Submitted to: International Journal of Food Properties
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2007
Publication Date: 3/10/2008
Citation: Brinely, T.A., Truong, V., Coronel, P., Simunovic, J., Sandeep, K.P. 2008. Dielectric properties of sweetpotato purees at 915 MHz as affected by temperature and chemical composition. International Journal of Food Properties. 11:158-172. Interpretive Summary: A process has been recently developed for converting sweetpotato roots into a high-quality puree wherein the material can be rapidly sterilized in a continuous flow microwave system operating at 915 MHZ, and aseptically packaged into flexible containers. The developed technology provides an alternative method to convert sweetpotatoes into a shelf-stable ingredient which would be readily available for the food processing industry and eventually result in increased consumption of this nutritious vegetable. In microwave processing, dielectric properties have a major role in determining the interaction between purees and the electromagnetic energy. The objective of this research was to determine the effects of microwave processing temperature (up to 145'C) and chemical components on the dielectric properties of purees derived from sweetpotato cultivars with yellow-, orange- and purple-flesh color. Results indicated that temperature, moisture, sugar and starch content had a pronounced effect on dielectric properties of the purees. Predictive equations were developed for dielectric constant and dielectric loss factor as a function of temperature, moisture, sugar, and starch. Overall, the developed predictive equations can be utilized in determining the heating patterns of sweetpotato puree for industrial microwave processing of sweetpotato purees.
Technical Abstract: A process for rapid sterilization and aseptic packaging of sweetpotato puree using a continuous flow microwave system operating at 915 MHz has been successfully developed. This development offers an opportunity for converting sweetpotato roots into a functional ingredient for the food industry. In microwave processing, dielectric properties have a major role in determining the interaction between purees and the electromagnetic energy. The objective of this research was to determine how dielectric properties are affected by temperature and chemical composition of purees derived from thirteen sweetpotato cultivars with varying flesh colors. Starch, sugars, ash, protein, minerals and moisture content were determined following AOAC methods. Fat content was determined with a pulsed proton NMR (nuclear magnetic resonance) using a Maran pulsed NMR. Dielectric constant (E’) and dielectric loss factor (E”) were measured from 15'C to 145'C at 915 MHz using an open ended coaxial probe equipped with a network analyzer. The chemical composition of sweetpotato purees (wet weight basis) was in the following ranges: ash (0.7% - 1.2%), fat (0.1 -0.2%), protein (0.3 - 0.7%), moisture (64.4% - 81.8%), sugar (5.3 -15.4%), and starch (1.8 - 15.7%). Temperature, moisture, sugar and starch content had a pronounced effect (p < 0.001) on dielectric properties. Dielectric constant linearly decreased with increasing temperature, while dielectric loss factor increased quadratically. Power penetration depth of all cultivars decreased with increasing temperature. Predictive equations were developed for dielectric constant (R2 = 0.82) and dielectric loss factor (R2 = 0.90) as a function of temperature, moisture, sugar, and starch. The predictive equations would be useful in determining the dielectric properties of sweetpotato purees for the microwave processing technology.