Room cooling involves placing the items to be cooled in a refrigerated room. Room cooling may be the most simple cooling method, the least expensive to build and operate, and the most effective when properly used. The refrigeration system cools the air inside the room, which in turn cools the produce by natural convection and conduction. Home refrigerators and the small “reach-in” coolers found in retail food stores are really just small room coolers. If the items or packages of produce to be cooled are not particularly perishable, already partially cooled (precooled), physically small, and have a relatively low respiration rate and adequate space between them, natural convection may cool the item fast enough to be effective. In some situations, such as small walk-in coolers, the fans on the refrigeration coils may provide sufficient air movement to create some forced convection and more rapid cooling, although this is not normally the case.

Some room coolers cool warm produce, although most are very common in produce packing facilities where they temporarily store precooled produce before shipment. Postharvest room coolers can vary in size from small walk-in coolers that are common in roadside stands to large rooms with pallet racks and roll-up doors for fork truck and truck access. Figure 3e-1 shows the interior of a large room cooler with pallet racks and mixed lots of produce in a wholesale warehouse and distribution center. The pallet racks increase room capacity and open access to every item. In contrast, small tightly packed rooms of mixed produce are a nuisance because of the “first-in-last-out” reality.

For active cooling and long-term cold storage, room coolers are used primarily for crops that have been harvested during the cool season and have a relatively low respiration rate. Examples are cabbage, onions, collards, late apples, and root crops. Room cooling alone is usually inadequate for packaged produce on pallets or in bulk containers at summer field heat temperatures that require quick cooling.

Tests have shown that the center of a 1000 lb pallet bin of field hot blueberries (90°F+) packed in clamshells in 12 pint flats actually increased in temperature a few degrees from the respiration heat when placed in a 36°F refrigerated room with no air movement. This was caused by the lack of heat exchange in the hot produce at the center of the mass. The center began to cool by conduction after a few hours, but the cooling was slow and required more than 24 hours to decrease to 45°F. During that time, the interior quality of the fruits had declined to the point that the produce was rejected by the buyers. If left in the room long enough, the produce would eventually approach the room temperature. In many cases, the produce will spoil before it cools adequately. The temperature of the outside containers of a pallet or bin can have a very different reading from the inside.

Room cooling is the most versatile of all methods because it may be used with almost all produce items in all types of containers. However, room cooling is generally not considered when large amounts of warm produce must be cooled rapidly. With the slow active cooling rate of conduction and natural convection, the refrigeration load per unit time of room cooling is often small when compared to other cooling methods. This makes it less expensive to build and operate because it may require only a modest refrigeration system sized for slow cooling and for offsetting conduction losses through the walls, floor and ceiling, as well as infiltration through open doors.

Room cooling is sometimes suitable for complementary cooling, such as keeping produce cool after it has been precooled by other means. This is not advisable because it involves moving cooled produce from one room to another.

Figure 3e-1. Interior of room cooler with mixed lots of produce awaiting shipment.

Source: M. Boyette.

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Figure 3e-1. Interior of room cooler with mixed lots of produce awaiting shipment.

Source: M. Boyette.

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In postharvest cooling facilities that are beginning with a very limited budget, building a cooling room can be a good first step. The room will provide some measure of cooling and may later be converted to a forced-air cooling facility through the addition of fans and greater refrigeration capacity.

Roadside stands, local farmers markets, and small produce retailers often build a refrigerated room for keeping fresh produce. A refrigerated room that is entered many times per day for putting in or taking out produce loses much cooling capacity through the open door. This “service load” represents wasted refrigeration capacity. Service load can be as much as 50% of the total. One way to reduce this waste is to install plastic strip curtains on the door.