Why does cooling happen

For a single vegetable surrounded by cold air or water, surface area relative to volume will be large, even for a round product such as a pumpkin. However, if product has been packed into a carton, or is still in the harvest bin, the effective surface area is the outside of the container, not the product skin. The surface area of a bin relative to its volume is very small, resulting in a slow cooling rate. It is possible to increase the effective surface area for product in bins or cartons by forcing air or water through the container.

This allows heat to be removed directly from the surface of the product. Forced air-cooling effectively does this, pulling air through the container across the surface of the products contained.

Hydrocooling also allows direct contact between the product and the cooling medium, greatly increasing the rate of cooling even if product is still inside the harvest bin. Thermal conductivity refers to how easily heat can be removed from a specific vegetable. This is affected by shape, skin structure and composition. Layers of air trapped between overwrapping leaves, such as a cabbage or lettuce, reduce thermal conductivity. This means they are slower to cool, especially when compared to a solid vegetable of similar size and shape such as a pumpkin.

Thin skins, such as that of carrots, and large surface area, such as leafy greens, also increase thermal conductivity. The difference in temperature between the product and a cooling medium is another driver for temperature changes. A large differential results in rapid cooling. However, as the temperature of the product approaches that of the surrounding air or water, temperature changes more slowly.

It can take hours or days for product to reach the same temperature as the delivery air. In some cases it never reaches this point, as the heat produced by respiration keeps the product fractionally warmer than its surroundings. Acknowledgement This project has been funded by Horticulture Innovation Australia using the vegetable industry levy and funds from the Australian Government.

Home : Postharvest fundamentals : Cooling and storage : Cooling rates. Cooling can occur by conduction, convection or radiation. Now when we talk about the temperature of something, we're talking about the average kinetic energy. Each of the individual molecules, they all have different kinetic energies.

They're all bouncing around in different ways and transferring the momentum in all different ways. And so you can imagine a reality.

Maybe this one has a fairly high kinetic energy. It's moving in that direction. This one has a lower kinetic energy. Maybe this one has a medium kinetic energy, moving in this direction. Maybe this one has a really high kinetic energy moving in that direction.

And so we've already talked about how hydrogen bonds in water between the partially negative end and the partially positive ends. That's what keeps the water together as these things move past and flow past each other. What gives the water its cohesion is these hydrogen bonds. But if all of a sudden-- remember, we're talking about the average kinetic energy-- but even if we're at room temperature, and the average kinetic energy isn't so hot, you might have individual particles, individual molecules that actually have quite a high kinetic energy and if they're in the right place, if they're near the surface and their kinetic energy is high enough to break the hydrogen bonds with neighboring water molecules, and to overcome the pressure in the atmosphere, so let's say that this is, these are just gas molecules in the atmosphere here.

But it's enough to break free and none of these things bounce into it and force it back to form hydrogen bonds.

This thing could actually break free and enter and become water vapor. And become in its gaseous state. And it'll be so far apart from other water molecules that it won't form hydrogen bonds anymore. So by vaporizing or by this process of evaporation, what's happening? Well if your highest kinetic energy particles or some of your highest kinetic energy particles are able to escape, what's going to happen to the average kinetic energy?

Well as the highest kinetic energy things escape and those are the ones that are most likely to escape, well then your average kinetic energy is going to go down. So average kinetic energy is going to go down.

Or another way of saying it, is that your temperature is going to go down. Your temperature is going to go down because as these molecules turn into water vapor, they're going to be the highest kinetic energy, energy is transferred to them, and then they escape. As this happens, it cools down our skin surface. This happens almost constantly to one degree or another. When we are exposed to an environment that is hotter than what is comfortable for us, the degree of perspiration or evaporation increases.

And it follows that the cooling effect increases. The more water molecules that are escaping from the liquid phase from our skin surface and from our pores, the more cooling effect there is. Again, this is because the liquid molecules, as they escape and become vapor, require heat and they take it with them. Plants do something similar, through a process called transpiration. Plant roots "drink" water from the soil and transport it up through the stem to the leaves.

Plant leaves have structures called stomata. These are essentially pores that you can think of as comparable to the pores in our skin.

One of the main functions of this process in plants is to transport water needed by plant tissues in other parts of the plant besides the roots. But this evaporative cooling effect also benefits the plant. This keeps the plant—which might very well be exposed to direct, intense sunlight—from overheating.

And this also explains why, on a hot day, if we enter a forested area, we feel considerably cooler. Part of that is due to the shade, but part is also due to the evaporative cooling effect from the trees through this process of transpiration.

Wind increases the effect of evaporative cooling, and this is a familiar concept.