Postharvest water loss

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, accelerateddevelopment of injuries.• Textural quality – loss of crispness, juiciness, etc.• Nutritional quality – e.g. vitamins A & C.• Thus, managing water content of commodities iscritically important.

Loss of marketability due to water loss

Commodity% water Loss

Asparagus8

Brussels sprouts8

Spinach7

Lettuce5

Celery3

Cabbage3

Water loss (%)Potential effects

0.5%Increased activity of some cell wall enzyme.

1%Increased CO2 and ethylene production. Faster

ripening, yellowing and abscission. Reduce wound

healing (periderm formation).

2%Reduced turgor. Reduced susceptibility to chilling

injury. Accelerated loss of volatiles.

3%Loss of membrane integrity.

4%Faster loss of vitamins A and C. Loss of flavor.

Discoloration of mechanical injuries.

5%Loss of color intensity and gloss. Increase severity

of pitting associated with chilling injury. Wilting and

shriveling.

6%Loss of textural quality: softening, limpness,

flaccidity, and loss of crispness and juiciness.

Pomegranate water loss

5%

10%

15%

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.

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

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%).

Properties of WaterHigh heat of vaporization (540 kcal/kg/oC).• Water that evaporates absorbs a great amount of heat =>

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.

Relative humidity (R.H.%)

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

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”).

• 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.

Water vapors and liquid water

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.

Trichomes

Stomates

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.

Factors affecting water lossEnvironmental 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.

Factors affecting water loss

Environmental factors:

Temperature:

• Higher temperatures → greater water loss.

Atmospheric pressure:

• Lower pressures (at high altitudes) increases water loss.

Water loss reduction

• Addition of water to some commodities

(cut flowers, potted plants).• Careful handling.

Injury and puncture of surfaces greatly increase water loss.

• Proper temperature, R.H.%, packaging, etc.

• Rapid cooling and keeping cold.

Water loss reduction

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

Water loss reduction

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

Water loss reduction

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.

Water loss reductionInfluencing 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.

Influencing the environment:

Use 0f crushed ice in shipping containers and in retail

display of commodities that tolerate direct contact with ice.

Water loss reduction

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.).