Forced Air Cooling of Cherries
Rapid cooling of fresh produce after harvest is one of the best ways to maintain high quality and to promote long storage and shelf life. Production of sweet cherries is highly seasonal and limited to relatively small areas. Even with good temperature management of the fruit in the distribution and marketing chain, storage and shelf life are short. No phase in the postharvest handling procedure to maintain quality is more critical than immediate cooling after harvest. Major reasons for cooling cherries immediately after harvest include:
- Reducing the heat of respiration
- Minimizing water loss from the fruit
- Retarding development of decay caused by fungi.
Heat of Respiration
Figure 1. Heat of respiration of apples and sweet cherries. (Adopted from USDA Handbook 66.)
Figure 2. Estimated temperature increases of packaged palletized cherries held in a cool room with negligible heat transfer between the fruit and room air.
For example, if the fruit is packaged at 35°F, the temperature will increase five degrees in a 5-day period. However, if the packaging temperature is 45°F, then the rise is 15 degrees. This substantial increase will aggravate problems of mold growth, moisture loss, and decay. Under longer holding periods, the self-heating accelerates rapidly. Figure 2 also illustrates the need for cooling the fruit to below 35°F prior to packaging or to immediately cool those cherries boxed at temperatures above 35°F. Shipping cherries at their lowest safe temperature ensures that their respiration rate will be at its lowest, so cherries arrive at their destination at a lower temperature, with the least amount of deterioration and a longer shelf life.
Cooling Prior to Packaging
Cooling of Packaged Cherries
Figure 3. Cooling curve with half-cooling periods shown.
The concept of half-cooling time refers to the amount of time required to remove one-half of the unwanted heat from the fruit. In the example in Figure 3, a total temperature drop of 30°F is desired (from 62°F down to 32°F). Half of the cooling, or a 15-degree drop, is achieved in the first half-cooling period, which requires 10 hours. Note that 10 hours is also required for the second half-cooling period, although only 7.5°F of the remaining 15-degree temperature drop is achieved. Half of the total heat is removed in the first half-cooling period, 75% is removed by the end of two periods, and 87.5% after three periods. The product is 97% cooled after five half-cooling periods, which in this example would be 50 hours after placement into the cooling room.
Cooling Methods for Packaged Cherries
- Room cooling, where cold air is discharged
horizontally by the evaporator fans at one end of the room,
allowing the cold air to flow over and down between rows of
palletized stacks of fruit.
- Forced air cooling, in a room as described above where cold air is forced through the space around boxes or through boxes containing the fruit.
This method is commonly used for reducing cherry temperatures after packaging. In the cooling room, fans direct the cold air from the evaporator coils across the room in a horizontal direction from one wall toward the opposite wall, across the top of the room. Then the air flows downward and back through the channels between the stacks and returns to the coils. Pallet loads with up to 11 layers of boxes are usually placed only one pallet high unless cool room space is limited. Ample air can flow around the pallets. However, no air can circulate through the pallet load if the boxes are stacked tightly together. Heat is transferred by conduction from the center of the pallet through the adjoining boxes to the outside surface of the pallet stack. Heat transfer by conduction from the inside of a pallet to the surface is too slow for effective cooling of the interior boxes. Some operators, expecting increased cooling, stack the boxes about 1/2 to 1-inch apart to provide air channels through the pallet loads. However, without forcing air through the spaces, operators will not gain significant reduction of cooling time. Figure 4, curve 1 depicts the slowness of room cooling of tightly stacked palletized boxes with an average half-cooling time of 100 hours for the inside boxes. For example, it would take 100 hours to cool the inside boxes of a pallet load from 50°F to 40°F and another 100 hours to cool them from 40° F to 35 ° F with a room air temperature of 30°F. This method is too slow for most packaging operators, unless the fruit is packaged well below 40°F, and several days of cooling time are available.
Figure 4. Cooling of cherries with various packaging and stacking configurations, using 30°F cooling air.
