The storage of the enormous grain stock necessary for the huge masses
of people today is practically always done in closed silo bins. One
differentiates between : high constructions and low-built cells, with or
without aeration facilities.
The grain kernel is a living organism which breaths (respirates). The microorganisms
(bacteria and mold (fungi)) living on it, also breath and play an important
role. Heat, as well as water and carbon dioxide is produced during respiration,
in the course of which oxygen of the air surrounding the kernel is consumed. The
respiration intensity increases with increasing moisture contents, and also
with increasing temperatures. It can be clearly seen that the respiration
intensity and therefore also heat development are both very small at a moisture
content of 14%, whereas the intensity increases quickly above 15%. A temperature
increase has a similar effect on respiration intensity as does increasing moisture
content. A rapid increase will occur though, if moisture and temperature
increase simultaneously, resulting in the following sequence. Over 14% moisture
content, temperature above 120 C:
a.More or less active respiration with water, carbonic acid and heat
development, depending on moisture. Heat cannot dissipate, temperature of the
wheat increases, e.g. to 200 C.
b. More intensive respiration than before, temperature increases
continuously, e.g. to 250 C, as does that of water content.
c. still more intensive respiration than in case b), due to higher
temperature and water content, a more rapid temperature increase, e.g. to 300
C.
d. Livelier respiration than in case c), due to higher temperature, a
more rapid temperature increase, e.g. to 400 C. The endosperm of the
wheat kernel begins to turn brownish
e. Further temperature increase to 50 – 700 C and higher. Kernels
become brown, microorganisms die off, respiration comes to a standstill. The wheat
lot is spoiled, color and baking properties are damaged.
The wheat moisture content should not exceed 14% in silo storage, in
order to avoid such chain reactions; the wheat temperature should also be as
low as possible, corresponding to a relative humidity of the air surrounding
the kernels of about 65%. No mold fungi can develop on the kernels at this
relative humidity, this will occur however at approximately 75% (corresponding
to approximately 16% wheat moisture content). Bacteria need 90% to be able to
live.
In cases where the conditions mentioned are not fulfilled, the wheat
can only be stored for a limited period and only if special measures of
precautions are taken e.g. aeration, during the course of which respiration
heat, as well as the excessive relative humidity between the kernels can be
removed continuously. Wheat with a moisture content of 20-25% can be kept sound
for several weeks in this manner, until an opportunity occurs for milling it or
for drying it down to 14% and lower for permanent storage. The technique of
preliminary grain storage with aeration, in connection with modern harvest
methods (combine harvesting), has been fairly well developed during the last
few years. A disadvantage of this type of storage is the low permissible height
of the grain, depending upon the grain moisture content, to avoid :
a.movement of the moisture into the upper layers with condensation and
b. the growth of fungi
Slow drying is od course preferable.
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| Degree of respiration of rye at various temperatures and moisture contents |
More or less of a loss of dry matter will occur through respiration of the grain during storage, since the developed carbonic acid is produced at the axpense of the kernel matter. This loss is influenced in a similar manner by heat and moisture, as illustrated in picture above. It can be very great under certain circumstances (high moisture content, high temperature, long periods of storage), even if the grain looks externally sound. This source of loss is often everlooked in storage bookkeeping accounts where quantitative deficiencies are studied.
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