Effect of controlled atmospheres on quality of stored grains

• National Postharvest Institute for Research and Extension, Philippines

Enclosing bag stacks in gas-tight plastic envelopes has potential for overcoming losses of grain and other food commodities stored in bags. The closely related ACIAR Projects 8307 and 8310 concern development of the technology for application in Malaysia, the Philippines and Thailand, with particular reference to the insect and moisture problems associated with storage in the humid tropics. However, understanding of the effects of various storage atmosphere on the quality of stored foods in inadequate. Recent Australian research has confirmed that some atmospheres cause defects in stored wheat, and other grains would probably deteriorate also. In particular, rice stored at more than 15% m.c. is said to become tainted when held under carbon dioxide, but some evidence suggests that this gas may provide a better storage atmosphere than phosphine and that the benefit involves water activity changes.
To resolve such uncertainties, the present project seeks to determine the influence of oxygen and carbon dioxide tension in the storage atmosphere, in combination with temperature and water activity, on the viability of stored grains, spices and pulses. It incorporates a study of the relations between water production and various storage gases during storage in closed systems.
Commodities for study will include milled rice, maize, soybeans, various spices (probably pepper, chilli and cardamom), cocoa, sorghum and barley. Samples of know provenance will be subjected to particular atmospheres under controlled temperature and water activity for various exposure periods.
Atmospheres will comprise different concentrations of oxygen in nitrogen and of carbon dioxide in air. Those chosen should provide a satisfactory range of 02 and CO2 activities for constructing descriptive models that will facilitate study of how these gases affect quality. Of the four temperatures chosen, 60C provides a possible model system for rapid evaluation of storage, with the influence of mould growth eliminated. At the low end, 30C directly reflects field conditions. Two intermediate temperatures - 35 and 47C - will provide data for a mathematical model.
Scientists in Australia will measure the water output of grains in the presence of various levels of CO2 and one reference concentration of phosphine. They will also conduct all storage experiments. Quality evaluation will be carried out both in Australia and in the Philippines, where collaborating scientists will test commodities for seed viability and for quality characteristics related to the end use of each commodity.
This research should develop the quantitative understanding necessary for efficient and safe utilisation of the storage technology emerging from Project 8307. In addition, it will contribute basic data in a suitable form for integration into the mathematical models of heat and moisture environments in storages that scientists in Project 8310 are constructing to promote further development of the technology.