Post Harvest Management of Horticultural Crops

Methods of creating modified atmosphere conditions

1. Commodity generated or passive MA
First produce is packed in sealed plastic film. As result of respiration the produce and non/selective permeability of the package to oxygen, the concentration of the O2 reduced and concentration CO2 increased due to respiration thus generating MA.
Film must allow O2 to enter the package at a rate offset by the consumption of O2 by the commodity and CO2 must be vented from the package to offset the production of CO2 by the commodity. This atmosphere must be established rapidly and without danger of the creation of injurious or high levels of CO2.



2. Active Packaging
This can be done by creating a slight vacuum and replacing the package atmosphere with the desired gas mixture. This mixture can be adjusted by the use of adsorbers or absorbers in the package to scavenge O2, CO2 or C2H4. Active modification ensures rapid establishment of the desired atmosphere. Ethylene absorbers can help to ensure the delay of the climacteric rise in respiration. CO2 absorbers can prevent the building up of CO2 to injurious levels.
Package that offer a level of control over in-package conditions and how they vary with produce (eg. ethylene production) and environmental (eg. temperature) factors. Eg. Polymer film that can increase or decrease in permeability to O2 and CO2 as temperature rise and fall, respectively.
Modified atmosphere conditions are created inside the packages by an active modification and also using O2, CO2 and ethylene scavengers within the package. These scavengers may be held in small sachets within the packages or impregnated in wrappers or into porous materials like vermiculate. The smart wrap, which contains a permeable membrane impregnated with an ethylene absorbent, an anti-fog material to avoid moisture condensation and a slow release fungicide to inhibit the mould growth.

Inadequate (high) temperature and differences in the gas diffusion across plastic films as compared to those of physiological process such as respiration increases the anaerobic condition may occur in sealed plastic film packages. This risk is minimised using safe foil (eg. low melting point polymer) or variable aperture devices (eg. bimetallic strips) to regulate formation and/or size of the holes. Advances in microelectronics, biosensor and polymer sciences may develops the film that actively sense and respond in a controlled way of stimuli, such increase in temperature.

3. Oxygen Absorbers
Most commonly available O2 absorbers include Ferrous oxide (FeO): Iron is main active ingredient in powdered form becoming Fe2O3 and Fe3O4 and its hydroxides after absorption of O2.

4. CO2 Absorbers - They are hydrated lime, activated charcoal, magnesium oxide.

5. Ethylene Absorbers/scrubber
Compounds that can be used for ethylene absorption within polymeric film packages are potassium permanganate KMnO4 absorbed on celite, vermiculite, silica gel or alumina pellets. They oxidize ethylene to CO2 and H2O. Squalene and phenyl methyl silicon can also be held in small sachets within the packages or impregnated in the wrappers or into porous materials like vermiculite.