Question of the Week
Answer: You have spent months working in the fields, and you now have a bountiful harvest of beautiful fruits and vegetables. You want to ensure that your customers will also enjoy this healthy harvest. How can you best maintain the quality and safety of your produce as it travels from the field to the table? How can produce be stored so that it does not need to be sold immediately? High-quality, disease-free produce with a good shelf life is a result of sound production practices, proper handling during harvest, and appropriate postharvest handling and storage.
Production practices have a tremendous effect on the quality of fruits and vegetables at harvest and on postharvest quality and shelf life. To start, it is well-known that some cultivars ship better and have a longer shelf life than others. In addition, environmental factors such as soil type, temperature, frost, and rainy weather at harvest can have an adverse effect on storage life and quality. For example, carrots grown on muck soils do not hold up as well in storage as carrots grown on lighter, upland soils. Lettuce harvested during a period of rain does not ship well and product losses are increased.
Management practices can also affect postharvest quality. Produce that has been stressed by too much or too little water, high rates of nitrogen, or mechanical injury (scrapes, bruises, abrasions) is particularly susceptible to postharvest diseases. Mold and decay on winter squash, caused by the fungus Rhizoctonia, result from the fruits lying on the ground and can be alleviated by using mulch. Broccoli heads are susceptible to postharvest rot caused by the bacteria Erwinia if nitrogen is applied as foliar feed—a grower should feed the soil, not the leaves. Beets and radishes are susceptible to soil-borne diseases when the soil temperature reaches 80º F; symptoms are black spots on these root crops.
Food safety also begins in the field and should be of special concern, since a number of outbreaks of food-borne illnesses have been traced to contamination of produce in the field. Common-sense prevention measures include:
• Don't apply raw dairy or chicken manure or slurries to a field where a vegetable crop such as leafy lettuce is growing.
• Don't apply manure to an area immediately adjacent to a field nearing harvest maturity.
• Don't forget to clean equipment that has been used to apply manure to one field before moving it to another field in production.
• Don't irrigate with water from a farm pond used by livestock.
• Don't harvest fruit from the orchard floor for human consumption as whole fruit or nonpasteurized juices, especially if manure has been spread or if animals allowed to graze.
• Don't accumulate harvested product in areas where birds roost.
A grower should constantly evaluate water used for irrigation and compost all animal manures before applying them to fields. There are many good sources of information on growing conditions and production practices that promote postharvest quality. Consult ATTRA publications, textbooks, Extension publications, and trade journals. You can also become involved with grower organizations to learn more.
Quality cannot be improved after harvest; it can only be maintained. Therefore, it is important to harvest fruits, vegetables, and flowers at the proper stage and size and at peak quality. Immature or over-mature produce may not last as long in storage as that picked at proper maturity. Cooperative Extension Service publications are an excellent source of information on harvest maturity indicators for vegetables and fruits.
Harvest should be completed during the coolest time of the day, which is usually in the early morning, and produce should be kept shaded in the field. Handle produce gently. Crops destined for storage should be as free as possible from skin breaks, bruises, spots, rots, decay, and other deterioration. Bruises and other mechanical damage not only affect appearance, but also provide entrance to decay organisms.
Postharvest rots are more prevalent in fruits and vegetables that are bruised or otherwise damaged. Mechanical damage also increases moisture loss. The rate of moisture loss may be increased by as much as 400 percent by a single bad bruise on an apple, and skinned potatoes may lose three to four times as much weight as non-skinned potatoes. Damage can be prevented by training harvest labor to handle the crop gently, harvesting at proper maturity, harvesting dry whenever possible, handling each fruit or vegetable no more than necessary (field pack if possible), installing padding inside bulk bins, and avoiding over- or under-packing of containers.
Postharvest and Storage Considerations
Packaging should be designed to prevent physical damage to produce and be easy to handle.
Temperature is the single most important factor in maintaining quality after harvest. Refrigerated storage retards the following elements of deterioration in perishable crops:
• Aging due to ripening, softening, and textural and color changes
• Undesirable metabolic changes and respiratory heat production
• Moisture loss and the wilting that results
• Spoilage due to invasion by bacteria, fungi, and yeasts
• Undesirable growth, such as sprouting of potatoes
One of the most important functions of refrigeration is to control the crop's respiration rate. Respiration generates heat as sugars, fats, and proteins in the cells of the crop are oxidized. The loss of these stored food reserves through respiration means decreased food value, loss of flavor, loss of salable weight, and more rapid deterioration. The respiration rate of a product strongly determines its transit and postharvest life. The higher the storage temperature, the higher the respiration rate will be.
For refrigeration to be effective in postponing deterioration, it is important that the temperature in cold storage rooms be kept as constant as possible. Exposure to alternating cold and warm temperatures may result in moisture accumulation on the surface of produce (sweating), which may hasten decay. Storage rooms should be well-insulated and adequately refrigerated and should allow for air circulation to prevent temperature variation. Be sure that thermometers, thermostats, and manual temperature controls are of high quality, and check them periodically for accuracy.
For more information, see theATTRA publication, Post Harvest Handling of Fruits and Vegetables, available at
https://attra.ncat.org/attra-pub/summaries/summary.php?pub=378. This publication covers postharvest practices suitable for small-scale operations and points out the importance of production and harvesting techniques for improving quality and storability. Various methods for cooling fresh produce are discussed, and resources are listed for further information, equipment, and supplies.
Additionally, the Western Sustainable Agriculture Research and Education project and ATTRA partnered to produce a Post-Harvest Handling and Food Safety Guide, available at https://attra.ncat.org/intern_handbook/harvesting.html.
Finally, a publication from North Carolina State University, titled Postharvest Handling and Cooling of Fresh Fruits, Vegetables, and Flowers for Small Farms. Part II: Cooling, may be helpful to have on hand in your washing facility. It is available at http://www.ces.ncsu.edu/depts/hort/hil/hil-801.html.
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