Room cooling is where a bin or carton of produce is simply placed inside a cool room. Unless there is rapid air movement, most cooling will occur by conduction— heat energy moving out of the product into the surrounding environment.
While cost and labour are minimised, this method can result in quite slow cooling rates. Room cooling can be particularly slow if the room is very full and/or liners are used, as in the example at right.
The centre of a half-tonne bin, for example, can take several days to cool from an initial temperature of over 20°C to below 5°C. This can be problematic if products have been harvested while hot, are susceptible to moisture loss and/ or have a fungal or bacterial infection. Moreover, as warm, saturated air from the centre of the bin cools, condensation on the product is likely.
Room cooling rates will be affected by the amount of air moving across and through the package. It is recommended that air velocities around packages should be >1m per second. However, whether this is sufficient will depend on the produce temperature, type and surface area relative to volume.
The surface area of a packed pallet or half tonne bin is relatively small, so it will be slow to cool. For example, the graph below shows the internal temperatures of chestnuts at the centre of half tonne bins placed in a cool room. Unvented wooden bins with and without liners were compared to vented plastic bins and a wooden bin fitted with ventilation pipes (shown below) to increase air circulation through the centre of the bin during room cooling.
In the example shown above, the plastic bin cooled slightly faster than the wooden bin, likely due to the vents on the bin floor and sides.
Product in the centre of the bins cooled fastest in the bin with the ventilation pipes. However, fruit at the top and in the base of this bin cooled at the same rate as the standard wooden bin.
Product in the lined bin was extremely slow to cool due to the prevention of air circulation and convection.
As products must be widely spaced to allow airflow, room cooling is also space inefficient. It can also increase the load on the refrigeration system if warm products are constantly being added to the room.
Forced air or ‘pressure cooling’ effectively increases the surface area being cooled from that of the package to that of the produce inside. Forced air-cooling can reduce cooling times by 10 times or more, compared to room cooling.
Forced air systems pull cold air through vented packages at rates varying from 0.1 to 2.0L/second/kg. A general guide to fan strength is that there should be enough pressure to hold a piece of A4 paper against one of the carton vents.
Most forced air systems are designed for two rows of stacked pallets (or bins) to be placed against a central plenum. A tarpaulin is draped over the top to block the gaps between the pallets, forcing air through the carton side vents.
The fan inside the plenum pulls air through the cartons, removing heat from the packed produce. The air may be exhausted directly back into the room or passed through a cooling system first.
For forced air-cooling to be efficient, cartons should have vents covering at least 5% of their surface area at the air entry and exit points. The vents must line up between cartons, even if pallets are cross-stacked. Note that one or two large holes allow more air movement than many small holes, even if the total area is the same, due to the edge effect around vents.
Moisture loss is not usually a problem during forced air-cooling as the process is quite fast. However, high-humidity systems are available if this is an issue.
Unlike room cooling, condensation does not occur with forced air systems. Condensation can increase disease and reduce strength of cardboard packaging. Without positive air movement, water vapour transpired by warm produce can condense on the cold product or packaging closer to the air delivery system. With forced air systems, the air warms as it moves through the produce, increasing its capacity to hold and remove water vapour, thus preventing condensation from occurring.
The energy efficiency of forced air systems varies widely. In some cases rooms used for forced air are also used for storage. This can reduce overall efficiency, especially if the fans are left on in between cooling cycles to keep the room cool. Efficiency of fans can also be a source of variability.
Water is much better at conducting heat than air, so hydrocooling can provide very rapid cooling of produce. Hydrocooling systems can be either continuous feed on a conveyor, or a batch treatment. Product may be immersed in a dip tank of cold water or drenched by a shower or spray to extract heat from the product. Drenching systems often use a pan with holes to distribute the water, as this requires less energy than generating pressure and spraying through nozzles.
To be effective, the cooling system for the water must have sufficient capacity to remove the heat absorbed from the produce by the treatment water. Also, treatment times have to be long enough to thoroughly cool the product’s core temperature; a brief shower or immersion treatment may cool the outer skin, but fail to significantly reduce core temperatures.
