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The term precooling refers to the removal of heat energy from crops, known as field heat, after they are harvested. Precooling is ideally done immediately after harvest, as many crops begin to degrade rapidly after harvest if kept at high temperatures.
Precooling and temperature controlled storage can:
For highly perishable products, such as strawberries, deterioration at high temperatures occurs quickly. For example, every hour delay in precooling strawberries harvested at 86°F will result in a 10% loss in shelf life (Brosnan 2001). Conversion of sugar to starch in sweet corn occurs 4 times more rapidly at 50°F than at 32°F (Boyette 1990).
Strawberries stored for 5 days at 32°F (left) next to strawberries stored for 5 days at 50°F (right). Courtesy Sensitech™ (2013)
|Table 1: Cooling temperatures which may result in chilling injury|
|Chilling injury will occur below these temperatures|
Not necessarily. It is important to determine the ideal temperature of the crop you are harvesting and ensure it is not cooled below the optimal temperature. Crops that are damaged by low temperatures can exhibit browning, texture defects, short shelf life, incomplete or failure to ripen, or changes to the flavor and color. These crops are referred to as 'chilling sensitive' crops.
The crops listed in Table 2 will best maintain their quality at temperatures just above freezing. Cooling to temperatures below 41°F, and as close to 32°F as possible, will increase their shelf life by days, and in some cases, even weeks.
Note: The range of shelf life listed below for each product can be attributed to the fact that the longest shelf life is obtained at 32°F, and is incrementally shortened as the temperature nears 41°F. Temperatures above 41°F lead to rapid quality loss.
|Table 2: Shelf life at refrigerated temperatures|
|Commodity||Shelf Life* if Stored between 32°F & 41°F|
|Cabbage||20 days to 3 months|
|Lettuce||14 to 28 days|
|Peaches||30°F to 36°F: 1 to 5 weeks, or greater than 45°F. Do not store between 36°F and 45°F if the cultivar is susceptible to internal breakdown.|
|Mushrooms||2 to 7 days|
|Sweet Corn||5 to 8 days|
|Strawberries||About 1 week|
Percent of food loss in the United States in 2008 attributed to fruits (9%) and vegetables (17%). Improper postharvest handling contributes greatly to these percentages.
Estimated amount of time to break even when investing in a mobile forced air cooling unit (Talbot 2002).
Produce is often harvested at high environmental temperatures, sometimes in excess of 80°F. During the time period between harvest and precooling, it has been shown that spoilage and pathogenic microorganisms can multiply rapidly under certain conditions. Precooling can slow the growth of harmful microorganisms if present and allow for a longer shelf life.
Precooling produce can be achieved by utilizing one or more of the common methods below.
Additional methods to precool include vacuum cooling and icing but are not outlined in this fact sheet.
|Top View - Room Cooling|
(Kitinoja 2002, Boyette 1991)
Room cooling utilizes a standard cold room of any size. These walk-in cold rooms are designed to maintain the temperature of produce, but not necessarily to rapidly cool it. There are several factors to take into account when determining how long the produce will need to be in the room cooler prior to reaching the ideal temperature. Factors under the farmer's control are: initial produce temperature, which can be controlled to a degree by harvesting in the cooler hours of the day, packaging (ventilation and size), and amount harvested at one time.
Forced air coolers draw air through packaging resulting in rapid flow of cold air over produce. This increased airflow results in fast removal of heat from produce. Forced air cooling is very versatile and can be used for almost any produce item.
|Forced Air Cooling Schematic|
Contact with water will increase the rate of spoilage of many produce items and can be affected by pH of the water. Many Colorado crops can contact water without losing quality. Asparagus, broccoli, beets, carrots, cauliflower, celery, cucumbers, cabbage, eggplants, leeks, peas, rhubarb, spinach, summer squash, sweet corn, chard, and cantaloupes are good candidates for hydrocooling.
Many small and medium sized growers will use a batch hydrocooler in which produce is cooled one batch at a time. Cold water is used to remove heat from the produce.
(Suslow 2001, Ritenour 2002)
Anytime water is utilized in a postharvest process, understanding and managing the quality of water used is critical to the safety of the product. Postharvest water is the single most critical point which can amplify a problem, and spread contamination to entire lots of produce. For all postharvest processes that involve water, the water must start out as being potable, or the equivalent of drinking water. Sanitizers also should be added to the water to prevent the spread of harmful microorganisms. Sanitizers are not added to 'wash' the produce or kill all pathogens, but rather to prevent the spread of microorganisms.
There are many options to sanitize postharvest water including, but not limited to, the addition of chlorine, peroxyacetic acid, and ozone. Chlorine is typically the cheapest and easiest sanitizer to use for postharvest purposes. When managing chlorine levels in the water, pH and turbidity (the amount of solids, leaves, and other organic matter) are two critical areas to monitor to ensure the sanitizer will work properly. A pH between 6 and 7 is recommended for chlorine use. Knowing when to change water in hydrocooling or bulk cooling tanks will also ensure chlorine is not being bound up by organic matter. Be sure to measure and record free chlorine, not total chlorine.
For more resources on sanitizing postharvest water, click here for the resources.
