Development of a nuclear magnetic resonance-based sensor to detect ripeness of fruit

Seong In Cho, Purdue University

Abstract

The possibility of detecting ripeness of raw fruit by measuring sugar content with an A-200 Fourier Transform (FT) Nuclear Magnetic Resonance (NMR) was investigated. Chemical approaches for understanding ripeness, carbohydrates and sugar content in raw fruit were described. Then, the samples of sugar-water mixtures and real fruit were tested with the NMR and the correlations of sugar contents to NMR spectra were obtained. Finally, an NMR based sensor to grade raw fruit nondestructively was designed. Sugar contents in 7 different water-sucrose mixtures were measured with the NMR. Sugar content was linearly correlated to the amplitude of sugar peaks in the NMR spectra. The amplitude of the highest sugar peak was the basis for the correlation. The error of the sugar content measurement was within $\pm$2%. The use of NMR spectra of raw fruit samples was investigated for promise in the development of nondestructive and noncontaminative grading technique for raw fruit. Ripeness level (or sugar content) of raw fruit was also linearly correlated to the amplitude of sugar peaks in the NMR spectra. Therefore, the amplitude of sugar peaks could be used to grade raw fruit into several groups according to their ripeness levels. At present, most of NMR machines are quite expensive for food and agricultural engineering applications. A low cost NMR sensor to detect ripeness of fruit could be developed, because it does not require high accuracy compared to chemical or medical NMR. The use of permanent magnets and the design configuration of the magnet structure were the key factors to reduce the cost. The NMR based sensor for the quality control of raw fruit was designed by using a data based expert system and an interactive graphics simulation with a finite element model of the magnet console structure. Corner magnets were used for higher magnetic field induction around the air gap center where fruit would be located. This resulted in a more than 50% increase of the magnetic induction. Also, the shimming plates and the pole surface plates were attached to improve homogeneity of the magnetic field around the center. The NMR sensor design configuration could easily be changed depending upon the kind of fruit to be tested.

Degree

Ph.D.

Advisors

Krutz, Purdue University.

Subject Area

Agricultural engineering

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