Quality determination, equipment and methods

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The term 'quality' has different meanings for those who are concerned with the handling, storage, processing and utilisation of grain, even though all will be looking for grain of 'good quality'. For example, grain-handling agencies will want dry, insect-free, undamaged grain which will store well; millers will want a grain which will yield a high percentage of finished produce; and consumers will be concerned with flavour, appearance or cooking qualities of grain.

Grain quality may vary with the variety or type of grain selected by the farmer. It will be influenced by climatic and soil conditions during the growing season, cultivation practices, weather conditions at harvest, and by harvesting techniques. Apart from short-term aging or maturation immediately after harvest, quality cannot be improved during storage, handling and processing - on the contrary, it is easily lost.

Every type of grain can be said to possess properties which contribute to its overall quality. A consideration of the various properties or qualities, either alone or together, allows the grain to be graded and valued, and enables the design and development of optimum methods for handling, storing and processing.

Research work into methods for the identification of varieties may hold the key to grain grading systems of the future. Consumers are becoming accustomed to buying fruit and vegetables by variety, so why not grain? Most cereal crops have been studied; polyacrylamide gel electrophoresis (PAGE) techniques have been used to discriminate between wheat varieties, and reversed-phase high-performance liquid chromatography (RPHPLC) techniques may be applicable to wheat and rice.

Assessment of grain quality

With over 420 standard test methods, including at least 75 internationally-applicable, it is apparent that there is a large diversity in grain character. This is obvious when considering the range of uses for grain: paddy to produce milled rice, barley for malting, durum wheat for pasta production etc.

Many assessments are commodity-, product- or end user-specific. Of the wide range of properties, bulk density and foreign matter are commonly assessed for most types of grain. In addition, the influence of moisture content on other grain qualities, as well as the simple economic fact, make it important for quantification.

(i) Bulk Density

All equipment for the determination of bulk density have features of (a) causing the sample material to fall from a standard container through a standard height into a standard volume weighing bucket, (b) levelling the surface of the material in the weighing bucket in such a way as not to influence its packing and (c) weighing the loaded bucket. However, differences in equipment design and procedural detail can result in very different values for bulk density, even when the same grain sample is used. It is essential, therefore, that only one type of apparatus is used for determining bulk density. ISO 7971 is a standard reference method with results expressed as mass per hectolitre.

The bulk density of a sample of grain can also be affected by the presence of foreign matter, and varies with mc Consequently it is standard practice to remove as much foreign matter as possible by sieving samples before carrying out bulk density determinations, and also to measure the mc of the sieved material.

(ii) Foreign Matter

Most grain quality standards state that the screens in sieves used for the assessment of foreign matter content should consist of perforated metal plate conforming to specifications laid down by national or international standards organisations. Such specifications cover the composition and thickness of the metal plate, the shape and dimensions of the perforations, and the arrangement of the perforations on the plate. Table 3.8 (see Table 3.8. Sieve Perforations for Grain.) shows some examples of perforation specifications for some grain types.

Operating Capacity of Sieves

The efficiency of a sieve is dependent upon two factors: the dimensions of the apertures in the screen, and the proportional volume of material which will not pass through the apertures. As a general rule, the percentage sieving area' of a screen with small perforations is less than that of a screen with larger holes, and its capacity for sieving efficiently is correspondingly reduced. Also, for a perforated metal screen of fixed specifications the sieving efficiency falls off markedly if the volume of material which will not pass through the apertures exceeds a certain quantity. Table 3.9 shows the recommended volume of grain that should be placed on a screen, to maintain its sieving efficiency.

Table 3.9. Grain Sieves, 200mm Diameter, Maximum Loadings.

Source: International Standard ISO 2591-1973

(iii) Moisture Content

The standard test method (ISO 712) for the determination of mc in cereals is by mass loss in a hot-air oven. The method is time-consuming and a variety of rapid methods have been developed for day-to-day use. These range through accelerated heating by infra-red source gravimetric tests to almost instantaneous readout by electronic moisture meter. Of the latter, two types are common; resistance and capacitance meters.

It is recommended that grain-handling agencies avoid using a mixture of meter types, because this can lead to conflicting results. Instead, the meter best suited to their particular requirements should be selected. The following factors should be considered when selecting a meter to determine moisture content:

Resolution - the ability of the meter to differentiate between moisture contents which are very close in value. Some meters have the ends of the scale compressed i.e. the scale is not linear. The resolution of the meter is therefore relatively poor for high and low readings.

Repeatability - a measure of the meter's ability to give a constant reading when the same sample is tested several times. Capacitance meters, due to variations in grain packing, may not produce such accurate results as resistance meters, which normally use a more homogeneous ground or compressed sample.

Reliability - a measure of variation between meters when measuring the moisture content of the same sample. Meters should be regularly checked and calibrated to ensure reliability.

Stability/drift of measurements - affects the frequency of the need to calibrate the meter against the standard test method.

Range of commodity - calibrations will be necessary for all the commodities of interest, and the meter must be capable of accommodating them.

Range of mc - in general, resistance meters cannot measure low mc i.e. lower than approximately 9%, whereas capacitance meters can - to 1 or 2% in some cases.

Sample size - meters use differing size of test samples: larger samples give more accurate results, and require fewer replications.

Sample weighing - most capacitance meters require the sample to be weighed, thus introducing an extra variable (and extra cost).

Ambient effect - meter readings vary with temperature, and correction is required. Some meters automatically display the corrected moisture content.

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