Post harvest diseases fruits and vegetables - Xavier University ...

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE CHAPTER 10 PHYSICAL MEANS There is increasing public awareness that many of the chemical treatments applied to fresh horticultural products to control decay are potentially harmful to consumers. This fact, along with the possible consequence that a number of chemicals may be withdrawn from use, has revived and increased interest in physical treatments that could serve as alternatives to fungicides. Cold storage and controlled or modified atmospheres are physical means discussed in the chapter on Means for Maintaining Host Resistance. The present chapter will discuss other physical means: heating, ionizing radiation and ultraviolet illumination. A. HEAT TREATMENTS Heat treatment may be applied to the commodity by means of hot water dips and sprays, hot vapor or dry air, or infrared or microwave radiation. However, practical systems have used mainly hot water or vapor (Couey, 1989). While hot water was originally used for fungal control and was extended to removal of insects from fresh commodities, vapor heating, developed for insect control may also serve to reduce fungal decay. Thus, the vapor heat treatment used for disinfestation of Carabao mangoes in the Philippines also significantly reduced the incidence of anthracnose and stem-end rot in fruit, although the onset of decay was not delayed by the treatment (Esquerra and Lizada, 1990). Similarly, the development of green mold on grapefruit, caused by Penicillium digitatum was inhibited by the 300-min treatment with moist forced air at 46°C used to provide quarantine security against the Mexican fruit fly (Shellie and Skaria, 1998). Hot humid air has also been useful in controlling decay in crops that would have been injured in hot water. For example, postharvest decay of strawberries caused by Botrytis cinerea and Rhizopus stolonifer was controlled by exposure of the fruit to humid air at 44°C for 40-60 min (Couey and FoUstad, 1966). The possibility of using hot water dips to control decay in citrus fruits http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 190 Postharvest Diseases of Fruits and Vegetables had already been reported in the 1920s (Fawcett, 1922), and during the following decades, hot water treatments were tested on and applied to various fruits and vegetables, although the intensive use of chemicals pushed them aside, along with other means of control. Today, with the trend toward less reliance on chemical treatments, the interest in postharvest use of heat treatment has revived. A list of commodities heat-treated to control postharvest decay is presented in the reviews by Barkai-Golan and Phillips (1991) and Coates and Johnson (1993). Short-term and Long-term Heat Treatments Short-term heating, where the fruit or vegetable is dipped in hot water at temperatures above 40°C (generally 44-55°C) for a short time (from a few minutes to 1 h), has been the main heat treatment studied over the years. The principle is in the use of temperatures that are high enough to inactivate the pathogen without causing significant changes to the host. Early studies had already confirmed that fruits and vegetables commonly tolerate such temperatures for 5-10 min, and that even shorter exposures to these temperatures is sufficient to control many of the postharvest pathogens (Smith, W.L., Jr. et al., 1964). Recent studies reveal increased interest in long-term heat treatments, in which the commodity is exposed to temperatures lower than those mentioned above (usually 38-46°C), but for a longer duration (12 h to 4 days) (Fallik et al., 1996; Klein et al., 1997). Both short-term and long-term heating, aimed at suppressing storage decay, could act directly by inactivating the pathogen, or indirectly via physiological and biochemical changes in the host, which enhance the resistance of the tissues to the pathogen. The Effect of Heat on the Pathogen Genetic differences among fungi are expressed in differences in their sensitivities to high temperatures and, therefore, in differences among levels which kill them or inhibit spore germination and hyphal growth (Sommer et al., 1967) (Fig. 30). For a given species, spore inactivation increases with both temperature and duration of treatment; conidia of Alternaria alternata may be inactivated equally by treatment for 2 min at 48°C or for 4 min at 46°C (Barkai-Golan, 1973) (Fig. 31). Spore sensitivity to heat is also dependent on their physiological state. Germinated fungal spores are markedly more sensitive to heat than non-germinated spores. It was found that water at 42°C does not affect dormant conidia of A, alternata but does inactivate many germinating http://arab2000.forumpro.fr
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