8 Special CA systems

Question 1

describe membrane CA storage

Answer 1

based on equilibrated gas diffusion from air through special films/membranes to achieve and maintain CA; employs special membranes with differential permeability for O2 and CO2.

Question 2

what is an example of a membrane used for membrane storage systems?

Answer 2

silicon elastomer: more permeable to CO2 than O2

Question 3

resulting CA condition depends on

Answer 3

• permeability, area of membrane, and concentration gradient
• respiration and quantity of produce
• temperature

Question 4

diffusion is represented by what law?

Answer 4

fick’s

Question 5

describe the rates of gas exchange in a porous (non specific) membrane

Answer 5

oxygen entry rate > CO2 exit rate

O2 in flow = CO2 out flow x (19/5)

Question 6

describe the rates of gas exchange in an equal permeability membrane

Answer 6

O2 permeability = CO2 permeability x 5/19; the amount of O2 absorbed = CO2 released

Question 7

describe the rates of gas exchange in a differential permeability membrane

Answer 7

usually the intent is to have higher permeability to CO2 than O2

Question 8

the air starts with 21% O2 and (basically) 0% CO2. what needs to happen to make a CA?

Answer 8

Ideally, we want to sustain 2% O2 and 5% CO2. That means 19% O2 needs to leave, and 5% CO2 needs to enter.

Question 9

what factors influence the design of a MA system?

Answer 9

• type of membrane
• type and quantity of produce
• CA conditions affect on concentration gradients and RR
• temperature
• membrane area

Question 10

what diffusion units were used in the 1960s?

Answer 10

• atmolysair

- marcellin

Question 11

what did the 1970s bring?

Answer 11

bulk packaging: impermeable membrane + silicon elastomer window to control the atmosphere inside

Question 12

what did the 1980s bring?

Answer 12

• MA packaging using polymers

- insertion of patches to scrub undesirable compounds

Question 13

what have we been using since the 1990s?

Answer 13

active packaging (modified polymers)

Question 14

describe hypobaric storage

Answer 14

produce is stored at substantially lower pressures under the continuous flow of saturated air

Question 15

what are the components of a hypobaric system?

Answer 15

• gas/air tight chamber
• vacuum pump
• pressure regulator
• flow regulator
• humidifier and humidity control
• refrigeration and temp control

Question 16

the typical conditions for a hypobaric system looks like:
Pressure = ?
RH = ?
Temp = ?
Air speed = ?

Answer 16

Pressure = 1-50 kPa
RH = 80-100%
Temp = -2 to -15C
Air Speed = 3 to 4 air changes

Question 17

air at lower pressure occupies a ______ volume

Answer 17

larger; based on P1V1=P2V2
1 = atmosphere
2 = low pressure system

Question 18

O2 concentration depends on (in hypobaric storage)

Answer 18

• chamber pressure
• temperature
• RH

Question 19

what are the advantages of outward diffusivity?

Answer 19

• better quality
• efficient removal of ethylene, CO2 and toxic volatiles
• very small concentration difference
• efficient flushing due to continuous flow

Question 20

LPS yields _____ heat of respiration

Answer 20

lower

Question 21

advantages of LPS?

Answer 21

• low heat of respiration and indirect heat removal
• reduced atmospheric composition at reduced pressures
• increased gas diffusivity
• reduced concentration gradients
• system offers good control of chamber oxygen levels
• change in diffusivity facilitates outward diffusion of other gases
• permits good air circulation without a fan
• high RH
• no need for 5% CO2 like in CA
• beneficial effect lasts even after removal from LPS

Question 22

what are the disadvantages of LPS?

Answer 22

• expensive
• special structures needed
• lacking flavors
• structures could leak, making humidity maintenance a problem
• inward implosion (insurance)