Biomedical Engineering Reference
In-Depth Information
Table 3.1
Chemical composition of wheat regions (Adapted from [
15
] ) (data expressed as
g/100 g dm)
Kernel
(%)
Starch and soluble
sugars (%)
Proteins
(%)
Lipids
(%)
NSP
a
(cellulose,
etc) (%)
Region
Ash (%)
Fruit coat
(
pericarp
)
5
14 ÷ 16
10 ÷ 14
1 ÷ 3
60 ÷ 74
3 ÷ 5
Seed coat
2
9 ÷ 11
13 ÷ 19
3 ÷ 5
53 ÷ 63
9 ÷ 15
Aleurone
8
10 ÷ 14
29 ÷ 35
7 ÷ 9
35 ÷ 41
5 ÷ 15
Endosperm
82
80 ÷ 85
8 ÷ 14
2 ÷ 3
1 ÷ 3
0.5 ÷ 1.5
Germ
3
19 ÷ 21
36 ÷ 40
13 ÷ 17
20 ÷ 24
4 ÷ 6
a
Non-starch polysaccharides
25-30% of total bran is hidden, into the flour of the bran layers [
13
] . Milling is
generally simpler for those cereals without furrow.
The physical separation of the various parts of the wheat grain is made possible
by the different composition of the three morphological areas of the kernel, that
determine different behaviour during processing (Table
3.1
). The separation of the
endosperm from the bran is not quantitative, since parts of the endosperm are lost
into the milling by-products and small percentages of bran fragments are inevita-
bly present in the flour. Therefore, the milling process has to reach a compromise
between “extraction yield” and “grade of refinement” (accuracy of elimination of
bran) of the flour. The main criterion for flour classification is based on the accu-
racy of teguments separation. The approach, used by legislation in many countries,
is based on the threshold value of a number of parameters, especially ash and
proteins.
The current technology for milling considers the following four stages: (1)
receiving, pre-cleaning and storage of the incoming wheat; (2) cleaning and condi-
tioning; (3) milling; (4) storage of the flours. Wheat coming to the mill is usually
transported in bulk in trucks, trains or ships and it is unloaded into large hoppers
with a grilled opening to facilitate the elimination of large foreign matter. This oper-
ation is preceded by rapid analytical inspections of samples representative of the
entire batch, to assess quality parameters. Preliminary cleaning operations made
before loading the wheat into the silos are intended to remove mainly coarse foreign
materials, in order to provide for better storage. The cleaning of the wheat is carried
out immediately before the milling process. This involves a sequence of operations,
each performed by a special machine, with the aim of removing impurities, foreign
matters and powders. Differences in size, shape and density compared to the whole
and sound wheat grains are used to achieve this aim [
14
] . The conditioning or tem-
pering of kernels is decisive to achieve an optimal milling. This operation includes
grain humidification and the successive resting time, to increase the water content
of 2-4%. The conditioning step toughens the bran, favouring its break off in form of
large particles, and mellows the endosperm, thereby facilitating the separation
between these two parts. The conditioning time (from 6 to 24-36 h), the quantity of
water used, and the ways to add water (one or two tempering steps) depend on
the initial humidity and on the hardness and vitreousness of the wheat kernels.
