The principle of adsorption is based on the property of solids (adsorbent) to bind certain gases (adsorbate) to their surface. Interactions between adsorbent and adsorbate are usually electrostatic, and therefore weak and reversible.
Food grains and seeds containing significant quantities of protein interact with carbon dioxide to form a complex by adsorption, causing the CO2 to disappear from the inter-grain space. In hermetic enclosures, this creates a "vacuum effect" by pressing the envelope against the product. For cereals, the higher the CO2 concentration in the enclosure, the greater the quantity of CO2 adsorbed onto the grain. On paddy rice, in a pure CO2 atmosphere, the adsorption of CO2 by the grain can reach 1 L per kg of grain, when inerted with 100% CO2. This adsorption takes place over several days after CO2 injection. Japanese researcher Mitsuda and his team have shown that the lower the grain temperature, the more intense the adsorption phenomenon (Mitsuda & Yamamoto, 1980). At 10°C, adsorption can be twice as high as at 30°C. This phenomenon of shrinkage of sealed envelopes under CO2 can be used as an indicator of enclosure tightness. Conversely, if the vacuum is too high, the tightness of the hermetic structure or cell may be compromised. For cells with a capacity of several tens of tonnes, a "mechanical" safety system has been developed in Australia (by CSIRO in conjunction with hermetic cell manufacturer Ahrens agri™) to absorb negative or positive pressure differences in large-capacity rigid storage structures (Fleurat-Lessard, 2019).