Biomedical Engineering Reference
In-Depth Information
Fig. 3.2
Sieving or grading section: plansichters (
above
) and middling fractions separated by
sieving action (
below
) (Courtesy of Bühler AG, Switzerland)
to the reduction system, the final stage of the milling process, with the objective to
reduce the size of clean middlings to flour. It consists of a sequence of several
smooth roll mills (up to eight to ten, according to the size and the expected starch
damage) and sifters. The milling diagram comprises a number of the above steps, to
ensure that the majority of the endosperm is converted into flour and that most of the
teguments are removed as by-products.
The flour extraction yield varies between 74 and 76%. Since the bran and the
germ together represent around 20% of the weight of the wheat grain, the flour
extraction yield is lower than the theoretical value. More refined flours have lower
extraction rates, since most of the external layers of the endosperm are eliminated
with teguments. Milling of durum wheat requires different diagrams, which are
characterized by a higher number of purifiers to improve the separation of bran
particles. Nevertheless, the yield is lower (68-72%), since semolina is mostly
formed by particles larger than 250-300 mm and contains only a minimum amount
of fine particles (lower than 200 m m).
The separation of the more external layers and germ of the caryopsis inevitably
causes a marked change of the chemical composition and, consequently, of the
nutritional value of the flour. It has a lower concentration of ash, proteins, vitamins
and soluble sugars than the caryopsis and a higher starch content. This difference
depends on the efficiency of the separation of the more external layers of the cary-
opsis from the endosperm. Consequently, refined flours, corresponding to an extrac-
tion yield of approximately 75%, contain only 5% of fibre, 45% of fats, 30% of
