Cereal Chemist, Wagga Agricultural Research Institute, NSW Agriculture and Fisheries. Wagga Wagga
The word “Protein” is derived from the Greek word “proteios” meaning “primary” or “holding first place” and is well chosen, because living organisms depend essentially on four classes of chemicals: proteins, nucleic acids, carbohydrates and lipids.
Typically the wheat grain will contain somewhere between 8-16% protein which can be grouped into two principal types: those that are actively involved in the metabolic life of the grain (the enzymes, hormones, etc) and those stored by the grain to provide a source of nitrogen upon germination.
It is this latter group, the endosperm storage proteins, that make wheat protein so unique in the plant kingdom. They comprise 80-90% of the total grain protein and therefore obviously dominate protein effect.
Protein - Hardness Interaction
Figure 1 shows the approximate range of grain protein required for satisfactory production of various products. Also shown is the range of grain hardness required for these products. Grain hardness is an artifice of protein created by the way the protein matrix surrounds the starch granules in the endosperm and it influences the way the grain fractures during milling giving rise to an effect known as Damaged Starch. The milling of hard wheats creates more damaged starch than in the milling of soft wheats. Damaged starch absorbs more water than undamaged starch. Thus to form a dough, hard wheat flours will absorb even more water than soft wheat flours of the same protein content.
This is of economic advantage in bread making because the baker can make more loaves per tonne of flour. But it is detrimental in cake production because the extra water causes cake collapse, and also in biscuits, which are a very low moisture product and any extra water has to be baked out at an energy cost. Thus hard wheats are preferred for bread-type products, and soft wheats for cakes, biscuits, etc.
Figure 1. The relationship between end product and wheat grain characteristics (protein and hardness). (Adapted from H.J. Moss, Bread Research Institute of Australia).
Functional Properties Of Protein
Note in Figure 1 that there are some products that require high protein soft wheats but there are no products that require low protein hard wheats. This is because various products exploit the functional properties of the wheat proteins in different ways. What is meant by “functional properties”?
The endosperm storage proteins can be broadly classified into two types:
gliadins and glutenins.
To explain their behaviour relevant to their functional characteristics it is important to understand that essentially there are two extremes of matter. An elastic material is one which returns to its original shape when a deforming force is removed. Viscous material keeps deforming after such a force is removed. When glutenin proteins are mixed with water they form a viscous liquid. When glutenin proteins are mixed with water they form an elastic solid. So when flour is hydrated and mixed it is going to have a viscous component derived from the gliadin proteins and an elastic component derived fromtheglutenins. Indeed the resultant protein complex is a viscoelastic material called gluten. Wheat flour and to a lesser extent rye flour are unique in that they have proteins that form gluten upon hydration and mixing, and it is the viscoelastic properties of wheat gluten that make wheat flour so important in the baking industry.
There are three important processes in bread production:
(i) Mixing: this hydrates the ingredients, develops the gluten and incorporates air into the dough. The mixers are designed to push, pull, squeeze and knead the dough to achieve these functions.
(ii) Prooving: after the intense work of the mixing stage the dough needs to recover somewhat. It recovers through a process called Structural Relaxation to a point that represents the permanent structural modification of the proteins due to mixing (= Dough Strength). Dough strength is the functional expression of gluten and depends on how much of the gliadin and glutenin proteins are present to form gluten, and on the rate and amount of work input during mixing. Also during prooving, yeast is starting to generate carbon dioxide gas which fills up the air cells that were incorporated during mixing. (Gas cells cannot spontaneously create in the dough -this is why kneading is so important during mixing). The gluten must be viscous enough to expand as the gas fills the cells but elastic enough to retain the gas. At this stage the viscoelastic balance is critical.
(iii) Baking: in the first few minutes of baking there is a lot of enzymic activity before the enzymes are inactivated by the heat, and thus an extra burst of gas production. The heat also causes gas expansion, so the loaf enters a phase known as “oven spring” and increases its volume considerably. Again at this stage, dough strength is important. The gluten protein must expand yet retain its shape until the starch gelatinises and sets the loaf.
Bread baking exploits the viscoelastic nature of gluten. If the gluten is too viscous the gas will escape giving a small loaf with an uneven and hole-riddled crust and an open coarse texture. Too much elasticity and the loaf will be squat and dumpy with a tight texture because no expansion has been possible. There has to be a balance between viscosity and elasticity. This balance is generally achieved at grain protein contents of 11.5 - 14%.
Figure 2. The relationship between loaf volumes and flour protein for several varieties from the crossbreeding programmes at Wagga WAgga.
Figure 2 shows the relationship between loaf volume and flour protein for several varieties from the crossbreeding programmes at Wagga. There is a spectrum of responses. Note that they tend to coalesce at somewhere around 10% protein. At this protein level suitable dough strength is barely developed and the discrimination between the crossbreds is minimal. We take this to be the minimum protein at which the visco-elastic properties of gluten can be exploited for bread products.
These are similar to bread. The basic difference is that muffins are roasted, whereas bread is baked. Usually a very high protein flour (13%) is used to carry a high water level and to impart the characteristic chewy taste. The texture is produced by a combination of the gases produced by yeast and the steam generated bywater vaporisation on the griddle. It is the steam that produces the characteristic blow-holes.
