How you harvest and handle your produce directly affects freshness and flavor. For most vegetables, rapid cooling after harvest slows deterioration, and high humidity prevents moisture loss. Different vegetables respond differently to the cooling method used, storage conditions required, and the temperatures where injury may occur (see table below). There are several ways to assure that the vegetables grown will maintain their freshness and quality, including cooling, harvesting and handling, washing, and storage conditions.
After harvest, vegetable quality is maintained through cooling and by slowing down the rate of respiration. Field heat is the temperature of the vegetable at the time of harvest. The heat of respiration is the heat produced by the crop when sugars, fats, and proteins are broken down after harvest. The byproducts of respiration are carbon dioxide, water, and heat. Initially, cooling removes field heat and holding the produce in a cool environment slows respiration. Slowing respiration slows postharvest growth, delays senescence and/or ripening, and decreases tissue breakdown. Lower temperatures also slow the growth of microorganisms, and thus decrease decay. Vegetable quality is reduced more quickly in high respiration rates and heat production. Produce with high temperatures also have increased rates of evaporation and transpiration, resulting in rapid wilting and loss of quality.
There are several ways to effectively cool produce after harvest, and different crops have different recommended cooling methods. Besides harvesting when it is cool, produce may be air-, hydroice-, or vacuum-cooled. Each method has different advantages and disadvantages.
The length of time required to cool produce depends on:
- Cooling method (air-, hydro-, ice-, or vacuum-)
- Temperature of the medium used
- Initial and final desired temperatures of the produce
- Type of crop (fruit, leaf, or root)
- Containers used and their size
Specific recommendations for cooling times vary with individual vegetable types. To be successful, measure the initial product temperature at harvest, and monitor the temperature during and after cooling. Don’t rely only on the air temperature in the cool room, but track fruit, leaf, or root temperatures. Remember that some leafy greens and many fruits are sensitive to chilling temperatures (between 35°F and 55°F). If possible, monitor temperatures during storage and also during delivery to determine if optimum temperatures are maintained.
Cooling Method and Handling Factors Recommended to Maintain Quality and Shelf Life
|Crop||Recommend Cooling Methods||Crop Handling-Storage Factors|
|Air||Water||Water||Vacuum||Temp. (F)||Relative Humidity (%)||Storage Life*||Chilling Injury**|
|Other Brassicas||+||+||+||+||32||90-95||2-5 w||I|
|Other Leafy Greens||+||+||+||32-36||95||1-2 w||I|
|Root Crops||+||32-36||90-95||2-6 m||I|
|Summer Squash||+||+||50||90-95||4-7 d||H|
|Sweet Corn||+||+||+||32||90-95||5-7 d||I|
|Sweet Potato||+||55-60||85-90||3-5 m||VH|
|Winter Squash||+||50-55||50-70||2-6 m||
*Storage life are days (d), weeks (w), or months (m) under the best conditions.
**Chilling injury sensitivity: I-insensitive; L-low; M-moderate; H-high; VH-very high. Sensitivity varies with stage of maturity for some vegetables. Information from USDA Handbook 66 (www.ba.ars.usda.gov/hb66/contents.html).
Harvesting and Handling
- Handle fresh produce with care. Avoid cuts, abrasions, and bruising damage to the tissue.
- Harvest produce at the peak of quality.
- Harvest during the cool part of the day (if possible). Produce is coolest in the early morning and lower temperatures reduce the rate of deterioration, extend quality, improvesshelf-life, and reduce cooling costs.
- If cold storage is not available, harvest only what you can pack or sell. Replenish roadside stands with freshly harvested produce throughout the day.
- Spread the harvest season through successive plantings and a mix of varieties.
- Shade harvest bins, trailers, trucks and market areas. Sort and pack in a shaded location.
- At fresh market stands, display only quality vegetables. Sort and remove poor quality produce during the day. Shade the sales display from the sun.
- Explain storage requirements to customers.
- For vegetables that lose quality rapidly, ensure that washing, handling, and cooling are appropriate to maintain quality
- All fresh produce has some bacteria and fungi present on the surface. When washing, the temperature of the wash water should be warmer than the produce temperature to prevent decay organisms from being drawn into the tissue.
- Be careful about using recycled wash water. Bacteria levels and dirt build up over time.
- Add chlorine to the wash water to destroy decay-causing microorganisms on the surface of vegetables. Chlorine concentrations in the wash water depend on the vegetable; chlorination is most effective at pH around 6.5 to 7.5.
- Monitor chlorinated wash tanks and spray washes with test kits to verify that the correct pH and concentration of available chlorine are present.
Many vegetables lose quality and show specific injury symptoms when exposed to ethylene after harvest. Ethylene damage includes: leaf spotting, green color loss, increased toughness or woodiness, bitterness, leaf yellowing and abscission, rapid softening, and development of off-flavors. While most know that ethylene increases ripening and softening of mature green tomatoes, it can also cause sprouting of potatoes. To avoid the detrimental effects of ethylene on vegetable quality:
1. Do not store or transport ethylene-sensitive crops with ethylene-producing fruits like apples, cantaloupe, bananas, and tomatoes.
2. Use electric forklifts in storage and transport areas. One byproduct of internal combustion engines is ethylene in the exhaust fumes.
3. Vent storage areas to reduce ethylene or install ethylene absorbers.
For more information on maintaining produce quality during harvest and postharvest, visit the Postharvest Technology Center for detailed information on how to reduce postharvest losses and improve the quality, safety, and marketability of fresh horticultural products.