While strolling through the local grocery store looking for ingredients to make bread, I found myself in the baking aisle staring at all the different types of flour. There was white flour, bleached white flour, flour labeled all-purpose flour, and specifically for bread making, but what are the differences among these choices?
We must first understand what flour is before we can distinguish between these types. Flour is traditionally made from wheat grains (which contain the protein gluten) which can generally be broken down into two major categories: hard and soft. Hard wheats, which make up 75% of the American wheat crop, contain stronger gluten proteins, while soft wheats, which make up 20% of the American wheat crop, contain weaker gluten proteins.
Flour is made by milling, or breaking down, wheat kernels into small particles and then sifting those particles to make a flour of a certain desired quality, whether that be a fine or coarse powder. Most flours are refined, which means they are sieved to remove the germ and bran layers from the endosperm. These three parts (germ, endosperm, bran) make up a grain of wheat as seen below:
The degree to which the flour has been refined is determined by the extraction rate, or the percentage of whole grain remaining in the flour (including germ and bran). Whole wheat flour has an extraction rate of 90% while most commercial white flours contain 70-72% of the whole grain. Bakers can create higher extraction refined flours by supplementing commercial white flour with a portion of whole wheat flour.
Protein content, both amount and type, in flour plays a major role in its use. Traditionally, flour used for bread is roughly 13-16.5% protein by weight while all-purpose flours are generally 10-13.5% protein by weight. As you may have guessed, gluten is the major protein in wheat and when wetted, reacts with the water and fellow gluten molecules to produce the sticky texture of dough we always get on our hands. These interactions between fellow gluten molecules and water can be either strong or weak. A lower amount of stronger gluten proteins present will create a denser and flavorful product, as some breads are. The opposite could also be said; a higher amount of strong gluten proteins will give a lighter, higher, and chewier loaf of bread. Pastry and cake flours have low levels of weak gluten protein to make baked goods.
That brings us back to the title of this article, why are some flours bleached? As flour ages for a few weeks in contact with the air, its gluten and baking properties improve. Oxygen helps to create longer gluten chains that create a more elastic dough. Millers began to add ascorbic (vitamin C) which oxidizes the gluten proteins and speeds up the process. Flour originally has a yellowish tint to it (as seen below) when first milled but becomes paler as it is exposed to the air. Once this was discovered, many bakers decided to add bleaching agents (peroxide) to whiten the flour and give the perception of being aged.
So when browsing through the store and you cannot decide whether you should choose white or bleached white, go with the regular white flour. If you have any follow up questions or would like me to elaborate on any topics discussed, leave a comment and I will do my best to answer!
Source: McGee, H. On Food and Cooking: The Science and Lore of the Kitchen. 2004
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