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Sullivan, G.H., L.R. Davenport, and J.W. Julian. 1996. Precooling: Key factor for assuring quality in new fresh market vegetable crops. p. 521-524. In: J. Janick (ed.), Progress in new crops. ASHS Press, Arlington, VA.

Precooling: Key Factor for Assuring Quality in New Fresh Market Vegetable Crops*

Glenn H. Sullivan, Lonni R. Davenport, and James W. Julian

  5. Table 1
  6. Fig. 1

Total domestic and export sales of fresh fruits and vegetables produced in the United States reached $65 billion in 1994-95. Marketplace demands for greater selection and consistent high quality, combined with consumers' rising nutritional expectations, have placed increasingly greater performance pressures on all segments the fresh produce marketing chain (How 1991). Producer success in the marketplace for fresh fruits and vegetables is highly correlated with maintaining postharvest quality and shelf-life consistent with current consumer demands (Sullivan et al. 1991). Precooling technologies that quickly remove the field heat and reduce internal respiratory processes were found to be critical in meeting these consumer demands for high quality fresh produce (USDA/ARS 1986). Forced-air and hydrocooling have been found to be the most efficient and economical in enhancing product quality for new muskmelon cultivars and sucrose enhanced bi-color sweet corn. Shelf-life was extended 7 to 14 days, and desired quality characteristics were maintained at near optimum levels.


In the past, the principal market for most fresh produce has been the traditional family. Today this traditional family unit accounts for only about 13% of all households in the United States. Individuals living alone now comprise almost 24% of all households, and single parent families account for nearly 22% of all households with children under 18 years of age. Today 40% of all shopping is done by men! Point of purchase selections in the fresh produce departments of retail grocery outlets increased from an average of 67 in 1985 to over 312 in 1995 (Hammel 1995). These trends signal major changes in the way consumers spend their incomes, and how they make their purchases.

Retail supermarkets are still the principal point of purchase for most fresh produce consumers, accounting for over two-thirds of all domestic sales (Fig. 1). But how that produce gets to the point of retail purchase has changed dramatically over the last few decades. Currently, almost 40% of all fresh produce sold through retail supermarkets comes directly from first-handlers in the major producing regions (grower-shippers, independent shippers, and value-added processors) through the retailers' own integrated wholesale distribution facilities (VanSickle 1985). Historically, central terminal markets in the major cities throughout the United States were the dominant assembly points for fresh produce. Today, only about 25% of the volume sold at retail comes through market wholesale institutions who buy fresh produce both from the terminal markets and shipping point markets (Hammel 1995). This shift in the way retail supermarkets source their fresh produce has resulted from the need to achieve greater control over the postharvest handling functions, and thereby better meet the changing consumer demands for higher quality and longer shelf-life at a competitive price. These trends place increasing pressure on the marketing chain to adopt postharvest technologies that help assure greater quality control (Sullivan et al. 1991).


When it comes to produce quality, every minute counts. Research confirms that lowering the respiration rate of fresh vegetables is essential to preserving market quality. The most important technology for lowering respiration rates remains proper precooling of produce within hours of harvest (Jones 1996). Proper precooling preserves product quality by: (1) inhibiting the growth of decay producing microorganisms, (2) restricting enzymatic and respiratory activity, (3) inhibiting water loss, and (4) reducing ethylene production (Hardenburg et al. 1986).

The most efficient methods of postharvest cooling fresh produce were found to include forced-air cooling, hydrocooling, and vacuum cooling (Table 1) (Junge et al. 1986). In new muskmelon cultivars and sucrose enhanced bi-color sweet corn, forced-air cooling and hydrocooling respectively proved to be the most effective and economical, preserving optimum quality and increasing market life 7 to 14 days depending on the product. Muskmelon cultivars were preserved at the highest levels of market quality when the internal pulp temperatures (average 95°F/35°C) were reduced to 42°F/5.5°C within 4 h of harvest using forced-air cooling technologies. Sweet corn quality was preserved at optimum levels when harvested at field temperatures below 75°F/24°C and cooled to 32°F/0°>C pulp temperatures using hydrocooling technologies (Sullivan and Davenport 1995).


Research that focused on the market factors influencing commercial success clearly indicated that vegetable producers must increasingly commit themselves to meeting the consumers' demand for higher produce quality. Retail buyers in today's competitive marketplace are increasing their purchases through direct-from-shipping point deliveries, but only from grower-shippers who provide the value-added services demanded by increasingly more sophisticated consumers (Sullivan and Davenport 1991). Product precooling was found to rank as the most essential of these value-added marketing services.

Given current consumer lifestyles and demand for variety selection in the produce section of retail supermarkets, new vegetable cultivars represent a unique opportunity for grower entry into the marketplace. However, research findings clearly confirm that maintaining an economically viable market position at retail requires new crop producers to focus increasingly greater attention on the product quality demands of an increasingly more dietary diverse consumer population. Precooling is among the most cost-effective and efficient quality enhancements available to commercial new crop producers.


*Journal Paper No. 15010 Purdue Univ. Agr. Expt. St., West Lafayette, IN 47907-1165. This research was supported in part by grants from the Indiana Business Modernization and Technology Corporation, Indianapolis, Indiana.
Table 1. Postharvest cooling methods and suitable commodities.

Cooling method Commodities Comments
Hydrocooling Most leafy vegetables, fruits and fruit-type vegetables, sweet corn, snap beans Very fast cooling; uniform cooling in bulk if properly used, but may vary extensively in packed shipping containers; daily cleaning and sanitation measures essential; product must tolerate wetting; need water-tolerant shipping containers
Forced-air cooling (pressure cooling) Most fruits, berries, fruit-type vegetables, tubers, and vegetables not susceptible to chilling injury Much faster than room cooling; cooling rates very uniform if properly used. Container venting and stacking requirements are critical to effective cooling. Economical and efficient.
Package-icing Most vegetables Fast cooling; limited to commodities that can tolerate water-ice contact; water-tolerant shipping containers are essential. Economical and efficient.
Room cooling All commodities Too slow for many perishable commodities. Cooling rates vary extensively within loads, pallets, and containers.
Vacuum cooling Leafy vegetables, iceberg lettuce Commodities must have a favorable surface-to-mass ratio for effective cooling. Causes about 1% weight loss for each 6°C cooled. A procedure that adds water during cooling prevents this weight loss, but equipment is more expensive, and water-tolerant shipping containers are needed.
Transit cooling
Mechanical refrigeration All commodities Cooling in most available equipment is too slow and variable; generally not effective for field heat removal.
Top-icing and channel-icing Most vegetables Slow and irregular, top-ice weight reduced net pay load; water-tolerant shipping containers needed.

Fig. 1. United States fresh produce marketing system.

Last update August 24, 1997 aw