How to determine the gluten content of flour?
The method for determining gluten content in grain and oilseed inspection (GB/T 5506-2008) is applicable for determining the gluten content of wheat and its flour. The determination methods specified in this standard include the determination of wet gluten and dry gluten.
1. Instruments and tools
-Balance: accuracy of 0.01 grams;
-Small enamel bowl;
-Measuring cylinder: 10 milliliters or 20 milliliters;
-Beaker: 100 milliliters;
-Glass rod or horn hook;
-Basin or large glass jar;
-A circular hole sieve with a diameter of 1.0 mm or a sieve equipped with CQ20 mesh cloth;
-Glass panel: 9.16 cm, two pieces, thickness 3-5 mm, with 0.4 mm thick white adhesive tape attached to the edges;
-Centrifugal device: Modified from a small electric centrifuge in the laboratory, it is equipped with a circular aluminum box with a diameter of about 11 centimeters and a height of about 6 centimeters in the middle. Two sharp objects are symmetrically fixed on both sides of the aluminum box, and a sieve plate with a diameter of 1.0 millimeter is placed on top to ensure the rotational balance of the centrifugal device;
-Surface dish and filter paper;
-Electric oven;
-Saltwater washing device: 5-liter ground mouth bottle with bottom opening.
2. Reagents
-Iodine potassium iodide solution: Weigh 0.1 grams of iodine and 1.0 gram of potassium iodide, dissolve them in water, and then add water to 250 milliliters. Used to detect whether starch has been washed thoroughly;
-2% saline solution: Weigh 100 grams of refined salt and dissolve it in 5000 milliliters of water. Filter to remove impurities.
3. Wet gluten determination
-Water washing method:
-Sample weighing: 10.00 grams for special first-class powder, 15.00 grams for special second-class powder, 20.00 grams for standard powder, and 25.00 grams for ordinary powder;
-Wamian: Place the sample in a clean enamel bowl, add room temperature water (20-25 ℃) equivalent to half the weight of the sample, stir with a glass rod, and manually knead into a dough until it does not stick to the bowl or hands;
-Let it stand: Place the dough in a beaker filled with water and let it stand at room temperature for 20 minutes;
-Washing: Gently knead the dough in water to wash away impurities such as starch and bran. Pay attention to changing the water multiple times until the water squeezed out of the gluten no longer appears blue;
-Drainage: Place the cleaned gluten on a glass plate and squeeze it with another glass plate to remove any free water from the gluten. After each press, remove and dry the glass plate, repeating until the gluten sticks to your hands or the plate (about 15 times);
-Weighing: Place the gluten on a dried and weighed surface dish or filter paper, and weigh the total weight;
-Calculation: Calculate the wet gluten content according to the formula.
-Saltwater washing method:
-Weighing and kneading: Weigh 10.00 grams of wheat flour into a small enamel bowl, add 5.5 milliliters of 2% saline water, stir the flour with a glass rod or cow horn hook, and knead it by hand to form a smooth dough;
-Washing: Place the dough in the palm of your hand, turn on the saltwater washing device to slowly drip the saltwater onto the dough, while squeezing the dough with the other hand until gluten clumps form (about 5 minutes), turn off the saltwater, continue to rinse and knead with tap water until the bran and starch are washed away;
-Inspection: Place the gluten in a bowl, add 5 milliliters of water, knead several times by hand, remove the gluten, add 3-5 drops of iodine solution, mix well, and let it stand for 1 minute. If the solution does not turn blue, rinse thoroughly; Otherwise, continue washing with tap water;
-Drainage, weighing, and calculation: The steps are the same as the water washing method.
4. Dry gluten determination
-Operation: Spread wet gluten on a surface dish or filter paper into thin sheets, bake in a 105 ℃ electric oven for about 2 hours, take out and cool, weigh, and then bake for another 30 minutes, cool and weigh until the weight difference between the two times does not exceed 0.01 grams. The total weight of dry gluten and surface dish (or filter paper) is obtained;
-Calculation: Calculate the dry gluten content according to the formula.
5. Calculation of gluten water holding capacity
-Calculate the water holding capacity of gluten according to the formula.
Ensure that the error between the two test results does not exceed 0.2%, take the average as the final measurement result, and keep the result to one decimal place.