Investigation of Dielectric Sensing for Fruit Quality Determination

Authors: Nelson, Stuart; WENCHUAN, GUO - NW AG & FRSTRY UNIV,CHINA; Trabelsi, Samir

Submitted to: IEEE Sensor Applications Symposium
Publication Type: Proceedings
Publication Acceptance Date: 12/21/2007
Publication Date: 2/12/2008
Citation: Nelson, S.O., Wenchuan, G., Trabelsi, S. 2008. Investigation of Dielectric Sensing for Fruit Quality Determination. IEEE Sensor Applications Symposium.

Interpretive Summary: New nondestructive techniques for sensing quality of fresh fruit or fruit that has been stored before consumption would be helpful to producers, handlers and consumers. Subjective quality standards for many products tend to be highly arbitrary, different for individual consumers and for sellers and buyers, and often subject to supply and demand. Thus objective measurements that can be made rapidly and nondestructively could provide improvements in sorting and handling operations and also be helpful to consumers in selecting produce of desired quality. Electrical characteristics of fruit tissue known as dielectric properties can be sensed with electric fields for possible detection of fruit quality. Therefore, honeydew melons and watermelons were grown and harvested with a range of maturities for dielectric spectroscopy measurements of the melon tissue to learn whether there might be differences in the dielectric properties that could be correlated with quality. The best criterion for quality is the soluble solids content, which are mostly sugars and therefore a measure of sweetness. This requires the extraction of tissue samples from the melons and measurement of expressed juice with a refractometer instrument, which has been calibrated to indicate percentage of soluble solids. Similar dielectric measurements were made on fresh apples during a ten-week storage period along with firmness measurements that determined deterioration during storage. The dielectric properties of the melon and apple tissues were measured over the frequency range from 10 MHz to 1.8 GHz along with refractometer determinations of soluble solids content (SSC). A high correlation was found between SSC of the melons and the dielectric properties as expressed in a complex-plane plot of the dielectric constant and loss factor, each divided by SSC. Through this mathematical relationship, SSC was calculated from measured dielectric properties, but predictions were not as high as desired, and further research is needed to assess the dielectric technique for sensing watermelon quality. Dielectric properties of apples were relatively constant during the ten-week storage period even though apple tissue softened significantly during that period. The challenge in using this new principle for practical nondestructive sensing of melon and apple quality is considerable, but development of such quality meters would be helpful to growers and handlers in the marketing of high quality produce for American consumers.

Technical Abstract: Permittivities of honeydew melons and watermelons, grown to provide a range of maturities, were measured with an open-ended coaxial-line probe and impedance analyzer at frequencies from 10 MHz to 1.8 GHz. Measurements on fresh apples were also made over a ten-week storage period. Data were taken on the external surfaces of the fruit and on tissue samples from the edible internal tissue. Soluble solids content (SSC), indicative of sweetness, was used for melons as the quality factor for correlation with permittivity. Dielectric constant and loss factor correlations with SSC were low, but a high correlation was obtained between the SSC and permittivity from a complex-plane plot of dielectric constant and loss factor, each divided by SSC. However, quality prediction from this relationship was not very high. Dielectric properties of apples were relatively constant during the ten-week storage period even though firmness of apple tissues decreased significantly over that period.