Horticultural products evaluated by thermography (00-PH-003)
Hellebrand, H. J.(*); Linke, M.; Beuche, H.; Herold, B.; Geyer, M. Institute of Agricultural Engineering Bornim (ATB), Max-Eyth-Allee 100, 14469 Potsdam, Germany. Tel:+49 (0) 331 5699 212 Fax: +49 (0) 331 5699 849 email
[email protected]
Introduction Different properties of horticultural products such as water status, water vapour permeance, ripeness and metabolic activity have a significant effect on the quality, the mechanical properties, and the shelf life of the product. Biological activity generates metabolic heat and this leads generally to increasing temperature of the products. The metabolism of fruits and other biological products can be measured in terms of oxygen consumption and release of carbon dioxide by respiration analysis. The loss of water through transpiration causes the temperature gradient at the surface due to the latent heat required for the transformation of liquid phase to vapour. The transpiration itself depends on the type and state of fruit (size, shape, skin, maturity, etc.), the temperature and humidity of the surrounding air, and on the air flow around and against the product. Thermal images reflect the superposition of heat and mass transfer of the samples in relation to the ambient conditions. Therefore, thermal imaging could have potential for non-contact determining the properties of horticultural products. Canopy and plant leaf temperatures have been studied for about 160 years. The application of thermography for the study of plants started about 40 years ago (Monteith and Szeicz 1962, Tanner 1963). Due to dropping costs within the last decade, thermography has become an important tool in engineering, in medical research, and in other branches. There are several publications, in which plants or plant materials have been studied by thermal imaging. Infrared vision technique was applied for the determination of physiological depression in crop plants (Inque 1990), leaf transpiration rates (Inque et al 1990), stomatal conductance (Jones 1999) and single cell studies (Zohar et al 1998). Differences in the flag leaf temperature of cereals, caused by diseases and wind gusts, were observed by thermography measurements (Nilsson 1995, Daley 1995). Although expected, surface damage (bruises) of horticultural products could not be verified so far by thermography because of not sufficient temperature resolution (Danno et al 1978, Beverly et al 1987, Tollner et al 1993). Materials and methods Experimental studies are performed using a commercial infrared thermal imaging camera, with commercial software for image evaluation. The images have a temperature resolution of 0.1 K. The MCT-sensor of the camera is nitrogen cooled and operates at a wavelength of 10 µm. The intensity of thermal radiation is related to the temperature of the object according to Stefan-Boltzmann's law for radiation to the power of four and is directly proportional to the emission factor of the sample. Since the sensor measures radiation intensity, the emission factor of the samples must be determined independently or taken from the literature, as is the case in the first studies reported here. The image analysis software makes it possible to determine the temperature at single pixels, to provide the temperature profile across a chosen axis, to set isothermal areas of pixels, to evaluate regarding mean temperature of selected areas, and to determine the
standard deviation of temperature pixels. The measured temperature differences can serve as a source to characterise plots of transpiration at the surface of the fruit, or, by using transpiration models, to determine transpiration coefficients quantitatively. In this way, the temperature distribution across the surface may be used as an indicator of the state and quality of horticultural products. Results Although traditional grading is based upon shape and colour analysis, thermography gives additional possibilities for differentiation of fruits by varieties (Fig.1). Products with defects or mechanical damage also give variable surface temperature, or a measurable temperature reduction. Defective fruit skins can often be detected by a localised temperature decrease of the fruit (Fig. 2). Apple variety Elstar
Apple variety Jonagold
Sound apple
Apple with notch
Fig. 1 Thermal image of Elstar and Jonagold apples. The dark stems indicate low temperature and thus higher transpiration via the stems.
References
Cut apple
Fig. 2 Localised temperature decrease caused by transpiration at defect in skin. Isothermal areas with T = 18.9 °C are coloured as black pixels, while isothermal areas at a level of 16.8 °C are coloured as white pixels.
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