What Is Hydrocooling – Learn About Hydrocooling Vegetables And Fruits

By: Amy Grant

When the temperature gets near the triple digits and you’re cooling off with a chilled watermelon wedge, you should thank the hydrocooling method. What is hydrocooling? The method of hydrocooling is used to quickly chill produce post-harvest so it can get to your dinner table. Read on to learn more.

What is Hydrocooling?

Very simply, the method of hydrocooling is a way to rapidlycool produce by running near freezing water over fruits and vegetablesimmediately after they are harvested. Without hydrocooling vegetables andfruits once they have been harvested, the quality of the produce begins todecline, hence its shelf life. So how does hydrocooling work exactly?

How Does Hydrocooling Work?

Temperature and relative humidity begin to affect thequality of produce immediately after being harvested. Heat may arise from fieldtemperatures or from natural respiration. Some farmers harvest at night tocombat field temperatures, but what about natural respiration?

Once produce is harvested, it is still alive and reacts tooxygen to form carbon dioxide, water, and heat which begins the process ofbreaking the produce down. This is called natural respiration. Harvesting atnight does nothing to stall natural respiration, which is where thehydrocooling method comes in.

With hydrocooling, you are rapidly running chilled waterover the freshly picked fruit and veggies, quickly dropping their temperatureand eliminating tissue damage, thus extending shelf life. Water may be eithercooled with ice, a refrigeration system, or a hydrocooling system specificallyfor hydrocooling produce.

During the process, the water is sanitized with one of avariety of products. Hydrocooling is used to lower temperatures quickly butcannot be used solely to cool and store produce. Instead, it is often used inconjunction with forced air cooling or room cooling.

While there are a number of fruits and veggies that respondwell to the hydrocooling method, here are some of the more common ones:

  • Artichokes
  • Asparagus
  • Avocados
  • Green Beans
  • Beets
  • Broccoli
  • Brussels Sprouts
  • Cantaloupes
  • Carrots
  • Celery
  • Cherries
  • Endive
  • Greens
  • Kale
  • Leeks
  • Lettuce
  • Nectarines
  • Parsley
  • Peaches
  • Radishes
  • Spinach
  • Sweet Corn
  • Turnips
  • Watercress
  • Watermelon

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TRJ offers a hydrocooler that is custom designed to meet your specific needs including integrated conveyors, booth, pumps and refrigeration systems. TRJ hydrocoolers are one of many precooling systems that provide fast, reliable and efficient means of cooling many water tolerant fruits and vegetables such as sweet corn and broccoli.

Booth Style Hydrocooler
to Cool Sweet Corn

Hydrocooling Systems are suitable for precooling:

Artichokes, Asparagus, Avocados, Green Beans, Beets, Broccoli, Brussels Sprouts, Cantaloupes, Carrots, Celery, Cherries, Endive, Greens, Kale, Leeks, Nectarines, Parsley, Peaches, Radishes, Romaine Lettuce, Spinach, Sweet Corn, Turnips, Watercress and more.

Our Belt Style Hydrocoolers are
Ready for Shipment


  • Extends shelf life by precooling vegetables and fruits fast
  • Cools crops up to 15 times faster than air
  • Hydrates some products, reduces moisture loss
  • Rinses while it cools
  • Flexible means of cooling vegetables and fruits packed in bins, bulk, or palletized
  • Relative low cost and little maintenance

Hydrocooling rates of Barhee dates at the Khalal stage

The objective of the current study was to determine cooling rates for Barhee dates at the Khalal stage using a batch type hydrocooling technique. Crushed ice mixed with water was circulated within an insulated tank using a 0.5-kW water pump. The temperatures of the ice–water mixture and the dates were monitored and recorded at equal time intervals (1 min). The cooling curves are presented and the half-cooling and seven-eighths cooling times were calculated using linear regression equations fitted for the different size of dates and different initial temperature of the product. The seven-eighths cooling times ranged from 6.18 to 10.23 min. Small sized fruit cooled faster compared to larger ones. However, the difference in size only explained a relatively small fraction of the differences in calculated cooling times. The hydrocooling process with an ice–water mixture was found to be a very effective method to pre-cool dates in order to extend their shelf-life and maintain their quality during distribution.

