Continuous flow microwave-assisted processing and aseptic packaging of purple-fleshed sweetpotato purees.

Pumpable purees from purple-flesh sweetpotatoes (PFSP) were subjected to microwave heating using a 60 kW, 915 MHz continuous flow system, followed by aseptic packaging in flexible containers to obtain a shelf-stable product. Initial test runs were conducted using a 5 kW 915 MHz microwave system to measure dielectric in-line properties and examine the puree temperature profiles. The results demonstrated uniformity in heating of the puree at sterilization temperatures (>121 degrees C), and the dielectric constants and loss factors were within the range of published values for orange-fleshed sweetpotato purees. The pilot-scale test runs in a 60 kW microwave unit produced shelf-stable puree packages stable at room temperature. Polyphenolic content of the PFSP purees were evaluated and the results showed that while total phenolics increased (5.9%) and total monomeric anthocyanins slightly decreased (14.5%) with microwave application, antioxidant activity determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and oxygen radical absorbance capacity (ORAC) assays did not significantly change as a result of microwave processing. Color values showed that microwave-processed samples differed from fresh puree in saturation and hue angle, but not in overall color change. PFSP purees increased in gel strength when microwave processed, packaged, and stored, but the gel could be easily disrupted into flowable purees. Overall, high-quality retention can be obtained by microwave processing and aseptic packaging of PFSP purees so that they can be used as functional food ingredients.

Anthocyanin content, antioxidant activity, and selected physical properties of flowable purple-fleshed sweetpotato purees.

With high levels of polyphenolic compounds, purple-fleshed sweetpotatoes (PFSP) have been utilized as a healthy food commodity and source of natural food colorants in Asia. In the U.S. sweetpotato industry, there are growing interests in exploring these market opportunities for PFSP. A locally grown PFSP cultivar was analyzed for antioxidant properties. The total phenolic content ranged from 313.6 to 1483.7 mg chlorogenic acid equivalent/100 g fresh weight (fw), and anthocyanin contents were between 51.5 and 174.7 mg anthocyanins/100 g fw. The DPPH radical scavenging activities and were 47.0 to 87.4 micromol trolox equivalent (TE)/g fw, and the oxygen radical absorbance capacity (ORAC) values were between 26.4 and 78.2 micromol TE/g fw. Unlike orange-fleshed sweetpotatoes (OFSP), the steamed roots of PFSP formed a thick paste, which required a process modification to produce flowable purees. Rheological testing indicated that adjusting the dry matter of PFSP to 18%-21% produced purees with flow properties similar to the OFSP purees. The PFSP purees had polyphenolic content and antioxidant capacity within ranges reported for various purple-colored fruits and vegetables.