NSW Agriculture, Agricultural Research Institute. Wagga Wagga. NSW
Wheat has a diversity of food uses. In many respects it is similar to other cereal grains. However, wheat differs considerably from other cereals because of the unique properties of the flour it produces. Indeed, the flour milling, baking, pasta, and many other food processing industries have evolved to exploit these properties.
Wheat can be incorporated as a whole grain or kibble into many products, or it can be steamed and rolled to prepare breakfast foods, or soaked and parboiled to make burghul, which is popular in Lebanon. For such purposes wheat has no real advantage over other cereals except, perhaps, the nutritional aspect that it produces slightly higher protein content. Processing is minimal, and the principal quality requirement is for plump grain.
As flour, wheat has several advantages. By exploiting different aspects of wheat's protein, starch, nutritional and physical characteristics, many different products can be made. Flour millers blend wheats from different areas to take advantage of the subtle differences that exist between wheat varieties and the modification in those differences due to seasonal conditions. This genotype * environment interaction is often more substantial than the differences bred into varieties and requires constant monitoring by millers to ensure flour of consistent quality is supplied to the food industry. Thus the processing quality of wheat is determined by the characteristics of the wheat variety selected as modified by the genotype * environment interaction.
Grain size is important, both as an indicator that extreme seasonal conditions or disease have not seriously affected the grain, and to ensure an economical flour extraction, but more important is grain hardness. This attribute is only slightly affected by the seasonal conditions. It determines how the grain fractures during milling, and consequently how the starch granules within the grain are damaged as they are reduced in size to form flour. Damaged starch granules can imbibe water quicker than undamaged granules, and are more easily degraded by appropriate enzymes. Because most products are prepared by mixing flour with water to create dough, damaged starch can have a major influence on the water requirement for a product. This in turn can influence how the starch and protein characteristics are expressed.
Wheat flour contains between 8 and 16% protein. It is the proteins of wheat that make its flour unique. Some of the proteins when mixed with water create gluten. Gluten is responsible for creating that cohesive network within a dough that retains gas and enables the production of light baked products, such as bread. Only wheat flour and, to a lesser extent, rye and triticale flours, have the ability to form gluten. For most products the specification of flour quality largely refers to the specification of gluten characteristics. Protein content is a good first approximation of gluten character, but is considerably influenced by the environmental interaction. Consequently not only is wheat traded on a protein basis but an industry has developed to extract gluten from flour for use as a supplement within the baking industry.
About 75% of wheat flour is starch. At room temperature wheat flour starch is capable of absorbing about 30% of its weight in water and so has a major role as a sink for water in dough formation. The amount of damaged starch generated during milling can have a considerable effect on this water absorbing ability. However, from a food processing aspect, it is the pasting properties of starch that are most important. As starch is heated in the presence of excess water, the amount of water it can absorb increases causing considerable swelling and a concomitant large increase in viscosity. Continued heating causes the starch granules to distort and rupture, releasing soluble starch and further increasing viscosity. This solubilisation of starch is continuous and in excess water is not complete until a temperature of more than 120oC. Upon cooling, the starch forms a gel that is influential in determining the product's shape. There is also a redistribution of water as the cooling starch releases much of that taken up during pasting. This process, known as retrogradation, manifests as staling.
In most products, water is not present in excess, and the temperature does not exceed 100o or the water present would boil. Therefore the extent of solubilisation depends on a product's formulation and processing conditions. Conversely the requirements of a wheat-based product need to be defined in terms of the pasting, gelling and staling characteristics of the starch component.
Nutritional characteristics are also a major consideration in food processing. Like other cereals, wheat protein, fibre, minerals, and vitamins to processed foods. Contrary to popular belief, wheat-based foods, particularly bread, are less fattening than meat. An equal portion of white bread contains approximately half the protein of T-bone steak, minimal fat (2% compared with 30-35%) and about 48% carbohydrate, much of which is complex carbohydrate. Consequently the calorific value is about 60% that of meat.
The protein composition differs. Like most cereals, wheat proteins are low in lysine. On the other hand, animal and legume cereals are low in methionine and cysteine, so a truly balanced diet requires cereal-based foods to be eaten in conjunction with meat or legume-based foods. However, given the cost of cereal-based foods compared with meat, cereal protein is approximately 30% the cost of meat protein.
Another popular misconception is that white flour is over-refined. While wholemeal flours do have fibre advantages, white flour is still an excellent source of minerals (calcium, iron, phosphorus, potassium, magnesium) and vitamins, particularly thiamine (vitamin B1). Wheat niacin (vitamin B6) is present but largely unavailable. However, wheat proteins are a good source of tryptophan which the body can easily convert into niacin. Wheat germ contains vitamin E, of which significant quantities remain in white flour after milling.
1. Development of new varieties
The Cereal Chemistry section at the Agricultural Research Institute is currently conducting two major research programs impacting on the processing of wheat. The main research program is the development of new varieties with quality attributes suitable for appropriate markets. In the Riverina the principal wheat grades into which wheat is received are the Australian Hard (AH), Australian Standard White (ASW), and Prime Soft (PS) grades. The AH grade consists of selected hard grained wheats at greater than 11.5% protein content. Only varieties with dough strength characteristics well suited for bread baking are segregated into the AH grade. The PS grade contains selected soft grained wheats at less than 9.5% protein content which are well suited to biscuit making purposes. The ASW grade is both the base grade upon which most processing relies, and also the residue grade into which is received varieties that fail to meet the requirements of other grades. Consequently it contains a motley of varieties and is expected to fulfil many tasks.
