Effects of moist-heat treatments on color improvement, physicochemical, antioxidant, and resistant starch properties of drum-dried purple sweet potato powder

Abstract

Purple sweet potato (PSP) is sensorially attractive and rich in a valuable source called “anthocyanins,” but this tuberous crop is highly perishable. It is largely found in hot and humid countries, such as the South East Asia region. This study applied the drum drying technique to turn PSP into a shelf-stable powder product, wherein an issue related to severe discolouration (browning) was identified. Hence, two moist-heat pretreatments, namely boiling and steaming, were performed prior to drum drying. The effects of pretreatment on color changes, physicochemical, antioxidant, and resistant starch properties in the final product were analyzed. Both pretreatments had successfully retained the purplish color in the final product, in which steaming displayed better effects when compared with boiling in powder yield (243.02 g/100 g vs. 180.87 g/100 g), total anthocyanin content (121.71 mg/100 g vs. 83.72 mg/100 g), and resistant starch content (3.06 g/100 g vs. 2.53 g/100 g). The study outcomes demonstrated the capability of steaming in producing PSP powder that retained the original color and the health-promoting compounds (i.e., anthocyanins and resistant starch), which appeared to be better than boiling, thus pretreatment on PSP prior to drum drying is highly recommended. Practical applications: This study successfully tackles the browning issue identified in purple sweet potato processing, via two simple and straightforward heating methods; boiling and steaming. The tuber is turned into a shelf-stable, ready-to-use powder that acts as a thickener in soups, bakeries and fabricated snacks. The final powder product displays an attractive purplish color, exceptional functionality (in terms of water absorption capacity) and is loaded with health-promoting compounds including antioxidant and resistant starch. These findings proved that moist-heat treatments retain the purplish color, anthocyanins and resistant starch content in drum-dried purple sweet potato powder well and provide additional health benefits besides acting as an ordinary food thickener.