What is vegetable drying and dew point?

The Science of Drying: What Every Grower Must Know Before Storage

Before any harvested crop enters storage, it must go through the most important stage - drying. Growers know it affects quality, but far fewer understand how the physics behind drying actually works. And exactly this gap leads to huge post-harvest losses.

So - Agrovent educational mode on.

Drying hundreds of tonnes of onions or potatoes in wet, unstable weather is nothing like “hanging laundry outside”. Every detail of the microclimate matters.

What Determines the Drying Process? Six Key Parameters

Drying of vegetable crops is governed by physical laws. The following parameters dictate whether the product dries or becomes wetter:

Air temperature
Relative humidity
Atmospheric pressure
Dew point
Product temperature
Airflow volume through the product

The first four are strictly interconnected - their relationship is shown in a Mollier diagram (yes, the one Google tries to dodge).

Quick Knowledge Test

Two statements. True or false?

HEATING = DRYING?

Not always.

HEATING = HUMIDIFICATION?

Sometimes — yes.

Why?
Because everything depends on the dew point.

When Does Air Dry? When Does It Humidify?

✔️ Cooling = Drying

If air is cooler than the product, it always dries it.
Always.

✖️ Heating = Drying

Not necessarily.
Warm air can both dry and humidify - depending on its relationship to the dew point.

Everyday examples:

Hairdryer → dries hair (classic forced drying).
Cold soda bottle “sweating” → humidification (warm air condenses on a cold surface).
Foggy car windows → AC dries faster than heating, because cooling removes moisture.

What Is the Dew Point and Why It Rules Everything?

Dew point - the temperature at which air becomes saturated and water vapor condenses.

Key rules:

Dew point is always slightly below actual air temperature.
When air temperature falls below dew point → condensation.
The closer dew point is to real air temperature → the more humid the air.
At 100% relative humidity, the air temperature equals dew point.

Examples:

Rainforest, tropical coastline → dew point almost identical to air temperature.
Desert → dew point extremely low.

Absolute vs. Relative Humidity

Absolute humidity

Grams of water per kilogram of air.
You can't measure it directly — only calculate.

Relative humidity

Percentage of how much water the air contains compared to how much it could hold at that temperature.
Measured easily.

When RH = 100% → temperature = dew point → air is fully saturated.

The Three Parameters That Control Drying Inside Storage

To dry crops effectively inside a storage facility, you must control:

Temperature in the drying chamber
Relative humidity
Dew point (temperature of the coldest surfaces)

Even if it's raining, foggy or the humidity is 98–100%, correct control of these three factors allows you to keep the crop dry.

That’s exactly where proper storage engineering becomes critical — the airflow balance, fan capacity, heat sources, and microclimate automation determine everything.

You can't “guess” the drying process.
You must calculate it.
One mistake - and the entire crop shifts from safe storage to mass rot within days.