Poor temperature management throughout the supply chain, from precooling to market, can result in large losses of value.
In addition to maintaining the value of your crops, many farmers who have invested in pre-cooling methods can choose to charge other farmers to cool produce:
Harvesting produce during the coolest hours of the day, and utilizing shade for harvested produce awaiting transport from the field, are two very effective methods to extend the shelf life and maintain the quality of your produce (Kitinoja 2010).
|Many farmers are taking advantage of a relatively inexpensive and easy to install technology call 'CoolBot™'. This technology allows you to convert a window air conditioning unit into a walk in cooler or small refrigerated trailer.|
The CoolBotTM controller costs $299 (2013). Additionally, you'll need to purchase the rest of the materials to build your walk-in room cooler or mobile cooler. Estimates to build small room or mobile coolers range from $2,000 to $4,000, depending on if you are using new or used materials (Perkins-Veazie 2013).
'Traditional' cooling systems may be seen as too expensive for many smaller growers. There are many resources available that outline different types of solutions to fit the needs of your operation.
Dr. Penelope Perkins-Veazie of North Carolina State University provides detailed instructions on how to construct a small refrigerated trailer. You can find these instructions in the DIY Postharvest Equipment section of the NC State Extension Website: http://ncfreshproducesafety.ncsu.edu/good-agricultural-practices/postharvest-handling/diy-postharvest-equipment
A mobile cooler utilizing the principles of forced air outlined on page 3 of this fact sheet was designed by the USDA several years ago. It is slightly more expensive to build than the unit above due to the additional cooling capacity necessary for forced air cooling. The advantage is that it can rapidly cool highly perishable products.
Fixed costs and return on investment of the USDA Portacooler is outlined by Talbot and Fletcher (2002).
Plans for construction of the Portacooler are available here: http://www.bre.umd.edu/portacooler3.htm
A common mistake in many produce operations is to over pack transportation vehicles. In order to make the most of your investment in refrigerated equipment, you'll need to use it properly. Be sure to leave adequate space for airflow between the walls and the produce, and do not over pack the trailer.
Commodity specific postharvest handling information:
Small-scale refrigerated trailer:
Boyette, M.D. (1990). "Postharvest Cooling and Handling of Sweet Corn." AG - North Carolina Agricultural Extension Service, North Carolina State University. AG-413-4.
Boyette, M. D. (1991). "Design of room cooling facilities: structural & energy requirements." AG - North Carolina Agricultural Extension Service, North Carolina State University. AG-414-2.
Brosnan, T. S., D. W. (2001). "Precooling techniques and applications for horticultural products - a review." International Journal of Refrigeration-Revue Internationale Du Froid 24(2): 154-170.
Kader, A. A., Ed. (2002). Postharvest Technology of Horticultural Crops, University of California.
Kader, A. A., Rolle, Rosa S. (2004). The role of post-harvest manaagement in assuring the quality and safety of horticultural produce, Food and Agriculture Organization of the United Nations (FAO).
Kitinoja, L., Kader, Adel A. (2002). Small-Scale Postharvest Handling Practices: A Manual for Horticultural Crops (4th Edition). Davis, CA, Postharvest Technology and Research and Information Center.
Kitinoja, L., Thompson, James F. (2010). "Pre-cooling systems for small-scale producers." Stewart Postharvest Review 2(2).
Perkins-Veazie, P. (2013). "Pack'n Cool Construction Summary." Retrieved October, 2012, from http://plantsforhumanhealth.ncsu.edu/2012/08/20/pack-n-cool/.
Ritenour, M., Sargent, SA, Bartz, JA (2002) "Chlorine Use in Produce Packing Lines." HS-761. Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Suslow, T. (2000). Chlorination in the Production and Postharvest Handling of Fresh Fruits and Vegetables. Fruit and Vegetable Processing. Davis, CA, University of California, Davis: 2-15.
Suslow, T. (2001). "Water Disinfection. A Practical Approach to Calculating Dose Values for Preharvest and Postharvest Applications." Publication7256. University of California Agriculture and Natural Resources.
Talbot, M. T., Fletcher, J.H. (2002) "A Portable Demonstration Forced Air Cooler." CIR1166, 12.
Thompson, A. K. (2003). Fruit and Vegetables: Harvesting, Handling, and Storage, Blackwell Publishing Ltd., Garsington Rd, Oxford.
Thompson, J. F., Mitchell, F. Gordon, Rumsey, Tom R., Kasmire, Robert F. Crisosto, Carlos H. (2002). Cooling Horticultural Commodities. Postharvest Technology of Horticultural Crops. A. A. Kader, University of California, Davis: 97-112.
Thompson, J. F., Mitchell, F. Gordon, Rumsey, Tom R., Kasmire, Robert F. Crisosto, Carlos H. (2002). Pre-cooling and Storage Facilities. USDA Handbook 66 - The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks J. F. Thompson.
Author: Ryan Friedman, MS. Special thanks to Marisa Bunning, Frank Stonaker, Larry Goodridge, Martha Stone, Gretchen Wall, and the entire faculty in Colorado State University's Food Science and Human Nutrition Department.