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Wheat for bread and other foods
R.J. Peña

Wheat has accompanied humans since remote times (as far back as 3000 to 4000 BC) in their evolution and development, evolving itself (in part by nature and in part by manipulation) from its primitive form (emmer wheat) into the presently cultivated species. The more important modern wheat species are hexaploid bread wheat (Triticum aestivum L.) and tetraploid durum wheat (T. turgidum L. var. durum), which are different from one another in genomic make-up, in grain composition and in food end-use quality attributes. Except for the very warm tropics, wheat adapts to all diverse climatic conditions prevailing in agricultural lands and, therefore, it is harvested in the world all year around. Its wide adaptation to diverse environmental conditions, along with its unique characteristic of possessing a viscoelastic storage protein complex called gluten, are the main factors making wheat the most important food crop in the world.

There are quite large differences in grain composition and processing quality among wheat cultivars within a species. Hence, one cultivar may be suitable to prepare one food type but unsuitable to prepare a different one. Quality differences among wheat cultivars have gained even more importance in grain trading due to important global economic and social trends. Recently, many countries have adopted, or are in the process of adopting, free-market economies, impacting positively on the income of the population, particularly of that concentrated in urban areas. Concomitantly, several of these countries are experiencing a trend towards increased urbanization and increased demand for traditional and new convenient, processed wheat-based foods. As a consequence, wheat processing industries now require various distinct wheat supplies possessing specific grain quality attributes. Thus, it is common to find that the value of a wheat crop in the market is generally determined by grain attributes associated with its processing quality.

The establishment of the open-market economic model now permits wheat industries to purchase in the wheat export market wheat quality types not encountered locally. Therefore, grain quality is important, particularly for large- and small-scale farmers growing wheat as a cash crop. When wheat is cultivated as a cash crop, farmers will look for wheat varieties satisfying both their grain yield expectations and the quality needs of the targetted market. This chapter presents an overview of the main uses of wheat, the grain compositional characteristics associated with wheat processing quality and the manipulation of grain quality traits through breeding.

WHEAT TYPES AND CLASSES

Roughly 90 to 95 percent of the wheat produced in the world, about 600 million tonnes (USDA, 1998), is common wheat (T. aestivum), which is better known as hard wheat or soft wheat, depending on grain hardness. Wheat is utilized mainly as flour (whole grain or refined) for the production of a large variety of leavened and flat breads, and for the manufacture of a wide variety of other baking products. The rest is mostly durum wheat (T. durum), which is used to produce semolina (coarse flour), the main raw material of pasta making. Some durum wheat is milled into flour to manufacture medium-dense breads in Mediterranean and Middle Eastern countries and some into coarse durum grain grits used to produce couscous (cooked grits) in Arab countries.

Wheat for the purpose of trading is classified into distinct categories of grain hardness (soft, medium-hard and hard) and colour (red, white and amber). It may be further subdivided into subclasses based on growing habit (spring or winter). Each wheat subclass may also be grouped into grades, which are generally used to adjust the basic price of a wheat stock by applying premiums or penalties. Wheat grades are indicators of the purity of a wheat class or subclass, the effects of external factors on grain soundness (rain, heat, frost, insect and mould damage) and the cleanliness (dockage and foreign material) of the wheat lot. Grain protein content and alpha-amylase activity (enzymatic activity associated with the germination of the grain) are frequently considered as grading factors in wheat trading. These two factors, which are important in determining the end-use properties of wheat, can be tested rapidly upon reception of the wheat stock. High alpha-amylase activity has a large negative effect on the properties of baking doughs, as it hydrolyses excessively the flour's starch. Grain lots having very high levels of amylase activity may be totally rejected as a food item and accepted in the market only as feed grain.

WHEAT-BASED BAKING FOODS

Trends in bread consumption

Wheat, in the form of bread, provides more nutrients to the world population than any other single food source. Bread is particularly important as a source of carbohydrates, proteins and vitamins B and E (Pomeranz, 1987). Bread consumption, particularly that of breads prepared with whole grain flours and with multigrain flours, tends to increase in developed countries (Faridi and Faubion, 1995; Seibel, 1995; McMaster and Gould, 1995). This is mainly due to an increase in a nutritionally conscious population that wants to reduce the consumption of simple carbohydrates, fat and cholesterol while increasing the consumption of complex carbohydrates, dietary fibre and plant proteins (Seibel, 1995). According to various researchers (Buckley, 1997a, 1997b; Owens, 1997; Prior, 1997), trends in bread consumption in developing countries vary depending on factors such as:

• degree of industry privatization and extent of government controls in wheat trading;

• degree of the change from a more rural to a more urban population, which is accompanied by changes in food habits and an increase in the preference for processed, convenience foods;

• rate of adoption of food habits of developed countries and rate of increase of the income of the individuals.

