Postharvest Handling and Storage

Harvested vegetables remain metabolically active and continue respiration, and other biological and chemical processes, after harvest. How produce is handled after harvest will directly affect quality characteristics such as appearance, flavor, texture, and nutritional value. Attention to postharvest quality can increase repeat sales and support higher prices.

Control of postharvest quality essentially comes down to limiting respiration rate (lowering temperature), controlling water loss (maintaining proper relative humidity), minimizing physical damage to the product (harvesting and handling with care), and avoiding contamination (handling, washing, and storing appropriately).

Limiting Respiration

Respiration is a temperature-dependent, biochemical process that converts carbon in plant tissue (mainly sugars) to carbon dioxide (CO₂) and water (H₂O) while producing some heat. Rates of respiration vary by the crop (see Gross 2016, p. 7 and pp. 68-75 in References at the end of this section), and should be taken into account when sizing cooling equipment. Fortunately, we can significantly reduce respiration, and therefore maintain high product quality, by reducing product temperature (precooling) and keeping it low (holding or storage cooling). This concept is known as establishing the “cold chain”; a continuous series of temperature-managed steps from the field to the consumer that helps maintain produce quality by minimizing respiration.

Product quality begins to decline immediately after harvest. Intentional precooling of produce directly after harvest helps quickly reduce the rate of respiration and initiates the cold chain. Examples of precooling include harvesting during cooler parts of the day, shading harvest before transport, using forced-air cooling with refrigeration, hydrocooling with clean cold water, and vacuum cooling through evaporation. Once cooled to storage temperature, reliable, refrigerated storage is necessary to maintain high quality.

Not all crops can be cooled to the same temperature without risk of chilling or freezing injury and some crops can be damaged by specific cooling methods Crops have different susceptibility to chilling or freeze injury depending on their physiology. Good guidance is available (see Gross 2016, p. 62-67) and is summarized in Table 17 of this guide. Common precooling methods are also noted in Table 16. Additionally, a computer-based crop storage planner is available for determining appropriate grouping of your crops and estimating overall respiration load (see Callahan 2016). Chilling injury is also an important consideration when considering particularly sensitive fall harvested crops and the possibility of lower nighttime temperatures, e.g. winter squash. Notes on chilling injury guidance for these crops are provided in the appropriate crop chapter and in the references described above.

Avoiding Ethylene Damage

Ethylene is is a natural plant hormone involved in ripening and senescence in many fruits and vegetables. Produce in storage will naturally release ethylene. Some crops are more sensitive to damage from ethylene and, left unmanaged, certain crops will wilt early, produce off flavor, and lose nutritional value. Minimizing ethylene in storage may be necessary for more ethylene-sensitive crops, especially when storing different crops in the same cooler for extended periods. Ventilation and separation of crops based on ethylene production and sensitivity can reduce the ethylene injury in storage. Commercial products such as potassium permanganate pellets can also be used to absorb ethylene.

Controlling Water Loss

The control of water loss requires careful attention to relative humidity (RH) of the air surrounding stored product in addition to temperature. RH is the amount of water vapor in the air compared to the maximum amount the air can hold. Most, but not all, crops are ideally stored at higher RH (90-100%) to prevent water evaporation into the air leading to water loss. The loss of water reduces the weight of the crop and also can lead to lower quality and poor appearance.

Some crops, such as onions, garlic, and winter squash, are purposefully “cured” or dried in order to dry outer skin and cure harvest wounds to allow long-term storage. Because this results in a paper-like layer, these crops are generally stored at lower RH to prevent development of postharvest disease. Other than these examples, most crops are best stored at 90-100% RH with specific guidance provided in Table 16, in the crop storage planner noted above, and in the literature (see Gross 2016).

Minimizing Physical Damage

Generally speaking, produce crops live a very gentle life until harvested. Starting with harvest, produce is moved and handled for the first time and, typically, many times after. With each movement there is a risk of physical damage. Even if the damage is not obvious, it can result in bruising or other damage that becomes evident later and can lead to postharvest diseases, which are encouraged by damaged cell tissue. Physical injury during harvest can reduce quality and increase susceptibility to decay. For the majority of crops, gentle handling, crates with smooth and clean surfaces, and conveyance with elastic and soft belts and rollers should be used.

Avoiding Contamination

Sorting and culling are also important practices at this stage. Sorting allows different sizes and grades of product to be stored and sold separately while culling removes damaged or lower-quality product from the main lot for different sale avenues, donation, or compost depending on the defect. The removal of obviously damaged product from the lot helps minimize cross-contamination with postharvest pathogens to a larger portion of the population.

Produce may be rinsed to remove soil and debris. When recirculated or batch water is used, an approved sanitizer is often added and monitored to reduce cross-contamination by plant and human pathogens (see the following references: LaBorde, Samuels and Stivers 2016, Bihn et al. 2014).

Once packed and ready for storage or transport, care should be taken to avoid contamination of product with other contaminants such as foreign matter and unintentional water such as condensate from refrigeration systems. Growers should follow current Produce Safety Rule requirements and label directions for sanitizer use, water quality, worker hygiene, and cleaning and sanitizing food-contact surfaces.

References

• Gross, Kenneth C., Chien Yi Wang, and Mikal Saltveit, eds. 2016. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Agriculture Handbook 66, U.S. Department of Agriculture, Agricultural Research Service, Washington, DC. Available at https://www.ars.usda.gov/ARSUserFiles/oc/np/CommercialStorage/CommercialStorage.pdf. Confirmed May 2, 2024.
• Callahan, Christopher W. 2016. Crop Storage Planning Tool. UVM Extension. Available online. http://blog.uvm.edu/cwcallah/2016/01/21/new-crop-storage-planning-tool/. Confirmed May 2, 2024.
• Bihn, E., Schermann, M. A., Wszelaki, A. L., Wall, G. L., and Amundson, S. K. 2014. On-Farm Decision Tree Project: Postharvest Water. National Good Agricultural Practices Program. Available online. https://gaps.cornell.edu/educational-materials/decision-trees/postharvest-water. Confirmed May 2, 2024.
• LaBorde, L., Samuels, R. and Stivers, L. 2016. Video Series: Using Sanitizers in Washwater. Available online. https://extension.psu.edu/using-sanitizers-in-wash-water. Confirmed May 2, 2024.

Table 17: Handling Produce for Higher Quality and Longer Market Life

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