Long-term storage of grain under plastic covers

• Department of Agriculture, Thailand
• National Postharvest Institute for Research and Extension, Philippines
• Malaysian Agricultural Research and Development Institute, Malaysia

Large amounts of grain and similar commodities stored in the tropics are held in warehouses in uncovered stacks of bagged product. This makes insect control difficult and often expensiveusually consisting of repeated fumigations and regular sprayings with insecticides. The CSIRO Division of Entomology has developed a method for the disinfestation and prolonged storage of bag stacks under sealed plastic sheeting, after initial disinfestation with carbon dioxide. In limited field trials, this process has successfully protected stored milled rice for up to 4 months under tropical conditions. However, its acceptance for wider routine use requires an understanding of some of the biological principles involved and an evaluation of the process in more extensive field trials.
Accordingly, this project seeks to determine the applicability of plastic covers for storage of grain in warehouses and in the open, and to determine the best design and size of enclosure, maximum permissible moisture contents and storage periods. Initially, it will concern bagged food and feed grains, but it will involve support studies relevant to both bagged and bulk grain. It comprises three sub-projects: field assessment of grain storage in sealed plastic enclosures; the protection against insect infestation offered by relatively low (less thant 40%) CO2 atmospheres; and quantification of moisture regimes in stored grain and related commodities.
Scientists in the team will coordinate their research with Projects 8310, on moisture movement in grain, and 8314, on controlled-atmosphere storage. They will carry out field evaluations initially in Australia, Malaysia, the Philippines and Thailand, and liaise closely with BULOG, Indonesia, in similar activities.
The first sub-project will investigate the reliability of the sealed sheeting storage method, in terms of protection from reinfestation and maintenance of commodity quality, and monitor gas holding and humidity within the stacks during storage to detect adverse changes within the atmosphere in the enclosure. Evaluation will involve a total of about 25 stacks (each 100200 tonnes) of bagged grain in each country. Analysis of grain samples will establish pre-trial quality and insect infestation. The team will then sheet the stacks, test the sealing and fumigate with CO2. They will include batches of test insects for bioassay as appropriate, and will open some stacks at pre-determined intervals to assess for changes in quality or infestation. This work will rely partly on data from the secon sub-project, for which it will provide field-testing facilities.
Studies on the effects of relatively low CO2 atmospheres on insects will seek to quantify the response of various developmental stages of some insect pests of stored grains, and will integrate the results with data currently being obtained for high CO2 contents so as to develop a broad model for the toxic response. The team will produce an optimal dosage regime for the use of CO2 and determine the degree of sealing needed to maintain this regime, and will confirm laboratory results in a parallel field assessment of mortality in the stacks in the first sub-project. Scientists in Australia will study the responses of Sitophilus oryzae over a range of CO2 contents between 10 and 40% at the Stored Grain Research Laboratory, while parallel studies in the Philippines will use other insects species.
Research in the third sub-project concerns moisture regimes in bulk-stored commodities. The moisture and temperature regimes of the humid tropics are particularly conducive to rapid deterioration, influencing the activity of insects and other deleterious organisms. However, reports of the water relations between commoditiesand their interstitial atmospheres differ. While the moisture content of the commodities has received much attention, that of their immediate micro-environments has more relevance to quality preservation and pest management procedures, especially in sealed or semi-sealed storage. Recent technology promises better measures of the humidity of interstitial air, and these measures coupled with improved understandingof the moisture relations with the commodities offer not only non-instrusive monitoring of quality but early warning of pest activity. Consequently, scientists in Australia will compile a comprehensive catalogue to list the diverse moisture sorption, desorption and equilibria data scattered through the literature, undertake a critical examinationof those data (to explain differences and assess reliability) and establish a standard test facility and use it to rectify deficiencies in the data. Using newly available semi-conductor peltier devices, they will develop a small dew-point sensor, to make the information derived above directly applicable within the bulk storage industries.