Bill Wilcke, Extension Engineer, and Vance Morey, Professor and Head, University of Minnesota, Biosystems & Agricultural Engineering Department
We occasionally get questions from fruit and vegetable producers about a portable, forced-air cooler that was built by the University of Minnesota Biosystems and Agricultural Engineering in the 1980s. Here is some background information and a brief history of the cooler.
Most fruits and vegetables must be cooled as soon as possible after harvest to ensure maximum quality and shelf life. There are a variety of cooling methods that can be used, but forced-air cooling (blowing large quantities of refrigerated air around and through the harvested product) is probably most practical for many Minnesota growers. Forced-air cooling can be used with a wide variety of products and it is relatively simple and economical compared to some other cooling methods. Researchers in the University of Minnesota Departments of Horticultural Science (Luther Waters, who left Minnesota for Ohio State University) and Biosystems and Agricultural Engineering (Vance Morey) came up with the idea of developing a portable, forced-air cooler that could be easily moved from one farm to another, or from one field to another, to rapidly cool produce soon after harvest. They received funding in the mid 1980s from the Irrigator’s Association of Minnesota and the Minnesota Department of Agriculture to build and test a prototype portable cooler.
The cooler consisted of a large, insulated aluminum chamber with doors on each side that was mounted on wheels. It was about the size of a typical semi-trailer. The cooler included a high-capacity refrigeration system and a large fan that circulated air through bags or boxes of produce. A tractor-driven alternator supplied power for the refrigeration system and fans. Warm, freshly harvested produce was packed into bags, boxes, or flats that had openings for air movement on the top and bottom. The containers were stacked one to three layers high on specially designed pallets that were then rolled into the chamber. The chamber doors were closed, and the fans and refrigeration system were operated until temperature probes indicated that the product had reached the target temperature. The cooled product was then loaded into refrigerated trucks for transport to market or moved into cold storage rooms.
The cooler was tested on several different farms using products like strawberries, broccoli, cucumbers, peppers, cantaloupe, and sweet corn, from 1984 to 1986. The cooler worked quite well and provided reasonable half-cooling times (the amount of time required to cool a product halfway from its initial temperature to the temperature of the cooling air). Cooling time depended on the size of individual fruits or vegetables, the total amount of product loaded into the cooler at one time, and the amount of open area in the containers. Half-cooling times ranged from 13 minutes for strawberries to about 100 minutes for cantaloupe.
After completion of the tests, we attempted to sell the cooler to several different growers, but none of the arrangements worked out. In 1990, using grant funds from the Agricultural Utilization Research Institute (AURI), the cooler was refurbished and moved to the Northwest Research and Outreach Center at Crookston, Minnesota. Gary McVey, a faculty member on the Crookston campus of the University of Minnesota (now retired), demonstrated the cooler at the center and on several different farms in northwest Minnesota from 1990 to 1995. The cooler has been stored in Crookston since 1995.
As was mentioned at the beginning of this article, the cooler was intended to be a prototype. Since the cooler seemed to work quite well, why hasn’t it been replicated and why aren’t large numbers of growers using portable forced-air coolers? We’re not sure, but one reason might be that it is probably more practical to incorporate forced-air cooling into a cold storage room than it is to build a separate portable cooler. While portability provides some advantages, it also complicates produce handling and adds expense.
Another reason that portable coolers haven’t caught on might be because produce buyers do not always provide economic incentives for growers to pre-cool their products. Although it’s true that quality and shelf life are much better when products are cooled immediately after harvest, growers aren’t always rewarded for using pre-cooling, or penalized for not pre-cooling their products. It’s been demonstrated that forced-air pre-cooling works, but now we need to demonstrate that it also makes economic sense.
If you would like more information about the portable cooler, or about other pre-cooling methods, contact Bill Wilcke at wwilcke@extension.umn.edu or telephone (612) 625-8205.
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This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/