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MSU Extension Food Preservation

Knowing how to properly handle and store produce can make for a more nutritious and tasteful experience as well as decreasing your food waste and making your produce dollars go farther.

Physiologically immature (top) and mature (bottom) pears.

“A little knowledge goes a long way” is a common saying. According to University of Vermont’s Meredith Niles, the average person in the U.S. wastes approximately a pound of food a day. That is roughly 1,150 pounds a year for an average U.S. family. This is food purchased and brought home that you were not able to eat before it spoiled. Therefore, if knowing more about proper storage of fresh produce reduces any of this loss, it is worth gaining that knowledge.

The first thing to know is that not all produce like to be stored at the same temperature. Each produce item has an ideal temperature or temperature range that is best for maintaining quality. Higher temperatures cause them to mature or spoil quicker. Lower temperatures will cause “chilling injury,” and if low enough (below 32 degrees Fahrenheit) they may freeze. For some produce, chilling injury can happen at what might be considered not that cold. Chilling injury shows itself as off color (bananas skin turns brown), or the flavor goes bland (tomatoes). Items that have frozen will look like they have been cooked and will have what is called a “water soaked” appearance.

Table 1 shows common storage conditions required for common fruit and vegetables. Most in-home refrigerators are set at 35 to 40 F, a range covering most of what is on the list, but some things do best at higher temperatures. As shown in Table 1, tomatoes, potatoes, bananas, squash, pumpkins, etc. store longer at temperatures above 50 F. Some high-end refrigerators now have a special compartment for storage of these higher temperature items.

Fruit and vegetables are categorized as climacteric or non-climacteric. Climacteric fruit reach a certain developmental stage and once attaining that stage, continue to develop to full physiological maturity, even when removed from the plant. Climacteric fruit such as peaches, plums, cantaloupe, bananas, pears and tomatoes continue to gain flavor and get sweeter by changing starch into sugar. Many also go from firm to soft and juicy (peach and plum), or at least softer (avocado and cantaloupe). They are also sensitive to ethylene gas, which they self-generate, further aiding the ripening process.

Physiologically immature (top) and mature (bottom) bananas.

Climacteric fruit can be left at room temperature until consumed or mature and then refrigerated. Since many climacteric fruit soften as they ripen, to minimize bruising during shipment and extend shelf life, they are harvested firm and physiologically immature. If not left to fully mature, this results in crunchy peaches, nectarines and plums green, starchy bananas and bland tasting cantaloupe. Many consumers are used to this condition and taste and think that is the way it is supposed to be. However, if left at room temperature for a few days, the fruit would develop more flavor and sweetness. Peaches, nectarines and plums reach optimum flavor when soft and juicy, bananas reach optimum when they are yellow and have brown speckling on the skin and cantaloupe when the fruit is somewhat soft.

Non-climacteric fruit are fruit that need to stay on the plant to reach full physiological maturity. Once removed, they will not continue to mature, gain flavor or sugar. Their highest eating quality is at harvest. Some examples of non-climacteric fruits, as shown in Table 2, are cherries, grapes, oranges and raspberries. For food safety, it is important that once all whole fruits and vegetables are cut-up, they need to be eaten or refrigerated within 2 hours.

Table 2 is a list of common climacteric and non-climacteric fruit. All climacteric fruit in Table 2 are labeled as ethylene sensitive in Table 1. Table 1 also has some fruit and non-fruit items as ethylene sensitive. As a gas, ethylene is hard to contain and many things respond in negative ways if stored with ethylene generating fruit. Cabbage and Brussels sprouts will turn yellow or crack and start to grow, carrots develop a bitter flavor due to ethylene-induced isocoumarin production and cucumbers deteriorate quicker. Other ethylene-sensitive items respond in different ways.

Knowing how to properly handle and store produce can make for a more nutritious and tasteful experience as well as decreasing your food waste and making your produce dollars go farther.

Table 1. Optimum storage conditions for common fruits and vegetables.

