11 Gluten

GLUTEN IN WHEAT FLOUR Introduction Wheat grains have from 8% to 14 % protein content. The amount and type of protein in flour affects the final product. The wheat proteins responsible for developing bread’s characteristic structure called gliadin and glutenin. These proteins get hydrated during the kneading of a dough forming the gluten which gives the dough its plastic properties. Gluten is a strong elastic substance which forms a network throughout the dough. The network traps carbon dioxide, produced by the added yeast and allows the dough to rise. The process of kneading dough helps develop the gluten network. Fat and sugar can inhibit gluten development. In products such as cakes, biscuits and pastry, flour with a lower protein content is mixed with fat to produce a crumbly and light texture. The quality of regular flour depends on the amount and quality of the gluten in it, so its determination is an important part of the quality control in bakeries. The minimum gluten content for a wheat flour should be about 24% (wet) and 8% (dry). The method of assessment consists in kneading a known mass of wheat flour with water, letting it stand to complete protein hydration and washing away the starch (starch is the main constituent of flour). The remaining elastic substance is hydrated gluten that is subsequently weighed, dried and weighed once again Gluten-free flour is most commonly used in products specifically manufactured for people with gluten allergy or coeliac disease. This used to be considered a rare condition, but new research suggests that 1 in 100 may be affected! Gluten free flour is made from wheat flour which has had most of the proteins, removed. Gluten free products are identified with the symbol you see on the right side. Aims To determine the gluten content of wheat flour Lab ware Beakers, balance, glass rod, watch glass, test tubes, teat pipettes Procedure 1- Weigh carefully 50,0 g of flour and put in a mortar or a dish. 2- Add 10 ml of water and mix with a rod. See that a dough is formed that will not stick to the walls of beaker or mortar. Eventually add 1-2 ml of water and keep on kneading. 3- Once the dough is formed, knead a further five minutes. You can use your fingers now but not before. 4- Let the dough rest for 10 minutes at least in the container. Cover it with a film or a sheet of paper so that it won’t get dry.

5- Take a small part of the dough with your fingers and kneading it continuously under a thin stream of tap water, rinse the starch away. A better procedure would be to rinse the dough with salt solution but for most purposes this method suffices. 6- When starch has been completely eliminated (you may test in a sample as indicated below) put the small chewing-gum-looking ball of wet gluten on a tared watch glass. 7- Once that all of the dough has been washed, the gluten balls in the watch glass are weighed and afterwards dried in an oven at 80º. We will leave it close to a lighted Bunsen burner below a wire gauze on a tripod. 8- Weigh the dry gluten and calculate: a- Percentage of wet gluten in flour b- Percentage of dry gluten in flour Test for starch Put some 5 ml of washing liquor in a test tube and gently heat it to the boil. Add some drops of Lugol (iodine dissolved in aqueous potassium iodide solution). If starch is present a deep bluish violet colour develops because of a complex formed between starch and iodine. It is a very sensitive test so a faint colour means almost no starch. The figure shows a negative test for starch (1) and a positive one (2) .