Wheat - The Structure & Physical Characteristics
Wheat is actually a grass and belongs to the genus Triticum species aestivum. Wheat is grown globally and is the second most important cereal crop in the world behind maize and ahead of rice. Below is a map showing the major cultivation areas across the globe.
Here in New Zealand wheat is primarily sourced locally or from Australia. Canadian or American grain is used when world pricing or customer requirements dictate.
In New Zealand Canterbury is the major wheat producing district for both autumn and spring wheats. Autumn or spring planting is determined by climatic factors such as rainfall and environmental factors such as disease. Spring wheats generally have better bread baking qualities than autumn wheats, so end-use is also an important consideration.
Wheat plants grow several side shoots called tillers from a crown which is just below ground level. Each of these tillers grows a long stem which bears a flowering head at its top. The length of these tillers varies greatly between different wheats, some being extremely short (less than 20cm) while other are extremely tall (over 1m!). Modern wheats are generally 80cm to 1m tall.
The wheat flowers are self-fertilised by the movement of pollen from the male part of the wheat flower (stamen) to the female part (the stigma). Each flowering head fertilises its own flower. Once this has occurred the grain begins to grow and develop.
Starch and protein are stored in the grain and used as an energy source by the new plant. The grain reaches its maximum size a month after fertilisation – this is usually in mid summer. Once the grain is fully developed the wheat plant begins to die and the grain slowly dries out. It is at this stage that harvesting interrupts the growing cycle of the wheat plant, as once the grain is dry enough, the wheat is harvested. The grain is harvested by a machine (called a combine harvester) which cuts the whole plant and separates out the grain. Grain can be stored in bulk bins if the amount of moisture in the grain is kept low.
The Structure of the wheat grain
Take a look at a wheat grain. It has a crease down one side. That is where the stalk lay by which the grain was attached and fed in the ear of the plant. If it was not that shape, but was round, it would be very easy to mill by grinding off the outer layers of the bran, similar to the process that is used today for the polishing of rice. Wheat grains are generally oval shaped, although different wheats have grains that range from almost spherical to long, narrow and flattened shapes. The grain is usually between 5 and 9mm in length, weighs between 35 and 50mg
There are several layers of bran. They protect the seed, and by being partly waterproof, ensure that the seed will not start to grow again immediately after harvest. At one end of the grain is the germ; that is the young plant that will grow when the grain is put into the soil and given the right conditions of moisture and warmth. The aim of the miller in making white flour is to obtain the greatest possible amount of the whitish interior of the grain, the endosperm (the grains food supply), with the least possible contamination by bran and germ. Bran is not wanted mainly because of its' colour and texture. Germ is not wanted because of its high fat content which reduces the keeping quality.
The grains are commonly a red colour, although many wheats have white grains and more unusually purple, black, brown or green/grey varieties exisit. The wheat grain (or kernal) is divided into several parts, as shown in the diagram.
Two other structures are important to us because of their food value. The aluerone is a thin layer between the bran and the endosperm. The Scutellum is also a thin layer between the germ and the endosperm. Both of these layers are very rich sources of vitamins essential for our health and which may be deficient in our diet. It is common practice abroad when very white flour is milled and these layers lost, to add synthetic vitamins to make up the deficiency.
If we look at the endosperm under a microscope we find it is made of a mass of brick shaped little boxes called cells. Inside each cell are found granules of starch and these are surrounded by a clear glassy protein. It is this protein when wetted that causes the stickiness and structure of dough. The wet protein, called gluten, has four very important properties: it swells in water, to hold about twice its own weight in water, it is sticky, it flows when pulled, yet it is also elastic like rubber. It is this curious combination of properties of the protein that makes wheat flour unique amongst the grains and suitable for bread making.
The structure of starch granules embedded in glassy protein is somewhat like the structure of concrete with shingle pebbles embedded in cement and sand. In the course of milling, pure chunks of endosperm are broken down by the reduction system, the result is much like breaking concrete with a hammer. we get a variety of pieces of different sizes ranging from lumps with somewhat like the structure of the whole concrete down to clumps of just two or three starch granules held together with protein, single starch granules and small bits of protein. These pieces jumbled together make flour, and the pieces cannot be separated by sieving alone because the small ones cling to the large ones. They can, however, be separated by air classification in a process called purification.
The protein content of wheat grains varies considerably, from perhaps 7% to 17%, depending on the climate and weather, the soil and the variety of wheat. Wheat grown in the interior of the main continents such as America and Russia ripen during the hot dry summer that causes a low yield of the crop but a high protein content. Due to the high protein content the wheat has a glassy, vitreous look, and is said to be hard wheat. It is in fact very hard and can be verified by biting it! On the other hand wheat grown in areas with cool and moist summers such as northern Europe, tend to have low protein content and the grain tends to look dull and whitish. They are called starchy, though this word does not mean that they contain more starch, and the wheat is soft, and easily chewed.
