When bakers begin discussing flour quality, the conversation quickly turns to gluten. During mixing of wheat flour and water, gluten forms. Gluten becomes the elastic framework of dough; it entraps the gas produced by fermentation and results in leavened dough.
Like other proteins, gluten becomes hard and dry on exposure to heat during baking (denaturation). Gluten denaturation, combined with starch gelatinization, contributes to the structure of most baked products.
Not all protein in flour is gluten-forming. The percentage of protein listed in the chemical composition of flour includes all proteins, irrespective of whether they are gluten-forming or non-gluten forming. Whole wheat flours contain more protein than refined flours, such as straight grade or patent flours. However, whole wheat flours produce a loaf of bread with smaller loaf volume. Bran and germ are high in protein of the non-gluten forming type. Although these increase the total quantity of protein in whole wheat flours, they decrease the proportion of gluten-forming proteins.
Gluten quality is generally assessed two ways:
- Based on sedimentation
- Based on the isolation of gluten by washing a flour/water mixture
Some mills and bakeries around the world rely on the percent of wet gluten and gluten index as important flour specifications.
Gluten washing is simply the process of using excess water to mechanically remove the starch and most other non-gluten components of the wheat flour. This can be done either by hand or with the aid of instrumentation. The end of washing time is determined when all visible signs of starch have been removed. The gluten is weighed, and the percent of wet gluten is calculated based on the starting flour weight. The gluten sample can then be dried, either in an oven or on a special hot press, and the dry gluten weight and percentage are recorded.
If gluten is washed instrumentally, a salt solution is generally used instead of water. After weighing, the wet gluten can then be centrifuged over a screen or sieve. The percentage of wet gluten remaining on top of the sieve, based on total wet gluten, is called the Gluten Index. If the gluten is very weak, most of the gluten will pass through the screen. If the gluten is very strong, most of the gluten will stay on top of the screen. The stronger the gluten, the closer the calculated Gluten Index value is to 100.
We know, from chemical analysis, that not all of the remaining gluten is protein. The average analysis of wet gluten is 67% water and 33% solids. If the dry crude gluten is analyzed, it will be found to contain an average of 75% protein. The other 25% is made up of crude fiber, ash, starch, fat, and other minor constituents. In hard wheat flours, about 15 to 20% of the total proteins have been determined to be non-gluten forming.
Gluten washing is mainly practiced for two purposes. First, the weight of a well-washed dry gluten provides some indication of the protein content of the flour. Secondly, a person who has had some experience in washing gluten is able to obtain an indication of flour strength and gluten quality from the physical and elastic properties of the washed wet gluten. They would be looking for such things as dispersibility or agglomeration during washing (does it want to come apart or does the gluten stay together?). Stretching/releasing the clean wet gluten also gives clues to its strength.
When weaker flours or flours from soft wheat sources are mixed into dough, and washed with water, it proves very difficult to recover any appreciable quantity of wet crude gluten. The dilution of the gluten by starch is so great that the gluten particles are not afforded an opportunity to aggregate into a cohesive mass during the washing process.