Forced Air Cooling
Using forced air cooling of palletized boxes greatly increases the speed of cooling over ordinary room cooling. In a forced air system, a pressure gradient is created across the two sides of the pallet load, which produces air flow through the air passages. A common way of obtaining a pressure gradient is to stack two rows of pallets about three feet apart across the room, starting from a wall. The top is then covered with plywood and a fan is placed at the front end of the stack (Figure 5). The end of the stacks must be set tightly against the wall and the pallets against each other to minimize air gaps. In operation, the fan pulls cold air through passages between stacked boxes into the interior space between the stacks and then out into the room at the fan end, assuring even air flow and even cooling of the boxes. Cooling rates depend on several factors, such as fan capacity, pressure gradient, air passages, channel depth, and temperature differential. Figure 4 illustrates these differences. Half-cooling rates for palletized loads is about 1 day (curve 2). When the boxes are stacked only one box wide, the half-cooling time is reduced to 10 hours (curve 3). If the boxes have an air space above the fruit through which cooling air can flow, the estimated half-cooling time is reduced to 7.5 hours (curve 4). In boxes without liners, having air flowing through the fruit, the half-cooling time is reduced to 5 hours (curve 5). But allowing air to flow through the fruit by eliminating the plastic box liners is not recommended for cherries, because this practice allows rapid drying of the stems and excess moisture loss from the fruit unless the cherries are waxed.
Figure 5. Diagrammatic view of a forced air cooling setup. Palletized boxes are placed to form a tunnel from which air is exhausted. The negative pressure causes the cold air to pass through the air passages between the staked boxes.
The increased effectiveness of forced air cooling methods allows a producer to package cherries at temperatures higher than when ordinary room cooling is used. For example, complete cooling of a pallet load packaged at 50°F would take about 2 days with forced air cooling in a 30°F room. If the boxes are stacked one box deep (an air flow channel one box deep), the 35°F temperature would be reached in 20 hours; given a vented air space between the top of the cherries and the box lid, a fruit temperature of 35°F would be reached in 15 hours (overnight). Without a liner, the cooling time would be only 10 hours.
Maintaining a high efficiency of the forced air cooling system requires close monitoring. Several factors need to be considered. They include: 1) static pressure and air volume, 2) air channel length, 3) uneven cooling in palletized loads, 4) air gaps between boxes, 5) air short-circuiting, and 6) temperatures of packed fruit and cooling air.
Static Pressure and Air Volume
In a correctly-designed and operated forced air system, both the static pressure and air volume are critical parameters to consider. A certain amount of air flow is needed to remove the heat and a certain amount of static pressure is needed to push the air through the air passages. Where the number and size of air passages is small, the amount of total air flow is low and cooling is slow. Generally, a forced air system is designed to deliver about 1 cubic foot of air per minute (cfm) per pound of fruit. This amount of air flow can be obtained with well-vented unlined palletized boxes, which offer little resistance to air flow. In palletized cherry boxes with polyethylene liners the air passage area is limited to the open space between the boxes unless special boxes with a 1-inch vented space below the box lid are used. With boxes that are filled to the lid the system capacity should be reduced to 0.5 cfm or less per pound of fruit. In this case, the cooling rate is reduced proportionally with air flow rate.
A static pressure of 0.25 inches of water is needed for most applications. Long channels offer more resistance to air flow than do short channels, reducing air volume. This can be compensated for by making the air channel areas larger. When air channels are long and spaced far apart (such as in palletized boxes), cooling takes place first where the cold air enters the air passages and along the walls of the passages. The last fruit to be cooled is that fruit farthest away from the air passage and near the air exit side. Thus, when monitoring the cooling progress, use the temperature of the warmest locations in the pallet.
Air that bypasses the fruit does little cooling. In any system the air takes the path of least resistance. Large air gaps (such as between pallet loads) must be avoided. These gaps allow too much air to bypass the fruit. For maximum efficiency in a forced air system, only that air which comes in contact with the fruit on the sides of boxes should be permitted to pass through the system.
Delivering the highest quality to the consumer requires good product temperature management from the field through packaging, shipping, and marketing. Effective handling and cooling methods during sorting and packaging are essential. These methods are:
- Harvesting during the cooler part of
- Removing field heat as soon as possible after
- Sorting and packaging cooled cherries as
rapidly as possible to reduce rapid warmup on the packing
- Packing cherries at temperatures between 35°F
and 40°F, which may require a second cooling operation
immediately before packaging
- Effectively using forced air cooling, which depends on how much air flows through and around the boxes.
Henry Waelti, WSU Extension Agricultural Engineer
Washington State University, Pullman, WA
Post Harvest Pomology Newsletter, Vol. 4, No. 1