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Lecture 4: Post-harvest physiology and pathology (Postharvest technology -…
Lecture 4: Post-harvest physiology and pathology
Postharvest technology - Introduction
Postharvest handling - stage of crop production immediately following harvest; include cooling, cleaning, sorting and packaging
Importance
Food safety
Increase shelf-life and marketing opportunities
Final step for producers
Factors
Post-harvest physiology
Post-harvest pathology
Post-harvest physiology
Scientific study of the
physiology of living plant tissues
after they have denied further nutrition by picking/harvesting
Importance
Fruit storage can be prolonged by preventing fruit tissue
respiration
Contribute to fundamental principles and mechanisms of respiration - cold storage, gaseous storage and waxy skin coatings
Sensitivity to ethylene and the rate of ethylene production groups fruits and vegetable into two groups
Climateric
can be picked from the tree at
full size maturity
but before it is 'ripe' and allowed to ripen off the tree
increase in flavor quality, juice, sugars and other factors
'ripening' is controlled by the fruits
production of ethylene
and a significant
increase in CO2 production
Example: kiwi, jackfruit, papaya, durian, tomato
Non-climateric
tend to
maintain
what ever quality they had harvest
without many beneficial changes
produce
little or no ethylene
and
no large increase in CO2 production
Example: lime, citrus, lychee, rambutan, jambu, grape, lychee
Post-harvest pathology
Introduction
deals with the issue of
microbial attacks
on the harvested produce
infection by microbes may occur -
growing season, at harvest time, during handling, storage, transport and marketing, or even after purchase by the consumer
Classes of specific causes of post-harvest losses:
parasitic, nonparasitic, or physical
Actual disease occurs when the pathogen starts to
actively grow in the host
. Classification of disease -
signs
(visible growths of the causal agents, and
symptoms
(distinguishable responses produced by the host)
Agents of post-harvest diseases
Microscopic bacteria
- common in
vegetables
, less acidic than fruit; capable of
very rapid multiplication
under the right conditions of pH, temperature and nutrition
Fungi
- common in
fruits
Factors that influences post-harvest pathology
2. Harvesting practices
Maturity of produce at harvest
determines how long the crop can be stored. Onset of ripening and senescence in various fruit and vegetables renders them more susceptible to infection by pathogens
Avoid/minimize produce injuries
(hand-harvest, transport)
3. Postharvest managements
Packing sanitation
Organic matter
(culls, extraneous plant parts, soil) - act as substrates for decay-causing pathogens
Flumes and dump tank
accumulate spores and may acts as sources of contamination.
Chlorine
readily kills microorganisms suspended in dump tanks and flumes if the amount if available chlorine is adequate (50-100 ppm for fungicidal activity).
Chlorine dioxide
- not corrosive and effective over a wide pH range
1. Preharvest factors
Choice of cultivars
- key factors in choosing cultivar:
geography and history of the farm
Weather condition
-
abundant inoculum and favorable conditions
for infection during the season often result in heavy infection by the time the produce is harvested;
high moisture
promotes survival of fungi
Physiological conditions of the crops
- determined by fertilizer and soil factors; example:
Calcium
more closely related to disease resistance than any other cation associated with the cell wall
Preharvest management
- certain
pre-harvest sprays
can reduce decay in storage. Examples:
Ziram fungicide, iprodione, cyprodinil
4. Postharvest treatments
(for decay control)
Fungicide treatment
Examples of chemical treatments:
thiabendazole, dichloran, imazalil
However,
Mucor piriformis
, major postharvest pathogen of apples and pears is not controlled by any registered fungicide
Factors
Best time for application
of the treatment
Maturity of the host
Location of the pathogen
in the produce
Environment
during storage, transportation and marketing of produce
Type of pathogen
involved in decay
Biological control of postharvest pathogens
Advantages:
exact environmental conditions
can be established and maintained;
biocontrol agent
can be
targeted
much more efficiently; expensive control procedures are more
cost-effective
on harvested food
Disadvantages: do not give consistent results; biocontrol efficacy is directly affected by the amount of pathogen inoculum present; compatibility with chemicals used during handling also important - biological control agents must be combined with other strategies
Examples:
Bacillus subtilis
(control peach brown rot),
Pseudomonas syringae
(controlled blue and gray mold of pome fruit),
Cryptococcus infirmo-minutus
and
Candida sake
(control brown rot and blue mold on sweet cherry and three diseases of apple)
Irradiation
low doses of
ultraviolet light irradiation
(254 nm UV-C) reduced postharvest brown rot of peaches; two effects - reduction in the inoculum of the pathogen and induced resistance in the host
gamma radiation
- controls decay, disinfestation, and extending the storage and shelf-life of fresh fruits and vegetables
Termperature and relative humidity
Fruit rot fungi grow optimally at 20-25 C; minimum growth at 5-10 C or -6-0 C
hot air can control decay in crops that are injured by hot water; hot water - more effective
fungal growth rate higher at intermediate temperatures (room temperature, 27 C)
Modified or controlled atmospheres
lowering the O2 or raising the CO2 above 5% can suppress pathogenic growth in the host
technologies - storage abd transport under low O2, and the use of carbon monoxide
Other decay control treatments:
heat treatments, growth regulators, SO2 fumigration, ethylene absorbers or scrubbers
Integrated control of postharvest diseases/decay
Early harvest:
low fruit nitrogen, high fruit calcium, yeast or yeast plus fungicide treatment and controlled atmosphere storage
reduced severity of blue mold and side rot
'Gala' apples -
heat treated
at 38 C for 4 days, followed by
calcium infiltration
with 2% CaCl2, and treated with the
microbial antagonist
,
Pseudomonas syringae
.
Food safety issues
Two most important causes of unsafe food -
microbial toxins
and
contamination of horticultural products by fecal coliforms
Microbial toxins -
bacterial toxins
and
toxins produced by fungi or mycotoxins
. Example:
botulinum
toxins (extremely toxic), aflatoxin (potent carcinogens in nuts and grains)
Interactions between plant pathogens and foodborne human pathogens (
Salmonella
and
Listeria
)