A trip to the market is usually an effortless experience, with a majority of the shopping-list items readily available. Whether we visit the neighborhood supermarket, hypermarket (big-box store) or a boutique market, more often than not, our needs and wants are met. There seem to be an endless supply of products.

Due to this abundance, most consumers have no idea what is required to get fresh produce from the farm to the fork. Grower expertise, experience, commitment and hard work converge to achieve the delicate balance of high quality, nutrient-dense and aesthetically pleasing fruits and vegetables.

Perhaps the most significant challenge a grower faces is keeping food fresh and edible during processing, storage and transport. Removing field heat from freshly harvested produce is an essential element in post-harvest handling. Field heat is the difference between the post-harvest temperature of the crop and the desired temperature, which is usually around 34°F (1°C), depending on the commodity.

Field heat accelerates decay: Once harvested, if produce temperature is not lowered immediately, field heat will accelerate softening, wilting and loss of water and weight. Deterioration can begin within one hour of harvest. Four hours after harvest, there is a 50 percent increase in water loss, and the aesthetic appeal of the product is affected irreversibly. At six hours post-harvest, produce will experience deterioration of more than 25 percent.

Field heat must be removed prior to storage and transport. Although refrigerated vehicles are utilized, they usually are designed to maintain temperature — not remove heat.

woman at a computer

RF technology can help growers confirm that their product has been cooled to the target temperature prior to storage or transport.

Post-harvest Handling Impacts Profits and Food Security

Harvest is the start of a race against the clock to retain as much product as possible through proper post-harvest handling. The Global Panel on Agriculture and Food Systems for Nutrition maintains that, “At any stage in the value chain, foods contaminated with pathogenic microorganisms such as E. coli or aflatoxins produced by toxigenic (molds) must be withdrawn or discarded to avert food safety hazards.”

Every grower, therefore, is challenged with finding the effective ways to slow ripening (decrease ethylene production) while minimizing spoilage and microorganism growth. An obvious result of this effort is an extension of shelf life, which translates to increased grower profits. Food loss — whether it occurs during harvest, post-harvest, production, storage and transportation of the produce — significantly affects grower profits. At the grower and processor levels, approximately 10 million tons of food are lost each year. Since 1950, food loss/waste in the United States has tripled, rising to 40 percent. Globally, approximately one-third of all food — and in most regions, half of all fruits and vegetables produced — are lost and wasted each year. Food also is wasted at the retail and consumer level due to regulation and other issues. With approximately 795 million people worldwide experiencing food insecurity, minimizing food waste has never been more important.

Grapes

Grapes should be precooled within four to six hours of harvest and brought down to a temperature below 39°F (4°C). Delays in precooling grapes have been shown to create a 15 percent loss in crop value.

Precool Methods

The first step in preserving produce is eliminating field heat through precooling. The precooling methods utilized most often are shade, room, forced air, ice, hydro and vacuum. The method used will depend on the commodity. For example, room cooling is appropriate for all fruits and vegetables while ice cooling is best for roots, stem and Brussels sprouts. Shade cooling is the most cost effective and easiest to perform. Compared to other methods, however, shade cooling involves a considerable amount of time to lower produce to the desired temperature. Conversely, vacuum cooling is a rapid evaporation process used traditionally for leafy vegetables and mushrooms. This process is quick and enhances both shelf life and quality.

Forced-air cooling — perhaps the most common precooling method —is appropriate for fruits, fruit-type vegetables such as tomatoes and pumpkins, tubers (potatoes, yams, jicama, etc.) and cauliflower. With forced-air cooling, pallets of newly harvested produce that has been placed in ventilated boxes are stacked in two columns with a fan positioned at one end. A tarp then is pulled over the top of the two columns and extended to cover the far end of the columns, creating a vacuum. Usually, this is usually done using a cold/refrigerated room (sometimes called a tunnel). The fan draws the colder air from the room and forces it through the ventilated boxes of produce.

Forced-air cooling

Forced-air cooling, perhaps the most common precooling method, is appropriate for fruits, fruit-type vegetables such as tomatoes and pumpkins, tubers (potatoes, yams and jicama) and cauliflower.

