Wheat preparation for grinding
When wheat lots arrive to the mill, they usually contain foreign or abnormal grains as well as pieces of rocks and straws. Lots are classified by the legislation. Grains have to be cleaned before grinding.
The cleaning equipments used by the miller are the following: separator cleaners, sorters, stoners, magnetic equipments, brushes as well as aspiration systems of these machines.
At the same time, the miller prepares the wheat with wetting and rest operations to facilitate the separation of the envelops from the albumen during grinding. These processes make the envelops more supple and more elastic which help avoiding the presence of envelops fractions in white flour and also facilitate the bran curage. Therefore, a maximal amount of albumen will be extracted and wastes will be avoided. Moistening increases the albumen friability and facilitates its grinding into flour.
Moisten wheat generally contains 16 to 17% water for grinding on cylinders, and about 15% if stone-ground. Cleaned and prepared, the wheat is industrially pure and ready for grinding.
To produce stone-ground organic flour, grains are sifted before being crushed between grinding wheels. Oils present in the germ scatter in the starch and stimulate enzymatic activities which generate the typical rich aromas of stone-ground flour.
The grinding wheels are composed of two layered parts: the mobile upper part, named the turning grinding wheel (speed of 200 rotations per minute) and the fixed lower part. Therefore, the speed is very elevated.
Grains arrive at the center of the turning grinding wheel where they are gradually crushed and the periphery is removed. The work is gradual but continuous with millstones, while it is divided in successive operations when ground on cylinders. The consequences of crushing and fraying of the millstone are the following:
Important heating which usually means a larger loss of water than when grinding on cylinders;
Reduction of the size of wheat envelops, which size becomes similar to other particles. Therefore, it is difficult to separate the components of the grains by sifting. The granulometry (the properties of flour particles) of a stone-ground flour is more homogeneous than a flour ground on cylinders;
A more homogeneous and ochre colour, which is the result of the fractionation of wheat envelops and the crushing of the germ. Stone-ground flours are therefore a little richer in lipids and needs special care to allow their preservation.
To insure a fresh ground and to reduce starch damages, we dressing the stone. This process means that we do the abrasion of the millstone over again every 150 hours with an air impact hammer. Therefore, the miller has to know the millstone very well. Usually, a millstone can last an average of 40 years.
After each grinding, wheat grains particles are aspirated through a pneumatic conduit to the planitscher located on the second floor where the flour is mechanically sifted. Whole flour (granulometry = 500_m with an ash ratio = 1.5%) and sifted flour (-250_um with an ash ratio -0.85%) can be produced. Thus, larger particles, such as the bran, can be removed. However, it is impossible to totally remove the bran of a stone-ground flour. Wheat bran is really nutritious and is rich in fibers.
Stone-grinding is really useful to transform grain lots of specific wheat varieties because of its artisanal nature and its readiness to meet specialized demands.
After sifting and granulometric separation, flours are bagged with weighters and automatic fillers in closed loop.
Then, the flour is stored and the merchandise can be delivered in the days following the receipt of an order.
Nowadays, most flours are ground on cylinders, a process invented during the last century.
Cylinders-ground flours are made from grains that are crushed in machines between two large ribbed steel cylinders turning in opposite direction. This system presents numerous advantages but one inconvenience. In opposition to millstones, cylinders never wear off, which allows complete separation of the wheat envelops from the kernel. Cylinders also isolate the wheat germ and eject it.
Wheat must undergo several operations to become flour. Grains are first cleaned, finely crushed, sprayed and sifted to remove the bran and obtain fine flour. The germ is also removed to allow better preserving the flour.
When the grains go through the cylinders, their envelops are gradually worn away to extract the wheat albumen (the interior of the grain).
The extracted albumen is constituted of larger and finer particles. This semolina is then crushed by smooth cylinders to transform it into flour. Semolina rolls are used for larger semolina while converter rolls are more suitable for fine semolina and middlings. Smooth cylinders, with slow differential speed, crushes almost instantly when in contact with the grains.
After each passing, grains are sifted on a succession of sift placed in planischters to classify the products accordingly to their size and to direct them to other processing equipment.
In opposition to stone-grinding (grains pass only once between the millstones), grinding on cylinders yield more uniform flours.
The choice of wheat is crucial for the backer’s value, but the work of the miller also influences significantly flour quality. The technique of the miller not only concerns performance and yielding of one type of grinding. Actually, the miller has to consider different techniques and handling of grinding to better meet quality requirements and optimize production management.
Grinding on cylinders allows obtaining a larger variety of flours, particularly white flours, while millstones do not provide good yielding for flours characterized with a low-mineral content. However, these flours are best able to allow optimal bread development. Moreover, it is also possible to add fine envelops parts to white flours in order to obtain whole flours.