Post harvest Fundamentals: Temperature, Moisture and the Cost of Delay

Between 5 and 25 percent of the fruit and vegetables that leave an Australian farm gate are never eaten. Disease and oversupply account for some of it. A great deal of the rest comes down to two things: temperature and packaging.

This is a guide to the fundamentals — where losses actually originate, why the hours immediately after harvest matter more than any other, and what a cool room is really doing to your produce while it sits there.

Where the losses actually come from

The figure most often quoted is Kader’s estimate that 5 to 25 percent of fruit and vegetables leaving the farm gate never get consumed. But the headline number understates the damage, because the economic cost of a postharvest disorder runs well beyond the value of the fruit that was thrown away.

There is the direct loss to the grower on the affected batch. There is the flow-on loss to packers and marketers handling it. If the problem recurs, there is the cost of the monitoring systems put in place to check every line and isolate the bad ones. And an intermittent, unpredictable disorder appearing in the market does something worse than any of these — it erodes trade confidence in the product, and that shows up as downward pressure on price across everything you sell.

The exposure is greatest for premium and branded lines. A differentiated product selling at a premium has more to lose from a quality failure than a commodity line does, because the premium depends on consistency. Years of work building a brand can be undone by a handful of poor consignments.

Harvest is a kind of amputation

Horticultural produce is alive and has to stay alive long after harvest. Like any living tissue it consumes oxygen and releases carbon dioxide.

Jobling’s framing is worth keeping in mind: for a plant, harvesting is a kind of amputation. In the field the fruit is connected to roots supplying water and leaves supplying food energy. Cut it away from both and it must inevitably die. Everything postharvest handling does is an attempt to delay that death for as long as possible.

Which means there is no universal answer. Spinach and apples, bananas and potatoes each have their own requirements, and the optimum handling for one is wrong for another. Growers, packers, exporters and retailers all need to know the specific needs of the product in front of them.

Field heat: the most expensive hours of the whole chain

In the paddock, the sun and the produce’s own respiration combine to heat it up — worst in the centre of a field bin, where there is no airflow and the heat has nowhere to go. This accumulated field heat has to come out fast, because every hour it stays in costs storage life at the far end.

The number worth remembering: strawberries respire nearly eight times faster at a field temperature of 25°C than at a storage temperature of 0°C. As Jobling puts it, one day left warm in the field sacrifices as much as eight days of storage life.

That ratio is why pre-cooling is not optional and why good intentions are not enough. Harvesting in the cool of the morning and keeping bins in the shade both help, but heat production of this order can only be controlled by active refrigeration.

Pre-cooling also needs more refrigeration capacity than cool storage does, which is why it is often best run as a separate step rather than expecting the cool room to do both jobs.

Method	How it works
Hydro-cooling	Cold water drenches; fast heat transfer, suits robust produce
Forced air cooling	Air pulled through the stack; depends entirely on proper air distribution
Packing with ice	Direct contact cooling; suits crops tolerant of wetting

Which one is right depends on the commodity. There is no default.

Why the cool room is drying your produce out

This is the part that catches people out, because it is the refrigeration itself doing the damage.

To hold a room at 0°C, the cooling coils have to sit appreciably below 0°C. Air passing over them gives up its moisture, which accumulates as ice on the coils. The lower the coil temperature, the more moisture comes out of the air. That now-dry, cold air circulates back into the room, warms slightly, and picks up moisture again — and the moisture it picks up comes from your produce.

So a well-run cold room is continuously pulling water out of whatever is sitting in it. Weight loss, shrivel and limpness are not signs the refrigeration is failing. They are what refrigeration does when nothing stands between the produce and the air.

Jobling states the remedy plainly: unless a plastic film or some other vapour barrier protects the product, water evaporates from it.

There is a second-order effect too. The more moisture freezing onto the coils, the more often the system has to run a defrost cycle — and every defrost cycle makes holding a stable temperature harder. Moisture loss and temperature instability are the same problem viewed from two angles.

Uniform temperature, and the short circuit that ruins it

A cool room average tells you very little. What matters is whether every part of the room is at that temperature, because only then can you safely run at the lowest and most effective setting for the crop.

If one corner runs warmer, the produce there ripens faster and your out-turn becomes mixed — the same consignment arriving at different maturities. Checking a wall thermometer will not reveal this. Core temperatures of the packages need checking at several points in the room, including at floor and ceiling level.

The hard part is that the room never stays still. Bulk bins and pallet loads are constantly rearranged as produce comes in and goes out, and each rearrangement changes the airflow. Maintaining even distribution through that is a genuine skill.

The specific failure to watch for is a short circuit — cooled air finding a direct path back to the coils instead of travelling through the stack. When that happens the system reports the room as being at temperature while whole sections sit starved of circulation and quietly warmer.

Maturity and handling

Maturity has no general definition, because it depends on what the product is for. We eat sprouted seeds, leaves, flowers, whole fruit, seeds and nuts — there is no single rule spanning that range. Maturity has to be defined per product, and in some cases per variety.

Standards matter for two reasons. They give the consumer a floor on quality, and they give a hand-picking crew something workable to follow. Most horticultural produce is still harvested by hand, and a simple colour and size guide is what turns an abstract maturity target into something a picker can apply consistently.

Care during harvest repays itself later. Fewer bruises and injuries mean less disease and better value at the far end. Reducing the sheer physical effort of picking helps here more than it might appear — conveyors for crops like lettuce and celery, cherry pickers for tree crops. A crew that is less exhausted has more attention left for handling the fruit gently.

Some compromise is unavoidable. Produce destined for long-distance transport often has to be picked firmer and less mature than is ideal for flavour, simply so that it travels. That is the trade being made every time fruit is available out of season, and postharvest handling is what determines how much of the eating quality survives it.

What this means in practice

The hours after cutting are worth more than the days after packing. On the strawberry figure, a day of field heat costs roughly eight days of storage life. No downstream intervention recovers that.

Pre-cooling is a separate job from cool storage. It needs more capacity than holding does, and expecting one room to do both means doing neither well.

Measure cores, not air. Several points, floor to ceiling. A room at temperature on average can still be delivering mixed out-turn.

Assume the cold room is taking moisture out of your produce, because it is. The question is only whether anything stands in the way.

A vapour barrier is doing two jobs at once. It holds moisture in the produce, and it reduces the moisture load reaching the coils — which means fewer defrost cycles and a more stable room for everything else stored alongside it.

Sources

Jobling, J. (2002). Postharvest management of fruit and vegetables. Good Fruit and Vegetables Magazine, January 2002. Sydney Postharvest Laboratory Information Sheet.

Kader, A.A. (1992). Postharvest biology and technology: an overview. In Postharvest Technology of Horticultural Crops, Publication 3311, University of California, pp. 15–20.

Wills, R.B.H., McGlasson, W.B., Graham, D. and Joyce, D. (1998). Postharvest: An Introduction to the Physiology and Handling of Fruits, Vegetables and Ornamentals. NSW University Press, Sydney.