Milling
Before milling, the wheat grain must have extraneous material from the field removed, be washed and have its moisture content adjusted for optimum results. The two principal methods of milling are by stone or roller mills.
Stone milling involves crushing the whole grain to a fine powder. This process produces most nutritious and flavourful flour because nothing is lost from the grain. However, its shelf life is limited by the tendency of the oil released from the crushed germ to go rancid after being absorbed by the crushed endosperm. A limited gradation of stoneground flour types can be produced by sieving out some of the larger particles.
Roller mills operate by a multi-stage process known as 'gradual reduction'. The grain is fed through a series of paired fluted cast iron rollers set with progressively smaller gaps between them. The grain is broken rather than crushed; this allows its components to be efficiently separated and then, often, reblended for making a variety of flour types.
Length
The term 'length' as applied to flour denotes the amount, expressed as a percentage, of useful flour abtained from the grist (the actual grains which are milled). The grist may consist of a blend of different grain varieties.
Strength
This term is used to indicate the ability of a flour to form strong gluten which is necessary for making a fine textured and well risen loaf. The higher protein content of the hard wheat varieties make flour of higher strength, hence they are more suitable for breadmaking. Such flours are marked 'strong' or 'for breadmaking' on the packet. Traditionally such grain had to be imported from North America and elsewhere, but this has largely been replaced by new home-grown hard wheat varieties.