2009_Book_FoodChemistry

15.3 Cereals – Milling 707
15.3.1.1 Storage
Cereals can be stored without loss of quality
for 2 to 3 years, provided that the kernel moisture
content, which is 20–24% after threshing, is
reduced to at least 14%. The low moisture
content prevents microbial spoilage, especially
by mycotoxin-forming organisms, and it also
lowers kernel respiration, i. e., metabolism.
The water is slowly removed from grains by
ripple-type dryers in a stream of hot air or burned
gas at 60–80 ◦ C (to the extent of 4% per passage)
to avoid damage to kernels by uncontrolled
shrinkage. Grains with high moisture content
can be stored for short periods of time in the
cold without quality deterioration. Stored grains
are fumigated for pest control. Aluminum and
magnesium phosphides are introduced. At 20 ◦ C
and 75% relative humidity, they decompose into
gaseous PH 3 . HCN or ethylene oxide fumigants
are also used.
Wheat and rye are suitable for the production of
bakery products, especially bread, and are called
bread cereals. Other cereals serve only as additives
for bakery products and are mainly used in
other ways, e. g., for porridge and pancakes.
15.3.1.2 Milling
The aim of milling is to obtain preferentially
a flour in which the constituents of the endosperm
cells predominate. The outer part of the kernel,
including the germ and aleurone layer (cf.
Fig. 15.2) is removed. Such a requirement is not
easy to accomplish since the kernel’s groove and
the unequal sizes of aleurone cells in cereals do
not facilitate simple dehulling. Therefore, the
grain has to be carefully broken, the particles
sorted and separated by size and, only then,
further disintegrated.
In a preliminary step to milling, the grain is
cleaned of impurities such as weed seeds, straw,
soil particles, spoiled decayed grains, dust, etc.
This cleaning step is based on the cereal’s kernel
size and specific gravity. Washing with water
is rarely done, since it promotes the growth of
microorganisms.
The next step is grain wetting or steeping in water
for 3–24 h, since an increased moisture content
to 15–17% facilitates the separation of starchy endosperm
cells from germ and hull. An alternative
procedure is wheat conditioning at elevated temperatures
up to 65 ◦ C; it is faster than steeping
and also favorably affects the baking quality. The
kernels are disintegrated stepwise. Each passage
through rollers involves particle size reduction by
pressure and shear forces, followed by flour separation
according to particle size using sieves in
the form of flat sifters (Fig. 15.22). Rollers are
matched to the product needed. Their size, surface
flutes, rotation velocity, gap between pairs
of rollers rotating in opposite directions at dissimilar
speeds – all can be selected or adjusted.
Wheat and rye are milled differently because of
structural differences in the kernels. The wheat
kernel is rather brittle; the rye kernel is gluey or
sticky. Therefore, rye is less suitable for coarse
grist milling than wheal. The wheat milling process
can be adjusted so that the first passages provide
the grist and the following ones provide the
flour.
The germ of the rye kernel, because of its loose
attachment, falls off readily during the cleaning
Fig. 15.22. Milling of cereal (1: roller mill, 2: sifters, 3 and 4: purifiers)