ON - LINE NMR EVALUATION OF AVOCADO FRUIT QUALITY

Abstract

The objective of this study is to develop a system using magnetic resonance as a quality sorting or grading technique for intact fruit and to construct and test a prototype on-line sorting machine. This study is performed to determine methods of utilizing NMR more efficiently from an engineering aspect. An avocado fruit is used as a sample for this experiment. Development of a real time sorting system for avocado fruit maturity included design of a data acquisition and processing using LabVIEW5.0. A robust data processing algorithm was developed to measure the maturity of avocados. A single pulse NMR experiment is used to acquire the proton free induction decay. The free induction decay is Fourier transformed and the magnitude spectrum is obtained. The proton magnitude spectrum yields two resonance peaks at low – field (25.9MHz), one associated with water protons and one associated with oil protons. The resonance peaks overlap and are fit to Lorentzian functions. The functions are then integrated to measure the amount of each component. The ratio of these quantities is directly correlated to solid content and hence avocado maturity. A microwave drying moisture / solids analyzer is used to validate the NMR results. The feed rate as well as the spacing of the fruit on the conveyor belt influence the NMR spectra. Higher feed rate and closer spacing of avocados result in greater distortion of the NMR spectra. Good predictions of avocado maturity were obtain at belt speeds of 25cm/s with a spacing between fruit of 20 cm. This NMR internal quality fruit sorting system can sort avocados by maturity into several category in real time at a feed rate of 1.25 fruit / sec.