QUALITY INDICES OF WHEAT
The importance of quality in relation to wheat has been emphasized to growers in many changes that have taken place in the wheat industry since the early 1970s.Consideration of wheat quality had been mainly in relation to suitability for bread manufacture, and new varieties were assessed only for this end-product.
These new initiatives have combined to give an increased emphasis to quality in the development of new varieties, to ensure their suitability for an expanded range of products in overseas markets. This range of products includes various pan and flat breads, steamed bread, noodles, biscuits and cakes. An important local end-use of wheat is in the manufacture of gluten and starch.
Grain quality is critical to all the role-players in the grain value chain – from the producer to the consumer. Grain quality is a term that refers to the quality of the grain. However, quality depends on what the grain is used for.
In food and feed production, properties like protein, oil and carbohydrates are important. In the milling industry, the health of the grain is the most important factor, and to producers who use seed for planting, a high germination percentage, the growth vigor of seed and seed dormancy are the main quality properties to be considered. To consumers, properties like color and flavor are important. The quality of grain is determined by physically grading it, when properties are determined visually and with the aid of instruments. During this process, the grain’s suitability for its intended purpose and the grade are determined, and these are linked directly to the value of the grain. The buyer of the grain decides based on the grading whether he can use the grain for his intended manufacturing and what he can pay for it.
Many factors contribute to variation in quality among different loads of wheat. Quality potential is largely determined by the variety, but the extent to which this potential is achieved depends upon factors such as seasonal conditions, soil type and management practices. In wheat breeding, considerable attention is given to improving the quality of varieties. Here quality characteristics such as grain hardness, grain size, milling, dough properties, and starch quality are important. In more recent times, additional varietal improvement has been sought in relation to reduced levels of black point and late-maturity alpha amylase. At the time of receival, quality tests are carried out to measure characteristics that that are influenced mainly by the environment and grower practices these include test weight, screenings, moisture content and protein content. Loads are also examined for the presence of stained and sprouted grain and insects.
Elements of grain quality
In general, wheat must be acceptable for consumption by people and animals. It should also be clean, ripe, and free of foreign matter and damage. The overall quality of grain is affected by various factors, including cultivation practices, time of harvest and harvesting practices, handling, storage and transport practices. Grain quality depends on the type of grain, genetics, cultivation practice and handling and storage of the grain.
Characteristics of grain quality
Grain quality is characterized into two main factors
(i) intrinsic factors, and
(ii) (ii) extrinsic factors.
The intrinsic factors of grain include, color, composition, bulk density, odor, aroma, size and shape. Color is an important primary factor for characterization and grading, trade, and processing of grain. It is a common criterion used in wheat trade.
The main compositions of grain are carbohydrates (energy), protein, lipids, mineral, fiber, phytic acid, and tannins. It varies significant depends on the type of grain, genetics, varieties, agricultural practice, and handling of the grain. Grain composition plays a significant role in grading and marketing of grains.
Bulk density is defined as the ratio of the mass to a given volume of a grain sample including the interstitial voids between the particles.
Size and shape are important factors in grain quality and grading; it varies between grain to grain and between varieties of the same species. It is commonly used in rice grading and key factors in milling industry.
The extrinsic factors include: age, broken grain, immature grain, foreign matter, infected grain and moisture content.
The kernel of wheat is approximately 1/8″-1/4″ lengthy. Tip of each kernel is protected by way of stiff hair referred to as brush. The grain is oval and round on each end. At one side, there is a folding of alueron and all covering layers known as crease. Wheat bran is largely manufactured from cellulose and consists of an excessive share of B group vitamins and nearly 50% of the mineral content of the whole wheat. The germ located at the bottom of the grain is rich in fat, proteins, nutrients B and E and iron. The endosperm is specifically starch and is separated from the germ by a membrane referred to as scutellum. Endosperm is the largest portion of wheat grain and contains mainly starch granules embedded in a matrix of protein. The outer edge of the endosperm is composed of a single layer of cells Basic Structure of all cereals referred to as the Alleurone layer rich in proteins than the endosperm, however is eliminated with the bran all through milling of the wheat.
Wheat (Triticum aestivum L.) is the world’s leading agricultural source of energy, protein and fiber; it belongs to a family Graminaea and genus Triticum and can be categorized into three main classes – hard, soft and durum. Wheat quality can best be described in terms of end-user, nutritional quality, milling, and baking and rheology quality. In general wheat needs to be sound, clean, well mature, free from foreign material and damaged.
Wheat (Triticum aestivum, Triticum compactum, Triticum durum) is expected to remain foremost among the staple crops for man, being a concentrated carbohydrate source with useful protein, fat, mineral, vitamin and fiber content. The ability to form a visco-elastic, gas-retaining dough also distinguishes wheat flour from that of maize, barley and to a lesser extent rye, and the attractiveness of the resulting foods gives this crop much of its economic significance (Wall, 1979). More than 70 % of the world’s cultivated surface is under cereals of which more than 30 % comprises wheat, followed by maize, rice and barley the importance of wheat, apart from being a staple crop, can be explained in terms of the following:
• adaptable, as different varieties tolerate a wide range of soil types, climatic and
• relatively easy to produce under favourable climatic conditions with a
harvesting procedure that is easily mechanised.
