Grain depth: Contribution to the performance of hermetic storage

Increased adoption by African farmers of Purdue Improved Crop Storage (PICS) bags has reduced losses caused by insect pests of stored grain. PICS owes its effectiveness in part to the multiple barriers to gas movement across the bag wall. Two liners of high-density polyethylene (HDPE) provide major barriers to the influx of oxygen which insects require to survive, grow, develop, and reproduce. The outer, woven polypropylene mesh bag provides strength to the bag, but also contributes in a small, but significant way to restrict oxygen flow (Martin et al. 2014).  It has likewise been suggested that the outer layer of seed pressed against the inner surface of the bag further reduces the available surface area across which oxygen can diffuse into the bag (Murdock and Baoua, 2014). Small holes or abrasions in one or both HDPE layers have sometimes been observed in the field. These holes are likely caused by insects that developed closest to the inner surface of the bag. Yet, damage to the seed has been observed to be limited to the immediate area around the site of damage. The level of seed damage had a negative correlation with the distance from an oxygen source. Previous research (Jayas, 1988) has demonstrated a relationship between grain depth and resistance to airflow, supporting the claim that increasing the size of the grain bulk can add an additional barrier and help limit oxygen flow.  Attempts to assess by experiment the contribution of the grain mass to the barrier properties of the PICS system will be described.