Respiration and Ethylene and their Relationship to Postharvest Handling

Introduction

Fresh produce is at peak quality when picked; its quality can only be maintained (or deteriorated) as it is handled and stored. Maintaining crop quality after harvest is an important consideration for any fresh market produce grower or handler.  This article addresses two important factors related to post-harvest produce quality: respiration and ethylene. 

Respiration

Despite having been detached from the plant, fruits and vegetables remain as living organs after harvest. Like all living tissues, harvested produce continues to respire throughout its postharvest life. During the process of respiration, carbohydrates are broken down to their constituent parts to produce energy to run cellular processes, thus keeping the cells and organism alive. Throughout this process, oxygen is consumed and water, carbon dioxide, and energy are released. Because this process occurs from harvest to table, the carbohydrates stored in the harvested plant portion are continually “burned” as energy to keep the vegetable alive; as respiration continues, compounds that affect plant flavor, sweetness, weight, turgor (water content), and nutritional value are lost. Thus, reducing the rate of respiration is an important consideration in extending the postharvest life of a fruit or vegetable and optimizing postharvest quality. Harvested fruits and vegetables of different plants have different rates of respiration; some respire at a faster rate (and thus are more perishable vegetables), while some respire at a relatively slow rate (less perishable vegetables) (see table below). In addition, storage conditions affect respiration, with higher temperatures leading to a faster rate of respiration; for every 10°C (18°F) rise in temperature, the respiration rate will double or even triple. Because of the significant effect of temperature on respiration, the amount of time a harvested product is exposed to heat should be minimized; the fruit or vegetable should be quickly brought to its optimal storage temperature

Ethylene 

Ethylene is a colorless gas that is naturally produced by plants and functions as a plant growth regulator. In this way, ethylene behaves in the same way as hormones in mammals. It triggers specific events during a plant’s natural course of growth and development, such as ripening. Through this action, it induces changes in certain plant organs, such as textural changes, color changes, and tissue degradation. Some of these changes may be desirable qualities associated with ripening; in other cases, it can bring damage or premature decay.

Fruits and vegetables may be classified depending on their response to ethylene. Climacteric species produce ethylene as they ripen, and the harvested produce is capable of ripening during the postharvest period. These commodities, such as bananas, apples, and peaches, tend to get sweeter and softer after harvest. Non-climacteric plants, such as leafy vegetables, do not continue to ripen after harvest; they will soften and rot, but this is due to moisture loss, decay, and tissue deterioration.

In addition to being naturally produced by plants, ethylene is produced by a variety of other sources. These include internal combustion engines, cigarette smoke, and natural gas leaks. Even low concentrations of ethylene throughout the postharvest life of a commodity can affect quality, so care must be taken to minimize exposure from both natural sources (i.e. climacteric fruit or veggies being stored with non-climacteric ones) or to artificial sources (engine exhaust, heaters, etc). All ethylene-producing sources should be considered when optimizing postharvest storage conditions as inadvertent exposure to ethylene can contribute to loss of quality in some fruits and vegetables.

Impacts of ethylene on post harvest quality of fruits and vegetables (Suslow, 2000)

• Russet spotting of lettuce (dark brown spotting on the mid-ribs of lettuce leaves)
• Yellowing or loss of green color (for example, in cucumber, broccoli, kale, spinach)
• Increased toughness in turnips and asparagus spears
• Increase or decrease sprouting in potatoes
• Yellowing and abscission (dropping) of leaves in Brassicas
• Softening, pitting, and development of off-flavor in peppers, summer squash, and watermelons
• Browning and discoloration in eggplant pulp and seed
• Discoloration and off-flavor in sweet potatoes
• Increased ripening and softening of mature green tomatoes

• Development of bitter taste in carrots and parsnips

Steps to reduce ethylene exposure during storage (Jobling, 2000)

• Do not store or transport green leafy vegetables in containers holding ripening fruit (apples, pears, mangoes, tomatoes, bananas).
• If possible, use electric powered equipment in storage areas versus gas powered.
• Remove overripe or rotting fruit from storage loads (these produce higher amounts of ethylene).
• Avoid storing ethylene sensitive products with products that produce high levels of ethylene.
• Increase the ventilation rate of the storage area, assuming that the outside air is ethylene free.
• Use ethylene scrubbers in storage areas to remove ethylene in the air.

References and Citations

• Jobling, J. 2000. Postharvest ethylene: A critical factor in quality management [Online]. Sydney Postharvest Laboratory. Sydney, Australia.
  Available at: http://www.ictinternational.com/casestudies/postharvest-ethylene-a-critical-factor-in-quality-management/(verified 24 Dec 2019).
• Suslow, T.V., 2000.  Post-harvest handling for organic crops.  Organic Vegetable Production in California Series. Pub. 7254. University of California Davis. Available online at: https://anrcatalog.ucanr.edu/pdf/7254.pdf (verified 24 Dec 2019). Editor's note: this was written in 2000 and some of the information is no longer accurate. Check with your certifier before using any product on your organic farm.
• Wilson, L. G., M. D. Boyette, E. A. Estes. 1999. Postharvest handling and cooling of fresh fruit, vegetables, and flowers for small farms. North Carolina Cooperative Extension Service. Available at: https://content.ces.ncsu.edu/catalog/series/158/ (verified 24 Dec 2019).