Keeping Cool
The subject of this article is some alternative cooling approaches that may be applicable to smaller vegetable handling operations: mobile cooling units and cooling tunnels. Several companies manufacture mobile precoolers, from hydrocoolers on skids, to refrigeration packages and air handlers installed in trailers for pressure cooling, to trailers and containers that can cool the load during transport.
Cooling Alternatives
The Carrier Transicold SmartAir system (http://smartair.us/) is primarily designed as a mobile pressure-ripening room, but the pressurized air flow design of these trailers allows the product to be cooled as well if necessary — at least to ripening-conducive temperatures. Researchers at the University of Florida Center for Food Distribution and Retailing (cfdr.ifas.ufl.edu) have recently helped Thermo King (www.thermoking.com) convert its standard Magnum marine container into a forced air precooler capable of 7â„8ths cooling 1.6 tons of produce in about two hours. Several of these units are being used commercially to cool fruits and vegetables in India, where refrigeration of produce is almost unknown.
Mobile Forced Air Cooling Services (www.coolforce.com) and Millennium Cooling (www.millenniumcooling.com) offer forced-air cooling units that can be used to cool loaded trailers and containers. These are mobile refrigeration and air handling systems with ports for docking two or three trailers or containers. The Mobile Forced Air “Cool Force” system is notable for using a “wet coil” system in which an ammonia-based dry coil refrigeration system is used to chill water to 36°F. The cooling air is blown through a “curtain” of the chilled water for cooling and humidification and back into the trailer.
Packaged Refrigeration Systems (http://prsmfg.com/) and 4 Star Ag Refrigeration (www.agrefrigeration.com) make several types of portable forced-air cooling units on skids that can be moved around to where they are needed. Mobile Forced Air also offers a 16-pallet capacity mobile forced-air cooling room. Millennium and Packaged Refrigeration also manufacture portable shower hydrocoolers.
Cooling Tunnels
An important axiom in cooling vegetables is that the better the contact between the product and the cooling medium, the more efficient the cooling. That’s why pressure cooling is faster than room cooling, and why hydrocoolers designed to cool individual vegetables are much more efficient than carton- or pallet-scale units. Cooling tunnels take that concept and apply it to cool individual vegetables as they move along a conveyor while being blasted with cold air.
Cooling tunnels are used extensively for cooling and freezing processed foods, but rarely for cooling fresh vegetables. However, used equipment is widely available and can be modified for small-scale cooling of fresh vegetables. Packaged Refrigeration makes a couple of different larger forced-air cooling tunnels for boxed or bulk produce. The mobile unit has an inflating air bag on one side to allow larger air volume and uses six, 38,000 cfm (cubic feet per minute) fans supplying chilled air at 2½ inches of static pressure. Pandura Farms in Australia has recently patented a cooling tunnel design specifically for fresh produce and uses it to cool table grapes.
The newest technology to discuss is the Turatti (www.turatti.com) Polar Wind cooling tunnel. Developed in a joint effort with compressed gas supplier Air Liquide, this system uses liquid nitrogen to chill vegetables to as low as 34°F to 36°F as they move along a short enclosed conveyor. This system combines the advantages of intimate cooling medium contact with the product and heat removal due to the phase change from liquid nitrogen to nitrogen gas.
At this point used primarily by the fresh-cut industry in Europe, these units have been shown to be capable of cooling about 660 pounds of salad per hour with nitrogen consumption of approximately 0.25 pounds per pound of product cooled to 37.4°F with a cooling time of about 105 seconds. It is currently being operated or tested by some handlers in North America for different types of vegetables.
No matter how large or small the produce operation, proper temperature management is critical for maintaining the best possible product quality. We hope that some of these alternative postharvest cooling technologies will help smaller companies take that important first step of cooling their product to its optimum storage temperature as quickly as possible after harvest.
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