area by convection currents and condensates.The waterincreasesthe humidity
of the commodity, causesfungal development and finallythe spoilage of the
commodity.
Storage structure
Storage structure is important as it facilitatesaeration, fumigationand the
transport of the commodity fromone place to other.
26.2.3 Temperature
Stored product insects are mainlyof tropical and sub-tropicalorigin.Because
insects cannotcontrol theirbodytemperature,theirratesof development and
reproductionincrease withrising temperatureand become inactive at low
temperatures.For eachspeciesthereis an optimum temperaturefor growthand
development,whichis between 28 and 34 ÎC for mostmajorstoredproduct
insects. Thereare also minimum(17±22ÎC) and maximumtemperatures (around
40 ÎC) between whichtheyare ableto develop.Temperature5±10ÎC above
optimum will be lethaland belowwill causea dropin theirreproduction and
feeding activity.
26.2.4 Theroleof moisture (humidity)
Moistureis the mostimportantfactorin storage. We have to rememberthat
wateris connected withlife and thereis no life without water.Therefore, the less
waterin the commodities, the betterwe can preservethem.For example, seeds
are in deepdormancy, and waterand oxygenwake themup. Thisis a negative
process that is not reversible and helpsthe fungito developthat finally spoilthe
commodity. Humidity can be dividedinto two parts:
- Waterinsidethe seedsor the commodity. The percentage of H 2 O in the
commodity plays an importantrole in the storabilityof the commodity.High
humidityresults in highactivity of bioticfactorssuchas microfloraand
causesthe spoilage of the commodity. H 2 O is an integralpart of the grainand
its products and it has the samecommercialvalueas the commodity. - Waterin the outsideair or relativehumidity of the air (RH). Cerealgrains
are hygroscopic and gainor lose moistureto achieveequilibriumwiththe
ambient air. If we storedry grainor othercommodity in humidair of 80±
90%RH, the grainor other commodity will absorb H 2 O until it arrives at
equilibriumwiththe moisture of the ambient air. In dry air of 50±40%RH
the oppositewill happen: wet grainwill lose H 2 O. The dynamic equilibrium
betweenthe moisture of the ambient air and of the commodity moisture is
calledthe equilibriummoisturecontent(EMC). Thisis one of the most
importantfactors in predicting the storabilityof the grain.EMCvaluesfor
grainvaryby differencesin grainvariety, maturity and chemical composi-
tion,(mainly oil content).EMCincreases, withincreasingrelativehumidity
and decreasingtemperature.
410 Handbookof hygiene controlin the foodindustry