Until recently, the domestic baking industry was the most quality conscious market for Australian wheat, and so the quality assessment program developed at the ARI has been largely based on grain hardness and the bread baking test. Hard wheats that bake well are targeted for the AH and ASW grades. Hard wheats that bake poorly are rejected. Soft wheats that bake well are targeted for the ASW grade, and soft wheats that bake poorly are targeted to the PS grade.
Over several years, this assessment system has worked reasonably well, particularly under regulated domestic marketing arrangements, but principally because of the requirement that ASW varieties needed to have good baking quality. However it suffers two major criticisms in today's world of market oriented production.
(i) The protein potential from most of the Riverina is below that desirable for bread baking. Very little wheat from the Riverina is sold for pan bread baking. Most of it goes to indiscriminate markets such as Egypt for their various bread types which are processed quite differently to our bread. The low prices for agricultural commodities, and the inability to recover the cost of necessary inputs to improve the protein content within the region, means that the protein potential from the Riverina is likely to remain low. This is exacerbated by the nature of grain receival that results in the ASW being a motley of varieties, both hard and soft, varying in quality depending on the mix. Thus it is unlikely that wheat from the Riverina will again form the basis of pan bread production. However, the region's protein content is suited to many other products, such as Chinese steam breads and noodles. Furthermore, research conducted principally at the Bread Research Institute suggests that the hard, strong wheats desired for the AH grade are better for Middle East flat breads when grown at lower protein levels, similar to those obtained in the Riverina.
(ii) Wheats of intermediate hardness are severely discriminated against by the current assessment procedures. This was deliberate. Harder wheats bake better bread and softer wheats bake poorer bread and make better bisuits. Wheats of intermediate hardness were considered too hard for the soft grades and too soft for the hard grades and were therefore rejected. However, evidence is being obtained that these varieties of intermediate hardness might be better suited to many Asian products.
As a consequence, at the ARI we are developing methods for selecting and evaluating new cultivars in terms of these other products for which wheat from the Riverina may well be used. Most of these products are quite unrelated to bread baking as we understand it, both in formulation and processing. Therefore, to describe quality in term of bread baking specifications would be quite erroneous. Our strategy is twofold. Firstly, to adapt the findings of the fundamental research on such products conducted by other research institutes to our selection program and, secondly, to complement and supplement such research with our own research. Currently we are investigating various means of measuring starch pasting properties more efficiently and effectively than the current accepted methods allow, so that ultimately we can better evaluate the contribution of and the interaction between gluten and starch during the processing of some of these products.
The outcome of this approach may well mean the development of varieties that are more product specific than suitable for general application as the ASW grade is now, and could require marketing authorities to establish more varietal segregation for specific markets. Some progress is being made in this respect in terms of an ASW-Noodle grade within the Riverina.
2. Dough Rheology
During the mid-1980s considerable pressure was put on breeding programs to increase the dough strength of candidate varieties for the Australian Hard and Prime Hard wheat grades to satisfy some of their overseas markets. This was successful and varieties such as Hartog were released. However, concomitant with this increased strength was an increased mixing requirement that has created problems within the domestic baking industry.
Then the wheat variety, Dollarbird, was identified, and released as having short mixing time and good dough strength. As such, it appeared aptly suited to satisfy both market requirements. An investigation of the characteristics of Dollarbird was commenced, particularly with respect to its mixing requirement-dough strength interaction, to identify what was different about Dollarbird and how it could be exploited by breeding programs.
With assistance from the Wheat Research Council a variable speed torque rheometer was purchased that has enabled the measurement of work input during the mixing of doughs. Coupled with a compressive stress test, to provide an alternative means of measuring dough strength to the accepted standard procedure, and the interfacing of test bakery mixers to computers, this work revealed that the mixing requirement of Dollarbird is similar to that of other strong hard wheats. This work also confirmed that the accepted laboratory methodology and instrumentaton is inadequate in terms of predicting commercial behaviour. Many of the tests used by cereal chemists were developed 50 years ago. The empirical nature of these tests has resulted in little change from then to now. However, baking processes and technology have changed, and laboratory instrumentation, particularly in the realm of data acquisition, has also changed. Cereal chemists need to reappraise many of their techniques. At the ARI we are pursuing this by continuing our research into dough rheology in a project designed to improve the measurement of dough strength and relate it to processing inputs.
In terms of product evaluation, much cereal chemistry thinking, instrumentation, and interpretation is in terms of the bread model. Bread is a yeast leavened product made with approximately 63% water addition to flour to create a viscoelastic dough. Most of Australia's wheat does not end up in such a product. Noodles are not leavened. The dough is not mixed like bread but rather is compressed into a sheet with only a 35% water addition to flour. Viscoelasticity as it is desired in bread does not exist. Some noodles have an alkaline lye added to develop a bright yellow colour. Under these conditions gluten as we understand it in the bread model is unlikely to develop.
Many biscuit doughs are also alkaline. Mixing is less intense than for bread and sheeting is an important processing step. Like noodles, plasticity is desired rather than viscoelasticity. Indeed a major requirement is control over dough movement so that the product can fit into the preformed packages. There is a similar low water addition and measurements reveal that less than 20% of the starch is gelatinised. Yet specifications for biscuits are usually written in terms of parameters measured on instruments developed for measuring the gluten and starch characteristics according to the bread model.
Middle eastern flat breads and Chinese steam breads also have much lower water addition than pan bread and undergo completely different production processes. Cakes, crumpets and wafers are batter products with water additions much higher than that of bread. Gluten development is minimised and product setting is more dependent on starch characteristics.
Therefore, I see three future directions:
1. more appropriate models are required for other products;
2. methods appropriate to those models are also required for the selection of new varieties and for quality control;
3. reappraisal of many of the existing methods used in wheat cereal chemistry.