Handbook of Hygiene Control in the Food Industry

direct sun. In storage with sacks, usually we see spoiled grain or other
commoditiesin the bottompart of the storagearea.Thisis becausethe flooris
coldand the moisture migrates fromthe upperhot places to the bottom.To
overcomethis problem, the sacksshouldbe storedon woodenbeams.

26.4 Dry-and wet-grain heating

During the metabolismof insectsand microflora, O 2 is consumedand heat,
waterand CO 2 are released. The very poorthermalconductivityof the grain
prevents the heatfromdissipating,and hot-spotsare formedwithtemperatures
that rise up to 40 ÎC. Thisprocessis calleddry-grainheatingand is causedby
insects. Hightemperaturesare unfavourable for insectdevelopment and the
active formsmoveawayfromhot spots to formnewones.In additionthe
metabolic waterin the formof watervapouris carried upwardthroughthe bulk
by convection currents and is deposited by condensation at the coolsurface.
Consequently, dry-grainheatingcaused by insects is oftenfollowedby wet-
grainheating causedby microfloraas moistureis transferredfromthe hot to the
coldpart of the grainbulk(Fig. 26.2).

26.4.1 Theprocessof heatproductionby insectsand microflora
Heat is producedby insectsand microfloraby aerobicor anaerobic processes.
Examplesof the aerobic processare:

∑ grainwithhighcarbohydratecontent, 1 moleglucose produce:

180 g glucoseá134 L O 2 ˇ!134 L CO 2 á108 g H 2 Oá677 kcal

∑ Grainwith highoil content, 1 moletripalmitin produce:

807 g tripalmitiná 1624 L O 2 ˇ!142 L CO 2 á883 g H 2 Oá 7617 kcal

Examplesof the anaerobic processare:

Lacticacidá22.5 kcal

1 M Glucose ˇˇ! Ethylalcoholá22 kcal

Aceticacidá15 kcal

The aboveequationsreveal that equivalentsof O 2 consumedand CO 2 produced
varywiththe typeof substrate oxidized. Also,the energyreleased per unit of
substrate in the aerobic processis much higherthanthe anaerobicprocess.
We differentiate between biological and chemical heating. Biologicalheating
is caused by insectsand microflora occursat a maximumtemperature of 50±
60 ÎC. It is caused by bothdry-grainheatingand wet-grainheating.
Dry-grainheatingis caused by insects and is characterized by:

∑ the moisturecontent of the grainor othercommodityless thanthe critical
moisture;

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412 Handbookof hygiene controlin the foodindustry