ARS | Publication request: Dielectric spectroscopy measurements for moisture prediction in vidalia onions

Submitted to: Journal of Food Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2012
Publication Date: March 7, 2012
Citation: Mckeown, M.S., Trabelsi, S., Tollner, E.W., Nelson, S.O. 2012. Dielectric spectroscopy measurements for moisture prediction in vidalia onions. Journal of Food Engineering. 111:505-510.

Interpretive Summary: Vidalia onions are a type of sweet onion grown in a region of southeast Georgia that are highly valued in the marketplace for their desirable characteristics. The 2010 Vidalia onion crop was valued at more than $126 million. Prior to storage, onions must be cured. Curing is an essential step that seals the moisture in the interior of the onion from the outside environment. This forms a barrier to the intrusion of fungal infections during post-harvest storage and shipping. Currently, the curing process is controlled simply by time and visual inspection. However, varietal and year-to-year differences require adjustments in the curing procedure. If a method for rapidly sensing the curing of the onions could be developed, it would enable a more efficient curing process. Since dielectric properties of many agricultural products can be used for rapid sensing of moisture content, a study of the dielectric properties of onions was initiated for potential use in sensing moisture content as related to the curing process. Dielectric properties of fresh Vidalia onions were measured at frequencies from 200 MHz to 20 GHz over moisture contents ranging from 8% to 91%. The variation of these properties with moisture content at different frequencies was studied, and linear relationships were noted between the dielectric constant and moisture content at the higher (microwave) frequencies. The correlation was improved markedly by expressing the relationship between a density-independent function of the dielectric properties and moisture content. This discovery permitted the prediction of moisture content from the dielectric properties with a high degree of accuracy. Thus, microwave sensing of moisture content might be developed for use in following the moisture changes in onions during the curing and providing information useful in improving the curing process, which would be beneficial for growers and handlers in providing high quality produce for consumers.

Technical Abstract: Microwave sensing offers an opportunity to determine nondestructively the amount of moisture in materials by sensing the dielectric properties of the material. Dielectric properties of Vidalia onions grown in southeastern Georgia were measured with an open-ended coaxial-line probe and network analyzer in the range from 200 MHz to 20 GHz. Frequency dependence and moisture dependence of dielectric properties were analyzed for moisture contents between 8% and 91%. Moisture content was linearly correlated with the dielectric constant at higher frequencies for the entire moisture range. A density- independent function that incorporates both the dielectric constant and loss factor was tested across multiple frequencies and moisture ranges. Use of this function enabled prediction of moisture content with high accuracy (R² = 0.99) up to 40% moisture content.