Technology and solutions

 When crops are being grown a variety of insects exists when still out in the field. Once harvested and stored, specific varieties of insects that thrive in warehouses only, are or become a part of the stored commodity. There are insects and pests that feed on the grain and have life cycles enhanced by sometimes hundreds of eggs per insect in a few weeks. These storage insects and pests like heat and moisture and can cause major damage to the stored commodity, not only causing weight loss, but often adding foul smell. Under tropical conditions insect damage can be 5-10% on an annual basis. Insects also can contribute to the spread of mycotoxins in the grain. 

Conventional treatment against infestation is by "fumigation", where a toxic gas is applied to the stack. The most common fumigant being "phosphine" (PH3) which is highly toxic for humans when inhaled.  Practically good fumigation is complicated as exposure of insects to the gas requires hard to achieve gas tight conditions. Storage pests and insects have become increasingly resistant to the fumigant and restrictions on the use of the gas at some a parts of the world because of environmental considerations, make a non-toxic solution desirable and part of the global effort to achieve "green" food and feed supply chains. 

"Modified atmosphere (MA)" solutions take the approach that insects and especially the eggs of these insects can be controlled by depriving them of essential oxygen and/or adding inert carbon dioxide.  The most classical form of MA is "hermetic" or "sealed" storage, where the commodity is stored in a gas tight container and the respiration of the insect population drives down oxygen levels and increases carbon dioxide levels to a point where the increase of the insect population becomes "stagnant" and controlled. 

 Most agricultural  commodities need to be dried  to a level where the moisture in the commodity can no longer cause fungal growth. Fungus develops when the humidity levels are 70% or higher.  A number indicating a moisture content of a commodity where it is at "equilibrium" (=balance) with 70% relative air humidity is common global practice.  This number indicating the "critical moisture content" of a commodity is also related to temperature.  Obviously at low temperatures fungal development is arrested and therefore in the US grainbelt  or in Heilongjiang Province in China, maize can be stored at 15 or 16% moisture. However in tropical climates that number goes down to 13% in the tropics. 

So in tropical climates proper drying to 13% for paddy and maize is crucial if long term storage is needed.  Since most drying is done in the sun, proper drying of grain is challenging and storage operations tend to be risky due to uneven or not properly dried grain. 

When commodities are stored in gas tight conditions and their moisture content exceeds the critical point, the respiration of microflora (fungi) adds carbon dioxide to the storage atmosphere. 

Elevated carbon dioxide levels  (in addition to the carbon dioxide generated by the insects)  create conditions that at one hand preserve the commodity and control infestation, but on the other hand elevated moisture contents increase chances for condensation when temperature fluctuations do exist. 

While storing under these conditions the stacks need to be closely monitored for convection currents and preventive measures are to be taken to reduce condensation damage. 

 The chemical composition of a commodity will change over time while stored: commodities contain fat or oil, starch and a range of chemicals that have impact on  taste and aroma, smell and color.  Oxidation is a break down process where chemical composition changes as the result of exposure to oxygen.  This process is intensified when heat and moisture are added.  All of us know what happens when nice crunchy peanuts are left for a day on the table:   the taste changes, they become soft and the taste ultimately becomes "rancid".  That is because peanuts contain some 50% oil and oil is very sensitive to exposure to oxygen. 

Green coffee beans are highly sensitive to moisture. Their critical moisture content is about 11-12%. When the moisture content slightly increases, the coffee will get a "stale" taste, which is easily recognized in coffee that has been stored too long. 

Cocoa beans contain like peanuts, a lot of fat.  When the beans are stored in the tropics, the level of "free fatty acids" in cocoa butter (the main product made of cocoa) will increase and effect the taste. 

Red chillies need to be stored for a year or more. While doing so the color will change from reddish to brown and the typical "pungency" (the "pepper taste") will decrease. 

In all cases and commodities mentioned, storage in an environment inside gas barriers keeping oxygen and moisture away, will preserve the quality of the stored commodity.

 The storage issues mentioned above are related to living organisms as insects and microflora, as well as chemistry: exposure to oxygen. Modified atmosphere technology addresses respiration, moisture ingress and control of oxygen. Respiration and oxygen are related, as any living creature uses oxygen and produces carbon dioxide. 

So by changing the composition of the air from almost 20.9% O2, the rest mostly nitrogen (N2) and  tiny amounts of other gases, such as 0.03% CO2, conditions can be created that have impact on the quality issues mentioned above. 

For instance insects at 7% O2 or less stop multiplying, at <3% O2 they die, including the insect eggs. 

Most molds (the "aerobic" ones) stop developing at O2 levels <1%, whereas reduced O2 of course slows down oxidational processes. 

Since these are all gases, changes in gas composition of the air in which a product is stored can occur only when the storage enclosure is made of materials that are as "impermeable" to gases as possible. Glass or metal would be good "gas barriers", but are not very practical when the stored volumes are large. For modified atmosphere technology to be commercially viable, it needs containers made of different materials. 

Permeability

 There is a wide range of flexible plastic films in the market.  Plastic is a good barrier against water vapor, but performs less well when gases as O2, N2 and CO2 are involved, which tend to leak through the plastic materials. There are standards for leakage or permeability rates, measuring the transmission of these gasses through the plastic film, measured  in milliters of gas per square meter per 24 hrs (at a given temperature and RH).   Common plastics such as PE and PP are highly permeable to these gases, where PVC performs better but still not very impressive. 

The plastic industry has developed the ability to make "co-extruded" plastics of multi (up to 9) layers, where each layer contributes specific properties to the film produced. 

Interesting is the development of  EVOH (Ethylene-vinyl alcohol copolymer) which when added as a layer through coextrusion into the film, creates extremely low permeability to O2. 

Choices in plastic materials and sustainability

No doubt that currently PE and its various co polymers can when co-extruded with EVOH , create excellent gas barriers to retain  the various gases playing a role in MA technology.  Increasingly recyclability of the materials plays a role, and PE and EVOH can be easily  recycled. 

PE is approved for contact with food and most of the food packing in the supermarket is made of it. 

The plastic industry has been able to develop films with excellent UV resistance. Modern  PE greenhouse films have life spans exceeding 10 years!

Whereas PVC has advantages in strength and weldability, it has clear disadvantages in toxicity, weight and price. 

Multilayer coextruded PE films with gas barriers  exceed PVC in barrier performance , in weight and if required, also in strength. 

Gas tight flexible containers  can be as small as a bag for a few kilos, to larger structures for storing bagged grain of up to a few hundred tonnes. 

Different forms of "modified atmosphere"

Storage atmosphere can be changed by  respiration only of all living organisms in the grain and then it is commonly called "hermetic", "gas tight" or "bio generated"  storage (HS).  This form of storage has been commonly practice  in semi arid areas around the Mediterranean and elsewhere, where large jars or underground pits sealed with clay and cow dung created environments of relatively low oxygen/high carbon dioxide. 

A more modern form of MA is when the storage atmosphere is artificially changed by inserting CO2 or N2  from a cylinder or a generator. This is called "controlled atmosphere" as the amount of gas inserted is regulated. In fact insects can be controlled by depriving them of oxygen or "poisoning" them with high CO2 concentrations.  Both methods are commercially applied, especially in the organic (bio) sector. 

In all cases the use of an excellent gas tight container is required to make gas concentrations effective.  Poorly designed gas tight containers will loose gas quickly, whereas well designed ones typically will not loose more than 0.5-1% per day,