The above is particularly true in China and Southeast Asia (Buckley, 1997a; Owens, 1997) and in Middle Eastern countries (Buckley, 1997b; Prior, 1997). Bread consumption in sub-Saharan Africa is low and varies widely from country to country. In most parts of sub-Saharan Africa, the main sources of nutrients for the population (mainly rural) are maize, sorghum and starchy roots.

Bread types

There are three general types of bread: leavened, flat and steamed. Although all three types are prepared from a refined (or whole-meal) flour-water dough, which is viscoelastic and cohesive, each bread type differs from one another on specific end-product properties, processing conditions and grain quality needs. Grain quality characteristics for various bread types and other wheat-based baking products are shown in Table 29.1.

TABLE 29.1
Wheat quality characteristics for various food types

Leavened breads are popular in almost all parts of the world. These are made with aerated, yeasted viscoelastic doughs, which expand by the action of gas produced by the yeast fermentation process to gain volume and decrease its density. Size, volume and density of a given type of bread are determined by a combination of bread formula, length of the fermentation stage, actual work exerted to the bread-making dough and time allowing the bread dough to rise before baking. The shape of the bread is determined by the panning mould (pan-type bread, hamburger and hot-dog buns, etc.) or given by the baker through hand manipulation of the dough before the oven stage (Plate 77). Hard to medium-hard grain is preferred for the manufacture of leavened breads. This is because the levels of damaged starch produced from these wheat classes are appropriate to achieve the high dough water absorption desired by the baker (high water absorption means high flour yield per unit of bread).

Both the type of bread and the bread-making process used, on the other hand, determine flour (or dough) strength requirements. In general, a mechanized bread-making process using high-speed mixing requires stronger wheat flour than does a manual or semi-mechanized one. Hard to medium-hard wheats, which yield strong flour doughs, are more suitable for the mechanized production of leavened breads, such as pan-type bread and hamburger and hot-dog buns (Faridi and Faubion, 1995; Wrigley, 1991). Those yielding medium-strong doughs are suitable for the production (generally semi-mechanized or manual) of French-type (yeast-fermented, hearth-baked breads, in general) and flat-type, such as Arab baladi bread, Indian chapati, Mexican flour tortilla, etc. (Plate 77) (Qarooni, 1996; Singh and Kulshrestha, 1996). Soft wheats, which produce weak doughs, may be suitable for Asian steamed breads (Nagao, 1995), although hard wheat with medium dough strength may also be utilized (Wrigley, 1991). Steamed breads demand the least dough strength since steam acts as the main leavening force during the baking process.

Soft wheat products

In contrast to breads, cookies, cakes and pastries (soft wheat products, Plate 78) are made with non-elastic, stiff doughs or with thick, viscous batters prepared with soft wheat flours. In preparing soft wheat-type products, the development of dough viscoelasticity is prevented by the addition of high proportions of sugar and fat in the baking formula (sugar may equal as much as two-thirds and fat as much as one-half the amount of flour used in the formula). Water vapour, air and chemicals are the leavening agents of these food systems. Spread (in the case of cookies) and batter or dough expansion (in the case of cakes and other pastry) are largely determined by the viscosity of the system and the availability of free water (water acts as a leavening agent as it evaporates) during the oven stage. Therefore, starch (damaged) and protein absorption are both highly undesirable in these baking systems (Miller and Hoseney, 1997). Low water absorption can best be achieved with soft wheat flours characterized by having low levels of damaged starch and low protein content.

Flour noodles

Flour noodles are widely consumed in East Asia and are a staple food in northern China (Huang, 1996). The consumption of flour noodles, particularly dried and instant (fried and steam pre-cooked) noodles (Plate 78), is increasing in the Western Hemisphere. There are two major types of noodles: white salted noodles (WSN), made with flour, salt and water, and yellow alkaline noodles (YAN), which in addition to the ingredients of WSN include alkali (roughly 1 g of alkaline salts/100 g of flour) to develop their characteristic yellow colour. The basic noodle-making steps are: making a stiff, crumbly dough by hand kneading or slow mixing of the ingredients, dough resting, sheeting, cutting of noodle strands and boiling (or drying). Mechanized noodle production predominates in Japan, while handmade noodles predominate in China (Nagao, 1995; Huang, 1996). White salted noodles should be bright, creamy-white, smooth in texture and soft but slightly elastic to permit a clean bite. Desirable noodle brightness and whiteness is achieved when low extraction (below 70 percent) flour having low ash content (less than 0.5 percent) is used. Acceptable softness and smoothness are attained when using flours possessing high flour-swelling volumes (high starch-pasting viscosity) and protein contents from 8 to 10 percent (Huang, 1996; Nagao, 1995; Ross et al., 1996). In contrast, YAN are from light yellow to yellow in colour, smooth but firmer than WSN, with an elastic bite greater than that of WSN. The greater firmness and elasticity of YAN is due to the use of flours having more protein content and gluten strength than flours used for producing WSN (Huang, 1996; Nagao, 1995; Ross et al., 1996).