Maximizing Throughput

Once precooling has begun, the product temperature must be monitored closely to determine when the ideal temperature has been reached. The time it takes to precool a crop depends on several things, including weather, the temperature of the crop at harvest, and the rate of heat transfer. Knowing precisely when the temperature target has been achieved allows growers to swap the cooled product with newly harvested product. This allows for a steady flow of harvested produce through the precool area.

Traditionally, growers have monitored precooling temperature by using a handheld thermometer with a probe, manually penetrating the pulp. These measurements typically are performed at 30-min intervals until the desired temperature has been reached. Measuring precool temperature in this way can raise some concerns:

  • Precooling can take multiple hours, and most growers estimate the time needed for cooling. This can result in misuse of money, energy and labor.
  • Cooling times vary depending on produce type. Some produce, known as “chilling-sensitive” crops, excessive cooling can lead to browning, wilting or shortened shelf life.[1]
  • Cooling times vary depending on pallet location.
  • Packaging may affect precooling times.
  • Produce can be damaged when manually pulped to obtain temperature.
  • Manual temperature monitoring requires opening the precooling room multiple times, which can negatively impact cooling time.

Temperature monitoring technologies can help mitigate these concerns. For instance, wireless radio frequency (RF) dataloggers can be used to reduce food loss and waste in the cold chain from the point of harvest. During precooling, for example, the datalogger derives the produce’s internal produce temperature using a mathematical calculation that renders a customized product emulation coefficient. This coefficient provides the exact pulp temperature, eliminating the need to probe the actual product. (Probes may be used with RF datalogging if preferred.) Whether one uses internal temperature modeling or probes, radio frequency technology provides automated tracking and reporting of temperature data, without the need to enter the precool room.

During forced air precooling, loggers can be positioned to simultaneously record produce temperatures at various pallet locations, providing real-time data on different cooling rates. Because it is impractical to monitor the temperature of every box on a pallet, mapping is used to extrapolate the temperature of the boxes that are unmonitored. The temperature data can be captured in real time. When the optimal temperature is reached, an alert notifies personnel so the pallets can be removed from precooling and replaced with a newly harvested crop.

crates full of tomatoes

Perhaps the most significant challenge a grower faces is keeping food fresh and edible during processing, storage and transport. Removing field heat from freshly harvested produce is an essential element in post-harvest handling.

Table Grape Study using Radio Frequency Technology

To demonstrate the effectiveness of radio frequency technology in precooling, a study was performed on a crop of table grapes. Grapes should be precooled within four to six hours of harvest and brought to a temperature below 39°F (4°C). Delays in precooling grapes have been shown to create a 15 percent loss in crop value.

To determine cooling times, four tests were performed over a two-day period. During all tests, product and ambient temperature were measured, and target product temperature was 31.5°F (-0.3°C).

Probes were placed on the interior and exterior of 70 pallets to be monitored. The data from each logger was recorded every five minutes and transmitted to the main receiver. The average starting and ending temperatures were 73.07°F (22.82°C) and 31.43°F (-0.3°C), respectively. The average temperature decrease was 41.64°F (23.13°C). The average time to reach 31.5°F (-0.3°C) was 12.06 hours, and each test averaged 16.25 hours.

Probes were placed at different points on the pallets to allow for variations in cooling times due to location. Cooling times based on location are shown below:

  • Interior, top: 12.51 hours to cool.
  • Exterior, top: 10.11 hours to cool.
  • Interior, bottom: 15.51 hours to cool.
  • Exterior, bottom: 8.87 hours to cool

To maximize precooling, the cold room temperature should be maintained at several degrees colder than the target produce temperature.

Obtaining this kind of data is important because it enables growers to streamline precooling of their crops, forecast sales and identify factors that could affect temperature. The ability to automate temperature monitoring during the precooling process can saves time, energy and labor. PC


Reference

“Precooling Colorado Crops,” Colorado Farm to Table, Colorado State University Extension.