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.
|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.