GRAIN QUALITY AND QUALITY IMPROVEMENT

The rapid increase in the world population demands parallel increases in food production, particularly of wheat. However, in order to preserve the environment and the present natural resources, further increases in global wheat production must come mainly from enhancing the yield potential of new wheat varieties and not from expanding the wheat production area.

Increasing yield potential without affecting negatively the quality of the grain is difficult, mainly because increases in grain yield are generally accompanied by a decrease in the grain's protein content, which is strongly associated with bread-making quality. Therefore, wheat breeders need to give grain quality aspects the same importance that they give to yield potential and disease resistance. Research objectives that are important for breeding programmes developing wheat cultivars targetted to specific food markets include:

• understanding the genetic control of specific grain components;

• understanding the relationship between grain composition and processing qualities;

• achieving rapid identification and manipulation of quality-related traits based on the use of reliable, fast, low-scale quality testing methodology.

Grain hardness

Grain hardness is determined by the way components are packed in the endosperm cells and refers to the resistance the grain opposes to being fractured and to being reduced to fine wholemeal flour or to fine endosperm particles (semolina or refined flour). Grain hardness is a grain quality trait associated with the milling properties of wheat (Miller et al., 1982) and with the baking quality of the resulting milling products.

Milling times, milling energy requirements and the level of starch damage produced in the milled flour are all influenced by grain hardness. Hard wheats require longer milling times and more milling energy, and produce a larger amount of damaged starch.

The genetic control of grain hardness is still unknown. Although grain hardness has been associated with a 15KD protein attached to the surface of the starch granule, starch from soft wheats tends to have more of this protein than does starch from hard wheats (Greenwell and Schofield, 1986; Rahman et al., 1991). The actual role of the 15KD protein (which is controlled by a gene on chromosome 5DS) on determining grain hardness is still not well understood (Rahman et al., 1991).

Rapid small-scale methods (based on grinding time, grinding volume, or particle size distribution) used to determine grain hardness make it relatively easy to screen for hardness as early as the F3 generation. Near infrared reflectance and transmittance (NIR, NIT) analysis of the particle size distribution of whole grain flour or analysis of the intact grain samples are particularly fast and useful in early generation screening.

Starch

Native starch, which is the main component of the wheat grain (70 to 75 percent dry weight), shows little influence on the functional properties of wheat flours used in bread, cookie and cake making. Damaged starch (mechanically damaged during flour milling), however, by exposing its components (amylose and amylopectin) to interact with other constituents of the baking formula, influences importantly the water absorption and fermentation time requirements of bread-making doughs, as well as the staling and crumb textural properties of bread. Some small amount of damaged starch is desirable in bread-making flours but highly undesirable in cookie- and cake-making flours, as it may reduce considerably the expansion capacity of cookie doughs (Miller and Hoseney, 1997) and cake batters during baking. This is the reason why the cookie and cake industries use soft wheat flour, which has a minimum, if any, of mechanically damaged starch and, consequently, low flour water absorption.

The swelling and pasting properties of native starch influence, on the other hand, the eating quality of wheat flour noodles, particularly white noodles, which are characterized for being smooth and soft but slightly elastic (Bhattacharya and Corke, 1996; Konik et al., 1994; Nagao et al., 1977). High starch swelling and desirable noodle softness (low firmness) have been associated with the absence (presence of Null4A) of the granule bound starch synthase (GBSS) protein controlled by genes on chromosome 4A (Ross et al., 1996). Other starch- and protein-related factors also influence starch swelling and noodle softness, and may mask the quality effects of Null4A (Ross et al., 1996). Thus screening for Null4A may not be effective to improve starch properties for noodle making.

