Submitted to: Postharvest Biology and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/24/2008
Publication Date: 9/1/2008
Citation: Qin, J., Lu, R. 2008. Measurement of the optical properties of fruits and vegetables using spatially resolved hyperspectral diffuse reflectance imaging technique. Postharvest Biology and Technology. 49(3):355-365. Interpretive Summary: Absorption and scattering take place as light impinges on a turbid biological material. Light absorption is related to chemical constituents of the material (e.g., sugar), whereas light scattering is influenced by structural/physical characteristics (density, particle size, and cellular structures). Hence it is important to understand and quantify light absorption and scattering in the plant tissue in order to design better optical techniques for quality evaluation of fruits and vegetables. Several techniques have been developed to measure the optical properties of biological materials for medical diagnosis applications. But these techniques are still not suitable for horticultural products because they are expensive in instrumentation and inconvenient or restrictive in collecting data from intact samples. This paper reports the application of a new optical technique developed in our lab for measuring the optical properties of horticultural and food products. We developed a sensing configuration appropriate for fruits and vegetables and algorithms for processing and analyzing optical data. We determined the optical properties of eight types of fruits and vegetables (apple, kiwifruit, peach, pear, plum, cucumber, tomato, zucchini squash) and demonstrated the usefulness of optical properties for evaluating fruit maturity/quality and estimating light penetration depths for these fruits and vegetables. Compared to other currently available techniques, the new technique is simpler, faster, and easier to use. Researchers now have a new, convenient technique to measure the optical properties of fruits and vegetables, which are needed for quantitative analysis of light interaction with the tissues of fruits and vegetables. Instrumentation engineers can use the technique to assist in designing better sensing systems for quality evaluation of horticultural and food products.
Technical Abstract: This paper reports on the measurement of the optical properties of fresh fruits and vegetables over the visible and short-wave near-infrared region using a spatially resolved steady-state diffuse reflectance technique. A hyperspectral imaging system in line scan mode was used to acquire spatially resolved diffuse reflectance images from samples of apple (three varieties), peach, pear, kiwifruit, plum, cucumber, zucchini squash, and tomato (at three ripeness stages) over the spectral range of 500-1000 nm. The optical properties of the samples were determined from the spatially resolved scattering profiles using inverse algorithms for a diffusion theory model. Values of the absorption and reduced scattering coefficient for the test fruit and vegetable samples were in the range of 0.0-0.5 cm**-1 and 8 -25 cm**-1, respectively. Absorption spectra were featured by major pigments (i.e., chlorophyll, anthocyanin, and carotenoid) and water in the sample tissues. Scattering spectra steadily decreased with the increase of wavelength with different levels and slopes for different commodities. Absorption pattern changes were observed for the tomatoes with different background colors, and the absorption coefficient for anthocyanin at 535 nm and chlorophyll at 675 nm and/or their ratios could be used for monitoring the maturity of tomatoes. Values of reduced scattering coefficient decreased with the firmness of tomatoes. Light penetration depths, defined as the depths at which incident light was reduced by 99%, ranged from 0.91 to 5.78 cm for the fruit and vegetable samples over the wavelength range of 500-1000 nm; they were influenced by major pigments and water in the plant tissues. The spatially resolved steady-state diffuse reflectance technique provides a convenient and efficient means for measuring the optical properties of turbid food and agricultural products.