Gluten is a special type of protein formed by the bonding of two other proteins, gliadin and glutenin, and is commonly found in rye, wheat and barley. As a result it is found in most types of cereals and in many types of bread although not all grains contain gluten. Some grains that do not contain gluten include wild rice, corn, quinoa and oats.
Gluten can be removed from wheat flour but it is almost impossible to remove ALL of the gluten. Even food labelled as gluten-free contains trace amounts of gluten.
Wheat flour is the most common type of flour used in bread baking. A grain of wheat is actually a seed consisting of 3 parts: bran, germ and endosperm. The bran is the tough outer skin, the germ is the embryonic wheat plant and the endosperm is used as a food source by the germ in early development. The bran is a rich source of protein, the germ a good source of vitamins, and the endosperm a great source of carbohydrates, plus some protein, minerals and oil.
All of these components have an effect on the bread making process, so understanding what your kneading is doing will almost certainly make you a better baker. So, the carbohydrates are used as fuel by the yeast, the proteins bond to form the all important gluten strands, and the minerals are used to strengthen the gluten strands. The oil helps to maintain moisture thus keeping your bread softer for longer.
When water is added to flour gluten is formed. Gluten helps make your bread elastic, providing that nice, chewy texture it has when eaten. For this reason, flour that has had most of its gluten removed produces a sticky dough that feels a bit like chewing gum.
As gluten is an elastic protein it can be really stretched to form long strands. The more it is worked, the longer and stretchier it becomes. These strands form a complex mesh which helps to trap carbon dioxide bubbles produced by yeast during fermentation, thus creating gas bubbles inside your dough. This is the process you are encouraging when kneading your dough, so as you can hopefully appreciate, well kneaded dough is a prerequisite for a well-formed loaf. Gluten also firms up when cooked and helps ensure the bread maintains its shape. The downside of this is that gluten is believed to be partly responsible for causing bread to become stale (along with starch granules). Gluten is also able to absorb liquid, which is why bread is capable of soaking up broths and soups.
Wheat flours tend to be graded by how much high quality protein (and hence gluten) is present in the flour:
Strong flour (or bread flour) has a high proportion of high quality protein which will yield a high percentage of gluten (around 15%). This is the flour needed for typical bread made with yeast. Yeasted bread is commonly known as leavened bread. Bread flour may be white, brown or wholemeal.
Plain or soft flour has poorer quality proteins and produce a lower percentage of gluten (7-9%). As the gluten is formed from poorer quality proteins it is less elastic and tends to snap. This means that the strands can’t form a mesh to trap the gas bubbles resulting in a crumbly texture. However, this property makes this flour perfect for making cakes and pastries.
Gluten is also important in coeliac disease. This is an autoimmune condition which inhibits the digestion of gluten.