The Amylograph/Viscograph and more recently the Rapid Visco Analyser (RVA) are used to obtain a complete profile of starch pasting properties. While the first requires a large sample size and a considerably long testing time, the RVA requires a 3 to 4 g sample and only a few minutes to reveal the pasting profile of the tested material. Therefore, the RVA is now considered a rapid test suitable for the early selection of wheat lines possessing desirable starch pasting viscosity for noodle making (Bhattacharya and Corke, 1996; Panozzo and McCormick, 1993).

Proteins

Grain protein content in wheat varies between 8 and 17 percent, depending on genetic make-up and on external factors associated with the crop. A unique property of wheat flour is that its insoluble protein forms, when in contact with water, a viscoelastic protein mass known as gluten. Gluten, comprising roughly 78 to 85 percent of total wheat endosperm protein, is a very large complex composed mainly of polymeric (multiple polypeptide chains linked by disulphide bonds) and monomeric (single chain polypeptides) proteins known as glutenins and gliadins, respectively (MacRitchie, 1994). Glutenins confer elasticity, while gliadins confer mainly viscous flow and extensibility to the gluten complex. Thus, gluten is responsible for most of the viscoelastic properties of wheat flour doughs and is the main factor dictating the use of a wheat variety in bread and pasta making. Gluten viscoelasticity, for end-use purposes, is commonly known as flour or dough strength.

Variations in grain protein content may significantly influence the dough strength properties of a wheat variety. Quantity alone, however, cannot always explain quality differences among wheat cultivars. Therefore, protein quality, in terms of the polymeric/monomeric protein ratio and the molecular size of the protein polymer (determined by the presence of specific glutenin subunits), is also important (see Weegels et al., 1996 for a review).

Wheat flour contains roughly the same amounts of glutenins and gliadins, and the unbalance of the glutenin/gliadin ratio may change its viscoelastic properties. The glutenin fraction is, however, the major protein factor responsible for variations in dough strength among wheat varieties. Fu and Sapirstein (1996) recently confirmed this; they observed that most of the variation in dough strength parameters was explained by the amounts of soluble and insoluble glutenin.

The Zeleny and the sodium dodecyl sulphate (SDS) sedimentation tests (Axford et al., 1979) can be used to obtain a semi-quantitative estimation of the amount of glutenin (or indirectly, of general gluten strength). These tests, which are based on the expansion of mainly glutenins (also known as gel proteins) in lactic acid or SDS/lactic acid solution, are currently the most rapid and reliable single small-scale tests (see Weegels et al., 1996 for a review). These tests are widely used to screen early generation wheat lines in relation to their general gluten strength type (strong to weak).

TABLE 29.2
Wheat gluten proteins and their genetic control

^a HMWG = high molecular weight glutenin; LMWG = low molecular weight glutenin.

Genes of the complex loci located in group 1 and group 6 chromosomes (Table 29.2) control gluten proteins. The Glu-A1 and Glu-B1 loci, in the case of durum wheat, and additionally Glu-D1, in the case of common wheat, possess genes coding for 0 or 1, 1 or 2, and 2 high molecular weight glutenin (HMWG) subunit components, respectively. Thus a common wheat variety may contain from three to five HMWG subunit components that can be clearly distinguished as discrete bands by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Payne and co-workers (Payne et al., 1979, 1981) observed that some HMWG subunits were distinctly associated with dough strength. It is now known that common wheat varieties possessing five HMWG subunit components generally have stronger gluten character than the ones possessing three or four components. There are also differences in quality effects among HMWG subunits of the same locus; for example, subunit 5+10 has a more positive effect on bread-making quality than does subunit 2+12 (Kolster et al., 1991; Payne et al., 1981). Quality scores for several HMWG subunits are shown in Table 29.3.

TABLE 29.3
Quality scores assigned to high molecular weight glutenin subunits on the basis of wheat quality-related parameters^a

^a Pogna et al., 1992.
^b Higher value indicates better quality effect.
^c Payne, 1987.

SDS-PAGE of whole protein extracts can be used in breeding programmes as an early generation technique to select lines possessing desirable HMWG subunit composition and in advanced stages to define desirable HMWG combinations in progeny of new crosses. These breeding actions have proved effective in improving the gluten strength in both bread wheat and durum wheat. Low molecular weight glutenin (LMWG) subunits (controlled by genes of the Glu-3 complex loci located on the short arm of group 1 chromosomes) are also important in determining gluten viscoelasticity (see Weegels et al., 1996 for a review). The role of specific LMWG subunits on gluten strength has been determined in durum wheat; LMWG subunit LMW-2 and its variants confer stronger gluten character than does LMW-1 (Pogna et al., 1988; Peña et al., 1994; Porceddu et al., 1998).

