Water relationships and temperature interactions in maize grain during maturation

The growth of microorganisms during maturation is influenced by several factors including temperature, moisture content and water activity (aw). In order to better understand the conditions in which microorganisms develop during kernel development, the evolution of the grain moisture content and water activity (aw) in a commercial hybrid (Pioneer PR34N43) has been recorded during maturation and the relationships between moisture content, aw and temperature have been analysed. In 2007, maize grain samples were collected in two fields during grain maturation at 60–17% (±1%) of grain moisture (wet basis). The samples were analysed for aw at 15, 25, and 40 °C and the moisture content was determined.The results have shown that the moisture content drops linearly during maturation, while aw is constant (aw = 1.000) until about 36–38% of grain moisture, and only after this moisture level, does it start to decrease. This would seem to indicate that: (i) the water that is initially lost is all surface or loosely held water and aw does not actually start dropping until late on the maturation phase, and that, (ii) consequently the conditions remain favourable to microbial growth and development until the last grain maturation phases. The measurements conducted at the three temperatures indicate that the effect of temperature only starts to be evident at a grain moisture content of about 30%: after this level, the higher the temperature, the higher the water activity at the same moisture content. Moreover, the lower the moisture content, the more the temperature influence aw.