Physicochemical and Viscometric Properties of a Sweetpotato Syrup

The sweetpotato [Ipomoea (L.) Lam.] has approximately 90% carbohydrate of its dry matter, which makes it ideal for glucose syrup production. However, data regarding the isolation and use of sweetpotato starch (SPS) in syrup production are scanty. The objectives of this study were to: i) develop a syrup from SPS; and ii) determine the physicochemical (refractive index and color), and viscometric properties during storage of the sweetpotato syrup. SPS was isolated from Hillbilly variety field-grown sweetpotatoes. The SPS was rehydrated, heated to 102°C, treated with -amylase at 90°C for five hours, cooled, and further treated with glucoamylase at 62.5°C for 12 hours. The syrup was filtered, evaporated and cooled. The refractive index and color were measured. To determine the viscometric properties of the syrup during storage, the syrups were stored between 21±4°C (room temperature) and 4°C. An AR-2000 Rheometer was used to measure the viscometric properties of the syrup. The refractive index of the sweetpotato syrup was significantly higher (P<0.01) than that reported in the literature (1.4±0.02 versus 0.7). The mean L*, a* and b* values of the syrup were 68.8±0.6, 0.7±0.1 and 18.7±0.6, respectively. The mean viscosity of the sweet potato syrup was 1.98 Pa·s at room temperature and 0.95 Pa·s after five days at 4°C. The viscosity for the sweetpotato syrup gradually increased as storage time increased. With syrups stored at both temperatures, increased shear stress decreased their viscosities. Under similar testing commercial corn syrup, exhibited no decrease in viscosity as shear stress increased. Overall, the sweetpotato syrup had acceptable refractive index, color and viscosity.