Hydrocooling systems generally recirculate water, making it important to include a sanitiser to avoid spreading human or plant pathogens. This is particularly important for fleshy products or those containing internal air spaces, such as capsicums or pumpkins. As the warm air inside the product cools, it contracts, creating negative pressure. This can draw water into the flesh or cavity.
If the water contains fungi or bacteria, this can provide them with an ideal environment in which to grow.In addition, not all products tolerate being wet. Soft rots and other diseases are more likely on wet products, especially if they are not dried before packing.
One advantage of hydrocooling is that the product loses no moisture, and some may even be gained.
Before refrigerated trucks and the common availability of cool rooms, ice was used for cooling produce. Ice is still used occasionally during transport, notably for broccoli and Brussels sprouts. The main reasons for using ice are that it keeps product cold and hydrated, it looks good when the carton is opened, and (most importantly) the customer expects it.
The freezers that make ice may lower the temperature to well below 0°C; even a simple domestic freezer operates at close to minus 20°C. The temperature of the ice itself may therefore be below the freezing point of the product. Even tolerant products such as broccoli and Brussels sprouts may suffer freezing injury as a result of contact with such cold ice.
As water changes from solid (ice) into liquid state (melts), it absorbs energy from the surroundings. Apart from the heat absorbed directly through conduction, ice mainly cools produce if it is melting. If the ice is still solid when the carton is opened then it was not needed.
If the ice has melted, thereby cooling the product, then the vegetable will inevitably be wet, and often sitting in water. This can cause splitting, discolouration and increased rots and disease.
Water used for icing must be of a high microbial standard. It must not contain human pathogens, and should also be free of fungi and bacteria that can cause plant disease. Making ice uses considerable energy and the process of top icing adds an extra step during packing, all of which increase costs. Adding ice to cartons also adds weight and increases volume, thereby increasing transport costs.
Vacuum cooling removes heat from vegetables by boiling off some of the water they contain.
Produce is loaded into a sealed container and the air is pumped out. This reduces the pressure from normal air (approximately 100 KPa) to a virtual vacuum (<1KPa). Under these conditions water boils at <7°C.
As water inside the vegetables changes from liquid into gas it absorbs heat energy from the product, cooling it. This vapour is removed by drawing it past refrigeration coils, which condenses it back into liquid water.
For vacuum cooling to cool vegetables quickly, they must be able to lose moisture easily. For this reason vacuum cooling is very well suited to leafy products, such as lettuces, Asian greens and silverbeet. Products such as broccoli, celery and sweet corn can also be cooled effectively using this method. Vacuum cooling is not suitable for products with waxy skins, or low surface area compared to their volume, e.g. carrots, potatoes or zucchini.
For every 5°C reduction in temperature, approximately 1% of the produce weight needs to be turned into water vapour. However, modern hydro-vacuum coolers address this issue by spraying water over the produce during the vacuum process. This can reduce moisture loss to negligible levels.
For suitable products, vacuum cooling is the fastest of all cooling methods. Typically, only 20 – 40 minutes is needed to reduce temperature of leafy products from 30°C to 4°C. In the example shown below, vacuum cooling reduced the temperature of harvested broccoli by 11°C in 15 minutes. Large vacuum coolers can cool many pallets or bins of product simultaneously, reducing demand on cool room systems. The process can even be used on packed cartons, so long as there is sufficient venting to allow air and water vapour to escape quickly.
Vacuum cooling is also the most energy efficient form of cooling, as nearly all the electricity used reduces the temperature of the product. There are no lights, forklifts or workers inside a vacuum cooler that can increase the temperature. The unit is sealed during operation so there is no issue with infiltration during cooling.
Hydro-vacuum coolers will be slightly less efficient as the water is also cooled. It has been estimated that 570L of cold water are lost from the system for each 400 carton load; water which must be replaced and re-cooled. Also, cold water on the sides of the unit cool the walls, potentially allowing heat transfer into the unit. However, such losses are relatively minor compared to those in most cool rooms.