Effects of corn oil on glass transition temperatures of cassava starch

Glass transition temperatures of cassava starch–corn oil blends were determined by means of differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA), in a moisture content range of 4–35% (dry basis, d.b.). The samples were equilibrated at ambient temperature (25 °C) during 4 weeks. A sub-Tg endothermic peak, occurring between 45 and 60 °C, appeared in samples with relatively low moisture contents (less than: 14.4% (d.b.), 10.1% and 8.5% for added corn oil levels of 1.1, 4.5 and 6.6%, respectively). In correspondence with DSC measurements, secondary relaxation temperatures (Tβ) at around 55 °C, measured by DMTA were also present at low moisture contents (less than: 18% (d.b.), 16% and 15% for added corn oil levels of 1.1, 4.5 and 6.6%, respectively). At intermediate moisture contents enthalpic relaxation events associated to ageing processes were evidenced by DSC first heating scans, while at higher moisture contents only the glass transition was exhibited by the samples. In all cases, increasing levels of added corn oil decreased the moisture content at which the samples exhibited the above described thermal events. The effect of water on glass transition temperatures (as determined during a DSC second heating scan and DMTA results) was well described by the Gordon–Taylor equation. It was found that cassava starch was better plasticized by water when the levels of added corn oil were decreased. The added corn oil was also found to be a plasticizer for cassava starch probably due to hydrophobic–hydrophilic type interactions with starch.