Spring, 1991 (v3n3)
by Lyra Halprin, SAREP
Marita Cantwell de Trejo, UC Davis Extension vegetable
crops specialist, heads a SAREP project on postharvest handling
of tomatoes and melons using hot water, hot air, and high carbon
dioxide regimes to control postharvest pathogens.
Cantwell de Trejo's preliminary work (1989) identified the time-temperature
ranges for forced warm air which are effective in controlling
pathogen growth on inoculated fruit without causing visible fruit
injury and other quality losses (especially firmness and flavor
in tomatoes.) An example of an effective treatment for tomatoes
was determined to be 60 degrees C (140 degrees F) for two hours
in non-humidified air.
Since 1990, the first year of the SAREP project, the focus has
been on hot water treatments as alternatives to postharvest use
of fungicides on fresh market tomatoes and cantaloupe melons.
Tomatoes were inoculated with Botrytis cinerea and treated
with hot water. Cantaloupes were inoculated with Fusarium
roseum and treated similarly. To date the project has identified
the limits of tolerance of the tomatoes and cantaloupes to hot
water treatments. "An effective treatment is considered
to be one that will stop the fungus dead in its tracks, i.e. there
will be no fungal growth or decay development," Cantwell
de Trejo said. Several hot water time-temperature regimes provided
effective pathogen control without causing visible fruit injury.
Heat treatments effective in controlling Botrytis were
also effective in controlling other common postharvest pathogens.
Although several regimes can efficiently control postharvest
pathogens for each fruit, only a few treatments resulted in fruit
of quality equal to that of untreated fruits, Cantwell de Trejo
said.
"Generally, higher-temperature, shorter-duration treatment
regimes had the least impact on fruit quality," she said.
Cantwell de Trejo said the hot water treatments found to be most
effective for tomatoes to date are: treated at 58 degrees C (136
degrees F) for three minutes; 55 degrees C (131 degrees F) for
seven minutes; 53 C (127 degrees F) for ten minutes (although
it softens the fruit too much).
Surface decay is the worst cantaloupe postharvest problem, Cantwell de Trejo said. Both fruit surface and stem-end decay were evaluated during the experiments. Lower temperatures for longer time periods (50 C for ten minutes) control pathogens better, but reduce fruit quality in comparison to shorter-term treatments.
Other effective treatments include 57 degrees C for 2.5 to 3.5
minutes.
"We're looking at ways to improve the higher-temperature/shorter-term
treatments," she said, adding that off-flavor problems could
result from the longer, lower temperature treatments.
Cantwell de Trejo said the principal benefit of hot water (or
air) treatments is that they can kill the organisms on and below
the fruit surface. "Postharvest fungicides only kill surface
pathogens," she said. She said the heat may affect ripening
behavior by slowing it, which could be good or bad. Heat treatments
are currently used as quarantine treatments for imported crops
like papayas and mangoes.
"The hot water treatments could be used in the future for
quarantines if this is ever necessary," she said. "This
is a side benefit that could result from our work if we know exactly
how these heat regimes work."
Cantwell de Trejo said her research group will be testing the
effects of modified atmospheres on the postharvest condition of
fruit in 1991. Researchers will be testing high C02
on fruits, both alone and in combined treatments with hot air
or hot water, she said. Investigators include Xunli Nie,
UC Davis vegetable crops, and Noel Sommer, UC Davis pomology.
"We have tried hot water on honeydew melons and observed
some benefit with pathogen control in longer-term storage, for
more than three weeks," she said. "Lately we've also
been playing around with onions. It looks like hot water might
be an effective treatment to control pathogens like Aspergillus.
There are other crops that could potentially use this treatment."
She said that research during the second year will also focus
on the application of these techniques on a semi-commercial scale.
Tests comparing the heat treatment with commercially applied
fungicides will be part of the evaluation.
In addition to controlling temperature and relative humidity and
modifying atmospheres, Cantwell de Trejo said that avoiding physical
damage to the crops is critical to the maintenance of quality.
"Putting masking tape over fingernails, avoiding drops of
the crop, these are not 'sexy' topics, but they are basic common
sense strategies for avoiding postharvest problems," she
said. "Generally, if proper postharvest techniques are used,
few crops require additional treatments such as fungicides or
hot air."
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