Post Harvest Management of Horticultural Crops

Modified Atmosphere storage

Modified Atmosphere storage (MAS )

MA storage implies a lower degree of control of gas concentration in atmosphere surrounding the commodity. The MA and CA differ only in degree of control, CA is more exact.
Advances in the manufacture of polymeric films with wide range of gas permeability have stimulated interest in creating and maintaining modified atmospheres within flexible film packages.

Biochemical and Physiological Basis of MA
The rate of respiration and metabolism doubles for every 10°C rise in temperature. Respiration can be therefore reduced by decreasing the temperature, O2 level and/or increasing the CO2 level in the storage atmosphere. Both O2 and CO2 levels exert independent effects on respiration. The net effect may be additive or synergistic. When O2 concentration is reduced below 10%, respiration rate is decreased. However, when O2 concentration falls below 2%, anaerobic respiration may set in, thereby leading to the accumulation of ethanol and acetaldehyde.
The desirable effect of MA on plant tissues is also attributed to lower pH, due to dissolution of CO2 in tissues. Ethylene action and biosynthesis are also effected besides water loss and chilling injury

Commodity	Temperature range (°C)	Modified Atmosphere
		% O2	% CO2
Asparagus	0-5	Air	5-10
Broccoli	0-5	1-2	5-10
Cabbage	0-5	3-5	5-7
Cauliflower	0-5	2-5	2-5
Sweet corn	0-5	2-4	10-20
Cucumber	8-12	3-5	0
Leek	0-5	1-2	3-5
Lettuce	0-5	2-5	0
Okra	8-12	3-5	0
Onion (green)	0-5	1-2	10-20
Pepper	8-12	3-5	0
Potato	4-12	None	None
Spinach	0-5	Air	10-20
Tomato(partially ripe)	8-12	3-5	0

Environmental factors affecting MA storage

a. Temperature and relative humidity

Ambient temperatures of the surrounding atmosphere affect the commodity temperature. Temperature changes also affect the permeability of the film, which increases with increase in temperature. CO2 permeability responds more than O2 permeability. Relative humidity has little effect on permeability of most film packages. Most common films are good barriers to moisture and vapour because they maintain high internal humidity even in dry, ambient conditions.

b. Light

Green vegetables consume large amount of CO2 and reduce O2 through photosynthesis and would antagonize the process of respiration which aids in maintenance of specified MA within the package. Greening of potatoes can cause loss in quality unless light is excluded. Hence, opaque packages should be used for such commodities.

c. Sanitation Factors

The high humidity maintained within MA packages may enhance the growth of plant pathogens. So care must be taken to ensure proper sanitation and to avoid conditions favourable to growth and reproduction of such micro organisms. Fungicidal treatment of packaged vegetables is thus very important.