Transpiration

Question 1

What is transpiration?

Answer 1

evaporation of water (from produce -> air)

Question 2

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

Answer 2

saturated

Question 3

What are the processes through which moisture loss occurs?

Answer 3

vapor diffusion

evaporation

Question 4

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

Answer 4

transpiration

Question 5

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

Answer 5

Cooling

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

Question 6

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

Answer 6

latent

produce; sensible

Question 7

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

Answer 7

Cp of water (4.2kJ/kgC)

consists of mostly water

Question 8

What are the equations for latent and sensible heat?

Answer 8

latent: Q = mass x L

sensible heat = Q = m(Cp) delta T

Question 9

advantages of transpiration:

Answer 9

cooling effect

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

Question 10

disadvantages of transpiration:

Answer 10

irreplaceable water loss

economic loss (weight loss)

quality loss (shrivel, wither)

Question 11

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

Answer 11

3%

Question 12

What permits transpiration in plant tissue?

Answer 12

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

Question 13

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

Answer 13

different # of stomatae on surface

different amounts of moisture

Question 14

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

Answer 14

vapor diffusion

Fick’s law of diffusion

Question 15

water vapor pressure outside the fruit is represented by:

Water vapor pressure within the fruit is represented by:

Answer 15

outside: Pwa (air)
inside: Pwi

Question 16

Pwi is equal to: ____.

Why?

Answer 16

Pws (saturated vapor pressure @ that temperature)

fruit is mostly water

Question 17

Pwa depends on: ____

Answer 17

amount of water present in the air (moisture content)

Question 18

What is Pws?

Answer 18

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

Question 19

If T increases, Pws will: ____.

How do we find the values?

Answer 19

increase

steam table

Question 20

What is WVPD? Why is it relevant?

Answer 20

water vapor pressure deficit
Pws-Pwa
DRIVING FORCE FOR TRANSPIRATION

Question 21

What is RH?

Answer 21

relative humidity

Pwa/Pws x 100%

Question 22

RH is meaningless without also giving _____

Answer 22

temperature

Question 23

What measurement represents the driving force of transpiration?

Answer 23

WVPD

Question 24

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

Answer 24

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

Question 25

if WPVD is positive, then the produce will: | The higher the WPVD, the (more/less) moisture is lost.

Answer 25

lose moisture | more

Question 26

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

Answer 26

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

Question 27

Why is sweating in produce bad?

Answer 27

more free moisture -> mold

Question 28

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

Answer 28

False: will cause high transpiration losses

Question 29

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

Answer 29

False: there will be CONDENSATION on produce

Question 30

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

Answer 30

1. positive WPVD, Pwa saturated (sweating) | 2. negative WPVD (condensation)

Question 31

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

Answer 31

curing; roots and tubers

Question 32

What does curing achieve? (2)

Answer 32

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

Question 33

Curing rooms require special control of what factors?

Answer 33

temperature, relative humidity

Question 34

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

Answer 34

``` WVPD surface area surface thickness temperature diffusion coefficient ``` Fick's Law

Question 35

What is "TC?" What does it represent and why is this relevant?

Answer 35

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

Question 36

How is TC expressed (units)? (2)

Answer 36

1. mg H2O per kg produce/MPa WVPD/s | 2. % water lost per day per unit WVPD

Question 37

What types of produce have the highest TC?

Answer 37

roots, leafy greens

Question 38

What characteristic will cause a product to have low TC?

Answer 38

skin/peel

Question 39

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

Answer 39

volume | greater

Question 40

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

Answer 40

produce temperature | air temperature

Question 41

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

Answer 41

RH | decrease

Question 42

How does respiration affect transpiration?

Answer 42

respiration produce HEAT | if not removed -> higher produce temp -> more transpiration

Question 43

How does mechanical damage affect transpiration?

Answer 43

increase | since increase respiration -> increase heat

Question 44

What controllable factors should be considered to control transpiration?

Answer 44

air flow temperature humidity

Question 45

What role does air play in controlling transpiration?

Answer 45

require air flow to remove heat from respiration

Question 46

If the air flow is too fast, what occurs? | If air flow is too slow, what occurs?

Answer 46

too fast -> remove more water (more mass transfer) | too slow -> doesn't remove enough heat

Question 47

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

Answer 47

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