Bruising

A bruise begins when applied force exceeds the strength of the tissue carrying it. Cell walls fail, membranes rupture, and cell contents that were held in separate compartments come into contact with one another and with air. Enzymatic browning follows in most commodities, which is why a bruise darkens; in others the damaged zone becomes water-soaked, translucent, or simply soft. The skin is often entirely unbroken above all of this, because skin is flexible and the flesh beneath it is not.

Two things follow from that mechanism and matter more than any description of appearance. First, the damage is done at the moment of loading, but the discolouration that makes it findable is a chemical process that takes time to develop — the defect and its symptom are separated in the clock. Second, because the visible surface is intact, there is often nothing at all on the outside to find. A bruise is not a mark on the produce; it is a region of ruined tissue underneath a mark that may never appear.

Impact damage and compression damage are genuinely different events. An impact is a single dynamic blow delivered in an instant — a drop onto a grading table, a throw into a bin, a fall against the side of a trailer. Compression is a sustained static load carried over time — an over-deep bin, an over-stacked pallet, an over-tight pack. Bruising is what both of them produce, and the resulting tissue damage is the same kind of damage in both cases.

There is a widely repeated cue: compression bruises tend to be broad and flattened where the produce meets a contact face or a neighbouring unit, while impact bruises tend to be more localised at the point of the blow. The cue is real enough to be worth noticing and far too weak to diagnose with. Bruise shape is modified by tissue firmness, by the geometry of whatever the produce was resting against, by how far the damage spread after the event, and by whether more than one force acted on the same unit — which, in a real chain, is usual. A broad flattened bruise on the shoulder of a fruit that also fell twice is not evidence of compression.

Bruising is under-recorded for three compounding reasons, and they reinforce one another rather than simply adding up.

• It is often invisible. Sub-surface damage under unbroken skin cannot be seen from outside, and only cutting, processing, or peeling reveals it. Non-destructive assessment of internal bruising is an active area of work, not a routine packing-line capability for most commodities.
• It is delayed. The browning that makes a bruise findable develops after the causal event, so the inspection nearest to the damage is the one least able to see it, and the inspection that finds it is far downstream from where it happened.
• It becomes something else. Bruised tissue is an open door for wound pathogens, and a rot that follows a bruise is recorded as a disease loss. The wound is frequently unrecognisable by the time the rot is the visible endpoint, so the handling failure disappears from the record entirely and the fungus takes the blame.

The practical consequence is that bruising rates measured at the point of packing understate the bruising a lot actually carries, and loss statistics that separate "mechanical damage" from "decay" understate mechanical damage in favour of decay. Where a lot shows disproportionate rot after an unremarkable cold chain and clean sanitation, the wounding history upstream is worth examining before the pathogen is treated as the primary problem.

Bruise susceptibility is a property of the produce as much as of the handling, which is why two lots through the same line can differ substantially. Commodity and cultivar set the baseline. Maturity moves it: over-mature, softer fruit generally bruises more readily, while very firm immature tissue may resist bruising and crack instead. Turgor matters — fully turgid tissue and wilted tissue fail differently under load. Tissue temperature changes the mechanical properties of the flesh. In potatoes, susceptibility to blackspot-type bruising is affected by the tuber's condition and history well before it reaches the store.

Commodity and cultivar

The dominant factor and the one least under a handler's control at the point of packing; cultivar differences in bruise susceptibility are well documented for apples and potatoes in particular.

Maturity and firmness

Softer, riper tissue generally bruises more readily; firmer tissue may resist bruising and fail by cracking or splitting instead, which is a different defect with a different record.

Turgor and hydration

Fully turgid and partly wilted tissue respond differently to the same load, so bruise outcomes vary with the water status of the lot at the moment it is handled.

Tissue temperature

Flesh temperature alters the mechanical behaviour of the tissue and therefore its response to a given force; the direction and size of the effect are commodity-specific and are set out in sourced guidance rather than generalised here.

None of these are given here as values, thresholds, or rankings, because none of them generalise across commodities. What generalises is the principle: a bruising result is the product of the force applied and the susceptibility of the tissue it was applied to, and a lot cannot be assessed against another lot's handling record.

Bruising cannot be treated once it has occurred. The damaged tissue does not recover, the discolouration does not reverse, and no downstream step restores the lot. Management is entirely upstream, and it acts on force rather than on symptoms.

• Reduce force at every transfer point rather than at the one where damage is noticed, since the bruise found in the packhouse may have been made in the field.
• Treat drops, throws, and hard contact surfaces as the design problem they are: padding, reduced fall heights, and gentle transitions act on impacts that no inspection will later see.
• Limit sustained loading — bin depth, pallet stacking, pack tightness — as a separate problem from impact, because it is a separate mechanism and the two are reduced by different measures.
• For potatoes and sweet potatoes, curing after harvest allows wound periderm to form over damage that has broken the skin, which genuinely reduces the route to decay; it does nothing for sub-surface bruising where the skin was never broken.
• Assess bruising destructively on a drawn sample when the question matters, rather than relying on a surface inspection that cannot see the defect.
• Investigate unexplained decay as a possible wounding problem, not only a sanitation problem.