Spring 1994 (v6n2)
	Project Update Fruit Decay Organisms in Hot Water by Chuck Ingels SAREP for an effective nonchemical method of controlling decay and physiological disorders of fruit after harvest. A team of researchers, led by Elizabeth Mitcham in the Pomology Department at UC Davis, was funded for $11, 730 by SAREP in 1993 to examine the effects of prestorage hot water dips on fruit quality. Hot water showed promise decades ago, but chemical treatments proved to be cheaper and more effective. Now, many chemicals maybe unavailable for use in postharvest treatments because of public pressure to reduce The search is now on chemical use and because many organisms have developed resistance to these materials. Controlled atmosphere (CA) storage, in which the carbon dioxide level is raised and the oxygen level is reduced in storage, can be very effective but is expensive. The Experiments. The hot water treatments in this study were aimed at reducing several diseases and disorders in stored fruit: storage scald (apples), brown rot (peaches, nectarines, and apricots), mealiness (peaches and nectarines), chilling injury (stone fruits and persimmons), and Botrytis rot (kiwifruit). Results of the research on storage scald and brown rot are presented here. Storage Scald of Apples. Apples are injured by prolonged storage at low temperatures. With apple storage scald, surface cells are killed, causing an irregularly shaped, diffuse browning of the skin. The researchers want to know if the hot water dips can effectively replace diphenylamine (DPA), a commonly used antioxidant for the control of storage scald. In the first year of the study, 'Granny Smith' apples were immersed for 5 minutes in hot water at 46 or 50 degrees C and stored in 1/2-degree C air for three or six months. Hot water-immersed fruit had only minor scalding after three months compared to the untreated controls, which were dark brown. After six months, however, more scalding occurred and the fruit showed some yellowing. Virtually no scalding occurred on fruit stored under CA storage. Other fruit quality indicators were not adversely affected by the treatments. Interestingly, the development of bitter pit, another physiological disorder which results in brown sunken lesions on apples, was also significantly reduced by heat treatments. The most promising treatments after three months of the second year study were 48 degrees C for 15 minutes and 50 degrees C for 5 minutes. Mitcham noted that results after six months will indicate whether the technique merits promotion or further study. Brown Rot. Brown rot is a serious disease of stone fruits in storage. To test the effects of hot water on brown rot control, the researchers dipped fruit in 52 degree C water for 2 minutes (apricots) or 2 1/2 minutes (peaches). The fruit was then stored at 5 degrees C and then at room temperature, both for varying lengths of time. With the apricots, heat treatment usually resulted in lower disease incidence and severity for both inoculated and noninoculated fruit. Similarly, brown rot on peaches was greatly reduced, however, some skin browning did occur as a result of the heat treatment. Limitations. Mitcham acknowledges that she would find herself in "hot water" if she suggested that this method alone will replace chemicals. Unlike many chemicals, she notes, hot water treatments do not leave a residue for prolonged control. The potential for extended storage is therefore dependent on the degree of sanitation and other conditions after the dip. Hot water can also reduce storage life if fruit injury occurs, as noted above. "The challenge is that there is a fine line between control and injury," Mitcham says. Energy costs are also an issue, but Mitcham believes that some of the heat produced during the cooling process could be diverted to heat the water baths. Mitcham and her colleagues have been awarded a second year of funding from SAREP to refine the hot water treatment techniques. Industry Response. According to Mitcham, hot water baths are already practiced for decay and insect control on oranges, mangoes, and papayas. However, she has encountered widely varying reactions from growers and handlers of temperate zone fruits. "Responses range from great interest and excitement to extreme skepticism, mostly because growers have always been told to cool the fruit as quickly as possible after harvest," she says. "The idea of heating the fruit first takes some getting used to." Chemicals have been the cornerstone of postharvest quality in the past. However, Mitcham believes that probably no single treatment will provide adequate control of decays and disorders in the future; it will likely take a combination of techniques. While CA storage and packaging hold the greatest promise, other methods may include biological control with yeasts, and increased sanitation and reduced injury during handling, along with hot water dips. [ Back | Search | Feedback ]