How are flour grades related to the amount of bran particles left in flour?

In cereal processing, the grade of flour is directly determined by its freedom from branny particles. During the milling process, wheat kernels are separated into various streams of flour that differ in their purity and the amount of residual bran they contain.123

The relationship between grade and bran content is as follows:

• Purity and Grade: Higher-grade flours, such as "patent flour," are selected from the purest flour streams and are remarkably free from branny specks and other impurities. Conversely, lower-grade or darker flours contain a higher concentration of these residual particles.31
• Ash Content: Because the mineral content (ash) of the bran is significantly higher than that of the pure endosperm, the ash test is a standard method used to determine flour grade. A lower ash content indicates a higher degree of purity and fewer bran particles, which results in a brighter, lighter-colored flour that is generally preferred by bakers.13

By selecting and combining different flour streams, millers can produce specific grades of flour tailored to different culinary requirements.3