2. Wheat physical properties

Different wheat grain properties are included in the grading systems. Quality parameters are related with stable properties, that is, grain hardness, size, shape and color; variable properties, that is, moisture content, contamination, damage to grain and bulk density and permanent properties, that is, fermented and foreign smells and faults that can be rectified [8]. While there is no uniform approach in the wheat industry to estimate the degree of quality, the stable and variable properties have extensive relationships with physical characteristics and provide a primary basis to determine initial grain quality control parameters.

Understanding the physical properties using rapid analysis methods is essential to estimate or predict with an acceptable level of certainty grain quality according to postharvest handling, storage and food processing. The market value of wheat grain is determined by various factors such as kernel morphology, texture, test weight and the shape of germ, crease and brush [9]. El Fawal et al. [10] cited that the physical quality of the kernel plays an important role for identifying the engineering characteristics of cereal crop grains, while Dziki and Laskowski [11] discuss that studies concerning the relationships between wheat kernel physical properties and milling properties have been carried out since the beginning of the cereal processing industry.

There are five main categories of data on physical properties of agro-food materials, which responds to physical treatments involving mechanical, thermal, electrical, optical and electromagnetic processes [4, 12]. For granular materials as cereal grains, the physical properties are classified by category or purpose, which in turn include more specific quality assessments; some of the major criteria applied to determine wheat kernel physical quality attributes included appearance and damage, size and shape and weight volume and density test, to cited a few of them.

Broadly, the geometric properties such as size and shape are one of most important physical properties considered during the separation and cleaning of kernels [13]. Study of physical properties in wheat kernels used are more complicated due to the inherent relationship among categories, that is, weight volume and density values are dependent on the shape, size and degree of kernel damage. Besides, physical properties of cereal grains are intrinsically linked to its moisture content level.

#### 2.1. Appearance and damage

main cereal in the human diet worldwide due to its agronomic adaptability, storability, nutritional value and diversity of products produced from it [3]. Wheat is unique among grains because its protein mixed with water and mechanical work yields a viscoelastic dough or batter capable of trapping gases produced by yeast or baking powders producing and array of leav-

Cereals grains are biological materials that differ in features due to many factors such as cultivar or genotype, soil fertility, growing conditions and agronomic practices [4]. The classification and grading play an important and critical role in the market because assures quality control guidelines. Furthermore, the standardization of grain quality allows a better marketing and grain processing to produce different products. Selected grain types for specific uses relate to their physical properties because they affect chemical composition, functionality and optimum industrial end use. The standardization of grain quality allows process a grain's lot with

Different methods for the classification of wheat grains are based on growth habit, end use and physical characteristics. The use of these methods can be referred to as technological classification [6]. Wheat-based products require different classes of the grain for their processing. Industrial quality is characterized by physical, chemical and rheological analysis related with roller milled refined flour, used especially in the manufacture of yeast-leavened breads (hard wheat) or chemically leavened cookies and cakes (soft wheat) or to produce semolina to

Physical evaluation of the wheat kernels offers a first and interesting quality control for selection as raw materials because the kernel physical features are related with design of equipment, handling, aeration and storage as well as to end use. Increasingly, analyses are implemented to assess the inherent characteristics of the grain to better know their attributes. The study of wheat kernel characteristics is necessary because new cultivars (with new and

This chapter reviews the principal physical properties of wheat kernels in three sections. The first covers aspects related to appearance and damage, the second addresses shape and size

Different wheat grain properties are included in the grading systems. Quality parameters are related with stable properties, that is, grain hardness, size, shape and color; variable properties, that is, moisture content, contamination, damage to grain and bulk density and permanent properties, that is, fermented and foreign smells and faults that can be rectified [8]. While there is no uniform approach in the wheat industry to estimate the degree of quality, the stable and variable properties have extensive relationships with physical characteristics and provide a

ened products such as breads, cakes and cookies.

276 Wheat Improvement, Management and Utilization

elaborate long and short pasta products (durum wheat).

different properties) are constantly being bred and produced [7].

primary basis to determine initial grain quality control parameters.

features and the third grain volume, weight and density.

2. Wheat physical properties

similar grade or quality [5].