Temperature (F)

Hydrocooling Analysis for Fresh Produce Safety

Worried about bacteria in your tomatoes? Research presented at the COMSOL Conference 2014 Boston shows where bacteria seeps through during hydrocooling — and how we can avoid ingesting it.

Hydrocooling Fresh Produce

Mm… Farm fresh tomatoes still warm from the sun… How about farm fresh tomatoes that were then cooled off in dirty water and shipped off to your local grocery store with bacteria inside of them?

Image of a tomato created by A. Warning and A.K. Datta and taken from their poster.

A Precooling Process

Hydrocooling is a form of precooling, which is a common practice at the harvesting site. Precooling is designed to keep the produce fresh longer without cooling produce off soon after harvest, it will continue to age and start to degrade. The hydrocooling process, specifically, involves putting freshly picked and hot-from-the-sun batches of produce into containers of cold water. The water will then cool off the fruit or vegetables. How long that takes naturally depends on the size of the produce, ranging anywhere from a few minutes to a full hour.

But, it turns out, a method meant to improve the quality of our produce might actually introduce new bacteria.

Contaminated Water Poses Health Risk

At this year’s COMSOL Conference in Boston, I met Alexander Warning of Cornell University during the poster session. He was standing by the poster he had created with Ashim K. Datta (also of Cornell University) on fresh produce safety during hydrocooling.

He explained to me that the water used during the hydrocooling process might be contaminated. For example, a farmer might touch soil containing manure for fertilization, pick a tomato, then dump it in the water with the rest of the batch. This then becomes a sort of bath tub of bacteria (my words, not his).

According to Warning, when you place warm produce in cold water, a negative pressure differential is established because of the condensation of water vapor inside the produce. Next, a vacuum is formed that may suck in water through an opening.

What about the tomato’s protective skin? The skin does a good job keeping contaminants at bay when the tomato is still attached to the plant, but when it’s removed from the stem, the tomato is no longer completely sealed.

A tomato undergoing the hydrocooling process. Image created by A. Warning and A.K. Datta and taken from their poster.

Hydrocooling Analysis to Keep Your Tomatoes Safe

Warning and Datta decided to create an engineering model to test how contaminated water might affect tomatoes during the hydrocooling process.

To begin, the researchers scanned a tomato wedge using magnetic resonance imaging (MRI). They used these images to create a geometry from symmetry for their porous media model. Then, they ran simulations to analyze the water infiltration in the tomato at different times and water temperatures.

The researchers used MRI scans of the tomato wedge to create the geometry. Image created by A. Warning and A.K. Datta and taken from their poster.

The tomato starts out at 35°C and is placed in water that is 5°C. After 15 minutes, the tomato has cooled down significantly on the outside, but is still relatively warm at the center (called the core and placenta). After 30 minutes, the tomato is roughly 15°C in the placenta and around as cold as the water closer to the skin (the cuticle and vascular bundles).

Analogously, the longer the tomato is sitting in the water, the more water seeps in through the stem scar, and the higher the concentration of bacteria. (View all of the results in the presentation and poster.)

Surface plots of the tomato at three different times. A) Spatial variation in tomato wedge of temperature. B) Pore water saturation. C) Gauge pressure. D) Bacteria concentration. Image created by A. Warning and A.K. Datta and taken from their poster.

Safer Handling of Produce

To combat the contamination issue, water used for hydrocooling should first of all be of drinking quality. If we can’t drink it, don’t bathe our produce in it. Another recommendation is typically to add some form of sanitizer to the water to keep microorganisms from spreading. One common sanitizer, which Warning mentioned during his live presentation at the conference, is chlorine.

When using chlorine, it’s important to control the pH and turbidity levels in the water. There are varying regulations for how much free chlorine can be used depending on whether the farm is organic or conventional. Intuitively, higher levels of chlorine can be used for produce grown conventionally than that grown according to organic standards. (Learn more about postharvest chlorination here.)

After talking to Warning at the poster session and hearing his presentation the following day, I couldn’t stop thinking about their research. While most people might cut the stem scars out anyway because of their indelicate texture, I used to not bother with it. I don’t know for sure exactly how the tomatoes I buy were precooled, but I’m no longer taking any chances…

Watch the video: What is the risky effect of hydrocooling?

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