Transpiration Flashcards
What is transpiration?
evaporation of water (from produce -> air)
all materials will lose moisture to the air, unless the air is _____
saturated
What are the processes through which moisture loss occurs?
vapor diffusion
evaporation
What is the second most important factor responsible for quality loss during storage/transportation?
transpiration
Transpiration has a (cooling/heating) effect. Why?
Cooling
Requires energy to evaporate water (state change) -> energy comes from the produce
The energy required to evaporate transpired water is ____ heat, which comes from _____ in the form of _____ heat.
latent
produce; sensible
heat capacity of the produce is assumed to be: _____ because of: _____
Cp of water (4.2kJ/kgC)
consists of mostly water
What are the equations for latent and sensible heat?
latent: Q = mass x L
sensible heat = Q = m(Cp) delta T
advantages of transpiration:
cooling effect
needed during plant growth for mineral/water absorption through roots, and mineral distribution
disadvantages of transpiration:
irreplaceable water loss
economic loss (weight loss)
quality loss (shrivel, wither)
The maximum % moisture loss by weight for high moisture produce is as low as ___
3%
What permits transpiration in plant tissue?
specialized surface tissue; with STOMATAE (tiny openings open/close continuously)
allows for gas/liquid exchange
Why do different produce transpire at different rates (structural difference)?
different # of stomatae on surface
different amounts of moisture
The process of ___ ____ in transpiration, in which water vapor travels through the membrane, obeys what law?
vapor diffusion
Fick’s law of diffusion
water vapor pressure outside the fruit is represented by:
Water vapor pressure within the fruit is represented by:
outside: Pwa (air)
inside: Pwi
Pwi is equal to: ____.
Why?
Pws (saturated vapor pressure @ that temperature)
fruit is mostly water
Pwa depends on: ____
amount of water present in the air (moisture content)
What is Pws?
saturated vapor pressure: max amount of water air can hold at temperature T
If T increases, Pws will: ____.
How do we find the values?
increase
steam table
What is WVPD? Why is it relevant?
water vapor pressure deficit
Pws-Pwa
DRIVING FORCE FOR TRANSPIRATION
What is RH?
relative humidity
Pwa/Pws x 100%
RH is meaningless without also giving _____
temperature
What measurement represents the driving force of transpiration?
WVPD
What should be done if the temperature of the produce and air are different when calculating WPVD? What about calculating RH?
WVPD: use temperature of produce to find Pws
RH: use air temperature to find Pws
if WPVD is positive, then the produce will:
The higher the WPVD, the (more/less) moisture is lost.
lose moisture
more
What happens if the WPVD is positive but the Pwa is already saturated 100%?
SWEATING: produce will lose moisture and transpire, but air cannot pick up; stays on surface (BAD)
Why is sweating in produce bad?
more free moisture -> mold
True/False: moving warm produce into a cold room is ideal
False: will cause high transpiration losses
True/False: having a negative WPVD value is good because the produce will absorb moisture
False: there will be CONDENSATION on produce
What WPVD conditions will cause excess moisture on the exterior of produce?
- positive WPVD, Pwa saturated (sweating)
2. negative WPVD (condensation)
Controlled transpiration can be used in the process of ____ on hardy produce such as ____, to extend shelf life.
curing; roots and tubers
What does curing achieve? (2)
Dries out surface -> resistant to further moisture loss, less microbial spoilage
Curing rooms require special control of what factors?
temperature, relative humidity
What are factors that affect the amount of transpiration? (5)
All these factors can be summed up by ____
WVPD surface area surface thickness temperature diffusion coefficient
Fick’s Law
What is “TC?” What does it represent and why is this relevant?
Transpiration Coefficient
amount of water vapor evaporated (per unit weight product, unit time, unit WVPD gradient, at a given temperature)
allows for comparison of transpiration between different commodities
How is TC expressed (units)? (2)
- mg H2O per kg produce/MPa WVPD/s
2. % water lost per day per unit WVPD
What types of produce have the highest TC?
roots, leafy greens
What characteristic will cause a product to have low TC?
skin/peel
The (smaller/greater) the surface area to ____ ratio, the greater the transpiration rate
volume
greater
Higher ____ or lower ____ will increase transpiration rate (temperature).
produce temperature
air temperature
At the same temperature, higher Pwa or higher ____ will (increase/decrease) transpiration
RH
decrease
How does respiration affect transpiration?
respiration produce HEAT
if not removed -> higher produce temp -> more transpiration
How does mechanical damage affect transpiration?
increase
since increase respiration -> increase heat
What controllable factors should be considered to control transpiration?
air flow
temperature
humidity
What role does air play in controlling transpiration?
require air flow to remove heat from respiration
If the air flow is too fast, what occurs?
If air flow is too slow, what occurs?
too fast -> remove more water (more mass transfer)
too slow -> doesn’t remove enough heat
What methods are used to control transpiration in storage? (5)
- maintain higher RH
- minimize temperature difference (produce vs air) - can precool
- protect from injury
- wax coating
- plastic wrap/packaging
