3 Transpiration and Psychrometry Flashcards
define transpiration
evaporation of water from produce to air
true or false: produce transpires at all times
false: when the air is saturated, it cannot transpire
transpiration has a (heating/cooling) effect on produce. explain.
cooling
the energy required to create vapor is removed from the plant the water
what is L of water?
2260 kJ/kg
what are the pros and cons or transpiration?
pros during plant growth:
- cooling effect
- enables mineral and water absorption
- distributes minerals
cons:
- decreases sellable weight due to water
- quality loss from damage (wilting, shriveling, shrinkage, drying)
transpiration is a ______ phenomenon
diffusion
transpiration obeys the basic law of _____ _____ also known as _____ law
gas diffusion; Fick’s
Here are the variables of Fick's Law. Tell me what they are, and which ones are directly proportional to J: J A D Pi Pa dx R T
J = gas (vapor flux) A = Surface Area D = Diffusivity (diffusion coefficient) Pi = Partial Pressure inside produce Pa = Partial Pressure of water vapor in air dx = thickness of surface layer/film R = Gas constant T = Absolute temperature
A, D, (Pi-Pa) are directly proportional to J
Where can you find Pws?
steam tables. as T increases, so does Pws.
the percentage ratio of Pwa to Pws is the:
relative humidity
expand WVPD
waver vapor pressure deficit
the difference between the saturated water vapor pressure and the partial pressure of water vapor in the air is:
WVPD
when WVPD is positive…
produce will transpire and lose moisture
produce with a higher WVPD will undergo (greater/less) transpiration losses than produce with a lower WVPD
more
placing warm produce in a cold room will result in a (greater/smaller) WVPD than warm produce at room temperature
greater
what happens when you have a positive WVPD but the RH is 100?
sweating
what’s a good way to reduce WVPD without making it negative?
bring cool (not cold) produce (harvested during cool weather) to room temperature, and RH of 50%
what happens to WVPD when you have cold produce brought to room temperature?
negative WVPD (condensation)
under what 2 undesirable conditions related to WVPD promotes the growth of mold?
sweating and condensation
differentiate between sweating and condensation
sweating: produce attempts to transpire, but the moisture can’t enter the saturated atmosphere so it stays on the surface of the produce
condensation: moisture attempts to enter the produce but the skin’s too good at its job, so it just collects on the surface
what is curing?
treatment for some roots, tubers and bulbs to extend their PH shelf life. helps surface layer to dry, offering resistance to moisture loss and decay via bacteria and fungi
what two mathematical methods are used to determine moisture loss
WVPD and TC
what is TC?
transpiration coefficient
what are the units for the TC?
mg/H2O/kg produce/MPa WVPD/s
as produce respires, the temperature will (increase/decrease) causing the transpiration rate to (increase/decrease)
increase, increase
what 5 methods are used to control transpiration losses?
- maintaining high humidity
- maintaining low temp difference between cooling coils and produce
- protect from mechanical and physical injuries
- regulate moisture loss (permeability)
- prepackage in polymeric films (allow CO2 escape, regulate oxygen levels)
the study of the thermodynamic properties of air water vapor mixtures is known as:
psychrometry
on the psych chart, what does the horizontal axis delineate?
dry bulb temp
on the psych chart, what do the curved liens delineate?
RH
on the psych chart, what do the diagonal lines delineate?
wet bulb temp
how is dew point read on the psych chart?
intersection of horizontal line through state point and 100% humidity line
stay at same dry bulb temp but increase RH, dew point, and humidity ratio
isothermal humidification
stay at same dry bulb temp but decrease RH, dew point and humidity ratio
isothermal dehumidification
stay at the same humidity ratio but increase dry bulb temp, decrease RH,
adiabatic dehumidification
stay at the same humidity ratio but decrease dry bulb temp, increase RH
adiabatic humidification
