Grain filling lasts for about twenty days in total. We have broken the time down into three sections, 11-16, 17-21 and 21-30 days after flowering and describe what happens to the endosperm, the cell layers surrounding the embryo sac and the embryo during each of these time periods. These periods correspond to the Medium Milk, Soft Dough and Hard Dough stages used by farmers to describe the grain in the field.
About 21 days after flowering the grains reach their maximum fresh weight. On the outside the grain is changing colour from green to yellow; it has reached the 'Soft Dough' stage. The inside of the grain is still moist but the endosperm contents are now semi-solid. It is at this stage that the crop is most vulnerable to lodging. The controlled programme of drying out will not start until after stage 3.
Cell division has stopped in the endosperm and during this stage two types of starch grains, Type 'A' and the smaller Type 'B', will be packed into the compartments that formed in the Medium Milk stage. As these storage reserves become tightly packed the cell layers surrounding them are stretched and crushed and we can observe this in detail in micrographs of grain sections.
The embryo has grown to half its final size and is digesting the endosperm cells that are nearest to it. These cells fail to develop as normal endosperm and will become the 'crushed cell layer'. This layer is important during grain milling as it is here that the embryo or 'germ' will be separated from the endosperm. The 'crushed cell layer' is also important to the brewing industry because it restricts water uptake during the steeping and malting processes.