmest grain for insects and the wettest grain for mites. Traps, which can be inserted in the grain, are very sensitive, detecting extremely small numbers of insects in bulk.
Unfortunately, no data exist to relate trap catch to the likelihood of pests being detected by normal “in transit” sampling, and traps cannot be used on moving grain. Therefore, insects found in a trap may or may not indicate that normal sampling will detect these insects. In these circumstances, the only reliable action is to carry out some form of disinfestation.
Whatever pest detection method is used in stored grain, the first step should be to measure temperatures. Sampling can then be concentrated, using hand probes or vacuum samplers, on the warmest parts of the bulk.
The samples should be sieved over a 2-mm mesh and the sievings examined for insects. The quantity of grain that is examined should be noted so that the number of insects found per kilogram can be calculated.
If traps are used to detect insects, these should be inserted into the grain surface at the intersects of a 3-meter grid. Leave the traps for seven days and then check for insects.
A few traps can be used on a rolling basis, being moved through the store at weekly intervals. If insects are found in only one or two traps, concentrate all available traps in this area to establish the extent of the infestation.
Collect, sieve and examine probe samples from the infested area. If insects can be found in these samples, the problem is likely to require control measures. It is much more difficult to make definitive recommendations about insects caught in traps because recommendations will depend on several factors, including the temperature of the grain.
If the grain is to remain in store for some weeks, the best approach is to continue to monitor with traps and also use aeration to cool the grain. If the number of insects caught increases, then control measures are likely to be needed.
Once grain has been moved out of the store and is transported in trucks or moved along a conveying system, insects become randomly distributed in small clumps. The practical effect is that although the infestation level is 1 insect/kg, one 1-kg sample of grain may have no insects while another will have 10. Therefore, the chance of detection is directly related to the amount of grain examined.
Most conventional pest detection methods use samples of 0.5 to 1 kg of grain, taken from a minimum lot of 20 tonnes and sometimes much more. If a single insect is detected in a sample using this type of approach, research has shown that the infestation level is probably between two and five insects per kilogram in the load or lot.
If no insects are found, this does not mean the grain is uninfested. The key rule to detecting insects in grain during transit is: the more grain that is examined the better.
Depending on the reason for checking the grain, it may be important to try to estimate the population density. This can be done very simply by counting the number of insects found in each sample.
It is important to understand that not all insects found in grain are pests. This is particularly true shortly after harvest, when several field insects may still be alive in the stored grain. Expert identification may be necessary.
Mites are even more difficult to detect in grain than insects. They are only about 0.5 mm to 0.6 mm in length and require the use of a 10x magnifying hand lens and a good light to detect. A better approach is to transfer all or some of the sievings to a dish and use a 30x binocular microscope. Live mites are easily spotted using this technique.
Moisture is the key to mites, which are only likely in grain with a moisture content of more than 14.5%. Problems often start in the surface layers of grain during late autumn and in winter, where the grain has adsorbed moisture from the atmosphere. Expert identification is often essential because predatory mites may also occur in the grain, sometimes triggering unnecessary control measures.
Robin Wilkin, an international adviser and consultant, worked for many years in the Central Science Laboratory of the U.K.'s Ministry of Agriculture, Fisheries and Food researching the control of stored product pests. This article is an excerpt from his manual “Grain, Seed and Feed: Sampling and Physical Characteristic Testing,” published by Samplex Ltd., U.K.Table 1 The effects of moisture content and temperature on the development of mites and mold in wheat
Table 2 Acceptable levels of screenings and sieve sizes specified for various European Union standards
| ||Temperature||Moisture||Equilibrium relative|
| ||(°C)||in percent||humidity in percent |
|No mold growth||5||14.9||60|
|or mite development||20||13.5||60|
|No mold growth;||5||15.8||65|
|very slight mite risk||20||14.4||65 |
|Risk of mold; mites||5||17.8||75|
|Quality standards||Level of screenings||Sieve size|
| Milling ||2% admixture||Slotted,|
| (NABIM* Code)||or 2% screenings,||2 mm and 3.5 mm|
| ||combined total|
| ||must not exceed 2% |
|Wheat & barley||2% Admixture||Slotted,|
| for export||2.2 mm barley|
| ||2.0 mm wheat |
|Feed grains||As defined by||Slotted,|
| ||purchaser, but often||2.2 mm barley|
| ||2% maximum||2.0 mm wheat |
|Malting barley||2% screenings||Slotted,|
| ||2.25 mm |
|Intervention grain||12% maximum total||Slotted,|
| ||impurities, but||2.2 mm barley|
| ||sliding scale of||2.0 mm wheat|
| ||penalties applies |
|Oilseeds||2% screenings||Round Hole Sieve,|
| ||2.8 mm and 0.5 mm |
|Feed beans and peas||Contracted||Stainless wire mesh,|
| ||percentage admixture||4 mm|
|*National Association of British and Irish Millers|