effect of baking and steaming on physicochemical and thermal properties of sweet potato puree preserved by freezing and freeze-drying

Thermal treatments could be one of the hurdles in applications of sweet potato purees for food different products formulation. Sweet potato purees (SPP) were prepared from raw, baked and steamed roots and they were preserved by freezing and freeze-drying. The effects of baking and steaming on thermal properties (melting temperature-Tm, melting transition energy - ΔH, and glass transition temperatures - Tg) of sweet potato (cultivar Beauregard), were measured by means of a Differential scanning calorimetry (DSC). The SPP made from baked roots had higher total and soluble solids (20.32 and 18.95%, respectively) than SPP made from raw and steamed roots. It can be also noticed that starch content was reduced by steaming and baking which reflected on amount of total and reducing sugars. The increase of reducing sugars level in baked SPP for 3.78% and steamed for 0.86% SPP was the result of yielding the maltose. The chemical changes of SPP also influenced the thermal behavior such that SPP prepared from baked sweet potato roots had the lowest initial freezing point (-2.80 °C) followed by SPP prepared from steamed (-2.63 °C) and raw (-0.71 °C) roots. The highest energy for melting (transition) was needed for SPP prepared from raw potato roots followed by steamed and baked roots, -103.79, -103.63, and -102.90 J/g, respectively. The glass transition in freeze-dried SPP prepared from raw roots was not detected. However, in the freeze-dried SPP prepared from baked and steamed roots the glass transition was detected in the range of 39 and 42 °C but with no significant difference (p > 0.05).