Harvested vegetables are living things that carry on the process of respiration and other biological and chemical processes. 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).
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, Table - p. 7 and pp. 68-75 in References at the end of this chapter), 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 chain of reduced temperature that connects the field to the consumer ensuring the highest quality produce possible by minimizing respiration.
From the moment of harvest, product quality will deteriorate. Intentional precooling of produce directly after harvest helps quickly reduce the rate of respiration and initiates the cold chain. Examples of precooling include scheduling harvest activities at cooler times of day, shading harvested product in the field prior to transport, forced air cooling through the packed product with refrigeration, hydrocooling with cool water, and vacuum cooling via evaporation. Once cooled to storage temperature, reliable, refrigerated storage is necessary to maintain high quality.
It is important to note that not all crops can be cooled to the same temperature without resulting in cold or freeze injury and some crops are sensitive to the method of cooling. 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 16 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 above.
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 a measure of the amount of water vapor in air compared the maximum amount that can be saturated in that air at a given temperature. Most, but not all, crops are ideally stored at higher RH 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 resulting in drier outer skin and curing 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 such as molds and fungi on this outer skin. Other than these examples, most crops are best stored at 90%-95% RH with specific guidance provide 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 led to postharvest disease and pathogens which are encouraged by damaged cell tissue. Even during harvest, crops can suffer “harvester blight.” 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.
Sorting and culling are also important practices at this stage. As the saying goes, “one bad apple can spoil the bunch”. Sorting allows for different sizes and grades of product to be stored and sold separately and culling can separate damaged or lower quality product from the main lot for sale, rescue donation or compost depending on the defect. The removal of obviously damaged product from the lot helps minimize cross contamination postharvest pathogens to a larger portion of the population.
Produce can be rinsed to remove soil and debris, and often a sanitizer is added to the rinse water to prevent cross-contamination of plant and human pathogens from one item of produce to another in the same batch (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.
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 July 12, 2019.
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 July 12, 2019.
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 July 12, 2019.
LaBorde, L., Samuels, R. and Stivers, L. 2016. Video Series: Using Sanitizers in Washwater. Available online. http://extension.psu.edu/food/safety/farm/gaps/video-series-washwater-sanitation. Confirmed July 12, 2019.