Continuous flow microwave reactors are alternative for pasteurizing, sterilizing sweet potato puree.

With high levels of nutrients and bio-active compounds, sweet potato puree can be considered a functional ingredient for use in processed foods. However, due to the puree's low heat conductivity and high viscosity, conventional thermal processing usually results in poor product quality and causes a considerable loss of nutrients.

Alternative technologies that create rapid and uniform heating, such as microwave energy, could help overcome these problems. Scientists at North Carolina Sate University evaluated the feasibility of aseptically processing sweet potato puree using a continuous flow cylindrical microwave system operating at 915 MHz.

The scientists measured the dielectric properties of the puree using an open-ended coaxial probe and an automatic network analyzer. Ten liters of sweet potato puree were pumped at 0.5 L per min to achieve a single-pass heating effect from room temperature to 90 C, 100 C and 121 C in a 5-kW microwave heating unit. The samples were rapidly cooled and analyzed for their color and rheological properties.

The researchers found that the dielectric constant values of sweet potato puree, processed at 10 C to 95 C, ranged from 70 to 62. Its loss tangent values ranged from 0.25 to 0.47. Based on these values, the maximum diameter of the applicator tube used was calculated to be 42 mm. The puree treated in the 5-kW microwave absorbed 88% and 90% of the microwave energy when it was heated and reached end point temperatures of 90 C and 121 C, respectively. The product's color and rheology indicated that the treatment at 121 C produced a good quality puree.

Subsequently, the scientists processed 100 kg of sweet potato puree using a two-stage 60 kW continuous flow microwave heater to reach a center temperature of 121 C in the product. They held it at that temperature for 20 seconds, cooled it to 20 C, and packaged the product in an aseptic bag-in-box filler. The resulting product was stable under refrigerated conditions. The investigators believe that the process is commercially viable.

Further information. Josip Simunovic, Center for Advanced Processing and Packaging Studies, Department of Food Science, 14C Schaub Hall, North Carolina State University, Raleigh, NC 27612; phone: [phone]; fax: [phone]; email: [e-mail].