Density is one of the important and widely used quality indices for agricultural products. Differences in density have been utilized for inspection of quality, structure, stiffness, maturity and seed viability. Density is calculated through measurement of mass and volume of an object. Mass can be measured precisely and quickly with the use of a digital scale. Volume measurement, however, is not so easy. There are two typical methods for measuring volume: the gas displacement method and the liquid displacement method. The liquid displacement method is simple and easy, but agricultural products or foodstuff, especially dairy products, may be damaged by immersion into liquid. The gas displacement method of measuring density does not affect food seriously. However, it requires quite a long time for measurement. A volume measurement system for foodstuffs through a Helmholtz resonator has been used to eliminate the above-mentioned problems. It had already been reported that this volume measuring system performed well ( ) while measuring volumes of water, clay, beans, apple and grapes. However, variations of resonance frequency as a function of volume are too small (0.1 Hz/cc) and need special analysis methods as well as expensive hardware. Therefore, in this research, one of the standing waves of a special Helmholtz resonator is used for estimation of walnut volume. This standing wave produced a wide range of variations for resonance frequency in response to walnut volume change (about 22Hz/cc). In this case one can employ the well-known Fast Furrier Transform (FFT) method toghether with inexpensive hardware for analyzing signals. Uncertainty of volume measurement by this method for spherical objects was ±0.47cc. Volumes of individual walnuts were measured non-destructively and with more precision than in the liquid displacement method. Correlation between the two methods was high ( ). This method and the instrumentation may easily be calibrated and used for other agricultural and food products where the water displacement method is not applicable.
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