Lecture 3: Transpiration and Psychrometry Flashcards
like most bitter tasting foods, bitter melon stimulates _____.
what is it a “cure” for?
digestion
diabetes
what is transpiration?
- evaporation of H2O from produce to air
how does moisture loss occur?
- through evaporation and vapor diffusion
- all moist materials lose moisture when exposed to air which is not saturated
transpiration is the second most imp factor for _____ during storage and transportation
quality loss
what kind of effect does transpiration have during plant growth?
how?
cooling effect
b/c E is required for H2O evaporation. This E comes from the produce. Hence when H2O is removed, it brings down the produce temp
transpiration can be used to estimate the degree of ____
cooling
for water to evaporate, _____ needs to be absorbed
latent heat (L) of vaporization
L = 2260 kJ/kg
heat absorbed: Q = m*L = 2260kG/kg x 1kg = 2260kG
what is Cp?
what is Cp of water?
heat capacity
4.2 kG/kgC
what are the advantages of transpiration during plant growth?
- cooling effect
- mineral and water absorption: absorbed by roots from soil
- mineral distribution
loss of water represents an _____ loss
why is this?
economic
moisture loss causes irreversible damage to product quality
what are specialized surfae tissues where gas and water exchange occurs?
stomatae, lenticels, wax, cuticle, cuticular membrane, etc…
what are stomatae?
tiny openings in the epidermis with guard cells that open and close continuously
transpiration involves _____ _____ process
vapor diffusion
what is vapor diffusion process?
a dynamic process where the h2o vapor from a produce diffuses out across the produce’s surface under the influence of a water vapor pressure gradient
what law is involved in the vapor diffusion process?
Fick’s law: the basic law of gas diffusion
what is Pwa?
pressure exerted by water vapor in the air
what is Pwi?
pressure of water vapor in the internal tissue of produce
the Pwa (partial pressure of water vapor) is due to what?
the mass of water present in the air (the moisture content)
what is Pws?
saturated vapor pressure of water at a given T (max amount of water vapor the air can hold)
what happens when you increase the vapor pressure past Pws?
condensation
Pws ____ (incr/decr) with T
increases
what is the unit for vapour pressure?
kPa
relative humidity is the ratio of what?
Pwa to Pws
RH = (Pwa / Pws) x 100 in reference to a temperature
what is WVPD?
- water vapour pressure deficit
- provides the driving force for moisture transpiration
- the difference b/w the saturated water vapor pressure and the partial pressure of water vapour in the air
WVPD = ?
WVPD = Pws - Pwa
what does a positive WVPD mean?
produce will transpire and lose moisture
what does a higher WVPD mean?
higher transpiration loss
what does a negative WVPD mean?
- negative transpiration
- condensation of moisture on the product surface
- condensing moisture comes from air
define curing
- a treatment given to some roots, tubers and bulbs to extend their postharvest shelflife
- helps the surface layer to dry
- offers resistance to moisture loss and decay caused by bacteria and fungi
define transpiration coefficient (TC)
- used to compare the moisture loss by different commodities
- quantity of water vapor evaporated at a given temp by a unit wt of the product in a unit time under a unit gradient of WVPD
why is it necessary to have a transpiration coefficient?
since different produce behave differently with respect to actual moisture loss (size, surface area, skin ,etc..)
what is the unit of TC?
mg H2O / kg produce / MPa WVPD / s
what are factors affecting transpiration? where are they from?
factors from Fick’s Law
- diffusion coefficient
- SA of the produce
- thickness of the surface layer
- water vapor pressure deficit
- temp
produce with protective skin/peel will have ____ (higher/lower) TC
lower
higher surface to volume ratios indicate ____ (higher/lower) transpiration rates
higher
which has the highest surface/volume ratio?
a) leaves
b) tubers
c) legumes
leaves
tubers have the lowest
increase in RH will ___ (incr/decr) TR and ____ (incr/decr) WVPD
decrease; decrease
increase in Pwa will ___ (incr/decr) TR and ____ (incr/decr) WVPD
decrease, decrease
higher produce temp will ____ (incr/decr) TR and ____ (incr/decr) WVPD
increase; increase
lower air temperature will ____ (incr/decr) TR and ____ (incr/decr) WVPD
increase; increase
why is air movement important to avoid product TR rising?
b/c respiration produces heat and moisture. If heat is not removed efficiently, prod temp will rise and RD inreases
what is desirable and not desirable about high air velocity movement in storage?
what is a solution to this?
high air velocity increases heat transfer (desirable) and mass transfer (not desirable)
solution: use medium flow rates
physical and mechanical damages ____ (incr/decr) TR
increases
what are 5 methods of controlling transpiration losses?
- maintain high RH
- maintain low temp difference between cooling coils and produce (by maintaining large cooling surfaces, better refrigeration capacity, high air speeds, etc…)
- protect from mechanical and physical injuries
- regulate moisture loss (wax coating)
- prepackage in polymeric films (bags should have perforations to allow CO2 to escape and to keep O2 levels inside the package low)
define psychrometry
- the study of thermodynamic properties of air water vapour mixtures
what are common psychrometric parameters?
absolute humidity
relative humidity
dry and wet bulb temp
dew point
what does a psychrometric chart describe?
the relatioship between psychrometric parameters (absolute humidity; relative humidity; dry and wet bulb temp; dew point)
in a psychrometric chart, what is the horizontal axis?
dry bulb temp
the temp registered by a regular nonwetted thermometer
in a psychrometric chart, what is the vertical RH axis?
what unit is this?
absolute humidity/humidity ratio
- moisture content of the air
- unit: mass of water vapor per unit mass of air
in a psychrometric chart, what does the curves represent?
relative humidity
when can RH be 100%?
when the vapor pressure of air is equal to its saturated vapor pressure at that temp
what is wet bulb temp?
the temp registered by a thermometer when its bulb is in dynamic equilibrium with the moisture of the surrounding air
what are the diagonal lines on the psychrometric chart?
wet bulb temp
differentiate wet and dry bulb temp
in wet bulb: if the thermometer is surrounded by a wick that is constantly kept wet, and evaporation of the moisture can occur, the evaporation will cause the bulb temp to drop. Thus, wet bulb takes into account this temp difference
define dew point
temp of air which when cooled, can no longer hold its moisture and initiate moisture condensation
define state point
a point on the psychrometric chart determined by any 2 coordinates. when the state point is known, all other properties of air can be found from the chart
what are examples of utilization of the psychrometric charts?
heating cooling humidification dehumidification dehydration
