Anotherimportant factoris the criticalmoisturecontent (CMC): at this
moisturethe grain is not resistant to storageand facilitatesthe developmentof
microflora.Eachtypeof seedhas its CMC.For example the CMCof wheat is
around 15%,whichis in equilibriumwith70±75% RH of the ambient air.
Usuallyseedswithhighoil contenthavemuchlowerCMC thanseedswithlow
oil content.
For safe storage, farmersuse a safe moisturecontent, which is usuallyabout
2% lowerthanCMC,and securesafestorage. For example wheat13%for a
periodof 1 yearstorage,11±12%for 5 years.
Methodsfor measuringgrainhumidity
∑ Directmethods± ovendrying. These are usually accurate methods. The
commonmethodis ovendryingprocedures.Wholegrainsor milledproducts
are driedin one step according to the temperatureand timespecified for each
graintype or its products. The moisture content can be calculated on a wet or
dry basis.Wet weightis calculated on the totalweightof the producttested.
Dry weightis the dry weightof the product testedafterthe waterhas been
removed.
∑ Indirect methods± electronicmoisturemeters. Theseare basedon electrical
conductanceor capacitanceof grains,whichare largelydependent uponthe
moisturecontent of the grain.Thisis a practicalmethodfor determining
moisturecontent of the grainor other commodities.Electronic meters are
reliable, fast, simple to operateand of low cost per test. Varioustypes of such
moisturemeters are availablein the marketfor different uses.
26.2.5 Atmospheric gases
By changing the percentage of the various gasesin the intergranular air,
increasing the concentrationof CO 2 or decreasing the concentrationof O 2 , it is
possible to controlor depressinsects and microfloradevelopment,for example
by usinghermeticstorageor modifiedatmosphere.
26.3 Moisture migration in the grain bulk
This is also called movement of moisture as a result of non-uniform
temperaturesin the grainbulk. In the intergranular spacethereare convection
currents, whichalso carry watervapourfromthe hot area to the coldarea of the
grainbulk.In the coldareathe watercondenses and the humidityof the seeds
increases, which acceleratesmicrofloradevelopmentand hot-spotformation,
whichcausebiological deteriorationof the commodities.In metalsilosthis
phenomenonis verycommon. Moisture migration is a limitingfactorfor the use
of hermeticstorage and in this typeof storageit is importantto storedry
commodities.To preventmoisture migration,it is recommendedto aerateso as
to coolthe grainand alsoto isolatethe partsof the storageareathat receive
Improvingthe control of insects in foodprocessing 411