Effect of soil temperature on starch properties of sweet potatoes

Starch was extracted from the tubers of two sweet potato cultivars (Ayamurasaki and Sunnyred) grown at four soil temperatures (15, 21, 27 and 33°C). The physicochemical properties of the isolated starches were analyzed in an attempt to investigate the impact of soil temperature on sweet potato starch. When the soil temperature increased from 15 to 33°C, the amylose content of the starch increased ca. 5.0%. The average granule size increased ca. 4.0 mu m as the soil temperature increased from 15 to 27°C. Highperformance anion-exchange chromatographic data indicated a distinct reduction in short chains of amylopectin with DP 6 and 7 with increasing soil temperatures. There were extensive variations in starch gelatinization characteristics as determined by differential scanning calorimetry. Large increases were found, values of onset (Ayamurasaki, 25.0°C; Sunnyred, 21.8°C) and peak (Ayamurasaki, 22.8°C; Sunnyred, 22.3°C) temperatures, as well as heat of gelatinization (Ayamurasaki, 5.4 J/g; Sunnyred, 3.3 J/g), as the soil temperature increased from 15 to 33°C. Starch pasting properties measured using the Rapid Viscoanalyzer varied according to soil temperature. Higher soil temperatures were generally associated with lower values of peak viscosity and setback. Thus, it was demonstrated that the soil temperature during the development of sweet potato tubers had an important influence on the starch properties.