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5) Post-Harvest > Lecture 3: Transpiration and Psychrometry > Flashcards

Lecture 3: Transpiration and Psychrometry Flashcards

1
Q

like most bitter tasting foods, bitter melon stimulates _____.

what is it a “cure” for?

A

digestion

diabetes

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2
Q

what is transpiration?

A
  • evaporation of H2O from produce to air
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3
Q

how does moisture loss occur?

A
  • through evaporation and vapor diffusion

- all moist materials lose moisture when exposed to air which is not saturated

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4
Q

transpiration is the second most imp factor for _____ during storage and transportation

A

quality loss

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5
Q

what kind of effect does transpiration have during plant growth?

how?

A

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

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6
Q

transpiration can be used to estimate the degree of ____

A

cooling

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7
Q

for water to evaporate, _____ needs to be absorbed

A

latent heat (L) of vaporization

L = 2260 kJ/kg

heat absorbed: Q = m*L = 2260kG/kg x 1kg = 2260kG

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8
Q

what is Cp?

what is Cp of water?

A

heat capacity

4.2 kG/kgC

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9
Q

what are the advantages of transpiration during plant growth?

A
  1. cooling effect
  2. mineral and water absorption: absorbed by roots from soil
  3. mineral distribution
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10
Q

loss of water represents an _____ loss

why is this?

A

economic

moisture loss causes irreversible damage to product quality

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11
Q

what are specialized surfae tissues where gas and water exchange occurs?

A

stomatae, lenticels, wax, cuticle, cuticular membrane, etc…

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12
Q

what are stomatae?

A

tiny openings in the epidermis with guard cells that open and close continuously

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13
Q

transpiration involves _____ _____ process

A

vapor diffusion

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14
Q

what is vapor diffusion process?

A

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

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15
Q

what law is involved in the vapor diffusion process?

A

Fick’s law: the basic law of gas diffusion

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16
Q

what is Pwa?

A

pressure exerted by water vapor in the air

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17
Q

what is Pwi?

A

pressure of water vapor in the internal tissue of produce

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18
Q

the Pwa (partial pressure of water vapor) is due to what?

A

the mass of water present in the air (the moisture content)

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19
Q

what is Pws?

A

saturated vapor pressure of water at a given T (max amount of water vapor the air can hold)

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20
Q

what happens when you increase the vapor pressure past Pws?

A

condensation

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21
Q

Pws ____ (incr/decr) with T

A

increases

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22
Q

what is the unit for vapour pressure?

A

kPa

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23
Q

relative humidity is the ratio of what?

A

Pwa to Pws

RH = (Pwa / Pws) x 100 in reference to a temperature

24
Q

what is WVPD?

A
  • 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
25
Q

WVPD = ?

A

WVPD = Pws - Pwa

26
Q

what does a positive WVPD mean?

A

produce will transpire and lose moisture

27
Q

what does a higher WVPD mean?

A

higher transpiration loss

28
Q

what does a negative WVPD mean?

A
  • negative transpiration
  • condensation of moisture on the product surface
  • condensing moisture comes from air
29
Q

define curing

A
  • 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
30
Q

define transpiration coefficient (TC)

A
  • 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
31
Q

why is it necessary to have a transpiration coefficient?

A

since different produce behave differently with respect to actual moisture loss (size, surface area, skin ,etc..)

32
Q

what is the unit of TC?

A

mg H2O / kg produce / MPa WVPD / s

33
Q

what are factors affecting transpiration? where are they from?

A

factors from Fick’s Law

  • diffusion coefficient
  • SA of the produce
  • thickness of the surface layer
  • water vapor pressure deficit
  • temp
34
Q

produce with protective skin/peel will have ____ (higher/lower) TC

A

lower

35
Q

higher surface to volume ratios indicate ____ (higher/lower) transpiration rates

A

higher

36
Q

which has the highest surface/volume ratio?

a) leaves
b) tubers
c) legumes

A

leaves

tubers have the lowest

37
Q

increase in RH will ___ (incr/decr) TR and ____ (incr/decr) WVPD

A

decrease; decrease

38
Q

increase in Pwa will ___ (incr/decr) TR and ____ (incr/decr) WVPD

A

decrease, decrease

39
Q

higher produce temp will ____ (incr/decr) TR and ____ (incr/decr) WVPD

A

increase; increase

40
Q

lower air temperature will ____ (incr/decr) TR and ____ (incr/decr) WVPD

A

increase; increase

41
Q

why is air movement important to avoid product TR rising?

A

b/c respiration produces heat and moisture. If heat is not removed efficiently, prod temp will rise and RD inreases

42
Q

what is desirable and not desirable about high air velocity movement in storage?

what is a solution to this?

A

high air velocity increases heat transfer (desirable) and mass transfer (not desirable)

solution: use medium flow rates

43
Q

physical and mechanical damages ____ (incr/decr) TR

A

increases

44
Q

what are 5 methods of controlling transpiration losses?

A
  1. maintain high RH
  2. maintain low temp difference between cooling coils and produce (by maintaining large cooling surfaces, better refrigeration capacity, high air speeds, etc…)
  3. protect from mechanical and physical injuries
  4. regulate moisture loss (wax coating)
  5. prepackage in polymeric films (bags should have perforations to allow CO2 to escape and to keep O2 levels inside the package low)
45
Q

define psychrometry

A
  • the study of thermodynamic properties of air water vapour mixtures
46
Q

what are common psychrometric parameters?

A

absolute humidity
relative humidity
dry and wet bulb temp
dew point

47
Q

what does a psychrometric chart describe?

A

the relatioship between psychrometric parameters (absolute humidity; relative humidity; dry and wet bulb temp; dew point)

48
Q

in a psychrometric chart, what is the horizontal axis?

A

dry bulb temp

the temp registered by a regular nonwetted thermometer

49
Q

in a psychrometric chart, what is the vertical RH axis?

what unit is this?

A

absolute humidity/humidity ratio

  • moisture content of the air
  • unit: mass of water vapor per unit mass of air
50
Q

in a psychrometric chart, what does the curves represent?

A

relative humidity

51
Q

when can RH be 100%?

A

when the vapor pressure of air is equal to its saturated vapor pressure at that temp

52
Q

what is wet bulb temp?

A

the temp registered by a thermometer when its bulb is in dynamic equilibrium with the moisture of the surrounding air

53
Q

what are the diagonal lines on the psychrometric chart?

A

wet bulb temp

54
Q

differentiate wet and dry bulb temp

A

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

55
Q

define dew point

A

temp of air which when cooled, can no longer hold its moisture and initiate moisture condensation

56
Q

define state point

A

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

57
Q

what are examples of utilization of the psychrometric charts?

A 
heating
cooling
humidification
dehumidification
dehydration

5) Post-Harvest (17 decks)

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