Appearance is a valuable physical characteristic for selective classification and rating for subsequent handling and processing. It is considered the single most important factor that determines the economic value of a certain lot of grains. A number of grading factors adversely affect the appearance of cereal grains [14]. For purposes of inspection and grading, wheat kernels are considered damaged if the damage is distinctly apparent, therefore recognized as harmful for commercial purposes [15]. More specifically, U.S. Wheat Associates [16] suggested that wheat kernels are damaged if badly ground, weathered, frosted or present heat, insect, mold or sprout damages. The inspection of grain appearance and damaged kernels are recognized to greatly influence wheat kernels quality, because sound kernels could be stored for longer periods of time or processed into better quality intermediate and finished products. A portion of a particular wheat sample is thoroughly examined in order to detect different sorts of damage by physical or biological factors. The most common tests related to quality are presented below.

#### 2.1.1. Color

Color is one of the first and most relevant characteristics related to grain quality. Wheat kernel color depends on the species and other factors and is mainly dictated to chemical components presents in the ripe seed coat. Bechtel et al. [17] mentioned that color of wheat caryopses varies from light buff or yellow to red-brown according to the absence or presence of red pigmentation in this layer. Wheat is commonly classed according to color as red or white. Sometimes the perception of grain color is affected by the texture of the endosperm since the soft portion presents air-starch and air-protein interfaces that impart the chalky or dull appearance. On the other hand, the absence of voids or microscopic spaces in the endosperm of hard wheat gives a glassy or vitreous appearance resulting in trends of the red coloration present in the seed coat.

Other factors may affect the natural color of kernels such as mold-infestation in the field or during storage, heat or frost damage and other caused by phytopathogens. Shahin et al. [18] reported that mildew growth on wheat kernels reduces grain quality due to the characteristic gray discoloration which negatively impacts color of refined flours.

Computer-aided image analysis has many applications in agricultural sciences. For assessing grain quality this technique is able to objectively determine shape and color. However, problems are encountered in practical applications of these methods because they rarely correlate with the examined attributes and therefore a multivariate analysis is required. Principal component analysis (PCA)—a variant of the above method—is used when a high number of variables have to be reduced to several components [19]. Image analysis is performed by algorithms that use different color, texture and shape features as input parameters [20].

Actually, interest in white wheat kernels by both the milling and baking industries is increasing, because refined and mainly whole grain flours milled are preferred for different products and applications. Hard white wheat has been reported to have distinct advantages over the conventional hard red counterpart. These advantages include higher flour extraction rate and lighter colored end products [21].

### 2.1.2. Insect damage

One of the most relevant wheat kernel quality parameters is insect damage. This particular assessment is considered one of the most critical degrading factors [22], because it relates to flour yield and color and increases the amounts of insect fragments present in flours and processed products which are considered as one of the most important quality factors related to food sanitation. Presence of insects induces losses in quantity and quality by insect consumption, grain weight loss, contamination (toxicity) with excrement, bodily fragments and chemical secretions that disfavor flour flavor and odor. In addition, insects increase heat and kernel moisture due to their metabolic activity. The potent enzymes produced by insects and by the grain respiration system are known to negatively affect milling and baking qualities [23, 24].

The main group of insects that cause serious damage in cereal grains includes beetles such as Sitophilus granarius, Tribolium castaneum, Trogoderma granarium, Tenebroides mauritanicus and Rhyzopertha dominica. Primary pests infest sound grain whereas secondary pests can attack only broken or cracked grains or milled products [25]. A large portion of the insect's life is spent inside the kernels; therefore, their detection is extremely difficult. Fortunately, the industry has adopted new techniques for identification of insect damage inside grains using Near Infrared (NIR) spectroscopy [6]. Needless to say, the internal infestation degrades the quality and value of grains [24].

Pest control begins with a preventive control and disinfection of empty storage containers and storing grain in pest-free conditions. Different improved methods include hermetically closed containers, heating or cooling, ionizing radiation, light and pheromone traps and storage under controlled atmospheres [25]. Postharvest control is essential in many countries and the traditional treatment is chemical fumigation [22]. Unfortunately, the use of synthetic insecticides has problems such as their persistent toxicity, development of resistance in insect populations and other adverse environmental impacts [26, 27].
