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Title: Maturity Effects on Dielectric Properties of Apples from 10 to 4500 MHz

item GUO, WEN-CHUAN - Northwest Agricultural & Forestry University
item ZHU, XINHA - Northwest Agricultural & Forestry University
item Nelson, Stuart
item YUE, RONG - Northwest Agricultural & Forestry University
item LIU, YI - Northwest Agricultural & Forestry University
item LIU, HUI - Northwest Agricultural & Forestry University

Submitted to: Journal of Food Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2010
Publication Date: 1/1/2011
Citation: Guo, W., Zhu, X., Nelson, S.O., Yue, R., Liu, Y., Liu, H. 2011. Maturity Effects on Dielectric Properties of Apples from 10 to 4500 MHz. Journal of Food Engineering. 44(201):224-230.

Interpretive Summary: Techniques for nondestructive determination of quality of agricultural products are helpful to producers, handlers and processors, those marketing the produce, and consumers. Visible and physical characteristics of many fresh fruits and vegetables are available for correlation with quality, and some of these, such as color, size, weight, density, elasticity, and firmness are used in automatic sorting of some produce into different categories for the market. Electrical properties of fresh fruits, known as dielectric properties, can be rapidly sensed with suitable measurement instruments that employ radio-frequency (RF) electric fields for this purpose. Therefore, if adequate correlations can be found between the dielectric properties of such fruits and their quality factors, it may be possible to develop new instruments for rapid nondestructive quality determination. In new exploratory studies, the dielectric properties of apples were measured at 24 'C over two months of the tree-ripening period to determine whether these properties might be used to determine quality factors such as soluble solids content (SSC), firmness, moisture content, and pH value. The dielectric constants and dielectric loss factors at 101 frequencies from 10 to 4500 MHz were determined for external surface, interior tissue, and juice measurements along with moisture content, firmness, and SSC (sweetness) and dc conductivity of juice expelled from the internal tissues. These new data are being reported for the benefit of other scientists and engineers, although no strong correlations were discovered that might be useful for nondestructive detection of the quality characteristics of fresh apples. Further research is needed to assess the potential for sensing quality of apples through their dielectric properties for the benefit of growers, handlers, marketers and consumers.

Technical Abstract: Dielectric properties of Fuji apples during the last two months of tree-ripening, from mid-August to mid-October, were measured with an open-ended coaxial-line probe and network analyzer at 24 'C over the frequency range from 10 to 4500 MHz. At the same time, firmness, soluble solids content (SSC, sweetness), pH, moisture content and electrical conductivity were measured to determine whether permittivity differences might exist that can be related to apple quality indices. Permittivity measurements were taken on the external surface of the apples, on internal edible tissue and on juice expressed from internal tissue. The frequency dependence of the loss factor of internal tissue and juice show that losses associated with ionic conduction dominate at lower frequencies, while those associated with dipole polarization account for the major loss at higher frequencies. A dielectric relaxation was noted at about 20 MHz in apple surface permittivity data. During the tree-ripening period, apple permittivity data and electrical conductivity did not reveal obvious trends. The moisture content and soluble solids content remained essentially constant, while the firmness of tissue decreased and pH value of juice increased with maturity. No obvious correlation was found between permittivity and firmness, moisture content and the pH. The linear relationship between surface permittivity and SSC was poor (R2<0.2). The best linear regression coefficient of determination between loss tangent of tissue and SSC was 0.61 at 4500 MHz. Between dielectric constant of juice and SSC, it was 0.67 at 4500 MHz. Further studies are needed to assess the potential usefulness of the dielectric properties for sensing apple maturity or internal quality.