Heat Damage

Dryer damage is an event. Air above what the commodity tolerates, or a drying rate that pulls moisture from the surface faster than it can migrate from the interior, imposes a thermal and mechanical stress on the kernel: proteins denature, the embryo is killed, starch and oil are altered, and the kernel may case-harden, stress-crack, or scorch. It is bounded in time, it is attributable to a machine and a setting, and it affects the material that passed through the dryer while that setting applied.

Self-heating is a process. Grain that is too moist for its storage duration continues to respire, and so do the moulds and insects its moisture permits. Respiration is exothermic. A bulk of grain conducts heat poorly, so what is released stays where it was released, and the warming mass respires faster — which releases more heat. The loop tightens on itself. A hot spot develops, usually where the moisture was highest or the aeration worst: near a wall, under a leak, at the peak of a bin, in a pocket of fines that resists airflow. Left alone it progresses through musty, caking, darkening grain to severely damaged grain, and in extreme cases to charring. It is a storage-management failure spread over time and hidden inside a bulk that looks, from the top, exactly like grain.

Once the kernel is damaged, the two routes are not distinguishable in it. Darkened, scorched grain from a dryer running too hot and darkened, scorched grain from a hot spot in a bin present the same way, because the same constituents were degraded by the same physics. The distinction between them is made by records — drying logs, moisture at loading, aeration and temperature monitoring history, where in the store the damage was concentrated — and never by appearance.

The single most important fact about heat damage is that its ordering is counterintuitive. The properties that fail first are the ones no one can see, and the property that fails last — colour — is the one the defect is named and graded by. Germination depends on an intact living embryo, which is among the most thermally fragile things in the kernel; it can be destroyed at exposures that leave every visible characteristic of the grain unchanged. Malting depends on that same embryo germinating on demand and on an enzyme complement that heat degrades. Baking quality depends on proteins whose functionality is diminished by thermal exposure long before they are visibly altered. Oil quality in oilseeds is degraded by heat that changes nothing about how the seed looks in the hand.

Germination and vigour

Lost first, and lost invisibly. A lot can be commercially normal in appearance and dead as seed; the only way to know is a germination test, not an inspection.

Malting quality

Requires a living, uniformly germinating embryo and an intact enzyme complement, both degraded by thermal exposure that produces no visible cue.

Milling and baking quality

Protein functionality and starch behaviour shift with heat exposure; the flour performs differently from a grain that looked no different.

Oil quality in oilseeds

Heat promotes changes in the oil and its stability that are measured in the laboratory and are not apparent in the intact seed.

Colour and scorching

The visible endpoint, and the last to appear. By the time a lot is visibly heat-damaged, the invisible losses are long complete and severe.

Grain darkens for many reasons, and heat is only one of them. Mould growth discolours kernels. Weather in the field stains and dulls them. Grain that stood too long before harvest, or matured under stress, darkens through ordinary senescence. Kernels wetted and re-dried in store are discoloured without ever having been hot. And varietal and lot-to-lot colour differences are wide enough that a comparison against an expectation is not a measurement. The characteristic that identifies heat damage in a grade standard is a defined factor assessed against defined reference material by trained inspectors — it is not the general impression that a lot looks dark.

• Mould discolouration — both darken kernels and both are associated with moist, poorly managed grain; they occur together for the same reasons, so the shared cue of dark kernels cannot separate a heated lot from a mouldy one, and a lot is frequently both.
• Weather and field staining — rain, dew cycling, and pre-harvest weathering darken and dull grain before it ever reaches a dryer, giving discolouration with no thermal history at all.
• Senescent darkening — grain that matured under stress or stood late darkens as a normal end-of-development change, sharing the appearance without sharing the cause.
• Dryer damage versus self-heating — the two are indistinguishable in the kernel, because the same constituents were degraded by the same physics; only records separate them.
• Varietal and lot colour variation — normal differences between cultivars and lots are wide, so "darker than expected" is a comparison to an assumption, not evidence of heat.

Self-heating and mould growth are not alternative explanations for a warm bin; they are companions. The moisture that permits respiration permits storage moulds, mould growth is itself exothermic and contributes to the heating, and the warm moist conditions inside a developing hot spot are close to ideal for the fungi that matter most in storage. A lot that self-heated is, on the evidence, a lot in which storage moulds had favourable conditions — so the discovery of heat damage properly raises a mould and mycotoxin question alongside it.

Heat damage's appearance does not answer that question, and neither does mould's. Visible mould does not establish that a mycotoxin is present: toxin production depends on the species, the substrate, and the conditions growth occurred under. More importantly, the absence of visible mould does not establish the absence of mycotoxin — toxin persists after the fungus that produced it is dead, occurs in material next to growth that was removed, distributes unevenly through a bulk, and is invisible and odourless. Cleaning, sorting, and inspection neither detect nor remove it. Presence or absence is established only by appropriate laboratory analysis on a properly drawn sample, against limits set by food-safety regulation for the specific commodity, jurisdiction, and intended use. This entry states none; EFSA and FAO publish the applicable assessment and management framework.

Heat damage is irreversible. A killed embryo does not recover, degraded protein does not re-fold, and a scorched kernel is not restored by anything done afterwards. Management is preventive on both routes: drying within what the commodity and its end use tolerate, and storing at a moisture and under a monitoring regime that does not permit self-heating to begin.

• Set drying air temperature and rate against sourced guidance for the commodity and, critically, for the intended end use — seed and malting tolerate markedly less than feed, so a single dryer setting cannot serve every lot passing through it.
• Follow the equipment manufacturer's manual for the dryer in use; drying limits are a function of the machine and the commodity together, and no universal figure applies across either.
• Dry to the storage moisture the applicable sourced guidance specifies for that commodity and storage duration, since residual moisture is the fuel for self-heating.
• Aerate and monitor stored grain so that a developing hot spot is detected while it is a temperature reading rather than a caked, damaged mass.
• Attend to the places hot spots start: wet pockets, fines accumulations that resist airflow, leaks, wall condensation, and the peak of a filled bin.
• Load uniformly moist material where possible, since a wet pocket in an otherwise dry bulk is a self-heating focus regardless of the average moisture of the lot.
• Treat a heat-damage finding as a prompt to consider mould status, and refer any mycotoxin question to laboratory testing under the applicable framework.