Post-Harvest Handling Decision Tool > Crop Groups > Bunched Greens

The primary consideration in the post-harvest handling of bunched greens is rapid cooling and re-hydration of the product, normally accomplished either by hydrocooling alone or in combination with package icing. Handling speed seems to be a function of personnel and size of the tanks, with larger tanks facilitating more rapid packing of crops.

The standard protocol on very large, commercial vegetable operations for rapid cooling and re-hydration of harvested product is a combination of hydrocooling, and, where the crops are suited, liquid icing of the product. In large scale hydrocooling, a pump moves chilled water into contact with warm produce; the warmed water is then re-cooled and recycled. Since water removes heat from produce fifteen times faster than air, this method provides rapid cooling, although the wetting of some products can promote the growth of decay organisms.

For crops that can tolerate top icing, large, commercial vegetable operations will supplement or replace this operation with the addition of crushed ice or a slurry of crushed ice and water that is pumped into open containers traveling along a conveyor under an injection nozzle.

On Upper Midwest market farms, hydrocooling takes place by moving the product through tanks of water. Larger market farms handle enough product to warrant investment in an ice machine for additional cooling and quality maintenance. The low level of mechanization means that the speed of the operation depends heavily on the personnel conducting it.

Water Tanks

Plastic livestock watering tanks, most often those manufactured by Rubbermaid, were used for hydrocooling bunched greens on the three farms included in this study. The plastic tanks, generally rectangular with rounded ends, have rounded edges and corners that facilitate easy rinsing of field dirt. Drains on these tanks are typically located on the long about one inch above the floor, and open and close by means of a threaded plug.

Most workers can easily move tanks sized up to 100 gallons. Because the drains on these tanks are typically located about one inch above the floor of the tank, workers still need to tip tanks over to completely change the water and clean any soil from the tank. 50-gallon tanks don’t provide much depth to keep product away from the settling dirt, but do empty and fill relatively quickly. One hundred gallon tanks have the same footprint as the 50-gallon tank, but provide more depth, as well as additional water to buffer the temperature changes. Tanks up to 100 gallons seem most suitable for a farm handling up to 500 bunches of greens per day, and facilitate handling up to 150 bunches per labor hour.

With two 180-gallon tanks, Gardens of Eagan reported handling up to 600 bunches of greens per labor hour. Two tanks enable alternate filling and draining, eliminating downtime for the workers. Gardens of Eagan modified their tanks by adding a two-inch PVC drain in the center of the bottom of the tank; the drains open and close by means of a ball valve.

For proper ergonomics, stock tanks of all sizes need to be elevated off of the floor. For tanks up to 100 gallons, plastic totes, wooden crates, sawhorses, and overturned tanks provide adequate support to prevent sagging. For their 180-gallon tanks, Gardens of Eagan welded racks with supporting crosspieces to position the tanks and prevent sagging; this also facilitated the location of the drain on the bottom of the tank.

Several Upper Midwest market farms use dairy bulk tanks for hydrocooling. Typically run without refrigeration, these have the advantage of large size, seamless stainless steel construction, and a drain positioned at the bottom edge in such a way as to facilitate complete drainage of water and solids.

The size and number of tanks to buy will depend on the flexibility needed in an operation, the rate at which tanks can be filled, and the number of units processed through them. In addition to bunched greens, Upper Midwest market farmers use bulk water tanks for handling peppers, cucumbers, bunched roots, and bulk salad greens, as well as pre-soaking bunched roots.

Drainage racks provide an intermediate step between hydrocooling and packing the bunched greens, and are typically constructed of a wood frame with galvanized hardware cloth for a screen. Hog’s Back Farm’s use of PolyMax panels on their work tables would seem to provide an easily-cleaned option, although they did not use a drainage step in their process.

Ice Machine

The decision about whether to add ice to bunched greens will depend on the demands of the market place, since an ice machine serves to improve the quality and shelf life of produce, not to speed its handling. In addition to bunched greens, many other crops benefit from top icing, including asparagus, broccoli, cantaloupes, carrots, cauliflower, green onions, and sweet corn.

Upper Midwest market farmers use chipped and shaved ice to further cool and rehydrate produce. The cost of liquid or slurry ice would generally prove prohibitive to market growers. Ice provides cooling as well as a moisture shield for produce, and may find an additional use as an aid to displays at roadside stands and farmers markets.

Ice machines consist of a refrigeration system, an ice-making mechanism, and an insulated bin. They must be hooked up to a water supply system. The insulated bin for collection of the ice may be separate or integral, and additional bins may be purchased or manufactured. Gardens of Eagan scoops ice from their integral bin into two supplemental, wheeled bins for storage in their walk-in cooler, as well as optimal positioning for adding ice to boxes of produce.

Machine capacity is measured in pounds of ice made per day. Flaked ice stores well for several days in insulated bins in a 34°F cooler. Extension publications cite needs of approximately sixteen pounds of ice for each 20 pound carton of broccoli; in practice, Gardens of Eagan uses about five pounds of ice per case. Precise data and calculations for ice requirements can be found here.

Ice machines require frequent cleaning and sanitizing, and have similar maintenance issues to commercial refrigeration units. These should be taken into account when considering an investment in an ice machine.

This information is part of a Post-harvest Handling Decision Tool developed by Chris Blanchard of Rock Spring Farm in Decorah, Iowa. The tool is a project of the Fruit and Vegetable Working Group affiliated with the Value Chain Partnerships program. This project was funded by the Iowa State University Extension Value Added Agriculture program and the Leopold Center for Sustainable Agriculture. Blanchard conducted case studies of three vegetable operations to gather information for this decision tool. Products referred to in this tool are not an endorsement by Iowa State University.