Reducing Postharvest Vegetable Losses due to Decay

By Mark A. Ritenour, Associate Professor, Indian river Research and Education Center Steven A. Sargent, Professor, Horticultural Sciences Department, Gainesville and Jeffery K. Brecht, Professor, Horticultural Sciences Department, Gainesville

Fresh vegetables live in a hostile world with pathogenic decay organisms just waiting to invade your valuable commodity. The good news is that commodities are normally very resistant to infection and decay; infection usually occurs only when commodities are injured or late in their development when tissues are senescing. Postharvest decay is also more prevalent in product grown under harsh or wet weather conditions. When handling fresh vegetables, it is always important to maximize the product's natural defenses, minimize opportunities for pathogens to contact the product, and create and maintain conditions that prevent or inhibit pathogen growth. Rigorous attention to these details will help keep product decay below customer and USDA grade standard limits and help prevent financial losses.

Decay is caused when pathogenic microorganisms (e.g., decay causing fungi and bacteria) grow within the commodity and cause the production of off-odors or visible symptoms (i.e., softening and discoloration). Decaying vegetables also reduce the quality of neighboring product by:

•  producing spores that discolor or infect nearby product

•  promoting ethylene production that accelerates aging or ripening or the development of physiological disorders (e.g., russet spotting in lettuce)

•  allowing the decay organism to spread through direct contact with adjacent product (nesting)

In addition, decaying vegetables may harbor human pathogens. For instance, researchers have found that Salmonella is much more prevalent and grows much faster on vegetables with bacterial soft rot ( Erwinia carotovora ).

Methods to reduce postharvest decay:

•  Practice good sanitation in the field and throughout the entire harvest and postharvest handling chain. There is a direct relationship between the population of decay pathogens in the environment and decay development. Thus, the presence of decaying plant material in the field (e.g., leaf litter, rotting vegetables, dead plants, etc.) and dirty harvesting and handling equipment results in higher rates of decay. Frequently clean and sanitize harvest and hauling equipment, packing areas and equipment, shipping containers, etc. Sanitize and frequently monitor the quality of all recirculated water systems and assure fresh water is free of pathogens.

•  Always harvest and handle commodities carefully to minimize wounds and injury. The epidermis or periderm is a commodity's first line of defense. Punctures, cuts, etc. not only break these natural physical barriers, but also rupture cells, which release their contents (water and nutrients), stimulating pathogen growth. Even injuries from bruises that do not rupture the epidermis, or exposure to injurious chemicals or atmospheres encourage decay because stressed tissues are generally less able to resist pathogen attack.

•  Hold commodities under conditions that promote wound healing and the formation of natural antifungal (phytoalexin) compounds (e.g., curing of potatoes). New chemicals are becoming available that do not affect pathogens directly, but rather stimulate natural production of these antifungal compounds. In some situations, hot water can be used to physically reduce surface pathogen levels and stimulate product resistance.

•  Use synthetic or biological compounds that eradicate or suppress the growth and development of decay pathogens.

•  Store and transport vegetables at their lowest safe (i.e., non-chilling) temperature. Most pathogens grow best at warmer temperatures. Therefore, except for those commodities that benefit from curing or other heat-treatments, exposure to warm temperatures after harvest usually results in increased decay. In addition, chilling susceptible commodities held at low temperatures develop chilling injury that also greatly promotes decay.

•  Maintain temperatures and humidity levels so that condensation does not form on the product. Fungal spores germinate under high humidity (i.e., >95%) or in the presence of free water (i.e., condensation on the product). Protect the product from rain and splashing water (i.e., from melting ice from a nearby load). For commodities that tolerate wetting, water quality and effective sanitation is critical.

•  Use controlled or modified atmospheres on commodities that benefit from their use. Maintaining tissue vitality longer delays decay development.

•  For some vegetables, wraps or coatings (incl. those with fungicides) help prevent decay organisms from “nesting” (spreading to adjacent product).

For more postharvest information, visit http://postharvest.ifas.ufl.edu .