Postharvest water loss

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Postharvest water loss
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Water loss

• 90 to 95 of the commodity is water.
• milk has more solids than cucumber (weight
  basis).
• water loss resulting in direct loss of salable
  weight, in addition to quality loss.
• Appearance quality - wilting, shriveling,
  accelerated
• development of injuries.
• Textural quality loss of crispness,
  juiciness, etc.
• Nutritional quality e.g. vitamins A C.
• Thus, managing water content of commodities is
• critically important.

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Loss of marketability due to water loss
water Loss Commodity
8 Asparagus
8 Brussels sprouts
7 Spinach
5 Lettuce
3 Celery
3 Cabbage
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Potential effects Water loss ()
Increased activity of some cell wall enzyme. 0.5
Increased CO2 and ethylene production. Faster ripening, yellowing and abscission. Reduce wound healing (periderm formation). 1
Reduced turgor. Reduced susceptibility to chilling injury. Accelerated loss of volatiles. 2
Loss of membrane integrity. 3
Faster loss of vitamins A and C. Loss of flavor. Discoloration of mechanical injuries. 4
Loss of color intensity and gloss. Increase severity of pitting associated with chilling injury. Wilting and shriveling. 5
Loss of textural quality softening, limpness, flaccidity, and loss of crispness and juiciness. 6
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Pomegranate water loss
15
10
5
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Effects of Water Loss

• Physical Effects
• Reduced turgor pressure from as little as 2
  water loss enhances
• - Wilting flaccidity of vegetables.
• - Shriveling and wrinkling of fruit.
• Shrinking produce within a package allows it to
  move or vibrate during transport damage.

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Effects of Water Loss

• Economic Effects
• Reduced quality or grade of the commodity
  reduces its value.
• Commodities are often sold on a weight basis.
• Less weight lower price

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Effects of Water Loss

• Physiological Effects
• Increased respiration ethylene production
  (1).
• Reduced periderm formation in some roots and
  tubers (1).
• Faster ripening, yellowing senescence (1).
• Reduction in volatiles (2).
• Faster loss of vitamins A and C (4).

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Properties of Water

• High heat of vaporization (540 kcal/kg/oC).
• Water that evaporates absorbs a great amount of
  heat gt cools the plant tissue (transpiration).
• High heat of fusion (80 kcal/kg).
• When water cools from a liquid to a solid
  (ice), it releases heat energy.
• Therefore, water absorbs energy at the
  transition from solid (ice) to liquid. This is
  the added benefit for top-icing.

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Relative humidity (R.H.)

• Corresponds to the ratio of actual water content
  of the air to the maximum water content at a
  given temperature.

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Water vapors and liquid water

• Air water content (vapor pressure or humidity
  ratio) increases rapidly with increasing
  temperature.
• Warm air can hold more water than cold air.
• When warm, moist air is cooled, RH increases
  until it reaches its dew-point.
• Air cooled below its dew-point begins to loose
  water as condensation.
• Placing a cold commodity in a warm room with
  moist air, cools the air that contacts the
  commodity to below the dew-point.
• Condensation will form on the commodity surface
  (sweating).

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Water vapors and liquid water

• Placing a warm commodity in room with cold,
  moist air will warm the air contacting the
  commodity and reduce the humidity around the
  commodity.
• water loss increased until the commodity is
  cooled.
• Delayed cooling results in greater water loss.

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Factors affecting water loss

• Commodity factors
• Surface to volume ratio.
• Cell type (epidermal cells vs. periderm and
  other cells).
• Structure of the surface Stomates, Curticular
  waxes,
• Lenticels, Trichomes ect.

Stomates
Trichomes
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Factors affecting water loss

• Commodity factors
• Physiological state of the commodity.
• Stage of maturity or stage of ripeness.
• Cultivar.
• Cultural conditions.
• Weather and growing practices.

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Factors affecting water loss

• Environmental factors
• Humidity Lower humidity ? greater water loss.
• Diffusion shells and air velocity
• Outside the epidermis, there is a thin layer
  of air that maintains high humidity (diffusion
  shell). Surface features (e.g. trichomes)
  influence the thickness of this shell.
• Faster air flow
• ?
• decreases thickness of the diffusion shell
• ?
• increases water loss.

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Factors affecting water loss

• Environmental factors
• Temperature
• Higher temperatures ? greater water loss.
• Atmospheric pressure
• Lower pressures (at high altitudes) increases
  water loss.

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Water loss reduction

• Addition of water to some commodities
• (cut flowers, potted plants).
• Careful handling.
• Injury and puncture of surfaces greatly increase
  water loss.

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Water loss reduction

• Proper temperature, R.H., packaging, etc.
• Rapid cooling and keeping cold.

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Water loss reduction

• Curing of certain root, bulb, and tuber
  vegetables.
• Waxing and other surface coatings.

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Water loss reduction

• plastic films (wraps) act as moisture barriers.
• Packaging Polyethylene or plastic liners.
• Wood or plain fiberboard boxes can absorb water.

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Water loss reduction

• Influencing the environment
• Additional moisture to the air (humidifiers).
• Minimizing air movement around the commodity
  and reducing room air exchanges.
• Maintaining temperature of refrigeration coils
  within 1oC of the air temperature.

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Water loss reduction

• Influencing the environment
• Moisture barriers
• In the walls of storage rooms and transport
  vehicles.
• Polyethylene liners or curtains within shipping
  containers.
• Polymeric films for packaging produce.
• Wet the floor in storage rooms.

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Water loss reduction

• Influencing the environment
• Use 0f crushed ice in shipping containers and in
  retail
• display of commodities that tolerate direct
  contact with ice.

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Water loss reduction

• Sprinkle produce with water during retail
  marketing.
• Can be used on leafy vegetables, some root
  vegetables,
• and some fruits (peas, sweet corn, summer
  squash, ect.).