• suitable for long term storage under appropriate conditions.
• multipurpose and versatile providing both human diet and livestock feeds.
• suitable for various uses in the food industry. Having a bland taste, it can carry
different flavors e.g. as thickeners in soup and in snack foods.
The grain quality of wheat can be summarized as the following properties:
? Test weight
? Damaged grain
? Black point
? Moisture content
? Protein content
? Milling quality
? Baking quality
? Falling number
? Starch quality
? Foreign matter
In general wheat quality can be divided into three main groups
(i) botanical (species and varieties),
(ii) (ii) physical
(iii) (iii) and chemical characteristics.
Physical characteristics of wheat quality includes color, hardness, grain size and shape, and grain weight.
Physical properties of grain play a very important role in the quality of the grain, and in final products such as flour. Main physical properties that influence quality of wheat are test weight, hardness, grain size and shape, vitreousness and color.
Test weight of wheat is considered the most common and simplest manner to quantify wheat. It is an important quality factor in wheat grading as it gives rough estimates of flour yields. The primary factors that affect the weight of wheat are kernel size and shape, kernel density, maturity of wheat and real wheat range.
Test weight is a measurement of the bulk density of the grain, expressed as kilograms per hectoliter, and gives a quick guide to the extent of grain filling and its potential suitability for milling to produce flour.
Hectoliter mass (HLM), also called as volumetric weight, is regarded as the most common and simplest way of quantifying wheat. It measures the grain mass density and is expressed as mass per volume. The HLM of grain is influenced via many factors, which include climatic conditions during the growth period of the grain, frost damage, climatic conditions during harvest and foreign matter in the grain. The basic factors affecting the HLM are grain size and shape, grain density, maturity of wheat, diseases and cultivar.
Test weight is obtained by pouring (until it just overflows) a well-mixed sample into a Schopper Chondrometer which consists of a 1/2-liter aluminum measuring cylinder equipped with a cup and a striking blade. The Chondrometer is weighed empty prior to use and in testing a grain sample with the use of the striking blade the grain can flow through into the cup and the excess grain is leveled off. The grain in the cup is weighed and divided by five to obtain the test weight.
The hardness of wheat endosperm is important in determining the suitability of wheat for various end products and affects the processing and milling of wheat. It is the common characteristic used by millers and trader to classify wheat. In term of hardness wheat can be classified as either hard or soft.
Wheat grain hardness is one of the most important characteristics that affect the milling process, its efficiency and the end-use properties of the flour. The hardness of the wheat grain affects the ease with which the endosperm is separated from the bran during the milling process, and controls some of the properties of the flour produced. If sufficient pressure and shear is applied during reduction of the endosperm, the starch granules of hard wheats are more likely to become mechanically damaged. The flour from hard wheats would therefore have greater proportions of damaged starch grains than the flours of soft wheats. This will influence the end-use of flours from wheats of differing hardness.
Grain hardness is determined mainly by variety. It can be measured in many ways, which include by the coarseness of wheat meal or flour, with the aid of convenience with which outer layers of the grain can be removed by abrading, by the amount of energy required or noise emitted in grinding a given weight of sample, or by the amount of damage which occurs to starch granules during flour milling. A common method of measurement is the particle size index test, which indicates the percentage of wheat meal passing through a fine screen – typical values for soft grained varieties are 26 to 30, whereas hard wheats give lower values, say 12 to 16. Higher damaged starch levels, which occur in flour produced from hard-grained varieties, result in higher water absorption levels during dough mixing and increased gassing power during dough fermentation in bread making.
In terms of color, wheat is classified into two classes
Hard red winter wheat:(i.e. red wheat and white wheat), hard red winter wheat is considered superior and commonly used for bread flour production,
White wheat: while white wheat is usually used for cake, chapattis, and pasta (macaroni), each types of wheat has different properties such as taste, baking quality and milling yields.
Wheat vitreousness is an optical property used by many countries to grade or quantify durum wheat. Based on vitreouness, wheat can be classified into 3 main classes: vitreous, mealy and piebald. Vitreous wheat differs from non-vitreous by kernel appearance (starchy and opaque); vitreous wheat is considered better quality than non-vitreous kernels, because of higher quality semolina protein, nice color and uniform coarser granulation.
vitreous kernels appear hard, glassy, and translucent, and have superior cooking quality and pasta color, along with coarser granulation and higher protein content. Thus, the vitreousness of durum wheat kernels is an important selection criterion in grain grading.
The screenings level is a measure of the amount of material which would be lost in the cleaning process prior to flour milling. Pinched grain because of a dry season, frost or disease will give higher levels of screenings. Varieties with inherently narrower grain may also give higher screenings in adverse seasons. Other contributors to screenings can include damage grain and foreign seed.
Screenings are determined by means of weighing the particles falling through a 2.0mm sieve after 40 shakes. Many laboratories and receival sites in the industry use the Agtator sieve shaker to ensure repeatability in shaking the grain over the sieve. A representative sample of wheat is taken and passed through a nominated sieve (normally 2.00mm). The screens are then collected and weighed. The results are calculated and expressed as percentage.