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Post harvest technology > Transpiration > Flashcards

Transpiration Flashcards

1
Q

What is transpiration?

A

evaporation of water (from produce -> air)

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2
Q

all materials will lose moisture to the air, unless the air is _____

A

saturated

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3
Q

What are the processes through which moisture loss occurs?

A

vapor diffusion

evaporation

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4
Q

What is the second most important factor responsible for quality loss during storage/transportation?

A

transpiration

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5
Q

Transpiration has a (cooling/heating) effect. Why?

A

Cooling

Requires energy to evaporate water (state change) -> energy comes from the produce

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6
Q

The energy required to evaporate transpired water is ____ heat, which comes from _____ in the form of _____ heat.

A

latent

produce; sensible

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7
Q

heat capacity of the produce is assumed to be: _____ because of: _____

A

Cp of water (4.2kJ/kgC)

consists of mostly water

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8
Q

What are the equations for latent and sensible heat?

A

latent: Q = mass x L

sensible heat = Q = m(Cp) delta T

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9
Q

advantages of transpiration:

A

cooling effect

needed during plant growth for mineral/water absorption through roots, and mineral distribution

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10
Q

disadvantages of transpiration:

A

irreplaceable water loss

economic loss (weight loss)

quality loss (shrivel, wither)

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11
Q

The maximum % moisture loss by weight for high moisture produce is as low as ___

A

3%

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12
Q

What permits transpiration in plant tissue?

A

specialized surface tissue; with STOMATAE (tiny openings open/close continuously)
allows for gas/liquid exchange

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13
Q

Why do different produce transpire at different rates (structural difference)?

A

different # of stomatae on surface

different amounts of moisture

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14
Q

The process of ___ ____ in transpiration, in which water vapor travels through the membrane, obeys what law?

A

vapor diffusion

Fick’s law of diffusion

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15
Q

water vapor pressure outside the fruit is represented by:

Water vapor pressure within the fruit is represented by:

A

outside: Pwa (air)
inside: Pwi

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16
Q

Pwi is equal to: ____.

Why?

A

Pws (saturated vapor pressure @ that temperature)

fruit is mostly water

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17
Q

Pwa depends on: ____

A

amount of water present in the air (moisture content)

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18
Q

What is Pws?

A

saturated vapor pressure: max amount of water air can hold at temperature T

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19
Q

If T increases, Pws will: ____.

How do we find the values?

A

increase

steam table

20
Q

What is WVPD? Why is it relevant?

A

water vapor pressure deficit
Pws-Pwa
DRIVING FORCE FOR TRANSPIRATION

21
Q

What is RH?

A

relative humidity

Pwa/Pws x 100%

22
Q

RH is meaningless without also giving _____

A

temperature

23
Q

What measurement represents the driving force of transpiration?

A

WVPD

24
Q

What should be done if the temperature of the produce and air are different when calculating WPVD? What about calculating RH?

A

WVPD: use temperature of produce to find Pws
RH: use air temperature to find Pws

25
Q

if WPVD is positive, then the produce will:

The higher the WPVD, the (more/less) moisture is lost.

A

lose moisture

more

26
Q

What happens if the WPVD is positive but the Pwa is already saturated 100%?

A

SWEATING: produce will lose moisture and transpire, but air cannot pick up; stays on surface (BAD)

27
Q

Why is sweating in produce bad?

A

more free moisture -> mold

28
Q

True/False: moving warm produce into a cold room is ideal

A

False: will cause high transpiration losses

29
Q

True/False: having a negative WPVD value is good because the produce will absorb moisture

A

False: there will be CONDENSATION on produce

30
Q

What WPVD conditions will cause excess moisture on the exterior of produce?

A
  1. positive WPVD, Pwa saturated (sweating)

2. negative WPVD (condensation)

31
Q

Controlled transpiration can be used in the process of ____ on hardy produce such as ____, to extend shelf life.

A

curing; roots and tubers

32
Q

What does curing achieve? (2)

A

Dries out surface -> resistant to further moisture loss, less microbial spoilage

33
Q

Curing rooms require special control of what factors?

A

temperature, relative humidity

34
Q

What are factors that affect the amount of transpiration? (5)
All these factors can be summed up by ____

A 
WVPD
surface area
surface thickness
temperature
diffusion coefficient

Fick’s Law

35
Q

What is “TC?” What does it represent and why is this relevant?

A

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

36
Q

How is TC expressed (units)? (2)

A
  1. mg H2O per kg produce/MPa WVPD/s

2. % water lost per day per unit WVPD

37
Q

What types of produce have the highest TC?

A

roots, leafy greens

38
Q

What characteristic will cause a product to have low TC?

A

skin/peel

39
Q

The (smaller/greater) the surface area to ____ ratio, the greater the transpiration rate

A

volume

greater

40
Q

Higher ____ or lower ____ will increase transpiration rate (temperature).

A

produce temperature

air temperature

41
Q

At the same temperature, higher Pwa or higher ____ will (increase/decrease) transpiration

A

RH

decrease

42
Q

How does respiration affect transpiration?

A

respiration produce HEAT

if not removed -> higher produce temp -> more transpiration

43
Q

How does mechanical damage affect transpiration?

A

increase

since increase respiration -> increase heat

44
Q

What controllable factors should be considered to control transpiration?

A

air flow
temperature
humidity

45
Q

What role does air play in controlling transpiration?

A

require air flow to remove heat from respiration

46
Q

If the air flow is too fast, what occurs?

If air flow is too slow, what occurs?

A

too fast -> remove more water (more mass transfer)

too slow -> doesn’t remove enough heat

47
Q

What methods are used to control transpiration in storage? (5)

A
  1. maintain higher RH
  2. minimize temperature difference (produce vs air) - can precool
  3. protect from injury
  4. wax coating
  5. plastic wrap/packaging

Post harvest technology (18 decks)

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