In contrast, much remains to be known about the relationship between LMWG subunit composition and gluten strength in bread wheat. This is in part due to the larger number of LMWG subunits in bread than in durum wheat (genome D proteins are not present in durum wheat). The large number of components (up to seven) comprising a single LMWG subunit makes it practically impossible to distinguish them by using the conditions for determining HMWG composition, particularly if the gliadins, which comigrate with the LMWG, are not removed as a first step (Gupta et al., 1990). A new sequential protein extraction procedure combined with one-dimensional SDS-PAGE has been developed to obtain well-distinguished patterns of HMWG and LMWG subunits in a single gel (Gupta and MacRitchie, 1991; Singh et al., 1991). This procedure has facilitated the study of the relationship between genetic control and quality of LMWG subunits. In spite of this, determining LMWG subunit composition remains more difficult than determining HMWG composition.

Minor grain constituents

Lipids, pentosans, soluble proteins and other minor grain constituents also play a role in determining wheat flour quality. Their effect on flour and dough functionality can be corrected, generally, by making adjustments to formulas (e.g. use of additives or improvers) and to the baking process. Therefore, from the grain quality improvement point of view, they are not considered major ones.

Grain and non-grain (environment, growing conditions and diseases) factors interact to make the issue of quality rather complex. In spite of this, applying selection pressure at the segregating stages using rapid small-scale quality tests (NIR analysis of grain hardness and protein, SDS sedimentation, SDS-PAGE analysis of gluten proteins and RVA analysis of starch) results in significant improvements on the general quality characteristics of the wheat germplasm. The development of advanced lines possessing more specific desirable quality attributes can be achieved by screening wheat advanced lines for dough viscoelasticity (with the Extensigraph or the Alveograph), for dough mixing properties (with the Farinograph or the Mixograph) and for end-use quality (bread, cookie, pasta, etc., using laboratory-scale methods). Applying quality selection pressure at both the segregating and advanced stages should allow breeders to develop wheat varieties that are as productive as cropping and environmental conditions permit while possessing the quality attributes required by the baking industry. Table 29.4 shows quality parameters determined in breeding programmes in the United States and Europe and at the International Maize and Wheat Improvement Center (CIMMYT).

TABLE 29.4
Quality parameters used by bread wheat breeders for quality screening in the United States and Europe and at CIMMYT

^a Edwards, 1997.

Biotechnology and the improvement of end-use quality

Novel biotechnology techniques have opened the possibilities of investigating the basic genetic and biochemical aspects of individual protein subunits and of other molecules contributing to the end-use quality of wheat. For example, experiments in which individual HMWG subunits (synthesized from DNA cloned in Escherichia coli) were incorporated into wheat flour showed that dough strength can be enhanced by increasing the amount of polymeric subunits in the gluten complex (Bekes and Gras, 1992; Bekes et al., 1994a, 1994b). These results prompted some researchers to generate glutenin genes and DNA clones to explore the possibility of improving wheat protein quality through genetic transformation (Altpeter et al., 1996; Anderson et al., 1996; Blechl and Anderson, 1996).

Genetic transformation combines embryo culture with direct insertion (bombardment of gold pellets carrying DNA is an efficient technique) of specific genes to transform the recipient embryos in relation to specific traits. This is achieved in one single generation. Some examples of traits that can be modified include: disease and pest resistance, the nutritional (e.g. essential amino acids) and utilization value (e.g. changes in the functionality of proteins and starch) of crops and the suppression of undesirable gene expression (by introducing antisense genes), such as that promoting the increase of hydrolytic enzymes associated with pre-harvest grain sprouting, or that controlling the synthesis of undesirable molecules such as secalin proteins from rye in 1B/1R translocation wheats (Blechl, 1998).

Wheat has been transformed in relation to HMWG subunit composition and starch composition; however, the level of expression of the inserted genes and their contribution to quality in wheat advanced lines ready for varietal release are still to be determined (Anderson et al., 1996; Blechl and Anderson, 1996; Blechl, 1998).

Molecular marker technology is also expected to increase efficiency and speed up the process of combining specific genes in new breeder's germplasm. Instead of selecting progeny of new crosses by determining gene action (for example, presence of specific HMWG subunits or determination of their quality effect), marker-assisted selection is expected to allow breeders to select directly germplasm carrying the desirable gene(s). Marker-assisted selection has been very useful in selecting plants carrying genes associated with resistance to pests and diseases, but has not proved yet as a better, more efficient alternative to the traditional chemical and biochemical small-scale methodology used in screening for quality breeder's wheat segregating germplasm.

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