Quality characteristics of grains

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Consumers have become accustomed over the years to demanding grain with particular qualities. Where consumers are close to the source of the grain, e.g. in local markets, their own preferences and the laws of supply and demand will control the quality of the grain. However, where grain is traded over large distances, particularly internationally, the consumer will have no direct influence over quality, and regulatory standards must be established and imposed to protect consumer rights. Therefore criteria of grain quality must be established and accepted by all parties in the grain trade. The criteria assigned to grain are the intrinsic varietal qualities and those which are environment- or processinduced. The more important quality criteria as they relate to grading of grain are described in the following sections.


Intrinsic Qualities

(i) Colour

Cereal grains are pigmented and range through the colour spectrum from very light tan or almost white, to black. Where extractive milling is required, highly-pigmented varieties may give low yields of white flour.

(ii) Composition

Composition, e.g. protein, carbohydrate, lipids and their breakdown products, qualitatively influences product acceptability, by affecting texture and taste. Quality changes evolve slowly in stored grain and more rapidly in milled or processed intermediary products.

Some grain components, for example husk, are inedible and quantitatively influence product yield and gross nutrient available to the consumer.

(iii) Bulk Density

Each type or variety of grain when in optimum health, fully mature, etc. has a characteristic bulk density. This is defined as the weight per standard volume measured in a standard manner. The same characteristic is variously known as 'test weight', 'bushel weight' or 'specific weight'. For details of how bulk density is measured see page 62.

If the bulk density varies the trend is usually downwards and indicative of reduced overall quality of the grain. Hence it is often measured in the grain trade. Factors which commonly affect bulk density are insect infestation, excessive foreign matter and high percentage moisture content. In wheat, bulk density is considered to be a reasonable indicator of milling yield.

Bulk density should not be confused with 'specific volume' as defined in the context of Chapter 6 of this bulletin. The terms are related, but the distinction is necessary because it is an established fact that the 'bulk density' of grain increases when it is stored in large quantities, bag or bulk, due to compaction.

(iv) Odour, aroma

Most grain types, when fresh, have a distinctive natural odour or aroma. This is generally accepted as an indicator of good quality, although some people prefer grain which smells 'old' or even fermented.

As with most natural produce, some grain varieties are better-liked than others because of their odour. Certain cultivars of rice, for example, possess aromatic qualities which are considered desirable by some consumers.

See also mixed variety.

(v) Size, shape

Rice, as a whole-grain food, is classified by size (length) and shape (length:breadth ratio). Other grains also have size considered in their specification. In general a small range in size assists with processing and handling.


Induced Qualities

(i) Age

During the post-harvest phase, grain undergoes complex biochemical changes termed 'aging'. Changes to carbohydrate, lipids and protein fractions result in, for example, firming of texture in rice on cooking, and increased gas-retention capability in wheat flour. For most consumers, the effects of these changes are considered to be desirable. When plotting consumer acceptability of a grain product against its age since harvesting, generally it is considered to be maturing during the upward curve of the graph, and deteriorates only when the curve changes direction downwards.

(ii) Broken grain

Grain is marketed normally in whole grain form and is considered to be of inferior quality if broken. Breakage may occur from fissures as a result of excessive drying/weathering conditions in the field or during handling. Breakage reduces quality by reducing acceptability and by increasing susceptibility to infestation during storage. This affects milling yield by contributing to weight loss.

(iii) Chalky or immature grain

Empty grains result from sterility and pre-harvest infections and insect attack. Immature grain content is affected by time of harvest. In rice, immature grains are greenish in colour. Thin white (usually opaque) grains are caused by incomplete grain filling and may result from pests or disease. Chalkiness is caused by incompletely filled starchy endosperm which disrupts light transmission, causing opaque regions. In most cereals, chalky areas have lower mechanical strength on crush tests and may break during handling. The broken portion is more easily invaded by certain storage pests.

(iv) Foreign matter

Dilution of the prime product by foreign matter reduces the value, and also may affect handling and processing. Foreign matter may be subclassified as:

animal origin - insects and their products, rodent excrete, etc;
vegetable origin - straw, weeds, seeds, dust, micro-organisms/toxins;
mineral origin - stones, mud, dust, glass, metals, oil products, pesticide residues.

Elements from all three subclasses may render the grain unfit for consumption. Potentially the greatest threat to health probably is from micro-contamination with the bacterial products of poor sanitation, and with toxins and chemical pesticide residues.

(v) Infested, infected grain

Grain mass, and therefore yield, is reduced by infestation. Contamination not only has direct food hygiene implications but also indirect ones, as invading micro-organisms may produce toxins under certain conditions which may lead to acute or chronic illness.

(vi) Mixed varieties

A mixture is an indication of poor pre- and post-harvest management and supervision, e.g. seed selection, lot segregation and treatment, contamination, etc. Grains differing in size and other characteristics affect processing potential. Whilst preference for a particular variety may be influential nationally or regionally, internationally-traded grain is recognised usually by grain type rather than by variety e.g. yellow or white maize. Exceptions do occur, e.g. basmati rice, (see odour, aroma).

(vii) Moisture content

Moisture content (me) of grain plays a crucial role in post-harvest processing and is associated with most of the induced characteristics. Water vapour will diffuse throughout a bulk of grain and the mc will tend to equalise. 'Hot spots' may occur at a site of increased respiration (caused by sprouting, infestation or microbial activity), and condensation may occur on cold grain or containers.

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