These are essentially unleavened breads and include tortillas, and pita breads. Although yeast is included in the formula it does not generate appreciable levels of gas to contribute to the final product texture, but is primarily there to help modify the gluten. As such the incorporation of air during mixing is not so important and indeed the gluten is developed as much by passing the dough through sheeting rollers as in the mixer. This gives a flat oval doughopiece which is rested for a short time before baking for 1 1/2 minutes at 350 C.
At that temperature the water vaporises and blows the dough piece up like a football creating the characteristic internal pocket. Starch gelatinisation has to be rapid and gluten strength sufficient to hold the mass together during this rather violent process. Thus the gluten must be developed sufficiently to withstand these forces and impart a degree of flexibility to the product: it should be neither brittle nor tough and rubbery.
To achieve these effects the doughs are undermixed at a relatively low work input so that compared with conventional bread more of the elastic-component than the viscous-component of gluten is developed. Hence the sheeting rolls play a very important function, and very high protein wheats are preferred, although this does not seem to be essential.
Mixing is the most important step in cake manufacture. The intention is to develop a foam by the incorporation of an enormous amount of air. However, unless that foam is stable it will not be able to withstand the stresses of baking. It is the protein of flour that contributes structural stability to the foam until the starch gelatinises and creates the final textural stability. Too little protein will give a slack batter that rises nicely in the oven but then collapses soon afterwards. Too much protein gives a stiff batter with thick inner cells that affect the taste. A good sponge or angel cake should literallymelt in the mouth as the salivaryamylase dissolves the starch. Too much protein thickens the cell walls and makes the cake chewy. Thus flour protein contents from 6-9% are preferred.
There are three important processing considerations in biscuit production that are influenced by flour protein.
(i) The final moisture content of biscuits is around 4%. One of the biggest expenses in biscuit production is the energy cost in baking out the moisture required to form the dough. Thus soft wheat flours at low protein content are preferred to minimise the water requirement of the dough.
(ii) The biscuit has to fit the packet. Once shaped, the baker wants that shape retained. Too much elasticity in the gluten and the dough will spring back to give smaller diameter but thicker biscuits. This upsets consumers because they buy fewer biscuits per packet. Too little elasticity and the dough may flow after shaping so that large diameter, thin biscuits are produced. If they flow too far they will not fit in the packet. But if by chance they do, the manufacturer is upset because he must put more biscuits in to fill the packet. So the balance of elasticity and viscosity of the gluten is critital. By using low protein flours and undermixing at low work input levels, what elasticity is present can be exploited. The addition of shortening to the recipe supplements the viscous-component of the flour to give the balance required to achieve good products.
(iii) Taste: Upon biting the biscuit should snap. For this a flour protein of 7.8 - 8.2% seems to be critical. Much less than this and the biscuit becomes brittle. Much more and it is chewy. Sometimes this is exploited to produce chewy biscuits.
In biscuit production these effects due to protein are crucial and only certain wheats with the desirable dough rheological properties and at the correct protein level can be used.
In Australia, bread is undervalued nutritionally. Common misconceptions are that bread is fattening and that white bread is over-refined and thus nutritively poor. Figure 3(a) compares the composition for 100 g portions of T-bone steak and white bread. White bread is low in fat, has no cholesterol, is rich in complex carbohydrate, has half the protein content of meat, and a lower energy contribution. Figure 3 (b) shows the same values calculated toan equal energy contribution (i.e. equal “fattening” amounts). Bread is able to provide more satisfying bulk (by virtue of the complex carbohydrates) and almost as much protein. As a protein source, bread is cheap: at $1.20/680 g loaf, bread will provide 45 g protein per dollar whereas T-bone at $8/kg only provides 18 g protein per dollar. However, bread protein is deficient in the amino acid lysine and as such cannot be used as a total meat protein replacement. Nonetheless it has a valuable contribution to make to the diet because of its proteins, its complex carbohydrates and also because it is a valuable source of many vitamins (B-group particularly) and minerals.
Figure 3. Comparative nutritional values of white bread and T-bone steak
(a) 100 g portions
(b) equal energy portions (bread 160 g, T-bone 100 g)
It takes 5 units of vegetable protein to create one unit of animal protein. Or to put it another way, 4 ha of land will produce enough beef to feed one person for a year but that same 4 ha can produce enough wheat to feed 15 people for a year.
It is time the wheat industry appreciated that in bread they have a product that ideally fits the guidelines now being proposed as part of Australia’s food policy, viz, eat less fat, less cholesterol, more complex carbohydrates. As such, bread should be marketed more aggressively as a cheap, nutritive protein source, and it is time many of the myths that surround bread were exploded. However, it is also time the baking industry produced bread that consumers want to eat.
A large portion of the wheat industry relies on the utilisation of the unique properties of the wheat endosperm proteins to create baked goods. Different products exploit these properties in different ways but each requires particular protein characteristics and has an optimum protein range to produce the optimum product. The value of bread protein as a cheap nutritive protein source has yet to be fully marketed.
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