When the protein content of the developing grain is low the structure cannot hold together as it dries during ripening and so fine cracks and air spaces appear in the structure. It is these air spaces that make the grain look dull and feel soft. In New Zealand we are between the two extremes. Some grain varieties such as Claire are definitely soft, and others such as Conquest are of the harder type. Others are in between and there may be hard and soft wheat within a crop, or even hard and soft patches within one grain. This last type of grain is called mottled. In general, high protein is requqired for breadmaking and low protein for cakes and soft biscuits, whilst spaghetti and cracker biscuits are best made from very high protein content that is usually found overseas in the Durum type wheats. Durum wheat is not grown in New Zealand. Recent developments in equipment and process technology however, now make it possible to make some of these products from normal hard wheat types.The varieties grown in New Zealand have been developed by breeding. This is done by crossing wheats with desirable qualities and characteristics as parent grains. Thus if we have a variety which yields well but has poor baking qualities, we may cross it with another with lower yields but has good baking quality. Most of the offspring will have poor yield and poor baking quality, but with luck a few will have the best of both characteristics, and these will be used for further breeding programmes.
Bread is made by mixing flour, water, salt and yeast and keeping the dough warm for a few hours so that the yeast may ferment and the dough "rise". This means that the microscopic yeast plants feed on sugar present in the dough and make bubbles of carbon dioxide gas. The protein swells with water when the dough is made and because it is sticky the whole dough mass becomes a continuous protein structure with starch granules lying in it. Later when the dough has risen it is baked. In the oven the dough gets hot and the starch granules start to swell and take up water that they steal from the protein. The granules swell until they touch and stick together making another continuous structure. Bread is unique in having these two structures of protein and starch; most baked goods have only a starch structure (sponge cake) or only a protein structure (cracker biscuits)To allow the dough to rise well, the protein gluten must be able to stretch in thin films that surround the gas bubbles. If the protein is too weak the bubbles will swell too much and burst, while if it is too strong the bubbles either cannot swell, or will burst while still small. Thus the quality of the protein matters as well as its quantity. The difference in quality can be seen in variation of stretchiness and toughness of glutens washed out of doughs.
Yeast works by means of enzymes that enable it to digest its foodstuffs. Some of the enzymes it requires are naturally present in the wheat and flour in just the right quantities. In particular it must have a continuing supply of sugar to ferment. Some of this sugar is present naturally in the flour and some the baker adds at the dough making stage. But for a lasting supply the yeast relies on the continuous breakdown of starch by flour enzymes to give sugars. The enzymes cannot easily attack whole starch granules but can attack those granules that have been mechanically damaged durring the milling process. So the amount of starch damage done in milling is important. It is important also because damaged starch granules absorb water when dough is made, whereas intact granules do not. And the amount of water required for a dough is important to a baker; too little and he is selling more flour and less water in the bread, too much and the dough gives trouble later in the baking process.
Starch granules are damaged in milling when the large chunks of endosperm are squeezed between the reduction rolls. If the protein structure is soft and can break down easily, then a little damage is done to the granules, but in a high protein hard wheat the granules cannot easily escape being squeezed and damaged. So in soft wheats it is sometimes difficult to produce enough damaged starch, whilst in hard wheats it may be difficult to keep the amount of damage low enough.
Another starch digesting enzyme is present in wheat that has sprouted because of wet warm weather at harvest time. This enzyme is called alpha amylase, and can attack undamaged starch granules and rapidly break them down. It also severely attacks the starch network of the bread as it is forming in the oven, giving it a sticky bread crumb that gums up the blades of the slicing machine. More severe attack prevents the bread structure from forming at all. The amount of alpha amylase formed during sprouting is not related to the size of the sprouts visible on the grain and conversely it is possible to have sprouts upto an inch long with scarcely any detriment to baking quality. A very small amount of alpha amylase is desirable, to give a moist crumb and improve the keeping quality of the bread, and in countries where sprouting is unknown it is usual to add small amounts of malt flour to provide this enzyme. Some varieties of wheat sprout more easily than others, and red grained wheats are more resistant to sprouting than are white ones.
Wheat protein is sensitive to heat. Wheat may become heated when damp wheat is kept in an unventilated stack and moulds and insects grow in it. Alternatively it may be overheated in a mechanical drier. Any way that the damage is done, the result is the same; the wheat and flour is unsuitable for breadmaking because the gluten has lost its elastic properties that enable the dough to rise.